Bottom Bracket to fix chain line?

Dr.Millah

Hey all.
Chain line is a bit out of line on my fixed gear. Cant replace cranks or change from the outer chainring to the inner (driveside crank attached to large chainring, no cash for new cranks)

If i was to buy a new bottom bracket that is shorter than my current one would it solve the problem?

Thanks
Neil

Single Malt

Possibly. What cranks are we talking here? And what's the current BB length?

Morgan

If it's a square-taper/octalink/isis bottom bracket you'd be able to change the chainline a bit by putting spacers under the driveside cup.

http://www.velosolo.co.uk/shopchain.html
(near the end...)

Oh, just realized you want to reduce the chainline - then a shorter bb would the way to go.

Dr.Millah

Cranks are a no name brand off a 20year old raleigh.

Current BB length is 120mm. Its an old style crank with all the parts exposed (square taper bar, 2 bearing rings like from a headset)

Single Malt

How much is chainline off by? A shorter BB, shorter by twice the chainline offset should sort it, as long as neither crank or chainring foul the frame. good luck with it (it may require some trial and error). BTW, common lengths below 120: 119, 118, 116, 115, 113, 109. The bold are shimano sizes, all should be available as spurious china brands

Dr.Millah

Single Malt wrote: »

How much is chainline off by? A shorter BB, shorter by twice the chainline offset should sort it, as long as neither crank or chainring foul the frame. good luck with it (it may require some trial and error). BTW, common lengths below 120: 119, 118, 116, 115, 113, 109. The bold are shimano sizes, all should be available as spurious china brands

Its off by around 2-3mm so i will get a bb 4 or 6 mm shorter. might try 115 to be in the middle. Have been looking on CRC and they are very cheap (some less than 10e) but, they seem a little different.

If you look at the attachment, the top BB is what i have, except i have threads on each end of the square taper which is used to bolt 14mm bolts to hold the cranks on. All these new ones have holes bored out to screw into? just a case of getting new bolts?

Morgan

Yes, you just need a regular pair of crank bolts.

LastGasp

Dr.Millah wrote: »

Its off by around 2-3mm so i will get a bb 4 or 6 mm shorter. might try 115 to be in the middle. Have been looking on CRC and they are very cheap (some less than 10e) but, they seem a little different.

If you look at the attachment, the top BB is what i have, except i have threads on each end of the square taper which is used to bolt 14mm bolts to hold the cranks on. All these new ones have holes bored out to screw into? just a case of getting new bolts?

I'm not sure what the experts on here think, but afaik most fixies (including mine) don't use two chainrings, normally just one, which is the easiest way to be sure you will have a good chainline. 2 will be a compromise. Apart from anything else the lack of the additional gear shifter, cable and derailleur adds to the "aesthetics" of the fixie.

Dr.Millah

LastGasp wrote: »

I'm not sure what the experts on here think, but afaik most fixies (including mine) don't use two chainrings, normally just one, which is the easiest way to be sure you will have a good chainline. 2 will be a compromise. Apart from anything else the lack of the additional gear shifter, cable and derailleur adds to the "aesthetics" of the fixie.

You have misunderstood me, i should have explained clearer. I am using the large chainring (52t) and i drilled the rivets off the inner smaller one. What i meant was that the inner chainring position is better for the chain-line than the outer one and that i could cut off the large chainring and bolt the smaller inner one on. But i dont really want to do this. So a shorter BB should solve the problem.

fixie

stupid question but how do you measure how much the chain line is out by?

flickerx

fixie: http://www.sheldonbrown.com/chainline.html
I've found its not an exact science in the real world. Depends on the shape of the crank. 110mm usually works as a good bottom bracket in my experience, for using with road bike cranks made into a singlespeed crank.

Dr Millah if you use a shorter bottom bracket to bring the crank/chainring closer to the frame, if you are running a 52t chainring then you need to be careful that by bringing it in, you dont end up rubbing the teeth off the chainstay. If its originally an outer ring, thats where its made to be, i.e. out from the frame.

Personally I think you're better off buying yourself a proper singlespeed crankset, you can pick one up in town for 25 quid these days.

Photos always help.

fixie

thanks flickerx
think i might just leave it cause that small ticking noise has pretty much dissapered

Dr.Millah

flickerx wrote: »

fixie: http://www.sheldonbrown.com/chainline.html
I've found its not an exact science in the real world. Depends on the shape of the crank. 110mm usually works as a good bottom bracket in my experience, for using with road bike cranks made into a singlespeed crank.

Dr Millah if you use a shorter bottom bracket to bring the crank/chainring closer to the frame, if you are running a 52t chainring then you need to be careful that by bringing it in, you dont end up rubbing the teeth off the chainstay. If its originally an outer ring, thats where its made to be, i.e. out from the frame.

Personally I think you're better off buying yourself a proper singlespeed crankset, you can pick one up in town for 25 quid these days.

Photos always help.

I have at least 15mm clearance between the large chainring and the chainstays and i wont be going anywhere near that far in. Il try get some picture up later on.
Very hard to know how far the chainline is out by, im just judging by eye at the moment. But whatever i do as long as i dont go too much shorter will be a nice improvement.

rflynnr

I know you said you didn't have the cash for a new crank but CRC are doing an amazing deal on a 7/8 speed Sora Triple chainset. The middle ring is a 42 so you can discard the inner and outer and use special short bolts to secure the middle. (Hubjub do them - possible others).

This may not solve your problem (and you'd want to be sure that the Sora was compatible with your existing bottom bracket) but it may be worth considering.

flickerx

fixie wrote: »

thanks flickerx
think i might just leave it cause that small ticking noise has pretty much dissapered

You'll always find that with new components, they're a bit clicky, they soon shut up with some wear and tear.