Shimano front shifts

Nick Russell

Shimano’s front indexing cable pulls have always differed between road and mountain. You’ve never been able to swap front mechs. I say never, but there used to be a trick you could pull by clamping the cable the wrong side of the bolt on some MTB front mechs – until Shimano changed the design.

Given road STI, you’re stuck with a road mech. These mechs will shift smaller rings, but not a whole lot smaller and only if you can slide the mechs down the frame far enough. (This adjustment is often limited by too-high integral brackets, inopportunely positioned bottle cage bosses and random tube shapes!)

Go too small with your outer and only the front of the mech can be the regulation 2mm from its teeth. Towards the rear of the cage the smaller ring curves sharply down and away from it, leaving a gap – perhaps big enough for the chain to jump through! I use a 46 outer with a Tiagra mech designed for 50, so four teeth smaller seems to be safe enough.

Then there’s the outer-middle difference. This is vital to the functioning of a triple, since it determines the proximity of the middle ring to the cage; and for reliable shifts, the middle ring must be as close up to the cage as it can be. How close is limited by the mech’s deep inner cage.

The cage of the Tiagra mech pictured is designed for 11 teeth outer-middle difference, but here is working fine with a difference of only 10. I think the sharper curve of smaller rings helps me get away with less outer-middle difference than Mr Shimano intended. These are 46-36. One might not be so fortunate with 48-38. My inner is 24, by the way. Inners can generally be as small as will fit the chainset.

Chris Juden

This was first published in the December 2012 / January 2013 edition of CTC's Cycle magazine.