jschen

Let me preface this by saying that I have no intention of changing any of my equipment at this time. This question is being asked more out of curiosity since random optimization problems are always on my mind. And perhaps it will help guide decisions on equipment at some point in the future.

Evidently, a shorter crank makes it easier to spin at high cadences. So does that mean a longer crank makes it easier to climb really steep stuff when you're running out of gears? Let's say you're on a steep climb, already in your lowest gear, and watching your cadence bog down. Presumably, since you get more leverage with a longer crank, you can generate more torque, and if you're out of gears and at a low cadence, more torque should equal more power. Argued from a slightly different angle, does a longer crank optimize for a slightly lower cadence, thus allowing you to take on a slightly steeper grade before your cadence starts bogging down?

So what do you think? For what it's worth, Sheldon Brown advocates calculating a "gain ratio" that takes into account crank length, and based on the numbers, he seems to advocate the belief that a longer crank equals slightly lower gearing. And I know Lance Armstrong went to a shorter crank to improve spin, but unlike him, us mere mortals do run out of gears. Since he's not running out of gears, I don't think the analysis I suggest above really applies to him or any other pro.

So what about us mortals? If you have a bike dedicated to very hilly rides (or if you always ride hilly stuff), would a slightly longer crank make sense? Of course, it's no substitute for proper gearing, but would it help you get past the occasional 15-20% pitch (or whatever "borderline too steep" means to you) in the road?

1955

Hey Jason,

I'll find out on the Pie ride, OC switched out the 172.5 standard DA crank on my bike for the compact crank w/175's he had "lying around". Since I won't have a chance to put more than a ride around the block with them, I will be your very own science experiment in the mountains of Sandy Eggo on the 28th. Wish me luck!

Ralph

jschen

Woohoo! My very own test subject to try it out on! Don't be surprised if your bike is sabotaged and your 25T and 27T cogs don't work. I need you to stay in a 34/23 (closest to a 39/27) for the sake of science!

Pico

A longer crank gives you more leverage, which has the same effect as using a slightly easier gear. Work = Force x Distance, with a longer crank the distance is greater so for a given amount of work the the force is lower.

Because you have less leverage with a shorter crank you will want to use easier gears and spin faster.

Pico

JS, don't you have a triple? Maybe we should remove that granny gear of yours.

fogrider

First, I think using different crank lengths on a regular basis is not a good idea. When I had a mt. bike, I was running 175s and my road bike had 172.5s and I noticed that there was a difference in my spin. I have switched to 170s for my road bikes and have a dedicated climbing bike with a compact crank...best upgrade yet!

jschen

That would be very amusing, though perhaps not to me. You guys probably would get to watch me fall over on the steeper portions. And Mandy would be assured plenty of company.

531Aussie

I agree that it's probably not a good idea to switch crank lengths -- routinely or infrequently -- especially if the length is increased for a hammer-fest up a climb.

Sheldon Brown and Andrew Bradley say that longer cranks can't provide extra leverage when gears are invloved. This is Bradley's opening statement on his site (https://www.cranklength.info/cranks.htm):
"If you know anything about physics you will have realised that more crank leverage, of itself, does not mean "more power". Any power gains via crank arm length would have to be due to physiological reasons." And these are Sheldon Brown's thoughts: "this is a common misunderstanding. The "leverage" of a bicycle drive train, also known as "gain ratio" depends on the crank length, wheel diameter and the sizes of both sprockets. Yes, if you go to longer cranks without changing any of the other variables, you will have more "leverage", which is another way of saying you'll have a lower effective gear...but on a multi-speed bike, you can change gears at will! Ay, there's the rub! Assuming you adjust your gearing appropriately, crank length has no effect on leverage, it just has to do with the range of motion of the knee and hip joints." https://sheldonbrown.com/cranks.html

But I reckon otherwise coz I've accidentally put myself in a blind study (n=1 ) using longer cranks without realizing it, and then wondering why I was flying up hills.

Some pros use longer cranks for time-trialing (Indurain used 190s), and Marco Pantani, the diminutive Tour winner, used 180mm cranks for mountain stages, but he spent a lot of time off the saddle on climbs, obviously negating a possibly compromised seat height. Other pros used longer cranks for mountain stages.

So, I don't care what the 'doctors' say, I reckon long cranks = more leverage

1955

If that were to happen, my truck just might "break down" on the way to the ride that morning.

My only concern is my saddle height, since our hours at work changed back I won't have a chance to ride at all until the Pie day. About the only thing I can do is put in some time on the rollers and make some VERY slight adjustments then if need be.

1) I've never ridden over 106 miles before.
2) I've never used 175mm cranks before.
3) And I've never been much of a climber either.

Man is this going to be fun!

Ralph

mooncake

I use 177.5mms and I have the femur length to handle 180mms and I get appreciably more power than when I experimented with 172.5mm cranks.

Don't anybody jerk you off here - longer cranks do give you more power just like a longer crowbar let's you do more damage than a short one.

How about these modern extra-long golf drivers that let the Average Joe hit a golf ball 300 yards? The fact that a dwarf like Pantani used them for climbing illustrates the advantage. BTW, Miguel Indurain used 190mms in his 1 hour record.

EURO

Yes. I've switched in the past from 172.5 to 175 and it feels like you just dropped a gear.

If you are a low-cadence, high pedal-force grinder, big cranks are a great idea. They work for me.

patentcad

>>A longer crank gives you more leverage<<

I'm going to err on the side of class and not use this as a setup line for a dirty joke.

MichaelW

Pick your cranks to match your leg length. For riders like Mooncake, that means selecting longer cranks but longer is not always better. If you are 4'6" tall, there is little advantage in selecting a 180mm cranks.
Once you have a crank that fits, then select gearing to match your conditions. If you are running out of low gears, simply fit some lower ones.

56/12 and 22/28

+1.

EURO

Pantani used 180's and was 5' 7"

simplyred

So... for a rec rider...
Attempting a mountainous ride with 170's/165's with a double 54/42 & 11-21 cassette is similar to jumping out of a plane without a chute?

-simplyred

bbattle

I'm using 175's and I'm 5'8". But their the only ones I've used on a road bike so I have no comparison. My fit is good on the bike so I think I'll keep them.

Corsaire

What no one isn't taking into consideration is factors like the bike frame and saddle height, it's all relative. Of course, I'll agree too that: longer leverage = easier climb. But that's just the physics book theory.
When I had 172.5 cranks, my knees would go a bit too high for my comfort (my inseam is 30 inch by the way). Now that I switched to 170s, I feel spinning with far more ease on the climbs, giving more torque to the pedals, but again that's just me, with my particular frame size and saddle height. To anybody else, there are a lot of variables. The best is to experiment with your own particular, current frame and saddle height and different crank lengths.
Corsaire

thad

I was thinking about this today actually. So we are talking about a difference of a quarter to a half centimeter right (170 vs. 172.5 vs. 175)? I was just wondering how much of a difference that actually makes, in terms of feel, leverage, etc. Just doesn't seem like much on the surface, but I know tiny adjustments can make a big difference in other areas.

jschen

Thanks for all the feedback, guys, especially from you people with actual experience with this. I realize switching crank lengths between bikes probably isn't the greatest thing, so if I ever switch, it would probably be a switch across all my bikes (once I commit to a particular crank length). Again, no changes planned at the moment.

Well, let's assume on the extreme, we're talking about half a centimeter. That's about 3% difference in leverage, max. I suspect it's just better to optimize for comfort and power under normal circumstances. But assuming you're about the same in a range of cranks, I could see a few percent being very important when you run out of gears on unusually steep portions. If you're at a too low a cadence to be effective, this minor difference gets magnified since slightly more power equals slightly faster equals slightly higher cadence equals even more power. Of course, all this is speculation on my part, and not running out of gears would be better...

terrymorse 

The difference between 170 and 175 is 3%. That's 3% more torque into the cranks.

I certainly can tell the difference of 3% in torque, especially on a steep climb. I use 175s and tried a bike with 172.5s. It felt like I was trying to pedal through the bottom of the stroke. My leg length is 88 cm, which is on the long side (classic ectomorph body type).

Corsaire

Wow!, your leg length is 34.6 inches. I couldn't possibly use 175 mm cranks with my 30 inch leg length, not on my current bike frame anyway, perhaps, and I mean perhaps I could get away with it in a small compact frame.

Corsaire

mayukawa

It's analogous to the bore x stroke of an engine. You make a compromise of top-end power or low-end torque. And like an engine (even though it's a thermodynamic engine versus a chemical engine), you have an optimal power (rpm) range. Pedal too fast or too slow and you get tired quickly. Maybe everybody needs an hp/torque (power meter) vs rpm curve on the various gear combinations using various crank lengths to find what's optimal for them?

Mr_Super_Socks

According to Sheldon's gain ratio calculator, 170mm cranks in 39/29 yields 2.6. The same gears with a 190mm crank yields 2.4. With 175's it's 2.6.
Since the crank length is the radius of the circle, and not the circumference, you are actually getting less of a % benefit than the straight percent increase in crank length.
i.e. pi (170^2) < 1.03(pi(175^2))
or less than 1% advantage.

Pico

The formula for a circumference of a circle is 2*pi*radius. The radius is not squared, you're probably thinking of the formula for the area of a circle.

Leverage tracks in a 1:1 ratio with crank length.