How important is bottom bracket length?
- Thread starter dfthe1
- Start date
Page may contain affiliate links. Please see terms for details.
uclown2002
Guru
- Location
- Harrogate
1mm extra per side shouldn't make any difference.
Last edited:
ColinJ
Puzzle game procrastinator!
- Location
- Todmorden - Yorks/Lancs border
I think it would be more of a problem if it had been too short!
accountantpete
OAP
See Q factor https://en.wikipedia.org/wiki/Q_factor_(bicycles)
Racing roadkill
Guru
I'd agree, you've not gone to a 127mm BB or something very different, you almost certainly will not really notice a 1mm each side.
Yellow Saddle
Guru
- Location
- Loch side.
It is not a Q-factor issue but a gearing issue. The front derailer only has a limited range of swing and a limited reach. The range is the effective distance it can go between left and right and the reach how far it leans over to the right in order to work between it's two range limits. If your BB spindle is too long or too short, it could put the FD outside it's range or outside it's reach, meaning that you won't have great gear adjustment.
If you fit a 115 instead of a 113, you may not have a problem providing that the 113 was in the first place the correct one. Sometimes the bike's history is not known and you cannot rely on the previous BB in there to give you the correct length. The centre point of the FD's range within its correct reach is published as a number in mm called the chainline. This would be the line measured from the centre of the seat tube to the centre of the middle chainring on a triple or halfway spot between the two chainrings on a double.
Q-factor has an indirect bearing but one doesn't change a BB spindle length to fiddle with the Q-factor. The Q-factor is determined by tyre clearance at the chainstay and crank clearance at the outside of the chainstays and FD chainline then chosen according to that by the manufacturer.
If you fit a 115 instead of a 113, you may not have a problem providing that the 113 was in the first place the correct one. Sometimes the bike's history is not known and you cannot rely on the previous BB in there to give you the correct length. The centre point of the FD's range within its correct reach is published as a number in mm called the chainline. This would be the line measured from the centre of the seat tube to the centre of the middle chainring on a triple or halfway spot between the two chainrings on a double.
Q-factor has an indirect bearing but one doesn't change a BB spindle length to fiddle with the Q-factor. The Q-factor is determined by tyre clearance at the chainstay and crank clearance at the outside of the chainstays and FD chainline then chosen according to that by the manufacturer.
Ajax Bay
Guru
- Location
- East Devon
So if one wanted to 'fiddle with the Q-factor' (measured as the lateral distance between the outside of the left (NDS!) and right cranks) how else could you do it? Change cranks? Move from a triple to a double?
If one sought to change the lateral distance between feet when pedalling, then, besides the above, longer pedal axles (cf Cavendish's reported preference) or moving the cleats (very limited adjustment normally available) seem the only options to a longer BB spindle or a different crankset.
Yellow Saddle
Guru
- Location
- Loch side.
You'd have to change cranks I suppose but I can't think of a reason why you would fiddle with it. It isn't something that's important, it is just something that is. Some bikes require more to clear the chainstays and others less. Our bodies easily adapt to small changes, be it a small enforced stride length, a step height, crank length or cadence.
* I saw that but I see that I've at least converted you half-way, since right was not annotated with DS.
Thanks all. The BB replacement is part of a bigger job to sort some indexing issues so I'm going to use the right BB to ensure I don't add some chain line problems as well. The BB I'm removing is the original one from when I bought the bike new so confident it's correct.
- Location
- the post-brexit wasteland
yellow saddle is right.
i once tried putting a double chainset on a bb that had been used for a ss setup. the end result was that the front shifter could not be adjusted to work properly, as the bb axel was too short.
i imagine the same would be the case when trying to put a triple on a double bb…
i once tried putting a double chainset on a bb that had been used for a ss setup. the end result was that the front shifter could not be adjusted to work properly, as the bb axel was too short.
i imagine the same would be the case when trying to put a triple on a double bb…
