170mm vs 175mm MTB crank

[NINER_boy]

Just imagine...

 

On a HOT summers day, sweat dripping, flies buzzing,

climbing Hells hoogte @ 7km/h, your lowest gear, RPM around 45 and your heart rate through the roof...

 

And your crank arm gives a too short lever to turn the pedals over

 

I'd be bitchin!!!!!!

 

 

[NINER_boy]

With all of this said....

 

Yes, from 160mm track cranks to 180mm Crank arms there is a BIG difference.

 

That 5mm, not so much for the average Joe,

Unless...

You are sprinting against Absalon for a stage win

Climbing against Shurter for a KOM

 

[Furbz]

With all of this said....

 

Yes, from 160mm track cranks to 180mm Crank arms there is a BIG difference.

 

That 5mm, not so much for the average Joe,

Unless...

You are sprinting against Absalon for a stage win

Climbing against Shurter for a KOM

 

you got me!

i'm Shurter!

[MarcBurger]

All the talk about torque and acceleration is completely redundant: That's what gears are for.

 

If you're among the shorter classification of human beings, go for the 170 as it will probably better match your leg lever length and stroke.

 

+5mm extra clearance (woo! that's an extra bug on a rock that might live)

[JA-Q001]

All the talk about torque and acceleration is completely redundant: That's what gears are for.

 

If you're among the shorter classification of human beings, go for the 170 as it will probably better match your leg lever length and stroke.

 

+5mm extra clearance (woo! that's an extra bug on a rock that might live)

 

You see, that's where you are mistaken, your length has the least to do with any crank length and if you think it does little, on torque or acceleration, then I guess science has got it wrong.

 

Now lets say you want to keep pedaling at 300Watts and 100rpm, you see Watts is a result of your Torque multiplied by your cadence(in radians per second). This gives you W = (2πN x T)/60 and T = Force x distance

 

So if you then take 300 = (2π x 100 x T)/60, then T = 28.6 Nm (there about)

 

Now to bring in your crank length, so Force = 28.6 / 0.175 = 163.4 N or Force = 28.6 / 0.170 = 168.2 N

 

So this gives you a difference of 4.8 Newton force or about 500 grams that you have to put out extra to keep the same cadence at the same wattage, it does not sound like a lot, but put that onto your max for 5 to 20 minutes, and you will suffer more.

 

But this is not the Alpha and Omega, some people are more prone to ride lower cadence and other higher cadence, so someone that prefers lower, will use a longer crank and someone that prefers higher will use shorter.

 

Then it is also dependent on use, like for example sprinters use shorter cranks, so that they can get a higher cadence, because the circle circumference is shorter on a shorter cranks, hence there legs can move at the same speed (meters per second) and they can obtain a higher cadence (revolutions per minute).

 

In the end, your gear ratios has very little to do with what wattage you put out and what torque is needed. Only reason why length is brought into it is because it is presumed that shorter legs turn slower that longer legs.

[NINER_boy]

 

 

You see, that's where you are mistaken, your length has the least to do with any crank length and if you think it does little, on torque or acceleration, then I guess science has got it wrong.

 

Now lets say you want to keep pedaling at 300Watts and 100rpm, you see Watts is a result of your Torque multiplied by your cadence(in radians per second). This gives you W = (2πN x T)/60 and T = Force x distance

 

So if you then take 300 = (2π x 100 x T)/60, then T = 28.6 Nm (there about)

 

Now to bring in your crank length, so Force = 28.6 / 0.175 = 163.4 N or Force = 28.6 / 0.170 = 168.2 N

 

So this gives you a difference of 4.8 Newton force or about 500 grams that you have to put out extra to keep the same cadence at the same wattage, it does not sound like a lot, but put that onto your max for 5 to 20 minutes, and you will suffer more.

 

But this is not the Alpha and Omega, some people are more prone to ride lower cadence and other higher cadence, so someone that prefers lower, will use a longer crank and someone that prefers higher will use shorter.

 

Then it is also dependent on use, like for example sprinters use shorter cranks, so that they can get a higher cadence, because the circle circumference is shorter on a shorter cranks, hence there legs can move at the same speed (meters per second) and they can obtain a higher cadence (revolutions per minute).

 

In the end, your gear ratios has very little to do with what wattage you put out and what torque is needed. Only reason why length is brought into it is because it is presumed that shorter legs turn slower that longer legs.

 

Sorted!!!!

[MarcBurger]

Now there I disagree with you completely. Yes your maths is correct P=tw, but what is crank length other than another form of gearing? To get the same torque on a longer crank arm requires less force yes, but to get that same torque duration, the applied time of that lesser force needs to be taken into account.

 

Maths:

 

P=Tw

w=rad/s

 

P=t.rad/s

 

P is constant (300)

 

T1. rad1 /s1 = T2. rad2 /s2 (rad1=rad2)

T1/s1 = T2/s2

 

For the same power P, the duration of the applied torque is inversely proportional to torque itself.

 

Which is what gears do!

 

the only thing that might make a difference is the friction differences between different cog sizes. Energy cannot be created or destroyed so the energy you put into your crank is what you get out on the wheels (-friction)

[X-Man]

Makes no difference to us recreational amateur cyclists...

[Sniffie]

 

 

You see, that's where you are mistaken, your length has the least to do with any crank length and if you think it does little, on torque or acceleration, then I guess science has got it wrong.

 

Now lets say you want to keep pedaling at 300Watts and 100rpm, you see Watts is a result of your Torque multiplied by your cadence(in radians per second). This gives you W = (2πN x T)/60 and T = Force x distance

 

So if you then take 300 = (2π x 100 x T)/60, then T = 28.6 Nm (there about)

 

Now to bring in your crank length, so Force = 28.6 / 0.175 = 163.4 N or Force = 28.6 / 0.170 = 168.2 N

 

So this gives you a difference of 4.8 Newton force or about 500 grams that you have to put out extra to keep the same cadence at the same wattage, it does not sound like a lot, but put that onto your max for 5 to 20 minutes, and you will suffer more.

 

But this is not the Alpha and Omega, some people are more prone to ride lower cadence and other higher cadence, so someone that prefers lower, will use a longer crank and someone that prefers higher will use shorter.

 

Then it is also dependent on use, like for example sprinters use shorter cranks, so that they can get a higher cadence, because the circle circumference is shorter on a shorter cranks, hence there legs can move at the same speed (meters per second) and they can obtain a higher cadence (revolutions per minute).

 

In the end, your gear ratios has very little to do with what wattage you put out and what torque is needed. Only reason why length is brought into it is because it is presumed that shorter legs turn slower that longer legs.

The fancy physics is not really needed. In simple terms; 165 mm is roughly 3% shorter than 170 mm. To maintain the same power (300 watt as per your example) it is obvious that you will have to apply 3% more force to the pedals. That is IF you keep cadence constant, but we are supposedly not talking single speed. By going a gear lighter you can up the cadence slightly, maintaining the same or even less force on the pedals while maintaining power.

 

What you are implying in your first sentence of your post is that a 1 m tall toddler would be just as comfortable on 170 mm cranks as a 2 m giant?

[Daniel J Méssem]

 

 

You see, that's where you are mistaken, your length has the least to do with any crank length and if you think it does little, on torque or acceleration, then I guess science has got it wrong.

 

Now lets say you want to keep pedaling at 300Watts and 100rpm, you see Watts is a result of your Torque multiplied by your cadence(in radians per second). This gives you W = (2πN x T)/60 and T = Force x distance

 

So if you then take 300 = (2π x 100 x T)/60, then T = 28.6 Nm (there about)

 

Now to bring in your crank length, so Force = 28.6 / 0.175 = 163.4 N or Force = 28.6 / 0.170 = 168.2 N

 

So this gives you a difference of 4.8 Newton force or about 500 grams that you have to put out extra to keep the same cadence at the same wattage, it does not sound like a lot, but put that onto your max for 5 to 20 minutes, and you will suffer more.

 

But this is not the Alpha and Omega, some people are more prone to ride lower cadence and other higher cadence, so someone that prefers lower, will use a longer crank and someone that prefers higher will use shorter.

 

Then it is also dependent on use, like for example sprinters use shorter cranks, so that they can get a higher cadence, because the circle circumference is shorter on a shorter cranks, hence there legs can move at the same speed (meters per second) and they can obtain a higher cadence (revolutions per minute).

 

In the end, your gear ratios has very little to do with what wattage you put out and what torque is needed. Only reason why length is brought into it is because it is presumed that shorter legs turn slower that longer legs.

Now there I disagree with you completely. Yes your maths is correct P=tw, but what is crank length other than another form of gearing? To get the same torque on a longer crank arm requires less force yes, but to get that same torque duration, the applied time of that lesser force needs to be taken into account.

 

Maths:

 

P=Tw

w=rad/s

 

P=t.rad/s

 

P is constant (300)

 

T1. rad1 /s1 = T2. rad2 /s2 (rad1=rad2)

T1/s1 = T2/s2

 

For the same power P, the duration of the applied torque is inversely proportional to torque itself.

 

Which is what gears do!

 

the only thing that might make a difference is the friction differences between different cog sizes. Energy cannot be created or destroyed so the energy you put into your crank is what you get out on the wheels (-friction)

 

Kind of agree with both of you

 

If cadence is kept constant, say at 100rpm (10.4719755 rad/s) then yes, the applied force is higher for the shorter cranks

 

But will the cadence really be constant ?

I would imagine that the applied force may be more constant (talking mtb not track road)

 

I'm thinking that a better fit might allow someone to increase power - or maybe it depends on what type of riding.

 

Have yet to see compelling evidence that crank length influences continuous power - haven't looked too hard i'll admit

[jcza]

For many athletes, the idea “longer is better” has changed in part because of Dr. Jim Martin’s 2001 study titled “Determinants of maximal cycling power: crank length, pedaling rate and pedal speed” (Eur J Appl Physiol (2001) 84: 413-418).

 

Jim’s study involved 16 bike racers of various heights doing maximal sprint power tests, typically less than four seconds duration. During the study, they repeated the efforts while systematically testing the following crank lengths: 120, 145, 170, 195, and 220mm.

 

Believe it or not, the test results showed no statistical difference in maximum power among the three middle crank lengths (145, 170 and 195mm). The saddle height (measured to the pedal) was maintained throughout and the researchers did not adjust fore-aft saddle position or handlebar height despite changes in pedal-to-knee relationship and handlebar drop with the various crank lengths.

 

For years crank length tests had been inconclusive and the general working knowledge came more from experience and intuition than science. Now athletes can choose the crank length they like without worrying they’re affecting power.

[IH8MUD]

Take both bikes for a spin . . . . IF one crank length feels better, faster, makes you less tired or produce any other perceived advantage over the other. . . . take that one.

[Albatross]

The 175mm SLX cranksets are all sold out on CRC, only 170mm available, speaks volumes to me (was always crap at maths & science anyway)

[HeadDown]

170mm. Weighs less...

[HeadDown]

But seriously - 170mm. They're made for us short okes.

[ACM]

I'm 165 with inseam of 79cm and I ride a 165 crank. Came from 175 and could feel the difference. Shorter crank for shorter legs are better; my 2 cents.