13 replies to this topic

[Supercar]

I was told that if you have a high-downforce car then there is no sense to have too much droop travel. So often the springs are somewhat preloaded.

But... won't the roll stiffness become unlinear (unpredictable car) if one of those rear shocks unexpectedly rebounds fully in a slow corner or over a bump? I am pretty sure this is not a desirable scenario.

So, will it be safe to design a car to have plenty of droop travel? It is going to hurt anything? Is preloading springs just a band-aid for not having enough droop, or there is some trick strategy to this?

Philip

[Ben]

As I understand it, preloading was the bandaid of choice for F3000 cars in the eighties. The more modern approach is to use a pitch/heave control spring (3rd spring) to allow vertical and roll modes to be partially decoupled.

Check out this discussion on preload but for cars without aero:

http://fsae.com/eve/...8/m/18910968321

Ben

[soubriquet]

Originally posted by Ben
As I understand it, preloading was the bandaid of choice for F3000 cars in the eighties. The more modern approach is to use a pitch/heave control spring (3rd spring) to allow vertical and roll modes to be partially decoupled..

Ben

I have seen numerous references to third springs/dampers, but never a clear description of why they are used and how they work. Ok, now I know it is to decouple vertical and roll movements. Anyone out there prepared to walk me through it?

TIA
S

[wegmann]

I think there was a thread a couple months ago about 3rd springs that you could search for, but here's my short version:

The 3rd spring (and conceivably 3rd damper, 3rd bumpstop, etc.) acts on what *both* wheels are doing. If both wheels go up, the 3rd spring tries to push them both down. However, if one wheel goes up and the opposite wheel goes down the same amount, then the 3rd spring does nothing.

A schematic might help, but I can't draw ... imagine an arm connecting the suspension pushrods of both wheels. Connect the 3rd spring to the middle of that arm. If both pushrods are pushed up, then the whole arm will go up, and the 3rd spring will resist it. If the car is simply rolling, then the arm will be up at one end and down at the other, but the middle will be in its original position, so the 3rd spring does nothing.

This tends to be advantageous for high-downforce cars. On the straight, at high speeds, the downforce is pushing down hard on the whole car. Without a 3rd spring, the regular springs have to be stiff to keep the car from dragging on the ground. Adding a 3rd spring allows the regular springs to be a bit softer, which increases grip in the corners.

The 3rd spring doesn't do as much in the corners - you're going slower which causes less downforce, plus the car is rolling which doesn't make the 3rd spring do much work.

[Supercar]

Originally posted by Ben
As I understand it, preloading was the bandaid of choice for F3000 cars in the eighties. The more modern approach is to use a pitch/heave control spring (3rd spring) to allow vertical and roll modes to be partially decoupled.

Check out this discussion on preload but for cars without aero:

http://fsae.com/eve/...8/m/18910968321

Ben

Wow, this is good stuff! It made me read it all and "chew" on that for quite some time. Still thinking about this stuff... I probably should join that club too.

So, no spring preload for me then. I never liked it anyway. This is what a small rear spring preload has done to me once (along with cold tires). No car damage occured.

http://supercar-engi...rica03/boom.wmv

Philip

[Fat Boy]

Originally posted by Supercar
So, no spring preload for me then. I never liked it anyway. This is what a small rear spring preload has done to me once (along with cold tires). No car damage occured.

http://supercar-engi...rica03/boom.wmv

Philip

You blame that on rear spring preload? That car was loose about 5 minutes before you started to countersteer! From now on, your nickname is 'slowhand'.

[Fat Boy]

Spring preload, be it at the front or the rear is not intrinsically good or bad. It is just another one of the multitude of available tuning tools. It can be very benificial to restrict droop travel to some extent or another at some tracks, while at others, it is good to allow as much droop travel as you can. In general, bumpy circuits need more droop travel than smooth ones. Another generality is that the rear tends to want less preload than the front. I've never been to a point where I thought I needed more preload than it took to get to zero static droop.

Having the inside tire top out it's damper stiffens that end of the car can either stiffen or soften that car in roll depending on the anti-roll bar rates and spring rates involved. If you have certain problems that need the roll couple of the car to change at a certain point in the corner, playing with spring preload/droop restriction can be a very good thing. If you try to live and die by it, I think you'll find it's not a cure-all.

Interestingly enough, drivers do not seems to report the ability to discern when the inside tire 'tops out'. We are dealing with several springs in a series and we are simply taking one out of commission and cutting another in 1/2. Apparently, with the combinations that I've ran (which is quite few), the elimination of one of those springs is not a big enough of a deal to feel (in terms of a discontinuity), but it is big enough of a change to feel the handling balance.

[Supercar]

Originally posted by Fat Boy
You blame that on rear spring preload? That car was loose about 5 minutes before you started to countersteer! From now on, your nickname is 'slowhand'.

The tires were cold. If you pay attention to my steering wheel, the car was pushing shortly before it started to oversteer. It's an AWD car, Fat Boy. You normally do not countersteer to catch an oversteer, you mash the gas pedal.

Want to see my tire warmup procedure later that year? That was after the rear preload was eliminated.
http://www.supercar-.../RA_Oct03-2.wmv

[Supercar]

Originally posted by Fat Boy
Having the inside tire top out it's damper stiffens that end of the car can either stiffen or soften that car in roll depending on the anti-roll bar rates and spring rates involved. If you have certain problems that need the roll couple of the car to change at a certain point in the corner, playing with spring preload/droop restriction can be a very good thing.

Could you please elaborate? I cannot think or a spring/bar combination that would result in a decreased roll stiffness of that end of the car if one of the springs tops out. Thanks.

[Fat Boy]

Originally posted by Supercar
Could you please elaborate? I cannot think or a spring/bar combination that would result in a decreased roll stiffness of that end of the car if one of the springs tops out. Thanks.

Consider this. The suspension springs are acting in parallel. When the inside is topped out, it can't move any more. The anti-roll bar only operates due to differential motion between the inside and outside hubs. If the inside can't move, then the A/R bar rate gets cut in 1/2 because its portion of roll stiffness is only due to the outside compressing rather than the inside extending and outside compressing. So while the roll angle would be reduced due to the cancelling of a spring (roll stiffness increased), the roll angle could be increased by reducing the effectiveness of the bar (roll stiffness decreased).

Making any sense?

[Supercar]

I was thinkning about that... I agree that the roll stiffness that comes from springs will increase - double, but the ARB stiffness (resistance moment per degree or roll) will not change. The ARB will be still twisted the same for the same roll angle.
---------

So it looks like you can add understeer, especially at the turn exit, with that decreased droop setup in the front. In the rear though, it could help the car spin into the turn, lol!

Philip

[Supercar]

Let me add another thing. It is well-known that understeery cars have a better steering response and an initial turn-in. So, that means that a car with a zero front droop will have a better turn-in response. They would feel like they have less understeer in the first part of the corner.

Then, mid-corner, the car will probably understeer soemwhat.

Then, at the turn exit, those BMW's and Porshes lift their inside front wheels. Some do it even mid-corner. When the wheel is in the air that zero droop, front ARB stiffness, and even front spring rate do not matter. The car runs on 3 wheels. So I guess this is how you make those German RWD MacPherson-strutted car handle... !

Philip

[Fat Boy]

Yes, the ARB will have the same stiffness for a degree of roll, but you are assuming that the car will roll the same amount with both droop scenarios, which it won't. If you look at the roll load taken by the ARB for a given lateral acceleration, it will be reduced with restricted droop because the roll load taken by the springs will be greater. The trick is whether this reduce roll load taken by the ARB is more or less of an issue than the reduced effectiveness of the ARB.

Zero droop has no effect on whether or not the tires come off the ground. That is a function of total lateral load transfer and roll couple distribution. Yes, the shock does have to top out before the tire comes off the ground, but restricting droop does not cause tires to come off the ground by itself.

You're getting the chicken and the egg mixed up on the 'understeery cars have good turn-in' statement. Cars with a stiff front end have quick response. That is a function of the frequency response of the system. In addition, car with a stiff front end will inherently carry a high front roll couple. This will mean steady state understeer due to roll couple distribution. Also, if the driver knows that the car is going to understeer steady-state, he is much more likely to yank aggressively on the steering wheel than if the car were prone to oversteer. It is not difficult to have a car that understeers and has a poor turn-in.

Many, many racecars spend at least some of their time around the track with tires in the air, most commonly the inside front on tight, bumpy corners. I always take it as a sign that I'm doing my job right. If the car couldn't produce a good combination of cornering grip and forward acceleration, then it would never develop enough load transfer to get that tire off the ground. I don't tune specifically to reach this point, I want to have all 4 on the ground, but if the car is fast, I'm not concerned to see it happen.

Porsches and BMW's have relatively narrow tracks, high CG's, and good grip. Stick this together and you get a wheel in the air every once in a while. Horrible suspension geometry on most Porsche's might have something to do with it as well.

[Supercar]

Ah, Fat Boy. I will have to make a colorful animation for the chicken and egg problem. I am becoming curius myself.

As far as ARB, I see your logic. You are thinking in terms of forces and therefore are getting confuzed. Just plug in any numbers into formulas to verify your thoughts. If you think in terms of stiffenesses, then there is no way that the zero-droop end can be softer and have more traction.

Also, if it is not the fully extended shock that is lifting the wheel, then it is not a limited(zero) droop suspension.

Philip