Altering crank arm

[Old timer]

Can I grind a small amount from the inside of the left hand pedal arm (left hand when sitting on the bike. The one without the chain wheels)

Mine is an ali Shimano Exage, around the square drive hole is a raised section (obviously to give a more wider area on the square drive) I need to shave around 1/8" and basically remove the raised section. This will give me more clearance for the pedal assist rotory magnet that I need.

You are supposed to fit the assembly on the chain wheel side( where looking at mine the bottom bracket spindle has a touch more length before the sq part of the spindle starts and would give the clearance needed) but as pointed out in an earlier thread it would mean removal of my small chain ring and also some grinding of the lugs that hold it meaning no going back.

I presume that the tapered sq in the crank arm is a tight fit along it`s length and removing the 1/8" wouldn`t just allow the arm to go on further? and I know that if the manufacturers made the arm that thickness then it was to allow a decent amount of contact area but wonder just how over kill that might be for an old boy like me that certainly can`t punch along like you fit youngsters and remembering that this will be ridden with assistance from the electric hub.

Your views would be much appreciated.

Dave

 

[fossyant]

If you have clearance on both sides of the Bottom bracket, I'd go for a narrower BB axel to bring in both cranks.

I wouldn't shave bits of the square taper....

 

[Old timer]

If you have clearance on both sides of the Bottom bracket, I'd go for a narrower BB axel to bring in both cranks.

I wouldn't shave bits of the square taper....

I probably didn`t explain it correctly. what I need is a slightly larger gap between where the sq section of the spindle ends and the bearing cap(not a lot) 1/8" will do it. So it`s just shave 1/8" from the inside of the left hand crank arm which has a raised section anyway. I wouldn`t ever touch the taper on the spindle.

Thanks fossyant

 

[fossyant]

Ah - just a bit off the crank arm....should be OK !

 

[gwhite]

Unless you are Reg Harris I can't see this doing much harm. The old crank arms were of much smaller diameter.

 

[BigEvo]

Measure the surface area of the full contact area (all 4 surfaces), then work out the surface area you are removing as a percentage of the entire area. If its less then 10% of the surface area then you should be OK. My experience of design engineers is that they will normally over engineer structural conponents by at least 30%.

 

[Old timer]

Thanks lads

Another idea(probably not so good) I have worked most of my life repairing hydraulic door closing mechs, up till recently when modern aluminium doors have seen the units fitted in the headers they were always in the floor, the bottom of the door had a metal or brass fitting with a tapered sq hole to match the sq drive spindle sticking up from the unit in the floor. Till recently when the Germans introduced height adjustment into the foundation boxes the only way we had to raise a door to the correct height was to add shims to the tapered pivot which lifted the doors by the amount of the thickness of the shim( we are talking some very heavy doors at times) and seeing as the mechs in the floor had strong springs to return the doors to the closed position which needed a fair torque to open the door (rather like turning the crank on a bike) I just wonder if some of my old shimming skils might just do the job. You don`t need much thickness of shim to make a big different. Our favourite material for using as shims was the steel packing case strapping material(which is now all plastic)



Anymore input?

 

[BigEvo]

Not sure about the shimming option, sounds complicated and if the crank arm does not seat correctly on all 4 faces, then that would be worse than removing the 1/8" inch form the tapered face.

 

[Amanda P]

I have successfully shimmed a square taper crank with aluminium cut from a coke can. It got us out of a hole on tour when the only crank we could find to buy to replace a broken one fouled on the chainstay.

For a permanent fix, I would have thought removing some metal from the inside of the crank would be a better solution, though.

Unless it's a steel crank, don't grind it - file it. A grinder will make a lot of mess, as it melts the alloy as much as grinding it off.

Put the crank in a firm vice and firm, two-handed filing will do it nicely. You're also less likely to remove more than you intended that way.

 

[Old timer]

I have successfully shimmed a square taper crank with aluminium cut from a coke can. It got us out of a hole on tour when the only crank we could find to buy to replace a broken one fouled on the chainstay.

For a permanent fix, I would have thought removing some metal from the inside of the crank would be a better solution, though.

Unless it's a steel crank, don't grind it - file it. A grinder will make a lot of mess, as it melts the alloy as much as grinding it off.

Put the crank in a firm vice and firm, two-handed filing will do it nicely. You're also less likely to remove more than you intended that way.

Phil

I ground it it is ali but I`m used to ginding(I should be so lucky anyway it worked and fully tightened up the magnet plate just about kisses the sensor so no problems.
BTW when I only wanted to shim a door a knats knacker I would often cut up a Golden Virginia tin ( and have dry baccy for a week:-)

Cheers

Dave

thanks

 

[fossyant]

Well done.