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?

(Did post in non scoob related but got no response )

Fat Boy

Basically right.Is the ARB an adjustable as well as that is usually also softened for the wet and sometimes disconnected at the rear as well. Rest is experimentation.

Dizzy

Thanks for the reply
no the ARB is a solid one.. I did see a chappy swap over to a slightly softer front one at bruntingthorp in the wet. Bit too much of a faff for us.

Shame that getting to the rears is such a pain in the @rse.

I've also found some really odd opinions on tyre pressures too.. IIRC we run around 25psi all round and some ppl recond to pump the up for sprints?!?! dont quite follow that. I can see for trackdays with Uber hard settings.

Fat Boy

Tyres are more vehicle/suspension/conditions specific. Generally harder than road for track/ sprints to stop reduce tread movements/heat build up, but again less in the wet. Again experimentation. Slicks a bit different due to sometimes different construction, some lower than normal road tyres some equal /slightly higher. Ask your supplier really.

Look up the Driving Developments, or is it Techniques, site on the web ( written IIRC by a couple of people from this bbs John Felstead ?)where there is a good section on track preparation including balancing the pressures/temperatures on tyres.

Cheers

ex-webby

Copied from reply in NSR...

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.

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

Fat Boy

Best of luck with all that Dizzy old Boy- God has spoken .

As Webmaster Simon de Banke knows just a little bit more than me (roughly 99% more)I'll get me coat and leave you in his tender mercy.

Cheers

Claudius

Erm, isnt that exactly the opposite??

Claudius

It depends on the damper, but generally you do not need to fully close the bump, as that would result in inefficient absorption. To avoid too much roll, close the rebound a little more (start with open and tighten until you get the handling you want AFTER having set the bump).

In the wet, you need a completely different set up with less bump and more rebound but possibly also different springs. It also depends on what tyres you use in the dry and in the wet...

ex-webby

Hi Claudius

What do you mean by "isn't that the oposite?"

Be careful about making sweeping statements like "wet set up - you need less bump and more rebound", this could quite easil cause the car to jack down under braking and have you running on the bumpstops at the front.

The ammount of roll will not be efficiently reduced by increasing rebound resistence and this is not the way to go. All that will be reduced is the speed at which the body rolls towards the position the springs and roll bars dictate, and it will also increase load transfer during the transition period and raise the centre of gravity in relation to the roll centre during the transition.

Roll should be reduced by roll bars and springs. Transitional states should mainly be controlled by dampers.

All the best

Simon

[Edited by webmaster - 7/15/2002 12:33:02 PM]

Claudius

Hmmm, Simon,

we are possibly saying the same thing except that I misunderstood you

Anyway, my point was that if you want more weight to the front, you should lower rebound resistance in the rear, ie let the damper come back up faster, and decrease the bump in the front to make even more weight go to the front so the car can get into the corner. Not sure what you mean by being on the bumpstops, I meant a click or two (I have 30 clicks for bump in the front).

ex-webby

Hi Claudius

Unfortunately you've mis-understood how weight transfer at the contact patch works and how it is created through the centre of gravity.

The ammount of weight on a tyre has nothing to do with the angle the car is pitching at.

In order to get more weight transfer to the front as the suspension is in it's transition as the body is pitching forward you can increase bump, not reduce it, and increase rebound, not reduce it.

The reason...

if the front suspension is going into bump, the more resistance you place between the sprung and unsprung weight, the more load is transfered between the two, placing more of the weight of the car through the suspension and onto the contact patch.

At the other end of the car, the more rebound resistance you have the less the tyres are pressed into the road as the rear of the sprung weight lifts (in fact if you put enough rebound resistance into the dampers you would be effectively lifting the tyres off the road (obviously not literally)).

This misconception is incredibly common, and is very closely linked to the misunderstanding that ARBs reduce weight transfer inside to outside by keeping the car body more level (where as it is quite the reverse).

Hope this makes sense.

All the best

Simon

ex-webby

ps...

on the bump stops thing...

this is caused by damper jacking... if you have too much bump in relation to your rebound setting (being too low) you get positive jacking over bumps where the suspension does not absorb the bumps enough and transfers the motion into the body of the car, then the reduced rebound allows the springs to accelerate the wheels downwards too quickly meaning the car is now (for a split second) riding higher than it was... now add another immediate bump and the same happens... go over series of bumps and you can be at the extent of the suspension in no time...

the same applies in reverse... too little bump and too much rebound (this is more common) and when braking over bumps, the bumps are absorbed into the suspension, but the springs cannot push the sprung weight back up to it's normal dynamic ride height quick enough for the next bump.. again, in no time, you can be riding on the bump stops.

All the best

Simon

Claudius

Please accept my apologies for invading this BBS with my erroneous suspension talk.

I understand what you mean about the weight transfer and the bump in the front.

Andy.F

Can one of you clarify something for me please.

In steady state cornering, assuming a 50/50 distribution fore/aft all else being equal, the end with the highest %age weight transfer will lose grip first.

But in transient cornering, the end with the greatest weight transfer grips most.

Hows that then ??

Andy

ex-webby

Hi Andy

You are absolutely right about theoretical steady state cornering... assuming 50/50, etc, etc the end that shares the load most evenly will grip most.

I'm not sure what you've picked the transient state thing from.

If you are comparing it to the getting more load onto the front to get it to grip more, the difference is that it only makes the front grip more.. and makes the back grip less.

Under braking, if you can share the load front to rear more evenly (and at the same time exploit the grip on the back with extra brakes) you will stop quicker as you have more grip.

Does that make sense?

Claudius.. don't be silly, I'm just discussing it with you.

All the best

Simon

Andy.F

What I was getting at was increasing the front bump in order to improve initial turn in. I visualised this as causing greater weight transfer across the front wheels and hence relativly less front grip.

But, I think I'm confusing weight transfer with transient force. It's possible then, under transient (lateral G not longditudinal G )conditions for the sum of the forces on the front wheels to exceed the actual mass suspended.

Is it fair to say that a higher front roll centre could create a sharper initial response due to the 'jacking' effect acting instantaneously on the wheel ?

Andy

ex-webby

Hi Andy

I see what you're saying and you're dead right... whilst the added bump will increase load transfer across the fronts during transitions, the total of the extra load at the front will be higher during the pitch transistion. This is why I limited it to "at the moment you hit the brakes". As with all things in chassis dynamics, one good thing creates another bad thing

I don't follow you on the roll centre thing. It may be a gap in my understanding, but I don't understand how the roll centre can affect the jacking effect. (getting excited that I'm about to learn something new!! (sorry, I'm quite sad! ))

All the best

Simon

Andy.F

Simon

For simplicity, lets talk cornering only, no braking.

Weight transfer takes place via springs, dampers, ARB and lateral links (TCA or wishbones)

The height of the Roll Centre above (or below) ground dictates the %age weight transferred by the lateral links (sometimes called jacking)

If the RC = ground level then wt trans via link = 0
If RC = 50% of height to Centre of Gravity wt trans via link = 50%
If RC = CG then 100% wt trans via link (and you don't need springs to resist roll )

For Springs or ARB's to transfer weight we require body roll, for dampers we require an 'attempt' at roll (bump resistance) or actual roll. This is a bit 'after the event' of steering input.

The lateral link force is transferred as soon as the tyre develops a slip angle ie as soon as the steering is moved.

So the higher the RC, the more instant the wt transfer.
As usual high roll centres can bring their own issues of scrub, bumpsteer, pitch angle etc but the thought triggered my original question of ....
Will a higher front roll centre improve initial turn in ? If the downsides can be overcome, would it be preferable to harder bump settings on front dampers ?

Andy

spence7

Just a quickie, I think. I think I have the common misconception about ARBs !

Basically I thought that ARBs reduce the difference in loading of left side to right side when load is applied during cornering, hence maintaining a more equal load on each side giving more potential grip? Problem is slow load transfer between left and right can be beneficial depending on the road surface i.e.more bumpy so this is why you shouldn't go too stiff cos you'll slide across the bumps?

I dunno that was my understanding can someone set me straight pls!

Cheers
Alex

SDB

Hi Andy.. cracking post!

The jacking I was talking about was damper jacking where the damping is too high in one direction to cope with the movement in the other direction.

That is seriously interesting about the amount of load transfered through the links being a function of it's distance between the ground and the centre of gravity. How does that work for roll centres below ground?

On the question of initial turn-in... so little is dependent of the initial load transfer through the links that only in the softest of set-ups would the height of the roll centre (in my experience) be considered as a means of controlling turn-in in any way.

The earliest point I have ever spent any time on is the geometry in order to produce the slip angle you want when you want it (and of course the camber, castor, etc) but the first big part of it is damping...

absolutely, if you had higher bump settings on the front, you are likely to improve initial turn-in bite.

Thank you for the insight!!

Spence
The simplest way to think about it is this...
If the body of the car is being held more horizontal, something must be restricting it. The only place you can have any leverage over the body of the car at the contact patch. So in order to lift the outside side of the car, you would need to press down on against the outside wheels, and lift the inside wheels, which means more weight on the outside and less on the inside.

It isn't QUITE as simple as that, as I have been picked up on before, there may be complications if you have exactly the same roll rates front and rear in a theoretical 50/50 weight distributed car, with equal roll centres / cg heights, and in a theoretical steady state situation... but in the real world in transient states (which cars spend all but every single second of their moving life in) the above usually applies.

All the best

Simon

Andy.F

Simon

With roll centres below ground level, the cornering force through the lateral link actually pushes upwards on to the wheel and down on the body, lessening the load on the outer wheel The load on the outer wheel doesn't actually decrease for long as due to the increased cg/rc leverage, the roll angle increases and the spring/arb force maintains equilibrium.

Most of the text books will recommend fairly low RC's but my preference is to maintain a relativly high RC and reduce the CG, what a pity they act against each other on most production cars ie reduce the cg 3" and you probably drop your rc by even more..............bu88er !

Andy

spence7

Simon, ok so basically an ARB, during cornering, effectively presses more on the side of the car which is experiencing less load to maintain a more even spread of load? So round a left hander the left side naturally loads up so the right side needs to be pressed down ... so that's what the ARB does doesn't it?

Cheers,
Alex

spence7

Simon, just read your post again makes more sense the second time ! OK, so round a left hander the weight is shifting towards the left and pressing more on the left side. So in order to take that weight away to prevent overloading that side, you push the left side of the car up with the stiffness of the ARB and this puts more weight on the right side hence spreading the load more equally? So the role is to spread the load more evenly but it does this not by pushing the inside up, but pushing the outside up to bring the weight over the inside?

I think that's what you're saying no?
Cheers
Alex

SDB

Andy

THANK YOU! It is amazing how long you can do this and still learn something that seems like it should have been obvious!! I am really grateful for the information!

spence...
no...

Round a left hander, the load is being shifted towards the right. The body will roll towards the right. In order to stop the body rolling that way, the suspension between the body on the right wheel needs to resist that roll. This means the tyre is being pressed into the ground (which means MORE load transfer = oposite of spreading load more evenly).

All the best

Simon

spence7

Simon, Ok nice one (btw in my post I meant to say in a right hand corner not a left - totally bu88gered the rest of the logic in the post !! doh!)

Yeh, ok so outside wheels are being pushed into the ground more than they would be without an ARB to resist the body's natural tendency to roll. So pushing the body upwards on the outside means less roll and more load transfer to the outside wheels during the transition or loading phase....but once settled, surely there is a more even spread of load because the new body angle means more weight is applied to the inside wheels - that's what I was trying to say about spreading the load earlier...So more load on the outside wheels initially during transition to bring the roll under control...but the new roll position is such that the inside wheels are loaded more than they would be if the body was without an ARB. I thought that was the idea of decreasing roll ?

Cheers,
Alex

Pavlo

Andy et all,

Roll centre: Point in space about which the car rolls.

What does this mean?

It's the point in space about which the angular motion takes place during roll. If the roll centre is below the car, as the body rolls to (say) the left, then the body will also shift left at the same time.

Will the body transfer weight to the inside?

Absolutely not, all the while the time the centre of gravity is above the roll centre. You might just get the roll centre close to the CofG in a Formula car, with the engine and gearbox on the floor, and the ability to put the roll centre up high, but it's not going to happen in any practical case.

Why is weight transfer bad?

The rubber in car tyres is very grippy stuff, it's also non linear. Classic (linear) friction theory assumes that if you double the vertical load between friction surfaces, you double the friction available. With rubber on a tarmac surface, doubling the vertical load on the tyre, will less than double the friction force. This means if you put all the cars weight on the outside in a bend, you will have less grip available overall.

So why don't we run very stiff antiroll?
If you did have 100% roll stiffness using an antiroll bar (not active suspension) you would lose the ability to control a single wheel of the axle over a bumpy surface, which even the smoothest roads and tracks are. You would also have no ability to absorb the roll energy, so you would effectively bounce off the tyre, momentarily unweighting the outside tyre.

That should keep you going...

Paul

Andy.F

Paul

I think you over simplify You fail to consider the jacking effect of the lateral links during cornering.
In the same way that a drag racing car uses anti-squat rear suspension to 'jack' load into the rear suspension, you can do similar with lateral G forces.

Don't get me wrong, not even I can change the laws of physics so a car with a fixed CG and track width will always transfer a set load from the inner pair to the outer pair of wheels proportional to the lateral G, regardless of rollcentre height.

It is the transitional phase that you can play with. Also the jacking can move the CG which WILL change the weight transfer.

Andy

Pavlo

Indeed, just didn't want people to get the wrong end of the stick.

Just need to make sure not too much antirole/dive/squat is used.

Paul