Setting up suspension on a FWD car?

[RR98ITR]

Every front wheel drive car is the same: they all need higher roll stiffness in the rear than in the front.

Forgive me, but I don't think that is a correct statement. I have rig tested many front drive cars. If I recall correctly, most have had a very similar roll stiffness split since 1995, when the advantage(s) of a high rear roll centre became apparent.

Hi Dave,

Same thing - different way of getting it. Your testing of many - race or road cars? How did they differ?

Thanks,

Scott

[DaveW]

Same thing - different way of getting it. Your testing of many - race or road cars? How did they differ?

You are right, of course. Generally, the inside rear wheel must lift first for a front drive car. Using rear geometry to carry to roll moment helps kerbing, of course.

A good part of my work is rig testing "touring" race cars.

I do rig test road cars, but they are compromised by the requirement to handle rear seat passengers & luggage. When setting-up a "track day" road car, is it a bit dangerous to ignore that fact.... The proverbial sack of potatoes in the boot helps, I'm sure.

[thisisatest]

my big beef with setting up a FWD car to lift the rear wheel in cornering is that, once the rear wheels is off the ground, there's no more work the rear end can do to try to "balance out" the car. and as i see it, for smaller lateral loads, the balance is initially to oversteer. as the loads increase, it transitions through neutral, to understeer at the limit.
i feel that the best way around this would be to make the rear tires narrower (or front tires wider, as long as they fit of course). then you could have the front and rear ends set up with similar percentages of roll stiffness in relation to weight distribution, and handling would be much more neutral more of the time. Audi's FWD-based AWD S3 (or is it RS3?) has a staggered tire setup from the factory, which i was very happy to see.
i understand that tire size is often rule limited, but there must be other ways of reducing overall grip on a tire, right? extreme negative camber, positive camber (?!), others im sure.

thoughts?

[autogyro]

Rear wheel steering, no steering on the front, with torque vectoring across the front drive axle to control steering effects.
That should work and allow a much more efficient design with fully controlled over to under steer.

[Belatti]

A real challenge would be to develop a suspension for an Argentine Super TC2000 racing car.
430HP on the front is quite a thing...

i´m sure you don´t want the same tyre size/construction/compound front rear for these or you need to setup the car to three wheel around the corners ?
Is the Argentine series allowing for front -mid engine installation- or is your engine position fixed in the oriinal position?

I remember Ricardo Divila did some wonders to RMLs Nissan S2000 primeras a while ago ...tying the engine in for structural integrity of the whole front for example and playing around with swingarm lengths .

there is trick stuff around to optimise Macpherson struts -especially the top mount is a critical area in my view ..

Argentinian STC2000 lets u use the same brazilian Pirelli tyre front and rear, engine is a V8 2.6L radical mounted at the front in transversal position, ARB mounted to the hub allowed. You dont use Mcpherson strut at the front, double A config with coilover damper from the structure to the bottom wishbone. At the rear is mandatory a solid axle.
Suspension components like hubs and wishbones are the same for everyone but geometry is free.

[Belatti]

There is another fwd touring car series i work with here that has 2 classes. One uses medium size cars with 280hp and the other small cars with 220hp. They have to use original goemetry (mcpherson front and whatever the model is sold with at the rear)

The best combo I have seen is renault clio rear passive steering. ARB rates are almos even like DaveW said. We use about 15lbs more of rear tyre pressure and we combine front new tyres with used rear. The qualy fast lap is optimised when you select the correct tyre grip combo taking into account front tyre temp growth, a drivable turn in oversteer (fast corners the most important) and minimum exit understeer.

[DaveW]

my big beef with setting up a FWD car to lift the rear wheel in cornering is that, once the rear wheels is off the ground, there's no more work the rear end can do to try to "balance out" the car. and as i see it, for smaller lateral loads, the balance is initially to oversteer. as the loads increase, it transitions through neutral, to understeer at the limit.

Note, I said: the inside rear wheel must lift FIRST for a front drive car. i.e. It is not a good idea to lift inside front wheel first....

The are several ways to delay the transition to three wheels, all basically aimed at controlling the axle based roll over moment. These include lowering the c.g. height, increasing track width, controlling the longitudinal position of the c.g, and the distribution of roll moment carried by the axles. Donsker covered most of these.

Centralizing the c.g. laterally is also a good idea.

Geometry has no effect on overall roll moment but it can, used in moderation, help the car to "kerb" by allowing a reduction in roll stiffness.

Matching tyres to the car is a good idea, even playing with tyres pressures can help (within reason). Slicks, especially, must be heated evenly to obtain consistent performance.

Suspension set-up is key to making the car drivable, balanced, and matching the car to its tyres. Arguably, that is what rig tests achieve, as well as helping to understand the vehicle.

Edit: Be suspicious of Macpherson struts that centre the springs along the body of the damper. The levels of friction introduced can be a disaster.

[DaveW]

Rear wheel steering, no steering on the front, with torque vectoring across the front drive axle to control steering effects.
That should work and allow a much more efficient design with fully controlled over to under steer.

Failure mode requires some thought. Have you ever wondered why conventional cars have just one reverse gear?

[Tommy Cookers]

Rear wheel steering, no steering on the front, with torque vectoring across the front drive axle to control steering effects.
That should work and allow a much more efficient design with fully controlled over to under steer.

steering geometry for RWS ?
no such geometry exists for natural (ie passive) centering, giving directional stability when cornering ?
self-alingning torque has become self-unaligning torque ?

the Citroen SM had powered (artificial, 'q feel' aircraft-like) centering, not PAS
so it could have had RWS ?
similarly, Thrust SST had RWS ?

BTW ..... CVT cars have in principle the same gear ratio regardless of direction
(but there is an inhibitor switch to hold the ratio down when reverse has been selected)
the Messerschmidt '3 wheeler' reversed the (2 stroke) engine direction for reverse motion (by using a 2nd set of ign points)
so could actually be driven at 55 mph in reverse if directional control could be maintained

[autogyro]

Rear wheel steering, no steering on the front, with torque vectoring across the front drive axle to control steering effects.
That should work and allow a much more efficient design with fully controlled over to under steer.

Failure mode requires some thought. Have you ever wondered why conventional cars have just one reverse gear?

Certainly needs some thought.
Although it is surprising how many power units can be run in the opposite engine rotation with little difficulty giving multiple reverse gears.

[DaveW]

similarly, Thrust SST had RWS ?

You should ask Andy Green about that....

The Messerschmidt '3 wheeler' .... could actually be driven at 55 mph in reverse if directional control could be maintained

There you go....

[gixxer_drew]

similarly, Thrust SST had RWS ?

You should ask Andy Green about that....

Very different animal... aluminum wheels with no tires, tiny Cl value resulting in a massive vertical loading. Different in every way.

You know its funny you mention about the roll center on FWD vs roll stiffness dist "since 1995". There was a certain factory effort a about ten years ago running rear rates on the order of F1 fronts. I still cannot consider it as having been optimal, even inside their rules (at least from everything I understand). However they won two championships like that so to this day I think back on that and wonder if I missed something or overestimated the rest of the field. The first option seemed more likely until just now.

[donskar]

I got nothing when I searched "RWD car" . . . (Mods, please move to appropriate thread)

My FWD 200 SX SE-R now is far faster than its rapidly aging driver. So now I am building a RWD 240SX (S14). I have a couple Carroll Smith books on the way from Amazon, but perhaps some of you will chime in and supplement my basic knowledge.

I'll leave fine tuning toe, castor, camber to Carroll Smith's input -- and yours. So far:

KYB shocks and Tein lowering springs (adjustable coilovers later)
Adjustable tension rods up front
Adjustable toe and upper control arm rear along with new traction rods
Strut tower bars front and rear
Tein steering arms and tie rods front

Next -- I think -- a stiffer front sway bar will help tame oversteer. (I am too old for sideways motoring).

Thoughtful input very much appreciated.

[Jersey Tom]

Next -- I think -- a stiffer front sway bar will help tame oversteer. (I am too old for sideways motoring).

A bigger FARB would probably push everything in the direction of understeer, sure (as would softer rear main springs). Probably also a cheap component change.

Something else to think about - what's the balance of the car kinda neutral throttle steady state in a corner... versus what is it when you get on throttle. If your oversteer is only on-throttle and the car is neutral or even understeer otherwise.. could think about better tires all around ($$) or a better diff ($$$).

But yes, a different FARB ($) is a nice option to have regardless. Can probably do quite a bit with a FARB option or two (that includes disconnecting FARB!) and a few sets of springs with varying rates.

[Yawpower]

Edit: Be suspicious of Macpherson struts that centre the springs along the body of the damper. The levels of friction introduced can be a disaster.

Are you referring to a "coilover" setup where the spring is concentric with the damper?