Bicycle Mechanics - Convert SS MTB to Road Crank?

I use a single speed mountain bike as a winter commuter. Would there be any benefit to swapping out the 104mm bcd crankset for a 130? One of the reasons would be to be able to use a stainless steel 42 tooth chainring that is only available in a 130 (I find aluminum gets eaten by salt fairly quickly). What is the easiest way to get the chainline right - I assume the BB spindle length will have to be longer. I have a Surly single speed hub on the rear.

Let's see...

I'd start with measuring the rear chainline (as per Sheldon Brown). I think the SS Surly hub chainline for their 135mm axles is 46.5mm while their 130 axle runs a 42.5mm line. My guess is that you're running a 135mm axle with a 46.5mm chainline (be sure to double check this tho).

If it's the 135/46.5 then it's probably just a matter of selecting the best spindle length to gain a usable chainline (again, as per SB).

The shorter axle might make things more difficult...

Once you know the rear chainline, you can start determining a ring size that will both clear your chainstay while meeting a "workable" rear chainline (by way of spacers, rear or front, as needed).

Thanks. It's the 135mm width. I think the chainline is around 52mm but I can check. It's a thread on freewheel so I am guessing you can't adjust the chainline on the rear. Does choice of crank matter? or do they all sit in the same place relative to the spindle?

Does choice of crank matter? or do they all sit in the same place relative to the spindle?

No, they definitely do not sit in the same place relative to the spindle (as far as the position of the chainrings.) If you get a different crank, especially when changing from road to MTB you will likely need a different BB/spindle.

here's my experience....

i had a nice spare road crank, probably sugino or SR (sakae) and a 97 gary fisher MTB. i thought i would switch cranks. seeing as how the MTB had a freehub, and i had a variety of spacers, i thought that chainline wouldn't be an issue. and as i turned out, it wasn't.

first off i should say that i did not replace the BB, at first.

i removed the old crank and put the drive side crank on. there was no chainring attached. the spider (sans chainring) would not clear the chainstay:notamused:. so i put on a different BB with a longer spindle. it cleared. yeah:thumb: so i put on a chainring (36 teeth, i believe) and it wouldn't clear the chainstay :notamused:, so i moved it to the outside (large chainring position). and it cleared. yeah:thumb: then i put on the non drive side crank arm and it wouldn't clear the chainstay :notamused::notamused:. i was out of BB's so i gave up. geometry's different. learned a lesson.

yea, +1 ...
Chainstay bend to clear a fat tire, on the inside, may limit the size of chainring,
you can run on the outside.

Ok, it looks like I was wrong about the 46.5mm chainline. The Sheldon Brown site indicates that most SS MTBs run a 52mm chainline, which is very close to your measurement.

Below is a link for general bottom bracket info. The next link down has quite a bit of specific BB information that might help you determine what you'll need for this conversion.

Note: most crankset manufacturers provide crankset specific information that explains which spindle length you'll need to achieve a given chainline.

http://sheldonbrown.com/gloss_bo-z.html#bottom

http://sheldonbrown.com/bbsize.html

HTH

here's my experience....

...... geometry's different. learned a lesson.
I ran into the same thing with a '93 Trek 7000. The fat and splayed out chainstays needed to clear large tires required an unusually long bottom bracket to clear both the chainrings and the non-drive-side crank arm. A Shimano 127.5 mm cartridge, the longest they made, just cleared.

Thought I would report back on the outcome of the conversion. I use a Surly single speed mtb 135mm rear hub with a ss freewheel. Got a used 105 crank and a Stainless steel Surly chainring (130 bcd). I didn't have to do too much but get a longer bb spindle since it wouldn't align with the rear. Played around with measurements and they didn't actually come out right. I just had to assemble it a couple of times - The final result was replacing my 110mm spindle with a 115mm. The 110 needed replacing anyway. Chainline looks really good and no noise. It's an older frame so the chainstays don't go out too far. Lots of room for the crank to go by the chainstay. I'm using a zinc-coated chain since this is a winter bike (that's why I wanted to use the stainless ring)

It seems to be more of an art than science though, to get everything lined up. I'm happy now though!

Congrats! Getting these single-speeds all lined up takes trial-and-error sometimes, and then you end up with a small pile of leftover parts. :thumb:

a given crankset should specify what width BB they think you should use and what standard CL that gives. lets call that bb0 and cl0.

now, you know what CL you need for you bike, so lets call that cl1. assuming a symmetrical cartridge BB here... we take (cl1-cl0)*2 + bb0 => bb1

see? simple.

example: your frame wants a cl1 == 52. a given track bike SS crank says its for cl=42 and uses a 108mm bb0.

so... (52-42)*2 + 108 == 128. you want a 128mm BB.

good luck with that. :)

a given crankset should specify what width BB they think you should use and what standard CL that gives. lets call that bb0 and cl0.

now, you know what CL you need for you bike, so lets call that cl1. assuming a symmetrical cartridge BB here... we take (cl1-cl0)*2 + bb0 => bb1

see? simple.

example: your frame wants a cl1 == 52. a given track bike SS crank says its for cl=42 and uses a 108mm bb0.

so... (52-42)*2 + 108 == 128. you want a 128mm BB.

good luck with that. :)

But how does that take into account chainstay clearance?

well, if the chosen chain ring size won't clear the chainstay at the required SS chainline, then nothing will work.

Except a cassette hub with 1 cog and spacers. I usually go this route anyway, as almost all my wheels have cassette hubs. The only disadvantage is running a wheel that's dished.