Suspension Design

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Originally posted by MCoach:
Z is just advocating very simple systems for a very simple competition. The cars should posses these systems and no others (to keep it simple):
<UL TYPE=SQUARE><LI>A single cylinder engine
<LI>A simple frame (preferably painted brown)
<LI>A single speed transmission
<LI>Longitudinally connected springs (help with bump capability)
<LI>Diagonally connected springs (to stiffen in pitch and roll)
<LI>Don't forget interconnectedly sprung left to right
<LI>NO FANCY SUSPENSION, beam axles front and rear
<LI>DIRECT actuating dampers that are cheap as dirt and don't work because you don't use them. The courses are smooth enough that you don't need to worry about wheel movement, but they are just bumpy enough that you need to spring all of the wheels together.
<LI>Pull/push rods are unnecessary
<LI>Suspension should be robust enough to enter the Baja SAE competition with
<LI>An aero package capable of 3G on an autocross couse but still is rules compliant with less drag than a non aero car. (but aero also isn't considered a big enough deal to try to move dampers and springs out of the air stream).[/list]Remember guys, keep it simple. http://fsae.com/groupee_common/emoticons/icon_razz.gif

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Z,

Assuming you agree with (at least some of) the above, where do you stand on:
<UL TYPE=SQUARE><LI>active yaw control?
<LI>four wheel steer?[/list]To me, many of the manoeuvres required for FSAE are not best achieved by conventional front wheel steer vehicle dynamics:
<UL TYPE=SQUARE><LI>why yaw the car when driving a slalom?
<LI>why create yaw moments via axle translation forces?
<LI>why pivot the car around a turn centre within the wheelbase
<LI>(etc.)[/list]With beam axles, many common parts could be used to easily achieve 4 wheel steer.
Additional actively controlled brake systems are not banned, as long as there's a simple passive braking system in place as well.

I can envisage a 'sidewinder' car with a very short chassis structure and the engine beside the driver. A beam axle front and rear, mounting off the front and back bulkheads. A 2nd brake caliper for each wheel linked to a control ECU. Mechanical linkage rear steering (no steer-by-wire) using cams or similar to program the rear steer characteristics for each event (accel, skidpan, autocross). If necessary, a selector lever, a bit like a gear shifter / ARB adjuster / weight jacker, to select different rear steer characteristics around the autocross lap.

Regards, Ian

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Originally posted by Jay Lawrence:
The wheels moving backwards in bounce reduces loading because you are reducing the vertical acceleration required of the assembly as it goes over a bump. Think about it: if your wheel hits a bump and the geometry tells it to move forward as it does so, you are causing the wheel to accelerate up the bump at a greater rate than if it moved backwards. Hard to explain, relatively easy to picture.

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Also, ride quality increases as you're using your suspension to absorb the wheel deflection over the front of the bump. Slow moving constant sinusoidal bumps, this is not as big of an issue since your wheel can follow the profile of the road vertically quite well, but with a sudden step bump (real life bump, rock, curb, etc.) your wheel needs to absorb a bit of the impact as it goes over the bump. If you're pushing straight up on the front of your tire, you're also pushing straight back in X. Allowing your wheels to move backwards a bit helps take up this force.

Probably talking in circles here and a diagram would surely help, but hopefully you get the gist. Bump forces aren't purely in the Z direction.

M. Nader,

"... a ride frequency around the 1Hz range (more or less) would be sufficient?"

Off-roaders usually have highly non-linear rate springing (which is not good for circuit racing). For example, using a really soft rate around ride height (as close to 0 Hz as possible), which then gets progressively stiffer at each end of the travel is good. This is typically achieved with conventional (linear but soft) main springs, backed up by quite long, rapidly rising-rate, hydro-pneumatic bump stops, themselves backed up by heavy duty bump rubbers. There are also rising-rate droop stops (often using "springy" seat-belt webbing) to lessen the shock when the wheels drop down. And multi-rate dampers with adjustable bypass channels for softer damping at mid-stroke, stiffer at the ends.
~o0o~

"Here is one the problems. for an offroader the aim will always be less camber change in bumps, but then when turning ...
... the relatively soft roll gradient ... would yield substantial loss of traction on both ends."

I see this as an unjustified bias, or prejudice, that comes from circuit racing (ie. "real racing" http://fsae.com/groupee_common/emoti...n_rolleyes.gif). Namely, that for good cornering grip the wheel must be upright, or with a few degrees of negative camber. IMO this is not so off-road.

The best way to objectively decide this is to do TTC type testing "in the dirt". This is unlikely to happen in a laboratory, so I reckon the off-road tyre companies just ask the off-road racers "How does it feel?". The massive roll-angles of the winning cars suggests "adverse tyre camber angle" is NOT a problem.

From my observations, the rounded profile, cross-ply tyres grip just as well when the knobs on their outer shoulder are digging into the ground, as when the knobs on their centreline or inner shoulder are digging in. The square section shoulders of radials often (not always) "bite" better with considerable positive camber. With negative camber they seem to "surf", or "skip", over the sand and gravel. Depends on the surface...

More importantly, off-road corners are never really flat enough to measure any sort of "inclination angle". Invariably, they are ruts-on-top-of-more-ruts, and good drivers try to drop their wheels into the best ruts that get them around the corner like a train. Furthermore, a lot (most?) of off-roading is simply about going as fast as the suspension will allow in a relatively straight line. Time spent, or lost, in corners is relatively small compared with time gained by going fast over bumps.
~o0o~

"A ground level RC/IC is nearly impossible to get in offroaders...,"

Not at all. Very easy! http://fsae.com/groupee_common/emoticons/icon_smile.gif

To repeat, I strongly suggest you aim for zero camber change and horizontal front-view n-lines (= ground level ICs a long way away). Also be very wary of following the fashion to lateral double-wishbones. As I said, these are now common on the top-level $500,000 cars "Because that's what real racecars use...". Unfortunately, the undeniable benefits of zero camber change result in equal length, parallel wishbones, which then result in massive jacking forces... Which many drivers hate, but have to tolerate because "That's they way everyone is going... It's progress...". http://fsae.com/groupee_common/emoticons/icon_frown.gif
~o0o~

".. would you care to explain why [wheels moving backward over bumps] would lessen the forces?"

As Jay and Axel pointed out, the upward acceleration of the wheel, as it hits the rising face of a steep bump, also results in a considerable rearward force acting on the wheel, and then to the body.

Furthermore, if the wheel is rolling at, say, 1000 rpm on the level ground, and it suddenly must roll up a 45 degree face of a bump, then for constant forward car speed the wheel must now roll at ~1400 rpm (= sqrt(2) x original speed). This sudden increase in rotational speed required of the heavy, large MoI, wheel, requires a couple of forces acting on the wheel, one of which has a rearward reaction on the body (do the FBD http://fsae.com/groupee_common/emoticons/icon_smile.gif). Allowing the wheel to move backwards, in effect slowing it down, lessens these forces.

BTW, the above "recession" issue (I think that's what they call it) is currently a big deal with production car suspension designers. It is mainly an NVH issue caused by low profile, wide and heavy belted radials, hitting lateral cracks/steps/seams in the pavement. The proliferation of 5-link suspensions is a response to this NVH problem. Namely, with a 5-link it is quite easy to fit the longitudinal links with soft rubber bushes (allowing the recession movement), while the lateral links are fitted with stiffer bushes (maybe even BJs) to better control toe change.
~o0o~

Oh, and an extra "Requirement for Off-Road...".

6. At each front wheel, have the CG of the wheel assembly as close as possible to the steer-axis.
Ie. aim for a "centre-plane" steer-axis, passing through the middle of the wheel. When the wheel CG is away from the steer-axis, then any inertial forces at the CG (due to wheel assembly accelerations) can result is severe steering kickback. This kickback is minimised when the CG is on the steer-axis, although the gyroscopic couples (eg. from camber change) still get through.
~~~~~~~~~~~~~~~~~~~~~~o0o~~~~~~~~~~~~~~~~~~~~~~~~


Ian,

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Assuming you agree with (at least some of) the above, where do you stand on:

* active yaw control?
* four wheel steer?

To me, many of the manoeuvres required for FSAE are not best achieved by conventional front wheel steer vehicle dynamics:

* why yaw the car when driving a slalom?
* why create yaw moments via axle translation forces?
* why pivot the car around a turn centre within the wheelbase...

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Yes, I agree with MCoach's accurate summary (except that interconnected springing is only needed if the tracks get a lot bumpier.)

Regarding "active yaw control" (eg. by differential longitudinal forces at the left and right wheels), and "four wheel steer", I agree that these can be done relatively easily, and can be extremely powerful. However, I believe that for FSAE type courses they might be MUCH TOO POWERFUL!

I accept that FSAE is meant to be "tight and twisty". But, from what I have seen, it requires nowhere near as much agility as, say, "Motorkhana" (or whatever they call it in other places). So nowhere in FSAE must the car "turn on a dime", do "snap spins", manoueuvre through impossible zig-zags, etc. For Motorkhana that sort of extreme yawing power is good. Also for some Trails, Rock-Crawling, etc. (though I note that some Rock-Crawling comps ban "torque-vectoring", because it is too effective!).

But I think too much yawing power might be counter-productive in FSAE, especially considering the amateur drivers. The slaloms and hairpins are not all that tight. Note that "differential longitudinal forces" can be utilised by having as much positive Ackermann as possible in the steering. This is good for the hairpins. Also some rear steer is possible by having some rear static toe-out, or adjusting the toe-links to give a bit of rear roll oversteer. In both cases you only need a very small amount for a big result. Best investigated with a lot of testing...
~o0o~

"If necessary, a selector lever, a bit like a gear shifter / ARB adjuster / weight jacker, to select different rear steer characteristics around the autocross lap."

I note that "telehandlers" (an evolution of farm tractors) nowadays have selectable steering. Either normal front wheel only. Or rear wheels steering in same direction as the fronts, for a "crabbing" motion (useful in slalom). Or rear wheels opposite to fronts, for extremely tight radius turns (useful in hairpins). I guess this is what you are getting at.

Again I wonder if the amateur FSAE drivers are good enough to benefit by switching between different steering modes between every slalom and hairpin? If a team had the resources to investigate this, I would love to see the results. Worst case, they just set the "steering knob" to "Normal" (front only).
~~~o0o~~~

Err..., better get back to work...

Z

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Originally posted by AxelRipper:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Jay Lawrence:
The wheels moving backwards in bounce reduces loading because you are reducing the vertical acceleration required of the assembly as it goes over a bump. Think about it: if your wheel hits a bump and the geometry tells it to move forward as it does so, you are causing the wheel to accelerate up the bump at a greater rate than if it moved backwards. Hard to explain, relatively easy to picture.

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Also, ride quality increases as you're using your suspension to absorb the wheel deflection over the front of the bump. Slow moving constant sinusoidal bumps, this is not as big of an issue since your wheel can follow the profile of the road vertically quite well, but with a sudden step bump (real life bump, rock, curb, etc.) your wheel needs to absorb a bit of the impact as it goes over the bump. If you're pushing straight up on the front of your tire, you're also pushing straight back in X. Allowing your wheels to move backwards a bit helps take up this force.

Probably talking in circles here and a diagram would surely help, but hopefully you get the gist. Bump forces aren't purely in the Z direction. </div></BLOCKQUOTE>

The best example to see it is to look at mountain bikes (Especially downhill bikes or all-mountain, not freeride which are made more to absorbs huge shocks and not road bumps). There suspension are not directly applicable to cars/truck because they have more constraints : very flat longitudinally, and need to reduce chain length change (though that might be good depending how you transfer motor torque). But the latest mtb suspension are quite good at minimizing impacts from bumps. A bump that hits high on the wheel slows you a lot on a hardtail bike, and also tends to throw you over because of your high CG. So the systems do absorb Up-Down and Front-rear bumps. And with suspension like the DW link minimize squat when applying torque to the wheel. There might be some inspiration there.

Z, there is a national competition here in the United States for high schools known as FIRST Robotics. It's meant to introduce kids to science and technology with control systems, autonomous controls, sometimes extremely basic AI, and overall, a goal oriented teamwork based competition. It looks flashy because it's partial goal is to gain the attention of high schools and be more like a sports match (footy games).


Anyway, down to the point. The robotics are all remote controlled, typically through joysticks. It's very common for drivers to be able to switch back and forth between different drive styles on the field. Tank drive? Click the left button. 4 wheel steer (crab drive)? Click the middle button. Ackermann steering? Click the right button. This also all happens on the fly. The speeds are nowhere near as high as FSAE, but the reaction speeds I would estimate to be on par with several other robots are maniacally flailing around in the same fashion and ability to adjust direction just as quickly.

Video for reference:
http://www.youtube.com/watch?v=JqJK0E9Jpmk

http://www.youtube.com/watch?v=kZHaTGiakZM

It's simple, no suspension, but control scheme dense.

I've done it, I've driven them, and it takes some thought to keep up with the available power you have in those styles of controls. However, one quickly adapts to these movements that would be very foreign to car and being able to readily and capably control them.

Ian, MCoach,

Here is an earlier thread about 4 Wheel Steer.

Thinking about it a bit more now, I still reckon that with the current Rules the biggest gains are available from better aero. But if the Rules were changed to greatly lessen aero, then I would probably go for greater "agility" from something like 4WS, before chasing ultimate horsepower, 4WD, etc. The mechanical cost/complexity of 4WS is quite small. It can be done with any of the simple suspension types, probably using very similar suspension components at each corner.

Ergonomically, the easiest for the driver might be two big thumb-actuated buttons on the steering wheel. When neither button is pushed the steering is normal "front-only". Pushing and holding the green button gives stable "same-steer" at the rear wheels, for low-yaw side-stepping through the slaloms. Very briefly pushing and holding the red (danger!) button gives small rear counter-steer, to get around the super-sharp hairpin.

The only way to know for sure is to build it, and then extensively test it. This would make a great "parallel design project" (using last year's car) for a well resourced team, and should be a lot of fun! http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

Z,

Look at what UWA is up to...

http://sphotos-a.xx.fbcdn.net/...8445_214229793_n.jpg

http://sphotos-d.ak.fbcdn.net/...810_1823465174_o.jpg

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Originally posted by Z:
... I still reckon that with the current Rules the biggest gains are available from better aero. But if the Rules were changed to greatly lessen aero, then I would probably go for greater "agility" from something like 4WS, before chasing ultimate horsepower, 4WD, etc. The mechanical cost/complexity of 4WS is quite small. It can be done with any of the simple suspension types, probably using very similar suspension components at each corner.

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I think you may be right, but there's a gain to be made from torque vectoring as well and it should be additive to the aero gain.

Practically, I think getting the aero right is not easy without independent facilities (wind tunnel, CFD cluster) while the torque vectoring is retro-fit to an existing car. Having developed a couple of hydraulic control systems recently I was pleasantly surprised as to how straightforward it can be.

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Anyway, down to the point. The robotics are all remote controlled, typically through joysticks. It's very common for drivers to be able to switch back and forth between different drive styles on the field. Tank drive? Click the left button. 4 wheel steer (crab drive)? Click the middle button. Ackermann steering? Click the right button. This also all happens on the fly. The speeds are nowhere near as high as FSAE, but the reaction speeds I would estimate to be on par with several other robots are maniacally flailing around in the same fashion and ability to adjust direction just as quickly.

Ergonomically, the easiest for the driver might be two big thumb-actuated buttons on the steering wheel. When neither button is pushed the steering is normal "front-only". Pushing and holding the green button gives stable "same-steer" at the rear wheels, for low-yaw side-stepping through the slaloms. Very briefly pushing and holding the red (danger!) button gives small rear counter-steer, to get around the super-sharp hairpin.

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That 'robot control' description was something I had in the back of my mind. Kind of like a first-person-shooter game where sideslip and yaw control have equal priority and tactical use (Google 'circle strafing').

I think a 4WS electronic interface might prove difficult or impossible to get declared legal. But what about another degree of freedom in the steering wheel:

Rotating the wheel is conventional Ackermann steering. Pulling the LH of the wheel toward the driver, and the RH away indicates a requirement for left side-slip for the slalom. This would be linked to the rear axle in the 'same steer' sense, generating the low-yaw slalom.

For 'counter-steer' hairpins the driver would need to learn to apply the opposite steer input, in this situation, with significant lock, the driver would pull the top of the wheel towards them.

Of course, the torque vectoring system would have sensors to detect both wheel steer and wheel 'push / pull' and use these plus speed and throttle / brake to determine what torque vectoring actuations to add.

Regards, Ian

http://motorsport.mech.uwa.edu...2012/day-1-to-4.html

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I have no problem with people wanting to follow the flock, but I don't think it at all good for society when the braindead insist that everyone else do as they do.

Much of this thread shows how obstinately some people resist change, and how fiercely they condemn others who want to try something different. And, for some reason, in this fight to maintain the status quo, never is a rational argument used... Confused

It's just not engineering! Smile

Z

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Hello "Z",

You miss a point there... engineering is not solely based on the idea to have the best concept ever... Engineering is to MAKE THINGS WORK ! And one of the aspect of making something work is risk management. Your ideas are perhaps great but they are a HUGE gamble. With the numbers of years we put in every FSAE years it's a huge gamble.

You also miss another point : gain.

What is the gain Z? We want laptimes comparaison, we want compliances study and weight study, we want drag and downforce figures, we want inertia and CG comparaison...

Give us figures! Science is based on facts. Be an engineer. The day you prove your point, well all FSAE will be rocking those beam axled side-engined mutli-steered cars on whatever tire you used with the frame type you used. And there is no problem with that.

You might have the best ever concept but a concept is just a concept. History is written on what is done not what is thought.