Smaller chain ring or shorter cranks ... or won't it matter?

[Dogtrousers]

I'm kinda with Sheldon on this one - from a scientific perspective at least. Bicycle gearing is a system of levers and pulleys that magnify the input (arc distance travelled by pedal) to give an output (distance travelled by bike). The best ideal measure that takes into account all of the devices - including crank length - is the ratio of input to output. Anything else is incomplete.

And do I use that ideal ratio? Of course I don't. I use gear inches.

 

[roubaixtuesday]

I'm kinda with Sheldon on this one - from a scientific perspective at least. Bicycle gearing is a system of levers and pulleys that magnify the input (arc distance travelled by pedal) to give an output (distance travelled by bike). The best ideal measure that takes into account all of the devices - including crank length - is the ratio of input to output. Anything else is incomplete.

And do I use that ideal ratio? Of course I don't. I use gear inches.

How do you know what is the "ideal" measure?

It could be that crank length is almost incidental, dependent on the biology.

Or it could be vitally important.

It might even have the *opposite* effect to what that ratio would predict if, for example, shorter maximum knee angle results in higher efficiency.

It's impossible to say without some data.

I think @Ming the Merciless quoted some research which would contradict the view that including crank length In the way Sheldon suggests is sensible.

 

[Dogtrousers]

How do you know what is the "ideal" measure?

It could be that crank length is almost incidental, dependent on the biology.

Or it could be vitally important.

It might even have the *opposite* effect to what that ratio would predict if, for example, shorter maximum knee angle results in higher efficiency.

It's impossible to say without some data.

I think @Ming the Merciless quoted some research which would contradict the view that including crank length In the way Sheldon suggests is sensible.

Nothing to do with biology. It's physics. It's ideal because it deals with the ratio of the input (distance travelled by pedal) to the output (distance travelled by bike) of the mechanism. Any other measure uses an incomplete description of the system.

Now I agree that whether this distinction is practically useful or sensible, when considering a human riding a bike, would need some more data. But I don't care about that For practical purposes I use gear inches which may be arcane, stupid and daft but at least they are familiar (to me)

 

[roubaixtuesday]

Nothing to do with biology. It's physics. It's ideal because it deals with the ratio of the input (distance travelled by pedal) to the output (distance travelled by bike) of the mechanism. Any other measure uses an incomplete description of the system.

Now I agree that whether this distinction is practically useful or sensible, when considering a human riding a bike, would need some more data. But I don't care about that For practical purposes I use gear inches which may be arcane, stupid and daft but at least they are familiar (to me)

No, the biology is also relevant.

There is more than one dimensionless ratio which describes the physics.

Whether the ratios should be used linearly to model the system depends on the biology.

You can't just assert that a particular ratio is the one true one, and should be used linearly.

If, as may well be the case, human power output is independent of leg extension in the range of interest, then that ratio is not merely incomplete, but downright misleading.

If you were calculating the weight required on a pedal to hold a bike stationary on a hill, then yes, it would be the relevant ratio.