no droop suspension

[golfpin]

Quite an interesting one here, have just had an old friend call to say hi, he is a former SA formula atlantic champion and Daytona 24 hour winner, good pedigree! So the conversation got onto guess... suspension engineering. He currently runs a very powerful cobra replica based on BMW suspension components and 600 bhp Lexus V8 .

A very well known figure in UK motorsport engineering who is out here on holiday, has suggested that cobra owner shortens the stroke on the front shocks to achieve a " no droop " suspension effect, He is out here to get away from that Brit weather.

I have not had much experience with this set up but did discuss it at length with the late Alan Stanniforth he saying, at the time. that the jury was still out on the merits or demerits of this layout. would love to hear comments from all,
Golfpin

 

[dynatune]

Depends on what you want to achieve with a "no droop" front or rear end. The "trick" is very old but not well known and allows one to change the balance of the car drastically to understeer or oversteer depending on what end of the car the droop stops acts first. In the late 90s and early 2000nd many OEM's used "rebound" springs to fruit from the "advantage". Unfortunately it comes at the price of a severe loss of ride quality, resp. ride grip and is solely to be recommended to try to "cure" a bigger evil like for instance an unfavorable weight distribution.

Cheers,

Dynatune,

http://www.dynatune-xl.com

 

[BrianPetersen]

Motorcycle front forks generally have top-out springs. They have the effect of raising the spring rate when the fork is extending beyond some position that generally corresponds to nominal ride height, and the thinking is that this keeps the suspension in a range where there is at least some springing action when the front is relatively unloaded (acceleration) as opposed to being hard-topped-out against a mechanical top-out stop. Usually the top-out spring has a high enough spring rate that the mechanical hard stop is not reached even with the suspension completely unloaded. The hard stop is just beyond the position where the force on the top-out spring balances the preload on the main spring.

 

[golfpin]

Good morning all,
thank you so much for the trouble you take to pen these replies.
On the issue of no droop can we discuss the dynamics as I, have tried to work them out, being far more practical than erudite.
1 The car rolls about the roll center
2 the suspension links on the loaded side do their thing
3 the unloaded c/r tries to do its thing by extending the shock leg and then it comes to a stop, so that at this point the car is trying to lift the entire corner but that means that tyre is basically off the ground as I see it, unloaded!! surely with what we know to day, not good. 4 wheels on the ground must be better than three!!!!!
My comment to my guest last night was that like use of massive toe out as a result of excessive understeer problem a last ditch effort to cure another ailment, I think this point is also covered by Dynatune. We will have to wait,for once the car is on Fresh Hancock rubber [not 4 year old stuff] we will have st see what works on the day, running in a 6 hour in a few months time. All comments crits most welcome.
With thanks,
Denzil Schultz aka Golfpin

 

[NormPeterson]

Seems to me that as soon as one corner tops out whether hard or through stiffish top-out springing the geometric roll center loses much of whatever meaning it has. Topping out one corner probably upsets grip at the other three, if maybe only slightly.

If top-out occurs with load still at that corner's contact patch, that corner of the chassis will no longer rise until the corner weight actually reaches zero (neglecting the change in tire deflection). Meanwhile the opposite side of the chassis with its suspension being in compression will continue to drop. I guess you'd see the CG drop a little as well.


Norm

 

[TC3000]

Golfpin

I think, it is beneficial, to (try to) understand what the "idea" behind a suggested setup is. I know that you try to do so, because you come here and ask the question. Just because some "well known" motorsport figure says something, doesn't mean, that it is the "be all/end all", because there is still a fair bit of "monkey see - monkey do" going on in the motor racing world, all the way up to F1.

Furthermore, I think it is worthwhile to keep in mind, that a lot of motorsport "engineering" decisions are (at least partly) "rule driven" and not necessarily "driven by engineering/first principles". In a nutshell, that means, you have to make your car/bike as fast as possible on the day, with what you got, and at times, that may includes things, that "fly in the face" of "normal engineering practice".
One case would be, that many (almost any) championship today has you run a mandated tyre. Now, you would hope, that this tyre is well suited for the application at hand, but I wouldn't bet the farm on this. In "lower" spec series ( and this can include national championships in some countries), it's quite often a "price driven" decision. So the manufacturer,who provides the tyres, may just uses what he "still has on the shelf" (from a discontinued other championship &/or stillborn series)instead of developing a tyre for the application at hand. In such a situation, and I have worked in such a championship, it's not always beneficial or better to have "four tyres on the ground". It can be a case of "he - who has the most load transfer wins", literally trying to get your car on "two wheels" in every corner.
That doesn't make a lot of sense, but if you consider that a slick tyre needs a minimum energy going through it to "switch on", things can change.
So, don't assume anything, if you don't have to - often there is more to it, then meets the eye. - with apologize, don't mean to sound like a smarta.... - but "be careful" with some broad/wide sweeping statements like this.

surely with what we know to day, not good. 4 wheels on the ground must be better than three!!!!!

See "coil binding" setups in NASCAR for another example. All these things may not being the "most elegant" solutions - engineering wise, but this brings us back to a "rule driven" statement, I made earlier. Just that something "doesn't make sense on first glance", doesn't mean, that it can't work, if you consider ALL things, which may come into play.

but coming back to your initial question, people do this stuff (zero droop / droop limited suspension) for different reasons.
if you find an Formula Ford team/racer, chances are, that he will know some things about it ( at least changes are high, that he uses such a setup). Because it is a very common FF setup (other single seaters too - but for an aero car, not everything, that "looks like" a zero droop setup on the setup patch (at no speed/no downforce) is a "zero droop" setup while driving on the track (airspeed --> downforce).
One thing worth to remember, in the FF context is, that and FF has an open diff, so to achieve good "power down" you will try to keep as much load on the inner tyre as you can. It think, we will all agree, that if you start lifting a wheel off the ground, you have achieved the maximum contribution of this axle (the one who will lift a wheel), in terms of it's contribution to overall roll stiffness.
Therefore, you can't "ask" more (roll stiffness) from this axle.
Other "typical" examples would include "older" Porsches (911), who used (and some still do) droop limited setups at the front, and I'm sure you will have seen Porsches lifting the inner front wheel on corner exit. This may ties in, with what Dynatune said about the effect of weight distribution on handling.

Which brings us/me to the final point I would like to make, you should look at "zero droop" setups not only in terms of "pure" lateral acceleration, but also in terms of "combined" (lat & long) acceleration. Chances are, that you will brake before entering most corners, and perhaps "trail" some brake (longitudinal acceleration/deceleration) all the way to the apex. In this conditions (nose/front down - pitched) you may have not "zero droop" anymore, but this may changes when you start "powering out" of an corner.
So, you could see this (and similar techniques) as an way/attempt to manipulate roll stiffness distribution as a function of longitudinal acceleration.

Other things to consider (especially but not exclusively for aero depending cars) are that if you don't have "pure roll" (pure roll = the outside raises as much as the inside compresses - CoG position remains constant), you will have a change in CoG position (a heave component in addition to the roll component), this in turn, can/will have a couple of effects ranging from kinematics (camber gain/loss etc.), over general load transfer - CoG height affects overall load transfer, to aero effects front wing/splitter/floor height with respect to the ground etc. etc.

So as you can see, and I have just touch on some of the stuff, there can be various reasons, why one may considers/favors a "zero drop/droop limited" setup. And as always, there are many ways to skin a cat, this type of setup is just one meaning to a/one specific end/aim - there are surely others.

 

[GregLocock]

The intention behind zero (or reduced) droop at the front end typically is to encourage limit understeer, by overloading the front outer tire. I'd be inclined to plot the vertical load on each tire as the car trundles around a skid pan at ever increasing lataccs. There's also going to be a more complex behavior in transients, but until you can sort the skidpan result out then everything else is too hard.

Incidentally the car very rarely rolls around the so called 'roll centre' or 'roll axis'. There obviously is an axis about which it rolls, I don't think I'm going to define exactly how to calculate it for a given trim, it would be a gruesome calculation.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[TC3000]

for one reason or the other, this BB doesn't seem to let you edit a post.

In the above post:

(pure roll = the outside raises as much as the inside compresses - CoG position remains constant)

should read:

pure roll = the outside (springs) COMPRESSES as much as the inside (springs) RAISES/EXTENDS - CoG position (height) remains constant.

 

[golfpin]

Good morning Greg,
Thanks for your comments on the no droop suspension but I am now totally lost.Your comment that "a car does not actually roll about the roll centre or roll axis" if that is the case what/where does it rotate about? Would you explain in laymans terms please.

With thanks.
Golfpin

 

[NormPeterson]

Golfpin - suppose you replaced the shock at one corner of the car with a solid steel rod?


Norm

 

[GregLocock]

It rolls about an axis, which is easy to measure but hard to calculate, and is rarely aligned with the (inertial) roll axis often mentioned in books, or the roll centres.

For instance if the car is up on two wheels the actual roll axis is very close to the line joining the two contact patches

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Komodo86]

Another effect of ZD setups that I have observed on my self built 3D kinematics models (and I do appreciate I might not have the load transfer theory 100% correct here) is that when the inside wheel tops out, the outside wheel continues compressing at it's normal rate. The effect is that the car is no longer rolling around an arbitrary point somewhere in the middle of the car (not the RC), but is instead now rolling around in the inside wheel contact patch.

This serves to reduce the roll rate and lower the car. Whether those effects are beneficial compared with the detrimental effects of a locked out inside wheel, is anybodies guess.

 

[golfpin]

many thanks for the few last tips gentlemen I think I,m getting the "drift'
PUNS .....an idiot,s delight.

Golfpin

 

[rikgray]

I've used droop straps for:
keeping the springs on the perch, expecilly when jacked up for tire changes
preloading the springs without increasing ride height
keeping the car from junping on rough roads

The shocks don't like being the droop limiter.
Straps made from fabric with a little give are better than, wire cables or chains, but I've used them all
Droop limiters do tend to give a big rise in roll stiffness, but at that point the inner wheel has little load anyway.
It's a question of balance, limit the front and rear.