Cycling UK Forum

I have just taken off an old (and very seized) square taper BB from my daughters Dawes hybrid. It is a 68/118. I need to replace the chainset which is a 6 speed triple 48/38/28. I have found a Shimano FC TY501 but it states must be a 122.5 BB. I didn’t measure the chain line of the old setup before I took it off. Am I right in thinking that to get the chain line specified by shimano I would need to stick to the 122.5 BB. If I go for the 118 it should fit but the chain line maybe out. Is that right?

Am I going to have to put the old stuff back together to measure the chain line?


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where the FD sits should tell you more or less what the chainline was.

If both BBs are symmetrical then you could have 2.25mm difference in chainline. In reality not all BBs are symmetrical which means that the chainline may be different by another amount

I measured most lengths of BB-UN55 here

https://forum.cyclinguk.org/viewtopic.php?f=5&t=105385&p=1004573&hilit=un55+length#p1004573

and this suggests that the chainline will vary by ~1.7mm between a 118mm and a 122mm bottom bracket of this type.

cheers

Brucey wrote:where the FD sits should tell you more or less what the chainline was.

If both BBs are symmetrical then you could have 2.25mm difference in chainline. In reality not all BBs are symmetrical which means that the chainline may be different by another amount

I measured most lengths of BB-UN55 here

https://forum.cyclinguk.org/viewtopic.php?f=5&t=105385&p=1004573&hilit=un55+length#p1004573

and this suggests that the chainline will vary by ~1.7mm between a 118mm and a 122mm bottom bracket of this type.

cheers

Am trying to understand what that means. It’s been a long day.

I have put the crank arm back on and measured. The chain line appears to be 50mm. Thinking out loud - I can’t go for the 118mm BB as Shimano state the 112.5 will give me 47mm. If I had the 118 the chain line could be 44.75mm (4.5mm difference divided by 2). Is that right?

Could I get away with the 122.5BB and the shimano chainset to achieve a 47mm chainline. Can it be 3mm out? Would I need to go even bigger and get a 125mm BB (if they do such a thing).


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50- 1.7mm = 48.3mm

probably this will be just fine, depending on how close the chainrings come to the chainstays, and if you have a little adjustment left in the FD. Its very easy to try this out.

cheers

Brucey wrote:50- 1.7mm = 48.3mm

probably this will be just fine, depending on how close the chainrings come to the chainstays, and if you have a little adjustment left in the FD. Its very easy to try this out.

cheers

Hi Brucey, where do you get the 1.7from?

Sorry was looking at this on my phone and did not see all of your message above.

Thanks for your help. As I said, a long day.


I'm a trendy consumer. Just look at my wobbly bog brush using hovercraft full of eels