Thursday, June 28, 2007

Physics of Crank Length - On my Bike of Course

I ran across an article about Crank Length on a bike. I was purposely looking for it so it would make sense that ran across such an article. I determined that if I had a longer crank that I could effectively increase my torque. You know if I had a long enough lever I could move the earth.

http://www.nettally.com/palmk/Crankset.html

He gives a formula for crank length.

The standard typically put on bikes is between 165mm & 175mm most bikes come with 170. Both my current bikes are 170. I had to measure inches and convert. It was fractionally off of 170, but close enough.

So, I went through the formula in the article above and discovered my optimal crank length should be 185mm. So, I did a little geometry and determined that the optimal angle between my foot and lower leg when the crank is ninety degrees from top dead center (horizontal compared to the ground towards the front of the bike) should be 58 degrees, based on 185mm crank. A picture would be helpful, but just trust me. So, with my current 170 mm crank I am closer to 61 degrees at this point. You understand this means my foot is not allowed to bend an extra 3 degrees on every pedal compared to someone who's optimal crank length is 170 mm, hence losing some of the power stroke. He has some theories about how this hurts a tall guy when hill climbing or sprinting.

So, I have been looking for a 185 mm crank. I have found some, none made by Shimano or SRAM. The one I found was over $400 (not a mis-print) for the crank, custom built by a dude in Colorado. So, off to eBay I went. A 185mm not happening. So, I checked 180mm. Saw one sell for $75 that was so well used that the manufacturer name was rubbed off the side of it. I looked the next size down which was 177mm, I have found them selling for about $35 to $40 dollars.

You may ask why do I care. I will tell you. From 170 to 185, I could achieve a 8.8% increase of torque at the 90 degree position. This probably does not translate into 8.8% increase in power over the whole revolution, but something higher than I am currently doing.

Now before everyone goes out and starts getting all crank happy, I must warn you that a smaller rider than I , must stay in his optimal range. A crank too short for you has never shown to do knee damage, but a crank to long has the potential. From what I have read, professionals have used them to get an advantage, but the advantage only lasts as long as your knee holds out from the constant increase in angle change.

I may not be able to get the 185, but am hoping for a 180, will settle for 177 which gives me 5.88% increase in torque, or 4.12% increase respectively. I believe I currently am losing power compared to someone with optimal crank length, so you could say I have crank envy. Ha Ha!

1 comment:

SJV said...

That is a very funny site, but I am in agreement with it. I always wanted the longest cranks I could buy. I still remember going to Bike World and getting the longest set they had combined with the biggest chainring available (at tyhat time, in Warren), after which I won every race I entered. Before that, I didn't. Now, I have a 170 mm, just like pretty much everyone else, but would love to get a longer one, and I could still use a bigger big ring on the front, just like the old days. If you figure out a way to do this, I will be interested. The BMX sites have them up to 190mm. I wonder if there is a way to fit those on a road bike.

On the other hand, improving our aerobic performance and form in each discipline will help us more than any technological change we can afford.

But it would be cool to have cranks that fit.