This sucks. Really sucks.
I went to change the cranks on my Coker yesterday and made the very unfortunate discovery that the threads on the hub had stripped. The hub is DOA. It’s a custom widened Miyata hub by The Unicycle Factory. I’m going to be needing a new hub and a new wheel build for the Coker now.
I’m not sure what caused the threads to break off like that. I always use a torque wrench when installing the cranks. I tighten the nut up to 40 foot-pounds. The nut has never been over-torqued. I use a torque wrench in part to make sure that this type of hub failure would never happen.
Some possible factors:
I use red high strength Loctite on the threads. Could the Loctite be strong enough to break the threads when removing the nut to change cranks? It has never been difficult to remove the nut using a regular 6 inch socket wrench by hand. I have never measured the torque necessary to break the Loctite bond when removing the nut. I don’t think it’s excessive because it has always been easy enough to remove, but with no torque measurements I can’t know for sure. Maybe using blue medium strength Loctite is a better idea just to be on the safe side.
The last cranks on the hub were 170 mm Black Widow Euro cranks. The cranks have always pressed well on the hub tapers. I didn’t realize how far they pressed on the hub tapers till I looked at them while investigating the broken threads. The ends of the tapers in the cranks were right even with the shoulder of the tapers on the hub. In other words the cranks were pressed on as far as the nut would push them on. The nut was at the end of the threads on the hub. Yikes! That’s not good. No more Black Widow Euro’s on Miyata hubs for me. The Black Widow’s just aren’t a match for the tapers on the Miyata hub. This could possibly be a factor in overstressing the threads on the hub.
It is a concern that 40 foot-pounds may not leave a margin of error for stripping or breaking the threads on a Miyata hub. I’m not sure if the red Loctite was a factor or if the poor fitting Black Widow Euro cranks were a factor.
I seem to recall that there may have been a batch of Miyata hubs that was more brittle and prone to failure than normal for the Miyata hub. Can anyone confirm? Is it possible that my hub was made from that batch?
The other side of the hub also has a small chip in the threads that is going to start a bigger failure just like the threads on the other side of the hub. Both sides got the same treatment when installing and removing cranks. Both sides failed at the same time. One side stripped a big chunk of threads while the other side just has a little chip that would grow and probably fail very soon.
My decision now is to decide whether to replace the hub with another widened Miyata hub from The Unicycle Factory or go with the UDC Super-Wide Chromoly Hub. If I go with the TUF widened Miyata hub I may be able to reuse the spokes since the hub will be the same. If I go with the UDC hub I’ll definitely have to get new spokes from The Unicycle Factory and those spokes are expensive.
I have a large high resolution picture of the broken threads in my Miscellaneous Stuff gallery (last photo on the page right now). And I’ll attach a smaller resolution picture here.
Right now I’m going to go try to cheer myself up with a muni ride.