Alloy wheel - Aluminum nuts shearing

johndburton · Mar 13, 2009

I had inspection carried out by Tech 9 ( brilliant ) - Their findings and following report was very detailed and comprehensive.

One item was to replace the Al wheel nuts as they showed signs of corrosion and aging ( my car is 993 varioram - 1996 - 91,000 miles well cared for)

I got the wheel refurbished, I then replaced then nuts with new (from 911 design) tightened to 70nm torque - spec say 130nm.

I had to remove the wheels for a recent suspension rebuild. I was very careful to get full hex contact on the nuts and used a new 19mm full depth ( hex and not bi-hex) socket with a protection outer sleeve that fitted snugly inside the counter-bore of the wheel.

I sheared 2 nuts on 2 of the 4 wheels, I had to drill / end mill out what was remaining to the nuts to remove the wheels - A NIGHTMARE of a job.

The nuts sheared at the base of the hex where the section changes to boss part that contacts the wheel.

I'm thinking the replacement nuts material quality was NG ( the Al material spec used to make the replacement nuts was inadequate to meet the shear load ). The remaining nuts inspection showed how the aluminum had deformed / distorted during un-tightening.

Before I take this up with 911 design can you help - Has anyone had a similar experience??

As a general communication be very careful I can't describe what a Ba...st..d of a job it was, I damaged the studs (replacement required ) and damaged the wheels, may be beyond repair.

Thanks Best regards John

Black80XSA · Mar 13, 2009

Why did you tighten to 70nm ??

was this the recommended setting for the replacements nuts ? I could imagine under high loads undertightened nuts could lead to some premature failure ?

barrysmith · Mar 13, 2009

Hi John, I spent 6 hours with cobolt drills doing the same on the locking wheel nut on a new audi only last week. the nuts are HT steel and the key is Ally, so I know what a job it can be,

I have owned 4 993 and never had a problem, other than the tread stripping,

I would ask you to speak to Brian Goff of Jasmine porschelink, Brian as sold genuine nuts for over 10years

01282 697171

Shapesman · Mar 13, 2009

Hi John

Thankfully I have not had this experience with my 993 but would tentatively suggest that using aluminium nuts with steel bolts may have caused this problem.

Dissimilar materials in contact in the presence of an electrolyte are often subject to the phenomenon of galvanic corrosion at the interface. In this case you have aluminium nuts in contact with steel studs in the presence of water (possibly containing dissolved salts). The answer is to use steel nuts.
I guess you could separate the two materials with a copper based release agent but I don't think I would risk that on wheel nuts.

barrysmith · Mar 13, 2009

ORIGINAL: Shapesman

Hi John

Thankfully I have not had this experience with my 993 but would tentatively suggest that using aluminium nuts with steel bolts may have caused this problem.

Dissimilar materials in contact in the presence of an electrolyte are often subject to the phenomenon of galvanic corrosion at the interface. In this case you have aluminium nuts in contact with steel studs in the presence of water (possibly containing dissolved salts). The answer is to use steel nuts.
I guess you could separate the two materials with a copper based release agent but I don't think I would risk that on wheel nuts.

Sorry to say but Porsche nuts have allways been aluminium on the 993

Shapesman · Mar 13, 2009

In that case the nuts are probably anodised. I wonder what the spec was for the replacements.
Interesting that the problem seems to be quite common.

IndischRot · Mar 14, 2009

They do sound like a sub-standard part; a quiet word with the supplier is in order.

clyde · Mar 14, 2009

Could you keep us up-dated on this one John? I got a set of nuts last year from Design 911 and wouldn't want to end up with one sheering. I didn't pay a huge amount of attention to them a couple of weeks ago when I put the summer wheels back on but I'm sure I would have noticed if they had been distorted in any way. I looked at the Design 911 site earlier and notice that they now give a choice of type, they didn't when I ordered mine and now I'm a bit concerned. I might order a new set from Jasmine for piece of mind.

I don't suppose you've been into a tyre shop since you put the new nuts on? I've always been concerned with the air wrench approach to removing and fitting wheels most of these places adopt. An ill-fitting socket on an air wrench could have damaged them but it wouldn't be noticable until you tried to remove them.

johndburton · Mar 14, 2009

Thank you for your reply.

What you say is correct, however on the nuts I did remove there was no evidence of fretting that would have been evident if there was insufficient clamp load.

The reason I tightened to 70nm was to minimise the force in the fastening, I was worried about the aluminum material from which the nuts are made having a relatively low resistance to tortional stress / resistance to shear.

The technical reason is as follows:-

90% of applied torque is required to overcome friction and 90% of friction is under the bolt head, the remaining applied torque is transferred to clamp load. Clamp load is the function of the fastening

Friction is effected by lubrication, I applied copper grease under the bolt head. The grease reduced the friction so by applying a reduced torque the same clamp load can be achieved.
I did not calculate torque / lubrication (friction) / clamp load relationship, but from experience I estimated the design intent clamp load could be achieved by applying between 70 to 100 nm torque. I tightened to the lower limit because I did want to risk shearing the nuts - WHAT WENT WRONG ??

The only way to be sure is carry out comparative material test to determine tortional strength. I'll keep you posted

johndburton · Mar 14, 2009

I have just checked 911 web site.

What you say is true - They offer 3 types
1. Budget
2. OEM
3. Porsche spec ( cost- nearly 3 times > budget 2 times > OEM)

I'll check, but I cannot remember when I ordered if there was an option?.

I will be speaking to 911 design on this matter - I'll keep you posted.

I will be replacing all the nuts, they will be Porsche spec.. I would strongly recommend you do the same when replacing wheel nuts.

Regards and Thank you everyone for your support and valuable help.

John

colin appleton · Mar 14, 2009

I always use Copperease on wheelstuds and bolts for all our cars(BMW no longer sell their special aluminium grease) and have never used a torque wrench but use a standard 4-arm wheel wrench-never had any problems.When I buy new tyres or return for puncture ,repair ,etc,I always insist that the nuts/bolts are cracked with a manual wrench before using the airtool & to be honest,unless it's a new man,they have always done this without asking on any of my alloy wheel/performance cars .

graham harvey · Mar 15, 2009

I have a the same thing happen 3 times now. Twice when having tyres changed. Each time the nuts were from Design 911 - i now get them from Porsche Shop.

fhauser · Mar 15, 2009

To me it would seem a material problem,or pos a differance in machining quality,Can you compare a sheared one to an original porsche nut,is there a differance in look at the sheared end.ie I would exspect a small radius for strength if not it would be weak at this point

Shapesman · Mar 15, 2009

John

Do you have a photograph of the sheared nuts so we can be sure of the failure mode?

38F3E1 · Mar 15, 2009

Personally I would never put anything other than OEM nuts on my cars. As in OEM in an OEM bag with proper Porsche number from the OPC. It's not worth the risk IMO.

Ian.

Guest · Mar 16, 2009

This sounds like faulty wheel nuts to me.

I'd agree with Maurice. Dry seems to be factory recommendation these days by many manufacturers.

I've always used Coppaslip though for my mere road cars of various "species" & torqued to factory setting. Never, ever, had a loose nut. Think I'd check my wheel nuts very very regularly if the car did track work though!

fhauser · Mar 16, 2009

If Porsche say 130 Nm then 130 Nm it is

b7gud911 · Mar 16, 2009

I have always done my nuts up to FT spec and never had a problem!!

vitesse · Mar 16, 2009

They might employ a few engineers,but that has not prevented several 996 engines blowing up because of seemingly,a design fault associated with a bearing selection and cylinder casting problems.
Frankly,I would rather have a barrier film between dissmilar metals such as Al nuts on steel studs,similarly,steel spark plugs in Al threaded cylinder heads rather than the potential metal pickup caused by stick/slip with a dry interface.

There are more important aspects with wheels such as-clean wheel/hub mating surfaces,clean wheel nut cones & seats,ensuring concentricity & even seating with progressive nut tightening,before applying the final torque,providing of course,that the nut alloy is to the correct spec & strong enough.

Andy B Aces High · Mar 17, 2009

This is a very concerning post, if some of the independent suppliers are making wheel nuts from a sub standard material the implications of this could be huge. Since the wheel nuts are a safety critical component then failure on the road could lead to a serious accident.

I work in an industry where "Reverse Engineering" (taking an existing component and manufacturing from it) is carried out. But the Reverse Engineering process has to take in other considerations than just the dimensional features of the component, the material and surface finishing process are also critical. Both of these can be determined firstly by carrying out chemical analysis of the original material, by determining the chemical composition you can determine from standard ranges what material has been used (sometimes this can be a special blend for the manufacture). The surface finish is a little more difficult, in this case the nuts are obviously anodised to give the black finish and is most likely a hard anodise (this can be determined by surface hardness testing) but this may not be on the threads as these can be masked during anodising.

Obviously these tests need to be carried out in a lab and usually the components become sacrificial as they need to take samples of the materials.

John, I will find out some costs for testing and if you wish I can arrange for testing to be carried out on the failed nuts, if anyone has an old Porsche nut we could sacrifice then we could use this as a comparator. Might even be able to pull in a favour cost wise, this information may be very useful if you are going to make a claim for damage to your wheels etc.

This is only wheel nuts we are considering here, there are many other non Porsche manufactured items on the market that may be ill conceived, for my money I will stick with Porsche parts on any safety critical components.

Sorry, long post again from me but like to put all the information over.

Andy B