Crank length, how important?

ravey1981

I'm looking to replace the cranks on my road bike. Currently have an FSA compact crankset (cant remember the model) on an ISIS bottom bracket. After several strip and rebuilds I can't get them to stop creaking so I'm giving up. I want to replace with a shimano Ultegra SL 6650 compact crank to match the rest of the groupset, I with I had just bought one on the first place.... These are getting rare to find new these days and prices seem very high although I've found a 170mm chainset for a reasonable price.

So... How important is crank length? I think from memory my current cranks are 172.5, does such a small amount really make a difference? I am 5'10" tall and wear 32 regular leg jeans for reference...

Thanks in advance for any input.

mitchgixer6

170mm should be fine for you and i'd doubt you'd notice much difference.

Bozman

I'm 5'10" and I'm using 170 on one bike and 172.5 on the other and in the past I've used 175, I've yet to notice a difference.

canoas

170mm is too small.

I'm 6 foot and use 175mm. I went from 172.5mm about 2 years and swapped to 175.

I noticed one difference, they are much better for climbing as you get more leverage. I used the Zinn book if I remember he says from 183cm upwards try 175. 172.5 is fine for most people.

I used 175 as one guy I meet on the Pyrenees who was the same height and weight as me before a sportive and he was a great climber, I was never going to be as good as him, anyway we were talking about setups over coffee and he suggested for me to go 175. I must admit its not a huge difference for climbing, other than this point I wouldn't bother changing from 172.5, 170mm is way too small for your height.

mitchgixer6

That's a load of tosh. I know for a fact that Obree uses 170mm cranks and he is about 5,10. If it's good enough for him.....

Shorter cranks will allow for a higher cadence whilst keeping the same power output which should be better for climbing. A lot of guys who ride TT's opt for longer cranks as it does give more leverage and this is preferred when holding a short cadence with large power output.

neeb

You are not going to get a definitive answer to this question...

It's like asking for a saddle recommendation, what works for one person isn't necessarily going to work for another. Some people swear by short cranks (even much shorter than 170mm for someone of average height), saying that it allows them to spin better and sustain higher power, others will say 180mm gives them more leverage and power for climbing...

There are a huge number of interacting factors involved - your individual metabolism and how this affects your preferred cadence, your preferred climbing style, muscular-skeletal factors like optimal range of hip/knee movement, etc etc...

The best thing would to be experiment, but unfortunately most people can't afford to buy several different cranksets to do this, and also it takes a while to re-adapt to one length if you have been using another.

But you probably won't notice a change from 172.5 to 170mm, unless maybe the 172.5 was shorter than is ideal for you to begin with.

StillGoing

Use the forum search function. You've now invited another thread with the same old arguments for and against when the last one can't be more than a couple of weeks old.

bahzob

It's next to impossible to get a definitive answer to this topic on a forum, there are too many variables that differ by individual.

In the final analysis if there is a difference then it will not be that large, at least between the standard sizes.

If you really want to find out the answer go to a good bike fitter and ask them what they recommend and, just as important, why. Then do a comparison to check if their theory matches your experience (this should be possible during the fitting itself, if it isn't then choose a different fitter)

imposter2.0

canoas wrote:

I noticed one difference, they are much better for climbing as you get more leverage.

Utter nonsense, as already pointed out. How much more leverage do you think an extra 2.5mm will actually give you?

StillGoing

Imposter wrote:

canoas wrote:

I noticed one difference, they are much better for climbing as you get more leverage.

Utter nonsense, as already pointed out. How much more leverage do you think an extra 2.5mm will actually give you?

The arguments about noticing any difference are applicable only to the individual. The individual's ability to notice/feel change is what allows an experienced motorcycle racer to notice a 1mm difference in suspension changes whereas a complete novice wouldn't even notice if the wheels were missing. In the same way some cyclists can notice the difference/benefits in crank arm length and some can't.

bompington

philthy3 wrote:

Imposter wrote:

canoas wrote:

I noticed one difference, they are much better for climbing as you get more leverage.

Utter nonsense, as already pointed out. How much more leverage do you think an extra 2.5mm will actually give you?

The arguments about noticing any difference are applicable only to the individual. The individual's ability to notice/feel change is what allows an experienced motorcycle racer to notice a 1mm difference in suspension changes whereas a complete novice wouldn't even notice if the wheels were missing. In the same way some cyclists can notice the difference/benefits in crank arm length and some can't.

Unlike a lot of component changes, you could actually do a blind test, as I doubt anyone could actually tell by looking whether their cranks were 2.5mm longer or not. I would be interested to see the results - and I suspect that very few could reliably tell the difference: but P3 may well be right, some might. Not sure I can see anyone being bothered to do the trials though ;-)

imposter2.0

philthy3 wrote:

Imposter wrote:

canoas wrote:

I noticed one difference, they are much better for climbing as you get more leverage.

Utter nonsense, as already pointed out. How much more leverage do you think an extra 2.5mm will actually give you?

The arguments about noticing any difference are applicable only to the individual. The individual's ability to notice/feel change is what allows an experienced motorcycle racer to notice a 1mm difference in suspension changes whereas a complete novice wouldn't even notice if the wheels were missing. In the same way some cyclists can notice the difference/benefits in crank arm length and some can't.

The 'leverage' argument is completely fallacious, regardless of whether anyone thinks it makes a difference or not. That's all I'm saying. Someone claiming it makes a difference does not change that.

neeb

Imposter wrote:

The 'leverage' argument is completely fallacious, regardless of whether anyone thinks it makes a difference or not. That's all I'm saying. Someone claiming it makes a difference does not change that.

Why? The difference in force required to produce the same effect between a 172.5mm crank and a 175mm one is about 1-2%. That's maybe borderline noticeable, but the difference between 170 and 175mm would be 3-4%, which should certainly be noticeable. That equates to a difference in cadence of around 5 rpm I think (?)

imposter2.0

Maybe we should all be on 200mm cranks then....we'd all go up hills faster....

dj58

I have to concur with what philthy3 says, I purchased my first road bike recently and it had 172.5mm cranks, after years of riding 170mm on my MTB road/commuting bike, I could feel the difference straight away on my first ride. While no doubt I could get use to them and they seem to be standard fitment on road bikes, (medium frame), I've swopped/upgraded my chainset to a 170mm as that's what I prefer. Personally I find them more comfortable when spinning up climbs.

bahzob

neeb wrote:

Imposter wrote:

The 'leverage' argument is completely fallacious, regardless of whether anyone thinks it makes a difference or not. That's all I'm saying. Someone claiming it makes a difference does not change that.

Why? The difference in force required to produce the same effect between a 172.5mm crank and a 175mm one is about 1-2%. That's maybe borderline noticeable, but the difference between 170 and 175mm would be 3-4%, which should certainly be noticeable. That equates to a difference in cadence of around 5 rpm I think (?)

This just reinforces my point that trying to reach a definitive answer for an individual in a forum like this is just like the tip of my pencil...pointless.

It is simply impossible to gauge the theoretical benefit in terms of leverage or anything else in the absence of other data that may complement or offset it. This data varies widely between individuals and even for the same individual may vary according to circumstances (e.g. you could speculate that on a turbo where power is uniform one crank may outperform another but the situation would be reversed in a road race where premium is on rapid acceleration. This may or may not be the case, it is,as stated, just speculation).

How Far is too Far

I have 5 bikes and I could not even tell you what length the crank arms are, I just ride them as fast as I can.

I doubt anyone would know if a blind test was done, you would notice more if you adjusted your saddle a bit.

imposter2.0

Just to pick up on what bahzob said - more torque (which a longer crank/lever will undoubtedly give you for a given rotation) does not necessarily translate into more sustainable power.

Sheldon has a piece on it here: http://sheldonbrown.com/cranks.html

By all means change your crank length for comfort/biomechanical reasons, but not on the expectation of going faster.

neeb

bahzob wrote:

neeb wrote:

Imposter wrote:

The 'leverage' argument is completely fallacious, regardless of whether anyone thinks it makes a difference or not. That's all I'm saying. Someone claiming it makes a difference does not change that.

Why? The difference in force required to produce the same effect between a 172.5mm crank and a 175mm one is about 1-2%. That's maybe borderline noticeable, but the difference between 170 and 175mm would be 3-4%, which should certainly be noticeable. That equates to a difference in cadence of around 5 rpm I think (?)

This just reinforces my point that trying to reach a definitive answer for an individual in a forum like this is just like the tip of my pencil...pointless.

I did make the same pointlessness point earlier..

Of course it's possible to find mechanical answers if you isolate/assume certain variables, but I agree of course, if you have all the variables varying and interacting with each other then the system becomes far too complicated to give definitive answers, even if all of the variables were easily quantifiable (which they aren't, especially the biological ones).

neeb

Imposter wrote:

Sheldon has a piece on it here: http://sheldonbrown.com/cranks.html

One thing he doesn't mention there is that while it's true that the leverage difference is offset by selecting different gears and that the real difference comes down to the range of movement of the joints, this range of movement interacts with the variation in power over the course of the pedal stroke. Also (but closely related), with different length cranks (but the gear and crank length selected to create the same power and the same cadence) there are differences in how force applied varies with leg extension. Some people might be better at applying a lot of force when their leg is within a relatively small range of extension variation, others may be better at applying the same force over a greater range of leg extension.

Basically, you can't say that anyone is "wrong" to say that they can notice a difference between crank lengths, nor can you say that any length is necessarily better for any individual. It's all too complicated to be solvable.

imposter2.0

neeb wrote:

Basically, you can't say that anyone is "wrong" to say that they can notice a difference between crank lengths, nor can you say that any length is necessarily better for any individual. It's all too complicated to be solvable.

I'm not saying anyone is 'wrong' if they feel a difference. If they 'feel' a difference, then that's great. In reality though, the 'difference' they 'feel' does not actually amount to anything in speed or performance terms.

DubaiNeil

I am 5' 11" and have 172.5 on one bike, 175mm on my single speed - and due to a shop error I even had a 172.5 DS and 175 NDS on a third bike...

I can't tell the difference to be honest...

neeb

Imposter wrote:

neeb wrote:

Basically, you can't say that anyone is "wrong" to say that they can notice a difference between crank lengths, nor can you say that any length is necessarily better for any individual. It's all too complicated to be solvable.

I'm not saying anyone is 'wrong' if they feel a difference. If they 'feel' a difference, then that's great. In reality though, the 'difference' they 'feel' does not actually amount to anything in speed or performance terms.

You can't really know that. Maybe some people fatigue more quickly if they have to apply power with a greater range of muscular motion, and maybe it's non-linear in a way that means 2.5mm makes a significant difference. Who knows?

meanredspider

Intuitively, it seems hard to believe that 1.5% difference in a dimension is going to make a significant difference. I'm sure that, whilst it might be "optimal" to have a certain crank length, it won't be a show stopper. I'm finding it hard to think of another bike dimension that comes in such small percentage increments.

FransJacques

neeb wrote:

Imposter wrote:

neeb wrote:

Basically, you can't say that anyone is "wrong" to say that they can notice a difference between crank lengths, nor can you say that any length is necessarily better for any individual. It's all too complicated to be solvable.

I'm not saying anyone is 'wrong' if they feel a difference. If they 'feel' a difference, then that's great. In reality though, the 'difference' they 'feel' does not actually amount to anything in speed or performance terms.

You can't really know that. Maybe some people fatigue more quickly if they have to apply power with a greater range of muscular motion, and maybe it's non-linear in a way that means 2.5mm makes a significant difference. Who knows?

I've done a lot of experimenting with cranks following several schools of thought (Lemond's, Zinn's, Serotta/SICI etc.). I've ridden170 to 180. If you have multiple bikes set up with very similar dimensions (why wouldn't they be?) and you switch between them from day to day of course you can feel a difference. You can feel the 180s are a benefit on a climb but trying to do a flat, full-on 300m sprint with the 180s is almost impossible. With the 170s it is much better. Your legs tell you what you're riding, it's gradual but they tell you.

Some riders are remarkably tolerant of variations. I'm more of a Prince and the Pea.

ravey1981

Thanks for the input guys, looks like I will be plumping for the cheaper 170's then as for me the most important variable is price. I'm not a racer nor do I own a power meter, I just ride my bike because I like doing so (but I do not like creaking cranks!)

Cheers

ps will remember to do a forum search in future before re-opening a can of worms...sorry