Favourite wheel nuts

[911hillclimber]

Yep, 65 on the Lola, but the draw bar thread is only 3/4 unf, designed by Williams for their F2 cars.
Remember hill climbers only do it for about 40 seconds!

[fetuhoe]

Torque measurement is only a fairly crude method of arriving at the preload in the stud.

If you don't lubricate the thread or the faying surface of the nut correctly you can end up with too little preload and the nut could come loose or the wheel could shuffle on the hub and start to fret.

I have never believed in either under or over tightening fasteners and would always use factory recommended values.

Porsche always recommended using a moly based grease on the domed end of the wheel nut and recently we have been using a Ceramic based product which is a lighter colour.

There are some issues with using the T4 Steel Wheel Nut which has quite a short dome and unless some care is taken the socket can bottom into the wheel and cause damage.

The Titanium Nuts are interesting but the only reason for Black Carbide coating is for appearance. Grade 5 Ti is extremely corrosion resistant in its natural state so adding a coating doesn't really add much and the carbide layer is extremely brittle and can crack if not handled carefully but I agree that they do look good.

We have been making Ti Wheel nuts for several years now but normally in a natural finish but we could produce them in black. The dome on these nuts is deep enough to prevent to socket from touching the wheel.

[911hillclimber]

At least I do something right!
I use moly grease on the 911 and the Lola!,

[anglophone1]

Chris
How much for a set of TI nuts.
C

[gridgway]

So is the Porsche 94-96 lbft spec for lubricated thread and tapered surface?
Graham

[911hillclimber]

I'm not qualified to say, but I would think yes.
The grease on those surfaces will give the best chance to get those torque values accurately and repeatably. I imagine in 1973 the nuts were hand assembled and torqued by hand, but far from that in 2003.

I can't imagine the factory would have greased surfaces on the nuts when assembling the cars, far too messy a process.
I put the light smear of the correct grease as simply good practice.

As an aside, I think engines are built with all fasteners dry, and there are a few critical torque levels involved in that process albeit of a smaller thread size, but perhaps I'm confusing the issue.

When I've worked on everything mechanical the male threads tend to have been cleaned with WD40 and rag-wiped dry, fresh bolts are left as is (dry as a bone) and all new nuts the same esp nyloc type.

In all this simply use the factory specs whatever you are doing if not sure.

There are 100s of engineers all over the globe who do this reliability testing to ensure a device stays together from a wheel on a car to a food blender, I used to be one of the car types a long time ago!

[fetuhoe]

Tightening nuts is a subject which always creates much debate and disagreement resulting in confusion and the potential for damage to occur when poor practices are used.

In the engineering world the majority of fasteners are used to clamp the faces of a joint and prevent any relative movement by reacting the forces being developed during use.

Measuring torque accurately and repeatably is only one aspect of the problem as high levels of friction tend to generate low levels of preload which can impact on joint integrity as the preload developed may not be adequate for the application.

In the case of cylinder heads the external force may be axial and in the case of a wheel the force may be torsional.

In every case the joint relies on the preload or tensile stress developed by the bolt during tightening to clamp the faces of the joint together

The ideal method for measuring preload is to measure the stretch developed during tightening but this can be difficult to achieve with a stud although modern ultrasonic 'stretch' gauges are becoming increasingly common and can deal with this problem.

The least accurate and repeatable method for determining the tensile stress developed in a bolt is to measure the applied torque and this has lead to various approaches being used to improve the situation including the experimental determination of ‘nut factors’ http://www.hexagon.de/rs/k.htm



If we look at the typical losses that occur when we tighten bolts it should be clear as to why ‘torque’ tightening is a relatively poor method and can result in problems. A variation in preload of +/- 30% is quite commonly reported when using a simple torque measurement and I would tend to believe that this is a conservative estimate as it generally doesn’t consider the ‘operator’.

The figures above also assume clean and undamaged threads so if we take this type of problem into account the variability in preload just keeps on increasing.



The impact of lubrication not only tends to increase the preload for a given torque but it does reduce the scatter in preload as it tends to produce more consistent coefficients of friction.

So where does that take us?

I would say that the Workshop Manual suggestion is that only the dome on the nut needs to be coated with Moly grease as the frictional loss produced by the dome is likely to be greater than the 50% of a standard nut face and could result in more variability if these faces are dry.

Aluminium does also have a tendency to gall more than steel

I have to say that I would tend to use a light oil on the stud threads as they are easily strong enough to deal the small increase in preload that could be developed compared to a dry thread and would tend to reduce variability.

There are other issues which could be discussed and these would be the finish on the bolts and these will vary depending on whether or not bolts are phosphate or zinc plated.

[911hillclimber]

Excellent as ever!
Said engineers would also try to arrive at a theoretical and by test, same or different value and then add a factor of safety which might account for poorly applied torque via a wrench with extension which in the wheel nut case is a real world condition.

How does all this stack up when the local tyre fitter puts the rims back on with a rattle gun?
My very local '2 men and a cat' tyre shop put coppa slip on the hubs of alloy wheel cars, nothing on the nuts and zap the nuts tight in a random order.
Job done.
Asked why the care in using coppa slip, the response is it makes their job easier next time they see the car.
They do ask that the nuts or bolts are checked correctly using a torque wrench after a few miles.
Doubt many understand that.
They are very very good at balancing, so I've used them for years for all tyre fitting and the rattle gun torque is around 70 to 80 lbft to undo.

Complex subject full of variables.