Crank length (3)

Triathlon Forum
1

I’m wondering if you all use “normal” cranks lengths or something different. I’m about 5’10", so the standard size for me is 172.5mm, which I have on both my road and tri bikes. But I think this sizing was mainly developed for road riding. Do people use longer/shorter (probably longer) cranks for tri training and racing? How much longer?

-Colin

2

I’m 6-3 and use nothing but 180’s on my TT bikes. My road bike has 175’s. Its easier to change cadence when the group jumps on rides with the shorter cranks. I’ve tried to go shorter on the TT’s but I always end up going back to the 180’s. (BTW- I’m getting old and still turn the 180’s just fine, so thats not a factor).

3

Great primer: http://www.cptips.com/crnklth.htm
.

4

I’m 5-11 and use 175’s.

5

Can anyone explain - really - how crank length making a difference? Here are my intuitive thoughts:

1 - The difference between 170 and 175 mm, when looking at a ruler, seems to be negligible.

2 - On a percentage basis, 5 mm’s are about 3% of 170 mm. But - when you look at cptips or others, they fit that 3% difference to people with limb length differences that vary by about 25% (35-28" inseams).

3 - From an effort basis (and I currently have a sore knee which I think is chondromalacia/overuse…it aches after hard bikes), I can’t understand how the leverage differences in cranks changes the effort I put out. My force is my force, no matter if it applied to a 7" lever or a 7’ lever.

4 - From a fit/orthpedic basis, since one of the starting points in fit is the knee-vertical-over-the-spindle but moving the saddle, it wouldn’t see that the length difference would not have much effect on knee problems.

5 - from a cadence basis, I could see some variation/value. But I haven’t worked out the physics to understand diameter vs. speed

It would be interesting to hear your thoughts.

I also remember reading something about Zinn creating a formula that tried to go beyond the basic choices and he felt that it came down to a marketing thing where vendors don’t to supply so many choices.

6

I’m an unusually tall rider. My inseam to the floor is 39". My road bike has 175’s and that’s all I’ve known. I’m about to buy a custom bike and I’m going to go with 210mm cranks. After reading Leonard Zinn’s article on zinncycles.com, I’m sold. His theory just makes sense to me.

I think he came up with his formula (inseam in mm multiplied by .216) from taking the average pro cyclist inseam and relating that to the 172.5 cranks common in the peloton.

I’m definately a masher and I just have to believe that extra length equals extra power. Basically we’re only talking 35mm in each direction, less than an inch and a half. But my leg length is several inches longer than the average joe, so I don’t think biomechanically it will be a problem. I posted a thread about this months ago and all I heard was “no free lunches, longer cranks would take more watts to turn.” Sure, but I’m 6 friggin 9 over here. I’m working with freakishly long pistons.

If your inseam is less than 34 inches I could see where going with the long cranks would not help any, but being a giant, I’m thinking they will help.

I’ll be glad to let you know what I think in a few months. Zinn is 6’6" and says they make a notable difference (and I know he is trying to sell the damn cranks, but he makes a good argument).

He also makes shorter than standard cranks for all of you pygmies.

7

Longer crank length can mean (two examples):

  1. Less pedal force at same cadence (but longer path of motion for foot)
  2. Same pedal force at slower cadence.
    In terms of physics at the pedal, the power is the same. The issue is one of comfort, repeatability (endurance), efficiency (whether pure biomechanical efficiency or endurance-type efficiency), etc. There are lots of things you could look at, and no one has the ultimate answer. Go for it and try your new cranks. Just make sure your BB is high enough that they don’t hit the ground! Let us know how the 210s work out.
8

There is not that much written about it because most people are less than 6’3". So, to 99.something percent of the population; who cares? I am letting the frame designer know to bring the bottom bracket height up so that my cranks won’t scrape the road every time I make a turn.

I think Jan Ulrich time trials on slightly longer cranks and he’s over 6 foot, and I think Indurian did to. But, I’m Shaq compared to those guys. Anyhoo, it will be interesting, i’ll let you know.

I’ll post pictures.

9

Longer crank length can mean (two examples):

  1. Less pedal force at same cadence (but longer path of motion for foot)
  2. Same pedal force at slower cadence.
    In terms of physics at the pedal, the power is the same. The issue is one of comfort, repeatability (endurance), efficiency (whether pure biomechanical efficiency or endurance-type efficiency), etc. There are lots of things you could look at, and no one has the ultimate answer. Go for it and try your new cranks. Just make sure your BB is high enough that they don’t hit the ground! Let us know how the 210s work out.

There are other factors to consider.

A 5mm increase in Crank length means having to lower your seat by 5mm – does not sound like much – but you can feel it.

Also the diameter of your pedel stroke increases by 10 mm – this means that while one foot if down – your up foot is 10 mm higher – or your knee is 10 mm closer to your chest – unless you are very flexible - you will feel that!!

Odds are you are not the most flexible and lowering your seat 5 mm and then raising your stem 5 mm so you can still bend equals being less aero – you will lose any power gain by having a longer lever.

A friend was riding 180s and was having IT band problems – I convinced him to swith to 175s and his IT problem lessened.

Even going from 172.5 to 175 can mess up a good aero position. You need to fit your bike and crank length to your own personal height and flexability.

10

I agree that crank length depends on height and flexibilty. And, for the majority of the population standard cranks 170-175 fit the bill.

Tall clydesdales are already at an aerodynamic disadvantage. Think about it.

“That’s just my opinion, I could be wrong.”
-Dennis Miller

11

I agree that crank length depends on height and flexibilty. And, for the majority of the population standard cranks 170-175 fit the bill.

Tall clydesdales are already at an aerodynamic disadvantage. Think about it.

“That’s just my opinion, I could be wrong.”
-Dennis Miller

As I said – need to fit the crank length to your leg length and flexability. Not good for someone 6’4" tall to be riding 170mm cranks and not good for someone 5’6" tall to be riding 180mm cranks.

Just seems like the discussion was headed more towards power gain without any consideration of other factors.