7

I cased a jump badly and I broke either my bottom bracket or crank, I’m not sure which. What can I do to fix or replace this?

enter image description here

Picture is of my broken bottom bracket or crankset.

  • Wow, I’ve never seen or heard of this kind of damage. I’m surprised your wheels and frame survived the impact. – Michael Jul 6 at 6:26
6

You've broken your bottom bracket's axle.

There is no way to safely reattach, so you're up for a replacement bottom bracket. Ideally you'd install a cartridge BB that duplicates what you have.

The crank arm (in your hand) is probably okay to reuse, though have a good clean and a close inspection before doing so. Dings are fine, chips maybe, and cracks are not. You need a crank puller to get the sheared-off stub out of the crank arm.

I'm not sure if your bike uses a square taper or one of the newer standards - the broken bit could be square or round. So your new BB has to match what the cranks fit onto, otherwise you'd need new cranks too and that's getting expensive.

Tools should be mostly common hand tools (ie a big spanner), and you will need a BB tool to fit the splines visible in your photo, and a crank extractor to push the nubbin out of the crank arm. That last might be possible using a press or a hammer and brass drift, but only because its already off the bike.


A follow-on thought - to break a bike like this means either a very hard impact or a lack of maintenance on your part.

Based on the corrosion I can see in the photo, there's a part which was already cracked and exposed to the elements for long enough for rust to start.

Upshot - there was a fairly hard impact, and this was focused on pre-existing damage that you hadn't noticed. I'd strongly recommend a good clean and de-grease of your entire bike, and go over it looking for other looming damage. Or ask someone else to help with this - its easy to miss things.

| improve this answer | |
4

As Criggie says, bottom bracket has to be replaced. Axles in cartridge type bottom brackets cannot be removed or replaced. They are held in by pressed in bearings.

I can see it's a square taper axle type the axles on Octalink and other splined types have a greater diameter). Those are common and not hugely expensive.

Different model cranks need different bottom bracket axle lengths to achieve the correct chainline, you'll need to measure your axle length to find out what axle you need. You also need to know if you have a 68mm or 73mm wide bottom bracket shell. MTB cartridge bottom brackets fit both, but you need to know if you have a 68mm shell that requires two 2.5mm spacers to be installed.

Note that special tools are required to remove and install bottom brackets, and a crank puller is needed to get crank arms off square taper axles. Bottom brackets and crank arms should also be installed with a torque wrench. It's especially critical to get square taper crank arms tightened properly.

| improve this answer | |
0

I'm going to throw this out there as even under close zoom, I can't tell for sure, but.... It's the crank arm that has busted leaving a chunk of itself still attached to the BB spindle (which is perhaps an Octalink or Isis interface. It appears to be round). Based on the photo showing misshapen, silver chunk I think that it is probably aluminum alloy which is a material of crankarms whereas BB spindles--especially hollow pipe ones like Isis and Octalink--are made from steel.

| improve this answer | |

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.