Anyone running a 138 crank on a 29. I’m getting ready to try some. The 125’s are too short for me and the terrain has to many hills. I’m referring to street riding.
I am. I think it’s a really nice combination, very easy to ride. I’d like to try shorter as my skills improve. I sure don’t want longer ones.
I tried different sizes of cranks on my 29er including the 137mm. I is a great improvement for road riding over the longer sizes (150/165).
Turns out that my legs are not complaining that much on 125 so I am running then currently. However, I noticed that the shorter you go the comfier it gets (within your legs/knees limits).
Having a handle-bar to prevent lateral motion and provide leverage with your body to keep the pace up (where legs only can be not enough) makes a huge difference IMHO.
Trying for a couple of rides a length can help you evaluate if this suits you (going longer doesn’t require that much of adaptation as going shorter).
I can lend you a pair for you to try before placing an order if you want (can be useful if you don’t want to end up with dozens of cranks :D).
Thanks for your offer on the cranks. I’ve tried the 125’s,150’s, and the 170’s. The 125 seem way to short so I ever really tried these at any distance. I went ahead and bought the 137’s since I have a 20, 24 and a 29 uni. So I can always used them on one of these uni’s. I just got back from a ride with the 137’s on my 29. Dang my thighs are SMOKED. I’m referring to riding on the rode. I can normally do about 8 miles. However I could only do about 3 miles with the 137’s. My route does have so pretty goods hills, I was fine on the small inclines and flats. But the hills killed me, down and up hill. Since I have no brakes the down hill is pretty tough as well. I think I’ll swap the 137’s to the 24 and the 15o’s back to the 29. That should just about even out. At least until I can build up some more leg muscle.
My 29er uni came with 145’s I think, and I’ve been thinking about getting shorter ones. The 145’s seem like a pretty awesome length for rough terrain and hill-climbing, but I do quite a lot of smooth, long runs (Not necessarily road, but not really ‘off-road’ either). Maybe I’ll try 138’s or possibly even those too-short 125’s 
Does anyone have any opinion on dual-hole cranks? Are they easy to switch-out halfway through a ride (A poor man’s gear change) Or would I be better to just get a size I like for everything?
I like 140 on XC trails, and 125 seems good for everything else. Some of the steeper/longer downhill runs are much more comfortable with a brake on the 125’s. 140’s are manageable without a brake.
I have a pair of square taper cranks with 140/120 dual holes that I have been using on the 29. I am moving them to my 32", and I put 100’s on the 29er. With the 100’s it’s amazing how much like a 36 it feels. Brakes in the future.
If your pedals have Allen heads on the spindle a long hex handled hex wrench makes switching positions easy and fast.
You can change holes within about 3-5 minutes. Just make sure to find a compact but decent pedal wrench.
I don’t change holes that often and rarely in a ride but the longer cranks are always there as a fall back.
I am running dual-hole cranks on my 29er and I never changed the hole during a ride (too lazy and never forced to).
I currently using it more with the road/off-road position 
They’re good- I used to have KH 125/150s on my 29-er, and, if you carry a crank wrench it’s an easy change over during rides.
One other thing is that due to the crank length change, until you’ve settled on a seat height that works with both lengths, you’ll almost certainly have to adjust the seat height as well.
So I was very happy with the dual-hole cranks until I decided that 125mm was too short for me on the 29er, and switched to my quax cranks which are, I believe, 140mm?
The 125’s work OK, surprisingly even climbing v. steep road hills (I seem to adopt a more ‘standing on the pedals’ style when climbing with them- the major negative for me was that they aren’t good on steep downhills: I suspect most people using 125s on 29ers either live in flat areas, or have brakes fitted.
For me, unicycle=simple, so brakes are a no-no until the day my legs are so old that they require them.