crank arm length and power meter

Crank arm length is something that can be very beneficial especially on TRI bikes. On my road bike I ride 175 cm cranks and my tri bike I have 165's. The main reason (at least for me), is in the aero position with the shorter cranks I can spin faster, and it is easier for me to crank out the miles, this has something to do with the different muscle groups engaging at diffent times (I don't even pretend to understand the science, I just know there can be a positive benefit). So if you have a tri specific bike I would look into shorter cranks.

Because a compact crank still has your legs performing the same size circle, where as a shorter crank arm you are reducing the size of that circle. When you are riding in the aero position your do not have as free of motion in your legs (up wise) than you do riding the hoods of the road bike. for the record when I bought my try bike I took the 50/34 compact double with 175 cranks and changed it to a 53/39 with 165 cranks (and yes I realize my error in my previous post where I said centimeters instead of millimeters)

Thanks..FWIW I have a standard crank and was looking at the compact crank .....I have to.double.check but on my Felt b 16 I have 170 on it now........I thought due to leverage I would generate more power with th e longer arms per rpm?? Or is that wrong?

the "t" means teeth. It's redundant to put 50t/34t, so they just put it after the second number. The first number is the number of teeth on the biggest chainring, then the next size, and so on (If you have a triple crankset, for example).

Many people say that compact cranks are better for triathletes, but it's really not any different than a 53 with a larger freewheel in the back. Chain distance remains the same.

John

Thanks. Yes, I knew what the numbers meant but thought I was missing something with the t since some had it as a selection when ordering and others did not.

Some athletes will go with a shorter crank on a tribike as it will increase separation between thigh and torso during the pedal stroke; if you go from say 175mm to 165mm, you would have to raise your seat by 10mm to adjust for the change in crank length and your knee would gain 20mm in separation from your torso.

So, if you find that you are often massaging your torso with your thighs or knees while riding aero or if you want to go lower but feel that you are limited in how much lower you could go before your knees run into your chest, then you may want to consider shorter cranks.

Changing crank length also changes the gain ratios so it may be possible to adjust the cassette you are riding in order to get the right gearing combinations based on the crankset (crank arm length and chainrings) you are running.

FWIW I currently have 175mm - 170mm cranks depending on the bike and have run as short as 165mm and as long as 177.5mm and I couldn't "feel" the difference between them as long as the position was adjusted properly. They all feel right when I'm on that bike so for me, changing crank length is a non-issue but there are others who say they feel a change of 2.5mm and prefer to have the same crank length on all bikes.

Shane

You may already know this
If you have problems going lower in front, a shorter crank length may be part of the solution.
Imagine this : let's say you get cranks 5mm shorter.
a) When you are with the pedal at the lowest position position your pedal is now 5mm closer to your seat because the cranks are shorter
b) When you are with the pedal at the highest position, your pedal is now 5,m further from you seat. This is the closed hip position

Because of a) you can rise you seat 5mm and preserve your knee angle
By doing that, you are now 10mm further from the pedal in scenario b, which opens up your hip angle.

Your drop (seat to bars) has increased 5mm due to the seat raise. Your back will be a little flatter
But your closed hip angle is opened up, allowing you to drop even more if you want

In theory, you should move you seat back 5mm to preserve you knee over pedal position,

So all this to say you can go lower in front and get more aero without requiring any additonal flexibility.


EDIT : pressed the submit button at the same time as Shane :-)



Edited by marcag 2013-10-30 7:24 AM

That is false. There are too many levers of action from pedal to your hip for that association to ring true.

It is actually true as posed but on a geared bike, it should not be an issue.

Assuming the same gear combo and cadence, longer crankarms will generate slightly more power due to the increased length of the lever arm.

However, since our ability to generate power to the pedals is very rarely limited by the mechanical systems of the bicycle, the increased leverage of a longer crankarm provides no benefit for an endurance cyclist.

Shane

This is a good discussion, since I've considered this route too. I find that I can't go lower on the front end of my SHIV, and even if I could and did, I'd run into flexibility limits and the fact that my thighs would bang against my chest. I've considered going to 165mm from 170mm. I'm kind of worried, not from the hip angle aspect, but there could be the situation that I could become less aero... I'd love to go to a wind tunnel to assess whether this change would improve things or not. What's everyones' thoughts?

There is an optimal crank arm length for every athlete. There are several thoughts on this (length of femur bone, flexibility, even running form). I don't pretend to know all of the science but I have had the opportunity to use a cycleops power trainer and try several different crank arm lengths while running the same heart rate to see the difference. It was huge but there were difference in power output. I found that with a 170 crank arm I produced 8 more watts at the same HR compared to the 172.5 that I was running at the time. As we got to extreme length differences the power output or lack there of was greater.

After doing this with several guys that we ride with we determined there is definitely a correct crank arm length for everyone and the standard that comes with your bike may not be the one.

Yah, I have read the studies too. Which crank did you settle on? 170? Did you try any shorter and the results? What is your height? Leg length?

I have long legs proportionally to torso. I have used the ratios in one of the abstracts, either way, it shows a cl of 165 to 171. I use 170 on the TT bike, and 172.5 on the road, but considering going to 170.