Gear Ratios

[Sandman]

Yesterday, I was trying to help my wife understand what she was doing when she shifted gears on the bike... pretty much ran along the lines of "you shift up to go fast, you shift down to pedal easier".

Unfortunately, my neighbor was there and either we understand it differently or one of us is just wrong, thus creating continued confusion for my wife. For a motor vehicle, he insists that to a downshift is from 1 to 2, while I maintain that a downshift is from 2 to 1.

Is this a schooling or geographic thing? Are we both right or is he flat out wrong? He says he thinks of it as a ratio thing, but I think his math is wrong.

Just to verify my math... on a bicycle... if I'm on the 32 tooth drive and the 16 tooth cog, I'm at 2 to 1. As I shift up for speed, I select the 11 tooth cog, I'm at 2.9 to 1, a higher ratio and faster gear.

Or... am I looking at this all wrong?

[FUNKED1]

For cars, a downshift is always to a lower numbered gear which is always a higher numerical ratio.  

"Tall" gears = High Numbers = Low Ratios
"Short" gears = Low Numbers = High Ratios

So your buddy is wrong about cars.

However on your bike, you got it backwards.  The convention is that gear ratios are expressed in terms of output divided by input.  In your example, 16:32 and 11:32.

In general on a bicycle, your wheel RPM is higher than your crank RPM.  On cars it is generally the opposite - higher engine RPM than wheel RPM.

[moose]

Downshifting
You'll want to downshift when heading down a steep slope to reduce riding the brakes or when greater acceleration is needed. Downshifting is much like upshifting: off the gas, press the clutch and move to a lower gear and get back on the gas as you let out the clutch. Getting back on the gas quickly is important since downshifting and not touching the gas will slow the car down as the engine compression fights the turning of the wheels ,which is what you want while going down a hill but not when you need to pass somebody. If you are already revving high in your current gear, downshifting may spin the motor beyond its rev limit, and you might throw belts or worse, throw a rod or valve which means it would be time for a major overhaul or engine replacement.

[Sandman]

Originally posted by FUNKED1

However on your bike, you got it backwards.  The convention is that gear ratios are expressed in terms of output divided by input.  In your example, 16:32 and 11:32.

Got it... thanx.


Edit... then again, maybe I don't... http://travel.howstuffworks.com/bicycle3.htm

[Sandman]

Originally posted by moose

Moose, that's the way I've always understood it.

[midnight Target]

On a bike you downshift to a bigger gear on your rear hub.

[Masherbrum]

Originally posted by midnight Target
On a bike you downshift to a bigger gear on your rear hub.

Sprocket!

Karaya

[Sandman]

Originally posted by midnight Target
On a bike you downshift to a bigger gear on your rear hub.

That's the way I understood it.

[FUNKED1]

Originally posted by Sandman
http://travel.howstuffworks.com/bicycle3.htm

It looks like the bicycle industry (or at least that author) uses a different convention than standard engineering practice.  All the stuff on that site is input divided by output.

[RTSigma]

Bikes are for lazy people. I prefer to drive on the sidewalk.