RubeRad

OK, so I have an early 80's Raleigh Technium Pre (Suntour Accushift 6sp, 126mm rear spacing) that I'm putting my (large) 12yo son on. A friend from work told me that, according to UCI regs, juniors are limited to a certain max gear ratio due to developmental/skeletal/physiological issues. (He couldn't remember what it was, he guessed 52/14?) Anyways, he gave me an old 50/34 crankset and a 118.5 octalink cartridge BB to swap in. The BB is too long as it was meant for a triple, but I put it on so I can make some measurements and figure out what size I actually need.

So thing 1, trying to eyeball center of frame, the inner ring is about 50mm outboard and the outer ring is about 58mm outboard, so the center is about 54mm outboard. Again a little rough, the midpoint of the sprocket (between cogs 3&4) is about 22mm inboard from the right dropout, which would put it 126/2-22=41mm outboard. 54-41=13mm I need to shave off the spindle length.

OK so then I looked at chainline; the inner ring lines up really well with cog 2, and I would rather it line up with cog 4 (or even a little inboard of it, so the big ring would be a little outboard of cog 3). SB sez my center-to-center spacing is 5.5mm, two cogs over is 11mm, and I want a little more than that, so again, 13 seems good.

But also I checked my chainstay clearance. I measured again and again, and I'm seeing inner ring thickness 2mm, and depth from outer side of inner ring, to chainstay, is bang on 14mm -- clearance 12mm. So it would seem I can't afford to chuck a whole 13. Maybe 10? 10.5? Is 1.5mm enough clearance for the small ring? How much does a 34T ring flex anyways?

And then there's the matter of what sizes actually exist. I can't find anything on the webs smaller than 113 in octalink from Shimano (unless going up to Ultegra, which is too spendy for this bike). 118.5-->113 buys me 5.5mm inboard, which is exactly 1 cog; center of chainrings would be aligned between cogs 2 and 3, instead of cogs 1 and 2. Is that even worth it?

RubeRad

In the end I counted the cogs, and the freewheel is already 14-28, so I think that's probably low enough. I put the original 52/40 square taper back on and the chainline is perfect (almost like they designed the bike that way!).

demoncyclist

Is your son going to be racing this bike? If not, the UCI regs don't mean a thing- he will likely never use the biggest gear available just riding around. That being said, if you really want to swap out the crank and BB, I would use what you have, unless you find that it won't shift into all the gears properly and consistently. Then you need to decide if you want to spend the coin for the Ultegra BB or try to find another more easily sourced crankset and matching BB.

RubeRad

Thx, no he won't be racing (at least that's not planned right now, but he's only 12, if he gets really into cycling who knows?)

I was just thinking if it might be bad for him, maybe give him bad knees early or something, I'd want to avoid it. Also, as-is he never uses the big ring, so I thought it'd be good to give him some lower gearing. I guess we can just leave it as is; teach him not to choose a gear, but to choose a cadence, and find the right gear for it.

JiveTurkey

Which version of Octalink does your crank take? There's v1 and v2 and they are not interchangeable. The crank will determine the spindle length as well.

See here to determine the Octalink version and spindle length: https://sheldonbrown.com/bbsize.html#shimano

Shimano standard chainline for road double is 43.5 mm (halfway between 41 and 46, for inner and outer, respectively).

Don't fret too much about the chainline in the rear. The center of the cassette/freewheel doesn't fall in line with the crank's chainline and that's okay.

Shortest BBs I'm seeing online are 109.5 mm (v1) and 113 mm (v2).

Edited to add: According to above link, 43.5 mm has been chainline at least as back as 7-speed. 7-speed cassettes are about 3-6 mm wider than 6-speed. BUT that extra width went a little to the inside and a little to the outside (126 to 130 mm frame spacing), so the chainline did't change much, relative to the crank.

So......put the ruler away and just get the BB that the crank calls for.

RubeRad

109.5 would probably be ideal, but recalling the length of the splines, I'm stuck with v2, so I guess you're seeing the same as me, 113 is the shortest. Maybe that more expensive shorter one I saw was a v1.

JiveTurkey

Well... I, personally, think "ideal" would be the length the crank was designed for. They'll both achieve a 43.5 mm chainline with their respective cranks. I know you want to get things just right, but the chainline will be as it was intended with 113 and it'll work just fine with the 6-speed setup. Slap it on and forget about it.

RubeRad

Yes, I think the stock sakae cranks/sq.taper do have 40-something. The ritchey crankset and 118.5 BB are 54mm, so 113 should be about 48.5, which is 5mm high if 43.5 is ideal. I guess that's as close as I can get, and I should just go with it. At least I won't have to worry about chainstay clearance. And my son can get some practice with the front shifter.

RubeRad

OK, I just ordered the 68x113, should be here next week.

NEXT QUESTION: so this crankset has one good self-extracting bolt (good because the original broke a long time ago and my friend that gave me this crankset replaced it) and on the NDS the self-extracting bolt disintegrated last time I took it off. Y'alls here at BF helped me figure out before that I can still use my extractor to pull cranks, and I think the bolt itself (minus self-extracting collar) looks solid. Can I just use the bolt to press and hold the crank on, and use the extractor later? Or do I need to pay $10-$15 just for a replacement self-extracting crank bolt?

JiveTurkey

I don't mean to impune your measuring abilities, but if Shimano says your crank + 118 mm BB = 43.5 mm chainline. I'm inclined to go with that.

Your "eyeball" method may be giving you off results. The easy way to measure front chainline is to measure the distance between the chainrings and the seat tube and add it to one-half the diameter of the seat tube. Remember, double-crank chainline is measured to the halfway-point between the two chainrings (2.5 mm from each).

So, I'd measure it thus: 0.5*(ST dia.) + ST to center of inner chainring + 2.5 mm. (Alternatively, measure to the outer chainring and subtract 2.5 mm.)

Re: crank fixing bolts: If you have non-self-extracting bolts and a puller that works with Octalink, then you have everything you need to install, secure, and later remove the cranks.

I haven't taken a part a self-extracting bolt, but if the bolt holds the crank on the spindle, then, well, it's doing its job. If they don't appear to work, then you can get non-self-extracting bolts.

RubeRad

I think you're confused. I don't think Shimano is saying anything about my crank or chainline.

The existing, original equipment is a Sakae 52/40 on a square taper BB, probably Sakae as well, I forgot to measure the length of the spindle, but the chainline (which I measured not too carefully) is in the 40s, thus I assume close to 43.5 if that's an understood standard.

What I am switching to is a Tom Ritchey octalink 50/34 compact crankset (I forget the letters: VCS V-crank?) that my friend gave me, along with an 118.5 octalink BB. With that setup, inner ring is 50mm from frame center, outer ring is 58mm, thus chainline is 54 (right?). I have no idea what the spec chainline or BB length is for this crankset, or how to look it up. But dropping from 118.5 to 113 should drop my chainline from 54-ish to 48-ish.

Thx for confirmation on crank bolt though, when the 68x113 I ordered arrives I will install with confidence.

RubeRad

HOLD THE PHONE, I just had my wife double-check the 118.5 in the garage against this picture from SB:



and sadly I need V1. I just (tried to) cancel the eBay order.

Looks like V1 is very hard to find, although here's a 109.5 for $45+shipping. I'm not sure I love my son that much though, he might have to live in his 39 ring for a few years...

JiveTurkey

Sorry, I assumed Shimano for your crank.

Unfortunately, Sheldon's database doesn't include Ritchey cranks that take Octalink. Look up any model names and especially numbers online and you may be able to find info on the matching spindle.

If you can't be certain on the spindle length needed, then measure with the method I stated in post #10 . Chainring spacing is typically ~5 mm, so I'd be surprised if the chainline of the Ritchey crank is 54 mm (between 50 and 58).

Also, what you're missing in your math in the original post is that any reduction in spindle length will bring the crank chainline in only ONE-HALF that. That's because the reduction comes off the left and right sides. So, going from 118.5 to 109.5 mm will bring the chainline in 4.5 mm.

So, if you measure and find the current set up (118.5 mm) has a chainline of ~48 (45.5 inner, 50.5 outer), then the 109.5 BB should get you close to the 43.5 mm chainline.

RubeRad

50/58 inner/outer is what I measured, using the 118.5 BB I have access to, and it's certainly not right. Gotta do crazy things with the limit screws to adjust the swing range of the FD.

WHOA! That is an important detail that I missed. So 113 really won't do me hardly any good. Good thing I got to eBay seller to cancel before they shipped! Nothing lost.