Square taper BB problems

kingofthetailwind

So I'm having roblems with the BB on my GT Grade. It takes a 68mm English Thread.

When I had te cranks off I noticed the FSA cartridge the bike came with wasn't sitting flush to the frame. Neither the cartridge on the drive side or the cup on the other are sitting flush - they're slightly crooked with a gap of a couple of mm at one side. I think this is causing the cranks to spin unevenly when fitted, which is causing front derailleur rub at one point of the crank rotation.

So I removed the FSA cartridge and reftited it twice and couldn't get it to go on straight. And to rule out the BB I bought a Shimano UN55 one. (107mm as nowhere seemed to stock 103mm) But it's exactly the same problem.

To be honest I could probably put up with this as the FD rub only happens in the big ring and smallest few sprockets, and only when out of the saddle giving it some welly

But the other problem is that both BBs are incredibly hard to fit. On mt previous winter bike you could screw the BB in almost the whole way by hand. With my BB tool and extra large wrench I can just about fit it. It's hard to turn on both sides.

Any idea what the problem could be? The threads look OK but it's hard to tell. They'r clean, greased and i'm being very careful to start screwing it in straight.

bill57

Sounds as if the BB thread has been incorrectly cut, not so much "off centre" but at an angle to the correct centre line.
A decent bike shop can certainly re-cut the thread to make fitting the BB easier, but making it straight is a different matter entirely.
Have you had this bike long? Sounds like something that should have been taken up with the dealer or manufacturer.

sheffsimon

https://www.sjscycles.co.uk/bottom-brac ... d-threads/

or similar, might be worth a look.

Google threadless square taper bottom bracket

Alex99

KingOfTheTailwind wrote:

So I'm having roblems with the BB on my GT Grade. It takes a 68mm English Thread.

When I had te cranks off I noticed the FSA cartridge the bike came with wasn't sitting flush to the frame. Neither the cartridge on the drive side or the cup on the other are sitting flush - they're slightly crooked with a gap of a couple of mm at one side. I think this is causing the cranks to spin unevenly when fitted, which is causing front derailleur rub at one point of the crank rotation.

So I removed the FSA cartridge and reftited it twice and couldn't get it to go on straight. And to rule out the BB I bought a Shimano UN55 one. (107mm as nowhere seemed to stock 103mm) But it's exactly the same problem.

To be honest I could probably put up with this as the FD rub only happens in the big ring and smallest few sprockets, and only when out of the saddle giving it some welly

But the other problem is that both BBs are incredibly hard to fit. On mt previous winter bike you could screw the BB in almost the whole way by hand. With my BB tool and extra large wrench I can just about fit it. It's hard to turn on both sides.

Any idea what the problem could be? The threads look OK but it's hard to tell. They'r clean, greased and i'm being very careful to start screwing it in straight.

I would get the thread checked by someone with experience and the right tools. You should be able to turn the BB in at least most of the way by hand if the threads are good.

However, this might turn out not to be your problem with front deraileur rub. Square taper cartridge BBs are very tollerant to badly cut threads and non-square BB faces, because the bearings are kept in alignment by the cartridge shell. It might turn out to be front deraileur fettling that you need.

Nick Payne

A bike shop with a decent set of piloted taps should be able to chase and face the BB shell for you. Piloted taps will be able to correct any misalignment in the threads in the shell. I routinely chase and face the BB of any frame I purchase - I can only recall two that I've had over the years that didn't need it done.

Getting derailleur rub when pushing hard out of the saddle could be a combination of frame flex, flex in the BB axle, and flex in the crank/chainrings.

kingofthetailwind

After a couple more efforts the bottom bracket is in properly. The removing/fitting seems to have sorted the thread out?

The FD rub remains though. I've a spare 50t chainring so I'm going to fit that and see if it makes a difference. Failing that, try another chainset.

Cheers for the replies.

dgunthor

FD rub - check the chainring isn't bent. I usually place the FD at a slight angle out towards the rear mech (not perfectly parallel to the front chainring, so the chain doesn't doesn't rub on the smallest sprockets).

Check the outer limit screw on the FD too is set ok.

kingofthetailwind

The H (outer) limit screw on the FD is all the way out. Has to be or the chain will rub the outer plate of the FD all the time. Seems like the big ring is too far outboard on this bike, as well as possibly being warped. I can move the FD further out by using more cable tension, but it's nearly at the max and any more tension and you start to lose the ability to trim.

I have the original 103mm BB fitted again, which seems to be the narrowest width available. So I can't fit a narrower BB to bring the chainring any further inboard. So I'm wondering if the chainset (FSA Vero) isn't a good match with this bike. I read that using a Hollowtech BB/chainset brings the chainline in? Fairly expensive upgrade though.

arthur_scrimshaw

Square taper cranks rely only on the taper to fit square to the BB, my experience is that you can influence this alignment by trying the crank in different positions on the taper (ie in 90 degree steps) or by using a soft mallet to fit the crank, turning it and hitting it in different places to bring it square using the bolt only to tighten it the last small amount.

kingofthetailwind

I see what you mean Arthur, I've just been using shoving the crank on and assuming the bolt would centre it. :oops: Cheers.

sheffsimon

102mm is available, might give you a bit more room.

http://www.wiggle.co.uk/token-tk867-iso ... m-bracket/

Nick Payne

I have the original 103mm BB fitted again, which seems to be the narrowest width available.

Phil Wood make square taper bottom brackets down to 96mm length - I have one of their 96mm BBs on one bike in order to use a set of Deore XT M737 cranks as a double on a road frame, and a 98mm in another bike. They don't show those lengths on their web site, but if you email them and ask, you can get them.