Eire Mick

I'm looking to upgrade my crankset and I want someone to explain the benefits/disadvantages of having a shorter crank arm set up. I have 170mm now, but I found a good 165 crankset with a 46t chain ring (I have a 46t chain ring with 170mm arms). My gear ratio is 46/17 fixed. Thanks.

kyselad

Shorter cranks give less chance of a pedal strike with the ground, and they're a bit easier to spin at high cadence. If you're riding on the street, 165 is generally a better choice.

K3NNY

Leg length also plays apart check out this dandy little chart: https://nl.tinypic.com/view.php?pic=315mwqt&s=5

jdms mvp

1. reduces overlap
2. reduces the occurrence of pedal strike

larsalan

I read sheldon saying a 175 gives 3% more gear in. than a 170. So, I would believe that a 170 is the same regarding a 165. As for the pedal strike, I don't see it being a problem w/ a 170. I have used a 175 and struck it. I think the real issue w/ striking is how wide your pedals are.

tmh657

I am 6'1" with a 34" inseam.
I have used 165mm, 170mm, 172,5mm. I now have 175mm Dura Ace track cranks which are 175mm. I got those because they were a smokin deal on CL. I have less pedal strike issue with these cranks than with 170mm Sugino RD's because the crank arms are closer to the frame than the RD's. They go almost straight from the BB and spider but the RD's have a curve out to them.

You will generate more torque with longer arms but can't spin as efficiently. There are tons of threads about it.

Gotta remember that BB clearance is the other factor in pedal strike. A conversion will most likely have a lower BB than a track frame.
My 2¢ worth.
BTW I love my DA cranks. 49t and 17t

elTwitcho

Why is it that people consistently ignore point number 3.


3. Reduces leverage on the chainring and requires more torque to pedal.


170s are noticeably easier to pedal (plug in the different crank lengths in a gear inches calculator) than 165s IME. This is slightly offset by the smaller circumference of the pedal stroke but personally, I think 165s suck.

Jabba Degrassi

Most fit sites actually (like the one above, for example) suggest 165s for my inseam. Anyway, couldn't you just compensate for the difference with a different size cog and/or chainring?

rotharpunc

I use 165 since I ride a smallish frame. Lack of torque isn't that noticeable to me when riding, but it does seem to make skidding a bit more difficult.

kyselad

Yes, you can compensate for the gearing difference. However, there's still some leverage advantage in a longer crank, which comes at the expense of a smoother spin. I imagine most folks here ride under conditions where pedal strike is more of a concern than leverage, but obviously that's not going to be true across the board.

elTwitcho

Not exactly. You can compensate with a step up in a cog, but then for instance (and these numbers are pulled from the air just to explain the example) you'd be running 48-19 so that it FELT like running 48-18. But you'd only be going 48-19 speed. So really, you end up going slower for what feels like the same effort.

That said, if you're more comfortable riding 165s as an issue of bike fit, ride 165s. Personally, having given them both a spin (165s and 170s) I hate the shorter cranks. Personal preferrence is everything though, ride what makes you happy. I'm just saying it bears pointing out that 165s are going to require more torque to move the bike.

kyselad

Shorter cranks don't fundamentally move you any more slowly for the same amount of effort -- they shorten the path your foot takes in rotating the cranks. You can get back the same effective ratio (Sheldon calls this the "gain ratio") by increasing the cog size. You don't end up going any more slowly for your effort, you just end up doing a little more rotation along a shorter path than with the longer crank. The radius is tighter with shorter cranks, and from a biomechanical perspective, certain lengths will work better with certain legs, and it's a matter of finding the sweet spot in the middle.

elTwitcho

It depends on what you define as "amount of effort". You can't get back the same "effective" ratio by changing cogs out. 48x18 will always move a given amount per pedal rotation, regardless of whether you're running 175 cranks or 165 cranks. However,

48x18 with 165 cranks will require more torque in a given rotation than 48x18 with 170 cranks to move the exact same speed. The tradeoff is that the circumference of your pedal stroke is shorter. What's defined as "effort" is arbitrary though so there's no objectively correct answer either which way.

octopus magic

If you are short, get 165's.

If you are tall, get 170's.

If you are cheap, you get whatever you can take.

It's really not that hard on track bikes.

dookie

not to be picky or anything, but the torque is also identical. T = F*r, where F is your leg pushing the pedal ('effort'), and r is crank length. keep T (ie: gear inches) constant...if r goes up, F goes down. which is the whole point...longer cranks provide more leverage ant the expense of pedal strike and/or lack of spin.

JohnDThompson 

You have less chance of pedal strike with shorter arms, so you can take corners faster. This is particularly an issue if you're riding a road bike conversion, as road frames tend to have more bottom bracket drop than track frames.

You get more leverage with longer arms which makes it easier to mash gears.

carleton

True dat.

It depends on:
- Your bike's geometry. If it has a low BB then pedal strike (your pedal hitting the ground when turning a corner) is a concern. If your pedal strikes when in the turn, you will likely go down.
- Your shoe size. Big feet make toe overlap (touching the front tire during turns) an issue
- Your gear ratio. Shorter arms make tougher pushing. You will notice the difference during starts and stops.


So, weigh the benefits of the shorter arms with the cons being that they will be harder to push and you won't get the full stroke of power from your legs (if you are a taller rider).

adriano 

im pumped to try out my 160s!

carleton

Man, that's REALLY short. Unless you are a shorter dude with a small bike, it's gonna feel like you are pedaling a tricycle.

adriano 

ive got a 47cm and a big heart.

carleton

If it didn't make a difference then why do cranks come in 2.5mm increments?

Heck, why are bike components measured in millimeters in general?


...because it does matter.

elTwitcho

I can't tell the difference between a tire inflated to 130psi and one inflated to 90psi just by looking at it either. You can tell subtle differences in your components when you actually use them though

carleton

Have a look here: https://www.cptips.com/crnklth.htm

octopus magic

Yeah, it does make "that much" of a difference.

Hey, a difference in frame size is only what, an inch? Why bother getting a frame that fits you correctly?

elTwitcho

Yes and yes. It's not a huge difference, but it's definately noticeable.