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If your crank's wobbly or making strange grinding noises, the chances are that you need to repair your bottom bracket. Here is a step-by-step guide on the best way to repair your bottom bracket and bearings.
Repairing
a Bottom Bracket
by Tim Brennan
The bottom bracket is a steel shaft that connects the two crank arms. It has two sets of ball bearings that commonly need replacing after a couple of years or use. A bike shop will be able to do this job for you for around £20, but it's actually quite satisfying to complete the job yourself.
To complete this task you will need a special crank removing tool. This tool should cost say £10 and is necessary to remove the crank arm from the bottom bracket. Also it is best to prepare yourself with replacement ball bearings because you probably will need to replace the old ones. In some cases it is necessary to replace the bottom bracket itself, as it can be damaged if the bearing races get mangled up - a common problem. Getting hold of a new bottom bracket shouldn't cost more than £5, but make sure that you are buying the correct size for your bike. it is best to buy ball bearings in packets of 20 balls -- don't bother buying races they often disintegrate causing damage to the bottom bracket.
Step One
First of all you will need to remove the crank arm on the opposite
side to the chain. Some bikes this have a dust cap that needs to be removed
in order to access the nut. Use a socket
wrench or your crank removing tool to remove the nut. Then use the
opposite end of the crank removing tool to screw into
the female end of the crank arm. It is important to fully screw
in the tool or you may damage the thread. Once the crank removing
tall is fully inserted, the central arm of the tool needs to be
rotated so that it pushes out and releases the crank arm.
Step Two
next you will need to remove the locking ring.if you don't have the right
tool to remove this rain it can often be undone using an old screwdriver
and hammer.
Step Three
once the locking ring has been removed the bottom bracket cup should unscrew
fairly easily from the hub. be aware that when the Is removed the
ball bearings may run out. I would recommend you get a clean glass
jar to hold the components in.
Step Four
the bottom bracket can be removed by pulling on the crank arm that is still
attached. Be careful not to lose the bearings. Now take some tissue
paper and clean away the grease so you can inspect the bearing surfaces.
If the bearing surfaces are damaged then you will need to get a new
bottom bracket.to completely remove the bottom bracket removed the
other crank arm the same way as the first one
Step Five
To replace the bearings you need to find out how many balls you should place
around the cup. On my mountain bike it was nine. They should not
be rammed in together, this will wear them out, that at the same time
it is good not to have massive gaps in between. Once you out decided
on the right number of balls, grease and the cup and pop the balls
into position.
Step Six
Place the bottom bracket into position and screw the cup back into it's hub.
Be carefull that the balls don't fall out in the process. Don't overtighten
the hub - this will wear the bearings out. It is a really fine ballance
between the bottom bracket being able to rattle side to side (too
lose) and the bottom bracket not being able to spin freely or smoothly
(too tight). Also consider that the smallest of play in the bottom
bracket will feel a lot worse when the crank arm is fitted - so aim
for as little as possible.
Step Seven
Put the lock ring back on, but when you tighten it make sure that
you don't also screw the cup or you will mess up the fiddling that
you did in step six. make sure the cup is tight so that it does
not come undone with vibrations over time.
Step Eight
To refit the crank arms back on you simply screw that nut on nice
and tight. There are four positions that the arm so make sure the
crank arms are on opposite sides - surprisingly easy to miss. I
know some chain wheels
are oval shape for more efficient peddling - so if your bike is one
of those then you must get the position correct. Remember the dust
cap too.
Extra Notes
Just as it is important to keep certain parts lubricated with grease, it
is just as important that you dont grease bits that you don't want
coming undone when you are careering down a steep hill. I would avoid
greasing the ends of the bottom bracket where the crank arms attach,
or the lock ring.
If the cup on the drive side needs replacing then you may be better
off getting the help of a bike shop. This is because the drive side
cup is tightly screwed up very tightly and is sometimes a left hand
thread. So it often can be a real headache removing this part.
Even if you do get it out then you have two be certain that you tighten
the new one or else it will come undone whilst you are peddling.
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