Bottom Bracket B*****y

[Tiberius]

I set out to replace the chainring on a bike. It turned out the easiest thing to do was to get a whole new crankset, and I thought I might as well replace the cup and cone bottom bracket with a new cartridge one. Ok, its another £7 but hang the expense - give the cat a goldfish, as my father used to say.

The old BB was 127.5 mm wide so I got a new one the same width. But the new cranks are now too far out and the chain line is way off.

It turns out the taper is different on the old and new cranks. Swapping bits around with other bikes suggests its the old cranks that are "unusual" and a special wide BB had been fitted to accomodate them.

Humph.

Nick

 

[flecc]

Bottom brackets can be a real pain. If the crank position isn't too bad, perhaps you can swap the chainring to the other side of the spider mount and if necessary using spacer collars to get the chain line correct.

I had to do this on my T bike conversion to get the chain line right.
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[Tiberius]

Swapping to the other side of the spider would take it in the wrong direction, but spacers might work. But both cranks are out, so I think I'll just get a new BB - 110 mm should bring each side in about 8 mm which looks about right.

Nick

 

[flecc]

That's a pity, fortunately they aren't that expensive for what they are.
.

 

[Tiberius]

"I don't believe it"

Some jobs are just out to get you. The thread size on BBs may be standard but it seems that very little else is.

I changed from a 127 mm BB to a 110 one, and instead of bringing each crank in by 8.5 mm, it brought one in by 12 mm and the other by 5, making the chainrings hit the frame.

I had to make a special spacer to get it back to the right position.

Nick

 

[flecc]

I sympathise Nick, that's typical BB stuff, hence what I said above in my earlier post:

Bottom brackets can be a real pain.

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