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-   -   Preferred crank arm length for Distance riding (http://www.bikeforums.net/long-distance-competition-ultracycling-randonneuring-endurance-cycling/924093-preferred-crank-arm-length-distance-riding.html)

dwmckee 11-27-13 11:01 PM

Preferred crank arm length for Distance riding
 
Hello - I have an old Bob Jackson 80's touring bike with 177.5 crank arms and a new Co-Motion Cascadia with 175 cranks. I am about 5' 11" and 175mm cranks are probably the right crank length for me but I keep finding my old Bob J is still far more comfortable for distance riding than my much more expensive (and custom fitted) Co-Motion. I am just curious if other distance riders find that longer than 'normal' cranks are more comfortable? Granted there are other differences between the bikes than crank length, but I'd like to hear other's comments about crank length preference.

Thanks,

Don

Carbonfiberboy 11-28-13 08:43 AM

Some of the randos I know prefer a shorter crank because they tend to pedal a little easier at a higher cadence than if they were sport riding. Otherwise, they ride regular crank length.

Chris_W 11-28-13 01:00 PM

I'm 5'8" and prefer short cranks, 165mm and 167.5mm depending on the bike. I also like spinning at a high cadence, and short cranks are well suited to my style and body.

I wouldn't say that this setup would be right for anyone else, but I think that more people should think about crank length and try different lengths to find what works best for them. Manufacturers and shops should also make a greater variety of lengths more easily available.

lhbernhardt 11-28-13 03:28 PM

To understand crank length, you need to understand 1) angular velocity (loosely, "rpm") and 2) linear velocity (just plain speed over distance).

For a given angular velocity, a shorter crank makes your foot have to travel only a slower linear velocity, because it is making a smaller circle. For example, if you mount two wheel speed sensors on your fork, one near the hub and one near the tire, they will both register exactly the same speed. But the spoke magnet closer to the tire will be moving WAY faster than the magnet mounted near the hub.

So on level ground, spinning away at 90 rpm, your legs don't have to move as quickly with 165mm cranks as opposed to 175mm cranks.

The downside of short cranks is that you don't get the leverage on a climb. Or, on level ground, you can choose to dump the chain onto a bigger gear, spin more slowly, and maybe ride faster.

Now having said this, I will admit that I have been using a fixed gear with 165mm cranks for the past few years. I have completed PBP 2011, LEL 2013, and a California Triple Crown in 2012, all on the fixie. In my experience, when compared with riding similar distances on a geared bike (a 1000km ride nonstop in 2010), I find that I am much fresher on the fixed gear and 165's after the first 100 miles, and it takes me much less time after the ride to recover. I think that bigger gears, longer cranks, and lower cadence serve to load up the muscles with lactic acid.

I say this even though I'm often way overgeared on the hills with the fixie. The advantage of having gears is that you can shift down (and I mean REALLY shift down, like to 39x23 for a 6% grade) and quite often get up hills faster than trying to leverage up with long cranks and a bigger gear (which tends to tie up your legs more). I think there's a lot to be said for spinning up hills at close to 90 rpm, god knows I've been left in the dust by a lot of riders using this strategy! And this is much easier to do with shorter cranks.

So, conclusion: I don't think I'd go any longer than 170mm, even at 6'1" (me) for distance events. It might be different for short (40km) time trials, or hilly road races. But if you're doing ultramarathons, I think spinning is the way to go. You want to stay fresh as long as possible.

Luis

Coluber42 11-29-13 01:56 AM

I use 165's too although basically all my distance riding is on a fixie, too. I'm 5'6". At a certain point years ago, around when I started doing a much higher percentage of my miles fixed, I realized that I felt like I was pedaling squares on the geared bikes (with 170's), on the rare occasions that I rode them. I switched everything over to 165's and it felt much more natural. I can't be sure that it isn't just what I'm used to, but I definitely feel more comfortable overall with 165s.

It's a complex picture, though. In addition to all the aforementioned stuff about angular velocity and linear velocity, there's the angle of your knee at the top of the pedal stroke, the amount of leg extension at the bottom, and the gears you have available. Longer cranks give you more leverage, but so do lower gears. See Sheldon Brown's discussion of what he calls "gain ratio" for more on that.

Generally, it's probably fair to say that riders with longer legs tend to prefer longer cranks than riders with shorter legs, but there are many other factors that can come into play. One of my idle theories is that riders with bulkier/heavier legs are more likely to prefer shorter cranks because the mass of the leg doesn't have to go up and down as far, but maybe that's just the result of descending so many long hills on a fixie (having legs on the bulkier side). I do think riding style and body type are important factors, and if you feel better with one crank length than another, that is more convincing to me than what conventional wisdom would dictate.

Steamer 11-29-13 04:59 PM

http://www.plan2peak.com/files/32_ar...gTechnique.pdf

summary: choose crank arm based on simple preference, as it won't make much difference to your power one way or the other.

Homeyba 12-01-13 05:19 PM

When you change your crank arm length all you are really doing is changing your effective gear ratio. Just like changing gears. If you go to longer cranks without changing any of the other variables (wheels/gearing), you will have more "leverage", which is just another way of saying you'll have a lower effective gear. On a multi-speed/geared bikes, you can change gears at will so what's the point?

Rowan 12-02-13 04:01 AM

Quote:

Originally Posted by dwmckee (Post 16285126)
Hello - I have an old Bob Jackson 80's touring bike with 177.5 crank arms and a new Co-Motion Cascadia with 175 cranks. I am about 5' 11" and 175mm cranks are probably the right crank length for me but I keep finding my old Bob J is still far more comfortable for distance riding than my much more expensive (and custom fitted) Co-Motion. I am just curious if other distance riders find that longer than 'normal' cranks are more comfortable? Granted there are other differences between the bikes than crank length, but I'd like to hear other's comments about crank length preference.

Thanks,

Don

The issue with comfort may not have much to do with crank length in regard to pedalling efficiency, but rather the geometry of your bikes, and the saddle height, which changes marginally between crank lengths. What are the differences between the angles on the Jackson versus the Cascadia?

I've generally stuck with 170mm cranksets after opting for that length with my fixed gear some years ago. I figures that on each revolution my foot will be moving just slightly less distance than with a longer crank, and of course that leads to what I rationalise as cumulative savings in energy output (argue away on that one because it is only a perception). An additional factor was the slightly reduced risk of pedal strike on cornering with a fixie.

The exception is the tandem, and I don't mind the longer 175mm cranks because it makes stopping and starting while in the saddle easier. I am around 5'11" tall.

rhm 12-02-13 01:50 PM

Crank arm length does not matter much. People as much as 8" shorter than I are able to use longer cranks than I and experience no discomfort with this. That said....


I'm 6' tall and use 165 mm cranks generally, 160 on one or two, 171 on one. Crank arm length matters more in the relative extremes:
The advantage of shorter cranks is that they are easier to spin at high cadences;
The advantage of longer cranks is leverage, which means you can power through tough situations at low cadences such as heavy snow, soft sand, etc.;

If you get used to short cranks, you may suddenly find longer ones uncomfortable.

delcrossv 12-02-13 02:38 PM

Quote:

Originally Posted by rhm (Post 16294639)
Crank arm length does not matter much. People as much as 8" shorter than I are able to use longer cranks than I and experience no discomfort with this. That said....


I'm 6' tall and use 165 mm cranks generally, 160 on one or two, 171 on one. Crank arm length matters more in the relative extremes:
The advantage of shorter cranks is that they are easier to spin at high cadences;
The advantage of longer cranks is leverage, which means you can power through tough situations at low cadences such as heavy snow, soft sand, etc.;

If you get used to short cranks, you may suddenly find longer ones uncomfortable.

I have 170's on the upright and 150's on the 'bents. I notice the need for shorter cranks on the bents much more than on the upright even though the hip to pedal distance is very similar (30 degree knee bend at max extension). I'd find 170's to be excruciating on the bent but have no problem with them on the upright.

My cadence is a little lower with the longer cranks - maybe a 10 rpm difference.


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