Porsche wheel weights

[evilC]

Originally Posted by Wally

Yep, very true imo, but....for a circuit racer it also matters what the grip level and handling caracteristic is for both combos.The lower side wall will have better handling due to stiffer side wall giving much better steering respons.
Its never that simple

In racing circles you will note that the formula boys all wear relatively tall sidewall tyres and the enclosed bodywork cars all have very low profile tyres. I am convinced that the main reason is brakes. Most enclosed body work race cars are resricted to an overall wheel diameter by the bodywork and very often by the requirement to run steel discs/rotors. That dictates the largest diameter discs/rotors to maximise the brake performance that also results in the shallowist profile tyre because the overall diameter is restricted. Where brakes and tyres are relatively free then the tyre profiles tend to be taller e.g. Formula 1 which is supposed to be the pinaccle of development.

Originally Posted by ricola

Big wheels = big space for big brakes too!

I agree but only if you can use them. On a bug for instance, it is doubtful that very large brake disc/rotor diameters would be useful as the suspension systems are not capable of transferring the braking performance to the tyre contact patch since all the bug suspensions suffer severe camber changes. That also mitigates against low profile tyres as the lower the profile the worse the tyres capability at coping with camber change.

evilC

[Wally]

Excellent post!

never thought about the camber changes in relation to side wall thickness and the tire's ability to cope with it, so its a very good point imo!

I feel already better changing from a 225/35 to a 225/40 tire last month Of course, main reason to change was the stickier compound, but nevertheless

Tnx,
Walter

[Simon]

Originally Posted by evilC

I agree but only if you can use them. On a bug for instance, it is doubtful that very large brake disc/rotor diameters would be useful as the suspension systems are not capable of transferring the braking performance to the tyre contact patch since all the bug suspensions suffer severe camber changes.

Which camber changes do you mean?

That also mitigates against low profile tyres as the lower the profile the worse the tyres capability at coping with camber change.

Do you have any data about this?

[evilC]

Originally Posted by $aint

Which camber changes do you mean?


Do you have any data about this?

The front on a link pin or ball joint suffer camber change in roll from near vertical to horribly positive. The Macpherson strut suffers less badly but still has some changes.
The rear on a swing axle is absolutely chronic in roll going from several degrees poitive in droop to several degrees negative in compression. The IRS again suffers less badly but does go from positve camber to negative camber in droop to compression. To add figures to all this would require significant data on a wheel alignment gig and a chapter of a book.

I am not aware of any data relating to the tyres ability to cope with camber change but I ask you to imagine a situation where there is excessive camber, the taller profile tyre will keep the tread more readily in contact with the road than the lower profile tyre because the sidewall is not having to deform as much. Put another way if the load on the tyre is constant in both instances the sidewall of the tall profile tyre will deform more than the low profile tyre.

evilC

[Simon]

Originally Posted by evilC

The front on a link pin or ball joint suffer camber change in roll from near vertical to horribly positive. The Macpherson strut suffers less badly but still has some changes.
The rear on a swing axle is absolutely chronic in roll going from several degrees poitive in droop to several degrees negative in compression. The IRS again suffers less badly but does go from positve camber to negative camber in droop to compression. To add figures to all this would require significant data on a wheel alignment gig and a chapter of a book.

I'm sorry, I always only have a McPherson/IRS beetle in mind, forgot to add that

Corcerning the McPherson strut, I assume you mean the camber changes that come from compliance in the bushings?
Or do you mean other camber changes?

I am not aware of any data relating to the tyres ability to cope with camber change but I ask you to imagine a situation where there is excessive camber, the taller profile tyre will keep the tread more readily in contact with the road than the lower profile tyre because the sidewall is not having to deform as much. Put another way if the load on the tyre is constant in both instances the sidewall of the tall profile tyre will deform more than the low profile tyre.

I've read some data about tires having the most grip when cornering when they have a fairly large negative (dynamic, ie total) camber (afaik it was >7 degrees).
Don't remember anything about the sidewall hight or strength being said in that article though.
I'll see if I can digg it up.

[evilC]

Originally Posted by $aint

I'm sorry, I always only have a McPherson/IRS beetle in mind, forgot to add that

Corcerning the McPherson strut, I assume you mean the camber changes that come from compliance in the bushings?
Or do you mean other camber changes?


I've read some data about tires having the most grip when cornering when they have a fairly large negative (dynamic, ie total) camber (afaik it was >7 degrees).
Don't remember anything about the sidewall hight or strength being said in that article though.
I'll see if I can digg it up.

The strut on a beetle is essentially pivotted at the top due to the compliance of the top bush and the bottom of the strut pivots on the ball joint in the track arm. at the other end of the track arm there is also a pivot. In roll the track arm describes an arc with the centre at the inner most pivot point of the track arm therefore, the outer ball joint moves laterally in relation to the chassis. As the bottom of the strut moves in or out because the top of the strut is a pivot point the angle of the wheel changes in relation to the body - usually it goes more negative. The actual camber or the angle of the wheel in relation to the ground would be the same if the roll centre of the suspension was at the inner point of the track arm but it isn't it is some distance further away that means the wheel camber would change. A further complication is that the geometry of a Macpherson strut means that as the roll increases the roll centre moves closer to the car and that affects the camber. All up, to establish the camber of the wheel to the road surface requires the input of the roll centre position, compression of the strut the ride height and the static camber.

To get maximum grip from a tyre the tread has to be flat on the road surface. To acheive that the geometry of the suspension and the compliance of the tyre (sidewall stiffness) all play a complex and interrelated part. You will see that on the circuit mechanics will be using a temperature probe to check the temperature across the tyre. The temp. is a function of how much a particular part of the tread is working. The aim is to have a constant temperature across the full width of the tyre

evilC