Retrofitting wheels to your Porsche, in Relation to Porsche TSB bulletins (TechSupportBulletin)

  • What's the Story ?
  • The 18" TSB
  • The 17" TSB
  • Jon Mitchell - Tech. Advisor, TiPEC
  • Porsche 911 world comments
  • Adrian Jaye - Thames Valley Porsche Club, comments
  • Click to go back

    • What's the story?

    Well it's taken a long while to get this on line and after a lot of discussion on various groups.
    I started to get more involved after an article in "911 World" in response to a
    readers letter on aftermarket wheels

    So I decided to get some information on this subject, I've included several views,
    and my own right at the end.
    The 18" TSB

    Installation of 18 Inch Wheels Not Approved By Porsche
    Group - 4
    "Number - 9305" Model - All

    Part Identifier - 4412

    Subject: Installation of 18" Wheels Not Approved

    April 15, 1993

    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.
    The 17" TSB

    Tires/Wheels - Installing Approved 17 Inch Wheels

    • Group 4
    • Number 9303
    • Model 4 Cylinder
    • Part Identifier 4412
    • September 28, 1993
    • Subject: Installing 17" Porsche wheels on vehicles previously equipped with 16" wheels.
    • ATTENTION: Service Manager / Service Technician
    • This bulletin replaces Technical Bulletin Group 4, Number 9303, dated 9-14-93.
    • Models Affected:
    • 944 from 1987 with M3O option
    • 944S from 1987 with M3O option
    • 944 T from 1987 with M3O option.
    • 944 Turbo 1989
    • 944S2 from 1989
    Concern: Installing 17" Porsche wheels on vehicles previously equipped with 16" wheels.

      Installation Information:

      A.

    • Front and rear fender well lips must be reworked and/or checked for adequate tire clearance.

      • B.

    • Suspension travel must also be checked. Parts such as brake lines, hoses and brackets must have adequate clearance.

      • C.

    • New caster eccentrics, rear mounts and nuts must be installed in the front control arms. This change is necessary due to a change in caster specification on vehicles equipped with Porsche 17" wheels. The specification for vehicles equipped with 17" wheels is 3°15'-45

      • D.

    • Vehicles must have a rear stabilizer bar.

      • E.

    • When adjusting vehicles to new caster specification, all alignment values should be checked at that time.

      • F.

    • When summer tires are used on vehicles with 17" Porsche wheels , snow chains cannot be used. If winter tires are used, only snow chains with Porsche Part Number 911 361 924 OO may be used.

      • G.

    • Tire pressures for front and rear 17" Porsche wheels is to be set at 2.5 bar (36 PSI) at approximately 20°C (68°F).

      • H.

    • When using this rim and tire combination, a 17 mm spacer plate must be installed on the rear wheel hub. The customer must be informed that if the emergency wheel is needed, the spacer must be removed.

      I.

    • Not for Cabriolet vehicles.
    Jon Mitchell TiPEC Technical Advisor
    The US models had a much increased ride height,which causes the wishbones to opperate through a different angle range, "the suspension experiences much more roll" (due to raised centre of gravity),the larger wheels cause (due to less give in the sidewalls) slightly increased stress on hub, suspension and chassis components. "A combination that may increase the possibility of problems." Also in the USA,
    liability is a bigger issue, so if someone crashes their car into a wall and it turns out they had fitted massive "bling" carlos fandango wheels,
    they would be able to sue PCA for not telling them NOT to fit "carlos fandango" wheels.
    • Porsche-Cars-of-America issued a TSB on the subject,
    • Porsche-Cars-of-Great-Britain or Porsche-AG never issued an official TSB.
    • PCGB issued warnings in the late 80's and early 90's to main dealers that the only wheels authorised for 80's/90's models were the ones fitted to those models, this was more of a blanket rule for liability and sales reasons,it was not an official TSB.
    Both sides of the atlantic people have been fitting 17 and 18 inch wheels to 911 and 944 models for years.. And I am yet to hear of a problem. I think the main problem was that the cars were designed to sit at the ROW ride height, so when the US models were deemed to have to sit higher, PCA decided that they would make a general rule that nothing in the suspension hub or wheel assembly should be messed with, as they didnt design the car, but in the event of a problem they would be liable.

    Comments from 911 World

    I've looked into this issue as far as possible and spoken with some of our specialists in the trade. John Mitchell's advice was also very useful.

    The opinion is that the 17- and 18-inch wheels can be fitted, subject to geometry check. As the geometry is not designed for these wheels, it is felt that the 17-inch will not perform as well as the 16-inch (depending on driver preferences), and that the 18-inch is unlikely to provide desirable ride and handling. Unlike the NAS cars, the low ride height of UK models restricts any modification to geometry that might make the combination more suitable. I'm not aware of any safety issues. The larger wheels, especially 18-inch, are expected to accelerate suspension component wear.

    The reader's question concerned a UK-spec car, so I am happy with the reply that he was given. But your information obviously highlights a concern for NAS models, and this will be communicated to any US readers requesting similar information.

    Again, thanks for bringing this to our attention. I will keep you informed of any further information that comes in.

    Best regards, Ed, 911 & Porsche World

    Adrian Jaye - Thames Valley Porsche Club comments

    Well, this has always been a bit of an argument point.

    Let me put forward a couple of views.

    1. TO modify: A car's a car right? as long as you know what your doing, get some advice follow all the rules, then there's no problems in modifying your car… Driving around town in a kicking pair of 16,17 or bigger cup alloys will just make my Porsche look good, get some good advice… no one on any of the groups or forums have ever reported any problems… Make sure the insurance company are aware then "batta bing, batta boom"
    2. NOT TO modify: A car's a car right ? however I tend to play safe along with what the designer of the car knows, all car's have a margin of safety, make changes, add wear, different owners, then your margin gets smaller and smaller. Certainly my experience of modifying cars has always come to some form of failure, grief or problems. With "16" with no "serious" mod's I must admit I've had no problems.

    In "this" case, there seems to be different factors, the NA Porsches appear to have a different drive height than the ROW ones, now, this Would cause a problem with the NA ones over the ROW's ones. QUESTION: How many people knew that the ride height was different? Not me for one until now.

    I had a lengthy conversation with a "man who can" that is, a guy with practical experience of over 40 years mechanical, design, in motor engineering. Involved in tank's, Helicopters, cars and all manner of things. He sat me down and drew me a diagram (see below) There are all manner of things a designer has to stay with in, to do with height, angles etc etc, safety margins, country legislation. Which a designer is "given" when he has to make a part, or parts,

    I think by the end of the conversation, the TSB's would appear to be "Not so much" snake oil, but protecting against legal action in case of failure. All told, as long as you keep the size the same. That is when you increase the wheel size you lower the height of the tyre, so that the overall diameter is the same, there should be no problems, of course weight is another factor that needs to be taken into account.

    The adage is "buyer beware", is looks more important than overall safety?

    My favourite quote and one which we should all remember

    "Any argument about engineering problems or merits have to be done on the basis of objective and not subjective basis. Engineering is a discipline and not inspiration. 1 is 1 and not 1.5"