Bottom Bracket - Help

[antcox06uk]

Hi

Had problems with chain slip for months. Replaced the chain and eventually the casette and it wasnt as bad but then came back to a point where the chain was slipping under load. Through using my other two front chain rings, i have determined that it was my front middle chain ring. Rather than replace one ring, i decided to replace the full front chainset. Bought from Wiggle and when it arrived, it was different colour to the one advertised and more importantly, it was octalink not square taper. So now i need to replace my bottom bracket.

As i use my bike for work, i cant take the bottom bracket out without having one to replace. Can anyone tell me from looking at my link to my bike (2015 Trek 7.4 FX Disc - Trek link to bike spec) what length bottom bracket i will need. I have limited knowledge and i just dont want to order the wrong part as it seems sealed octalink BB are limited as where to get them.

 

[Ajax Bay]

I would strongly recommend that you return the Octalink chainset to Wiggle.
Then either swap for a square taper (SqT) one, or purchase a SqT one elsewhere (Spa Cycles good source). Your SqT BB is much better that Octalink (search on here for the latter's design flaws), and it's already fitted and perfectly OK.

 

[Cycleops]

As AB says return the Octalink for the correct chainset. A Hollowtech would have been a good upgrade but an Octalink!!

 

[I like Skol]

I wouldn't suggest Hollowtech as a bottom bracket upgrade! In my experience they have a much, much shorter lifespan compared to Shimano square taper bottom brackets.

Why not save all the expense and hassle by just replacing the worn chainring - https://www.sjscycles.co.uk/chainri...0-104mm-bcd-4-arm-middle-chainring-black-36t/
I would return the octalink one if I were you.

 

[Cycleops]

I have a Hollowtech on my Specialized gravel bike ands it’s been perfectly fine. Of course the bearings won’t last as long as as a UN55 but they are only about a tenner and the whole thing is a lot lighter if that’s important.

 

[ianrauk]

Of course the bearings won’t last as long as as a UN55 but they are only about a tenner and the whole thing is a lot lighter if that’s important.

Agreed

 

[I like Skol]

I have a Hollowtech on my Specialized gravel bike ands it’s been perfectly fine. Of course the bearings won’t last as long as as a UN55 but they are only about a tenner and the whole thing is a lot lighter if that’s important.

Well if you're happy with a bottom bracket that only lasts 2-3 thousand miles then I suppose the fact they are light and cheap is a bonus. Personally I think a UN55 which is at most about twice the price but are almost fit and forget items, are far better.

 

[antcox06uk]

I wouldn't suggest Hollowtech as a bottom bracket upgrade! In my experience they have a much, much shorter lifespan compared to Shimano square taper bottom brackets.

Why not save all the expense and hassle by just replacing the worn chainring - https://www.sjscycles.co.uk/chainri...0-104mm-bcd-4-arm-middle-chainring-black-36t/
I would return the octalink one if I were you.

I did consider that but couldn't work out what BCD I needed and to do it seemed to need degree in mathematical calculations so gave up and thought better to go with a new complete crankset. Bike is only for getting work so doesn't have to be anything fancy or light.

What tools would I need to remove the chain ring.

 

[faster]

2-3 thousand miles! That's bad luck.

The hollowtech BB in one of my bikes is on just under 10 thousand miles of all weather commuting with zero maintenance. Another is up to just under 4 thousand.

Both are still perfectly smooth.

 

[Ajax Bay]

couldn't work out what BCD I needed

Sheldon Brown's cribsheet gives the BCD calculated from the 'adjacent bolt' dimension (which the OP can measure with a ruler).
The Trek spec linked to in the OP says: "Crank - Shimano Acera M391, 48/36/26 w/chainguard"
https://si.shimano.com/pdfs/ev/EV-FC-M391-3213.pdf
• "Bolt circle diameter: 104 mm...(For outer gear and middle gear), 64 mm...(For inner)" [4-bolt].
Obviously check.
Tools: An allen key and a 8mm flat screwdriver.