Alloy wheel - Aluminum nuts shearing

[clyde]

I have still got my old and original nuts from when I replaced them last year. I'm happy to send one or more on if required, although they are all in various stages of self destruction.

I believe that the nuts fit a great many Porsches, so this would be of interest to quite a number of owners. Any rough indication on how much testing would cost Andy? In view of the potential interest, could testing be something that the club would pay for as a service to their members? If not perhaps we could have a whip-round.

 

[pncarrerars]

Do you by any chance have shim (5mm ish) spacers fitted? These can cause nut failure due to too short thread engagement.

Pete

 

[graham harvey]

I believe this is the result of the socket that is being used. My mechanic has a socket soley for Porsche wheel nuts and both times they have sheared is when a 3rd party has used an alternative socket.

 

[Andy B Aces High]

Clyde, I have made some enquiries and cost wise looks like it would be around £70 to carry out material analysis of the two nuts, I am not at work this week but will try another company next when I am back to see if I can get this done for less. Obviously against the price of a new set of nuts this is costly but in John's case he has damaged wheels etc and for piece of mind for those out there with after market nuts it may be worth them clubbing together to cover the cost. This testing will include a formal summary showing the chemical and mechanical properties etc of the two materials.

Out of interest I called my OPC and they quote £7.50 each plus VAT for the nuts, I bought 4 nuts to replace my locking ones last year from Jasmine Porschelink, these did not come in Porsche packaging so not sure if these are genuine or not, think they were around £2 each plus VAT which seems a lot cheaper than Porsche ones and raises the question how can they be supplying them that much cheaper ?

Here's another question to throw a spanner in the works...each time a nut or bolt is torqued a certain amount of elongation of the thread material will occur, any material only has a certain limit to this before failure will occur and in allunimium this will be sooner than in steel. Since our cars are now 15 years ish old and the wheels would have been removed numurous times with the nuts being retorqued, perhaps we should be considering changing these as a matter of course. My OPC advise that Porsche do not list this as a recomendation so it is only my own thoughts and something that I have been considering for my car for a while, especially as I have had the wheels off my own car quite a few times since owning it for various maintenance operations.

 

[clyde]

It seems do-able at that price Andy, I thought it would be much dearer. I'm happy to chip in if we can get a few others to sign up.

Once prices have been firmed up we should maybe put on the general forum or perhaps specific forums for models that use these nuts.

 

[Andy B Aces High]

No problem Clyde, will get a formal price next week when back at work then see if we can post on all forums.

 

[johndburton]

Thank you for your kind offer. I am trying to get SEM analysis of material :-
1. 911 supplied nut
and
2. Original Porsche
If no success I will take you up on your kind offer.

I studied the fracture under microscope - typical of torsional failure - grain flow shows a progressive transformation to failure indicatiing a low spec material , however this by itself is not conclusive.
I've checked hardness (Brinell) there is a significant difference when comparing 911 supplied nut to Porsche nut.
Again SEM analysis will show material composition- I suspect the porsche nut to have elements of zinc and manganese.

Dimension across flats also shows a difference resulting in a smaller cross section in teh 911 supplied nut, I measured by taking a section of the 911 supplied and Porsche nuts and measuring on a shadow graph. Next is to measure thread root diameter.

I have contacted Porsche 911 and requested:-
1. Dimension and material spec used by their supplier.
2. Suggested they stop sale of anything other than Porsche spec until this matter is concluded.
3. The matter of replacing nuts / studs and damaged wheels is under discussion , however their response so far has been most helpful and positive.

I am rather busy ( work I'm afraid ) but will try to conclude ASAP.

Thank you everyone

Best regards John Burton

 

[johndburton]

There are 3 methods of tightening
1. Yield controlled tightening - Accurately measuring both torque and angle
- Monitoring the slope - y axis torque - x axis angle of turn on
- The fastening is taken to a snug torque and the torque is applied.
- When the slope relationship changes the yield point is detected and torque application is stopped
YIELD TIGHTENINGS CAN USED TYPICALLY UP TO 6 TIMES
Usually used for con rod fastening / cylinder head etc where high clamp load with high level of assurance is required

2. Torque and angle tightening - Fastening tightened to a snug torque. From this point angle is turned on.
- Used for important fastenings - Designer will have tested clamp load accounting for variations in UTS of fastener, surface finish, friction and dimension variation etc..
In my experience these fastening should not be re-used - the decision should be taken understanding effect of fastening failure.

3. Resultant Torque - EG Wheel nuts - Applied torque. general application.
- Designer will decide clamp load required in the joint and select the fastener and its material spec ( Ulimate Tensile Strength ) appropriate to application and specify applied torque with some safety factor. The resulting spec should not take the fastener into yield .
However because of variations in dimension and friction critical fastenings should be replaced.

With reference to the wheel nuts
In my view the Porsche spec of 130 NM is high and therefore I expect the fastener material will require a high spec.. The designer will have consider some reuse and therefore fixed an appropriate factor of safety to fastener dimension and material so as to avoid tightening into the yield range. The designer will have made no allowance for low grade material UTS.
I am not surprised with what I have seen to date that the 911 supplied nut failed on untightening, the untightening torque requiring to overcome the clamp load and "sticksion". I also think that this nut went into yield and permanent deformation at 130NM , however this will require more testing to validate.

Hope this helps regards John

 

[polar964]

Interesting thread! As i too have purchased a complete set (16 off not including the locking wheel nuts) from design 911 last year to replace the existing ones. They were purchased as individuals and not in a boxed set of 20, which was an option at the time. As others have stated, i too torque to 130 Nm grease on the thread and not on the chamfer which makes contact with the wheel. Again as stated lubricated threads will withstand a greater torque than dry threads. 70 Nm is a scarey low torque by the way [&:]. I have noticed that on removing these design 911 wheel nuts, that every one shows signs of having had minimal contact with the chamfered surface which interfaces with the wheel, infact less than 1mm circumference.
In the attached picture the nut on the left is the wheel nut purchased from design 911 the one on the right was what i replaced, which i assume to be an original OEM part (documented history indicates this). When i first discovered this i thought i was being over cautious! from what you folks are describing i too may have an issue!

 

[Andy B Aces High]

Keep feeding back the info John, it is good that Design 911 are openly discussing with you and hopefully you will have a satisfactory conclusion. Not sure about the big picture for others out there that have these nuts, if it was Porsche they would have a recall.

Polar964 they are interesting pic's looks like the seat angle may be slightly different ?

 

[polar964]

ORIGINAL: Andy B Aces High


Polar964 they are interesting pic's looks like the seat angle may be slightly different ?

That's right Andy .... and with 130 Nm applied. All 16 nuts identically marked. I think Design 911 may have a serious issue!

 

[clyde]

Paul

I don't suppose it could be that numerous removal and re-fits have worn down the chamfers of the original nuts and the new ones have still to go through this process? Presumably the nuts will be softer than the wheels? I know I'm probably just clutching at straws but I'm trying to remain positive.

 

[polar964]

ORIGINAL: clyde

Paul

I don't suppose it could be that numerous removal and re-fits have worn down the chamfers of the original nuts and the new ones have still to go through this process? Presumably the nuts will be softer than the wheels? I know I'm probably just clutching at straws but I'm trying to remain positive.

Clyde, I would have thought that the nuts would have the exact same chamfer as the seating face of the wheel. I'll give Design 911 a call tomorrow and see what they say.

 

[iangray100]

Interesting thread and to me it proves you pay for what you get..which is why a year ago I needed some 964 wheel nuts ( same as 993 and many other Porsche models ) I brought Porsche as I have this same conversation way back when I had an Impreza Turbo and guys were swap wheel nuts for non Subaru ones and the issues started arising .....

Being a high performance car, quality components are key

Hopefully 911 will step up to the plate here ...

 

[Andy B Aces High]

Clyde, Think if the increase contact marking was caused by the removal and refitting over time that you would see an undercutting of the surface so I believe the problem is incorrect chamfer angle on the new nuts unfortunately.

 

[polar964]

ORIGINAL: Andy B Aces High

Clyde, Think if the increase contact marking was caused by the removal and refitting over time that you would see an undercutting of the surface so I believe the problem is incorrect chamfer angle on the new nuts unfortunately.

I would agree.
I spoke with Design 911 today. Explained in detail the issue i have and that there appeared to be a problem with the chamfer angle of the wheel nuts which they had supplied. They didn't seem to convinced! so i have emailed the picture which i have posted on this thread. I await their response.
Irrespective of price, i would have expected Design 911 to have sold competitively priced wheel nuts, not inferior quality. Wheel nuts ain't rocket science!!

 

[Porker993]

I always use Porsche parts for safety critical applications. As far as I recall the OEM wheel nuts are fairly cheap anyway - why would you use anything else ?

 

[polar964]

Contacted Design911 today spoke with a guy called Graham. To cut a long story short, they are going to take the chamfer issue up with their supplier and send me a complete set of Porsche original wheels nuts! excellent customer service. Although, I think they need to consider contacting customers who have recently purchased this item from them. Together, with John's material quality issue, there's obviously a serious problem. I hope John resolves his issue and for the sake of others I hope there are not many of these wheel nuts fitted to our cars.

 

[johndburton]

Dear Polar,
I believe I have found the direct rootcause of the failure.
The indirect causes are material and dimensional spec. although below the porsche spec by themselves would not result in the failure experienced.

I found the ROOTCAUSE through a detailed inspection of the remaining nuts I removed through un-tightening.

I don't have the time now to detail my findings but will do so this weekend.

Your photo shows witness of strong clamp load and therefore sound fastening - however the witness marks location along the outer edge is not correct and supports the findings of incorrect dimensions.

The problem with the nuts on my car was very different. The tightening and un-tightening torque was taken up to overcome the friction in the thread, there was almost no witness of clamp load between wheel and nut clamping face, the nuts body however showed serious torsional deformation from the applied torque.

Hope this helps - Regards

John

 

[polar964]

Hi John,
I had only brought my issue to light as a result of reading your post, because initilly, i had thought i may well have been a bit pedantic, even to the point of ocd !! (which i probably do suffer from []). So up until your post, the offending wheel nuts were removed, put aside and the originals replaced, having not taken issue with Design911. So thanks to you, i now have a set of Porsche originals!!
Will be very interested to read your findings and very much hope that Design 911 compensate you likewise, for your investigations, time and effort in bringing this issue to their attention.