Gain Ratio?

[mercalia]

I picked up a free app for my Windows phone that gives various types of gear combination info from Gear Inches to Gain ratio and speeds at various rpms. What puzzled me was the gain ratio that I googled takes into account the crank length. Must say I was underwealmed by the idea. I suppose might be useful if you have lots of bikes? only.

[Chris Jeggo]

Many years ago the CTC Handbook contained a specification for a touring bike, including a normal gear (expressed in inches as usual for the UK) of ten times crank length, a top gear of twelve times crank length and a bottom gear of five times crank length. This is one way of expressing the fact that you can push a higher gear (as expressed in inches) if you have longer cranks. Another way of encapsulating the same idea is to use gain ratio, but since we are not familiar with that and are familiar with our inches, we stick with the latter.

[andrew_s]

Crank length does have a real effect (as does tyre size).

Switching from 170 mm cranks to 180 mm has the same effect as changing from a 17T sprocket to 18T, in terms of how hard you have to push on the pedals. What does differ is that your cadence isn't affected, so you are getting a slightly lower gear without having to pedal faster.

However, most people don't switch cranks regularly, and judge what gear inches mean by how hard it feels to ride up a hill, which means it's only relevant that a person with 180 cranks has a slightly different view of suitable gear inches than a person with 170 cranks if discussion of one-upmanship along the lines of "I can ride this hill in a 20 inch gear but you need a 19" gear" starts up.

[Brucey]

Crank length does have a real effect (as does tyre size).

Switching from 170 mm cranks to 180 mm has the same effect as changing from a 17T sprocket to 18T, in terms of how hard you have to push on the pedals. What does differ is that your cadence isn't affected, so you are getting a slightly lower gear without having to pedal faster.....

Thats not been my experience; I can't spin longer cranks as fast (when flat out) and my average cadence drops with the longer cranks too. Which kind of makes sense, otherwise you might be getting something for nothing.

cheers

[Tigerbiten]

Crank length does have a real effect (as does tyre size).

Switching from 170 mm cranks to 180 mm has the same effect as changing from a 17T sprocket to 18T, in terms of how hard you have to push on the pedals. What does differ is that your cadence isn't affected, so you are getting a slightly lower gear without having to pedal faster.

Not true.

With longer cranks, you feet move further per revolution of the cranks.
So more energy is needed per revolution to speed your feet up and then slow them down again.
So you slow your cadence down to lower your energy expenditure.
Therefore the longer your cranks are, the slower your cadence is for the same effort.
The opposite happens with short cranks.
That why it's common to have short cranks on recumbent which you then spin uphill.

Luck ........

[hercule]

I switched to short cranks on my recumbents a few years ago (a very good move for me personally, YMMV). Using Sheldon’s gain ratio setting on his gear calculator was helpful to check what my equivalent ring combinations needed to be as I shrunk the cranks. That said, reducing the rings by the same ratio as the cranks achieves the same outcome! A bit more experience suggests that I can comfortably reduce the rings by the same ratio but can cope with adding another 10% or so, especially for the big ring.

[NATURAL ANKLING]

Hi,
Foot Speed!