Re: Seat tube motor bolts keep breaking?!
Ok, this is some seat of the pants engineering that will be a little difficult to explain. Here goes. I am suspect of the thickness of the plate you are using too unify the motor and "U" clamp.
It is possible that as you tighten down the two nuts on the U bolt and against the plate it is warping inward as you achieve a grip on the seat tube. Although the warping in itself is tolerable, the two engine studs may well be being torqued laterally by the warped plate. Studs have strength longitudinally but they are not designed to counter shear forces.
And actually plate thickness issues and warping aside, your studs are not being used longitudinally at all regardless. The motor weight is resting in a shearing state across the studs. Lets add some constant vibration. Metal fatigue. I'm not trying to be a smart arse I just like to think (too much).
I just noticed the shear point which eliminates the warped plate theory. However the motor is still relying on the shear strength of the studs unlike a typical clamp set up where forces are distributed by well fitting blocks that can not rock by virtue their fit and the studs lengthwise strength.
Last edited by K.i.p; 12-23-2009 at 04:48 PM.
Reason: further observation