981s track tyres - diameter question

[rego]

I’m new to tracking cars with a staggered tyre setup (previously raced Evo’s and an MX5) but I’ve purchased a set of 718 18” Cayman wheels to use as a set of dedicated track wheels for my 981 Boxster S so I can save my factory 20” wheels for the road and at least see what sort of time the car can do with a decent set of tyres and alignment.

The reason I went with 18” is because I paid next to nothing for them and I also want to run proper R-Spec tyres, either Yokohama A050’s or Hankook Z221’s which I’ve had a lot of experience with on previous cars (I will put more camber on to make use of the tyres, can trailer the car to tracks further away) but I am looking at the tyre sizes to compare rolling diameters and it looks like all the track tyres will be significantly smaller and different relative diameter front & rear due to the differing profiles.

20” Stock 265/35/20 & 235/35/20
26.5” F - 27.3” R (-2.9% F/R)

18” Semi’s 265/35/18 & 235/40/18
25.4” F - 25.3” R (+0.4% F/R)

The other option is that I could get a set of a lesser semi slick like NT01’s/DZ03G’s in the same profile or get a set of 19” wheels to run Cup 2’s which are probably more suitable for a road car but I’m happy running proper semi’s on 18” wheels if I can get the right size.

Has anyone got any experience running tyres with smaller diameter than stock and different relative diameter front/rear than stock? Anything I should know or is this difference small enough to be negligible?

[Timm]

2 hours ago, rego said:

I’m new to tracking cars with a staggered tyre setup (previously raced Evo’s and an MX5) but I’ve purchased a set of 718 18” Cayman wheels to use as a set of dedicated track wheels for my 981 Boxster S so I can save my factory 20” wheels for the road and at least see what sort of time the car can do with a decent set of tyres and alignment.

The reason I went with 18” is because I paid next to nothing for them and I also want to run proper R-Spec tyres, either Yokohama A050’s or Hankook Z221’s which I’ve had a lot of experience with on previous cars (I will put more camber on to make use of the tyres, can trailer the car to tracks further away) but I am looking at the tyre sizes to compare rolling diameters and it looks like all the track tyres will be significantly smaller and different relative diameter front & rear due to the differing profiles.

20” Stock 265/35/20 & 235/35/20
26.5” F - 27.3” R (-2.9% F/R)

18” Semi’s 265/35/18 & 235/40/18
25.4” F - 25.3” R (+0.4% F/R)

The other option is that I could get a set of a lesser semi slick like NT01’s/DZ03G’s in the same profile or get a set of 19” wheels to run Cup 2’s which are probably more suitable for a road car but I’m happy running proper semi’s on 18” wheels if I can get the right size.

Has anyone got any experience running tyres with smaller diameter than stock and different relative diameter front/rear than stock? Anything I should know or is this difference small enough to be negligible?

I don't have any specific experience with that on a boxter, but I would say it should be fine. Staggered diameters are only a problem with awd drive cars where the front and rear wheels are physically linked and therefore need to be rotating at the same speed.

For track work a smaller tire diameter than stock will probably serve you well, as it effectively gives you a higher final drive ratio.

[rego]

1 hour ago, Timm said:

I don't have any specific experience with that on a boxter, but I would say it should be fine. Staggered diameters are only a problem with awd drive cars where the front and rear wheels are physically linked and therefore need to be rotating at the same speed.

For track work a smaller tire diameter than stock will probably serve you well, as it effectively gives you a higher final drive ratio.

Thanks for that.

Makes a lot of sense it making a difference on AWD cars and the final drive benefit.

I think I might just pull the trigger on some Z221's..