Derbi Crank Support for use with HPIs

Noah K /

I'm making ongoing improvements to my Derbi with Gilardoni, and one thing I read on here was that people were blowing up cranks. Especially in the case of using an internal rotor HPI like the Italkit Selettra. Both because those ignitions will produce higher RPMs and because there is less weight on the left side of the crank to counter the heavy weight of the variator on the right.

So to hopefully address any crank killing vibrations, I made a crank stabalizer that supports the crank on the variator side. The idea is to reduce the potential for crank wobble due to the heavy variator at high RPMs and the high torques being transmitted through it. To do this I made a support plate, a vibration issolated bearing holder, and what is essentially an extended nut for the end of the crank. This nut is actually a steel rod, drilled and threaded on one end to screw on to the crank, and machined on the other end in the shape of a hex nut so it can be tightened with normal sockets.

Overall setup:

Close up of the back of the bearing block. There are rubber washers used on both sides of this to absorb vibrations.

View without the plate, just the supports and extended nut:

Top down view to show the pedal shaft extension, made from a cut pedal crank and pedal shaft welded together:

It feels like there is less vibration through the pedals. But short of doing some data recording with an accelerometer on the bike with and without this, it will be hard to know for sure how much vibration has been reduced. I'll just have to wait and see how long the crank lasts.

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