Simple Gear Ratio Problem

1. Sep 9, 2012

alexcc17

1. The problem statement, all variables and given/known data
In a bicycle, the ratio between the size of the wheel sprocket to the size of the crank sprocket is 2.5 which means...

An example of one of the answer choices is:

The rear wheel turns ___ times for every full rotation of the pedals.

2. Relevant equations

3. The attempt at a solution
I feel like that should be a really simple question if I had formula. We were given that:
ωwheel=(R1/R2)ωpedal
R1 is the radius of the crank sprocket
R2 is the radius of the wheel sprocket

This would mean that ωwheel=(2/5)ωpedal, but that doesn't really help.

2. Sep 10, 2012

Pkruse

No complicated math required. The wheel turns 2.5 times for every one turn of the crank.

3. Sep 10, 2012

CWatters

Rearrange to give..

ωwheel/ωpedal = R1/R2

But perhaps it would help to understand it from first principles? Lets say TP is the Tooth Pitch in inches. If the crank sprocket had 50 teeth each revolution of the crank advances the chain a distance of..

50 * TP

Then if the wheel sprocket had 20 teeth how many revolutions would it make...

= 50*TP / 20*TP

TP cancels

= 2.5

It's quickly obvious that the gearing depends on the ratio of the number of teeth on each.

In fact the ratio depends on the size of the gears regardless of how you specify the size. For example you could specify the size in terms of:

Teeth
Circumference