Project Weight-Weenie: Cranks

My goal is to produce the lightest mountain unicycle possible without sacrificing strength and reliability. I plan on backpacking / traveling with a 27.5" mountain unicycle throughout South-East Asia in the near future. Dropping as much weight is going to be important while I am hiking and lugging it around.

I figured I will share my progress, I am going through each component individually and determining what I can do to shave those grams.

Starting with the cranks. I used KH Spirits 127/150mm as the Baseline model and my mass (63kg) at a 3g impact force.

The 3 variations:

  • Baseline Model
  • 3 x 16mm Diameter Holes @ 25mm Spacing
  • 1 x 16mm Diameter Slot @ 50mm Long

Summary of Results (per crank arm).

  • 3 Holes: Mass dropped 9% (18 grams). Strength decreased only 3% (based on yield strength). Negligible change for factor of safety (3.8).

  • Slot: Mass Dropped by 15% but strength dropped by 27%. This would be too risky for mountain unicycling, but would be sufficient for purely road unicycling where impacts aren’t as high. Dropped the factor of safety from (3.8) to (2.0).


How did you do this simulation?
Really cool is there any other way to lose weight but still keep the strength?

You appear to just be modelling bending stress there. How about torsion (which is where the slot particularly will have a drastic effect)?

18 gram saving per crank would be 8.333% according to the 432 grams claimed on the KH website.

To put that into perspective. (18*2)g = 0.0794 lbs.
1 oz of water is .065 lbs,

You did say weight weenie, and I get where you’re coming from, but that’s an awful lot of effort for that amount of weight. I know you’re doing this with other pieces of gear too so that it adds up. . . I take it this is a different hobby in itself at this point. : P

Do you plan on swapping the crank size out much? If not, ventures (376g) would save you more weight)

Personally, if I was in south east Asia, I would be more worried about reliability and repair-ability more than I would weight. A 26" wheel is more common in the rest of the world (so I hear), so I’d probably ride that. You could get a workable replacement tire if you were desperate without having to build another wheel. I’d have replacement bolts, a little repair kit, etc. That’s just me though.

FiberFix Emergency Spokes are amazing. If you don’t have one, it’s invaluable for a touring kit.

Anyway, everybody is different, but the way I see it is that it’s not a race. It’s an adventure. I frown upon anything that could cause my adventure to come to a grinding halt… like a crank arm breaking in the middle of nowhere.

Remember to round those edges. No sharp corners. They create stress points.

crazyguyonabike has some cool light weight touring articles. Tyvek packs and things like that can really shave a lot. Some people point out that you can carry a lot less than you think, and just about every item you have should have multiple uses (with the exception of some tools and safety gear).

Put a waterproof bag inside your helmet for a convenient soup bowl! :roll_eyes:

On a serious note, Qu-Ax cranks are even lighter than Ventures… Hell, you could almost bring two pairs and be about the same weight as one pair of the KH, if dual-hole functionality is your thing. Yes they’re not as strong, but they’re probably stronger than running your modded KH cranks…

Nice stress simulations !

Purely out of curiosity, this is very interesting. However, on a uni-packing project, I doubt that the uni weight will be the biggest source of weight save given that you can have a stock bombproof uni under 6kg for a 26" and you can build a sub-5kg custom uni still plenty strong if money is not an issue (without modding parts & jeopardizing strength).

On the other hand, all the food+water+tools+eating equipment+living equipment makes most of the volume and weight. That is were the gain is to be searched and only being supported by somebody on another vehicle (being it a bike or anything else) can really make the trip as lightweight as possible :stuck_out_tongue:

Which quax cranks are you referring to

If it is these ones I break them playing unicycle hockey regularly. I would not trust them on a muni. I have them on my 29" at the moment and I swear they feel like they twist as the pedals don’t feel perpendicular when I put weight on them

These ones are strong but I didn’t think they were that light

My personnal tip is to save weight anywhere else before drilling the cranks.

I got my 27,5’’ Muni down to 4,6kg with stock parts (ok… not stock… but without customising existing parts).

Things that saved weight on my uni you should consider:
-Carbon rim, suprisingly reliable, I rode some pretty intense DownHill on it yesterday, still awesome. Yet, this only saved 75g from my older aluminum rim.
-32h wheel, bike rims are usualy way lighter than KH, Nimbus or Qu-Ax rims. You get plenty of strong choices too.
-Smaller tire, I ride a 2.35’’, good enough for all my riding, I wouldn’t go bigger on my setup. With a small tire, you need to find a good one though.
-Tubeless tire, you still have to carry a tube on you in case your tire burps (Make sure you have a tubeless ready rim too for more reliable setup!)
-Ti bolts! Ti bolts are almost half the weight of a steel bolt. One bolt wont make a big difference, but all of them does.

My 2 cents, Drilling cranks is cool, but as said earlier in this thread, you should choose reliable before weight. I found that my uni is as reliable as it could be while being the lightest uni I’ve owned (lighter than my trials uni at the moment). The only part I wouldn’t trust was my tire, I changed it last week after I broke it the second time and now it’s all good and ready to kill the trails!

Lots of low hanging fruit before you get to machining cranks. Lots of good options for light, low profile flat pedals. I’m also a huge fan of tubeless for muni but it could be a pain traveling with it (have to carry a CO2 inflator to reset burps-but not allowed to have the cartridges on planes, have sealant, spare tube anyway etc.) I’d invest in some nice light tubes, and hesitate at running anything over a 2.4" tyre. Get a CF seat base- and streamline your handlebar set-up (if you use one). With Jakob on the CF tubeless ready rim and 32 spoke count- would have liked to do that myself but run a schlumpf so stuck with the 36 hole rim.

I’m based in Vientiane, Laos at the moment so hit me up if you are passing through on your trip.


With less than 300 gr. my cranks are lighter, and with tubular design I believe also stronger than solid CNC’ed alloy.

Checked out your cranks, those are absolutely beautiful. Nice welds!

Yes I meant those ones! :smiley:

Sure, they’re no where near as strong as Ventures, but are they going to be any weaker than these drilled-out KH’s?

Yes they are weaker than drilled out KHs… I’m sure those cranks bend by looking at them too hard :stuck_out_tongue:

I suggest that you thread those drill holes, which will effectively give some gear options, allowing you to save lots of energy. :slight_smile:

The spirits are thinner there due to the cutout in the back and the relief in the front. There may or may not be enough material to safely tap and thread pedals into. You would also need a proper flat plate for the pedals to stop against due to the curved relief on the front of the crank. I think Moments are a better option for that sort of thing.

Yep, J.A. You are correct. The lower Q factor on the moments and the extra thickness makes them more suited for threading

I’ve seen completely drilled out kh moments that have survived multiple years of big drops in street.
Those quax ones can’t handle 6 months of unicycle hockey that requires minimal to no hopping with all hopping being like 5cm at most. Would never be game to use them on a muni. It’s scary enough using them on my 29 :stuck_out_tongue:

Did you machine those cranks in the first picture yourself, or do they just look different from the normal KH finish? Very interesting to see those stress and strain simulations, a friend once claimed he felt the spirits twist when landing a big drop, and I was skeptical, but seems realistic now. If you didn’t, you should rerun the tests with a load scenario that includes the torque the pedals introduce to the system, it is probably not negligible. Most street riders have bent cranks, even the super heavy duty Moments bend from drops, and the line Mad4ones snap along seems to be always from the bottom of the isis interface to the top of the pedal threads in the riders jumping stance. (those seem to be almost the only cranks that snap and not bend).

Personally, like many said I would not bother with shaving weight at the cranks much, a 32 hole wheelset would do much more though, especially since it will be easier to find replacement rims if anything goes wrong. 36 grams is nothing, I would rather sacrifice a row of knobs on my tire than drill out spirits (says the guy with drilled moments on his trials, but I had to try out that drillpress…)