kenshireen

I have tried to figure out what are the advantages/disadvantges pf a 170/172.5/175.
Why does the crank length make a difference.

Thank you

jyl

In theory, shorter (longer) legs -> shorter (longer) crank. However, I've never seen any data showing that 2.5 mm (1/10 inch) makes a difference.

fietsbob

ah, typos, no meth jokes need apply ..

Homebrew01

The longer the crack, the more likely it is to brake.

etw

Here is some info that you might find interesting

Bill Kapaun

I have one knee with a limited range of motion.
5mm made a hell of a difference, increasing my cadence from 70 to 85. (170>165)
2.5mm would have helped some.

habilis

On a fixed gear bike, crank length can make the difference between safely negotiating a corner or being thrown because of a pedal striking the pavement.

Hermes1

The longer the crank in theory the more power can be brought to bare, there are some schools that say this is does not bare out in reality. But depending on the length of your legs a longer crank can induce knee issues or aggravate existing knee issues.

HillRider

Mostly it seem to be personal preference and what you are used to. Some riders with joint or mobility problems (e.g. Bill) prefer shorter cranks for their smaller circle diameter and lesser vertical travel.

Several years ago Lennard Zinn did an extensive study putting riders of a great range of heights on a variety of crank arm lengths far greater than the 165 mm to 180 mm commonly available. He concluded crank arm length has almost no bearing on power output and was only a comfort factor. The GCN video above seem to support that conclusion. Recommendation: ride what's comfortable.

tcarl

Here's my personal experience. I have a 31" inseam and have been riding enough years to be able to feel some difference between crank lengths. 170mm and 172.5 are both comfortable and I can't really tell the difference between them. With 175's it depends on the situation. They're fine for me in any high-torque situation, such as mountain biking , loaded touring or hilly terrain. On flat land they're too long for me. I can feel my feet are going around in too big of a circle and my knees are going up and down too high and low. So I'd say crank length is a combination of leg length and intended use. From previous discussions on this topic (you may want to do a search, this subject comes up several time a year) many of the people responding say they can't tell any difference, so you may not either.

McBTC

Imagine walking and not thinking about your stride--just doing what feels natural. It's not hard because generally we don't think about it -- we do what's natural depending on the pace we want. Generally, the higher the pace we want the shorter and quicker the stride will be. Given the arbitrariness of a fixed crank length, it's the "shorter" aspect of turnover that is missing but in cycling, instead of 'stride' it's foot speed that you cannot make 'shorter' with a fixed crank length. So, if you wish to have a higher pace you know you'll need more turnover (gearing aside for the moment) and the only way to 'shorten the stride' will be to preselect a shorter crank length so as to lessen foot speed --i.e., so you can do what feels natural. And, while it may be different for different folks, the optimum may be higher cadence with lower foot speed -- achieved with shorter cranks -- while upping the inches per revolution by shifting to a higher gear.

dim

I read that a shorter crank gives you a faster cadence

a faster cadence (in the same gear), gives you more power, which gives you more speed

thats what it's all about (speed)

watch this video:

https://www.youtube.com/watch?v=L1uJ0oL98ps

exmechanic89

Um.. ok.

HillRider

I don't think you read all of it. A higher cadence in the same gear requires more power, it doesn't give you more power. You have to provide that yourself. Generally, a higher cadence is used in a lower gear for the same power and speed. The video you linked to shows training yourself to use a higher cadence and to develop more power through that training.

bradtx

The only verifiable difference I observed with my bikes (mix of 165 mm-175 mm crank arms) was that the longer crank arms required more effort to spin at a high cadence. I did switch the crank arm length on my then dedicated distance roadie from 170 mm to 175 mm, when I switched to a triple due to a nagging knee injury. The idea being that my torque sensitive knee could use the extra length to keep torque as high as possible. I don't really know if the extra length helped, but it didn't hurt in my situation.

Unless I rode two bikes with different crank arm lengths back-to-back, I couldn't feel any difference. The feeling went away rather quickly, however.

Brad

kenshireen

If you have sore knees is a shorter crank better

Falcon3

Also in crit races, you usually pedal through the corners, so a shorter crank arm can prevent pedal strike

habilis

Disclaimer: I'm neither a doctor nor an engineer, just a cyclist and an observer of mechanical things.

To help your knees, you should focus on two things: proper saddle height (to keep your legs as straight as possible while pedaling) and more rapid cadence. Bent knees hurt more than straight knees while you are exerting force against the pedals. Likewise, slowly struggling in a gear that's too difficult for the situation is bad for the knees.

Long cranks give more mechanical advantage than shorter ones, but only in a particular gear. Change to a lower gear, and you increase the mechanical advantage of the shorter cranks. Long cranks may actually force your knees to bend more near the top of the pedal stroke, which could aggravate knee pain.

Long cranks were once recommended for single-speed BMX bikes, where the crank provides mechanical advantage not available from multiple gears. But the same advantage could be obtained through a different combination of chain ring and cog.

A technique that helps reduce knee strain is standing while accelerating or pedaling uphill. This takes practice and can be exhausting until you get accustomed to it, but it really saves the knees by keeping the legs straighter.

Retro Grouch 

That's me too only, in my case, it's my left hip that's the problem. For me the sweet spot seems to be 165 mm cranks. I've got 170 mm cranks on my beater. That works OK, but I can tell the difference. I've tried cranks as short as 148 mm on my recumbent bikes. They're OK too but feel a little "choppy".

Bill Kapaun

On my quest, I tried 160's.
They were just too short and made me feel like I was pedaling a kids tricycle. It was too unnatural.

Paramount1973 

For most people it won't make any real difference in efficiency. You should be able to ride what feels best to you without worrying if you are reducing power output or efficiency. 170 mm cranks feel 'right' to me I, or so I thought, but I swapped the crankset on my commuter from 170 to 175 to get a better chainline (already had the Deore crankset in my stash) and really couldn't tell the difference after an hour or so. Six months later I'm still riding it.

https://wattagetraining.com/files/JMa...gTechnique.pdf

andr0id

Unless you have a really, REALLY low Bottom bracket, that is not going to happen with any reasonably modern road pedal.

I run 172.5 on my fixed gear road bike and don't even come close to hitting the pavement.

I run 170 on my track bike and have only bumped the track at walking speed on the bank. You just kinda thunk through it. A bit of a surprise the first time, after that, no big deal. It's a 44 degree bank at Frisco though. It's really hard to lean a road bike over to 44 degrees.

habilis

I like to exaggerate for dramatic effect.

Falcon3

I have hit a 175 around a corner before. I also like running a 165-170 due to having a little more clearance from most curbs- the curb can easily catch your pedal on a straightaway, especially if you're being squeezed by some cager, which is the worst time to get a pedal-strike on a curb.

McBTC

True... all other things aside, it takes more torque (that you provide) to increase the cadence while in the same gear and even more torque if you increase the rate of gain in elevation (and/or to overcome an increase in wind resistance), and that equates to an increase in [horse]power (or, a higher wattage).