13" WHEELS

[Bud]

Hi

I am entering the speed series this year     and need to get some new tyres. I currently have 15" wheels with 205x50x15 tyres on. I have been told that although this size of wheel and tyre look good on the car that I am over tyred and that smaller would be better.

Therefore what are the benefits of going down to 13" rims and narrower tyres, say 195 or 185 for the front and 195 for the rear? Apart from the obvious cost and weight. Also are there any disadvantages other than the speedo will probably read wrong.

One option I have thought of is putting some Toyo T1Rs on the 15" to use in damp or wet and then by a new set of 13" rims with R888's for the dry - what do you think?

Apologies if this has been discussed recently - I seem to remember something a couple of weeks ago so I did a search but could not find that post.

Thanks

Bud

[Asterix]

You're on the right track.

Smaller wheels: -

(usually) cheaper tyres

(usually) lower unsprung mass

Lower gyroscopic resistance

Higher potential acceleration (assuming smaller wheel/tyres at rear)

Whether smaller tyres all around are of benefit is an arguable question.

For a sprint, there is an argument (that I would ascribe to) that smaller tyres on the front lead to less understeer (as you can heat the fatter tyres at the the rear easier than you can those at the front).

Disadvantages: -

(Potentially) chosen tyres may not be available in size you require

Speedo will be wrong (but that's correctable and it's a minor error)

Lower top speed (assuming smaller wheel/tyres at rear) - this is low impact for majority of sprints

Your option sounds fine.

[Terry Everall]

If you are going to use 1B tyres ( 888 for example) you will go into class E or F rather than "road going " classes A,B,C,D

[Kevin Jones]

Bud has an aeroscreen I think so F.

I would not worry too much about wets if you want to save £. It would have to be very wet for a 1A tyre to be significantly better than a 1B in my experience. Occasionally that happens but not that often.

On sizes, you may want to think about putting 205 on 7" on the rear. You can run less but there is a limit to how much power 6" will take. Having 7" all round allows you to rotate them, but as mentioned heat in the fronts might be an issue.

[davidgh]

An investment in 13" wheels is of questionable value now that ACB10s are going to be banned in the SS. With radial 1Bs there is probably more tyre choice in 15", and the reduction in unsprung weight will not be nearly as large.

[John Loudon - Sponsorship Liaison]

13 inch wheels will always be of benefit if fitted with anything other than list 1A tyres. OK, so there are 15 inch list 1B tyres available but they are more expensive and heavier than 13 inch ones plus there is a wider choice in 13 inch for 1Bs and especially slicks

[davidgh]

there is a wider choice in 13 inch for 1Bs

John

Not sure that holds for V70As, which are many peoples "radial 1B of choice".

David

[John Loudon - Sponsorship Liaison]

More manufacturers do a 1B in 13" than 15" IMO

[Bud]

Thanks for the replies,

Kevin is right, I will be in class F. (should have left the windscreen on) so want to use 1B tyres.

John and David I am considering the Toyo R888 tyres which I thought were widely available for 13" wheels.

205 rears on 7J rims sounds good but is it overtyred for a 2L zetec producing 150bhp at the wheels? or is it about right? If 195s will be as good then I'll probably go for them with 185s on the front again all on 7Js.

Bud

[Martin Keene]

FWIW, I think 13" are more suited to the car than 15" regardless of use. I had more 'moments' in the wet with 14" wheels and proper road tyres than I have had with 13" and A048R.

I firmly beleive that if your car is reasonable light weight, mine is ~550kgs with 2/3rds tank, then the car doesn't have the weight to push the propoer road tyre through the water to the road surface, therefore you are better off with the softer track tyres.

Also the car will also feel a lot more 'pointy' and generally lively and lighter on it's feet with 13".

Going to 13" rates as one of the best modifications I have ever made.

[chrisfopp]

Bud,

Please appreciate that I'm not a racer but my Sport 2000 is lightish and puts out 175 bhp and I spend most of my time on track these days so could be equivilant to your 150bhp.  I wouldn't bother with T1-Rs for the wet, I use those on my Capris that weigh nearer to a ton, proving Martins advice above as I use R888s as road tyres and wet track day tyres on my Westie and they're excellent.  I use 205/50/15s alround so I can rotate easily but everyone else will give you the right advice on 13s vs 15s etc and you've got the added consideration of what you're allowed to run which I don't.

I'd love to have the time to compete but other sports / activites take precidence, best of luck and enjoy yourself!