Korina

Oh we have one, it's just another complication, another delay in getting her rideable.

I know the first rule of upgrades is "It won't fit", but it's forking ridiculous; want to change shifters? You'll end up changing out the entire drive train. New brake levers? Yep, better be ready to buy new brakes and cables. Great Bird forbid you want a different handlebar.

/rant

dedhed

I don't know that I agree with that. I find a lot of compatibility, particularly with Shimano, over many generations and levels.
Between manufacturers, sot so much. With Shimano once you understand the compatibility and limitations within it, it's not hard to mix and match to get what you are looking for. BB and cranks (threaded square taper) can cause some issues, but in stock locally I can pick up a BSA threaded BB in just about any size for <$30.

JohnDThompson 

The way Mark Stonich at BikeSmithDesign shortens cranks is to drill and tap a new pedal eye closer to the spindle and then cut off and smooth the excess material along with the old pedal eye. Of course, not all cranks are amenable to this type of modification, but some are.

Short Bicycle Cranks

AnthonyG

165mm Mountain crank arms are not readily available. We are talking about going down from 175mm to 160mm or less.

63rickert

Sugino XD is readily available down to 150mm and all sizes in between. Complete triples well under $200 even if you aren't patient and don't shop. Origin 8 has short ones too, lower price, lower quality. But not bad.

Not even sure what the MTB incompatibility is supposed to be about. Buy the cranks and put them on the spindle you've got. See where it fits. If you need a longer spindle you get a longer spindle. Have done, what, 5 very short crank conversions past two years, all kinds of bikes, no significant problems yet. Bikes built 1973 to 2018. Road bikes, city bike, mountain bike. 4 of 5 the existing spindle was fine. Working on bikes 45 years old you get used to spindle exchanges.

63rickert

The Sugino and Origin cranks are 110/74bcd. Minimum chainring 24 teeth. You really don't know what gearing you are going to need with the shorter cranks. Not short myself so only working from what my friends and wife tell me. What they all find is their pedal stroke is completely changed. Much more of the pedal circle is available for generating power, as opposed to just getting feet around to where they will next be able to work effectively. So far everyone is using same gearing as before and getting up hills much easier than before. Higher top end too. And in the wife's case, riding up hills that had become walkers even when she was significantly younger.

Mad Honk 

Well I guess I'm stumped here... I know that 175 crank arms won't work on a very small mountain bike. Toe clearance and every other aspect... But there are shorter arms out there and they can be fit onto the shorter frames. A good mechanic should be able to do the job easily and quickly. Bottom bracket spindles should be a minor concern if going from a triple to a triple. I'm going to have an adult beverage and go to sleep on this one, I'm baffled! MH

Korina

Hot diggity! A new source for short cranks! They run 130 to 175 mm and are available in silver (the correct color) or black (if you must). They're also $150 and coming from Japan, so I'm afraid to look at shipping. Nonetheless, they look pretty great.

Polaris OBark

$28.

bargainguy

For reference, I had Mark Stonich of Bikesmith Design shorten these cranks for me quite a while back.

Both are ~148mm, shortened from 170mm. From what I remember, he had a CNC setup in his basement that allowed him something like 22mm as a fixed shortening measurement he could always rip out. He cut off the crankarm ends, buffed out the new end curves, and drilled a new pedal hole to replace the original.

I also remember removing all the chainring bolts ahead of time to save money. He used to charge $5 extra apiece for cranks that arrived with chainrings and bolts attached, because he'd just have to remove them anyway. Yeah, if I was doing all that crank shortening, I'd probably get sick of removing and reinstalling chainring bolts too.

rsbob 

This site is full of them.

icemilkcoffee 

they call for 113mm BB. But the beauty of square taper cranksets is that you can use whatever length spindle to fit your bike. Typically you would add about 5 mm going from road bike chain line to MTB. So I am guessing 118mm would be right for your MTB application.

icemilkcoffee 

that crankset appears to be for a 68mm wide BB. Your MTB is probably 73mm. So don’t order that until you clarify with the shop whether it can be used with a 73mm BB

pdlamb

Darn, beat me to it. Though there's also a population of tall cranks, too!

Frkl

I ride 150 or 152mm on all my bikes (I'm 6'3").

For bikes that should look nice, I use Stronglight Impact Kid, which are really Sugino XD2, and available many lengths from something like 130 to 160. They cost about $75 as a double. Silver, JIS 107mm, 110 BCD, nice.

For utilitarian or really low gearing, I use the Suntour XCT jr, 152mm. 4 bolt, 104 BCD, black. But under $30!

Both are adapted adult cranks so plenty strong.