Daft BikeBuild Question #3 - Drivetrain

[John the Monkey]

Hi all,

Just after a bit of a sanity check on my choices for the bike build.

In the bits box I have;

Front Derailleur - 105 9 speed triple
Rear Derailleaur - Tiagra 9 speed Long Cage
Hollowtech BB
Hollowtech Tiagra triple cranks
Rear Cassette - Tiagra 9spd 13-25

The rear spacing is 135mm, so the rear wheel will be built on a Deore MTB hub.

The original plan was to take the square taper BB and crankset from my current bike - I may still do this if I can shift the old BB, otherwise the hollowtech BB and crankset will go on.

Do I need to worry about chain line &c here? Would it be wiser to go with a square taper bb with slightly longer spindle (I think the current one is 113, maybe I should go to 118 to account for the extra 5mm at the rear?).

 

[PpPete]

My first thought was - no, surely only 2.5 mm needed as the extra 5 mm should be split symmetrically?

However Sheldon suggests that Shimano road gears are built for a 43.5 mm chainline and MTB for 47.5 mm, so presumably the Deore hub will place the cassette 4 mm further out than the ideal for Tiagra chainset.

If you're going for square taper the hollowtech cranks won't be any good anyway, you could leave the (presumably JIS taper) BB as is and put ISO cranks on it. Sheldon says they will sit 4.5 mm further out.

Someone with more experience than me PLEASE check this ? Bonj? Spandex? Jimbo?

 

[John the Monkey]

The chainset is a triple, which makes the chainline 45mm, according to shimano (the FD is for triple too).

The square taper bb feels a little graunchy, so changing it for a new one won't be a problem. If I go with a square taper bb, the current road bike gets the Hollowtech (it's all tiagra anyway, so no problems there) and the current chainset (Truvativ touro) goes on the new frame.

 

[John the Monkey]

Hmm - don't know why I didn't think of this before, but I've just had a look at how Surly build up the bike themselves (it's available built as well as frameset).

http://www.surlybikes.com/lht_comp.html

They spec a Tiagra front & rear derailleur with a 110mm spindled BB, so I guess I'll stop worrying about it.