Drewster67

I was looking at the Technical Information and Service Recalls for our beloved Sharks, at All Data, and included is a topic of "unapproved 18" wheels"

49305 APR 93 Wheels - Retrofitting Unapproved 18 Inch Wheels
49305 MAR 93 Wheels - Unapproved Installation of 18 Inch Wheels

Ia this to say, 18" can be mounted but we (Porsche) do not approve of the installation.

Anyone like to add to this .............

Jerry 87 928S4

My understanding is that many have put 18 inch wheels on and they work just fine. I have heard that they are more suseptible to damage just due to the size. Attached is the tech bulletin you refer to so you can see what Porsche said. ( I dont believe alldata gives you the actual bulletin)

Drewster67

Thanks Jerry, it's as plain as Black and White.

For those of you who do have 18" wheels - can you reply with any good dirt on your experiences from having 18" wheels.

Thanks again Jerry for the actual bulletin.

Chris Lockhart

Drew, I'm on my second set of 18" wheels. FIrst set succumbed to a large chunk of asphault that had been thrown out of a pothole. I swerved to miss the pothole, and hit the chunk with the right front and then the right rear wheels. They were toast, and could not be straightened due to the fact that the hub faces were warped. I had them on the car for about a year and a half before this happened. They were Cup 2 replicas, so they weren't a real strong wheel. My current wheels are AT Rivas, and they are a sturdier wheel, and correspondingly heavier. Had these for a little over a year, and they have handled everything the roads have thrown their way. I really like them, and wouldn't go back to 16's for anything. They look and handle great. Just my 2 cents worth.
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tdelarm

Drewster67

This topic was debated on the Socal 928 board awhile back and varies in opinion. Below is the link and an excerpt from the thread I posted to. I run 18” wheels and have had no problems. At first, I considered 18” as an unwise investment but further research concluded Porsche’s Tech bulletin to be nothing more then a legal statement releasing them from any liability or damage claims by those running 18” on the 928 due to Porsche non-commitment to further research a model heading out of production. Why would Porsche waste any further research money on a soon to be killed model when typing a disclaimer tech cost nothing more then the paper typed on. Have you noticed how many 911 series models now run with 18” and larger wheels?

I don’t believe 18” wheels on a 928 are any more acceptable to bending then on any other Porsche model or for that matter any other car running 18” or larger period! It just would not make any difference or sense for that matter.

Has anyone on this board bent an 18” wheel on there 928? Or for that matter any other Porsche model you may own??? If so, I would be interested in hearing how big the hole was!

Chris, thank God that chunk of road didn't flip up and get the side of the car with damage.


<a href="http://groups.msn.com/socal928/sharktales.msnw?action=get_message&mview=0&ID_Message=2098&LastModified= 4675377784338377962&all_topics=1" target="_blank">18" Wheels Tech bulletin from Porsche</a>

At first I thought the bulletin had to do with the fact that Porsche didn’t have the chance to endurance test the 18’ wheel on the 928 while in production but the first bulletin replacing the previous months bulletin #4-9305 (yes, they posted in April) was posted March 93’…during 928 production and, we know they had the chance but possible passed knowing the 928 end was in sight and no further upgrades were planned therefore cost prohibitive. The bulletin states they did do some endurance testing of 18’s but omit which model they tested…“Retrofitting 18’ wheels on Porsche vehicles which do not have 18’ wheels as original equipment is NOT ALLOWED”…They continue by stating “may” several times and never actually confirm damage if any caused by running 18’s...“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 wheels carriers”... vague…maybe. Also, keep in mind the 911/964/993/996 series all run with the option of 18’s without problems but then again, that body style has been around since dirt so that’s of interest to Porsche to equip as such.
As for 18’s bending, -difference’s I can see would be the additional weight impact placed upon each 928 wheel. The 928 weighs 778 pounds more then Porsche’s lightest model currently on sale, which equates to an extra 194 Lbs. per 928 wheel…you would think this is not an issue. So that being said, why would an 18’ wheel bend anymore on a shark then other Porsche models? Is the 928 suspension any less forgiving in shock absorption then other models? Again, you would think not. However, it is recommended via Devek that street rim thickness be between 4-5mm while racing rims can be 3mm.
Concerning structural stress and performance. Maybe the shark places negative structural and geometry loads upon the wheel’s footpath creating adverse stress to suspension components…or creating a negative resonance’s causing cracks within the suspension system causing catastrophic failure, as can happen to helicopter rotor blades…maybe…I know there has been talk per Devek about pinning the weissach axels to help reduce toe effect when running 275mm or 10.826772 inch larger wheels…in the hopes of controlling & decreasing rear tire scrub/toe effect by 50% but only to help performance, not to enhance structural safety.
At this point in my best educated guess, until shark owner’s start coming out with catastrophic failure reports running 18”, I’d say this is clearly a legality issue for Porsche and therefore they feel the need to go on record with the bulletin.

Tim Delarm
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P.S. I love my 18” and have no future plans of releasing them from shark duty!!!

Drewster67

So, the moral of this story is.

18" wheels can and will fit but, since Porsche did not test nor designed the 928 for 18" wheels, the tech Bulletin is just to cover their ars for insurance and liability reasons.

Jerry 87 928S4

Drew, I dont completely agree.

Porsche is saying the 928 was not designed with 18" inch wheels in mind. Remember, early 928s including mine came with 16" and Porsche has no problem with the the change to 17".

18inch wheels will work as evidenced by those that have them, it is just there is some risk. I had the chance to go to 18" wheels and chose not to and I am very happy with my 17".

Brent 89 - GT

Will someone please explain to me what the difference is? Why would an 18" wheel transmit any more force to the cars suspension components than a 16 or 17 inch wheels? When going to a plus 1 or plus 2 inch rim size the sidewall hieght of the corresponding tires drops and the overall diameter remains essentially the same doesn't it? If the overall diameter of the tire/wheel combo remains the same how exactly do the forces change? I could see a point if the diameter changed and the leverage against the suspension components was changed.

For instance. The rear tires on a 928S4 are 245/50-16, that is a ~25.65" tall tire. If you go to 265/35-18 in the rear, the tire is ~25.3" tall. So actually the 18" wheel/tire in this size has less leverage on the suspension.

I guess none of the postings so far have provided a scientific type of reason to not run the 18's. I would have to conclude that it was a liability evasion technique.

DougM

10 years ago, 18" wheels were far from the norm and probably not many available with similar offsets. At the time, they probably had a similar bulletins for 911's for which they are now a standard. Personally, I have had no problems with my 18" cups. To me, the choice was strictly for visual appeal between the 17" and 18".
Doug M
'89 GT (for sale)

Drewster67

It seems to me that this topic carries many variables at which needs research and calculation.

If the 928 was not designed for 18" wheels, why are there 18" wheels made for the 928?.

Does the 18" throw calibration off on the Tach,Speedo or Odo?.

How do the 18" handle at ....say 150 mph.

I just came in from a 145 mph blast - NO B.S.. and my heart is still beating fast. Man I love it!.

The Tech Bulleting says . Loosened Bolts or Threaded Carriers for non approved 18" wheels may occurr. That thought alone will scare me from dropping the hammer if I owned 18".

I like the look but is being unsafe and hazardous worth being "Hollywood"?. (No offense to those of you who do have 18" - maybe I'm just jealous in a way)

So I guess the two real questions are

1. Is Porsche releasing liability with the Tech Bulletin.
2. Why would the bolts or Carriers loosen more so than their 17" counterparts

The debate goes on ............. Please feel free to add your two cents. Or 4

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chris928

I'm not sure what the problem is, especially considering that I just saw some 20's on a jag.

The loads imparted to the suspension are mostly due to two things the TIRE diameter and the wheel OFFSET.

If you use 18 inch wheels with exactly the same tire diameter and offset, your components such as brakes, hubs, control arms, struts, etc will not know the difference between the two! The one caveat is that the larger rims will have thinner tires and the thinner tires will put a little more impulse loads into your car, all things being equal. But Porsche does not warn against low profile tires on smaller wheels and the shock loads are greater on smaller wheels than larger wheels.

My vote is that porsche is just covering their ***.

Make sure all your bits are up to snuff (ball joints, tie rods, bushings, etc) and buy only quality wheels and you should be fine.

Chris Lockhart

The fastest I've run is 145 with my 18" wheels, and I've had no problems with handling. The car is smoother at that speed than at 70. It seems the faster you go, the faster it wants to go. No problems in 2 1/2 years with anything loosening up, or falling off. BTW, I drive my GT more than most, racking up 40K miles in less than 3 years. It is a true daily driver, as it's MY only mode of transportation. She never fails to thrill me. LOL.
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