The science behind wheel sizes

[R59]

Better buy a Nova then, I just happen to know where there is one languishing.....

This is all a bit off topic as it all started with wheel diameters..

With it understeering Denis, your addition more bar made the front end stiffer, which will make it understeer. You needed to balance the addition of more bar, with reduction in front spring rate. You obviously then have to be careful of bottoming.......so you need some good quality bump rubber to stop it.

When I worked on some prod-saloons, and Rover 216GTi's many years back, we used to stuff the travel up with bump rubber. Using a softer rubber on top of a harder compound, then packing with C-washers. This was done because you couldn't change the springs or the dampers from the one's in the spec, which were pretty well standard. The cars were running on slicks, so they could cope with very stiff rubber.

I used to do a similar thing with the Nova when I ran on Spax dampers. They were perfect in the wet, but too soft for the dry, so I used to pack them out with bump-rubber. The type of rubber depended on the circuit, obviously down to to the grip levels which varied all over the place. If it rained, I used to just pull it all out.

Bringing that into the context of your car, you could use bump rubber to stiffen the car either experimentally, or on a more permanent basis.

It sounds to me like you really need to get some shorter dampers made up which give you enough travel based on the ride height of that Corolla, which is only just off the ground!

Back to wheel sizes, and the origin of this thread. One thing that you have to remember is that when you change wheel diameters, the roll centre of the car will change, and this can dramatically change the handling characteristics. Might be better! could be worse!

While reducing the sidewall depth can reduce the amount of squirm in the tyres, and thus improve the size of the footprint, it also dramatically increases the effective spring rate as your tyres form part of your suspension.

Yes, moving to larger diameter rims can increase the unsprung weight, but choosing a quality lightweight rim can offset this, or even improve it. Which reminds me of the Carbon Fibre rims that I looked at a few years back. One set of them weighed about as much (less!) as a single magnesium alloy rim! But at £3000 per rim....

One final factor in rim size is the availability of rubber.
My latest car had 16" rims on it, with 12" wide rears, and 9" wide fronts. I had to run it on Dunlops, and the only tyres they produced that I could use were for Gp.C2 cars, and no way would they cope with either the weight of the car, or the torque. So I was advised to go to 18" rims and radials which is where the development is. GT's mostly run on 18" rubber, so I had to come down 1" at the rear, and went up 1/2" at the front to suit the availability of the best rubber.

There lies another subject for discussion...

Rob.

[Goran Malmberg]

Originally posted by racing59
Better buy a Nova then, I just happen to know where there is one languishing.....

Yes, moving to larger diameter rims can increase the unsprung weight, but choosing a quality lightweight rim can offset this, or even improve it. Which reminds me of the Carbon Fibre rims that I looked at a few years back. One set of them weighed about as much (less!) as a single magnesium alloy rim! But at £3000 per rim....

Rob.

One should not forget that the weight of the wheels is rotational. So diameter and therby the location of the mass has great influence on the car. It is sort of like adding wheight to the flywheel.
Goran Malmberg

[dtype38]

Originally posted by racing59
Back to wheel sizes, and the origin of this thread. One thing that you have to remember is that when you change wheel diameters, the roll centre of the car will change, and this can dramatically change the handling characteristics. Might be better! could be worse!

While reducing the sidewall depth can reduce the amount of squirm in the tyres, and thus improve the size of the footprint, it also dramatically increases the effective spring rate as your tyres form part of your suspension.

Correct me if I'm wrong, but I think the roll centre of the car would only change if the rolling radius of the tyre was changed. Bigger rims with lower profiles of the same rolling radius wouldn't make a difference.

On the last bit, I've always worked on the principle that runing deeper profile tyres with more compliance in the sidewalls will put more tread on the ground than a low profile. Deeper profile tyres also seem to be more forgiving of bump-steer and imperfect camber ratios.

[Goran Malmberg]

Originally posted by dtype38
Correct me if I'm wrong, but I think the roll centre of the car would only change if the rolling radius of the tyre was changed. Bigger rims with lower profiles of the same rolling radius wouldn't make a difference.

On the last bit, I've always worked on the principle that runing deeper profile tyres with more compliance in the sidewalls will put more tread on the ground than a low profile. Deeper profile tyres also seem to be more forgiving of bump-steer and imperfect camber ratios.

Roll centre is complex to say the least. But the way the question is asked I should say that the same radius tire
gives the same Rc.
Roughley the Rc is a projection of the A-arms to an instant centre and back to the contact patch centre of the tire, no matter what is rubber or rim. This projection is named "forcline" an has the effect of producing geometric corner weight distribution. This means that wheel travel alter this geometric over-understeer balance.

Tires has different sidewall stiffness depending on size but in general, a lower profile tire generate slightley less footprint. But the lower profile tire might need another airpressure for an even working temperature which must be taken in to account.
What is more important is that the lower profile tires is much more sensitive to camber change. Easely creating uneaven footprint area load. This lowers the total friction of the single tires in the same way as a stiffer bar creates less friction via uneaven load between the tires over the axle.
Goran Malmberg