5

I just fitted a Tiagra crank arm to my bike. I noticed that there was a gap between the bottom bracket and arm and was concerned whether I fitted the arm correctly.

I tried to force the crank arm further down the axle to reduce the gap but was unable to do so. The bottom bracket is an Ultegra BBR60. Is the gap normal or do require a spacer?

Appreciate any advice.

Ps the chainset is also Tiagra.

gap between crank arm and bottom bracket

1
  • Certainly for a traditional BB there's nothing wrong with that gap. Dunno about newer stuff, though. Jan 30 '16 at 3:38
3

Space for squirm, plus tolerances.

Firstly, cranks are made in many different shapes and styles, and not all are the same thickness. So some slack is left to allow for this. Imagine how annoyed you'd be if changing cranks meant a new BB cartridge or axle?

Secondly, modern funny-shape crank/axle joints are less prone to this, but a square taper fitment would creep further onto the axle over time. The more times the crank was removed, the further the crank can move up the square taper. We're only talking fractions of a millimetre.

So if your previous crank was original on the bike with the bottom bracket, they may have been better matched and the gap could have been smaller.

No you don't need a spacer - the axle does all the load bearing by itself. A spacer would simply add to friction and weight and provide no benefits.

1

Since you provided only a photo of the non drive side it is difficult to say for sure how many spacers you need. The number of spacers will vary with bottom bracket width and derailleur type. The spacer is needed to apply the preload on the bottom bracket bearings. This requires that the crank arm be butted up against the bearing. Then the plastic nut in the center of the arm must be tightened to 4-6 inlbs. The crankset should have come with 3 spacers. I strongly suggest you check either the Park Tool or Shimano website for step by step installation instructions for your particular set-up.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.