Bottom bracket size

[Alan Farroll]

Hi,

If I wanted to change my bottom bracket on a road bike from a dual compact to a triple, how do I know what size or type of bottom bracket to purchase. Also how do I tell if my chain will be the correct length?

Thanks in advance

Alan

 

[lpretro1]

If u are on HT bb then it will be no different. If you have square taper or splined bb then exact bb size will be determined by frame shell width ( will be either 68mm -very common - or 73mm - less common) and axle length dep on crankset u choose. If u are press fit then a whole different world unveils itself!!

 

[Alan Farroll]

Thanks so much for the response Ipetro1 but how do I find out if my bike has a HT, square taper or splined bottom bracket? According to the bike specs the bottom bracket is - FSA BB-7420ST

Thanks again

Alan

 

[Alan Farroll]

If this link I found is correct, is my bottom bracket a square taper one?

http://www.chainreactioncycles.com/fsa-th-square-taper-bottom-bracket-bb-7420/rp-prod105994

Regards

Alan

 

[Tim Hall]

If this link I found is correct, is my bottom bracket a square taper one?

http://www.chainreactioncycles.com/fsa-th-square-taper-bottom-bracket-bb-7420/rp-prod105994

Regards

Alan

Yes, that's a square taper, and looks like the part number of the one you quoted.

 

[lpretro1]

Yes square taper and the part no you quotes show s a 68mm shell - so now you need an axle length. But what you need depends on the crankset u buy. A triple normally uses a slightly wider bb than a double or compact. What crankset are you planning to put on?

 

[davidphilips]

with square taper bbs some chainsets sit further onto the tapper than others so might be best to just buy the chainset you want and try it on your bottom bracket and test as for chain lenght again you may get away without changing anything.

 

[Alan Farroll]

Yes square taper and the part no you quotes show s a 68mm shell - so now you need an axle length. But what you need depends on the crankset u buy. A triple normally uses a slightly wider bb than a double or compact. What crankset are you planning to put on?

Thanks again Ipetrpo1. Forgive my lack of knowledge but enjoying learning about this (I haven't been riding a bike regularly since my teens which was many years ago and my goodness bikes have changed). But what exactly is the axle length?

I think going by what yourself and David Philips says, I will buy the crank set and triple derailleur ( I can get one of those for £15) and take it from there. Some cranksets are advertised at 170mm or 175mm - is this measurement the length of the crank arm?

It might help if I gave you a link to the actual bike I have
https://www.giant-bicycles.com/en-gb/bikes/model/2013.giant.defy.5/11833/56807/

Thanks again

 

[Alan Farroll]

with square taper bbs some chainsets sit further onto the tapper than others so might be best to just buy the chainset you want and try it on your bottom bracket and test as for chain lenght again you may get away without changing anything.

Thanks for feedback David Philips. Yes, after your response and Ipetro1's response I will buy the crankset and derailleur and take it from there

 

[lpretro1]

Thanks again Ipetrpo1. Forgive my lack of knowledge but enjoying learning about this (I haven't been riding a bike regularly since my teens which was many years ago and my goodness bikes have changed). But what exactly is the axle length?

I think going by what yourself and David Philips says, I will buy the crank set and triple derailleur ( I can get one of those for £15) and take it from there. Some cranksets are advertised at 170mm or 175mm - is this measurement the length of the crank arm?

It might help if I gave you a link to the actual bike I have
https://www.giant-bicycles.com/en-gb/bikes/model/2013.giant.defy.5/11833/56807/

Thanks again

Yes 170 or 175 is crank length - good idea to stick with what you already have - it can usually be found stamped on back of crank arms near pedal

 

[C.J]

If you look at the cranksets on spa cycles website they normally put what BB is required for each particular crankset.

Also if looking at shimano cranksets, if you go on shimano website for the technical stuff, and put the model number
of crankset in, it tells you what BB you need. ( i think thats right, it was a few years ago when i put a triple on my bike ).

As for your crank length if your happy with what's on there get the same. I have 175mm but i'm 6 ft 4" with long legs.

Just a thought if its to help with the hills, would it be cheaper or easier to change the cassette to maybe a 11 - 32 or
even a 34? and a long cage derailleur.

 

[TheDoctor]

That's what I'd do, as fitting a triple usually entails a new front mech and STI lever(s), which starts to get expensive.