Does changing tyre width change wheel rate?

[J Chapman]

I’ve got a technical query regarding wheel rate. I run a Van Diemen RF03 Zetec in Mono 1800 and F4. Until now I have only ever run the standard width Zetec tyres; 6.0” front, 7.0” rear. Over the winter I have acquired a set of FF2000 (Pinto) tyres; 6.5” front, 8.2” rear.

Whilst the centre of the tyre will remain unchanged the outer edge will move further away from the chassis by 0.25” at the front and by 1.1” at the rear.

My question is: Does this increase the leverage applied to the car and hence the effective wheel rate? And if so will I have to fit a high rate of spring to compensate for this?

Any thoughts / experience will be greatly appreciated.

James

[tristancliffe]

No, the leverage value won't change, so assuming the wider tyres are the same spring rate themselves, then the overall wheel rate will remain the same.

However...

The wider tyres will weigh more, and the amount of grip front:rear will change, so the balance of the car and the optimum springs will probably therefore alter. You will also find that the ideal camber may (or may not!) change, and the cold tyre pressures might have to be fiddled with. The brakes may feel worse due to the increased inertia of the wider wheels and greater amount of rubber, and the acceleration will change. If you are changing tyre size (I assume you are) then you'll find the gearing will change too. The change of unsprung weight may change the damping required too. You probably already knew/realised all of this, so apologies if I'm teaching you to suck eggs (an odd phrase). It may be that everything is so small or cancels out, and the car is perfect as it is but with wider wheels.

[guilleracing]

Hi,

Something you may also have to consider is aero drag.
Wider wheels will have a greater surface area and may have an effect on any aero devices close to them.

I have seen some impirical evidence of top speeds being reduced a tiny amount by wider wheels.

Interestingly enough, Tyrrell F1 once ran some "Fronts" on the back of their cars at the Monza F1 GP to get their top speeds up in qualifying.

[J Chapman]

Many thanks for your input. I'll see how we get on. I'm expecting proportionally greater rear grip, however I've never struggle with understeer before, so hopefully this will if anything even up the balance. It's got to be worth a try since the tyres were free!! Unfortunately I can't go to the full spec. 9" tyre that most of the front running F4 cars run and the FF2000 tyre seem to fit straight onto the standard FZetec wheels. So I'll give it a go and if all seems ok I'll order up myself a new set.

Many Thanks

James