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Crank length change .


FirstV8
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I have ridden 175mm cranks my whole life , on road , track and now Mtb . I recently bought a Dura Ace group set with 172.5 length cranks . I have been riding them for about 2 months on my road bike and feel comfortable and have no niggles in my legs . My Mtb still has 175 cranks . Do i leave the  saddle on my road bike as is or should i raise it the 2,5mm difference . Will it help anyway or i just leave well alone . . 

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172.5 requires less torque and you will get on top of the gear quicker and also get to peak power quicker.

 

I would change the mtb if possible to 172.5 and have them both them same. The difference will be felt almost immediately like you have on the road bike.

 

2.5mm is a difference when turning it over, it is not about the shorts propelling you up, that’s like saying adjust the saddle height to accommodate. It’s about the crank and turning it over and being economical.

 

This is often forgotten between road and mtb

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172.5 requires less torque and you will get on top of the gear quicker and also get to peak power quicker.

 

Interesting comment. I would have thought that 172.5 would require more power/torque than 175 as the lever length is now shorter. I would believe that it would be easier to spin and maybe there is where the difference is.

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IMHO, it's really a bit of a red herring. I have 175 mm on my indoor trainer and 172.5 mm on the bikes I actually use on the road and on the trail. Doesn't seem to make any real difference in my world. This article pretty much supports the foregoing perspective:

 

https://www.trainingpeaks.com/blog/the-benefits-of-reducing-your-crank-length/

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I normally ride 175mm and have always tried to keep it that way.

 

My last build was 172.5 and can feel absolutely no difference between this bike and my other 5 road bike and 3 MTB's

Same. 175mm to 170mm. Zero difference. Just like i couldn’t feel the difference between a round chainring and an oval.

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Interesting comment. I would have thought that 172.5 would require more power/torque than 175 as the lever length is now shorter. I would believe that it would be easier to spin and maybe there is where the difference is.

 

Longer lever = more torque. 

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Longer lever = more torque. 

 

For the same force.

 

I don't want to question your logic because I'm sure that in a practical sense you know way more than all of us combined, but from a physics point of view your statement doesn't quite make sense and I agree with TDFN's comment.

 

A longer lever (crank) will give you more torque, and all other things being equal (cadence), your power will be higher with a longer crank, as power is a function of torque and rotational speed. Stating 172.5 requires less torque is incorrect, as the crank length is a function of torque.

 

But the more I'm reading your statement, the more I think I'm understanding the point you're trying to make. I think if you had said that 172.5 requires less force, it would make sense. And that will allow you to easier get into the "power zone" of the crank rotation whereby the force of your leg is more efficiently converted into forward motion.

 

But, I could be totally wrong. But I am curious to understand your comment nonetheless.

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So many different factors at play w.r.t. bike setup too.

 

Frame size is fixed, so can’t change that. Variables, to get your setup dialed in, are:

saddle height

setback.

Leg angle at top and bottom of pedal stroke

 

I listened to an episode on the Training Peaks CoachCast, where they talk about it. Very interesting in that they claim shorter cranks assist going faster. Read the summary here.

 

https://TrainingPeaks.com/blog/the-benefits-of-reducing-your-crank-length/

 

Speed = Comfort + Power + Aerodynamics.

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For the same force.

 

I don't want to question your logic because I'm sure that in a practical sense you know way more than all of us combined, but from a physics point of view your statement doesn't quite make sense and I agree with TDFN's comment.

 

A longer lever (crank) will give you more torque, and all other things being equal (cadence), your power will be higher with a longer crank, as power is a function of torque and rotational speed. Stating 172.5 requires less torque is incorrect, as the crank length is a function of torque.

 

But the more I'm reading your statement, the more I think I'm understanding the point you're trying to make. I think if you had said that 172.5 requires less force, it would make sense. And that will allow you to easier get into the "power zone" of the crank rotation whereby the force of your leg is more efficiently converted into forward motion.

 

But, I could be totally wrong. But I am curious to understand your comment nonetheless.

 

You can question anything I post, I have no problem with that. Your 1st paragraph if I understand correctly is saying with a longer crank power is more? I wasn't personally referring to that, I was referring to your last paragraph where you said "im reading your statement" You are correct with what you said in relation to what I was trying to get across information wise. You will get the typically quicker with less force and then be able to hold it. 

 

Some people as they have mentioned here is that they cant feel difference in a change. However, many (like myself) do, there is a reason why many women, even tall, do not run 175mm cranks. Its just to much effort in layman's terms to turn over effectively. Same for Short or tall people no one at 1.6m should be using a 175mm crank. 

 

Then there is a biomechanical element that also comes into effect, hip angle for example. 

 

There is also a reason riders use shorter than normal cranks ie: 170 vs 172.5 on a TT bike to to the above. 

 

Hope this answered alittle more clearer. 

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