Replacement bottom bracket help please

[BADGER.BRAD]

Hello All,

I have tried to get away again ( to no avail) with chaining just my drive chain which has made my gears jump like hell , no surprise really as on closer inspection there are teeth missing and the rest are badly worn. As the crank is creaking a little I have decided to replace the bottom bracket at the same time. Can anyone tell me what I need to know or measure in order to make sure I order the right one. I have never messed with the newer types, The bike is a mountain bike if that make any difference.


Many thanks

Brad

Ps I'm a little gutted as I'm going to have to scrap a good crank set as It seems much much cheaper to buy a new chain set than just the rings ( I hate not reusing serviceable equipment !) Maybe it will have some use when I finally build a single speed/fixie in the future !

 

[cycleruk]

try this
http://en.wikibooks....racket_overhaul
and just to add to this you will need to know the length of the bb(one end of the axle to the other)
and the bore size (most are 68mm but these do vary very slightly so you will need to measure)

 

[3tyretrackterry]

take the old one out and take to LBS and ask or one the same or that is compatible if the old one is a sealed unit then it may have the measurements on it when you take it out
HTH

 

[accountantpete]

Chainrings are really hard wearing - the chipped teeth and filed edges are more likely to be designed rather than the result of wear.

I would just replace the chain and test it out.

Then change the rear sprockets if it is still causing problems.

Finally admit defeat and replace the chainset.

steps 1 and 2 usually solve the problem.

 

[PpPete]

the crank is creaking a little I have decided to replace the bottom bracket at the same time.

More likely to be the interface between the BB spindle and the crank than the BB itself. Have you tried tightening up the crank bolts ?

That said, the definitive guide to Bottom Bracket sizing is here.

9 times out of 10 on an MTB or Hybrid with a square taper it will be 68 x 118.... but YOU (and only you) MUST measure the width of the BB shell on your bike, and the spindle length of the old BB before you buy.

The 68 mm tells you with 99.9% certainty that it's "english threaded" as the "French" and "Swiss" threads are rarer than hen's teeth, especially on MTBs

Actually 70 mm "Italian Thread" is pretty rare on MTB's too - but like the old cutting adage.... measure twice, buy once.

 

[BADGER.BRAD]

Many thanks all, much appreciated. The chain wheels front a rear really are shot with teeth fully missing and the rest really worn, what made me try the chain only is a mystery ! ( think it was my inner cheapskate) As I will change the crank arms with the new front chainset ( as I said cheaper than chain wheels only) I will try that first before the bottom bracket and see if the creaking stops. The chain jumps like crazy at the moment I cannot go uphill or try and go at a pace as the chain just spins around the chain wheels.

Again thanks everyone at least now I know what I'm after.

Brad

 

[Tim Hall]

Did you change just the chain?

Was it very worn?

If so changing the cassette/freewheel is likely to give you better results.

The sprockets tend to wear to accommodate the wear in the chain, effectively changing the pitch between them. The new chain is at the proper pitch and doesn't fit, so it skips under load.