jchabalk 

Hi there, I currently run a Sugino XD600 triple with a 110x68 bottom bracket.

I'm going to change over to a compact (50/34) and am wondering if i should change out my bottom bracket to something a little shorter.

The way it's set up right now the chainline is pretty much perfect. Chainline and choosing bottom bracket spindle length is not my strongest pursuit. If i keep going the way i have been my bottom bracket collection will look like john popper's arsenal of harmonicas. unfortunately i don't have a 108 yet or i'd just try them both out.

operator

The compact will tell you what length BB it needs - at any rate who makes a square taper compact?

jchabalk 

thanks operator, sugino makes one, the Swiss Cross Compact. there may be others out there but this is the only one i've found. my friend has one and is really happy with it. I've used their triples for a while and always liked 'em.

EDIT: i don't know that the crank says what bb spindle length it needs. with the frame i have i think the "ideal" chainline is 43.5 with a compact, which if i knock 2mm off a 110 (45mm chainline will get me there). the frame is a gunnar crosshairs mostly, if anyone knows let me know, otherwise i'll experiment a little and let you know what i figure out.

operator

Recommended BB axle width: 110mm(for a 45mm chainline).

jchabalk 

cool thanks operator, i think we're on the same page. i'll start with my existing 110 which sounds like it might be (probably is) right.

operator

Standard road chainline is 43.5...

TLCFORBIKES

Shimano has the 2350 Compact Crank. It uses a 110 square taper BB and comes in 170 & 175 lengths with 50/34 gears. Cost for both crank & BB is under $100.

DaveSSS 

The chainline is not measured the same on a double as a triple, so comparing the two directly will produce the wrong spindle length. A 45mm chainline on a triple is measured to the tip of a tooth on the middle ring. The 43.5mm chainline on a double is measured to a stupid point out in space, centered between the two chainrings.

In reality, the big ring on a double will measure 47-48mm to the tip of a tooth and a triple measures about 5mm more, or 52-53mm. If the crank's are made the same it would require a 10mm shorter spindle, but that's not always the case. You can't assume that using a 10mm shorter spindle would move the big ring 5mm to the left. The manufacturer should always list the spindle length needed to produce the normal 43.5mm chainline for a double crank.

Suburban Grind

I run a Sugino XD-600 on a 68 x 113mm bb. The bb, a Shimano UN54, is the limiting factor in my setup, not the clearance from the smallest chainring to the drive side chainstay (almost 6mm on my frame). I wonder if the OP had the crank faced or filed to get the 110mm bb to work?

I'm also considering a second compact crank setup for strictly road work, the Sugino XD-800 Alpina with 48 and 34 teeth chainrings on 110mm B.C., and will probably use the recommended 110mm bb for that. I was unaware of the Sugino Swiss Cross Compact, maybe I need to look into it.

SmallieBiggs

I think this is a case of marketing jibber-jabber confusing people.

Most road bike cranks have for years had a bolt-circle diameter (BCD) of 130mm. This is fine for large chainrings, but limits the smallest chainring to about 38 teeth. Therefore, for low gearing you needed a triple. Then somebody realized that by using the older mountain bike standard of 110mm BCD, a smaller inner chainring would fit (down to about 34 teeth) and there is less need for a triple. The stupidity is that instead of just calling it a ‘mountain crankset’ or ‘110mm BCD crankset,’ somebody decided it would be better to make up a new term – “compact crankset’ – to describe 110mm double chainring cranksets.

Well guess what - The Sugino XD600 is a mountain bike crankset with a 110mm BCD crank. You already have a ‘compact’ crankset. All you need to do is remove the small chainring and adjust the front derailleur*. Unless there is some other feature you have found on a ‘compact’ crank, or if your Sugino crank is broken, you are wasting your money by changing it.

*switching to a slightly shorter bottom bracket, and/or a double-chainring-specific front derailleur might give you slightly better shifting, but are not necessary.