After spending about 8 months riding a unicycle, I don’t understand why bikes don’t have shorter cranks. I’ve periodically had issues with my knees riding a bike, but after learning to use the brake on my uni, I feel like my knees have never felt better than after riding around on 125-150mm cranks.
Good question, I’d love to know. I mostly ride unicycles as bicycles really stuff up my knees (well, more that unicycles do anyhow). On unicycles, I’m trying to reduce crank length where possible, it seems the way to go for several reasons. I’ve just got back onto my 36" and for now am stuck using 150mm until I get some more skill and strength (which is taking time), but that crank size is just so tiring on the road.
There must be long standing reasons for bike cranks being 165-175mm, but seems strangle considering those guys get to have gears!
I am sure there is some good ergonomic reason for it but also a lot of ‘tradition’.
I too have knee problems and have moved to 165 mm cranks on my bicycles, which gives me much less trouble, primarily because I can ride with less bend in the knee at the upper end of the revolution.
My daughter’s small frame ladies bike came with 175 cranks?!
I noticed that pedelecs are getting shorter cranks.
Anyway, unicycle sessions are definitely better for my knees than bicycle. 
I just need to get better and manage to use it for my commute. 
Probably because bikes have gears, and the longer the crank is, the higher is the torque you can put on the pedals.
Thanks to multiple gears, you don’t have to spin fast on a bike.
On a unicycle we don’t have gears (except schlumpf hubs) so the only way to spin fast more easily for a long period is to reduce the cranks’s size.
Geared unicycles are generaly equiped with 137-150’s cranks because you have more control in high gear.
geared 36ers are very rarely equiped with 110’s or shorter.
By the way mine is, but during the 2 only rides I have done with this set up, I never tried to shift up.
I have made hundreds of km with G36x150, and few rides with G137, both shifting up and down.
I’ll probably try to shift up with the 110’s but I don’t think it’s useful, and It makes me a bit nervous.
the top speed is higher, but accelerations (and decelerations) are slower when you shorten the cranks.
I do many more miles on two wheels than one, and I’m convinced that it’s keeping my battered knees from seizing up altogether. (There is a plausible medical explanation: low impact mobilisation of the joint, and improved circulation of synovial fluid, good for the remaining cartilage).
I think the reasons for bike cranks being longer are: 1) gears of course 2) longer cranks on unis tend to cause a snake like trajectory. As a road bicyclist, unicycle gearing seems incredibly low to me in theory (but not in practise!).
I don’t like the sticky sweaty feeling I get behind my knees when riding a bike on a hot day. Shorter cranks means less knee bend and no sticky-back-of-knee feelings!
Riding shorter unicycle cranks convinced me to try shorter cranks on my fixed gear road bike, so I switched it from 170 to 155 mm. And I moved my single-front-ring MTB from 175 to 165.
Advantages are similar for both - smoother spinning for better high end speed, a little better ground clearance. Overall I’d say they’ve been a good move.
Short cranks are a little hard to find, though. Shimano only makes the 165mm in the XT line (fairly expensive). Origin8 makes short square taper cranks, but to fit my current chainrings I had to buy a road triple and grind away the extra stuff to convert to single speed.
There is an optimal length of crank for an average rider and average riding. When you ride with very short cranks, you are using less of the available range of muscle movement.
From an engineering perspective, it’s best to optimise the “engine” to produce the best balance of torque and power, and then transmit it via gears. This is not an easy option on a unicycle, but is simple on a bicycle.
To understand this in simple terms, take it to extremes: imagine riding your unicycle on 300 mm cranks and then on 20 mm cranks.
On 300 mm cranks, you would have plenty of torque, but it would be inefficient and uncomfortable because your feet would be making huge unwieldy circles.
On 20 mm cranks, in theory you could spin really fast, but it would be fidgety, and would focus all the work on a tiny muscle movement.
As unicyclists, we have to work within a range from somewhere around 90 mm (for fast spinning but low torque) up to around 170 mm (for high torque, but more laboured spinning) and most of us choose a length somewhere between about 110 mm and 150 mm.
That range of 110 to 150 is fine with the very low “gearing” on a unicycle. Even a 36 way below “bottom gear” on a road bike.
On a bike, a combination of experimentation and tradition has established that the ideal range for most people is around 165 mm to 175 mm. That allows you to put out a good amount of torque at a decent rpm, for best results.
Other factors are that on a bike you are usually just applying forward pressure on the pedals, you can freewheel (coast) on most bikes, and a bike has a more ergonomic riding position.
Note that in the bike world, “short cranks” means 165mm.
As Mikefule alludes to, I think there has been a lot of scientific research into optimal crank length relative to tibia and femar length. Of course, this relates to maximum performance racing with gears and a freewheel.
I think the reasons are such:
- I think the large majority of road racers on which such research was based are between 5’8" to 6’4" (173-193cm) so this falls in the 165-175mm range (and probably most in 5’8" to 6’1").
- to reduce mass production costs, bikes come with a “standard” crank length as offering custom lengths would be more costly (higher quailty children bikes usually come with 125mm cranks). I think some women’s bikes come with 165mm cranks, but in the small frame size, the cranks should probably be proportionally much smaller…
-> i.e. finding cranks outside the 165-175 range is hard and requires (expensive) non-standard virtually custom-made equipment. - as stated, high-revolution spinning is not necessary on a freewheel bike.
- even if someone does some research with shorter cranks, it will most likely be based on top cyclists (i.e. someone who already rides a lot and whose body has adapted to the given crank length), so a short-term study may not actually bring any benefit even if in the long term it might. It would probably be necessary for a rider to train and race an entire season on shorter cranks, but few professional racers would likely want to take such a risk just for research…
I think it is a very intersting and relevant question in track racing, where like unicycling there is no freewheel and usually no gears. Although I did some track racing way back when, I didn’t do much. I think it’s such a niche sport these days (even though amazingly it was the most popular sport in the world around 1900!), that is it really only “important” for the olympics. I do not know but I would guess that for modern olympics there has been experimentation with varying crank lengths in track racing. I decided to try and search and found something but not much
Hmm… maybe it’s even a rule: long cranks are usually prohibited on the track as they can be dangerous with pedal strike in the banks, so maybe the recommendation for 165mm cranks b/c of the upper limit of 170/175mm just became a rule that also requires a minimum length just because??
In recent years it is pretty well recognized, that shorter riders should use shorter cranks, e.g. BikeDynamics - Bike Fitting Specialists - Crank Arm Lengths
Having been a cyclist for many years before unicycling, upon learning to unicycle I found the 140mm cranks on my first 24" unicycle unbelievably short, but now am totally used to 125mm and even 100mm or 89mm… It would be interesting to try something like 125mm on a bike, but I wouldn’t know where to find short cranks (I guess I could use square taper but I don’t presently have and old school bikes).
–> it takes time to adapt and get used to a change in setup.
So it would be interesting to see some fixed gear track racing research with “normal” height riders switching from 165mm to even shorter cranks in the 125 to 145 range and riding and training for a whole season in oder to adapt their style and muscles to the shorter crank length.
Ah, this supports my theory above that veodromes effectively prohibit shorter crank lengths (in the goal of safety related to long cranks).
i.e. tradition.
Cobb makes cranksets down to 145mm https://speedandcomfort.com/collections/short-cranksets/products/alloy-short-cranks