Crank arm length

wacka

Hi,

What difference would an increase in crank arm length from 172.5mm to 175mm do with regards to cadence, power and overall effort?

Zerotails99

You will notice that your feet is 2.5mm forward. This will give you extra wattage

MichaelW

Zerotails99 wrote:

You will notice that your feet is 2.5mm forward. This will give you extra wattage

[/satire]

I depends how well your current legs fit your cranks.
If you are a short-arz, then the small extension in crank length may make them too long for your little legs.
If you are medium sized, then it will make a small difference to pedalling style.
If you are tall you will breath a sigh of relief at having cranks that fit better.

Longer cranks mean a pedalling style of slower revs, more work per rev, further distance per rev (via higher gearing). You won't generate more power or go faster.

drummer_boy

My main bike has 175 cranks and I just built up a winter bike with 172.5 cranks. BTH I can't tell any difference at all.

I have average length inside leg.

ai_1

You'll probably hardly notice.
I have one bike with 175mm and one 172.5mm. I'm not sure I could distinguish between them.
Main difference is that your knees will come 5mm higher at the top of the stroke with the longer cranks since the bottom of the stroke will be in the same place but the diameter of the pedal stroke increases from 345mm to 350mm. If I get a Tri or TT bike I may consider trying a small crank length (165-170mm) to get lower with a more open hip angle but other than that I don't think it's something you need to worry about too much.

top_bhoy

It seems a negligible change to consider but if you change cranks, don't forget to also adjust the saddle height to accommodate the 2.5mm difference in cranklength.

ai_1

Top_Bhoy wrote:

It seems a negligible change to consider but if you change cranks, don't forget to also adjust the saddle height to accommodate the 2.5mm difference in cranklength.

Agreed. The change in diameter isn't going to be noticable but the change in bottom of stoke position will be and will require a small saddle move

Dippydog2

Somebody do the maths please.

If I add 2.5mm to the cranks, how much does my cadence change to get the same power output?

I suspect it's almost nothing.

Dippydog2

Ai_1 wrote:

Top_Bhoy wrote:

It seems a negligible change to consider but if you change cranks, don't forget to also adjust the saddle height to accommodate the 2.5mm difference in cranklength.

Agreed. The change in diameter isn't going to be noticable but the change in bottom of stoke position will be and will require a small saddle move

Yes, about 2.5mm should do it.

bompington

Dippydog2 wrote:

Somebody do the maths please.

If I add 2.5mm to the cranks, how much does my cadence change to get the same power output?

I suspect it's almost nothing.

The Maths is pretty simple, it's the physics you need to think about. Imagine you are measuring the power at the wheel - it depends only on 2 factors, the speed you are doing and against what resistance: how you are generating the power makes no difference. Different gear, different cadence, different crank length - no difference.
The answer is therefore ZERO: you will have to push with slightly less force to generate the same torque (=turning force) though.

BrandonA

MichaelW wrote:

Zerotails99 wrote:

You will notice that your feet is 2.5mm forward. This will give you extra wattage

[/satire]
Longer cranks mean a pedalling style of slower revs, more work per rev, further distance per rev (via higher gearing). You won't generate more power or go faster.

How does that work? The crank arms are attached to the chain rings. Therefore one revolution of the chain rings equates to exactly one revolution of the crank arms. This 1 to 1 relationship is constant regardless of crank arm length.

Because of this 1 to 1 relationship one turn of the pedals will travel you the same distance regardless of crank arm length.

Your feet will be moving 7.86mm further each revolution due to the diameter of the circle being 2.5mm larger.

ai_1

Power = Speed x Torque

The length of the crank does not appear in this equation but it is relevant in calculating the torque since:

Torque = Force x moment arm

Moment arm reduces to approx 98.57% of it's original value due to change of cranks from 175mm to 172.5mm
So to maintain the same power with crank A (175mm) and crank B (172.5mm):

ForceB = ForceA/0.9857

In other words you need to apply 1.45% more force to the pedal, on average during your pedal stroke, to maintain the same power. You do not need to pedal faster.
If you wish to pedal faster to maintain a lower pedal force you'll need to change gears. But don't you do that anyway?

dennisn

Zerotails99 wrote:

You will notice that your feet is 2.5mm forward. This will give you extra wattage

WRONG. You can't buy something that will GIVE you more wattage. More power to the rear wheel is only going to come from more power applied to the pedals, not by buying a longer or shorter crank.

smoggysteve

The length of your legs will depend on how much difference it makes but for any positive there will be a trade off.

If you have particularly long legs the difference will be negligible. But if you are shorter the difference in the angle of you knee will differ. The shorter crank arm may help if your knee comes slightly too high at the top of a power stroke but the ability to climb out the saddle is reduced as you have less leverage. The longer crank is pretty much the reverse. But still the effect will be quite small to none in performance on an overall ride.

A track athlete would probably go for shorter cranks as they would help on a flat smooth track where tempo is the only concern. A climber may prefer a longer crank to help get the extra leverage on a steep climb but would do little to help on the flat. These are extremes where the marginal gains come into play. For the average rider they are insignificant.

imposter2.0

Wacka wrote:

Hi,

What difference would an increase in crank arm length from 172.5mm to 175mm do with regards to cadence, power and overall effort?

In practice - and assuming you are able to disregard all of the tedious theory being quoted elsewhere - the difference will not be noticable. I ride bikes with 165, 170 and 175mm cranks and cannot tell any difference whatsoever. But that's just me...

simon_masterson

I'm 6'0 with a 34.5"ish inseam, and I mostly use 170mm cranks. I use them because I nearly always have, but nevertheless they do seem to suit my tendency to high cadence, and I find them comfortable. I have 175mm cranks as well and I do like them; there's a definite difference, but the main effect of using them is that I seem to prefer to pedal 1 gear higher at a slightly lower cadence. I imagine that anything longer would be too long. Building another bike this year, and I haven't decided what I'm going to use.

StillGoing

Dippydog2 wrote:

Ai_1 wrote:

Top_Bhoy wrote:

It seems a negligible change to consider but if you change cranks, don't forget to also adjust the saddle height to accommodate the 2.5mm difference in cranklength.

Agreed. The change in diameter isn't going to be noticable but the change in bottom of stoke position will be and will require a small saddle move

Yes, about 2.5mm should do it.

Depends on the angles of the leg at the top and bottom of the stroke. Imagine the leg angles were at their optimum with the shorter cranks. Although the crank arm is only 2.5mm longer, it means the seat needs lowering to accommodate the leg angle at the bottom of the stroke to avoid over extending. So drop the seat by 2.5mm. In doing so, you then compress the leg at the top of the stroke by 5mm (2.5 crank length + the 2.5 you've just dropped the seat by) which has a detrimental effect on the ability to get the power out. So instead of dropping the seat by the additional length of the cranks, you may need to come to a compromise to avoid both over extending and compressing the leg at the top and bottom of the stroke.

I use short cranks on both road bikes and found I can notice the difference, mainly in the ability to spin at a higher cadence than with longer cranks.

hypster

http://www.stevehoggbikefitting.com/bik ... which-one/

mmacavity

viewtopic.php?f=40020&t=12962489

matt-h

MyRetul bike fit put me on shorter cranks.
165mm 31" inseam
I didn't notice the difference but the motion camera noted an improved lateral stabilisation in the knee.
This should mean less chance Of an injury apparently.

Took his word for it as he knows what he is doing.

Matt

MichaelW

BrandonA wrote:

MichaelW wrote:

Longer cranks mean a pedalling style of slower revs, more work per rev, further distance per rev (via higher gearing). You won't generate more power or go faster.

How does that work? The crank arms are attached to the chain rings. Therefore one revolution of the chain rings equates to exactly one revolution of the crank arms. This 1 to 1 relationship is constant regardless of crank arm length.
Because of this 1 to 1 relationship one turn of the pedals will travel you the same distance regardless of crank arm length.
Your feet will be moving 7.86mm further each revolution due to the diameter of the circle being 2.5mm larger.

7.86mm further means more work per rev (at constant foot pressure).
Your natural tendancy to select the optimum gear will mean a shift to a smaller sprocket with longer cranks.

ai_1

MichaelW wrote:

BrandonA wrote:

MichaelW wrote:

Longer cranks mean a pedalling style of slower revs, more work per rev, further distance per rev (via higher gearing). You won't generate more power or go faster.

How does that work? The crank arms are attached to the chain rings. Therefore one revolution of the chain rings equates to exactly one revolution of the crank arms. This 1 to 1 relationship is constant regardless of crank arm length.
Because of this 1 to 1 relationship one turn of the pedals will travel you the same distance regardless of crank arm length.
Your feet will be moving 7.86mm further each revolution due to the diameter of the circle being 2.5mm larger.

7.86mm further means more work per rev (at constant foot pressure).
Your natural tendancy to select the optimum gear will mean a shift to a smaller sprocket with longer cranks.

Why are you assuming "constant foot pressure"?
If pedal force is maintained then you would do more work per revolution and if you maintain RPM I agree this means increased power. However, I don't think your claim that you'll necessarily tend to select a lower gear is valid. I suspect on average riders will produce similar power regardless of small crank length changes and either pedal force, cadence or both will be modified slightly to provide this.

MichaelW

Ai_1 wrote:

If pedal force is maintained then you would do more work per revolution and if you maintain RPM I agree this means increased power. However, I don't think your claim that you'll necessarily tend to select a lower gear is valid. I suspect on average riders will produce similar power regardless of small crank length changes and either pedal force, cadence or both will be modified slightly to provide this.

the crank is a lever and part of the transmission.
Longer lever = a change to the gearing ie lower. the rider would sense this lower gear and return to their optimum gear using...the gears.

My understanding is that with a longer crank:
Pedalling force and power o/p are constant.
Cadence is reduced .
Work per rev increases
time per rev increases
Distance per rev increases (because you change gear)
Speed remains the same.