Tuesday 5 March 2013

Bottom bracket business

The bottom bracket on my electric bike was a bit wonky. This afternoon I'm visiting the dentist (a filling fell out, no big deal) so this morning, I decided to tackle the bottom bracket.

The bottom bracket is the main bearing and shaft that the cranks are attached to. The cranks are the things that the pedals are on. So you can see, it's a pretty important element of any bike. And it was wobbling.

Wobbling probably means that there's something wrong with the bearings. But these days, you don't replace the bearings; the whole bottom bracket, shaft, bearings and ball-race, is one sealed unit. And it just so happened that I had one, left over from an abortive attempt to repair a previous bike (the bottom bracket didn't fit into it).

So, first I removed the cranks; to do this, you use a special tool that screws into the cranks, and then you screw a nut that slowly extracts the cranks. That worked well.

Then I removed the bottom bracket. To do that, you need a special tool that engages with a slot on the collar that keeps it in place, and then you can turn the collar and unscrew it.

At that point, several ball bearings fell out and lost themselves amongst the gravel. But I didn't really care, I wasn't going to be reusing those. I was able to pull out the crankshaft, whch looked terrible, and the ball races which looked even worse.

Then I had to take off the other side of the collar that keeps the crankshaft in place, and that needed a special tool; a 36 mm spanner. Which I didn't have. So, a trip down to the bike shop and £18 (which I have to say seems like a lot for a spanner) and I had the right tool. And with that (and after remembering that it was a left hand thread, so if you tried to unscrew it in the usual way, you were actually tightening it), I got the second collar off.

Then I tried to put the replacement crankshaft on the wrong way round. It's clearly marked with L and R, but I had the bike upside down, which meant that L and R were the other way round. Soon, I realised this, and put the replacement crankshaft on the right way round. And then I used the special tool to tighten it up. Replacing the cranks was easy, and for once, didn't need a special tool; I tightened the cranks on by doing up the bolt that holds them in place.

I oiled the chain, because it needs oiling every so often, and scraped the mud out from inside the handlebar. The key for the motor had fallen apart, so I repaired that using a length of spoke. And now we're ready for another bike trip around Huntingdonshire.

  ...  update ...

I've ordered an 18 inch adjustable spanner from Ebay, with jaws opening up to 50mm. Next time I need a spanner bigger than any I have, I'll be ready.

Also, I did a bit more research on bottom brackets, and I'm not sure that the one I installed has the correct length. If it's too long or too short, the back gears won't line up with the front gears. So I took the bike out for a quick ride test, and apart from the gear change cable needing to be adjusted, I think it's all good.

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