Tyre Question 17" Cups

[kasbridge]

All,
Quick tyre question.

I have a set of 17" diameter 8"/9.5" OE Cups as used in the Carrera Cup in 91 I think.

They were bought as road wheels years ago and currently have unused Mich PS N3 on them sized 225/45 and 255/40.

I now intend to put Toyo888s on them and use them as track wheels, any recommendations on sizing?

I also have 225/40 and 265/35 on 18" Speedlines with 888s and am very happy with this setup on those wheels.

Thanks,
Kevin

 

[Hacki]

If the offset is 52 mm front and 68 mm rear, these will be the original cup wheels from the 17" years.
Tyre size 225/45 and 255/40 is the right choice - these are also the sizes Porsche recommended officially for Cup cars on 17" with street legal tyres.
Note: 21 mm spacer on the rear required!

I have no experience if the general set-up (camber, sway bars, toe in, ride height, etc.) has to be changed when a car which is set up for 18" gets 17" wheels.

Hope that helps,

Hacki

 

[kasbridge]

Hacki,
Thanks for the answer, very helpful.

The 18" setup is pure factory Geo and anti-roll bars are in centre positions - will be interesting to test the 17" and see the difference.....

Cheers,
kevin

 

[Damen]

Hi Kevin

225s are a little too big on the front and then will catch the inner wing on full lock almost regardless of the offset and/or spacers you use. Better to find some 215s if you can.

As you know I use 17s on the road but prefer 18s on track (plus 18s give more tyre choice, including MPSCs).

Damen

 

[ian harvey]

Hi Kevin,

My 888s are 225/45/17 and 255/40/17. My set-up is bog standard RS and whilst things look tight I do not get any rubbing/catching. I do have 6mm spacers on the front only. This arrangement works very well for me on track but I prefer the correct 205s on the front for road use. The steering is much lighter especially at parking speeds yet there is loads of grip and feel. They always look really weedy when I remove them but they are great in use.

Back to the 888s, I was concerned when I first bought them that the car would feel over-tyred but it never has, in fact it is spot-on. Mine are now almost worn-out and I will replace them with the same again without hesitation.

 

[Hacki]

ORIGINAL: ian harvey

This arrangement works very well for me on track but I prefer the correct 205s on the front for road use.

Same experience here.

225/45 on 8"x17" 52 mm offset (no spacer) will be fine on the front. Using them for decades (!), never had a problem with rubbing or catching.

 

[kasbridge]

Great inputs, thanks to all!

 

[Steve Brookes]

ORIGINAL: Damen

Hi Kevin

225s are a little too big on the front and then will catch the inner wing on full lock almost regardless of the offset and/or spacers you use. Better to find some 215s if you can.

As you know I use 17s on the road but prefer 18s on track (plus 18s give more tyre choice, including MPSCs).

Damen

215/45 changes the rolling radius hence why you should use a 225/45 to match the 255/40 rear.

 

[Dekker]

I have 215/45 on my Antera wheels with 8J front and 9.5J rears.
Even with those they catch slightly when backing out of a sloping drive on full lock so a 225 would be slightly worse.
Is the rolling radius that important as the speedo isn't affected and it doesn't affect the AB system?

Has anyone tried the 275/35's on the rears, these are recommended for this size wheel as it gives a bigger footprint on the road?

 

[Hacki]

ORIGINAL: Dekker

I have 215/45 on my Antera wheels with 8J front and 9.5J rears.
Even with those they catch slightly when backing out of a sloping drive on full lock so a 225 would be slightly worse.
Is the rolling radius that important as the speedo isn't affected and it doesn't affect the AB system?

24 years ago I had 215 on my 964 Cab - as you say, ABS and speedo were not affected, I never had a problem. This size seems to be within a certain tolerance of the rolling radius. If the wheels catch on full lock (prob on the inside of the wheel arch?), that´ll be a problem of the standard 964 and its suspension travel and amount of lock. IIRC the wheel arches of the 964 (front) had to be modified (with a hammer that is) to give these tyres enough space or the amount of lock had to be limited. The extra space was just a little, but it was part of having the bigger wheels street legal.

I used mine for 35k km without the hammer modification and never had a problem, although I did some trackdays with that 964 Cab.

I hope, that helps a little - I gave all the paperwork away when I sold the car in 1992, so no copies possible.


Hacki

 

[Steve Brookes]

Hacki here is the Porsche TSB about the modifications you are referring to when fitting 17 inch to early cars:

http://www.pcarworkshop.com/images/7/70/4405_1991_640691geolab.pdf

Obviously no such modifications needed for an RS []

 

[Hacki]

ORIGINAL: Steve Brookes

Hacki here is the Porsche TSB about the modifications you are referring to when fitting 17 inch to early cars:

http://www.pcarworkshop.com/images/7/70/4405_1991_640691geolab.pdf

Obviously no such modifications needed for an RS []

Right! You´re a mine of information!

A wheel manufacturer like BBS had additional paperwork for wider rims or tyres and different offsets - the rear part of the inner front wheel arch was the problem then. (See above- hammer)

No mods necessary for the RS nor for the Cup. In racing the even wider slicks and extreme camber would cause contact in certain situations, the wheel arches of the race cars tell the story!

Hacki

 

[Hacki]

ORIGINAL: Dekker

Has anyone tried the 275/35's on the rears, these are recommended for this size wheel as it gives a bigger footprint on the road?

No, never. IMHO the factory standard wheel sizes with 205/50 front and 255/40 rear are perfect for road use. The wider you go, the worse it gets on the road.

Porsche recommended 255/40 also for the 9,5" rear wheels.

 

[Dekker]

Hacki I got the info' from here
http://p-car.com/technical/tires.htm

 

[Hacki]

ORIGINAL: Dekker

Hacki I got the info' from here
http://p-car.com/technical/tires.htm

I see. The factory made the Cup car street legal with the 9,5" 68mm offset wheel on 255/40 tyres. IIRC the 275/35s were used on some 965 turbos, I never saw a Cup car or RS on 275/35x17s.
For driveability the smaller tyre is prob the better option.

Additional, for the Cup car a 21mm spacer was required on the rear, not a 17mm spacer.

For the bigger footprint 18" is the way to go I think, but for me 17" works fine on the road - with 205s front and 255s rear.
I use that combination on my 993 road car and it´s perfect.

 

[Dekker]

Hacki

For the bigger footprint 18" is the way to go I think

I understand the foot print is more to do with the width of the tyre ie 275 being wider than 255. There's a good site that shows you how the proposed changes to tyres and wheels affect your car.

http://rimsntires.com/specspro.jsp

 

[h_____]

different rolling radii front and rear, will affect rake, but this can be sorted with the flat floor, but worth considering if you swap tyres and dont get the car setup again.

 

[Hacki]

ORIGINAL: Dekker

Hacki

For the bigger footprint 18" is the way to go I think

I understand the foot print is more to do with the width of the tyre ie 275 being wider than 255. There's a good site that shows you how the proposed changes to tyres and wheels affect your car.

http://rimsntires.com/specspro.jsp

The 18" tyre (265/35 IIRC) had a little bigger diameter -thanks to the site you mentioned it´s easy to calculate precisely- and was one centimeter wider. Both these facts add to the bigger footprint.

I doubt the 275/35 on the 9,5" wheel fits in the wheel arch of the small body - as said, I saw them only on turbos, i. e. widebody.


Great discussion,

Hacki

 

[kasbridge]

Hacki,
On the 9.5/18" Speedline wheel which I think is et52 the 265/35 clears the inner wheel arch and chassis leg OK but is very close to the outer wheel arch when running approx -1deg of camber.

I think that the 275 could be made to fit (just) but you would be juggling camber settings to make it work and would it would run very close to the inner wheel arch/chassis leg.....

Regards,
Kevin

 

[Melv]

FYI I'm running following on the Cup:

WETS:
225/45 17
255/40 17

SLICKS:
235/620 17
265/630 17

No rubbing issues but I'm convinced there are stops on the steering rack because it has the lock of a Land Rover!

Strangely v. slight wheel arch lip issue with Pirelli slicks, but not Dunlops -same size tyre -go figure!