I have an ‘83 Trek 520 frame/BB that originally was fitted with a SR Super Custom triple chainring crank. I have an SR Super Custom double chainring that I was hoping to use. Does anyone know if the axle length was a different for the two crank arms? Does this make any sense, haha?

It's highly unlikely that the triple and double cranks would've had the same bb spindle length- the triple would have a longer spindle than the double would. You probably can get away with just slapping the double on, though your chainline won't be ideal. If there's enough clearance between the seat-stay and the small cog of the freewheel you could put a spacer behind the freewheel which might give you a slightly better chainline...

Yes, the double chainring crank would use a shorter spindle than a triple crank. BITD, we’d try different spindles until we found one that worked with the frame and crank we had. If the chainring contacted the frame we’d try longer. If there was too much clearance, we’d try shorter.

A rough guess would be a spindle 4mm shorter than the existing one. That’s assuming the difference is only on the right side- which is a tall assumption.

If it were my bike, I’d install a cartridge BB that’s s little bit shorter than the existing BB. As long as the chainring and crank arms clear the frame you’ll be OK.

The chainring doesn’t contact the frame at all when installed…

With freewheel, maybe. With cassette, I wouldn't try to space the cassette out at all; I wanted to do that because my chain would sometimes jam hard between the small cog and dropout, so I wanted to make that gap smaller, and put a spacer behind the cassette, however that greatly reduced the engagement of the threaded lockring, and small cog on the freehub shoulders, enough that I did not feel things were sound, so removed the spacer immediately without riding it that way. I looked online, no one seems to make a lockring with longer thread engagement. And changing the rear derailleur from the inferior Dahon compact design to a normal RD and mounting location, cured the chain dropping off the small cog.

Agree, though on an '83 I think it's more likely to have a freewheel than a freehub (if the wheels are original.....). [edit] actually, a 520 would've originally had Helicomatic hubs, which kind of come with their own kinda headache, so adding a spacer probably isn't a good idea..

Actually, I bought these wheels to use for this bike: https://www.ebay.com/itm/335875445800 I got them for $50 for the pair, which fits my budget(:

A spindle designed for a double will usually be shorter than one for a triple. The ideal outcome is where the chainrings lands (after tightening the crankarm bolts down hard) about 5mm from the right chainstay and the crankarm does about the same. A larger gap will result in a higher Q-factor for less pedaling efficiency, and a wide separation between the rings and the frame will lead to poor chainlines and front shifting.

Recommendation: seek out your local bike co-op and sift through their bin of old spindles. Find something that has the same specs as your old spindle, but a few mm shorter on the drive side.

Other recommendation: do not ride wheels with steel rims. At our local Co-op, we chop these up and recycle as soon as they land, as being a risk to life and limb.

No worries, aluminum wheels here! I’ll check some of our co-ops locally and look for a shorter spindle.

Miche and SRAM lock rings tend to be a thread longer than Shimano. Wheels Manufacturing might have longer lock rings, like those supplied with the Campy 8-speed conversion kits I bought from them years ago (NLA).

If the spindle is offset (longer on the drive side), you maybe able to flip the spindle to bring the drive side double closer in. You might have a little extra spindle on the non-drive after that, but it will possibly give you a better chainline. I remember reading about this, and in all likelihood I may have done it on one of my bikes and have since forgotten.

Y'know, I'd heard of Helicomatic but never knew what they were about. Just looked it up, HAH, basically a freehub with the same bearing positions, downfall seems to be the balls are a bit small and says more failure prone.

Thanks! That may be usful info for the future.

Congrats on the vintage Trek. I generally like the vintage components over the modern day replacements, but as another member stated earlier, the Bottom Bracket is where I generally replace a vintage part with a modern part especially if the original bb is not a high end unit. IMO, a cheap modern sealed cartridge bb is smoother than a cheap vintage bb. Also if the bb shell has not been faced, a sealed unit is the way to go. A new sealed modern cartridge bb can be had for less than $20 on sale.

All true. However...

Cartridge bottom brackets are great, but for me, start to get a tiny bit of slack after a year of daily commuting use, no way to readjust, and then wear accelerates due to lack of proper bearing preload, the force concentrates on much fewer bearings.

Cup/cone/loose-ball BBs can be adjusted to take out slack, but the insides are not sealed from contamination via the seat tube, etc.

Changing to a 2-piece hollow spindle ("Hollowtech II" style) crank was a revelation, and I found one in vintage style, 5 arms on 110 BCD, crank/chainrings/bearings complete for about the cost of 2 chainrings alone. The external bearings (in my case, the common standard of ISO External) are smooth as silk, well sealed, and readjusting for preload takes all of a minute, so much more durable; Loosen left arm clamp bolts, retorque spindle end cap, retighten clamp bolts. (I also clean out any dirt between arms and bearings on both sides as part of this.) And it's all backwardly compatible into BSA/English BBs. One of THE smartest designs I have ever seen. But no adjusting for chainline with a different length cartridge, the chainline spec for the crank needs to be perfect from the start.

Interesting, I have zero experience with external bearing bb. I can see how they are an improvement technologically over cup and cone and cartridge bb. I like traditional (press on??) cranks, one of the best looking parts on a vintage bike imo. Can external bearing bb be used with traditional cranks?

Nope, it's a complete system. I find most of the new cranks ugly, most especially the Shimano 4 arm asymmetical cranks. But if you look, you can find traditional looking hollow spindle cranks. This is the one I bought, it came with steel or aluminum rings, was $60 at the time including bearings. Wrench for bearings was $20 but I bought a 4-way one that will fit 4 different bearing spline patterns. I also like that this crank is "road" so has low Q-factor, straighter arms and pedals closer together than a mountain crank.

The chainline and Q-factor will be no different than a triple crank, which shift just fine. It's only a 4mm difference - the thickness of a chainring. That amount is not normally going to matter at all.

I would ignore Duragrouch's bearing pre-load religious beliefs. He has been told before that he doesn't know what he's talking about, but just keeps on spreading this baloney. Cartridge BBs of all type last a very long time, and none of them are built to be preloaded because they don't have angular contact bearings - including external bearing types.

I admit not knowing much about this stuff, but that’s exactly what I was wondering. 4mm seems so small a difference…

Don't get me wrong - it is enough of a difference that you can tell looking at it, and it would bother me on my bike, but it can usually be made to work just fine. I would be inclined to just bolt it on and see if it works for you or not.

Some modern index front derailleurs may not like it, though.

Using old Cyclone front/rear derailleurs.

I doubt you'll have any problems.

I have an SR double crank on one Trek and a triple Shimano crank on another. The double has a spindle length of 119mm and the triple has a 121.5mm.

Proooobably on most systems. I will only say that, due to the extremely fat seat tube and large FD adaptor thickness on my bike and an excessively large inboard flange lip on my FD, I had to space my crank outboard 4mm in order to be able to shift to the inner ring. Dropped chains when shifting to the inner ring when on the inner cogs (and left crank arm came loose due to 4mm less clamping length on the hollow spindle). Nope. I took a grinder to the oversize FD lip in the area of interference, removed the spacers so that chainline was exactly as designed (43.5mm), and the bike thereafter shifted perfect. I don't recall if the chainstays on my folder are shorter than typical, but 4mm off spec made a big difference. 4mm too far inboard also would have been a problem, as with the larger difference on a 50/34 versus old 52/42, the chain would have been rubbing the inside of the large ring when on small ring and outer cogs. A lot of specs can be stretched a bit, like RD capacity on many derailleurs, but some specs need to be dead on, at least on some frames.

Thank you, that’s super helpful!! Mine is a 121.5mm stamped with “3SS” on the shaft. I went ahead and installed my double and this is how far the inner ring is from the chainstay tube: