sapporoguy

I'm posting in case it helps anyone.
The bike: 2005 Co-Mo speedster, rear disc brake, DT Swiss Hugi hub with threading for Arai drum or disc adapter.
I'd noticed a bump-bump-bump when braking. Finally took a closer look and found this crack and another halfway around:


with disc off, cracked at bolt holes, 2 o'clock and 8:


Fortunately, Mel at Tandems East makes and sells these: DT Hugi Disc Rotor Screw On 145 (20mm wide), $44, aka the "disc donut."
Ordered by email. Mel phoned with advice: Check if there's a set screw and turn it out first, or you'll damage the threads; check threads for damage and use a thread chaser such as a same-sized nut if they are; use never-seize when putting the new donut on.

I screwed the old donut off just a bit, and it did this (a bit alarming!) all on its own:


I had no set screw, the threads were clean and I used never-seize on the shiny new donut:


I wonder why the failure. Fatigue? Material failure? Over-torqueing by prior owner (not me!)?
In any case, thanks to Mel for the quick solution.

Alcanbrad

It's hard to see much detail as the pictures are very small, but given the symmetric failure points right where the body of the adapter changes radius makes me wonder if there is a stress concentration situation going on. Is there evidence of cracking at the 4 and 10 oclock position of the second picture? Additionally, if the donut is aluminum there may be some galvanic corrosion coupled with a stress concentration that could weaken the metal.

Can you post larger pictures?

sapporoguy

Here you go. Let me know if you want still bigger or other angles. No cracking at 4 and 10. It does appear to be aluminum. Thanks.

LV2TNDM

I'm not sure how the prior owner could have "over-torqued" it. Isn't it simply tightened every time you slam on the rear brake? That thing must get INCREDIBLY tight. And it seems to me no set screw would be able to counter the torque forces created by the rotor.

The incorporation of wrench flats on that donut appear to be its Achilles heel. Dumb to reduce the thickness on something that sees so much force. That sure looks to me to be the reason it cracked in two. Shoulda forgone the wrench flats and simply utilize the rotor bolts for removal. It would require some tooling, but that seems like a much better way in the long run. Or utilize titanium or another material with higher strength.

sapporoguy

I meant: overtorqued the disc-mounting bolts. The cracks are in the thinnest point right there at the bolt holes.

LV2TNDM

Oh, I see. Possibly, but I doubt it. Either way, that design is flawed and it's too bad they made the walls so thin on either side. Bummer.

sapporoguy

OP here. Just to add: After swapping in the new disc donut, the bump-bump-bump was gone when I bedded in the sanded disc and pads.

Alcanbrad

I agree, I think it is a stress concentration at that point.

Sapporoguy, you might want to pick up a second adapter and have it in your stash of bike parts. It is fails again in the future, you might not be able to find a replacement.