Staggered Tyres

[Rusty]

I was recently sizing some tyres for a friend's Porsche and got to thinking about staggered wheels on the S2.

Using 17" wheels as an example, I started to look into the diameter of the available, suitable sizes:

205/50R17 - 636.8mm
215/45R17 - 625.3mm
225/45R17 - 634.3mm
235/45R17 - 643.3mm
235/40R17 - 619.8mm
245/40R17 - 627.8mm

Ideally, I wanted less than a 1% difference in diameter, to keep the centre diff on the straight and narrow...

The two tyre sets that look the closest are:

Option 1

F -225/45R17
R -205/50R17

(636.8-634.4)634.3*100 ~ 0.4% difference

Option 2

F - 245/40R17
R - 215/45R17

(637.8-625.3)/625.3*100 ~ 0.4% difference

Option 1 is a safer bet, as it offers more clearance up front with a ET35 wheel and also uses more commonly available tyre sizes, as 225/45R17 are fitted to nearly everything and 205/50R17 are standard fit on Ford Mondeos etc. A 7J wheel could be used at the back to save some weight, with a 7.5J or 8J up front, or 7.5J all round.

Option 2 is a more aggressive setup with the possible need for spacers up front and rarer tyre sizes. Wheel sizes would need to be 7.5J or 8J all round, or even 8.5J up front, 7J rear...

7J rears would need a spacer to make it not look too narrow, perhaps something in th the 15-20mm region on a 205 and 10-15mm region for the 215.

[Error404]

255/35 18 and 235/40 18 are also doable

[Rusty]

Holy front steering arm clearance Batman!

[Error404]

Its OK, I test fitted it last week
You minimum 8.5J ET32 and might need the arches massaged slightly if you are running low ride height

[Rusty]

Looking at the numbers, 225/40R18 are a better match to 255/35R18

215/40R18 - 629.2mm
225/40R18 - 637.2mm
235/40R18 - 645.2mm
235/35R18 - 621.7mm
245/35R18 - 628.7mm
255/35R18 - 635.7mm

F - 255/35R18
R - 225/45R18

(637.2-635.&)/637.2*100 ~ 0.2% error

With

F - 255/35R18
R - 235/40R18

(645.2-635.7)/635.7*100 ~ 1.5% error

The closest matching option is

F - 245/35R18
R - 215/40R18

(629.2-628.7)/629.2*100 ~ 0.1% error

[Rusty]

To complete things, for 16" wheels

F - 225/50R16
R - 205/55R16

0.1% error

F - 235/50R16
R - 215/55R16

0.2% error

[driftin jo]

Holy front steering arm clearance Batman!

You can gain a bit of clearance on the front steering arm by grinding a flat on the
rounded end , weld it up along the ground flat as it is two pieces of sheet that forms it.
I did this a few years ago on my old 90q, worked a treat.

[Rusty]

One for the more confident among us there but a good way of sorting the main issue with wider tyres.

[Sonic]

This is perhaps a little OT but seemed like the right place to ask!

Given i have 3 heavily worn 225/40/18 R888's after the one that destroyed itself at Silverstone, would it be a problem to replace it with an also heavily used 235/40/18 R888's which i've been given by a friend?

I figure it has to be better than running around on 3x R888 and 1x normal road tyre and probably not much worse than replacing it with a new R888 given the tread depth difference between the new and old anyway.

I have a new set of 595 RSR's to fit but given the R888's do have some life left it would be good to keep them if possible, or is it a BAD idea to try and run 3x 225 and 1x 235, especially on track?

[Fat Audi 80]

Gut feel says bad idea. I can't imagine it is a good idea to mix sizes across an axle either...

[rynaqui]

Bad idea

[Error404]

+1. Flog the part worns on ebay and ft the new-uns. Mismatching an axle pair is not road legal anyway AFAIK its an MOT failure. Tyres aren't something to compromise on IMHO.

[andy10v]

Bad idea.

Different front to rear grip is fine, side to side is not (unless its Nascar)

[Henkjan]

the MTM RSR had 265/35 17 on the front, and 235/45 17 at the rear

[Sonic]

Its a shame i didn't get both 235/40 tyres then as it sounds like that would have been an option, whereas 3x 225/40 and 1x 235/40 isn't. I checked the MOT Alex and you're right across mis-matching tyres across the same axle.

I think i'll just ebay then as you say.

Now the same scenario but in reverse - my spare is a 225/40/18. I'll be fitting a full set of 235/40/18, is it going to be safe to run the 1 225/40 to get me back from Belgium or is that pushing the limits of limping home?

I'm not fussed about MOT etc, but wondered if the rolling radius was similar enough to not cause a problem with the car. No point in taking the spare if it is!