queenmildew

hi all -- working on a build for a surly cross check. I have a pair of 110/74 bcd Sugino GT cranks that were born as a triple but I'm hoping to convert to a lower-geared double, something like a 40t x 24t. Sugino made a double version of the GT but it can only accommodate 110bcd chainrings, limiting the low end to 34t. I commute on a triple right now (different bike) and enjoy the range but have little use for the high end, & could use a slightly spinnier ratio than my current 1:1 30tx30t for some of the steeper grades & hillier trails in my city.

The CC frame has a standard 68mm width shell & the cranks require a JIS square taper BB. According to velobase, the triple model of this crank wants 3U spacing, but the purpose-made double is listed as 124.5mm. I've seen 122mm recommended to another user doing a similar conversion on the same crankset. Given that I will be removing a chainring, I imagine the latter figure would be more relevant, but just wanted to check and make sure I'm not missing anything.

I'm usually good with numbers but there are just too many standards to keep track of here and so many weird equivalencies that even the Sheldon database is not helping! I found a Shimano cartridge BB that is English threaded, JIS spindle, and 68mm x 122.5mm spaced. would this model work well for my purposes? and/or have any experience doing similar conversions? thanks all!

pdlamb

Sanity check on BB standards? That's like sanity check on the insane asylum.

Subject to correction by more knowledgeable folk, I think the axle length depends more on the crank. What have you got on now, and what exact model are you planning to put in there?

oldbobcat

The only way to know for sure, unless you have the crank manufacturer's data sheet, is to put the rear wheel with cassette on the bike, install the bottom bracket and crank, and determine that the centerline from between the two chainrings to the center of the cassette aligns with the centerline of the bike, plus or minus a millimeter or two.And if the spindle falls a little short on the drive side, it can be made up with a 1- or 1.85 mm Shimano/SRAM freehub spacer. With the information you're giving us, the 122.5 mm spindle should work.

FBinNY 

There are dimension charts for these spindles.

Start by checking chainline, decide how much you want to move your crank inward, then look up the current spindle and find the nears match with the right side shorter by the right amount.

The underappreciated beauty of classic BBs is that you could change chain line easily because there were so many options.

queenmildew

perfect ,thank you! I wish i had a spare one sitting around to test it this way but the sealed shimano ones are only $20 anyways so I'll take a chance on the 122.5 :-)

SoSmellyAir

Why not just reuse this crank with two chain rings: the larger one mounted in the middle position, and the smaller one in the inner position? That way you can use the same BB and same spindle length.

What I have learned, from doing a 3X to 1X conversion, is that while you can measure and calculate the new spindle length, the simpler way is to (1) just install the new crank onto the existing spindle, (2) see how far off the chain line is with respect to the cassette, (3) buy a new BB with the appropriate spindle length based on #2.

Lastly, for $5 to $10 more, you can buy a much better BB:

Tange LN-3922 Square Taper Bottom Bracket – The Bikesmiths

Tange LN-3922 Bottom Bracket | (benscycle.com)

Hondo6

OP: in case you were wondering about pdlamb's comment, this goes a long way towards explaining it. BikeForums automatically converts URLs here to HTTPS (secure HTTP), and this URL is nonsecure. Simply cut and paste what follows into your browser's URL window and then replace (slash) with the character "/" and (colon) with the character ":".

http(colon)(slash)(slash)pardo.net(slash)bike(slash)pic(slash)mobi(slash)b.bottom-bracket-standards.html

Germany_chris

I run a TA double with a 110mm bottom bracket on my Cross Check