Wheel offset on S vs S4
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From: Clemson, SC
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From: Oman
When i first raised a question about the NSR some 17 or whatever years ago when folks were ***** nilly talking about this wheel and that wheel no one seemed to know or understand why Porsche did what they did. It just happened to be that I knew full well why a degree of NSR was specified and it is intended to provide a means for the steering to self correct [i.e. maintain course] if a yaw inducing event occurs. As I am aware I can think of three such contingencies viz standing water to one side, a caliper piston seizing and a problem on the dual circuit braking system.
The NSR is controlled by the steering geometry [pretty much fixed] and can be varied by fitting front wheels with different offsets. The early models were specified to have a NSR of 16mm and the later models a NSR of 10.2mm as defined by the WSM and both are achieved with an offset of 65mm. Porsche may well advise what something is but rarely if ever advise why thus to understand what they did one has to surmise what their logic may have been.
In the case of the earlier models with NSR of 16mm I concluded years ago that the Porsche engineers reckoned they needed such to cope with a failure in one circuit of the dual circuit brake system the point being that the earlier models had side by side channels so if one channel failed the induced yaw would need a NSR of this value to enable it to keep a straight line. When they introduced the later models the dual circuit system was revised to a front/rear configuration that logic suggests would have no impact on yaw control and therefore they figured that they could reduce the amount of NSR they needed to specifiy.
From time to time when wheel threads come up I often throw in the chestnut about the CS/SE/89 GT models because they conflict with what the WSM says. With a front wheel ET of 60mm they deploy a NSR of 5mm. They clearly knew what they were doing and why and figured that the benefits outweighed the debits whatever they may be, a bit more track oriented-whatever. Mr Bulvot came up with the same logical conclusons I came up with quite some years ago [well done] and if such were so successful one might have expected with the GTS an option for front wheels with ET60 but no such option was ever offered and one cannot help but wonder why.
Porsche had no shortage of other wheels with offsets like 50mm, 55mm etc yet never offered them as options so the obvious question to me was why would other owners want to fit them. Worse still, they woud come up with other after market wheels with incorrect offset, load bearing capacity of unknown quantity, no idea of what quality control they have been through and best of all you could get them for US$500 for a set of four- what could possibly go wrong?
The other recurring theme I noticed was the stock comment "I have used them for years and never had a problem"- well if the contingency Porsche designed for has never happened then the problem Porsche strove to prevent happening will never occur but as I have tried to put across on numerous occasions the time to find out that the system really does need it is not when you are going into a tree at 100mph. It never ceases to amaze me as to how folks appear to go gaga when discussing this simple concept. Maybe it is the difference between someone who has the mind set of an engineer who knows the ins and outs of the engineering development process supported by the best part of 50 years experience working on and managing such.
Sounds like it is time to go get some Carlos Fandango super wide wheels I suspect!
The NSR is controlled by the steering geometry [pretty much fixed] and can be varied by fitting front wheels with different offsets. The early models were specified to have a NSR of 16mm and the later models a NSR of 10.2mm as defined by the WSM and both are achieved with an offset of 65mm. Porsche may well advise what something is but rarely if ever advise why thus to understand what they did one has to surmise what their logic may have been.
In the case of the earlier models with NSR of 16mm I concluded years ago that the Porsche engineers reckoned they needed such to cope with a failure in one circuit of the dual circuit brake system the point being that the earlier models had side by side channels so if one channel failed the induced yaw would need a NSR of this value to enable it to keep a straight line. When they introduced the later models the dual circuit system was revised to a front/rear configuration that logic suggests would have no impact on yaw control and therefore they figured that they could reduce the amount of NSR they needed to specifiy.
From time to time when wheel threads come up I often throw in the chestnut about the CS/SE/89 GT models because they conflict with what the WSM says. With a front wheel ET of 60mm they deploy a NSR of 5mm. They clearly knew what they were doing and why and figured that the benefits outweighed the debits whatever they may be, a bit more track oriented-whatever. Mr Bulvot came up with the same logical conclusons I came up with quite some years ago [well done] and if such were so successful one might have expected with the GTS an option for front wheels with ET60 but no such option was ever offered and one cannot help but wonder why.
Porsche had no shortage of other wheels with offsets like 50mm, 55mm etc yet never offered them as options so the obvious question to me was why would other owners want to fit them. Worse still, they woud come up with other after market wheels with incorrect offset, load bearing capacity of unknown quantity, no idea of what quality control they have been through and best of all you could get them for US$500 for a set of four- what could possibly go wrong?
The other recurring theme I noticed was the stock comment "I have used them for years and never had a problem"- well if the contingency Porsche designed for has never happened then the problem Porsche strove to prevent happening will never occur but as I have tried to put across on numerous occasions the time to find out that the system really does need it is not when you are going into a tree at 100mph. It never ceases to amaze me as to how folks appear to go gaga when discussing this simple concept. Maybe it is the difference between someone who has the mind set of an engineer who knows the ins and outs of the engineering development process supported by the best part of 50 years experience working on and managing such.
Sounds like it is time to go get some Carlos Fandango super wide wheels I suspect!
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Joined: Aug 2003
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From: Le Mans, France
Just for general information, the X38 GTS's had 8X17 ET 52 front rims, vs the usual 7,5 X 17 ET 65. But the front fenders were modified to cover the wheels.
I would guess that they use rear axle 968 rims (for winter tyres), ref: 993 362 126 00, but I need to check this when possible.
I would guess that they use rear axle 968 rims (for winter tyres), ref: 993 362 126 00, but I need to check this when possible.
Last edited by 928cs; Feb 3, 2022 at 11:58 AM.
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From: Tampa, FL, USA
Just for general information, the X38 GTS's had 8X17 ET 52 front rims, vs the usual 7,5 X 17 ET 65. But the front fenders were modified to cover the wheels.
I would guess that they use rear axle 968 rims (for winter tyres), ref: 993 362 126 00, but I need to check this when possible.
I would guess that they use rear axle 968 rims (for winter tyres), ref: 993 362 126 00, but I need to check this when possible.
