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tristancliffe (Automotive) (OP)
18 Jan 08 7:42
Hi,

I have been running a 1988 F3 car in a UK based championship for the last year (Monoposto), and have been trying to climb the steep learning curve.  One thing has caused a lot of confusion, and I've not really been able to find or deduce real answers:

Spring Preload!

Our car uses rising rate (rocker) inboard suspension with pushrods.

People in the paddock have said that increasing preload effectively increases the spring rate, but I don't beleive it does, assuming the car is running off the dampers' internal rebound stops.

As far as I can tell increasing spring preload only increases the ride height, limits rebound/droop travel, and increases available bump travel.  With the rising rate more preload (and correcting the ride height via pushrod lengths) moves the rockers so that wheel rate actually DECREASES - completely the opposite of what I'm told by the fast guys.

And then, to throw a spanner in the works, I'm told that some cars (often FFords) use lots of preload to give zero droop suspension to limit understeer out of corners - but I don't understand how that works either - surely limiting the droop will REDUCE tyre load, and therefore INCREASE understeer, unless it's achieving more tyre load by limiting CoG movement and hence load transfer.

I've visited this forum as a guest for a while on and off, and it struck me that I'm more likely to get sensible answer (perhaps with a bit of theory) here than almost anywhere else.

Can you help?   I can supply more (generic) information if required to set the scene a bit if it helps.

Thanks,

Tristan
Helpful Member!  murpia (Mechanical)
18 Jan 08 8:21
You are right and 'the fast guys' are wrong in terms of the effect of preload on spring rate or wheel rate, especially with rising rate rockers, as you correctly argue.

Zero-droop suspension does seems to work in a number of racing categories, but I've yet to hear a convincing explanation beyond it's what the stopwatch or driver demands. No doubt it can be analysed and understood given enough supporting data, but that rarely seems to be offered...

Regards, Ian
GregLocock (Automotive)
18 Jan 08 18:59
As I understand it the advantage of running zero droop is that it is the fastest way of getting the car back onto the ground after running off a kerb.

Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

JHarshbarger (Mechanical)
24 Jan 08 12:44
Wouldn't the zero droop condition cause load to transfer faster to the outside wheel? The inside wheel's tendency to lift off the ground will quickly transfer weight in a corner, assuming typical suspension geometry. I've always been confused about the uses of adjustable preload and zero-droop suspension.

Greg, wouldn't the car return the wheels to the ground faster if it did have more available droop? The spring will push the wheel back down to the ground faster than the car will fall.

_ _
Joel Harshbarger
GregLocock (Automotive)
24 Jan 08 17:42
No we're talking about aero, not wheel contact.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

murpia (Mechanical)
25 Jan 08 10:44
If we include aero cars, rather than the FFords of the original post, then there's a clear mechanism for improved performance with zero droop and / or preload:

Lower rideheight = more downforce from ground effect...

Regards, Ian
bhart (Mechanical)
30 Jan 08 0:49
Regarding the wheel rate:

Things with an "s" after the underscore refer to a spring quantity, things with a "w" refer to a wheel quantity.

F_s = force in spring
F_w = force at wheel (vertical)
z_s = deflection of spring
z_w = deflection at wheel (vertical)
MR = deltaz_s/deltaz_w = dz_s/dz_w(some people define MR as the inverse of this, but this is how it is defined here)

By conservation of energy, the effect of the spring at the wheel can be characterized by F_s*dz_s = F_w*dz_w

Therefore, F_w/F_s = dz_s/dz_w = MR

Combining equations from above:
F_w = MR*F_s,
(from here down I take some derivatives, if you haven't taken calculus, it will be hard to follow, but the other guys here will call BS if I'm wrong or made a mistake)

dF_w/dz_w = k_w (wheelrate) = dF_w/dz_s *dz_s/dz_w
= d(MR)/dz_w *F_s + MR *dF_s/dz_s *dz_s/dz_w

substituting dz_s/dz_w = MR, and dF_s/dz_s = k_s (spring rate)
--> d(MR)/dz_w *F_s + MR *k_s *MR
= d(MR)/dz_w *F_s + MR^2 *k_s

rearranging just a bit,
wheelrate = k_w = MR^2 *k_s + d(MR)/dz_w *F_s

the MR^2 *k_s term is tha classical motion ratio effect on wheel rate, and the d(MR)/dz_w *F_s term is called the geometric rate

if you consider the geometric rate term, is your wheel rate higher with more preload?

If so, then maybe this is the effect your competitors are refering to w/ regard to effectively increasing the spring rate - or maybe they just percieve it as stiffer b/c it travels less and feels harsher (spring rate is infinity until the preload is overcome)

Please note, the above analysis only applies if the car has overcome the preload, and the suspension is free to travel according to the fitted spring rate (not infinite spring rate)

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