My wheel fell off on a trackday

At 137 km/h coming into a long right turn, my inner rear wheel fell off. A loud clonk and I could see how the inner rear wheel was bypassing me! I slowed down slowly standing on three wheels and could keep the car on track the whole time. A guy ran out on the track to help me, and we could easily push the car out from the track by lifting at one corner while pushing.

The loose wheel had hit the rear wing rivets loose. Other than that, no damages at all! No scrape marks, nothing. I put the other set of wheels on and continued driving the rest of the day as nothing happened.

But how could this happen?

The wheels have been used three seasons. Magnesium centres are known for the need to be inspected every season - but this is aluminium and they're supposed to hold together. Then I noticed that the rear centres had larger bolt holes than the fronts!

I've emailed the manufacturer and got the following replies:

Cracks forming on the other rear

I have spoken to two other owners  with the very same design billets,  I think I only made 5-6 sets like your before changing to the later profile as  the power and weight of customer cars was  going up so I thought to beef them up a little, The two I have spoken to  have no problems and have used them now for three seasons hill climbing here in the uk. I do not have an answer for you why  there has been a failure, but I can Only assume fatigue, The later billets you have are now used on lots of similar cars to your including some saloon circuit  cars around the 850kg mark with no problems reported 
I am of course more than happy to send out replacement centres .

And later:

I see the  old wheel bolt holes are larger, this is because when I originally worked with Meteor  we were going tom use floating taper washer that needed a bigger hole, using these washer didn't work out so I changed the hole to smaller dia, the set you have were simply a set form stock that were used up.

I see the damaged inner rim. No problem for me to send a replacement under the circumstances.

After investigating the new design I'm confident they're much stronger than the one that broke, and I feel assured they'll hold. If you have similar wheels, I suggest you check them and see if you have the larger or smaller holes, and replace them if you do. If you have the new style centres I wouldn't worry.

I must say I got an immediate and professional response from the manufacturer, and they did everything they could have done in this situation. However, I don't think I should have gotten the weaker style centres in the first place.

New style centre from the inside.
 Note that this picture doesn't show that the centres are also thicker than before.

Old centre, with big bolt holes.

Rear hub nuts

Both sides loose - scary! Left side so much I could turn it with my fingers.

41 mm socket.
270 Nm (yes that is a lot!)

Handbrake cable warning!

My handbrake cable was routed so it was in contact with the fuel tank. As a result, it the cable has dug itself into the aluminium and created quite a big scar in the tank!

So everybody out and check your handbrake cables!

Cross-ply rear camber

Crossply tyres such as Avon ACB10 don't need as much negative camber as radial tyres, CR500, 888 etc.

On the CSR you adjust the rear camber by using spacers. But what happens if your bolts through the spacers are not long enough?

The wheels fall off!!! Well, almost. This is what happend to me at the "NGK"-turn at Knutstorp last spring. I'm glad I stayed on the track!


This is how the rear suspension looks like:

What happened was the threads on the two horizontal bolts ripped, which caused the two vertical bolts to rip as well.

So if you have a cross-ply set-up with a bit less negative camber, check that you have enough threads left on the horizontal bolts, and maybe replace them with something longer.