DDK - Die Deutschen Klassiker

To be fair I didn't spot them as I currently have the 2.4 on axle stands sorting the brakes ... the 964 was in for a service with david @ car-tech and care ( who I'd recommend and has looked after all my later cars ) and he spotted them whilst do the valve clearances ...

These have been on the car for about 20 years as I have the paperwork for the TUV approval when fitted. From my engineers point of view I think I'll have more confidence in longer studs and slide over spacer than replacing like for like

fetuhoe wrote:Jasmine makes them - great quality fair price

http://www.jasmine-porschalink.co.uk/pa ... 6028-p.asp

Genuine question ... would you have more confidence in the bolt ons over longer studs and slip ons ?

Interesting problem and the manner in which they have cracked is interesting.

What seems odd is that there appears to be a hoop stress present.

The crack is most likely to follow a region of maximum stress and the question is why has it propagated radially.

If the stresses were generated by cornering forces you may expect the deformation of the spacer to cause a shear stress in a different plane and the crack propagation would then be through the thickness.

If looks as if the fit of spacer may be at fault and then tightening them up has introduced an undesirable stress distribution and subsequent alternating loads have made them fatigue.

Long studs correctly tightened shouldn't be an issue but if for any reason preload does fall then the studs could shear so long studs are generally considered less 'fail safe' than hubcentric designs hence the current MSA Regs.

E12.8.2 Not to be fitted with any wheel spacer exceeding 2.5cm in thickness or of less than hub diameter. Multiple or laminated spacers and extended studs are prohibited.

Even if you use long studs the question would you still use a 'hubcentric' location.

I am not sure that an R14 ball seat will centre the wheel correctly compared to a 60degree cone and if the fit of the spacer is poor then you could see the same type of cracking.

Tricky decision - I tend to think the failure you have seen is a result of a spacer that has not been made very well rather than a fundamental problem as I have never seen a spacer of this type break in this manner but I will now check them more routinely.

thanks for the info Chris

based on the MSA regs, I've bought some of the same style , H&R bolt on 15mm spacers , which are TUV approved , but this time will regularly check them

Just thinking at 13mm there isn't much "meat" on them if you consider the forces involved. Longer studs and a spacer sounds a better solution in my mind.

A 25mm+ thick adapters look far more substantial, but then that's probably not suitable for your application.

Nige wrote:Just thinking at 13mm there isn't much "meat" on them if you consider the forces involved. Longer studs and a spacer sounds a better solution in my mind.

A 25mm+ thick adapters look far more substantial, but then that's probably not suitable for your application.

I'm sort of with you on that .. these 15mm are I think the minimum because of the tapers in the lug holes etc it is quite a small area between the lugs and the studs. Those ones that came off the car are pretty old and probabally been on & off a couple of times . it can get pretty hot near there too , especially by the cat so who knows what heat cycles theyve been through.

I'll see how these new TUV approved ones are .. if I'm not happy after a few months I might go the long stud route ( which are only 15mm longer so not that much difference )

fetuhoe wrote:


E12.8.2 Not to be fitted with any wheel spacer exceeding 2.5cm in thickness or of less than hub diameter. Multiple or laminated spacers and extended studs are prohibited.
.

Hope all is well Chris. Really interesting interpretation, I had taken differently. I read it that spacers with extended studs were prohibited, so effectively bolt on ones were ruled out. I can see, on reading again, that this may not be the intention....working in insurance we can be a literal bunch, and I do not consider an entirely new stud to be an extended stud!

Bizarrely, I read this in the rule book and acted accordingly about 7 years ago. On the 914, which uses 914 hubs, I drove out the old splined studs and replaced them with long studs to allow me to use spacers. The studs are quite long, and need a deep socket, but allow me the option of loads of spacers / wheel options. I know I am only sprinting and hillclimbing, but never once has this been questioned by a scrutineer. (I acknowledge the rule quoted is common and therefore applies to all disciplines)

Fast forward to the Boxster. Bigger wheels, heavier wheels, after 2 seasons of even my relaxed maintenance regime I was getting fed up of trying to lift the wheel, centre it, and put in the bolts. So over the winter, I replaced with 4 studs, and retained one free bolt hole for a locking wheel nut. I did my research, and used the VHT loctite at £45 to put them in. Last weekend at Anglesea, no interest at all from the scrutineers, and you could argue that my 4 to 1 approach actually draws attention to the mod, so I might rethink that....

I have to say, I am struggling to see how a thin spacer with studs attached can be stronger than a properly fitted longer stud, but then we have already proved that my eye is not in the detail!

Cheers
Wayne

I agree that banning extended studs was the initial cause of this rule but then the use of thick spacers also seems to be an issue.

This rule effectively bans a Clan Crusader as they had extended studs on the rear wheels from the factory which is a real PITA.

I don't think the thin spacer with studs is stronger than the long studs and the same thickness of spacer.

The issue is that if the wheel loosens then the longer stud will see more shear stress as the weight of the car will be applied to the stud.

Other than that potential issue I would agree that the longer stud with a thin spacer is probably better.

If you eliminate the hub centering feature as well then I would also worry that the ball seat may not centre the wheel so there could be balance issues.

If the thin spacer fitted with integral studs is used then there will be a force due to cornering which will try to pull the spacer outward and away from the hub face and this will only be constrained by the spacer and it will develop a shear stress but though the thickness on the spacer.

What I don't understand is how this has created the hoop stress that has driven the crack radially around the spacer

It would be interesting to try to carry out an FEA Analysis for different thicknesses. I have looked on line and can't find any published data.

I think that using spacers is always a difficult decision and I am not sure there is a 'right' answer.

On a competition car which is inspected regularly then it is likely problems can be spotted early.

...final solution is to buy some wheels with the correct (or deliberately incorrect!) offset, or just leave the wheels in the place the original chassis designer intended!


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I've got 21mm spacers on the back of my SC and the same design as yours James. They came from Mittelmotor about 16?years ago and are TuV gepruft, but given that yours were too I have just added this to the list of things I will be checking more often. Also posted over on impactbumpers as a warning

Ian