Sugino RD-48 and BB-7418AL Compatible?

James said:

I have a 4-yr old road bike with a 9spd Tiagra triple. I only use the
bike for commuting on a flat route. The crankset weighs a ton, and I
carry that weight for nothing. No hills, not much need to shift the
front gears. I was thinking of going with a single chainring and
removing the front dérailleur. That would lighten the bike by almost a
pound and make the bike simpler (more reliable), while still leaving
me with 9 gears to choose from.

I'm considering the Sugino RD-48, a 48-tooth single speed crankset.
One vendor says it's supposed to use a 110mm road BB or a 103mm track
BB.

My existing BB is a Tiagra BB-7418AL square taper 68x113. Will the
RD-48 square taper hole and the BB spindle match up? Also, what about
the 110mm crank spec vs. the 113mm BB length -- does that 3mm
difference matter?

If you were going to make the bike a single speed or fixed, then
having a precise chainline would make a difference. Since you have
gears in the back, the exact placement of your chainrings makes no
difference. The 113 should work fine.

Not having a front derailleur I've heard makes having the chain bounce
off over some bumps a (perhaps remote) possibility.

Joseph

James said:

I have a 4-yr old road bike with a 9spd Tiagra triple. I only use the
bike for commuting on a flat route. The crankset weighs a ton, and I
carry that weight for nothing. No hills, not much need to shift the
front gears. I was thinking of going with a single chainring and
removing the front dérailleur. That would lighten the bike by almost a
pound and make the bike simpler (more reliable), while still leaving
me with 9 gears to choose from.

I'm considering the Sugino RD-48, a 48-tooth single speed crankset.
One vendor says it's supposed to use a 110mm road BB or a 103mm track
BB.

My existing BB is a Tiagra BB-7418AL square taper 68x113. Will the
RD-48 square taper hole and the BB spindle match up? Also, what about
the 110mm crank spec vs. the 113mm BB length -- does that 3mm
difference matter?

BB-7418 is an old Dura-Ace model, not Tiagra. (congrats!)

Taper should match fine, length you can fudge a little since you're
not going single-speed. I'd just worry about the chain falling in your
smallest gear, since the chain angle will be similar to a big/big
combo, but no FD to keep it from derailing. I'd say try it, and if it
doesn't work, get a UN-54 or similar in a shorter spindle. With the
crank mounted, you can measure your chainline, and calculate how much
shorter it needs to be. The chainring should line up with the 5th
sprocket of your 9-speed cluster. Measure the distance from that
sprocket to the dropout and subtract it from 65mm (the center of a
130mm hub), then measure from the center of the seat tube to the
chainring. Those distances should be pretty close if the BB length is
correct.

Hank Wirtz said:

James said:

I'm considering the Sugino RD-48, a 48-tooth single speed crankset.
One vendor says it's supposed to use a 110mm road BB or a 103mm track
BB.

Quoted message said:

My existing BB is a Tiagra BB-7418AL square taper 68x113. Will the
RD-48 square taper hole and the BB spindle match up? Also, what about
the 110mm crank spec vs. the 113mm BB length -- does that 3mm
difference matter?

BB-7418 is an old Dura-Ace model, not Tiagra. (congrats!)

Taper should match fine, length you can fudge a little since you're
not going single-speed. I'd just worry about the chain falling in your
smallest gear, since the chain angle will be similar to a big/big
combo, but no FD to keep it from derailing. I'd say try it, and if it
doesn't work, get a UN-54 or similar in a shorter spindle.

Thanks to you and Joseph for your informative replies. I'm going to
give it a try.

James said:

I have a 4-yr old road bike with a 9spd Tiagra triple. I only use the
bike for commuting on a flat route. The crankset weighs a ton, and I
carry that weight for nothing. No hills, not much need to shift the
front gears. I was thinking of going with a single chainring and
removing the front dérailleur. That would lighten the bike by almost a
pound and make the bike simpler (more reliable), while still leaving
me with 9 gears to choose from.

I'm considering the Sugino RD-48, a 48-tooth single speed crankset.
One vendor says it's supposed to use a 110mm road BB or a 103mm track
BB.

My existing BB is a Tiagra BB-7418AL square taper 68x113. Will the
RD-48 square taper hole and the BB spindle match up? Also, what about
the 110mm crank spec vs. the 113mm BB length -- does that 3mm
difference matter?

FYI:
With a 103 mm bottom bracket, that crank will give you a 42 mm chainline
on the outside position (40 mm on the inside).