I'm putting together a bike with a BB92 bottom bracket (oversized shell with inboard bearings) and I'm getting in a twist over chainline.
Perceived wisdom always used to be that the ideal chainline was to line up chainset middle with cassette middle. However, I can't seem to achieve anything like it.
The Crankset is a Hollowtech II which calls for a 50mm chainline. With 1 x 2.5mm spacer fitted (as specified) the chainline is more like 53mm but the Q factor is equal i.e. pedals are exactly equal distance from middle at 73mm. However looking down the sprockets middle chainring lines up more like sprocket 8 instead of 5 as it should.
If I move the spacer to non-drive side I can improve chainline but then throw the pedal line out by 2.5mm.
What's more important better chainline or keeping pedals equal distance off centre?
Anyone got any experience fitting BB92 bottom brackets? I haven't fitted the chain yet so I may be worrying unnecessarily but it just doesn't look right.
Chainline question
Moderator: Moderators
If you have your pedals central on other bikes, keep them central. I've previously given myself physio problems by having my pedals offset, and it's better to wear out bits of your bike than bits of your body.
You may be able to fix the chainline issue though. Is the BB long enough that you can fit another spacer in the other side, to even up the Q factor? Alternatively, I now have a thick washer in between the right hand crank and the pedal, which works for me.
You may be able to fix the chainline issue though. Is the BB long enough that you can fit another spacer in the other side, to even up the Q factor? Alternatively, I now have a thick washer in between the right hand crank and the pedal, which works for me.
I spent a while looking at all my other bikes. They seem to have a variety of chainlines from lining up with middle sprocket exactly to lining up with 7th or 8th. All have pedals equal distance though so, as you say, that should be left alone.
With Hollowtech you have very little room for playing with the alignment. It works on a fixed distance of 96.5mm between crank faces and you basically add your BB shell width, BB cups + either 1 or 3 x 2.5mm spacers to achieve that (for normal BBs, 68mm shell = 3 spacers, 73mm shell = 1. However BB92 uses oversized shell and thinner cups). I don't know if the splines have enough length on them to still tighten up if I loose the 2.5mm drive side spacer ...but even if it did it would bring the pedal in at the same time.
The wheel hubs and dishing is correct so I think I'm stuck with it as a 'feature' of the frame design.
With Hollowtech you have very little room for playing with the alignment. It works on a fixed distance of 96.5mm between crank faces and you basically add your BB shell width, BB cups + either 1 or 3 x 2.5mm spacers to achieve that (for normal BBs, 68mm shell = 3 spacers, 73mm shell = 1. However BB92 uses oversized shell and thinner cups). I don't know if the splines have enough length on them to still tighten up if I loose the 2.5mm drive side spacer ...but even if it did it would bring the pedal in at the same time.
The wheel hubs and dishing is correct so I think I'm stuck with it as a 'feature' of the frame design.
With Hollowtech II, I'm sure you should be able to run the BB without a spacer at all, which will improve your chainline. You can then space the right pedal 2.5mm outwards, either with washer round the thread (there should still be enough thread to hold the pedal in) or else with a longer axle. I know Speedplay make longer axles, I don't have a clue about any of the other pedal companies.