Crank Arm Legnth & Power Cranks --Advice Needed

Triathlon Forum
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A friend of mine asked me the following question:

While studying the possibility of buying a set of Power Cranks to improve my cycling I’ve discovered that my road bike has a different crank arm length than my tri bike, 172.5 vs 175 respectively. In addition, my road bike has 700c wheels while my tri bike has 650c. I never gave much thoughts about it but now I have al sorts of questions:

What is the ideal crank arm length for someone who is 6’1.

Is there a real difference or one just has to get use to it

Power transfer is better with one or the other (short vs long)

How is cadence affected by the length of the crank arm

If I train 85% of the time on my road bike with 175 crank arms and 15% of the time on my tri bike with 172.5 cranks arms, how does this affect my performance? Is there any risk of injury?

What adjustments I need to make to make both bikes comparable

If I’m going to buy the power cranks what size should I buy them 175 or 172.5

Thanks in advance for your valuable opinion

2

Whats your inseam? Some recommend your inseam in cm times .216 is your crank length. So, both 172.5 and 175 may be too short.

I bought the adjustable PC’s so I could try at different lengths. The last time I used them they were at 200mm. I have a 37 inch inseam.

Dave

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What is the ideal crank arm length for someone who is 6’1.

There is no real ideal cranklength, there is a formula out there that says crank length should be about 20% of your inseam, search old threads for that.

Is there a real difference or one just has to get use to it

You’re only talking about a difference of 2.5 mm, take a look at a ruler and look how small that is. Your body will have to adapt, but only slightly. Your body has much larger things to adapt to when going from your tri bike to your road bike, this will barely be noticeable.

How is cadence affected by the length of the crank arm

Assuming the same power output, cadence will be slightly slower with a longer crank arm (more torque). Power is torque x rpm. More torque corresponds to slower speed.

If I train 85% of the time on my road bike with 175 crank arms and 15% of the time on my tri bike with 172.5 cranks arms, how does this affect my performance? Is there any risk of injury?

Performance, the crank lengths wont effect much. Injury, probably not. 85%/15% seems a bit road bike heavy to me. I like to do as much training as possible (even in the off season) on my tri bike. It keeps me better adapted to the position.

What adjustments I need to make to make both bikes comparable

Try to get your hip angle the same at the top and bottom of the pedal stroke. It will be impossible to get it perfect given your crank arm discrepancy, however it will be identical to the degree of what is actually measurable with a goniometer. Get the bend in your knee identical also, not your saddle height. If you’re like most triathletes you’re rotated much further forward on your tri bike and probably sitting more forward on the saddle, therefore it’s not realistic to measure saddle height btwn. the two and equate them.

What bike do you plan on putting the PCs on? I’d say get the same size as your tri bike cranks so you don’t worry about it.

hope this helps,

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What is the ideal crank arm length for someone who is 6’1.

This is individual and most people would just tell you to stick with 175s, but I am an inch shorter and use 190mm PCs that Frank made for me. (Inseam is 34.5 in/87.6 centimeters)

Is there a real difference or one just has to get use to it

You answered your own question—you didn’t even notice a 2.5 mm difference. However, there is a huge difference in my 190s and I like them much better.

Power transfer is better with one or the other (short vs long)

This is debatable. If 170s provide a proper range of motion for someone of a particular size, why would a taller person not need a much longer crank? However, the people who would tell you that nobody needs longer than 175 probably have never tried anything longer. If you were to suggest that they use 150s they might look at you like you had two heads, but that is essentially what they are tell you when they say a tall person should just use 175s.

How is cadence affected by the length of the crank arm

On regular 190s I found I could easily match my previous cadences. On the 190 PCs I was a little low in the beginning, but it came around until I averaged 95 rpm on PCs in a 14-mile bike leg, around 90 for a 40K and 87 for a 53k race this past spring.

If I train 85% of the time on my road bike with 175 crank arms and 15% of the time on my tri bike with 172.5 cranks arms, how does this affect my performance? Is there any risk of injury?

2.5 mm is nothing, but I’m never quite sure why people don’t do the bulk of their riding on their race bike. Unless the position is nearly identical you are not training much like you are racing.

If I’m going to buy the power cranks what size should I buy them 175 or 172.5

Maybe neither. I would suggest you buy the adjustable cranks for a number of reasons. It is much easier to ride PCs in the beginning with shorter cranks. Start out riding them at 170 and work you way up.

Try out the .216 times your inseam in centimeters (be as exact as possible) and work your way up to the recommended length over time. That way you will know if you like longer cranks and won’t have to buy multiple cranks to find out. Frank actually makes two types of adjustable cranks—one has a huge range, like 160mm-220mm and the other has a smaller range of adjustability from 165-185, but is also much lighter weight. I race on mine and think the second option is the best way to go. Others might not agree.

Chad

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Thank you all for your advice.

Hector