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.

New rims with stickers on

New 13" rims from Force Racing, 7" and 9" wide with brand new Dunlop slicks - "stickers".

I bought the rims directly from Force Racing this time and they were very easy to deal with and I can really recommend them.

The rims are very lightweight - only 3792g/3528g according to my kitchen scale* and look stunning.  If you want to order yourself, refer to this post and they know what to do.

Note that you need to modify your rear calipers to fit 13" rims on a CSR!

The slicks are the same dimensions I usually run - used Radical PR6 slicks 230/570/13 and 190/535/13 - but this time in soft compound instead of medium which the Radicals run. We'll see how that goes. I bought them from HP Tyres. The rims are a bit narrow for these tyres, but I think they work ok.

I plan to practice on worn used slicks and put these on when I really want to go for it. I've already started building a simple rack for my trailer.


* - my kitchen scale is known for messing up recipes 

Trackday slicks part #4 - how did it go?

During the winter I modified the rear calipers to fit 13" rims, and this weekend was the first test how it worked out.

Well..

First impression was that it didn't. Lots of scrape marks on the inside of the rims and I was really concerned that the rims were ruined.

After a closer inspection, I believe the scrape marks are not from the calipers themselves, but from gravel trapped between the calipers and rims. They are very shallow and I don't think it's anything to be bothered about except maybe for visual appearance. Brake dust, old tire rubber for sure doesn't improve the look of things...

I might give the calipers another go with the grinder just in case.

UPDATE:


After another track day I did have another go with the grinder. After trying several different ways to measure clearance I found the best way was to tape a small drill on the inside of the rim and see where it touches the caliper. Now I'm certain I have at least 2 mm clearance everywhere.

I also took a black pen and painted the edges of the caliper, so if it touches the rim again I can see exactly where I need to grind some more.

Trackday slicks part #3 - Fitting 13" rims

The rear brakes on road CSR's are to big to fit inside 13" rims, which makes the availability of tyres a bit limited. Most suitable slicks are 13" and if your're looking for second hand, 15" slicks are like water in the dessert. (see my previous email conversation with Avon)

I recently found myself a source for second hand Radical slicks at a very good price, so I decided to do whatever it takes to make 13" rims fit my car.

I ordered these nice looking split rims from Simon at 7tips, who were very helpful.

After a bit of grinding on the caliper cooling fins the rims fitted with 1.5-2mm clearance. The most grinding was on the inside part of the fins. It was an easy job to grind the calipers. Put the car on axle stands and start grinding. Measure caliper clearance by crawling under the car and measure from the inside with a feeler gauge.

Before

After

Unfortunately the hydraulic brake hose connector just barely touched the inside rim edge, so I had to either modify the connector, or put a thin spacer plate on the wheel hub.

Brake hose connector touching. Photo from the inside

Since I was concerned the wider tyres might touch the rear fenders I decided to use a banjo fitting to the calipers instead of spacers. After lots of searching for a what I thought would be a suitable banjo adapter I ended up ordering new brake lines with pressed banjo fittings. Unfortunately there was a misunderstanding with the supplier (who also was very helpful), but to keep a long story short I bent the banjo fittings myself. No problems, and the end result was good, but this can probably be done in a more clever way. It might not be necessary to have new hoses made up if you put some more effort into this. I also had to grind a small edge to make the banjo fit the caliper.

Rims
7" x 13" et40 - 139mm backspace
9" x 13" et30 - 158mm backspace

Simon at 7tips said my requested backspace on the rears was between sizes, so the backspace I got differs some than the CSR rims from Caterham (but I haven't checked how much). Be careful not to have too little backspace or the wheels will foul the rear fenders. With about 1.5 degrees of neg camber I have just about 5 millimetres to spare with my wide Dunlop slicks.

Weight

I compared the weight between my 13" 7tips wheels and 15" Caterham.

15" Caterham rims with very worn ACB10: 11.3 kg front, 14.3 kg rear
13" split-rims with wider Dunlop slicks: 9.0 kg front, 10.6 kg rear

That is a difference of 12 kg rotating inertia mass total! Using this spreadsheet that would estimate around 70kg lower chassis weight.

New brake lines (instead of spacers):
Banjo 12mm (long version)
Hose connector straight 3/8-24 conc female
Hose length 60cm (minimum, plus connectors)
Banjo bolt M12x1mm
Washers (two per caliper)
I ordered mine from NBM Hydraulic

The standard brake lines are 3/8-24 in both ends.

Spacer plates (instead of banjo fittings)
PCD 4-108 63,3mm M12 Ford standard.

They sure look fast!

Disclaimer
Beware, this is on your own risk! I have no idea how this affects caliper strength or heat transportation. I also don't know how much the calipers flex when braking hard, and I still haven't test driven this. But from my limited knowledge I don't think this will affect strength much.

If you believe it was too much grinding, MOG Racing sells a kit which also includes modified brake hose connectors.

UPDATE: Now I have tested them on track.

See also
Trackday slicks part #1, Trackday slicks part #2 and Trackdays slicks part #4