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has anybody used bmx cranks, bb's, and chainrings for a singlespeed? or is there some sort of glaring compatibility issues i didn't notice...like different crank arm lengths, or bb widths?
 

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A lot depends on which BMX bit you're trying to use. Those I have looked at seem solidly built, but often heavier than you'd want on a XC bike.

There are some great SS rear hubs for BMX, but too narrow for most MTB applications.
 

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BMX cranks (i.e., tubular chromoly arms with a single bolt chainwheel) will work fine with an SS MTB. The only issue is spindle length. Most BMX spindles are made to work with 110mm rear spacing. Profile makes a longer spindle for MTB applications (135 mm rear spacing), though.
 

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one chain, two sprockets
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Profile cranks & BMX sprocket:
Bicycle tire Wheel Bicycle wheel rim Bicycle part Spoke

My frame has an American BB, so I was able to use the Standard Profile BB. You could get their Euro BB kit and be good to go. In the back I run a White FW. To get a proper chainline I had to move my sprocket out further than I initially intended. So, make sure you get the proper length spindle - you can call Profile if you're unsure what you need...

Tom P.
 

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BMX Parts

I also have Profile cranks (180mm) on my Monocog. I used a Redline euro bottom bracket with a Profile spindle and DK BMX chainwheel. The chainwheel is a 36T -- much larger would probably not have had enough frame clearance with my combo (Nexus hub). I'm also using a KMC BMX chain.




The bottom bracket outer center bolts sandwich everything together. The correct chainline is obtained by using the proper shim washers between the bottom bracket bearing and the crank arm. The washers are included with the bottom bracket. As far as the correct spindle length, figure out the chainline distance from center, account for chainwheel thickness, add spindle insertion length into crank arm (about 2/3 insertion or so for the Profiles), and multiply by 2. I also made sure the crank arms had about the same frame clearance on each side.

I'm also using 4" rise Robinson BMX cruiser handle bars, a 69mm reach Marzocchi BMX stem, and Animal BMX bar end plugs. I like the more upright position, the chrome, and the sweep of the bars.




As for the BMX chain rings, there are all kinds of 110mm 5-bolt BMX chain rings that could be used.
 

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one chain, two sprockets
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blue78,

I would have used that stem if I'd found it! My original target was 70mm, but do to the severe mixed-use I planned for this bike I settled for 53mm Fly BMX stem. Now that I'm acustomed to it, I'm not sure the additional 16mm would be worth the effort. Still, it's nice to know it does exist!

Tom P.
 

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I'm new to this forum, so I don't know if I'm committing a crime by not introducing myself first, but here goes...

On my current singlespeed (which is actually my old geared hardtail reborn with new dropouts), I'm running 180mm profile bmx cranks with a euro-bb conversion and 135mm spindle. The one difference I'm seeing between my setup and the rest of you guys is my use of a four-bolt "spyder". Its similar to the five-bolt two piece sprockets of old but allows the use of newer, conventional four bold mtb chainrings in tooth counts that are strange to the bmx world, like my 32. And yes, autoduel, MOST of the old two-piece bmx spyders were 5-bolt 110 pattern. I can supply pictures of my setup if anyone wants. My spyder is mfg by profile, it's a real ***** to find on their site but is available thru them or your lbs. If lbs can't find it, tell them to look on qbp's website, it's there, only available only in silver, not black as advertised. Be prepared. I almost forgot, for a tooth-count as small as 32, you need to take some material the ends of the spyder closer to the chainring bolt, or the chain rides up on top of the spyder. A 34t can be run with no modification. Um...

Profile spindles in 135 width are ti or solid chromoly only. The cromo ones are heavy mother-effers, and the ti ones are stupid expensive. A few years back, fsa or someone was making a cheap-ish ti profile spindle, but I haven't been able to find one new anymore. I also haven't looked very hard, because I'm fat and don't really trust ti spindles with my weight combined with the length of my crank arms. I'm thinking about just milling the **** out of my cromo spindle to drop some weight and add a little flex without too much risk of breakage. Anyway, I think that's it... fitting bmx cranks on your mountain bike is easy-cheezy, as long as you can get a spindle long enough.
 
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