nicktf

Hi,
I have a new (second hand) bike with one of these bottom bracket. It has a slight knock under maximum load.

Looking at it, it has about 1mm clearance from the crank to the bearing housing, with no spacers that I can see and the (non chain wheel) crank is held by two pinch bolts with a pre loading bolt at 1.5nm.

So 2 questions:
1) What does this pre loading actually do. Given there is 1mm gap isn't the shaft free to float left and right by this gap? As it does not move I don't quite understand why!!??

2) Are the 2 washers needed to transmit the preloading effectively to the megadrive shaft?

Thanks
Nick

FBinNY 

If the preloading bolt is correctly named, and expected to do anything (except preventing the crank from creeping outward) the crank would need to apply axial preload on the bearing's inner race, either directly when moved inward, or by way of a spacer.

FWIW - I'm not familiar with that specific BB, but suspect that someone may have lost or forgotten a critical part.

FBinNY 

One note.

I didn't mention it earlier, possibly giving you too much credit. So a reminder that, like with threadless headsets, the cranks cinching bolts have to be loosened a bit so the preload bolt can push them in, then tightened to lock everything in place.

Disregard if you already know that.

nicktf

Thanks for your help 👍 I did loosen the pinch bolts! The preload washers are missing but even with these in it still has a slight clunk under load, no matter what the pre load torque was.

When I put the washers on the shaft I noticed it seemed to be a very sloppy fit onto the cup bearings I was expecting to see a tight-ish interference fit. Is
​​​ a tight fit normal, and if not this seems the likely cause of the clonk?

Thanks
Nick

FBinNY 

A few observations.

The spacers are just that --- spacers. There's no precision needed since all they have to do is gill the gap and transmit compressing from the crank to the bearing's inner race or centering cone (depending on the design), so no worries there if that condition is met.

So, there are two possibilities.

1- something, possibly corrosion is binding the race or cone on the spindle, keeping it from sliding and actually preloading.
2- your initial theory is wrong, and the noise has nothing to do with preload. Possibly there's movement at the outer race, the entire assembly within the frame, or elsewhere. Or the bearing is actually damaged (though I doubt that), or the problem is someplace entirely, ie. a pedal, or the crank/spindle connection.

The key to mechanical problem solving is to form a theory, test it by making a change, yet remain open minded to other possibilities.

In your shoes, I'd try to create bearing play to where I can feel it in the cranks, then readjust and preload. If changing preload has no effect at all then Id suspect that the bearing isn't actually preloading. Otherwise, I'd preload correctly and look elsewhere.

nicktf

Thanks FB,

I can't really feel the play, its only when its under alot of leg pressure (up a hill in a high gear, legs not spinning) and thats when it clonks. If I change down gear it normally stops.

I take your pint about the preload, the fsa system has a 'normal' and 'wavy' washer which don't seem to alter the clonk situation under any torque. Its not coming from the pedals as I have tried 2 sets.

I was just trying to ascertain what is the expected/normal binding between the bearing surfaces and the axle shaft, as I was wondering about axle wear. Mine has pitting both sides but I have no idea if this is within an acceptable limit.

When I removed the non chain crank, the axle and chain ring crank slide out under gravity with the bike at a very shallow angle. I was expecting to have to tap out the shaft! Hence my tack of trying to get an idea of what is 'normal' binding between the axle and bearing surfaces.

nick

Freerojo

MegaExo usually have a “wave washer” between the crank and BB on the non drive side. Mine does and I’ve replaced it.

nicktf

Thanks for the opinions and advice!
nick