18" wheels

[Tony]

I know there's been plenty of threads about wheels and tyres but has anybody explained why Porsche stated 18" wheels should not be used - and then introduced them on the 3.8 & later cup cars. Presumably something changed in the suspension spec (other than spring/damper rates) but what?

Tony

 

[clubsport]

There is a similar thing with early 993's that 18" shouldn't be used, they fitted a steering rack support bracket on late '95 models which meant they were safe to fit.

Part number or the bracket is 993 347 131 02

Maybe this is the difference. I have been in a couple of 964 RS on track with standard suspension and !8" wheels....The 18's were fine.

 

[Dave Wilkinson]

Tony,

As far as the 964 is concerned I believe the proverbial hit the fan after some Cup cars suffered axle failures after running 18" rims fitted with racing slicks during the Cup races in the early '90s.

There are many theories about the reasoning but most centre around the required use of lower profile tyres (to maintain overall diameter). The lower profile tyres have natually stronger and stiffer sidewalls whch, in turn, impart larger forces to the wheel and, hence, axle and suspension. The actual magnitude of such increased forces is somewhat debateable although I'm sure the factory would have assessed these forces. However, the official word on the subject came from Porsche North America who issued TSB 9305 in April 1993 largely, it is assumed, because of potential litigation in the USA in the event of any subsequent failures.

The reality is that in a road (or even track day) scenario, the forces experienced by the axle and suspension whilst using road legal tyres are unlikely to come anywhere near those of the racing situation on slicks and there are certainly countless cars on the road fitted with these "supposedly" illegal wheels (my own included). It is obviously the owner's decision and risk whether to fit the 18" wheels. For confirmation I have reproduced below the text of the original TSB.

Text from 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.


Regards

Dave

 

[Mark_Bennett]

Hmmm...

Now that's interesting. I would have thought road cars are MORE likely to experience harsh shocks to the suspension (potholes etc) than racing cars (on smooth tracks). I always understood the ban on 18" wheels was because the smaller sidewall was unable to cushion such impacts/shocks than the higher profile tyres, and therefore more likely to break the pick-up points etc?

 

[Dave Wilkinson]

Mark,

I think it's quite difficult to speculate on matters such as this when nobody has any factual figures to offer but my feeling was that the short sharp shock sustained when driving over a pothole is likely to be damaging anyway, regardless of whether you have 16", 17" or 18" wheels. The stresses (as opposed to shocks) while driving at racing speeds around corners on very grippy slick tyres are likely to be significantly more than you would experience with even agressive road driving on regular road tyres.

Maybe I'm wrong, but that was my take on the situation. The tone of the TSB was also seen by many to suggest that it's purpose was merely to relieve Porsche NA, and hence, I assume Porsche AG, of any culpable blame should any ill befall a car with "prohibited" wheels fitted. We all know the effects of the litigeous US law system.

Regards

Dave

 

[SimonExtreme]

While I do think that the Porsche NA warning is to cover their behinds, it shouldn't be dismissed out of hand. Larger wheels, even with the correct profile tires, put significant extra stress on suspension and brake components. The main point in changing to 18" wheels is to improve grip and handling characteristics and doing that means more load on components. Higher cornering speeds also load the chassis more. Larger wheels also reduces brake efficency unless the larger wheels have had their weight reduced to negate the extra centrifugal forces.

However, I am sure that it would take masive neglect on the maintenance side for there to be a sudden, catastrophic failure. They will, however, make a trackday car go through components quicker and add to the cost of maintenance. The same is true from running stickier tires or slicks. Slicks can also be dangerous on some vehicles as I have seen catastrophic component failure due to the higher cornering speeds and loads.

 

[lee fulford]

I run my RS with 18's.

The 17" mags, when fitted with the somewhat heavy Pilot cups are only fractionaly lighter. Furthermore the car seems to become harsher to drive on the road. These tyres I suspect have heavy/thick and stiff sidewalls adding weight and less suppleness over bumpy bits..

I'm sure that Porsche in the US build in a MASSIVE safety margin.

 

[Guest]

I am no expert but it seems that some common sense could be applied to this one. Does anyone question the use of r-compounds on 17s? These are much stickier and usually have much stiffer sidewalls than road tyres and would increase the loadings to a greater extent than 18s with road tyres. Common sense would dictate that with 17 inch r-compounds or 18s you have an increased programme of maintenance and just generally keep an eye on things. I think anyone doing track days should be doing this anyway. There is no question in my mind that track days put a much greater stress on components than road driving.

If the Porsche health warning came as a result of race experience, I think you can be comforted that the Supercup cars got subjected to stresses that will never be repeated on the road. If you watch how professional race drivers throw the cars over kerbs and then look at how rough/sharp/raised the kerbs often are, you would see that public roads rarely if ever offer the same kind of stress to the car - IMHO.

Richard

 

[Laurence Gibbs]

As I understand it the 3.8 RS has the same underpinnings as the basic 3.6RS and the 18" wheel ban was only applicable to non RS 964 models. All 964RS models should in theory be able to use the 18" wheels

 

[Mark_Bennett]

ORIGINAL: Computamedic

Mark,

I think it's quite difficult to speculate on matters such as this when nobody has any factual figures to offer but my feeling was that the short sharp shock sustained when driving over a pothole is likely to be damaging anyway, regardless of whether you have 16", 17" or 18" wheels.  The stresses (as opposed to shocks) while driving at racing speeds around corners on very grippy slick tyres are likely to be significantly more than you would experience with even agressive road driving on regular road tyres.

Maybe I'm wrong, but that was my take on the situation.  The tone of the TSB was also seen by many to suggest that it's purpose was merely to relieve Porsche NA, and hence, I assume Porsche AG, of any culpable blame should any ill befall a car with "prohibited" wheels fitted.   We all know the effects of the litigeous US law system.

Regards

Dave

I'm not disagreeing with you, I said it was interesting []

I know other TSB's regarding 18" wheels have concentrated on suspension/chassis strengthening - this one is worded differently, but still seems to indicate the chassis is not designed to accept the forces, however caused.

 

[SimonExtreme]

Before people think I am getting carried away (or should be carried away []) I would state that I don't see a saftey issue here. However, when Lee reports that his 18" wheels are only a fraction heavier, the overall weight is only part of the picture. Firstly, the important thing isn't the weight as such byt the distance the weight is from the centre - in other words moment of inertia is the key issue. The 18's will have more weight towards the rim and this, as well as the extra weight, is what has the effect of stressing components.

In comparing ride quality of the 2 sizes of wheel, you need to keep the tyre type constant. A 17" wheel with a very stiff sidewall tyre will feel far harsher than an 18" with a lower profile but less stiff sidewall combination.

An interesting aside to this is that most people assume that stiffer sidewalls leads to improved roadholding and handling. This does apply to some cars but by no means all. I haven't the experience with the RS to know what sidewall stiffness would best suit the car but I have been told by one "expert" that the RS is better suited to having some sidewall flex.I am assuming that is based on using standard suspension on a standard car and must be to do with balancing the car (changing tyre pressure has a similar effect). As for me, I will continue to use my 17" wheels with some nice S02's which seem to suit me fine.

 

[lee fulford]

My point being that there are a lot of RS guys out there who run cup sports or similar on the road and track. I doubt whether there have ever been any related issues.

ORIGINAL: Richard Bernau

If you watch how professional race drivers throw the cars over kerbs and then look at how rough/sharp/raised the kerbs often are, you would see that public roads rarely if ever offer the same kind of stress to the car - IMHO.

Oh and Richard...You obviously haven't seen my wife's driving..[]

 

[Laurence Gibbs]

Any tsb issued to PCarsNA will have no bearing (no pun) on european RS's which were not sold in USA.The 3.8 RS was fitted with 18" wheels as standard. It's suspension and chassis is virtually identical to the 3.6RS except for a dual spring set up and the obvoiusly wider wings fitted to cover the wider rubber.

 

[Melv]

FYI:

18" were standard fit on Cup Cars from 1992 due ready availabilty of 18" tyres and easier repairable Speedline spit rims (but try to get a separate piece now!!)as used by the DTM series -the spin off was faster lap times and the ability to fine tune the suspension to a greater degree than with 17" wheels.

 

[Tony]

So to summarise,
Although originally not approved by the factory - the development of different tyres and further experience in the cup series lead to the adoption of 18" rims on the 3.8. Everybody using them is happy, although there is nothing in writing from the factory that this is 'a good thing'.

Just to add some thoughts to the road/track which is tougher discussion - they're different. There's greater chance of 'shock loading' on the road - pot holes curbs, puts damper mounts and expensive rims at risk. Much higher progressive 'G' loads on track, hard on suspension, very hard on wheel bearings and stub axles.

Tony