# Bicycle Mechanics - How to use Gear Roll out?

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View Full Version : How to use Gear Roll out?

vrkelley
08-19-04, 11:23 PM
What is gear roll-out and how can I use it to increase speed?

Fat Hack
08-19-04, 11:55 PM
Gear roll-out is the distance the bike will travel with one revolution of the cranks for any particular gear ratio. I believe it's also called "gear development".

Gear roll-out chart.
http://www.analyticcycling.com/PowerMeasAnalyGear_Disc.html (SCROLL DOWN)

You can use it to determine what gears you need for any particular speed or rpms.

For instance, you might wanna know what revs you'll be doing at a particular speed.

For example: "what revs will I be doing at 60kmh (37.29 mph) with a 52/12 gear ratio"

So, you go to the chart which tells you your 52/12 gear ratio = 9.135 meter roll-out...then.....

60 kmh / 60 = 1 km in one minute
1 km x 1000 = 1000 M in one minute
1000 / 9.135 = 109.47 rpm

You can obviously do this the other way around. For example, you might wonder "hmmm...if I was riding at 105rpm in my 52/12 gear, how fast will I go?"

So, you go to the roll-out chart which tells you that 52/12 has a roll-out of 9.135 meters.

Therefore: 105 x 9.135 = 959.175 meters per minute.
Therefore 959.175 x 60 = 57550.5 meters per hour
which obviously = 57.55 kmh

It is also used for juniors because they have gear limitations to reduce the chances of injuries.

This is different to the old "inches" calculation, which is (# chain-ring teeth / # cog teeth) x 27

eg: (52/12) x 27 = 117 inches.

08-20-04, 12:58 AM
I wonder what overall wheel/tire circumference is assumed with this chart.

Gonzo Bob
08-20-04, 01:14 PM
I wonder what overall wheel/tire circumference is assumed with this chart.

Easy enough to compute: Gear Rollout = Tire Circumference * Chain Ring / Cog

So... Tire Circumference = Gear Rollout * Cog / Chain Ring = 9.135 * 12 / 52 = 2.108 meters which is about a 700x25C

vrkelley
08-20-04, 06:39 PM
OK that maps to Cadance at Speed and Speed at Cadance Where:

Cadance (rpm) at Speed = (rearCog*5280*12)/(60*3.14*Gear inches)