Suspension Design

[Buckingham]

Well, if you have independent suspension and a RC at CG height, then you can have quite significant jacking problems, which can be quite bad...

How significant is 'significant' ??

We built a car with 4" / 7" RC F/R (not at the CG, but plenty above ground) so that we could run soft springs and no ARB because the competition at the time was at a paved baja course known as the Silverdome (04). Virtual swing arm length was equal to 1/2 track width. ARBs seemed really dumb to me... compared to just running more spring - they stiffen 1 wheel bump and roll, but leave pitch soft... at a track that was known to scrape the rivets off the pan under your feet because of bumps and transitions in braking zones.

Was there a lot of Jacking.. yup. Did we figure that out ahead of time.... nope. Was the car super quick in slalomns... yup. Could the car handle rough bumps... like a champ. Was the front outer wheel cambered the wrong way during cornering because of jacking, short swing arm, and soft springs... yup. Did the car win the skidpad event anyway... yup. Pretty sporty in AutoX and Enduro.

Not something I would do for a smooth test track, but if I was going back to Pontiac it would be a good starting point.

[Z]

For people like you that want to learn there is a specific tool available. Have a look at it : www.dynatune-xl.com.

Dynatune,

Can your software REALLY help Mitchell design the type of suspension he is planning?

That is, can your software model Mitchell's (or UWA's++) type of TWIST-SOFT, interconnected springing???
~~~~~o0o~~~~~

MCoach,

... It turns out that having a 'helper' spring with high [low???] spring rates help tire grip when the primary springs are at high spring rates because it allows the system to deform to the road with[out] disturbing the chassis and causing unwanted load transfer. However, the stiff spring rates allow for proper rate of load transfer during maneuvers that occur in high performance situations so the helper springs are only allowed a few mm of travel...

Assuming you meant "low"-rate for the helper springs, then this is what I was suggesting earlier. The spring-rate curve for the wheel, from full droop to full bump, has a soft (= low-rate) zone in the middle, about +/- 1 cm either side of ride height. This means you only need quite low damping in this zone to snub out any unwanted oscillations. During high-G cornering the suspension is in the stiffer (= higher rate) zones, and these rates determine LLTD.

This multi-rate springing can be achieved with two conventional coilsprings in series, with one becoming "solid" (= coil-bound) at the changeover point from soft to harder. Or you simply use appropriate bump rubbers. And, of course, the definition of "full bump and droop" implies that the spring-rate becomes VERY MUCH stiffer there. Nothing in the real world stays "linear" for very long.

The benefits of this sort of multi-rate springing are rediscovered every few years, for as long as I can remember. But interconnected springing works better, and it can (and should) also be multi-rate.
~~~~~o0o~~~~~

Buckingham,

How significant is 'significant' ??
...
Was there a lot of Jacking.. yup. Did we figure that out ahead of time.... nope. Was the car super quick in slalomns... yup. Could the car handle rough bumps... like a champ. Was the front outer wheel cambered the wrong way during cornering because of jacking, short swing arm, and soft springs... yup. Did the car win the skidpad event anyway... yup. Pretty sporty in AutoX and Enduro.

Not something I would do for a smooth test track, but if I was going back to Pontiac it would be a good starting point.

I agree! Something is "significantly bad" when it costs you "significantly". Your high RCs did not seem to cost you much at all, so NOT a "significant" problem! Which is why I proposed, sketched, and recommended the "Swing-Axle" suspension (quite a few pages back). This seems to be functionally very similar to your short-FVSA, highish-RC, suspension, although if yours was with double-wishbones, then too complicated!
~~~~~o0o~~~~~

I would like to restress to students that (in my very "non-expert" opinion) ANY of the suspension types covered in this thread can be used on an FSAE car capable of winning the comp. The questions that are asked of the suspension by the currently very smooth FSAE tracks, are so simple that even the "stupidest" suspension can get a pass mark. (Well, the suspension can be "kinematically and elastically stupid", but it should be structurally strong and stiff.)

It is only when the organisers start to put a lot of real bumps in the tracks that you students will have to start thinking about "cleverer" suspensions. As Buckingham noted, that means softening the springs. But, I suggest, you should think of the "right" sort of softness to add ...

Z

[mdavis]

This means you only need quite low damping in this zone to snub out any unwanted oscillations. During high-G cornering the suspension is in the stiffer (= higher rate) zones, and these rates determine LLTD.

Z

Z,

How would you propose having the variable displacement damping?

We (Bilstein) have a system that would work (it came from a necessity to pass a certain European standard crash avoidance test), but it doesn't seem like it would be very well when applied to a "typical" FSAE car. If the car had direct acting dampers, then it would be possible (the Bilstein system adds a lot of dead length to the damper, which eats travel for a given length), but is still somewhat difficult to use, and would not be easily adjustable (dampers would need to be rebuilt to change valving, which is rather time intensive with that product).

-Matt

[Z]

Matt,

If you mean by "variable displacement damping" that the damping rate (ie. force/velocity) varies according to position along the damper's stroke, then this is very common on off-road racing dampers.

Briefly, these "monotube" dampers have a number of holes along the working length of the cylinder, and these holes are interconnected via external bypass tubes that allow different amounts of fluid to flow past the piston, at different piston positions along the stroke. The damper valving is actually very easy to adjust or change on these systems, because most of the valving (mainly for low-speed) is fitted to the external bypass tubes. From memory (?) the piston itself still carries the main high-speed blow-off valves.

But I was suggesting that a soft spring-rate zone in the centre of the suspension stroke allows a lower than usual, but CONSTANT DAMPING-RATE, to suppress unwanted oscillations.

Again briefly, if a "stiff" spring-rate suspension has dampers that give, say, only 0.2 x critical-damping, then you can expect several cycles of oscillation after any "event". These oscillations can be of the wheel (= hop), or car-body (= heave, pitch, or roll). To keep the oscillations down to about half-a-cycle you need more damping. In practice about 0.7 x critical is enough. But the nasty truth is that more damping means less grip on bumpy surfaces (ie. more damping = more Fz variation = less Fy capability).

Putting a soft spring-rate zone in the middle of the stroke means that exactly the same dampers as above now have, perhaps, 2 x critical-damping in the soft zone. So any oscillation movement that passes through this soft zone is, in effect, very heavily damped.

Note that having the soft spring-rate for the full range of suspension travel, and the same level of damping as above, gives a very highly damped suspension with no chance of oscillations. But the suspension springs are now so soft that the car goes onto its bump or droop stops, in heave, pitch, or roll, during any sustained high-G manoeuvres.

So add extra spring-rate, but only towards each end of the suspension stroke.

Z

[Z]

I am posting this here because this thread already has several mentions of the Citroen 2CV.

My Citroen-Mechanic/Sportscar-Wheeler-Dealer mate is nearing the end of a restoration of a Porsche 911, so is looking for the next project. He recently became aware of the following kit that can be bought from the UK. He has acquired a very cheap 2CV body and chassis and is preparing for the upgrade.

Sparrow Automotive Citroen-2CV+BMW-flat-twin-engine website.

From that webpage is this YouTube video of the car at Mallory Park.

Sparrow Automotive 2CV-BMW at Mallory Park.

Ok, so it is only a "track day", and the other cars are NOT super-high-performance racecars (only "hot hatches", etc.), and are probably being driven by amateurs.

But noteworthy is that the engine is only a medium performance bike engine with ~95 hp, and the 2CV has a chassis and suspension intended for "the world's cheapest car", suitable only for peasant French farmers (and occasionally for comic relief in James Bond movies...).

Nevertheless, the video shows this crude-as-possible package blowing away a lot of supposedly (?) much higher performance cars.

So, please, NEVER, EVER say that a Leading-and-Trailing-Arm suspension "WILL NOT WORK" on a race track!

(Interesting is the driver's very relaxed "truck driver's" steering style. For example, see the "Samurai sword grip" at 7:00 and 8:02! )

Z

[Big Bird]

Pantsing sports cars in a flat black 2CV - now that is my idea of a fun afternoon!

Nice find, Z

[theTTshark]

So, please, NEVER, EVER say that a Leading-and-Trailing-Arm suspension "WILL NOT WORK" on a race track!

Z

You may not be able to make a pig into a race horse, but you can make a damn fast pig.

[mdavis]

You may not be able to make a pig into a race horse, but you can make a damn fast pig.

We used this line many many many times to describe our 2013 car.

-Matt

[stever95]

Hi all. I graduated and am no longer a part of blue hen racing's formula team, but I've been reading more about vehicle dynamics.

In "Chassis Engineering," Captain Herb makes a couple of statements that have really thrown me off. The first concept I've been struggling with (for too long...) is the following: "When a car rolls due to the cornering force, the tires usually roll with the car and develop a positive camber angle to the ground Combine that with this: "when...camber gain is over 3/4 degree per degree of body roll...This means if the car rolls at a 4-degree-angle, the outside tire will decamber at 3 degrees, so the outside tire will lose 1 degree of camber in relation to the track."

My opinion is (or was) that a tire does not gain positive camber due to roll. In fact, camber should change however your suspension (and compliances / deflections) geometrically prescribe. What Herb seems to be saying is that when a car rolls, it acts as if it's at the limit of tipping over (ie ROLLING OVER) with infinitely stiff springs, which would cause a positive increase in camber.

I haven't read anything that clears this up in RCVD, Valkenburgh's Racecar Engineering and Mechanics, or Carroll Smith's stuff. And I've drawn more FBD's than I care to admit.

Help greatly appreciated.

Thanks!!

(edit: speeling)

[Pete Marsh]

I think this is just a terminology definition issue.
The more correct term for what you are reading about is "Inclination Angle"

That is the angle the wheel plane makes to the road

You may be confusing this with the "Camber Angle", the angle the wheel plane makes to the vehicle body reference plane. When playing with kinematics, and saying 'I have xx camber gain" this is relative to the body, not the road, so if the gain is less than 1, you will in fact loose inclination angle in roll. If your roll centres are above the ground, jacking will cause a further loss.

Pete