When your bottom bracket is badly corroded and you strip the pedal threads out of a crank, you really have no choice but to replace both. I finally found some time to do these little jobs on my Commencal Normal Disc ‘08 winter commuter bike.
Fortunately, I’d already removed the old parts (with a Large Spanner and extreme prejudice) so fitting the replacements was made all the easier. The new parts aren’t rusted and seized in place so they just screw right on in without too much trouble.
 The new chainset... |
 ...and bottom bracket |
I’d opted to replace the chainset like-for-like with a Truvative Isoflow unit with 175mm cranks and 42-32-22 pitch. These cogs are going to take a beating from the weather and road dirt so I don’t think there’s any point in buying a more expensive model that will wear out just as quickly.
The replacement bottom bracket, however, is a Shimano sealed cartridge unit to replace the open bearing design of the old part. Being sealed and therefore protected from the elements, I hope this new one lasts longer!
Partially fitted Bottom Bracket
Fitting the bottom bracket is reasonably straightforward. It comes in two pieces – a metal main body and a Nylon locking ring – which screw in to the frame separately from either side.
Fit the metal body to the drive (right hand) side and remember to grease the threads before fully securing it. You don’t need to grease the Nylon lock ring!
The mechanic at the local bike shop where I bought the new BB advised partially fitting the Nylon ring first, by screwing it into place by just a couple of threads, so that it acts as a guide for fitting the main body. This tip seemed to work well and the main body went in without trouble. The lock ring took quite some spanner pressure to secure though.
Securing the new Bottom Bracket
Something else to watch for is that the ring and body are properly aligned relative to the frame before screwing them home, as the threads are quite fine and cross-threading (and so wrecking the bottom bracket or your frame) is a real possibility.
Fitting the new cranks was even easier still. They are simply secured with two 8mm crank bolts (supplied) that compress them on to the tapered end of the bottom bracket axle. Again, remember to grease the bolts and axle before fitting the cranks to help avoid the parts seizing together later.
Fitting the new crankset
I could have made the process simpler by removing the chain first, but I live on the edge, me. You are free to choose.
Finally, I fitted my existing Shimano M424 SPD pedals.
All new Bottom Bracket and Chainset
I’m still not back to using the Commencal as a commuter as I’ve also purchased some SKS Chromoplastic Mudguards and I want to fit those first before finally switching bikes for the winter. These guards come highly recommended and they certainly look sturdy, but initial examinations show I may have some problems fitting them to the Commencal’s disc specific (and slightly weird anyway) frame.
Stay tuned.
