Steering with Pitman Arm.

[Ahmad Rezq]

I do believe that i'am missing something very great because I'am wondering why almost all the FSAE teams go for Rack and Pinion steering ?! I didn't even see any FSAE car through my search on the internet or during FSG 2015 use Pitman steering

In my University we haven't had a good experience with R&P. Baja And formula teams tried to design and manufacture R&P in the past and the final result was below the limit. Last year Formula team bought R&P from Titan and it was very expensive.

I think this solution was suggested by Z in the beam axle discussion
859

Why a simple and cheap solution -IMO- like Pitman is rarely used in FSAE?

[Jonny Rochester]

In 2001 / 2002 UTAS did use a steering box and pitman arm. I was not involved at that time.

For road going production cars, I think a rack and pinion is used for least number of moving parts and the most direct linkage (least flex) for a sporty feel. So any sedan or coupe that is meant to be direct and sporty has rack and pinion. But for a car that is not meant to be harsh, eg Mercedes, it may have a steering box with linkage steering so a bit of compliance gives a vague remote feeling that Merc drivers like.

But if you find a design that has only the same number of parts as a rack and pinion design like Z's illustration, then go for it. But feel will depend of friction in all those parts and compliance from bending and torsion.

[Mostafa Habib]

Ahmed,
Very good topic specially for egyptian teams :confused:
==
As one of my country and live the same problem in importing components from abroad because of high prices -_-
I think we did not try sufficiently to find a solution to the problem of R&P .
- Zagazig attempt which was bad
-And you say that your attemp was the same
-Mansoura also manufactured r&d few days ago and we are waiting their results !
- in Egypt There are currently two attempts of the amendment to the steering system of the " 128 car " :
Zagazig - although there is no result till now.
Menoufia and I think it's apparent that his performance was suitable !

So let's try again !

[tromoly]

I think this solution was suggested by Z in the beam axle discussion

Z was talking about a system that's used on Karts, this picture kinda shows it:

https://s-media-cache-ak0.pinimg.com/736x/40/d8/9d/40d89d1e403932f0a6a1882216ee6a9f.jpg

From what I understand, a rack-and-pinion gives better feedback to the driver because forces from the tires can back-feed up into the steering shaft, while in a steering box I don't believe that's possible due to the worm gear arrangement.

[Claude Rouelle]

Everything looks fine with such a system...on a kinematics software on on your CAD.... until compliance is taken into account.

Simply look the best you can in your workshop at your steering ratio from measurement (steering wheel angle Vs average of inside and outside wheel steering angles) and its hysteresis. Make 10 full left to full right steering round trips to have enough data. Or from recorded track data at your steering wheel angle Vs lateral acceleration or your steering wheel torque Vs lateral acceleration and their hysteresis. Make 2.5 graph colored by speed windows.

Just wonder how much this hysteresis influence the car response and the driver confidence.

Claude

[mech5496]

Coming from a team that tried pitman steering, I can elaborate on why we ditched it. To get a reasonable packaging and steering effort, a gear reduction was needed between steering wheel and pitman arm, which could offset any advantage in terms of complexity. Of course there are other advantages that can be found in a pitman arrangement; if you are willing to devote some serious development time and out of the box thinking, it could work excellent for you.

[CWA]

First consider the overall steer angle your FSAE car needs for an FSAE event. Consider your event's hairpin radius; about 30 deg of front wheel steer is not far off what you will need IIRC based on your expected wheelbase / understeer gradient. This is a lot of steer angle for a race car to accommodate.

A pitman arm, applied like in karts, directly to the steer column (as in the previous pic) gives less than 90 degrees of travel each way. You will never get around this. Depending on the amount of non-linearity from this part of the steering system you are willing to accept, I'm sure in reality you will at best get around 70 degrees of useful steer angle each way from this layout.

So to accommodate this hardware into your system, at your hairpin you design the car to steer its front wheels 30 degrees as needed, when your driver is steering the hand wheel 70 degrees. No big deal on the face of it I suppose, but let's explore the implications. Lets say your car achieves 1.5g Aymax; you are negotiating this hairpin at 10m/s. Are all your corners going to be this slow? Of course not, your lapsims should tell you it is very likely that you will get up to 25m/s for parts of the course.

Now at this higher speed your driver has (70/2.5=) 28 deg of HWA to modulate between Ay=0 and Aymax=1.5g. 1 degree of hand wheel angle gives 0.6m/s^2 (0.06g) of lateral acceleration. Now consider how small your chosen steering wheel is, for cockpit packaging reasons. 1 degree of angle for a wheel this small is a very fine movement of the driver's hands. Further consider that your steering system isn't without friction, and also remember that your driver will be trying to finely modulate the steering whilst he himself is trying to resist all the g's he is inducing. All these combined; your driver will find it impossible to modulate HWA with 1 deg resolution. With the best intentions he may only be able to modulate HWA in 5 degree steps, consequently modulating lat acc in increments of 0.3g.

Not only might this be a problem when the driver is trying to maintain a straight path between the cones of a very narrow FSAE course, but the driver will also surely struggle to confidently and consistently approach and maintain Aymax while cornering. The point here is, with a steering ratio so low, you will likely find that the steering is far too sensitive. You can increase your understeer gradient to reduce the sensitivity caused by this lack of steer ratio somewhat, but you can't rely on this mechanism fully without compromising your cornering capacity.

Before going further, consider, is all this acceptable to you? There is only so much 'response' a human likes from a car / plane before it is deemed 'too sensitive'. Personally I am sure you will find / exceed this limit by implementing a pitman arm. If you think the high levels of response your car now has at high speeds is acceptable, be very sure that your driver agrees before continuing the design process:

Now you have to consider how much steer torque your system has. Whilst the overall steer torque level that reaches your driver is very much dependent on the (very low) steer ratio you have already chosen, it's not the end of the world, you do have tuneables at your disposal to ensure the hand wheel torques are not too high. If you have low KPI, no scrub radius, low caster, soft suspension, low castor trail, not-very-wide tyres, your steering torque at the front road wheels may be low enough so that when it passes through your pitman arm steering system with such a small amount of mechanical advantage, your driver won't have to work too hard.

BUT you don't have a clean slate here; values for so many of these parameters have already been pretty much chosen for you. You need some scrub radius / KPI because of your wheel / upright / hub packaging situation. You need some caster angle / trail because you want to align your front axle peak MZ with front axle peak FY for the driver's benefit. Your tyre choice has been dictated by other factors, and the chosen tyre is pretty wide. Your suspension is stiff because your dynamics team say it should be. So now all these things, combined with your very low steering ratio are going to contribute to a steer column torque that is very likely to exceed 10Nm at max lat acc for your average corner. Is this acceptable to you? Test your driver somehow and confirm this for yourself, but I suspect it won't be.

The answer I think you should finally come to is that the degradation to handling / control of the vehicle for your driver is not outweighed by whatever gain you thought there is by fitting a pitman arm instead of a rack & pinion (what was this benefit again; cost, weight, complexity?). You could always modify your approach to the pitman arm option and add another 'ratio' in series, maybe use some kind of gearbox in between your column and the pitman arm? But isn't this now getting as costly / heavy / complex as a rack and pinion? Now which is the better way to go? I'll leave you to reason this decision yourself.

[BillCobb]

Dang. I had to play two hockey games this morning and thus CWA took the words right out of my drive home thoughts. All I would add is that it makes an Excel-lant (or Matlab) optimization tool example. Given some front weights, caster offsets, tire data, track curvature, speed limits and some human factors and wheelhouse geometry constraints, you can solve for a front weight or an overall steering ratio that gives the driver a +- 80 deg wheel angle limit AND a 35 Nm steering wheel torque max AND a positive tierod load gradient at max lat.

Ain't gonna happen with a 2:1 or 3:1 ratio obtained from the one pictured. The control sensitivity will be way too high and the steering effort will be also. Plus the tierod front view angles at large steer angles are going to produce some peculiar understeering action affecting max lat. Either run it iteratively or let the solver do it. Make sure your steer arms are within the wheelhouse !

[Z]

Ahmad,


"Why a simple and cheap solution -IMO- like Pitman is rarely used in FSAE?"

The short answer is that FSAE, like most other areas of social life (especially Engineering!), is driven primarily by FASHION.

The thinking is,
"All the other Teams are using a R&P, therefore we must too!" [Insert sound of bleating sheep...]

Some Teams have used Pitman-Arm steering in the past, and quite successfully (UWA comes to mind). But because it was buried in the footbox of the car and not highly visible, the "goodness" of the idea never spread. It never became the latest fashion, never the "new black".

A R&P with direct-shaft-drive and no UJs (or maybe only a single "flex-disc" at the pinion) can be a very good solution. It is simple, low parts count, etc., but it also has the disadvantages listed at end of this post. This R&P solution is especially good when the rack can be mounted at about mid-wheel height or a bit higher, because it puts the driver's steering-hand-wheel at a good ergonomic angle. But this is really only possible if the driver's feet are behind front-axle-line, and the R&P is mounted at about mid-height of front-bulkhead (this being normal on most other racecars).

But a R&P on the floor and under the driver's knees, as is common in FSAE, either has a direct-shaft and non-ideal hand-wheel angle, or it requires a means of "bending" the drive from hand-wheel down to the R&P. Multiple UJs can do this, but when poorly executed they give terrible results (much floppiness!), and they increase parts count, cost, and build time.

The next alternative is to add a Bevel-Gear-Box (BGB) to do the "bending" of the drive-shaft. But this solution, again very common in FSAE, strikes me as utter madness. It is a blind, or very myopic, approach of constantly adding more and more COMPLEXITY to the problem, whilst NEVER STANDING BACK TO SEE THE BIG PICTURE!

As should be obvious when you take that step back and look at "the whole", you should see that once you have the BGB, the R&P becomes completely redundant! :)

More below...
~~~o0o~~~

CWA,


BUT you don't have a clean slate here; values for so many of these parameters have already been pretty much chosen for you. You need some scrub radius / KPI because of your wheel / upright / hub packaging situation. You need some caster angle / trail because you want to align your front axle peak MZ with front axle peak FY for the driver's benefit. Your tyre choice has been dictated by other factors, and the chosen tyre is pretty wide. Your suspension is stiff because your dynamics team say it should be. So now all these things, combined with your very low steering ratio are going to contribute to a steer column torque that is very likely to exceed 10Nm at max lat acc for your average corner. Is this acceptable to you? Test your driver somehow and confirm this for yourself, but I suspect it won't be.

The first two emboldened sections above can easily be adjusted to give light and precise steering-feel for the type of system Ahmad is considering. Which is...


You could always modify your approach to the pitman arm option and add another 'ratio' in series, maybe use some kind of gearbox in between your column and the pitman arm?

This system is, of course, right in the middle of the sketch Ahmad posted! Actually, just to right-of-middle, and, importantly, with NO UNNECESSARY R&P to push up "cost, mass, complexity", etc.

I repost that sketch and another similar one here, so more easily visible to all.

This first "TBW" (http://www.fsae.com/forums/showthread.php?1324-Beam-Axles-Front-Rear-or-both./page3) sketch is from early in the "Beam Axles..." thread.
https://lh5.googleusercontent.com/-8ABMaKebyXs/Txtdpl2DZEI/AAAAAAAAAIs/fCJ-84VnwMc/s800/TwinBeamWing.jpg

Here is a more recent sketch - "BA5 - Front-Beam with BGB+Pitman-Arm-Steering" (http://www.fsae.com/forums/showthread.php?1324-Beam-Axles-Front-Rear-or-both.&p=121937&viewfull=1#post121937)
https://lh6.googleusercontent.com/-smRoViUfOQY/VG2_TYH_lKI/AAAAAAAAASA/otRZi2dRMLU/s800/Beam-Axle-5.jpg

Both Beam-Axle steering systems are explained in words in the linked posts. Of course, similar "BGB+Pitman-Arm" steering can also be used with wishbone suspensions.


But isn't this now getting as costly / heavy / complex as a rack and pinion? Now which is the better way to go? I'll leave you to reason this decision yourself.

Some reasoning given below.
~~~o0o~~~

Short List of Pros and Cons of R&P vs BGB+PITMAN-ARM.
==============================================
(Note that a R&P is effectively a BGB with very large radius crown-wheel.)

R&P.
=====
Pros:
* Everyone else uses them, so you get a warm, comfortable, feeling inside...
* Can be bought "off-the-rack" :), so no engineering effort required.

Cons:
* Potential for more friction/stiction because the surface between rack and housing has large sliding velocity.
* Potential for more slop/compliance, especially when steering-tie-rod is at angle to rack-centreline, rack-end has large overhang from its housing, and housing is short. This often called "rack-rattle".
* Massive slop/compliance of typical FSAE-style racks, because they hardly ever have the spring-preloaded teeth-mesh that is standard on every production car R&P that I have ever seen. This is quite easy to fix properly, but requires some engineering! The typical-in-FSAE screw or shim adjustment of teeth-mesh as they wear just gives slop at straight-ahead, and bind at higher steering angles.
* Less scope for good Ackermann tuning, because fewer sinusoidal "non-linearities" to work with.

BGB+PITMAN-ARM
==============
Pros:
* Minimal friction/stiction because low sliding velocity between mainshaft and housing, and easy use of needle-roller-bearings.
* No slop of the "rack-rattle" type.
* No slop of the loose-teeth type, because easy to spring-preload the "teeth-mesh" (as I explained on posts with the above sketches).
* Potential for very good Ackermann. In fact, much better that any FSAE car I have ever seen!

Cons:
* You are not part of the flock, so no warm and happy feeling inside. Yep, you have to get used to feeling like a stupid old goat...
* Cannot buy complete unit off-the-rack, so you will have to make some bits yourself.

Z

[CWA]

Z, I must admit I did not check the picture of yours that Ahmad posted in the o.p. I looked at the second pic from tromoly, read through everyone else's posts, and assumed everyone was considering fitting rack and pinion directly to the column without another ratio in place. Now I think I was incorrect to assume this.

I hope my reasoning against this particular application of the PA (with no GB) is still useful though, for anyone who was perhaps wondering why they can't just get rid of the GB, mimic a kart, and have even more 'complexity reduction'. If anything I hope my post helps provide anyone interested a way to arrive at a sensible steering ratio before the car is built and tested, no matter what hardware solution they use. Perhaps everyone knew to be careful of steering sensitivity already, but it is something our team overlooked the importance of in first year.

Anyway, I completely agree with the reasoning you've provided for BGB+PA versus R&P. It is certainly a viable option, I guess my post does but I did not mean to strongly imply otherwise. BGB+PA certainly has scope to provide plenty of benefit, just as you've described, if only one can convince others in their team (usually those with authority over the budget) that it is not a big risk despite being unfashionable and less common. This attitude was certainly prevalent in both teams I was a part of, and it's something I've seen plenty of times in industry too. It can be frustrating at best, and poison to creativity and objective reasoning at worst.

Finally, Ahmad, put 'bump / roll steer' on the pro/cons list Z has made. If you do not plan to use the BGB/PA solution on a beam axle as in Z's pic, and would use it alongside SLA suspension, your bump/roll steer will be very different to a) Z's implementation of it on a beam axle, and b) what you may be able to achieve with a long rack. I'm not saying which option is better or worse for this metric, but it's something else you will need to consider.

[Ahmad Rezq]

In terms of cost and complexity.
Provided is a sectional view from our Titan R&P.
864.
Total Cost -If i do remember- was $1100, and we were running a low budget.
We didn't even consider PA with GB in our decision matrix last year. Only we were comparing between designing and manufacturing R&P or Buying one.
I may agree with Z that we were copying what the steering system in a FSAE car have to be.

[Charles Kaneb]

Three factors to consider:

1) In a Formula car I do not want the driver to be crossing hand-over-hand or shuffling the wheel around. Check out onboard videos of the faster FSAE cars - you'll rarely see more than 90 degrees wheel movement, no matter what steering system is used.
2) Pitman arm steering is nonlinear, as you'll find if you play with a kart on a stand or make some drawings (when you draw it, make the steering arm longer than the pitman arm). Even better, it's falling-rate, so it can be fast on-center when effort is low and slow well off-center when effort increases.
3) This is a sprint race. The driver can apply more force to a steering wheel for an 11-minute enduro than he or she can for a 3-hour Le Mans, Grand Prix, or Indianapolis race or for a NASA spaceflight. If your best driver's a big Texan SOB, you can reasonably demand that he be strong. If your best driver's a small lady, the weight on the front tires (and thus the force required to keep the car balanced) is reduced, cutting down steering effort (1). Draw "reasonable-looking" steering geometry, find inputs (KPI, trail, steering arm length) that work for the tires, then calculate how much steering force you'll need both near-center (fast-corner) and near-lock (slow corner) - then USE that as an input to the driver's strength training program!


(1) In most FSAE cars, it's not a bad assumption to say that the CG of the driver's torso, head, and arms are roughly at the x-location of the CG of the car/driver combo and the driver's legs have a CG near their knees. If your car has a mass of 160 kg, then your driver's mass of 80 kg is 1/3 of the combined mass, and 40 kg of that is the driver's legs...

[CWA]

So you think a pitman arm straight from the column is a realistic option do you Charles? I know the suitability of this option shouldn't be dictated by the answer to these questions alone, but have you ever seen it done on an FSAE car? And is this what your car is fitted with?

Sure you don't want to cross hands, but you can easily get to ~150 deg before that has to be an issue for the driver. Yes you can technically achieve 90 degrees with a pitman arm, but you have to 1) make sure your tie rods won't hit your column, 2) protect yourself against over-centring, 3) deal with how incredibly non-linear this system is at 90 degrees. Which is why realistically you aren't really going to be able to use more than 70 degrees.

So your effort levels and response are already twice what they need to be. Ignoring effort levels, maybe the driver CAN control the car with this level of response, but would they 1) prefer less sensitivity, 2) perform better with less sensitivity? I guess the only real proof is in the testing, and I've never tested a car with this much response so unfortunately can't comment further. But I would be very surprised to hear that this would be favoured.

[DougMilliken]

A variety of steering gears are shown on this page, http://what-when-how.com/automobile/steering-components-automobile/ Brainstorming (I've never seen one of these), take a look at "Fig. 27.53. Screw and nut re-circulating ball gear mechanism".

Instead of generating motion with "pin-in-slot and rocker arm", what if the motion of the ball nut pushes-pulls on a tube that is concentric with the steering column? This in turn pushes on one arm of a bellcrank--and the other end of the bellcrank would be the Pittman arm (called "drop arm" in the figure).

Obviously many details to work out, but this could be a lightweight solution that also allows a lot of packaging flexibility? To position the inner tie-rod ball joints correctly use a relay rod and idler arm, see http://what-when-how.com/automobile/steering-systems-automobile/ see "Fig. 27.49. Split track-rod with relay-rod and idler steering linkage layout".

Ball screws and matching nuts are commercially available in a wide variety of sizes and thread pitch...

[BillCobb]

Take a look thru a junkyard at all the sophisticated steering setups used in today's finest lawn mowers. Heck, my John Deere F935 is rear wheel steer by hydraulic hose. Plenty of good used and new replacement parts available for these machines, too.

[BillCobb]

Wondering why so much trailing (rear) steer since pulling on the high force tierod is easier that pushing on it. I'm used to seeing front steer on rear drive high performance cars because of the N/S motor and there's no transverse transmission inconveniently in the way. There's some relief from the mass compounding effect with this, too.

As you can see, pushing the steer axle on this Advance Steam tractor probably wouldn't cut it. It was worm gear, though, too.

And another example of creative steering geometry, the FSAE Ratrod Class, also chain steer so to speak...

[Charles Kaneb]

CWA - It depends on what sort of car you're building! Karts have response like that and I cannot drive well WITHOUT it. I'm pretty awful at autocrossing passenger cars. Motorcycles are even more sensitive.

My wing-kart, 375 mm steering wheel, mass 80kg, zero trail, with a wheel 200 mm outboard from the kingpin axis and 15 deg kingpin inclination, doesn't have problems with excessive (> 200N) steering effort at 3+ g. It has a 50ish mm pitman arm and 125ish mm steering arms. If you build a 250 kg understeering dump-truck with 80 mm mechanical trail and a 50/50 weight distribution, this could be painful. If your ideal FSAE car is close to my ideal (150 kg, 40/60 F/R with a driver, <200 mm CG height, diff open at low torque, 25 mm mechanical trail) the numbers might work out for you. It was a close decision for A&M's -13 car.

[Charles Kaneb]

Bill, really odd steering & suspension geometries can be found all over The Henry Ford.

[mech5496]

That's a nice discussion going on here. Last year we considered multiple "unconventional" approaches on steering; pitman arm (exactly like Z described) was the one chosen. Angular gearbox is NEEDED because of the implications described prior on the thread by others, in our case the ratio was close to 4.5:1 to make it work. That being said, upright pickups were constrained in being almost identical with last years' so we could reside to using an RnP as emergency, if pitman steering failed us. The guy who designed the system got it to a point that ackerman and steering response was almost "perfect" (far better than any RnP system I have seen so far). However we really struggled to get a reasonable bump/roll-steer response (whether this is an issue or not is up to you). Now the really "crappy excuses" part starts; the guy responsible for the steering ditched the team, and that let our transmission guy struggling to manufacture the bevel gearbox, housing, mounts etc. in less than a month, while also fixing his issues with the transmission. Bottom end; a 250$ junior drag rack, modified to suit our needs in less than 2 days.

[Kevin Hayward]

ECU's steering is a little different using a planetary gearbox driving a pitman arm, then to rockers. Really great for getting good control of ratios throughout motion, and allows for a super low nose (which was the main motivation). If it wasn't for the front suspension the rockers could be ditched and run it just using the planetary. The planetary was designed and built in house using off the shelf gears. Cheaper than a rack and super low slop due to the high number of contacts.

I will say that it is worth being careful with alternative steering systems as the loads can get quite large in the mechanism and mounts. Much of this is dealt with internally in a R&P.

Some tuning the guys have done this year has been quite eye opening with regards to the steering. Improved control over dynamic toe (ackermann) and rate can have some interesting effects that cannot be achieved with a conventional R&P. These cars can be super-sensitive to steering so it may be worth allowing for a little weight to have a better system.

Kev

[CWA]

Charles, naturally I'm talking about FSAE cars. Two things stand out for me that I believe make the case of your kart different to an FSAE car:

1. You say your kart steering wheel is around 375mm = 15". IIRC our FSAE steering wheel was close to 8" diamater, I assume most other FSAE cars are similar to this. Again, ignore steering effort, but consider the 'response' of the vehicle to be it's Ay as an output caused by steer displacement as an input. Except this time consider tangential displacement of the steering wheel rim at the driver's hands, or driver arm displacement, rather than angular displacement of the HW / column. For any given vehicle, if you fitted it with A) the FSAE hand wheel, then B) the kart hand wheel, I believe the effective response of case A could be considered as double that of case B.

So, perhaps your kart does have favourable 'response' levels with a pitman arm and less than 90 degrees total lock. But if you tried to achieve this with an FSAE hand wheel of half the size, the vehicle may be deemed too responsive by any normal human pilot.

2. In my experience, karts (vs FSAE) do not have to accommodate such a range of vehicle speeds / corner radii (and hence range of steer angles). This is an issue for FSAE cars as per my previous reasoning - you have a limited amount of HWA you can utilise with a pitman arm, the max of which has to deliver quite a large amount of steer angle for a slow hairpin, meaning your steering response for the high speed corners is perhaps too high. The equivalent high-speed corners for a kart may be long sweepers and may be more forgiving than short duration FSAE slaloms / gates for a driver trying to modulate his steering system which is highly sensitive? I do not know if this is true for your kart / race series, perhaps your range of speeds are more equivalent to FSAE events than I realise. I wonder if you have any Ay vs Steer data to give insight into this aspect?

Kevin, that system sounds very interesting. I'd love to see some pictures / CAD if you happen to have any you'd be willing to share?

[mdavis]

The University of Cincinnati for the last 2 years has run a pitman arm steering system, but with no gear reduction of any kind. U-joints were used to get the steering wheel angle at an appropriate position, compared to a straight shaft. The 2014 car did not run, but I had a chance to drive the 2015 car at a test day this past summer. Beyond the spring/damper setup issues that the car had, the steering was incredibly heavy. It had a very fast ratio (slaloms were accomplished with ~+/-10-15 degrees of HWA, if that), but the weight was way too high to be fast over an endurance length. I would have struggled to drive the car flat out for 2 autocross laps at MIS, it was so heavy. More seat time with the car, and training would have helped a lot, but the steering was brutally heavy. More rear weight bias, different steering ratio, and/or some rear steer could have mitigated a lot of the design issues, IMO.

Having driven a car with a pitman arm setup, I think it is very possible to get a lighter than RNP solution in an FSAE car that provides appropriate weight and steering response. I would venture that neither of the UC setups were all that well analyzed or very well executed, and because of this, the team has (sadly) gone back to a RNP for 2016.

Kevin, I would also be interested to see any pictures/CAD you're willing to share of the ECU steering setup. No need to share the tuning details, unless you/the team want to.

[Z]

Ahmad,


Provided is a sectional view from our Titan R&P.
...
Total Cost -If i do remember- was $1100...

Well, at least for $1100 you get a spring-preloaded rack. But ...

If the bushings in the end of the housing are a close fit on the rack, then the spring simply pushes the rack against those bushings, and, when the R&P teeth wear "at centre", ... you still have slop-at-centre!

On the other hand, if the end-bushings are made loose enough to allow the sprung rack to be pushed against the pinion's worn teeth for no slop, and also move away from the unworn teeth for no bind, then ... you always get rack-rattle!

Ahh, ... $1100 ... because racecar! :D
~~~o0o~~~

Doug,


... this could be a lightweight solution that also allows a lot of packaging flexibility?
... see "Fig. 27.49. Split track-rod with relay-rod and idler steering linkage layout".

Yes, the idea of having two "idlers" has significant advantages, though not obvious.

Consider two "idlers" (aka bellcranks/relays/???) mounted at either side of the upper-frame, and pivoting about vertical axes. The lower end of each idler has a Pitman-Arm that drives a short steering-tie-rod going out to the wheel, with this tie-rod at the most suitable height between lower and upper wishbones. The upper end of each idler is at steering-Hand-Wheel-height and is driven from the HW by any suitable means. One way would be via a R&P mounted near the top of the FRH/footbox.

But I would drive the idlers via cables! Fit multi-groove pulleys at the end of the (short) HW-steering-shaft, and also at the top of the idlers, then use multi-strand (7 x 19?) steel cables about 3 to 5 mm diameter to transfer forces back and forth. Of course, you need two sets of cables, one pulling each way. Or use roller-chains and sprockets as on Bill's link. See also speedboat steering systems, or the many (small to very big) aeroplanes that use similar.

One advantage here is that by making the pulleys non-circular it is very easy to get a large range of different Ackermann behaviours and/or "variable ratio steering". In essence, you have two separate variable-ratio steering systems, one for each wheel. So set a little static toe-in for stability on the straights, then get the inner-wheel to turn fastest for early and large "dynamic toe-out" for lightning fast turn-in. All this is very lightweight, easy to make, robust, and slop-free (because you spring-loaded the cables). This is why it is so common on planes.

But the really big advantage is that the chassis floor can now be flat and obstruction free from driver's bum all the way to Front-Bulkhead. No need for the common "stepped-floor" that is necessary for the R&P to be at right height wrt wheels. Lowering the floor also lowers the driver's quite heavy lower legs, and lowers the whole of the footbox structure, including quite heavy pedal-assembly, FB, and IA. (BTW, I believe this "lowering of footbox" is the major rationale for ECU's system.)

So, more complexity by adding the two idlers, but overall a simpler chassis to design and build, and a significantly lower CG, and possibly less total mass.
~~~o0o~~~

Feasibility of PURE PITMAN-ARM Steering.
=================================
The main disadvantages I see with pure PA (as also noted by Mdavis above) are;

* Because of the various practicalities of the full linkage (ie. Ackermann, bump-steer, etc.), only about +/- 60 degrees of steering-HW-Angle is possible (ie. HW turns 60 degrees from centre to full-lock-one-side). Better would be HWA = +/- 45 degrees only. But this could mean too high driver arm-loads, and/or too fast car response to small hand movements.

* For typical FSAE location of driver wrt front-axle-line, the plane of the HW might be too horizontal, or "bus-driver-ish" (ie. near vertical steering-shaft).

But I reckon all these problems can be overcome. Taking them in reverse order;

* A car that has the driver entirely within the wheelbase (ie. feet on or behind the front-axle-line) can have the PA in front of driver's feet and at a suitable height that gives the right ergonomic angle for the HW. And "driver-inside-wheelbase" is generally a good thing for FSAE dynamics, because it gives more R%, lower Yaw-Inertia, simpler, lighter chassis, etc.

* The "too fast car response" is not a problem at all, IMO. In fact, it is what you should be aiming for!

Total car response time = time for driver to initiate the action + time for car's tyres, etc., to respond and make the car move. Reduce the driver-action-time to zero and you still have to wait for the tyres and the rest of the car to do their thing. That can still take a long time for a car with soft tyres, large Yaw-Inertia, etc.

Consider also that modern fighter jets are controlled by a rigid side-stick that barely moves at all. It is strain-gauged and responds to the pilot's hand forces, rather than responding to large motions. But to give the planes really fast response times the designers had to make them dynamically unstable as well (ie. negative "static margin"). Just reducing pilot-action-time to zero is not enough.

* So the problem boils down to finding a way to lower the HW forces required for such quick steering of, say, +/- 45 degrees of HWA. Simply moving the driver, and hence also the car's CG, rearwards, is a big step in that direction. Making the whole car lighter also helps. And centre-plane steering geometry helps a lot, with just enough Trail for the right feel (many FSAE cars already have this, so it is certainly "feasible").

But the "guaranteed to work" solution is found in almost all modern vehicles. I have a book (picked up cheap at school fair) that lists the specs of "All The World's Cars -1964". It lists many big American cars with manual steering that have at least 5 turns lock-to-lock! No modern car has steering that slow, because nowadays they all use POWER-STEERING! Even the very small cars. Even some Quad-bikes these days come fitted with Electric-Power-Steering. And a Quad-bike's handle-bars only turn about +/- 45 degrees. Hmmm...

But all things considered (ie. total mass, electrical load, speed of response, ++), I reckon a small car's hydraulic-PS would be the best choice. Quite easy. Fit an off-the-shelf belt-driven PS-pump+tank on engine, two hoses to an (off-the-shelf) valve-in-steering-shaft, and then an appropriate hydraulic-actuator on PA. The return you get from this investment is lightning quick steering that requires almost no effort from the driver, and is all wrapped up in a simple mechanical linkage.

Worst case, you remove all the hydraulics, reduce the Trail to ~5-10 mm, and slap the Enduro driver on the back and say, "You can do it!!!"

(FWIW, I once bought a 30 ton front-end-loader to help me do the gardening. It had a steering-box with about 3+ turns lock-to-lock. This broke one day (it was a conventional 'box like in Doug's link, but the balls managed to escape the worm, and then break other stuff). Of course, given the loader's weight and its "bend-in-the-middle" steering (ie. picture two 15 ton lumps mutually pivoting about a single, central, vertical axis), it had to have power-steering (... via 2 x ~5"-diameter-rams!). So I threw the steering-box away and replaced it with a homemade Pitman-Arm welded to the end of the steering-shaft. Yep, +/- 60 degrees HWA, and I had the biggest go-kart in the neighbourhood! Worked great, much better than the original. :))

Z

[apalrd]

But the "guaranteed to work" solution is found in almost all modern vehicles. I have a book (picked up cheap at school fair) that lists the specs of "All The World's Cars -1964". It lists many big American cars with manual steering that have at least 5 turns lock-to-lock! No modern car has steering that slow, because nowadays they all use POWER-STEERING! Even the very small cars. Even some Quad-bikes these days come fitted with Electric-Power-Steering. And a Quad-bike's handle-bars only turn about +/- 45 degrees. Hmmm...

But all things considered (ie. total mass, electrical load, speed of response, ++), I reckon a small car's hydraulic-PS would be the best choice. Quite easy. Fit an off-the-shelf belt-driven PS-pump+tank on engine, two hoses to an (off-the-shelf) valve-in-steering-shaft, and then an appropriate hydraulic-actuator on PA. The return you get from this investment is lightning quick steering that requires almost no effort from the driver, and is all wrapped up in a simple mechanical linkage.

Why not electric power steering?

There are a number of quads and snowmobiles and such that use electric power steering, as well as a large number of production road cars. Less parts, less weight, and less interference with the engine.

[CWA]

mdavis - very interesting to hear of your experience with pitman arm to column in fsae. Perhaps in your case the driver would have actually been fine with the response if the effort wasn't so great after all. What kind of steering axis geometry did your car have, out of interest?

Z, the strain-gauged flight control column is interesting to learn about, I am keen to read into this further. R.e. your comments on response, I had not considered a lack of steer ratio to cause what I’ve heard referred to as a ‘transient response’ issue, as it sounds you are describing. Ignoring transient input / output delays (which I agree, are really not much good at all), I believe the steady-state Ay/Steer gain can often still be too much. This is what I was referring to in my previous posts. Do you not think this could be the case with a lack of steer ratio? Of course this does not tie in with how your aforementioned fighter pilots now manage to control jets with only column force, not displacement. Perhaps the difference is that an fsae car will never be able to actually achieve a linear relationship between input torque and output Ay / yaw rate response (too much hysteresis?), so a driver always has to rely on displacement correlating with response for intuitive control?

Anyway I agree that with cooperation from other designs areas of the car, a steering axis could be designed to reduce steering efforts to acceptable levels with pitman arm to column.

[Kevin Hayward]

I have attached a GA of the first steering version as requested. This years is simpler, stiffer and lighter.

As Z indicated the initial reasoning was to reduce COG height by lowering the nose and driver. These were lowered by around 100mm. Note that this is a larger effect on COG than lowering an engine with a dry sump (while being simpler and lighter).

However through iteration and tuning it looks like what can be done with steering geometry is the larger advantage.

Excuse me not putting too many details of the design, but it is fairly straightforward to work through with some thought. Planetary gearboxes are not too hard to make and is probably not a bad call for someone wanting to go close to a pure Pitman arm while not having to deal with the high steering effort. ECU designed a "pancake" style gearbox instead of the normal output on one side, input on the other. This is possible due to not needing full rotations on the output.

Cheers,

Kev

[mdavis]

But the really big advantage is that the chassis floor can now be flat and obstruction free from driver's bum all the way to Front-Bulkhead. No need for the common "stepped-floor" that is necessary for the R&P to be at right height wrt wheels. Lowering the floor also lowers the driver's quite heavy lower legs, and lowers the whole of the footbox structure, including quite heavy pedal-assembly, FB, and IA. (BTW, I believe this "lowering of footbox" is the major rationale for ECU's system.)

So, more complexity by adding the two idlers, but overall a simpler chassis to design and build, and a significantly lower CG, and possibly less total mass.
~~~o0o~~~

Feasibility of PURE PITMAN-ARM Steering.
=================================
The main disadvantages I see with pure PA (as also noted by Mdavis above) are;

* Because of the various practicalities of the full linkage (ie. Ackermann, bump-steer, etc.), only about +/- 60 degrees of steering-HW-Angle is possible (ie. HW turns 60 degrees from centre to full-lock-one-side). Better would be HWA = +/- 45 degrees only. But this could mean too high driver arm-loads, and/or too fast car response to small hand movements.

* For typical FSAE location of driver wrt front-axle-line, the plane of the HW might be too horizontal, or "bus-driver-ish" (ie. near vertical steering-shaft).

But I reckon all these problems can be overcome. Taking them in reverse order;

* A car that has the driver entirely within the wheelbase (ie. feet on or behind the front-axle-line) can have the PA in front of driver's feet and at a suitable height that gives the right ergonomic angle for the HW. And "driver-inside-wheelbase" is generally a good thing for FSAE dynamics, because it gives more R%, lower Yaw-Inertia, simpler, lighter chassis, etc.

* The "too fast car response" is not a problem at all, IMO. In fact, it is what you should be aiming for!

Total car response time = time for driver to initiate the action + time for car's tyres, etc., to respond and make the car move. Reduce the driver-action-time to zero and you still have to wait for the tyres and the rest of the car to do their thing. That can still take a long time for a car with soft tyres, large Yaw-Inertia, etc.

Consider also that modern fighter jets are controlled by a rigid side-stick that barely moves at all. It is strain-gauged and responds to the pilot's hand forces, rather than responding to large motions. But to give the planes really fast response times the designers had to make them dynamically unstable as well (ie. negative "static margin"). Just reducing pilot-action-time to zero is not enough.

* So the problem boils down to finding a way to lower the HW forces required for such quick steering of, say, +/- 45 degrees of HWA. Simply moving the driver, and hence also the car's CG, rearwards, is a big step in that direction. Making the whole car lighter also helps. And centre-plane steering geometry helps a lot, with just enough Trail for the right feel (many FSAE cars already have this, so it is certainly "feasible").

Worst case, you remove all the hydraulics, reduce the Trail to ~5-10 mm, and slap the Enduro driver on the back and say, "You can do it!!!"

Z

On corner entry, the UC 2015 car actually responded very well, almost to the point where I didn't like it. I think that was more of a rear end suspension geometry issue than anything else (RC too high), where I could feel the rear end unloading. More static rear weight would have helped this, and the team missed their weight distribution goal by a decent amount. That car also had very high yaw inertia (I'd have to ask the team what the value was with 160lb driver, then add some for 210lb driver. The front wing (specifically end plates) were very heavy and this caused issues. The rear wing endplates had similar issue, which raised CG, exaggerating the rear weight transfer issue on corner entry. All things that are relatively easy to fix with an iteration of that car. Sadly, the team is going back to a RNP for 2016, so we'll never know if the system could be implemented in an FSAE environment. The pitman arm manufacturing also left something to be desired, but could be easily improved.

I have thought numerous times about building a larger version of an FSAE car for SCCA's A-Modified Solo II class, and the idea of a completely flat bottom of the chassis has some huge advantages for frame construction. You can also run the LCA's of an SLA suspension very far inboard (under the driver's legs) as UC has done in the past (2011) to the point where none of the extra parts would be necessary for reasonable bump steer.


mdavis - very interesting to hear of your experience with pitman arm to column in fsae. Perhaps in your case the driver would have actually been fine with the response if the effort wasn't so great after all. What kind of steering axis geometry did your car have, out of interest?

I think with a few geometry changes in the rear end (or possibly just simple setup changes), the car could be very good. I drove it after I had graduated, and was more on a "consultant" level than anything else. In exchange, I provided a fairly thorough write-up to the team about the whole car, packaging, and behavior from the driver's seat. The response was quite good on turn in, though steady state was pretty terrible. Some changes were made after I drove the car that seemed to trend in the right direction, but tire wear was far worse, especially on the front end. I don't think this was something that couldn't be compensated for after some more setup changes, but I was just suggesting gross changes to make big swings at the balance. If I had to guess, I'd say 2-3 seconds worth of time could be gained on an autocross course with that car from just setup changes.

As for the steering geometry, I do not know the specifics, but I think there was close to 3/4" of scrub radius on that car (7" wide wheels, with a 4" inner half is somewhat limiting here, which is why we ran 6" wheels in 2013), and if I had to ballpark caster and KPI, I'd estimate both were designed to be ~5 degrees each (maybe 1-2 degrees either way for those quantities) and I'm not sure what the mechanical trail was. Regardless of what the numbers were, I think re-designing the outboard suspension could allow for adequate steering weight to not need a bevel box or anything with a pitman arm steering.

If anyone questions whether or not drivers can achieve max lat with very minimal steering inputs or changes, you need to go watch a dirt oval kart race. Watch the hands of the really fast karts during qualifying. If I had to guess, they're moving less than 5-10 degrees HWA from straightaway to corner. This is a racing event where 20/100+ karts make the feature, and it could be a matter of .0015 seconds covering the entire 20 kart feature.

Kevin, thanks for that CAD. I hadn't seen the rocker mechanism in the pictures on the Facebook page last year. Very neat and tidy system, and that brace could provide a nice place to mount pedals.

[Kevin Hayward]

mdavis,

The original system did have the rockers mounted of the pedal box mounts. However it was not stiff enough and the brace you see in the CAD was stand-alone. It could have been made to work, but the team didn't have the time to redo the pedal mounts at that point. The team also started with the gearbox near the driver, however the steering shaft twist was far too high.

The new system eliminates nearly all the mounts and is much better integrated with the existing chassis structure.

Kev

[Z]

Andrew,


Why not electric power steering?
... Less parts, less weight, and less interference with the engine.

I would argue that compared with Hydraulic-Power-Steer, EPS is more complicated, heavier, and messes more with the engine. The main difference between the two is the "source" of the power.

An EPS system "piggy-backs" on the car's existing electrical power source/store, whereas HPS typically requires the addition of an extra "hydraulic-pump + oil-tank" (ie. analogous to "alternator/E-pump + battery" in EPS). Well, unless the car is an older Citroen, which comes standard with the hydraulic-pump + regulator + accumulator to power its self-levelling-suspension, brakes, clutch, gearbox, etc. So on Citroens, HPS is a miniscule addition of hardware (= 1 x very small valve on pinion-shaft + some o-rings around the rack...)

Anyway, add an EPS to a typical FSAE car and you will very likely have to fit a bigger alternator. In recent years Monash have run an extra belt-driven "E-pump", because their standard bike-engine's alternator was not enough for FSAE E-loads. When you add an often demanded extra few hundred watts of EPS to all the other E-stuff on a typical FSAE car, then I reckon you will need a BIG back-up alternator! Any power-steering system, E or H, will have to work quite hard, and almost continuously, in FSAE/Autocross conditions.

But I agree that EPS from a Quad-bike would be very EASY to fit to an FSAE car. And Quad-bike steering is the archetypal Pitman-Arm steering. It could be almost "bolt-on and drive". Just make sure your engine's E-pump is big enough...
~~~o0o~~~

CWA,


... I believe the steady-state Ay/Steer gain can often still be too much.
... Do you not think this could be the case with a lack of steer ratio?

No. Consider a car that is set-up correctly as a whole (see below), and has a "rigidly mounted steering-HW" fitted with strain-gauges that control lightning fast actuators that steer the road-wheels, such that the front-road-wheel-angles change (almost) "instantaneously and effortlessly" in direct proportion to the driver's hand forces (and, yes, I know all this is probably illegal in FSAE, but...).

I reckon good drivers would find this car great fun to drive, but, after a while, they would wish that the whole car would react MORE QUICKLY to their steering demands! Furthermore, if this type of PS was fitted to a variety of FSAE cars with different overall-masses, tyre-size/stiffnesses, yaw-inertias, etc., then the drivers would very easily differentiate the soft-tyre/high-inertia cars from the faster reacting cars, and would much prefer the faster ones.

The key here is to get the "whole car set-up" right. Briefly, the car should be stable with "hands off the wheel" (in simple terms this means toe-in front and rear, stiffer cornering-compliance rear-tyres, positive static-margin (*), etc.). Then, as soon as the driver signals "go left", the car becomes "unstable" to the left (front-wheels turn left and develop big toe-out, etc.). When driver lets go of HW the car proceeds stably in its current direction (well, after a minimum amount of yaw overshoot).

(* I haven't checked the sign convention for SM. Here and in previous post I assume "positive = good/stable", "negative = bad/unstable", ... like a bank balance. Corrections welcome. :))
~~~o0o~~~

FWIW, the Ford Model-T (a reasonably successful design!) has a planetary-reduction-box just under the steering-HW, and a Pitman-Arm at the end of the output-shaft from that planetary-box. The planetary-reduction was ~5:1, so the T's 1.25 turns lock-to-lock of HW gave +/- 45 degrees of PA motion.

Z

[CWA]

Right then. Let's not worry about steer ratio chaps. Get some wheels with a nice big positive offset, chuck in a vertical steering axis, no worries over steering effort, slap on your pitman arm with no gearbox, job's a good'un.

[MCoach]

I briefly considered go-kart type linkage, but found that without attempting to compromise desired geometry, either the forces were too high or the linkages would bind around the steering shaft due to their close proximity. The steering system ended up weighing nearly nothing anyway, so I wasn't too bothered by over complicating the steering system with a rack & pinion system.

With regard to EPS, this is likely a more feasible option. It's difficult to find a sufficiently sized engine that has PTO (Power take off) shaft for accessories. Monash may be a bad example because fitting an alternator rather than upgrading the stock stator is more of an outlier.

"The key here is to get the "whole car set-up" right. Briefly, the car should be stable with "hands off the wheel" (in simple terms this means toe-in front and rear, stiffer cornering-compliance rear-tyres, positive static-margin (*), etc.). Then, as soon as the driver signals "go left", the car becomes "unstable" to the left (front-wheels turn left and develop big toe-out, etc.). When driver lets go of HW the car proceeds stably in its current direction (well, after a minimum amount of yaw overshoot)."


I would disagree that toe-in front and rear is a good idea on these cars. Quite the opposite really. I think the car you will find that outperforms this one is the statically unstable car that is ready to dance with the devil but still on a leash. Toe out front and rear, with a large, fast available control input from the tire construction itself. Rear steer could aid with this strategy. This could double down with "side force" generation from big MF endplates, restoring vehicle stability, even though you are "hero driving" into the corner nearly sideways and at full throttle. ;)
Soft tires for life.

Kevin, I want to say I saw one of the Canadian Hybrid teams with a similar set up. They did so to clear their front electric motors.

[Claude Rouelle]

Kevin,

No worry about compliance? My experience is that the more you add parts the more the causes for flexibility...

The long link between rocker and the steering bracket (that is bolted on the upright) tension, compression, buckling.....the rocker itself, the brackets that hold the rockers..... Was a load case made?

Claude

[Claude Rouelle]

From the reading of all these posts (and especially the one of mdavis) it seems that steering torque calculation is not (or at least was not) part of the design process... I find that a bit worrisome.

[Z]

I would disagree that toe-in front and rear is a good idea on these cars. Quite the opposite really. I think the car you will find that outperforms this one is the statically unstable car that is ready to dance with the devil but still on a leash...

MCoach,

And what is that "leash"?

A steering linkage that has "static toe-in" plus some self-centring from positive Trail gives the car good stability when travelling straight-ahead. If this linkage quickly develops large "dynamic toe-out" as the wheels are steered away from straight-ahead, then the car has rapid turn-in from this destabilising effect. Win, win.

The "static toe-in" is the leash. The "dynamic toe-out" unhooks the leash. How else would you provide the leash?

The "computer controlled, rigid HW, active steering" system I described (ie. like a fighter-plane) would do the same as above, possibly also on the rear-wheels, but with minimal physical effort from the driver.

Z

[BillCobb]

Kevin,

No worry about compliance? My experience is that the more you add parts the more the causes for flexibility...

The long link between rocker and the steering bracket (that is bolted on the upright) tension, compression, buckling.....the rocker itself, the brackets that hold the rockers..... Was a load case made?

Claude

Now Clausde, look at the bright side. These flimsy mechanisms, loaded by discalculated forces and ignored moments will result in highly understeering cars that will perpetuate the textbook notion that oversteering cars (as determined from using ONLY tire force/weight data) are the best setups for any form of vehicle "dynamics" based on a "statics" representation. Then, student and armchair designers of cars having lots of 'yaw overshoot' and characterized as lack of 'yaw damping' can continue their religeous zeal towards the perfect roll center migration strategy. Now someone with a brain (at least the left side) tell me how an oversteering car having only first order (as in exponential) yaw velocity and sideslip responses can have any sort of overshoot going on. Oh, that's right... a first order response has it's peak frequency at 0.0 Hz. The same pre-'engineer' needs to tell me how a higher order roll responce with a fixed damped natural frequency will interact with this newly characterized yaw peak frequency as the yaw peak frequency changes from being below the roll frequencey and migrates to above the yaw frequency as the speed changes (actually speed squared). Why hasn't anyone connected the dots between roll natural frequency and the speed dependent^2 yaw natural frequency? Is the roll center religion so pius that it's effect' on roll dynamic interaction with the other state variables just so much unorthodox heresy? Then we add the soggy stiffness of a torque sensor in a hydraulic, electric or spaghetti power steering mechanism. As for steering effort, Just take a moment and add up the moments. Right now, some of the statements we are reading are being shot from the ankle.

Man, you folks need some Control System Engineering education instead of the agricultural zone you are in. If any of these cars ever get measured at an event, make sure there are enough ambulances to cover all the attempted suicides. Just sayin'...........

[BillCobb]

The beatings will continue until morale improves. Remember, I measured the understeer of my freakin outboard bass boat using only a VBOX. Different with 3, 4, or 5 bladed propellers. This is high school project work.

[MCoach]

Bill,

better?
I can't seem to load a bigger picture than posted...

[Kevin Hayward]

Kevin,

No worry about compliance? My experience is that the more you add parts the more the causes for flexibility...

The long link between rocker and the steering bracket (that is bolted on the upright) tension, compression, buckling.....the rocker itself, the brackets that hold the rockers..... Was a load case made?

Claude

Claude,

Obviously slop and compliance were both forefront in the designer's minds. Calcs were done, as well as careful design of all components in the path. There were many iterations of this and various other steering systems when the car was being designed. Despite a typical floor mounted rack and pinion (with spur gears) may have fewer components and simple installation it has big problems with slop, compliance, and packaging. The initial design was to run a Pitman arm straight off the planetary, but could not get bump steer working well without having a high nose (big COG penalty).

Compared to previous R&Ps with double UJ's the design has both less slop and less compliance. The planetary gearbox has less slop than a rack and pinion, and the rockers/rods are all sized similar to a push/pull rod suspension system. Rockers were designed with reasonably long actuation radii to minimise the effect of slop. All holes in sufficiently thick material reamed to size. The long rods are not as small as most steering tierods being used and are designed with buckling in mind. The bolted steering bracket at the upright is not a problem at all (nor should there be any reason that it is) and the upright structure is significantly reinforced around that area. Loads were measured on the car to validate load calculations.

I think that a lot of the teams are not aware of how much the universal joints are deflecting during actuation.

Note that the extra parts added (compared to a conventional setup) are the rockers and one more tierod, in the process no universal joints were needed.

The image posted was the 3rd version that was manufactured after a number of improvements to reduce compliance, the team has since further improved the system. The 2014 car with that system had the least slop an compliance of all the ECU vehicles. this includes 2 vehicles with a drop box to a R&P and 3 vehicles with 2 UJ's to a R&P.

It is not a perfect system by a long shot but does a pretty decent job of offering steering geometry design freedom and significant lowering of the COG.

Again I am passing on details of student work here. While I am present at design reviews and advise in a few areas I may be short on a few of the details. I have driven the car and found it to be quite a good steering system, with no noticeable problem with slop or compliance. Sitting in the car you wouldn't be aware that the mechanism is any different to a well setup R&P apart from having a little less slop than most I have driven. One real bonus is that over time the R&P designs tend to wear pretty bad, both in the UJ's and rack and pinion itself. In this system the gears are generously sized and with 4 contact points there has been no noticeable wear over 2+ years. It still has the same performance as when it was installed.

But for those still in doubt do the calcs/design/construction/testing and see for yourself.

Kev

[MCoach]

The leash being selecting tires that are capable of generating a high enough Mz/steer gain to save it, something I never found quite good enough in the 13" R25B tires to like them.

[tromoly]

Kevin, I want to say I saw one of the Canadian Hybrid teams with a similar set up. They did so to clear their front electric motors.

Not my picture, this is from Formula North 2013, could this be the car you're thinking of?

https://scontent.xx.fbcdn.net/hphotos-xfp1/t31.0-8/963826_524056850963970_1605001212_o.jpg

[MCoach]

That's exactly the car I'm thinking of. Excuse my previous post, it's not their electric motors, it's the entire chain drive system...

[Z]

This "two idler" steering is similar to what I (edit: and Doug) was getting at back on page 3 (http://www.fsae.com/forums/showthread.php?12197-Steering-with-Pitman-Arm.&p=125205&viewfull=1#post125205). This system has its idlers pivoting around longitudinal axes, but a similar system with idlers on vertical axes would be better IMO (better Ackermann, and less bump-steer).

https://scontent.xx.fbcdn.net/hphotos-xfp1/t31.0-8/963826_524056850963970_1605001212_o.jpg

Again, the biggest advantage here is that it allows a lower footbox with a flat floor for the full length of frame, which gives a significantly lower CG and easier build (as ECU have proved!).

Z

[BillCobb]

See the USA in your 1914 Chevrolet Baby Grand. Check out the steering reveal.

[MCoach]

http://49.media.tumblr.com/ad52e511d7a08b62073cab8e238c2b20/tumblr_nadv0z2Dje1qbuqcio1_500.gif

[tromoly]

Bill, correct me if I'm wrong, that system looks very similar (the same?) to what current Sprint Cars use for their steering systems.

http://www.world-sprintcar-guide.com/images/steering_main_image.jpg

http://image.hotrod.com/f/14802563+w660+h495+cr1/ctrp_0904_06_z%2Bsprint_car_setup_technology%2Bgui de.jpg

[BillCobb]

And to just about every vintage farm tractor. Looks like a bit of casual tire rub on the old drag link.

[Charles Kaneb]

CWA,

TAMU used to use roughly 10" steering wheels (250mm). Obviously, that will increase force required and per-inch-of-hand-movement response by 50%. If a vehicle is "too responsive" for a driver - you either have another vehicle dynamics problem (unstable rear toe, parts dragging on the concrete, excessive rear brake force, a rear wing that has just exited stage right) or you need to build or get a faster driver. You're looking for someone who can win a shifter kart race in intermittent rain.

At East Lansing Kart Track speeds without the wings range from 7.5 m/s (17ish MPH) to 23 m/s (60ish MPH). This is roughly equivalent to my FSAE experience.

-CPK

[Charles Kaneb]

After reading what Z had to say about power steering, I have a question for the professionals in chassis engineering.

With full analyses of K&C for the car, dynamic analyses of response & input, performed as part of the design process, then extensive testing with pro drivers at real proving grounds with good instrumentation - why does every power steering system I've ever driven on a passenger car suck?

Quantifying that assertion - examine system backlash on-center (<5 deg at the wheel for manual systems, >5 deg at the wheel for typical PS systems, for ~10N at the wheel), examine the force gain (a manual rack has increasing force with increasing lateral acceleration in an understeering car until self-aligning torque drops faster than the increase due to mechanical trail, but hydraulic and electric systems barely change effort when you go around corners faster - maybe 20% extra force for 100% more lateral acceleration), and ESPECIALLY examine lateral vibration response (Picture a car rolling along the road. Feed in a sinusoidal lateral forcing function at the front of the tire of a given amplitude, then sweep frequency from 1/10 hz to 100 hz. Measure the magnitude and phase shift at the wheel. Then do the same thing at the front of the rear tire).

I can't tell what's going on at the front wheels of my 2013 Fiat anywhere near as well as I can in my 1991 Honda, and it's not an FCA issue - a 2013 Honda is just as bad.

[rrobb]

Charles,

I'd guess that what you and I call "suck", the average car buyer actually prefers.


Cory

[BillCobb]

After reading what Z had to say about power steering, I have a question for the professionals in chassis engineering.

With full analyses of K&C for the car, dynamic analyses of response & input, performed as part of the design process, then extensive testing with pro drivers at real proving grounds with good instrumentation - why does every power steering system I've ever driven on a passenger car suck?

Quantifying that assertion - examine system backlash on-center (<5 deg at the wheel for manual systems, >5 deg at the wheel for typical PS systems, for ~10N at the wheel), examine the force gain (a manual rack has increasing force with increasing lateral acceleration in an understeering car until self-aligning torque drops faster than the increase due to mechanical trail, but hydraulic and electric systems barely change effort when you go around corners faster - maybe 20% extra force for 100% more lateral acceleration), and ESPECIALLY examine lateral vibration response (Picture a car rolling along the road. Feed in a sinusoidal lateral forcing function at the front of the tire of a given amplitude, then sweep frequency from 1/10 hz to 100 hz. Measure the magnitude and phase shift at the wheel. Then do the same thing at the front of the rear tire).

I can't tell what's going on at the front wheels of my 2013 Fiat anywhere near as well as I can in my 1991 Honda, and it's not an FCA issue - a 2013 Honda is just as bad.

Write yourself a detailed simulation of hydraulic or electric power assisted steering control and answer your own questions. Don't forget consideration for 'Lumpy Steeering' effect from poorly designed or poorly installed u-joints in the intermediate shaft. AND, some of your statements are not true. Try to keep in mind the purpose of 'power steering' and go from there. Your lateral excitation is called lateral runout. Another manifestation of it is 'high speed shake'.

Perhaps your own built in torque sensors need some more experience, calibration and higher sensitivity to rim force.

[BillCobb]

Here's an uplevel sporty mid-sized sled. Steering is a big 'heavy' at speed but because the steering gain is so high (g'100 degSWA), the 6.0 steering work sensitivity makes it a bit too 'nimble' (responsive) to the slightest hand movements at 100 klicks. Not the car to drive while eating a Big Mac double out on the motorway. It has a parabolic valve, and relatively low front cornering compliance (without counting the steering system contribution). With 51% weight distribution and some stiff tires (as indicated by the rear cornering compliance), the elements to tune are: valve profile, t-bar (torsion bar in the valve) size, flow rate of the pump, i-shaft stiffness and tire Mz and relaxation factors, you might like this car after a few hundred miles. Except parking it will break your elbows.

These are just a few of the design quandries facing the industry. Oh, and that older Honda steering pump sucks up way too much HP to be allowed by fuel economy rules. Same for the stiff tires (which are run flats, BTW, can't drag the weight of a spare around when the likelyhood of ever needing it is very very small. Yeah, this ain't no Fix It Again, Tony.