Re: [CR]Crank length, today and yesterday

(Example: Framebuilding:Brazing Technique)

In-Reply-To: <12c.4d8aa3a.28dbcc6e@aol.com>
References: <12c.4d8aa3a.28dbcc6e@aol.com>
Date: Thu, 20 Sep 2001 20:08:41 -0400
To: LouDeeter@aol.com, classicrendezvous@bikelist.org
From: "Sheldon Brown" <CaptBike@sheldonbrown.com>
Subject: Re: [CR]Crank length, today and yesterday


LouDeeter@aol.com wrote:
>I don't know about science, but I've ridden 170, 172.5 and 175 on 52-55
>frames. What I have found is that with 170s I can spin fast, but don't have
>the leverage for hills. As I have gotten older, I don't spin so fast either.
> With 172.5, I can still spin and have good leverage for hills.

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.
> With 175, my knees come up too much, causing me knee problems.

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 joint.

Too long cranks cause excessive knee flex, and can cause pain/injury if it causes your knee to flex more than it is used to.

On the other hand, there doesn't seem to be any deleterious effect from shorter cranks.

I've been experimenting with this a bit myself lately. I commonly ride 165 mm cranks with a 42/15 ratio on 700c or 27 inch wheels, when I'm riding fixed. This gives a gain ratio of 5.8.

On multi-speed bikes I usually ride 170s. I'm 6 feet tall with long legs and a short upper body. I tried 180s for a while, they came on a used bike I bought. They made my knees hurt every time I rode that bike.

My latest experiment is taking place on plastic Trek frame I picked up in a barter deal. I had a pair of TA 150 cranks that used to be on my kids' Cinelli BMX bike, so I've put these on the Trek. I'm running a 45/17, which gives a gain ratio of 5.9, just a bit higher.

When I first get on the bike after riding with longer cranks, it feels a bit funny at first, but within a very short distance it's just fine. I go just as fast, climb just as well. For a given speed, my pedal rpm is higher (though my pedal _speed_ is the same) but the short cranks make it easy to spin much faster than I normally would.

After riding this bike for a few miles, when I get back on "normal" cranks, they feel a bit weird and long at first, then I get used to them after riding a couple of minutes.

I think people really obsess too much about crank length. After all, we all use the same staircases, whether we have long or short legs. Short legged people acclimate their knees to a greater angle of flex to climb stairways, and can also handle proportionally longer cranks than taller people normally use.

I plan to write an article on this topic soon, indeed, this posting could be considered the first draft.

Those unfamiliar with the gain ratio concept can read about it at http://sheldonbrown.com/gain.html

Sheldon "Too Long Is Bad, Too Short Doesn't Much Matter" Brown Newtonville, Massachusetts +-------------------------------------------------+ | To stay young requires unceasing cultivation | | of the ability to unlearn old falsehoods. | | --Robert A. Heinlein | +-------------------------------------------------+
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