wheel+tyre size....gearing effect

[browrz]

Wondering about whether my cerb with 18' spiders was really designed for this when I'm now running 40-depth tyres (R888's). I wonder if it would perform (on the track) better with 17's. Forget wheel arches, have had them hollowed out enough. But the car must be slower accelerating with this set up surely?

[JR]

browrz said:

Wondering about whether my cerb with 18' spiders was really designed for this

No.

browrz said:

I wonder if it would perform (on the track) better with 17's.

Yes. Better on the road also.

browrz said:

But the car must be slower accelerating with this set up surely?

Yes.

[JensenA]

The Cerb is better on 17" wheels. Depending on the width of your tyre, the circumference of the tyre could well be out from the standard, and so that would affect your gearing.

Check here .... http://www.tyresave.co.uk/tyresize.html

Edited by JensenA on Sunday 18th September 09:29

[browrz]

JensenA said:

The Cerb is better on 17" wheels. Depending on the width of your tyre, the circumference of the tyre could well be out from the standard, and so that would affect your gearing.

Check here ....http://www.tyresave.co.uk/tyresize.html

Edited by JensenA on Thursday 15th September 22:30

Thanks guys. So 'standard' should be considered as 17' with 40 depth tyres, you'd say? If the 4.2 came with 16's in theory then I wonder if the T5 box is geared slightly different to allow for the smaller wheels?

[JR]

browrz said:

Thanks guys. So 'standard' should be considered as 17' with 40 depth tyres, you'd say? If the 4.2 came with 16's in theory then I wonder if the T5 box is geared slightly different to allow for the smaller wheels?

The rolling radius is the same for 16, 17, and 18" wheels. The increase in wheel dia gets taken out of the tyre sidewall.

You appear to be looking for track performance so have you considered a set of road legal 17s with R888s before their sale gets banned next year?

(As a footnote the acceleration is worse, marginally, with larger wheels because of the increase in second moment of inertia.)

[browrz]

JR said:

browrz said:

Thanks guys. So 'standard' should be considered as 17' with
40 depth tyres, you'd say? If the 4.2 came with 16's in theory then I wonder if the T5 box is geared slightly different to allow for the smaller wheels?

The rolling radius is the same for 16, 17, and 18" wheels. The increase in wheel dia gets taken out of the tyre sidewall.

You appear to be looking for track performance so have you considered a set of road legal 17s with R888s before their sale gets banned next year?

(As a footnote the acceleration is worse, marginally, with larger wheels because of the increase in second moment of inertia.)

Thx. I have R888's on my 18' spiders. Which are good. But I'm not sure that 17's with the same '40' depth tyres have the same rolling circumference. Surely they have to be smaller and hence quicker acceleration, less top speed. ?

[Tanguero]

Personally I much prefer the handling on 16" rims (Oz Saturns) with R888s for sprinting. It was a huge improvement over 18" Spiders with the same tyres, much more progressive at the limit. As an added bonus the on road ride to and from sprints is a lot better too. I haven't tried 17" wheels but logic suggests that they are somewhere between the two.

The rolling radius depends on exactly which tyre if fitted to which rim. There are numerous online calculators that will tell you the differences for various combinations of wheel/tyre and gearing.

Edited by Tanguero on Friday 16th September 11:23

[JR]

Hi Richard, to add to Tan's reply, it may well be that you have oversized tyres on your 18s. Many people do the maths and find that a very, very low profile is unacceptable to them for comfort, cost, availability etc so choose a taller profile. Also remember that profile is a proportion of width and people vary the width as well. I wouldn't worry too much about your present set up but where you are heading.

(As for your comment about top speed it doesn't always work out that way. A larger rolling radius will give a larger top speed as long as the engine has the torque to allow the power to be developed. FWIW I think that it's very academic - the car will be very fast either way and there is likely to be very little in it.)

[browrz]

JR said:

Hi Richard, to add to Tan's reply, it may well be that you have oversized tyres on your 18s. Many people do the maths and find that a very, very low profile is unacceptable to them for comfort, cost, availability etc so choose a taller profile. Also remember that profile is a proportion of width and people vary the width as well. I wouldn't worry too much about your present set up but where you are heading.

(As for your comment about top speed it doesn't always work out that way. A larger rolling radius will give a larger top speed as long as the engine has the torque to allow the power to be developed. FWIW I think that it's very academic - the car will be very fast either way and there is likely to be very little in it.)

Thx chaps. The other issue is choice of R888 sizes. Don't think they go that small in profile in 18's.

Anyhow, conclusion I'm coming to is prob 17's with low profile to keep closer to what the car seems to have been intended to run. Thx again