I've got some used middleburn cranks going on my existing square
Taper BB. As I torque up the cranks they reach the end of the taper and butt up against the face of the BB axle, so that the actual taper is not fully engaged. Only noticed after a test ride and both cranks creaked under heavy pressure. removed with the crank puller and there clearly wasn't much tension in the taper.
The crank tapers looked good before, still do, and so does the BB axle. Torqued to 30 lb/ft and greased the taper (but let's not open that can of worms)
So, are the crank tapers completely worn or have I got the wrong BB? BB Is a questionable SR suntour item that came on the frame, might go pick up a un-52 tomorrow. Surely the SR BB is JIS sizing though, as are the middleburn cranks?
I'd say the crank tapers are stretched, I've had it happen with Middleburn in the past. [Bodge]You could try grinding a bit off the end of the BB axle to allow you to pull the cranks on a bit further.[/Bodge] 😯
Had this before too. Think I bodged it with some slivers of a coke can. More than likely the crank tapers gone but I've also had a dodgy b/b axle
I've never had aluminium shims work on a crank taper, the pressures are too high and it works loose.
The metal in the crank arm is usually softer than a bottom bracket axle, so it is possible that the inside surface on the crank arms has changed internal dimensions through overtighting on a prior bottom bracket, or from repeated fittings. To me the obvious solution is find a bottom bracket with a longer taper - which of course also eliminates the possibility of the cause being too short a taper on the bottom bracket.
Note a longer taper is not the same as a longer axle. I suspect you get that 🙂
Have to say Dibbs solution is the opposite of the what is required, unless my head is on backwards. Skimming metal off the inside of the crank arm at the end where it mounts on the bottom bracket is a similar suggestion, that to me is inferior to the new bottom bracket idea. Removing material anywhere is a bad idea as it reduces the interference contact area increasing pressures and reducing ability for the mechanical transfer of torque.
Hmmmm, from experience I'd say not a problem with the cranks or tapers, I'd try a slightly longer bottom bracket axle. I've had this with Middleburns before - iirc (it was a good few years ago) I tried fitting my cranks (RS7s) to a 110mm b/b resulting in the inside of the crank surface butting up against the bearing cover (BB was new and cranks were in good condition - maybe it's a Middleburn thing, I don't know). Anyway, off to the lbs for a 113mm bottom bracket and job done (actually could've been a 113mm initially and then swapped for a 115 but I guess the principle is the same). Do longer b/b axles actually have longer tapers?? Maybe?
The OP states the inside of the crank arms butt up against the end of the BB taper, not the outside of the bottom bracket itself. So a longer taper is required, whether or not that means a longer axle could probably only be judged on sight.
Thanks for the excellent replies people. Glad to hear some have found this before. I think I'll go get a 113 mm un52 and try my luck. I think the current BB is too much of an unknown quanitity at the moment. Once I've eliminated that I'll consider grinding some extra clearance from the arms if they won't go on. If that don't work then the cranks are for the bin anyway so it's worth a go.
And to clarify, though I think you all understood me correctly, the inside face of the crank bottoms out on the BB axle where the taper section finishes and the axle is much wider, I.e the taper is too short and or too narrow. The cranks don't go on as far as the actuall BB body, it's not that bad. So it may just be an incompatibility between this brand of BB and middleburn, I hope.
I have ground the backs of cranks and the ends of BBs loads of times
infact I did some just the other night .
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Just screws up your chain line a bit and it means you're over-torquing your cranks if you're doing this regularly. Do you lube your tapers?