Steering with Pitman Arm.

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
Attachment 859

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

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.

Ahmed,
Very good topic specially for egyptian teams :confused:
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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 !

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Originally Posted by Ahmad Rezq

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

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

https://s-media-cache-ak0.pinimg.com...2216ee6a9f.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.

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

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.

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.

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 !

Ahmad,

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

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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,

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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.

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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...

Quote:

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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?

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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" sketch is from early in the "Beam Axles..." thread.
https://lh5.googleusercontent.com/-8...inBeamWing.jpg

Here is a more recent sketch - "BA5 - Front-Beam with BGB+Pitman-Arm-Steering"
https://lh6.googleusercontent.com/-s...eam-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.

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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.

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

R&P.
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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
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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

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.