replacing the bottom bracket
replacing the bottom bracket
servicing the bottom bracket
the bottom bracket is the dohickey that connects your two crank-arms
together, and secures them within the bottom of the bike frame.
the bottom bracket has threads around the outside that secure it into the
hole in the frame. inside, bearings surround a rotating spindle -- this lets the
pedals turn free from the frame. each side of the spindle has a mechanism to
connect your crank. this may be square tapered bolts, a toothed ring (splined
bottom bracket), or even a threaded bolt. the bottom bracket type must match the
crank type (a crank with a square hole must be mounted on a bb with the square
receiver).
bottom brackets also come in different widths, and with different spindle
widths: the bracket width should match the frame, and the spindle width should
put the cranks out away from the frame, and allow the front derailleur to
transfer the chain onto the smallest ring without hitting the bike frame. the bb
will usually have its size printed on the cartridge, but you have to take it out
to read the numbers. for example, a 68x116 bottom bracket is 58 mm wide as it
fits into the frame, and the spindles are 116 mm apart. if you wanted the pedals
a bit closer together, you might buy a 68x112.5 bb.
sealed bottom bracket: if your bottom bracket is a sealed cartridge
unit (as most are nowadays), it can't be serviced or
adjusted. if it grinds, wobbles, or catches, it must be replaced as a unit. just
remove the old cartridge and replace the entire thing.
take the chain off the chainrings, and lay it on the frame next to the bottom
bracket. with the bike clamped in a workstand, shove the base of the crank-arms
side-to-side to detect motion in the bottom bracket. now try to pull the cranks
away from, then back towards the bike to detect wobble. turn the crank and
listen for noises within the bottom bracket. these tests will help you identify
a damaged bottom bracket.
to replace the bottom bracket, you have to do some major
stripping! you must remove both the left crank and the right crank arm
plus chainrings. see the crank section for
additional information.
use a crank puller to remove the cranks. this tool extracts
the crank off the spindle without damaging the bottom bracket.
check the photo below. if you don't see gear-like teeth around the inside of
a ring surrounding the spindle, you have a different type of bracket. it may
take different tools to remove the bracket. scan down to the next section.
once both spindles are visible, apply a bottom-bracket
tool to the teeth on the lockring side of the bracket (usually left). this tool fits into the
teeth on the inside of the bracket cartridge, while the outer half is
turned with a wrench (usually a 32 mm headset wrench).
it's easiest to remove the bottom bracket with the bike lying on its
side, so take it out of your workstand for this part.
unscrew the bottom bracket. one side is the lockring, the
opposite side is the cartridge containing all the guts of the bottom
bracket. i have modern bikes with both configurations: bracket left with
lockring right (chain side), and two with bracket on the right and
lockring left.
begin by removing the lockring. on new shimano cartridge bbs, the
lockring is on the left-hand side, opposite the chain. turn counter-clockwise.
you may need to apply solvent. if the wrench won't turn, have someone
tap the bottom bracket tool gently with a hammer as you apply force.
(if it turns out you're turning a wrong-sided bottom bracket cartridge,
the opposite side's lockring may come loose as you turn the bottom bracket. that's ok.
just unscrew them both.)
after removing the lockring, flip the bike over, and take out the
bottom bracket. use the same bracket tool and wrench. note: the bottom
bracket unscrews in the opposite direction. turn it clockwise!
(photo shows a left-sided bottom bracket. yours may be opposite.)
notice all the dirty water that was bathing this bottom bracket.
clean the threads on the inside of the bracket housing, and clean the
threads on the lockring.
lube the threads on the inside of the bracket housing. a good option is: use teflon pipe tape on the bottom bracket threads.
if you have a titanium frame, this is highly recommended.
thread in the lockring. leave it slightly loose. now flip the bike back over to expose
the other side.
insert the new bottom bracket, and screw it in until it's tight. snug
up the lockring. now
reattach the cranks.
other brackets:
cheaper bikes may come with odd components. for example,
this kid's bike has a spindle that terminates in a bolt. first, remove the
cranks. (see the crank section for information.) a lockring secures the
bottom bracket on the left-hand side. remove the lockring with a pipe
wrench.
now apply a cone wrench to the bottom bracket and unscrew it
from the housing.
note: remove the chain-side crank before turning the bottom bracket! in
this photo, the chain-wheel is still lying loosely on the bracket spindle.
adjustable bottom bracket: in standard old bottom brackets, the bearings can be serviced and
adjusted. in others, the unit is sealed, so the bottom bracket must be replaced
if it misbehaves in any way. to replace or service the bottom bracket, you need
a set of "bottom bracket tools," consisting of a lockring tool and a pin
wrench to immobilize the bearing cup.
looseness of the bottom bracket is detected by a subtle
"floppiness" of the cranks as you apply inward or outward pressure on
the end of the crank-arm. if it's a standard bottom bracket, you can adjust the
bearing tightness.
adjusting the bearing tightness: first remove the left
crank from the spindle. you may need a crank extractor tool. apply a
lockring removing tool to the notches on the lockring and twist about 1/2 turn
to loosen it. now adjust the bearing cup -- turn it slightly counterclockwise to
loosen the bearings, clockwise to tighten. while keeping the bearing cup
immobilized with the pin wrench, tighten the lockring.
removing the bottom bracket, standard bearings: remove both left and right cranks
from the spindle. you may need a crank extractor tool. loosen and remove the
lockring from the left side. (since you are removing a standard -- non-sealed --
set of bearings, place a cloth under the bike to catch any loose bearings. now
apply the pin wrench to the bearing cups and turn counterclockwise until the cup
and bearings are removed. pull the spindle out from the right side and catch the
bearings there also.)
servicing the bearings: clean and inspect all parts. replace any parts that
are damaged or corroded. pack both bearing cups with bearing grease. push the
bearings into the grease. put the bearing retainer rings into the cups, making
sure that only the balls (and not the retainer) contacts the cup. if the fixed
bearing cup was removed, reinstall it, turning counterclockwise. insert the
spindle from the left side. now put in the adjustable bearing cup (on the left)
and adjust the tightness so the spindle turns smoothly without wobbling. tighten
the lockring down while restraining the bearing cup with a pin wrench.
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