Bill Verburg

The Tire companies generally provide 2 specs for their tires wrt wheel width
1) measuring rim
2) recommend range of wheel widths

the nominal size on the sidewall is affected by the wheel width, a wider wheel stretches the carcass(but not the tread) and shortens the sidewall, a narrower wheel does the opposite. The measured wheel spec is there to provide a starting point datum for the actual installed width x height of the tire. The wheel range is self explanatory. If you talk to the tire engineers they will universally recommend the widest wheel in the spec range for max track type performance, and the narrow end for max comfort. For racing use under very closely monitored conditions they will even sometimes recommend a wheel wider than the street spec, which because of the nature of our legal system has to be ultra conservative.

DaveConn

Bill,

Thanks for the additional info. Food for thought as I continue to feed my track addiction...

One of the things I love about this place is that, even after a number of years, folks like you are still around and willing to chime in. And, I never stop learning!

Regards,

Dave

Cupcar#12

the 225/255 combo is a great starting point.
The Cup's used 235/265mm but these are not obtainiable any longer i believe in 17", only in 18".

The US cup series was slated to use the RA/1 by Toyo in 235/275mm 17"

D666S

US Cup Series was intent to use 8" an 9,5" Cup I rims, if i'm right. I have one of those sets, was an option here in Europe for the Carrera RS.

David

cobalt

I thought this originally also. But after some research I found documentation showing they were intended to use the 17" 7&9" cups with a 235/275 tire. Against everything discussed but it does work although not Ideal but I guess if all the cars were required to use the same rubber and wheels it would make the playing field even.

Cupcar#12

the US series was spec'd with 8" & 9.5" wheels
No 7"s
The cars were delivered to customers after the collasp of the series w/ stock wheels so 7" &8"s but that was not the race spec

cobalt

Do you have documentation proving this? The info I have is from a very reliable source and they claim otherwise.

Cupcar#12

why yes i do.
http://carreracupusa.org/Documents/Tech%20Specs.pdf

Technical Specs: 1992 Porsche IMSA Carrera Cup Race Car

i did screw up on the tire spec<sorry) it is 235/255 using Toyo F1-SR (a non-treaded full race tire)
it also states that they are pressure case Alloy wheels (not Magnesium)

D666S

That is for sure, alluminium. I would have to check out (wheels are in my lumber room) but remember they have not very common offsets (rears are 9,5" with 68 mm. offset, can't remember fronts).
If someone needs it could go and check it.

Also I think they are very heavy and actually i would like to turn into Fikses or alloy lighter ones.

David

cobalt

Thanks this is quite interesting and typically confusing as I find most info from Porsche. I picked up some RA-1's 235/275 on 7&9's from 38D 2 years ago and I didn't feel they were safe so I did quite a bit of research after talking to someone who claims they were to be part of the tire support effort for this series. He had similar but conflicting documentation clearly stating 7&9 235/275. Needless to say based on this info I ran a season of AX with them and had no issues. In fact they worked quite well. So to my chagrin I see this document states otherwise.

This is reminiscent of other Porsche factory info regarding other factory race prepped cars of the time that inevitably proved to be changed before the final product was released. The info I had seen might have been preliminary with changes made later. I will need to follow up on this would be interesting to compare documents.

This might also answer what the 9.5" wheel was used on. There was a thread regarding the 7.5 and 9.5" x 17" width Cup I's. There were some comments stating the 9.5's were used on the America Roadster and after checking again I see the TSB does state that these were approved for this model however all other documentation clearly states the AR came with same running gear and wheels as the 3.3l turbo which is the 7&9". The 7.5's were supposedly offered on the 968 but again all the factory technik documents and spec books state the 7&9's were only offered as an option. Although a 7.5 was used on the 928 GTS but a different wheel accommodating the RDK tire pressure monitoring system.

Occasionally these wheels 7.5 ET 65 965.362.124.05 and 9.5" ET 68 965.362.128.05 do surface so they must have been used on something. BTW the info I have states the 9.5 ET 68 weighs in at 27.5 pounds or .75 pounds more than the speedline 10 x 18" wheel. The only 8" width cup I's I am aware of are the ET 52 p/n 965.362.126.00 weighing 22 pounds and the 965.362.126.01 weighing 23.3 pounds. These are essentially the same wheel manufactured by 2 different companies with different design of the inside spokes. I would be curious what weights you get if you get a chance to weigh them.

So I stand corrected and appreciate the documentation, always something new to learn and will look into why this other person has documentation stating otherwise.

D666S

Off topic again but related to track day performance.
I red that "Fikse FM5" and "Profil 5" are truly lightweights (about 16.0lbs. (7.3kgs.) for a 17"x8" width) so become an interesting option to reduce unsprung weight. Any experience?

David

cobalt

I use the Fikse Mach V in 8 and 9.5 x 18" 225/265 for the track and I have to say they are incredibly light weight. I have never weighed them but would say the tire and wheel of my 9.5 weighs less than a 9" x 17" cup I without a tire. I am assuming the 17" wheels would weigh less but I don't think by much. If you buy Fikse pay the extra and have them coated. If not they corrode quickly if left out in the rain. I think they are great for track use where the surface is smooth but would be concerned with them for street use. The barrel is still cast although the center is forged and machined and they will bend if you hit an average sized pot hole.

Cupcar#12

on the Cup I's
the 9.5" ET68 offset wheel as used on Turbo/Turbo Look was pt# 965 362 128 05 (no year is given)
the 8" ET 52 is listed "for winter use" in my copy of Pet and is 965 362 126 01 - this is listed for the Carrera 2/4, M 403 (a mistery to me right now) , & Turbo/Turbo Look

D666S

Thanks Anthony, I will try 17" first...

cobalt

Although our numbers agree. I have all the TSB's and factory technical documents. Volumes of info about 35 different red and gray 4 ring binders. I even have factory training manuals, tool descriptions, warranty info and multy colored 5 layer (press hard) documents and lots of other interesting PCNA and factory info. (BTW PCNA and factory technical documents don't jive about 25% of the time) With all this info the earlier published documents circa 1989 through 1995 does not show the turbo or turbo look as actually receiving the 9.5" wheel and nobody has ever noted having a 9.5" wheel out of all of the posters in the 964 turbo forum. There isn't even an option code for 9.5" wheels. M 403 is only listed as Pressure cast 17" wheels This was the option used on the 92 on up 964 upgrading the 16" D90's to the Cup I's in 7 and 8" widths. The technical spec books only reference these wheels also. The only odd ball mentioned is the 964RS which shows 7.5 and 9" but there is no mention of material used although cast magnesium is the known alloy. the 7.5 mag wheel has an ET of 55 along with the 9 x 17. 965.362.124.15 965.362.128.10 These should be the part numbers for those.