Monday, May 11, 2009

Chamfer Pedal and Crank

The pedal to crank joint is a pretty poor design.  Obviously thought up before bolt joints/preload was understood, nonetheless it is functional.  For example, a 9/16" bolt can easily be preloaded to around 5 tons of force! Which one should not do or you will deform your crank threads (or maybe even make a miniature slinky).  So the 9/16"  axle size is obviously chosen for stiffness.   Yet if not fairly snug, movement and fretting can occur and your cranks may form a crack and fail.  It doesn't help that a sharp shoulder is digging into the aluminum.

To digress, a better design would simply use the pedal axle as a "nut" and run a bolt from the back side (with a proper washer) sandwiching the crank.   a high grade 5/16 bolt would probably be adequate with around 3000 lbf of preload...

Now, I have never had a crank fail at the pedal threads.  However evidently older campagnolo cranks sometimes would fail.  I remember reading Jobst Brandt's postings (5-10 yrs ago..),  his fix for this (he had cranks that failed)  was to simply  to chamfer the interface so that the pedal force would no longer be forcing threads or a sharp shoulder into the aluminum, but rather a chamfered smooth surface.   Good idea !

My main reason for chamfer is to move the pedals in closer for reduced q-factor (bowleggedness), and thus increased bio-efficiency (ok not proven), increased aerodynamics (less "width"), increase cornering clearance, and just general comfort.  Nothing worse then pushing down and to the side when you pedal, I like to push straight down !   

This also why I am still using square taper spindles..  can mix and match for less "q-factor".  Fortunately my feet line up straight and my heels don't hit the chainstays.. 

I bought a little 45 deg dovetail cutter.  Using old pedal axles, made a bushing that is first installed into the crank, and then the dovetail cutter is put in and turned to cut a concentric chamfer.  I set it up in the lathe with a spring for smoother and more controlled cutting. No I did not turn the lathe on!, only hand turned the chuck.  
The axles were removed from the pedals and cut with compound set at 45 deg. 

2014 comment:s I still have these tools and will do this.  Not necessary, but if I can get away with a lower q-factor than why not...