Dizzy

I've done a couple of trackdays and sprints in my westfield now and I was wondering how to set the suspension up best for wet and dry. I undestand its down a bit to personal taste but I need a starting point. I have spax full adjustable shocks all round and an Antiroll bar at the front (were looking for a rear one atm)

all you dedicated trackday /race goers please help
Dry - Set it as firm as poss?
Wet - A couple of clicks softer all round?

ex-webby

Hi Dizzy

OK... this could be a big one...

What adjustment do you have on your shocks? Can you adjust bump and rebound seperately?

Is the ARB adjustable?

Can you adjust ride-height?

I'll assume yes to all... (and please bare in mind this is a RIDICULOUSLY complex suject, so all of this is speculation until you've done some testing)

Rideheight...
The reason for lowering a car is to reduce the overturning moment which is the torque operating between the centre of gravity and a function of the contact patches...

The less weight transfer you can from inside to outside, the more potential grip you have...

BUT...

McPherson (sp?) strut suspension design is such that lowering the ride height also lowers the roll centre (the point at which the suspension rolls around) further than the centre of gravity which means that the car body will effectively want to roll MORE not less...

why is roll bad?
Roll is not the devil, in fact it is very useful, but it should be limited to the point where you are not deforming the contact patch to the point where the reduced load transfer is countered by the reducing in contact patch width / performance.

So... lower to the point where you are balancing load transfer with contact patch deformation. Most cars have a happy medium.

Remember the centre of gravity and almost certainly the roll centre are at different heights at front and back (and in fact all the way through the body of the car) so ride height at front almost certainly has a different optimal that rear, but you should always have the rear higher than the front (rake).

ARB...
The main use of an arb if it is adjustable and you only have one is to trim the roll-couple distribution (the difference between the roll rate of the front and back)...

In short (and as all of this, this is a rule of thumb rather than facts and figures) the end that has less roll resistance (actually lower roll rate) will grip more than one with more. Set the rear up stiffer in roll than the front and your car should oversteer more.

Dampers...
COMPLEX... Springs have greatest effects in long sweeping corners and when the springs have settled into their load baring state, dampers on the other hand only have effect when the suspension is moving.. they resist speed of movement, not the amount of load they can support (like springs do).

So... think of the dampers from a performance point of view as a way of moving load around the contact during transition states.

So...

More bump on a corner means that load will be transfered to that contact patch (and therefore grip) quicker when weight is transfered towards that corner.

More rebound resistance and load will be taken away from that contact patch quicker when load is moved away from that corner of the car.

Left and right front should be the same, left and right rear should be the same.

So..

if you wanted more bite in the front suspension at the moment you lift off the throttle (weight moving forward) you could try adding a little more bump on the front, or a little more rebound on the rear.

BUT...

rebound should be reduced only to the point where it can still control the force of the springs accelerating the wheels (unsprung weight) away from the body (sprung weight).

An easy rule of thumb way to spot this is to have someone drive the car on fairly high bump settings all round and watch the body of the car as it rides slow bumps. The body should not oscilate but should rebound once and settle. Once you have the minimum rebound setting that acheives this, this is your rule of thumb minimum rebound setting.

TESTING...

There is NOTHING that ANYBODY (no matter how experienced they are) can know for sure until you test the car.

Make big extreme changes to ONE thing at first and see which feels better.. test the setting in as many situations as possible (fast corners, slow corners, trail braking, short shift bends, full acceleration, heavy straight line braking stability, etc, etc), then move to smaller changes.

have fun!

All the best

Simon

ex-webby

sorry.. wet set-up...

softer suspension all round generally as the tyres do not resist the forces acting on the centre of gravity as much as they do in the dry...

Higher tyre pressures can help the tyres bite through deep water...

more rear brake bias if you can adjust it as there is less weight transfer from rear to front.

Also... i forgot to say...

If you are using bias-ply tyres rather than radial, you have less negative contact patch deformation with roll. The benefits of the two against each other is too huge a subject ot go into, and probably not necessary.

All the best

Simon