Front mech vs Bottom bracket axle sizes

[79stef]

In trying to establish the correct bottom bracket axle length size do you need to account for the sizing of the seat tube to keep the shifting distance correct?

For example, from what I've read, Suntour's MD XC Pro/ Comp crankset needs a 115mm Axle for its BB. But then do you need to account for this if the seat tube clamp for the front derailleur is 35mm as opposed 31.8mm?

 

[MuchAlohaNui]

In trying to establish the correct bottom bracket axle length size do you need to account for the sizing of the seat tube to keep the shifting distance correct?

For example, from what I've read, Suntour's MD XC Pro/ Comp crankset needs a 115mm Axle for its BB. But then do you need to account for this if the seat tube clamp for the front derailleur is 35mm as opposed 31.8mm?

Short answer: yes.

Seat tube diameter plays when measuring. If you have the parts in hand, install everything. Then measure. General rule of thumb is 47.5 mm to 50 mm but for oversized you're closer to needing 50 mm chain line.

 

[79stef]

Makes sense. So as its currently unknown, I'll need to buy the bottom bracket that I think is correct and employ some trial and error. Thanks

 

[Markybeau]

https://www.sheldonbrown.com/chainline.html

 

[MuchAlohaNui]

Makes sense. So as its currently unknown, I'll need to buy the bottom bracket that I think is correct and employ some trial and error. Thanks

If you have a bike stand, you can get away with using a ruler and straight edge and mock up some cardboard taped to the seat tube out to where the center line may be for the middle chain ring teeth and then calculate your bb spindle width.

If you don't get it right once your cranks and bb are together you can cheat by using chain ring spacers or shimming the actual cups on the bottom bracket a mm or two from one side or the other in regard to the bb housing.

 

[79stef]

Exactly what I needed. Thank @Markybeau and @MuchAlohaNui

 

[hamster]

One other point to add is that old style front mechs (high band) generally can run closer to the seat tube than low-band types.

 

[79stef]

One other point to add is that old style front mechs (high band) generally can run closer to the seat tube than low-band types.

Thanks. It will indeed be a high band front mech.