charlesj

So I have heard countless times that it is highly recommended not to run an 18" wheel on a regular 964 without the motorsports uprights. I have heard the problem it creates. What I have not heard is a god explanation of why. And does this apply to all 18" wheel/tire combinations? Would it be OK with a very light 18" wheel that maintained the same rolling diameter? What is it specifically about an 18" wheel that causes problems on the track?

Strega(UK)

Here we go again!!!

From all the things I have read the problem mainly comes from the widths of typical 18" wheels causing them to rub on the exhaust heat sheilds and the oil line in the rear r/h arch when under high load. This I believe is why it was ok to fit to late cup cars, they had a completely different exhaust.

aeroman

lol

cobalt

There are hundreds of threads on this. Yes it does work, no I wouldn't do it. the light weight wheel will crack bend etc. I use 18's on the track but for street the 17"s offer a better ride.

charlesj

The exhaust is a different issue. I am referring to the fix by using the motorsports cup uprights. I understand that with a big chunky heavy 18" wheel that there are issues. But take a wheel like the Volk Te 37...forged and light. Would there be any problem in running that? The decreased ride quality from a lower profile tire cant be the problem because nobody complains about using stiff suspensions setups which would also put more stress on the hub from impacts.

I know this has been discussed several times, but what I am looking for is a more technical answer than what has been offered.

cobalt

There is a TSB regarding this which I am sure you can find in an old thread. IIRC it has to do with more than the upright. The 94 turbo is the first Porsche approved to use 18's and I believe they made other modifications to handle the additional stress loads associated with running 18's.

I have 18's on my C2 I use on the track. I know many people that run 18's on the track like myself with no issues. However, street use is a different story and the constant pounding the car takes on poor road surfaces I think is more an issue than anything else. I find no matter how strong the wheel even my heavy Speedlines will bend and if really bad hit crack. The handling improvement gained by using an 18 on the street does not outweigh the poor ride quality of driving on city streets with 18's nor the chance of damaging the wheels or car.

If you are interested for track use, I would just go ahead and do it for street i would stay with 17's. I run 17's for street and autocross and 18's for track.

warmfuzzies

Then the search shall be your freind.

items such as cup cars, alluminium hubs, slicks and more shall be your answer.

Kevin.

donoman

I also tried searching for the answer to this. I can't search for 18" or 17" as they are under the minimum search word length. From what I gathered a lot of people are on either side of the fence but nobody has seen a failure (on this forum) due to 18" wheels. At least my feeble attempts at searching didn't show up any failures.

Sure, running bigger wheels makes you more susceptible to potholes but that's the risk we take by trying to get faster transition times and shorter sidewalls. One big thing that is keeping me on the fence is that a lot of the wheels I like are only in 18" + and there is only one type of wheel that I like (CCW Classics) that is in 17".

joey bagadonuts

I'm not sure if the TSB advised against track use but 18's are the preferred size for most 964 PCA Club Racers. Now that's not to be mistaken as a guarantee nor assurance that you won't experience problems with 18's, just an observation. Do with it what you will.

The point is, you won't find a black and white answer, here. All you can do is gather as much information as you can and then make the call.

Ritter v4.0

FWIW I have 18 hollow TT's with no issues on the street and have only recently begun to track the car.
The roads in Ga are good for the most part. I also stuck 275's on the back and again no issue so far.
I did however flatten the already somewhat flat oil line thats on the '92.

Ritter v4.0

Joey- did you retro-fit the flat oil line to your '90? Any idea of the part number by chance?
i need to do this for the new '90.

charlesj

I am actually specifically wanting to use 18s on the track. I was looking at using the 18" Enkei NT03+M. For the street a have my 17" Kinesis Cups.

I have searched on this and like the other person mentioned, this is a hard topic to search. I have actually found more information just from browsing.

I have run 18" wheels with equally low profile tires on countless cars and only ever bent a wheel once. I really think that has to do with the roads you are on and the quality/construction style of the rim hoop.

I am still looking for that very technically answer of what it is specially about wheels over 17" (when maintaining the same weight and rolling diameter) that causes undo stress to the hub and bearing that a very taught suspension would not cause....or maybe it does and just no one is talking about that. Also, is the primary problem the weight or the short sidewall?

Also, the track I generally go to is Sebing which has some bumps in 1 and 17....would this factor into running the 18s on the track?

TR6

I run 18" Kinesis super cups on my RSA with no issues. Street and track.

JasonAndreas

Look for posts by user named "Adrian" and a good thread to start with would be HERE.

There was a recent one (past two months or so) in New Hampshire that was attributed (or theorized) to 18" wheels.

If you're going to run 18" wheels at least consider installing the additional support bracket that Porsche created for the 993, it has an MSRP of $67.98USD. It might not solve the actual problem but every little bit helps! Here's the TSB





Wheels - Unapproved Installation of 18 Inch Wheels
Group
4

Number
9305

Model
All

Part Identifier
4412

March 23, 1993
Subject:
Installation of Porsche 18" Wheels Not Approved

ATTENTION:
Service Manager/Service Technician

Models Affected:
All

Concern:
Retrofitting 18" wheels on Porsche vehicles which do not have 18" wheels as original equipment is NOT ALLOWED.

General Information:
Endurance tests have shown that equipping certain Porsche vehicles with 18" wheels in an application not already approved by Porsche Cars, N.A. may result in loosened bolts and/or threaded connections on the front wheel carriers. This, in turn, may cause fractures in these components. Fractures may also occur in the front strut mounts and in the body in the area of the rear suspension as a result of the installation of 18" wheels not already approved for installation for this specific application.

Such damage can occur without warning and the vehicle may become very hard to control or steer as a result of this improper installation.

Porsche dealer personnel coming in contact with a vehicle that has been retrofitted with 18" wheels in an application not approved by Porsche, should inform the owner of the vehicle of the potential for damage to their vehicle and loss of steering control. Porsche Cars, N.A. should also be notified of any vehicles retrofitted with 18" wheels.

Millemiglia

Thanks Jason!
I run 18" both on street and track, with no appearent problems so far (knock on wood...), but this is definitely a mod I'll install this winter.