Broken brake disc

[lbcnut]

Having just fitted a new set of Minilites to my '69 S4, I took her for a check drive. I'd gone about ? mile when I got a horrendous noise from the left front. Returning (slowly and gingerly) home, I had great difficulty removing the wheel; the center nut was tighter than I had any way to make it. I finally was able to loosen it by jumping on the end of the three-foot lug wrench handle...since I weigh 220 pounds, that translates to 660-plus foot-pounds. When I removed the wheel I found three of the five drive pegs missing; the other two fell out of the wheel rim. The brake disc was broken, the inner part that bolts to the hub having separated from the braking surface.

Fortunately the hub and the new wheel are undamaged and I have ordered a new rotor and a set of drive pegs, so it could have been worse!

Might I have caused this by failing to engage the drive pegs into the holes in the back of the wheel, and in tightening it pushed the pegs out of the hub? Could one or more of the pegs have jammed between the brake caliper and the rotor, causing the rotor failure? This sounds unlikely, especially since I can find no mark on the rotor that would confirm it, but I can think of no other explanation for this occurrence.

11/26/18 update: I've pretty much decided that my error caused the broken rotor. Note two attached pictures; one of the two drive pegs, showing distortion, and the other of the rotor, showing the nature of the break. Also, further inspection shows a fresh scar on the caliper. One additional observation: in a related thread, a member posted that the pegs are not "drive pegs" because they have no "drive function". The pegs are there to deny any relative movement between the wheel and the hub. In my case, loss of that function led to the self-tightening of the center nut to the point that I was almost unable to remove it.

[Bud English]

The brake disk could have simply broken. I've found two cracked (but unbroken) disks, both front, in the forty years I've had my car.

[nigelrbfurness]

I've come across a number of cracked brake disks over the years, almost always it has been caused by the owner attempting to force the brake pads apart with a screw driver or pry bar. Often the discs are very worn, suggesting a general lack of maintenance and regular inspection.

[joe7]

There 2 different types of drive pegs. Make sure you get the correct ones. Check out RD listing regarding these.

[Quart Meg Miles]

On my S130 I once bolted a wheel on carelssly and it simply pushed the pegs out of the hub.and they fell on the ground while I was doing it.

[MarkDa]

I had a brake disc shear off like this once many years ago when I was reversing in a car park.
It had rusted through at the root and was testament to my being too cheap to replace worn discs.
Since then I've been a bit more circumspect about chipping off the rust build up one gets outside the friction area.

[cobraboy]

Looking at the photo of the brake disc there appears to be a darker area on the cracked surface at around 10 o' clock on the photo.
If you examine the cracked surface carefully you may find that there are areas that have started to rust indicating the crack was present before you fitted the Minilites.