DiamondBack Topanga bottom bracket removal, check & repair?

[Offroader]

Hello,

1st time checking a bracket since my old Townsend required use of a ball joint remover & hammer combo to bash off the cranks way back in the early 90's so I want to make sure I'm doing this right without damaging anything.

Reason for repair due to crank slipping when cycling up hills (& there's a lot of hills round here so this workhorse of a bike has been given quite a test the past 5yrs), the steeper the hill & the lower the gear = more slipping & high gears on slight rises at low speed also result in some slipping, my suspicion is that something in the bottom bracket has finally given up the ghost as chain, derailleurs & cables are all set correctly.

No idea how old the Topanga is precisely, but it's fitted with its original Shimano Altus A20 derailleur etc (+ presumably a BB-LP20 bottom bracket). There's an 8mm hex bolt either side of the crank, easy enough to remove, but I need a crank removal tool to get to the bottom bracket... it looks potentially quite rusty on the visible part of the bracket so who knows how easy this will be to remove.

If necessary the whole BB will be replaced but does anyone have any tips re: removal, what to be wary of, what to make sure is & is not done?

Thanks.

 

[ishaw]

Re:

If things are slipping, it will usually be the chain, chain rings and cassette due to wear. The bb bearings could be worn which would give side to side play in yourcranks, not slipping.

The axle interface between the crank arms and bb axle coukd be worn, but again, that would produce play in the arms and not slipping.

Does the slip occur in all hears or just some?

 

[Offroader]

Re:

Thanks for your reply.

The slipping/catching (by which I mean the crank slips before catching itself again on the bracket) mainly occurs in the lowest gears when cycling up hills (the bike cannot be cycled up hills now as the horrible crunching is a consequence of attempting to do so). I can feel that there's something wrong with the bottom bracket when this happens & this is where any sound suggestive of the source of the problem emanates, as well as this something has come to feel loosened within the bracket (& this is a very recent sensation whilst peddling especially in highest gears when cycling along a flat surface).

Chain seems fine, it does not slip off any cogs whilst the crunching occurs & gears change smoothly remaining in place without issue.

 

[ishaw]

Re:

Could be the freehub. The pawl within may not be catching on the ratchet which will cause slipping. Sound is often transferred via the drivetrain/frame.

Can you drop the chain and try moving the various parts when they aren't connected. Might help isolate where the issue is.

 

[hamster]

Check in order:
1 Worn chain. Measure with a ruler. As they wear, chians lengthen adn thne don;t mesh smoothly with the sprockets, hence the slipping.
2 Worn cassette. As above.

The bottom bracket doesn't transmit drive (that's the chain and sprockets) so a loose BB won't cause slip.

 

[Offroader]

Could be the freehub. The pawl within may not be catching on the ratchet which will cause slipping. Sound is often transferred via the drivetrain/frame.

Can you drop the chain and try moving the various parts when they aren't connected. Might help isolate where the issue is.

Check in order:
1 Worn chain. Measure with a ruler. As they wear, chians lengthen adn thne don;t mesh smoothly with the sprockets, hence the slipping.
2 Worn cassette. As above.

The bottom bracket doesn't transmit drive (that's the chain and sprockets) so a loose BB won't cause slip.

Thanks for your replies I'll take a close look at the chain etc asap & get back to you both.

 

[Offroader]

Re: Re:

Could be the freehub. The pawl within may not be catching on the ratchet which will cause slipping. Sound is often transferred via the drivetrain/frame.

Can you drop the chain and try moving the various parts when they aren't connected. Might help isolate where the issue is.

Check in order:
1 Worn chain. Measure with a ruler. As they wear, chians lengthen adn thne don;t mesh smoothly with the sprockets, hence the slipping.
2 Worn cassette. As above.

The bottom bracket doesn't transmit drive (that's the chain and sprockets) so a loose BB won't cause slip.

Definitely looks like the cogset (there's quite some wear on some of the "teeth") & the chain are the cause of the slipping, thanks to you both for your advice it's saved me unnecessarily taking the bottom bracket to pieces (though I'm not looking forward to tackling that when it finally requires repair or replacement).

 

[legrandefromage]

pictures pictures pictures!

whats often mistaken for wear can often be the shift pattern on the teeth to aid shifts between the rings

chainrings are most often the last part to wear out enough for a chain to slip. The middle ring is usually the first to go and the teeth end up very sharp and pointy.

Cassettes often wear in a favoured ratio leaving the rest of the cassette ok but one cog is knackered

chains wear as the bushings become loose making the chain appear to 'stretch' - an easy way to check is to wrap it around a large cahinring and just tap it, if it looks loose on the ring or appears to rattle wehn you tap it, its pretty much too worn, If it fits reasonable tight, its good to presume that its ok.

If the chainset/ cranks move side to side then the BB is worn or loose. This would not make the gears 'slip' unless everything was wildly out of place, but then by this time it would have worn through the rear stays on the frame.

Old BBs can be serviced with new grease and bearings. If the BB is a cartridge type, remove note the size and replace.

 

[Offroader]

pictures pictures pictures!

7 photo close-ups of the cogset & chain:

 

[Splatter Paint]

Re:

There looks to be heavy wear to the cassette and chain which would explain the slipping. The front rings will almost certainly be past it too.

Compare your sprockets to the tooth profiles on this and you'll see what I mean: https://www.wiggle.co.uk/shimano-cs-hg4 ... -cassette/

The good news is that 7 speed cassettes and chains are dirt cheap:

https://www.wiggle.co.uk/shimano-hg40-6-78-speed-chain/

https://www.merlincycles.com/sram-pc850 ... 44531.html

Rings though :? more of an issue.

SP