stilltooslow

I have a Sugino 75 170mm on my fixed, which I feel is a great compromise for road routes, and recently put a 172.5 on my SS for that added leverage on the slightly longer, hilly routes I cover with it. I recently read an article promoting the virtues of very short arms, like below 150, citing a supposed mechanical advantage with all kinds of anecdotal evidence utilized as supportive data. Seemed at least compelling.

To be honest, I rode with 165's for years but always felt they were too short and feel better with longer arms, but being the curious (and actually open minded…..sometimes…) sort that I am, I wanted to hear some of your perspectives on this issue. BTW, I'm 6 ft. with a fairly long 34 in. inseam, if that matters.

prooftheory

The track guys in here (TT and carleton) always say that optimal crankarm length is different for each individual and can't even be predicted based on body geometry. TT is very short and uses 175s for instance. If you are more comfortable at 172.5 then use that. I use 165s and I feel like I'm pretty efficient with those but I don't notice it much if I go up to 170. The only real argument for shorter on fixed is pedal strike. It may be worth a couple millimeters less than optimum to avoid crashing.

TejanoTrackie 

Huh ? The longest cranks I have on any of my bikes are 170mm, and all my fixed gear type bikes have 165mm. However, I can hardly tell the difference unless I'm spinning really fast or racing. Far more important is getting your seat position dialed in and having a smooth efficient pedal stroke. As to pedal strike, BB height is just as significant a factor as crankarm length, and some SS bikes are based on road frames, which have low BB's.

prooftheory

It is weird that I have a distinct memory of you posting that you had ordered longer cranks because they were going to help you spin faster. I must have dreamed it.

seau grateau

I rock the 170s for no reason other than they came stock on the bike or the cranks I wanted were most readily available in 170. Never saw any reason to change. I don't imagine it's an issue I'll have have to consider seriously.

gregjones

Some folks say 165, some say 170.

Mine are 167.5. That's how to decide.

They go round and round and so follows the rear wheel. No problem.

TejanoTrackie 

Longer cranks definitely do NOT help you spin faster. Think of high revving engines, that have less crankshaft offset for the same reason to keep piston speeds under control.

prooftheory

That is definitely the conventional wisdom as passed down from sheldon but my memory is that you were actually bucking the said wisdom and pointing out that leg length didn't necessarily have an impact on optimal spinning. Again, some weird bfssfg acid trip I had.

TejanoTrackie 

Leg length =/= crank length

prooftheory

Obviously. What I meant was that the leg-length to crank-arm length ratio wasn't predictive of optimal spinning. At this point, I can find @carleton posting similar claims but I thought you had joined with him in these assertions and I can't find it, so it is simply a false memory. Idiots sometimes get those.

Coluber42

For another perspective on crank length, gearing, and mechanical advantage, read here: https://sheldonbrown.com/gain.html
The basic point is that if you talk about leverage, you should talk about the leverage of the entire drivetrain together, meaning the total mechanical advantage from the gearing and the cranks combined. So in theory, if you want to keep everything the same but change your crank length, you would have to change your gear as well to wind up putting the same amount of force on the pedals for the same power output (but at a different cadence).

Aside from that, I think ideal crank length has more to do with the mechanics of your body, and what you're used to (and what you need to prevent pedal strike). If you have longer legs, you're more likely to want longer cranks. Short legs, short cranks.

But here's another reason you might want to go shorter on a fixed gear: If you change crank lengths *without moving your seatpost*, two things happen. One is that your leg extension at the bottom of the pedal stroke changes. Shorter cranks mean your leg is extended less, longer cranks mean it's extended more. The other is that the angle of your knee at the top of your pedal stroke changes. Shorter cranks mean your knee is bent less at the top, longer cranks mean it is bent farther. You put more strain on your knee joint when you put pressure on it when it's bent farther, and less strain when it is closer to straight. That's why riding a bike with the saddle too low will make your knees sore in short order. (Not to say that you can't or shouldn't move your seat height - just describing the geometry of the situation)
On a fixed gear, you wind up grinding in too high of a gear a lot of the time, because you only have one. So using shorter cranks means that at least you aren't putting the pressure on your knees at as much of an angle. At the same time, it is easier to spin fast more smoothly without bouncing if you have a little slack at the bottom of the pedal stroke. Shorter cranks get you both of those things simultaneously.

Personally, I always had 165's on my fixed. I used to have 170's on my geared bike, but at a certain point I realized that since I rode it so little, I wasn't really used to the 170's anymore and sort of felt like I was pedaling squares... plus I kept scraping my pedal in corners because the road bike doesn't have the clearance the fixie has. So I put 165's on everything now, and it's much more comfortable that way. I'm 5'6" with a 30" inseam, FWIW.

One last little aside about crank length - have you ever noticed how many bikes that come in really small sizes still come with 170 cranks, because no one wants to bother with more than like two crank sizes? (170mm cranks on frames smaller than maybe 56cm and 175's on everything bigger) And then all the really short riders keep asking why they always feel like their knees are hitting their chest when they ride and the bike shop dudes don't have any idea?

OneGoodLeg

What kind of riding were they doing with these 150 and shorter cranks?

I'm just curious, because in my experience there is a balance (equilibrium?) that is reached at somepoint between the crank legnth and the speed produced from spinning. To me too short a crank arm just means you'll spin out sooner in certain circumstances.

The only benefit to the short cranks would be for a brief burst of speed, like when doing BMX tricks.

Nagrom_

Quite the opposite actually.
Shorter crank arms give you less leverage, meaning less torque for the same amount of power input. This means you have less of an ability to accelerate.

Theoretically, using shorter crank arms will give you a higher top end speed if you can produce the same foot speed as you can on longer crank arms. Not in terms of revolutions per minute, but rather distance per minute.

Carleton and I did the math a couple years ago in another thread, but it turns out its pretty linear. We used a base crank arm length of 170mm. For every 5mm increase/decrease in arm length there is a ~3% increase/decrease in leverage and a ~3% decrease/increase in in cadence given the same foot speed, respectively.



Remember:
This is all physics on paper. We aren't robots riding bicycles in vacuums. Physiology dominates nearly all the physics in this case. Just because it's better on paper doesn't mean it's better in real life, and in particular, it doesn't mean it's better for YOU.

Xgecko

When I rode fixed used 165mm cranks mostly because I live in a small town with lots of little maze like streets where I had to turn while riding. Since I switched to SS I use 175mm cranks because the big arms make climbing easier

stilltooslow

I wish I could find the article, although it was actually more of a sales pitch for the company that made the extra short cranks. If I recall correctly, the main subject of the 'case study' was a triathlete type who greatly improved his time after making the switch. Again, it was all sales-y, but still quite compelling. The author went on to mention others who are moving to shorter arms, where he brought up Lance Armstrong as well.

I'll try to find it, but if anyone else does, feel free to post the link.

Soil_Sampler

https://www.powercranks.com/cld.html

Dannihilator

170mm here.

rms13

165 because I thought it was an understood standard on a fixie/track bike. But I am riding a frame with proper track geo and bottom bracket height. My road bike has 172.5 and they feel pretty damn comfortable to me

stilltooslow

Thanks SS….that be the one.

Soooo…….hype and sales propaganda, or is there some element of truth in there??

europa

You're only going to notice the difference in crank length if you ride lots of different bikes and to honest, I don't think it makes any difference on the street. The track is different of course.

My Hillbrick is running 170s but she was built as a fg bike and has the higher bottom bracket to suit. I have never suffered pedal strike despite using relatively wide pedals.

My Europa was built as a road bike. When she was fixed, I started with 170s and got the occasional pedal strike. When I upgraded the cranks, I went for 165s and I never suffered pedal strike. She's now geared again and is using 170s. Thanks to all my fg riding, I keep forgetting I can coast through corners and quite often have pedal strike.

JohnDThompson 

I have 165mm arms on both my fixed gear bikes (for clearance) and 170mm arms on my road bikes. I don't notice any real difference beyond less pedal strike with the shorter arms on the fixed gear bikes.

stryper

wow. much photoshop. such excellence.

TMonk

172.5 on all road bikes, fixed gear roadie

165 on track bike

ThermionicScott 

165mm on mine. No reason but peer pressure, but it works well enough that I have no reason to switch.

Leukybear 

165mm track
170mm road