Hi,
I'm looking to take a mountain triple, remove the outer chainring and use it as a double on an 80's road bike. The triple I have is a Sugino RD2 600 26-36-46 which is designed for a 113mm bottom bracket. The bike has a 110 (I actually measured 111, it has french parts, so who knows) bottom bracket. I took the original double off and put the Sugino on, just to see if I could rig it up. The inner chainring does clear the chainstays, however it appears to be significantly farther out than the double it replaced. Just on a visual, the small chainring seems to line up with the 4th cog (with the 1st being the largest) of the 6 speed cogset. Looking at the two cranksets visually, the original is a double 42-52 with a single spider (and spacers between the spider and inner chainring). If you look through it, between the chainrings, the mount (where it mounts onto the spindle) is almost exactly between the chainrings, so the inner chainring sits inside the spindle mount.
When I do the same on the Sugino, the mount is pretty much even with the inside of the inner chainring. I haven't measured the actual dimensions, but it appears that the Sugino inner ring, on the same bb, sits 5-10mm farther out than the original.
So my initial thought is, and I've never worked on cranksets, could I add spacers to the inner and middle chainrings of my Sugino to decrease the chainline? Or do they make mountain triples that would have a lesser chainline? I didn't buy the Sugino for this purpose, I pulled it off another bike that I sold, so I'm not stuck on using it, in fact, it will likely go on my next commuter.
I suppose another option would be to try and get a smaller (lower tooth count) double crank, however I haven't seen any that have small ring in the low thirties, high twenties. Rivendell sells a low wide double with 29-40, but it is essentially a Sugino RD2, just like mine, but with a chainring guard in place of the outer chainring.
I suppose I could try to run it as is and see what happens, but with the shorter chainstays of a road bike, I think the performance will be marginal at best, and at the worst possible place, the low gear.
If it is possible to add spacers to the Sugino, I would want to know what to search for, I don't know how they are spec'd, etc.
Thanks!
Jon

I should also mention that swapping out the chainrings on the original double is not a likely option as it is an old crankset with an odd BCD.

You'll want a shorter spindle, to improve your chainline. Just how French is this bruiser? What style of square taper BB does it have-- ie, is it a loose-ball, or a cartridge BB? Is the crankset a 135bcd? Which frame? What's your vision for this build?

Basically, give us s'more info of what, specifically, you're working with. Sugino triples are a well-known character in these drivetrain stories, but you have to introduce us to the other key players....

I don't know that I can find a shorter spindle, this one is already close to 110. I've never taken apart the bb, but it must be loose ball - from the mid-80's. The crankset is 118bcd. It's InterCycle branded, made by Sakae. My goal is to get to lower gearing, the cogset is 14 - 28 six speed, which I'd prefer not to tinker with to avoid cold setting the rear dropouts. I have a shimano un55 bb 113mm, and it looks visually to be practically the same as the current spindle. It was on my previous bike with the Sugino crankset.

triple crank the middle chainring should be in about the right chainline..
to single:
remove the big and granny .

screw in the inner stroke screw on the FD so its a chain keeper then , and you can remove the cable and shifter.

3>2 , where the middle chainring Is of a triple, that is the space between the chainrings of a double..

I did something similar on a Cannondale that had quite tall gearing and it worked out fine. I removed the granny though and ran center and big ring. The center fell on the center and the big ring lines up nicely with the small cogs. I later lowered the FD.

It's close, but not quite. The middle ring of a triple sits about 1.5mm further outboard than the centerline between the two rings of a double. (45mm triple chainline vs. 43.5mm double: https://sheldonbrown.com/chainline.html)

If using the middle ring mounting position of a triple crank for a single ring front, ideally, you'd use a 3mm shorter bottom bracket than intended for that crank. Of course, it's such a small difference that you'd likely not even notice.

For the OP, a MTB triple chainline is 47.5mm (possibly 50mm if on an oversize tubed frame). A double outer chainring on a road bike should be at 46mm so again you'd want to move the crank in by 1.5mm (or 3.5mm if oversize) if using only the two inner positions.

Maybe on yours , but not as a broad generalization I can only paint with a broad brush ,

not having the bike on the repair stand in front of Me. in Person.

I'm not quoting my own numbers but the industry standards published on Sheldon's website. If he's wrong then I've given out a lot of misinformation over the years.

It's not a life-and-death thing anyway. Some people prefer to have the big ring in line with the very smallest rear sprockets, others (like me) use the shortest BB they can get away with so that the big ring works nicely with the whole cassette.

As with any gearing selection, the best answer is entirely dependent on the person riding the bike (and how and where they ride).

I am very familiar with Sugino cranks and Tange/Shimano BB's. I think you mean an XD 600(110 BCD), as an RD cannot use a 36t midle ring(130 BCD). They are designed for a 45 or 47.5mm chainline with a 110 or 113 cartridge BB.. A 107 and 110 Tange or Shimano has the same drive side length. A 103 will move your chainline in 2.5mm, but as these cranks have built in 74mm spacers, depending on your frame, they mat come close to hitting your frame.

Using just the two inside rings and outside chain guard is a great idea if you can get the gears you want. Many use say a 30/42 or 44 or even 46 for Road riding. Anywhere from a 24t on up for MTB use. It depends on you and your frame clearances.

I just took a visual on the chainline when I put the sugino on. The bb is Sakae and I measured it at 111mm, so it's either an odd size or I'm off by 1 mm. I have a shimano cartridge 113mm bb and I just held it up to the bb and visually it seemed very close to the same spindle length that was already in it. I would prefer not to take the bb apart unless I have to. I may well be able to go with a 107mm bb since the inner ring of the Sugino is 26T, I don't think it would hit the chainstays.
At any rate, I did not even try to put a chain on it, as the inner ring seemed to line up with the 4th cog (counting from the larger to the smaller). I'm even curious now to put the original double back on and just visually see where it lines up, I don't see how it could be more than one cog closer to the wheel. Maybe it will work fine, I'll give it a shot and see what happens.

A Triple is naturally going to demand a wider chainline of course, measured from the center of the seat tube, to the middle of the middle ring. A double is measured to the middle, in between the two rings.

Your Sakae BB and the cartridge BB you have, although they may appear equal or near equal drive side length , may not set equally in the crank arms, making for slight differences in your chaninline. Yes , a JIS taper is supposed to be a JIS taper, but they are not precise and do vary from brand to brand.

Jon
Did you run the numbers on going from 42-52 to 26-36?

I like to use this program.
https://home.earthlink.net/~mike.sherman/shift.html

On the bike I showed in the above post I also had a 42-52 with a 13 thru 24 cassette (7 cogs)
I would have liked the 26t chain ring maybe but would have lost too much on the high end doing it that way even if it had cleared the chain stay and I could have lowered the FD enough both I couldn’t do.

The chain line was much better working off the larger rings 32-44 and the reduction in low gearing was quite a bit and as practical as I could have without losing too much on the other end. The center position (32t) works across the whole cassette and gives me gear inches between 35 and 64. The 4 inner largest cogs are redundant so I don’t care about chain line but the 3 outer smallest cogs that chain line straight extend my range of gear inches 72 thru 89. For me that was exactly what I wanted for the hills around here. The nice thing about the gear calculator is it will show you at what speed and RPM you will spin out. My cadence doesn’t get much above 80 RPM and that gave me a top speed of about 22 MPH. More than fast enough for me and on down hills I just coast.

What gears do you have in the back?

So I put the old double back on and measured the chainline, and visually saw that the inner ring lined up with the third cog. Then I put the Sugino on and it's inner ring lined up with the 5th cog and the chainline was over half an inch farther out. So I took off the bottom bracket and put in the Shimano 113mm, then put the Sugino back on, and what do you know? Same chainline as the double had. So I threw a chain on it and the inner and middle work fine. However it's geared quite low now, so I will probably try to use all three rings, which will require a new rear derailleur.
Thanks again for all the input, I really appreciate it.
Jon