Reborn

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for some reason I could not paste the code which would allow for the image to show in the thread

Quoted post said:

Originally Posted by dot [IMG]/img/forum/go_quote.gif[/IMG]

I bought a road bike this year, it came with 172.5mm cranks. Before this road bike I'd never ridden any other cranks except for 175mm on all my mountain bikes. I tried 170mm once and it didn't work - I felt like I'm wasting my effort and my loop times on a well-known course were worse than on 175mm. I kinda feel the same with 172.5mm cranks but the effect is much milder.

The only time there is a difference is when you are at an extreme such as exerting maximum torque and are unable to turn the shorter cranks. Yes, there are such times.

But the reality is that unless you are at an extreme the difference, (175-172.5)/172.5, is unnoticable and unmeasurable by the typical devices we have.

To answer the OP factually, we can use applied math.

Power (W) = Torque (N.m) x 2.PI x rotational speed (rps) If we apply the above formula:

To deliver 350W at a cadence of 90rpm, for a crank length of 170mm the average force applied to the crank = 22.28kg, or 49.01lbs.
To deliver the same 350W at the same cadence, for a crank lenth of 175mm, would need an average force = 21.64kg, or 47.61lbs.

Or, we can look at it differently:

If we apply an average force of 45lbs (20.45kg) to the crank to deliver 325W we would need a cadence of 91.02 RPM with a 170mm crank, or a cadence of 88.42, with a 175mm crank.

The changes are really small, what is more important is the most comfortable position, or being able to spin a bigger or smaller circle. For climbing, lower cadence work, a 175mm is a good choice, for spinning and speed on the flats a 170mm is a good choice, IMHO.

WillemJM said:

To answer the OP factually, we can use applied math. Power (W) = Torque (N.m) x 2.PI x rotational speed (rps)   If we apply the above formula: To deliver 350W at a cadence of 90rpm, for a crank length of 170mm the average force applied to the crank = 22.28kg, or 49.01lbs. To deliver the same 350W at the same cadence, for a crank lenth of 175mm, would need an average force = 21.64kg, or 47.61lbs. Or, we can look at it differently: If we apply an average force of 45lbs (20.45kg) to the crank to deliver 325W we would need a cadence of 91.02 RPM with a 170mm crank, or a cadence of 88.42, with a 175mm crank. The changes are really small, what is more important is the most comfortable position, or being able to spin a bigger or smaller circle. For climbing, lower cadence work, a 175mm is a good choice, for spinning and speed on the flats a 170mm is a good choice, IMHO.

Factually, discounting a low power output due to a sub-optimal fit, power is not constrained by the crank length but by the energy output per unit time of the rider. The bike itself generates zero power. All that crank arm length does is allow the rider to find the optimal position w/ respect to his leg geometry. All things being equal if a rider has a maximal power output of P0, then with changing crank arm lengths the rider's cadence and force applied to pedals will vary to produce P0. That's shown in your numbers. Restating the power equation with a slight difference you get: P=constant=T⋅2π⋅f, where f is revs per second. I only point this out because not all riders understand that gear ratios and crankarm lengths don't change your maximum power output (again, assuming that power output is not fit dependent).

Don't overlook the point that mountain bikes are generally ridden differently than road bikes, and very often come with a disparity in crankarm length in comparison to their equivalent sized road counterparts. Generally the torque requirements are higher under the circumstances in which a mountain bike is designed to be ridden. My takeaway... choose your crank length for the job at hand. Are you planning on cranking out big gears at the local TT or spinning up to 140-150rpm at the crit finish line?

Quoted post said:

Originally Posted by alienator [IMG]/img/forum/go_quote.gif[/IMG]

Factually, discounting a low power output due to a sub-optimal fit, power is not constrained by the crank length but by the energy output per unit time of the rider. The bike itself generates zero power. All that crank arm length does is allow the rider to find the optimal position w/ respect to his leg geometry. All things being equal if a rider has a maximal power output of P0, then with changing crank arm lengths the rider's cadence and force applied to pedals will vary to produce P0. That's shown in your numbers. Restating the power equation with a slight difference you get: P=constant=T⋅2π⋅f, where f is revs per second. I only point this out because not all riders understand that gear ratios and crankarm lengths don't change your maximum power output (again, assuming that power output is not fit dependent).

That's an excellent point. For some reason people tend to get so fixed with the calculations about crank length effect to required force that they completely forget having very handy mechanism in their bike to adjust that force, namely gears.

5mm the difference between 170mm and 175mm cranks is about 3%. One might be able to tell the difference in crank length by the 3% difference in force on the pedals. But it is unlikely.

It is unlikely that the difference in length has a significant effect on the pedaling mechanics of most riders. I have use 170, 172.5, and 175 cranks during the same year on different road bikes and could not tell the difference.

Short cranks are good for ground clearance. Important for mountain bikes.

Quoted post said:

Originally Posted by alienator [IMG]/img/forum/go_quote.gif[/IMG]

Factually, discounting a low power output due to a sub-optimal fit, power is not constrained by the crank length but by the energy output per unit time of the rider. The bike itself generates zero power. All that crank arm length does is allow the rider to find the optimal position w/ respect to his leg geometry. All things being equal if a rider has a maximal power output of P0, then with changing crank arm lengths the rider's cadence and force applied to pedals will vary to produce P0. That's shown in your numbers. Restating the power equation with a slight difference you get: P=constant=T⋅2π⋅f, where f is revs per second. I only point this out because not all riders understand that gear ratios and crankarm lengths don't change your maximum power output (again, assuming that power output is not fit dependent).

Power IS constrained by the crank length, if you had a very short crank you would not be able to produce the same power since the torque required would be in excess of what you can produce.

To answer the orginal question, I see it like this. If you keep the torque the same and increase crank length, then power will increase since the same torque is being applied over a greater distance. The difference is small however. Is it noticable? Possibly. Did I notice any change going from a 170mm to a 172.5mm crank length? May be. Changing crank lengths will make a difference to the potential power output, but whether that is discernable is up for debate.

Dr Lodge said:

Power IS constrained by the crank length, if you had a very short crank you would not be able to produce the same power since the torque required would be in excess of what you can produce.

That's a physiological constraint that is dependent on the rider in question and is a function of his biomechanics. In that respect it's not anything related to the crank itself.

Quoted post said:

To answer the orginal question, I see it like this. If you keep the torque the same and increase crank length, then power will increase since the same torque is being applied over a greater distance. The difference is small however. Is it noticable? Possibly. Did I notice any change going from a 170mm to a 172.5mm crank length? May be. Changing crank lengths will make a difference to the potential power output, but whether that is discernable is up for debate.

Torque and power are given by these: T=rFsinθ and P=Tω where T is torque; r is the crank length; F is the force applied to the crank; θ is the angle between the crank arm and the direction in which the force is applied; P is power; and ω is the angular frequency (radians/second or degrees/second) of the crank. You can see from the two equations if T is constant as you suggest in your example, power can only change if the angular velocity changes. You can also see from the torque equation, if you want to maintain a constant torque as you increase the length of a crank arm, the force applied to the crank arm has to go down......otherwise torque won't be constant. Unfortunately, there is no valid equation that relates power and torque with respect to a given body or body part dimension. Given that going from one size crank arm to another, say from 170 to 172.5mm is a very small change, it could very well be that what crank arm size is most effective is based on something subjective due to rider bias.

Pretty good discussion on crank length vs. power here: http://www.cervelo.com/en/engineering/ask-the-engineers/crank-length.html

Bottom line, across as narrow a range as 175mm to 172.5mm there isn't a measurable difference in ability to sustain power. But in certain situations like TT fitting in an aggressive aero position it can make sense to go with shorter crank arms but that's related to minimum hip angle at the top of the pedal stroke as a limiter.

-Dave

Quoted post said:

Originally Posted by WillemJM [IMG]/img/forum/go_quote.gif[/IMG]

If we apply an average force of 45lbs (20.45kg) to the crank to deliver 325W we would need a cadence of 91.02 RPM with a 170mm crank, or a cadence of 88.42, with a 175mm crank.

The changes are really small, what is more important is the most comfortable position, or being able to spin a bigger or smaller circle. For climbing, lower cadence work, a 175mm is a good choice, for spinning and speed on the flats a 170mm is a good choice, IMHO.

And if you really dig into it, your feet are moving at the same speed pedaling 91.012rpm with 170mm and 88.42 with 175mm cranks. The angular velocity is different as indicated by the cadence, but the tangential pedal velocity is equal. Your muscles don't work on angular velocity; muscular contraction rate is related to tangential velocity so. There should be very little difference once the body adapts to the slightly different range of motion.

Assuming you have plenty of gears available, shorter cranks offer advantages in TT fit, but also give additional clearance pedaling through a corner and are probably a tiny bit lighter.

jollyrogers said:

And if you really dig into it, your feet are moving at the same speed pedaling 91.012rpm with 170mm and 88.42 with 175mm cranks.  The angular velocity is different as indicated by the cadence, but the tangential pedal velocity is equal.  Your muscles don't work on angular velocity; muscular contraction rate is related to tangential velocity so.  There should be very little difference once the body adapts to the slightly different range of motion. Assuming you have plenty of gears available, shorter cranks offer advantages in TT fit, but also give additional clearance pedaling through a corner and are probably a tiny bit lighter.

Uhm, angular velocity is just a metric, so it's okay to use. Muscle contraction rate is related to angular velocity given there's a relationship between tangential velocity and angular velocity. Which specific metric you use does not matter in the slightest. Besides, which metric is preferable isn't related at all to the OP's question.

Quoted post said:

Originally Posted by alienator [IMG]/img/forum/go_quote.gif[/IMG]

Uhm, angular velocity is just a metric, so it's okay to use. Muscle contraction rate is related to angular velocity given there's a relationship between tangential velocity and angular velocity. Which specific metric you use does not matter in the slightest. Besides, which metric is preferable isn't related at all to the OP's question.

Yes tangential and angular velocity are "related" but they aren't equal. 88 rpm on 175mm cranks and 91 rpm on 170mm cranks are equal in terms on tangential pedal velocity i.e. how fast your feet are moving. The same cadence across different crank lengths is not the same foot speed, measured in terms of tangential pedal velocity. How fast your feet are moving = how fast your muscles have to contract.

jollyrogers said:

Yes tangential and angular velocity are "related" but they aren't equal.  88 rpm on 175mm cranks and 91 rpm on 170mm cranks are equal in terms on tangential pedal velocity i.e. how fast your feet are moving.  The same cadence across different crank lengths is not the same foot speed, measured in terms of tangential pedal velocity.  How fast your feet are moving = how fast your muscles have to contract.Â

Who claimed that angular and tangential velocities are the same? Scale angular velocity by the radius and you get tang vel. Again it doesn't matter what you use. Just as muscles don't work on angular velocity, they don't' work on tangential velocity since the contraction of any given muscle in the leg does not move the foot in a circle. I think everyone understands the relationships at hand. Neither velocity is directly measured and needs to be scaled. Which is used doesn't matter in the slightest.

Quoted post said:

Originally Posted by alienator [IMG]/img/forum/go_quote.gif[/IMG]

Who claimed that angular and tangential velocities are the same? Scale angular velocity by the radius and you get tang vel. Again it doesn't matter what you use. Just as muscles don't work on angular velocity, they don't' work on tangential velocity since the contraction of any given muscle in the leg does not move the foot in a circle. I think everyone understands the relationships at hand. Neither velocity is directly measured and needs to be scaled. Which is used doesn't matter in the slightest.

Angular velocity is directly measured (in rpm) and the point I was making, in replying to the post that compared cadences is that while the two cadences noted are different angular velocities (88 and 91 rpm), the actual speed that the foot is moving is equal. As far as whether muscular contraction rate is tied to angular or tangential velocity, I'll take Dr. Coggan's word over yours.

jollyrogers said:

Angular velocity is directly measured (in rpm) and the point I was making, in replying to the post that compared cadences is that while the two cadences noted are different angular velocities (88 and 91 rpm), the actual speed that the foot is moving is equal.  As far as whether muscular contraction rate is tied to angular or tangential velocity, I'll take Dr. Coggan's word over yours.

You know you can tie it to angular velocity by dividing the angular velocity by a constant.....like.....uhm.......crank arm length. The point is that it doesn't matter because you can "tie" things to all manner of physical quantities by using equations and simple algebra. It happens all the time that things are "tied" together like this. Very little in science is measured directly.

I have just moved up to 172.5's from turning a 170mm for the last 30 years. So far the only change made to bike setup in the swap was lowering the saddle about 2mm.

Physiologically my inseam is 31.75" which presents as slightly shy for 172.5 optimal length using some charts (32"-34"😉 but by another formula I supposedly need an even longer crank. My cadence usually falls into the 90-105 range. 85 is slogging for me.

It would be easy for me to imagine folks who turn lower cadences or who don't race (with different riding demands) not perceiving much of a difference but I can say after having ridden 172.5's for the last 2 weeks, my efficiency when accelerating in the same gear especially up small rises, i.e. by increasing cadence, is noticeably compromised.

The cadence acceleration compromise is enough that it's challenging the otherwise lovely honeymoon with my beautiful new Power2Max crank. I'm hoping to get used to it but holding off till the end of the season before I throw my old crank on eBay.

Quoted post said:

Originally Posted by danfoz [IMG]/img/forum/go_quote.gif[/IMG]

I have just moved up to 172.5's from turning a 170mm for the last 30 years. So far the only change made to bike setup in the swap was lowering the saddle about 2mm.

Physiologically my inseam is 31.75" which presents as slightly shy for 172.5 optimal length using some charts (32"-34"😉 but by another formula I supposedly need an even longer crank. My cadence usually falls into the 90-105 range. 85 is slogging for me.

It would be easy for me to imagine folks who turn lower cadences or who don't race (with different riding demands) not perceiving much of a difference but I can say after having ridden 172.5's for the last 2 weeks, my efficiency when accelerating in the same gear especially up small rises, i.e. by increasing cadence, is noticeably compromised.

The cadence acceleration compromise is enough that it's challenging the otherwise lovely honeymoon with my beautiful new Power2Max crank. I'm hoping to get used to it but holding off till the end of the season before I throw my old crank on eBay.

Have you adjusted the fore-aft position of the saddle, yet?

Quoted post said:

Originally Posted by alfeng [IMG]/img/forum/go_quote.gif[/IMG]

Have you adjusted the fore-aft position of the saddle, yet?

Not yet, that could certainly be part of the problem.

Ok lets think about the whole picture. First of all it is well established (thanks to Jim Martin's research) that as you increase crank length or decrease it you CPV (circumfrential pedal velocity) stays the same. This means that in order for the pedal to move at the same speed, when you move to a shorter crank you are going to increase cadence and the opposite if you decrease. In the same gear you have essentially changed the gearing of the bike now, that is to say for a distance of circumfrential movement you are moving the wheel farther. So it is natural to spin in lower gears, at a higher cadence at the same steady speed with shorter cranks. Now, upon switching cranks, it is possible that muscle firing patterns could take some time to adjust if you have lots of time on different length cranks as you are now spinning different size circles.

Now lets get to the "torque" issue everyone likes to discuss. A longer lever does create more torque, but we must remember that you are not interested in creating torque around the BB, but around the wheel axle (the stuff that makes your bike actually move). Given what we know about how the gearing of the bike physically changes when you switch crank lengths, we will find that torque around the hub does NOT change. Yes it is easier to push a given gear with a longer crank, because that given gear is essentially smaller and you are going slower in it. (Sheldon Brown had a great way for accounting or this in his gearing calculations known as "gain ratios". The best way to compare gearing!!)

In the end, what it comes down to is ride what you feel comfortable riding, and fits your body. An overly long crank for your body size will result in a hip angle that is very acute on the top end of your stroke for an optimal extension. Too short and you have the opposite problem. For this reason, many people attempting to get a very aggressive TT fit will go with a shorter crank: it allows them to get aggressive without running into hip angle issues. The academic research I have seen points to limited to know power generation difference in the range of crank lengths people actually use. plan2peak.com32 article JMartinCrankLengthPedalingTechnique.pdfOpen ↗