This coudl be a long thread....8" vs 6" rears for sprinting

[andrew mckay]

I'm running 6" ZZS rears on a 310R for sprinting....mainly because I can't be bothered finding some 8 inch rears and buying more tyres.

Has anyone done any back to back comparisons of 6" vs 8" other than more subjective comparisons?

On the face of it you might expect more grip with a bigger contact patch but there's less weight on the contact patch and it's not like a 310R has lots of power.

Plus there's what happens in the wet. Lets assume it's dry for now as there  may be advantages in the wet.

Caterham use 6" for all of their motorsport series with the exception of the R300/420R cars with 180 bhp.

My gut feeling with 150 ish bhp is there;s very little in it and the lower unsprung weight may be beneficial in some areas?

I'll keep an eye on the 64ft times to see how i compare but there's other factors at play...check out the class 1 64 foot times. Someone's ability to control the start may be much more beneficial than the benefit the tyre width gives.

Your thoughts?

 

 

 

 

 

[Jonathan Kay]

No idea about sprinting, but some thoughts about physics.

On the face of it you might expect more grip with a bigger contact patch but there's less weight on the contact patch... 

What are you comparing with what when you say "less weight"?

Jonathan

[andrew mckay]

Sorry car weight supported by more of a contact patch on bigger tyres. Force per square cm less with bigger tyres. 

[Jonathan Kay]

Depends on how you set the tyre pressure. 

I don't know much more massive the 8" wheels and tyres are but apart from that effect on weight the width of the tyre doesn't independently affect the area of contact: that only depends on the corner weight and the tyre pressure. The width of tyre is likely to affect the shape of the contact patch.

Jonathan

[Simon.Rogers]

Someone may prove me wrong but given there is not much time to get anything warm through the run and certainly not before the run I will take width/contact patch size every time.

You can not compare class start times as lower power cars as just som much easier to get off the line.

Then ask Shaun about getting a Busa off the line.

[Richard Price]

Andy,

It's a question I've also asked myself!

With a car in otherwise very similar spec, I've always (until last year) used the same wheel and tyre size all around, and always enjoyed reasonable balance. Infact, my quest has normally been to reduce understeer rather than worry about lack of rear grip.

Last year, when starting to use the new generation of list1b tyres, I swapped to ZZS's with 185 front and 215 rear. I was expecting to struggle with understeer, but the opposite has been the case.......

[Jonathan Kay]

... I will take width/contact patch size every time.

Same question as before: at what pressure do you run the 6" and the 8" tyres in order to get that bigger area of contact?

Jonathan

[andrew mckay]

I will run lower than Avon recommended pressures. Circa 18 psi in both though having not tried 8" I can t say fo sure what pressure i would use. Obviously im not going to try to over inflate the 8" to reduce the contact patch.

[andrew mckay]

Simon that's my take too though I want to use the car for circuit racing too and didn't want to chop and change.

[andrew mckay]

Richard,

Perhaps we could put my wheels on your car at a Curborough test day and see what the numbers tell us? I'm thinking of organizing one in July.

Your laps are much more consistent than mine so we might be able to put this one to bed?

 

Andy

[Richard Price]

Your laps are much more consistent than mine

Unfortunately, the consistency that I enjoyed on Kumhos has evaporated since changing to Avons....

I was horribly inconsistent yesterday. I struggle to get off the line effectively. If I spent the wheels I may loose a couple of tenths. But bog down using lower revs and don't put as much energy into the tyres, and then find the back loose.....

 

 

[andrew mckay]

I thought you'd be dialed in now with the Avon's.

[Simon.Rogers]

Alway use 16psi cold.  All types of tyre.

It gives a good balance between a responsive car and one that has some deflection in the tyre.

No testing or analysis ever done other than by changing pressures up or down over 15 years of competition.

I always end up back on 16psi

[andrew mckay]

Thanks. I was using 17 in A539's but have started on 18 with ZZR's. Only done 2 events so will try a few different ones in practice.