Suspension Geometry

[Maurotoro]

Hello,

I am currently in the process of designing a low cost hillclimb racer, based on a transverse rear engine taken from a Renaul Clio, (configuration similar to the Ariel Atom), while running simulations in Lapsim I have found the car to oversteer greatly and have played with the front-rear roll stiffness balance like crazy making the front very stiff and the rear very soft, this seems to mitigate the problem to some extent. If anyone has any insight on what can be done to reduce this oversteer it would really be appreciated. I also have various questions about the suspension geometry, since the tires I am going to use are low profile high-performance street tires, I have chosen a suspension geometry with very little camber gain and a low rollcentre, this to ensure the maximum contact patch surface and minimum jacking, As a result of the low rollcenter I will have to run quite a bit of ARB especially at the front to reduce body roll. Well that's pretty much what I have, if anyone can help me with representative figures for anti-dive anti-squat and feedback on what i'm doing it would be gratly appreciated.

[Nero]

Do a bundorf analysis prior to the kinematics

[Maurotoro]

Originally Posted by Nero

Do a bundorf analysis prior to the kinematics

what are representative cornering stiffnesses for street tires? how do I correct them to take weight transfer, etc into account? Plus do you know anywhere I can get good info on Bundorf analysis because I have read about it in Race Car Vehicli Dynamics but the description is not very detailed

[Goran Malmberg]

I have newer looked att hill climb cars, but why are you thinking about antisquat and dive? Are there a need for this?
Goran

[Maurotoro]

I believe so, because it is pretty important to limit pitch, due to the soft springing usesd in the car (to maximise mechanical grip) and the high camber sensitivity of low-profile tires especially in braking and accelerating which it's important do be optimal at in the tight turns encountered in hill climb.

[Goran Malmberg]

Okej, if we have swaybars to limit roll and antisquat and dive to limit pitch roll,
in reality you want a roll stiffer car in all directions but soft in heave, is that right understod?
Goran

[Nero]

Mauro best you email me and I can share information.

[Maurotoro]

Goran,

You are right, that's more or less what i'm looking for, especially because your average hill-climb is a lot bumpier than you average race track so it's important to keep the tires on the ground as much as possible.

[phoenix]

Originally Posted by Maurotoro

......based on a transverse rear engine taken from a Renaul Clio, (configuration similar to the Ariel Atom), while running simulations in Lapsim I have found the car to oversteer greatly and have played with the front-rear roll stiffness balance like crazy making the front very stiff and the rear very soft, this seems to mitigate the problem to some extent. If anyone has any insight on what can be done to reduce this oversteer it would really be appreciated.

We have a race car with a similar layout - the transverse engine and gearbox from a front wheel drive car mounted in the rear - and not much weight at the front.

Driver and fuel on board, our weight distribution is 44% front/56% rear.

Our rear wheel frequencies are a little (10%) lower than the front - i.e. rear springs are softer than the front, relatively speaking.

However, due to having a wider track at the rear than the front on our car (something we can't change) roll stiffness from the springs alone is only 39% front with 61% at the rear. Without a front anti roll bar the car would oversteer like crazy!

To get neutral handling, our roll stiffness (a combination of the springs and anti roll bars) has to be set to around 57% front/43% rear.

If your numbers are similar to these then don't be too concerned.

If you can incorporate a wider front track that would help. The other thing I suggest is to get the engine and box as low to the ground as you can which if you sim it you will see can improve matters greatly.

If all else fails just go for the big front anti roll bar that the simulation points to!

[Nero]

I have a 900kg project car with mid V6 engine, wider track at the front, 58R/42F weight distribution, vert. CoG 482mm, Bundorf analysis showed slight oversteer characteristic, will be tuned with bars.

[Maurotoro]

I suppose I will tune it with bars too, I have also tried raising the front roll centre and managed to increase front load, But this is just as a means to be able to run a lighter, less expensive and less loading bar. I will continue the shematics of the car and it's suspension and hope to publish CAD renderings of the suspension shortly.

[Maurotoro]

Nero,

Is it possible you e-mail me a copy of the Bundorf Analysis you did for your project? An example would be really useful.

Thanks

[Nero]

RC's for a competition car are quite different to a road car. Driving style, track surface and track style also affect choices such as camber gain.
I cannot share the bundorf data, my work-though I cannot guarantee is correct and I am paying a skilled professional engineer to assist me in this area. I can provide a copy of his summary if you wish if you would like to email me via www.racemagazine.com.au and I will send a .pdf of the article.

[EdLeake]

Slightly off topics gents but hope you don't mind me asking, is lapsim a worthwhile tool to employ?

[Maurotoro]

well the main advantage of lapsim is it's free at least the basic version of it, Plus you can validate your results by getting actual data from your car into the program. It's pretty much entry level but if you tinker a little you can find it's quite useful especially to help understand roll stiffness and aero balance effects on a given car on a given track. If you have a budget get chassis sim if not Lapsim is quite nice.