I’ve read through various crank length topics, but I haven’t found the answer to one specific question:
Why is it, that the longest uni-cranks (~150mm) are shorter than the shortest bicycle-cranks (~160mm)?
Professional road-bikers ride 165-170mm cranks, because this seems to enable the highest power output. For a geared bicycle, efficency (input to output ratio) seems to be more or less the only constraint.
For the uni, speed is another constraint, and I understand that you need shorter cranks to go fast, given a certain wheel-size. And it makes perfect sense for many use cases to choose a crank size which is way below a “physiological efficiency optimum”.
What I don’t understand, is why all uni cranks appear to be well below that optimum.
I switched from a very old KH24 with 165mm cranks to a KH27.5 with 137mm/117mm cranks. Obviously I am faster, but I lack control on downhills which are on my technical limit. Also on uphills my incline-limit is lower than with longer cranks.
I am aware that the wheel-size contributes to this as well, but the wheel sizes also comes with advantages like rolling smoother over obstacles. So, if I wanted to go faster, according to my current understanding, I would rather lean towards a 29er and a crank length which is closer to what has proven to be efficient on road bikes, than to (further) reduce my crank length below that efficiency sweet spot.
Do you think, I am missing something?
And, as far as I’ve seen the longest I can go for my Q-Axle cranks is 150mm/127mm, or is there a longer alternative? (Perhaps 150mm will be just fine for me)
Going by my experience, if you choose a crank length that is comparable to anything on your b!ke, you’re gonna be slower than needed, yet you end up more exhausted. Why this is, I don’t know.
With my 150mm crankscurrently on the 36er and 145s on the 29er I still seem to be riding with way longer cranks than most people here. At the moment I even have 165s on the 26" fat muni but they will replaced (again) with 150s when I mount shorter cranks to the Ferris wheel.
Yes, I’m slowly making my way to shorter cranks on all of my unis. Just takes time to adapt both skill-wise and as far as fitness is concerned. So far, everytime I went with shorter cranks I started to ride faster (but then again, I’m not in 36"/45mm territory )
Qaxle standard is based on Shimano Hollowtech standard. So, if you can source Shimano cranks, you could have a longer alternative
As per the other questions, there are a few points to think about:
a few decades back, when the first munis have been introduced, some of them were using bike cranks whose length was about 170 mm. This made the wheel size/crank size ratio feel really weird. I have tried such cranks on a technical uphill and it didn’t seem to give any advantage compared to shorter cranks. Sure, you get more torque but it makes the uni feel unstable.
similarly, as you noted, short cranks theorically help you to go faster. Put the other way around, you go slower with long cranks. Based on the Total Gear Ratio concept that has been theorised on this forum before, you never really need that much amount of torque whichever uni you ride. Instead, you’d prefer to be able to go faster… Thus, no need for the manufacturers to provide longer cranks [1].
nowadays, short or mediumly-short cranks shouldn’t be an issue for downhill because you should be using your brake to control your speed (assuming that you are used to a good disc brake).
By the way, this comparison topic between short and long cranks has been revived every now and then. There are probably lots of topics talking about it. You may want to look for archives on this website
[1] This may be a lie as I am looking for 137/155 ISIS cranks for my G36er to tackle some very steep uphills (grade > 20%). But that’s not an usual use case, so manufacturers don’t focus on that need.
That’s a good point!
I’ve noticed more stability uphill, but I contributed that more to the bar and the wheel-size than to the crank-length.
My total downhill experience on a rim-braked KH24 is ~5000m elevation, and ~2000m elevation on the disc-braked KH27.5. So I am certainly not skilled with using the brakes, and I am also struggling with the 1-finger braking a bit, which wasn’t a thing, when I was mountain-biking.
Certainly I was using the torque of the long cranks of my KH24 to power myself through tricky parts of my downhill trail, where a more advanced rider would have used less force on the legs and more technique on the brake.
Another concern is the uphill: In my area, the uphills (which lead to interesting downhills) happen to be at least 10% steep, which appears to be roughly my current limit with my KH27.5 and the 137mm cranks.
If I could add 2 or 3%, it would open a lot more uphills for me. (Non of them are technical, they have all pretty smooth surface, if not tarmac.)
That’s why I came up with the idea, that 137mm is too short for my specific use case. Also I am a bit taller than average, which should also shift my ideal crank-length upwards
That’s also a good point. My brain is wired for the 165mm cranks on the 24inch wheel, and the re-wiring might simply take some time.
I’m not sure that Shimano bike cranks would work on a uni. Every set I have had has a one piece right side crank and axle. It’s only the left side that has the pinch bolts that qu ax and kh are now using. So to fit them on a uni you would need 2 left arms and could run into problems with the right pedal unscrewing as it would be threaded the wrong way then.
If we’re talking about the required effort for a given (high) speed, shorter cranks are definitely the way to go. For a 36" wheel 125-100 mm cranks are probably optimal on flat to somewhat hilly terrain. Anything longer would (for me) be very detrimental to the average speed. A skilled rider could ofc. given the right terrain take advantage of even shorter cranks. Having used both 75 & 100 mm cranks i would say 89 mm is probably perfect although i have not actually tried it.
I am aware, that on the downhills I am mostly struggling with a skill issue and not with too short cranks!
But I am not yet convinced about the uphills: Of course, with more skill (and strength) my current limit of roughly 10% incline might increase to let’s say 15% incline. But there’s got to be a crank length which allows the steepest incline, given a certain skill-level and wheel-size. And I would bet, that’s not 100mm.
When it comes to why unicycle cranks are typically shorter than bicycle cranks, I think it mostly comes down to the fact that most unicycles are “geared” really really low compared to a bicycle. A single speed bicycle with an all-around gear ratio is running somewhere around 70 gear-inches, whereas even a Schlumpf 36er in high gear reaches only 55-ish gear-inches.
The average unicycle is probably more in the 20-30 gear-inch range, and our cranks are fixed to the wheel (99.9% of the time). This means that if we want to get anywhere at a good pace, we will be spinning, everywhere, all the time. Shorter cranks help facilitate that.
When it comes to uphills, running short cranks is a bit of a compromise. I live in a relatively flat area with gentle elevation change, so I can easily get away with running 114mm cranks on my 36er and be fine most of the time. There is a bridge that has a steep incline beforehand that I occasionally have to cross, and with short cranks I usually have to stomp to get across. Being that it’s so inconsequential to the rest of the ride, I can live with that. However, if I knew I was going to be riding in a rather hilly/mountainous area, I’d maybe opt for 142s.
E-bike cranks work. I’ve seen it done and I actually have a set of them that came with a second hand uni, but I have no desire to use it…
Studies seem to suggest that this optimum is fairly shallow (https://www.pinkbike.com/news/why-shorter-cranks-are-better-according-to-science.html is a decent overview article linking scientific studies), so going outside of that range does not seem to come at a big cost considering maximum power output or efficiency. (Which are quite different targets, you conflate them a bit in this statement and the title).
I think if you were riding faster than fast walking pace at a sustained climb of more than 15%, you must be extremely fit aerobically. Most riders just dismount and push when they are at walking speed, so cranks are not typically chosen for that situation.
Another consideration is that riding position is slightly different between bike and unicycle. Cranks are more or less directly below the rider on a unicycle in normal riding position, while they are in front of the rider on a bike. I’m not sure how much effect this has, but it does change around the angles of your joints a bit.
I hadn’t thought of e bike cranks, I’ve never ridden one. Though now you mention it I did see a rather nice looking set of Hope 155mm e bike cranks on Facebook marketplace that looked like they would fit an Isis hub.
That’s interesting - thanks!
Altough I don’t think that maximum power output and efficiency are very different targets, because maximum power output also comes down to efficiency, since the human power input is more or less given. So to maximize power output you have to maximize efficiency.
That’s a good and actually obvious point, which I still missed:
There’s not much of a point in uphill-cycling with walking speed, if you could as well just walk. It’s indeed better (also for my intended use case) to optimize the uni for signiificantly above walking speed, and be somewhat fast when it’s flat (and on the downhills) and walk when it’s steep, instead of being forced to be slow all the time just to avoid a bit of walking.
You’ll see a road cyclist out of the seat in a sprint, but sitting down at lower intensity. Out of the seat can produce higher power because more muscles can be engaged, sitting down is more efficient because less muscles have to be engaged. To me that shows how they are different targets, otherwise the same technique would be used for both.
No one has mentioned it yet, but shorter cranks also make your leg movement smaller. It makes the ride smoother (less wobbling) and less tiring. I ride 127mm cranks on my 27.5’’ Muni and riding longer just feels weird. With proper technique this setup will go up the same stuff I can pedal on longer cranks. On the downhills, I find it much smoother, the brake does most of the work anyways.
On my Mountain Bike I have custom drilled cranks at 153mm. I can ride the same gear for longer amount of time when sprinting because I’m able to spin faster due to the shorter leg movements. On long riding days, I can feel the muscles working more (requires more force on shorter cranks) but for most of my rides I wouldn’t want to use any longer on my bike either.
On my 29er I have used 125 and 150 cranks and have not realized much differences in speed from moving down to 125. A few miles per hour, hardly noticeable.
When I was mentioning maximized power output on a road bike, I was mostly thinking about sustained power output for something like an hour. But even if we include sprints and standup riding, we’ll still have to consider efficiency imo.
The goal is to maximize standup efficiency, which will require a certain crank length, that might differ from the crank length required for maximum seated efficiency.
One of my main issues with standup pedalling is that the testicles tend to touch the saddle with each half revolution / each stroke. That’s not particularly comfortable and it’s distracting. Lowering the saddle is a possible remedy, but that is detrimental to normal riding, so not really an option - except for a drop seatpost, but I don’t think they exist for the 25.4mm seatpost diameter of my unis. Kinda wish the main manufacturers made 27.2mm their standard many years ago.
Another way to mitigate the problem would be to tiptoe with the lower foot. Have to give this a try; not sure I have the ability to concentrate on this when I’m already in a challenging situation, such as climbing a steep ascent when usually already being quite exhausted…
I don’t feel, like standup riding on the unicycle significantly increases my power output. It mostly delays fatigue because it requires slightly different muscles.
I am not talking about the non-steady standup riding from horizontal crank position to horizontal crank position, which happens to be not faster than walking speed for me, but about smoother steady pedalling.
About your issue: I assume, you have tried tight (bike-) pants already?
I think it could be beneficial for my knees. But I honestly haven’t standup ridden long enough distances to really tell. All I know is that riding up a hill takes its toll on my knees, especially as the cadence drops
Ok. I usually stand up when I’m getting exhausted, when I’m starting to feel some knee pain, or when it’s too steep to continue riding seated. But I’m really only at the start of my hillclimb career
I might not be the most relevant people to speak about long cranks since I have basically never used anything longer than 127 (I’ve used 145 for unicon 20’s road climb and unicon 21 uphill and 135 for a timed climb of my local “mountain” which is 100 m high), and my position is very different from the typical unicyclist’s position due to aerobars and large handlebars. However, I’ve used basically everything below 127 I use 117 on my 29S and I’ve used 125 on it for quite some time, I’ve used 75, 89, 100, 110 and 127 on the same setup without a geared hub before, and I use 127 on my muni.
The simple answer on flat rides (with rather moderate speeds) is that you want to pedal as fast as possible, and have basically no power to put on your wheel. To achieve that, you want short cranks since you spend less energy accelerating your legs and can go faster. As an example, my personnal best with 100/89 (I’m not sure what I used) on the 10k is 24 minutes. With 75 it is 20’36".
However, unicycling usually involves hills. To counter it, you need to be able to put power to the ground. Basically. Long cranks allow you to have a lot of torque, but you end up going slow and if you start stalling with vertical cranks, you just end up going half rotation after half rotation, which is rather slow. If you have enough power, it is often better to use shorter cranks and give it all you have (it doesn’t work for the longer hills though).
For flatter rides, most riders won’t go all the way down for several reasons: it feels unstable, it is harder to mount, any small bump can throw you off the uni AND you end up meeting aerodynamics that slow you down and require some power.
So basically, most unicyclists use shorter than optimal cranks because at their speed, they don’t need longer cranks which would actually slow them down due to cadence limitations instead of allowing them to put more power.
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