2/07/2008

Gear inches and ratios demystified

Track bikes are simple beasts aren’t they? The have one chain ring up front, one cog in the back and the bike moves when you pedal, even if that pedaling is backwards. They don’t even have brakes. When you want to stop, you stop pedaling. It couldn’t be much simpler.
Then someone starts talking gear inches or gear ratios and things get a lot more confusing. Hopefully, we can clear up some of this confusion.
The gear ratio of a fixed gear bicycle, or for any single gear on a multiple geared bicycle for that matter, is found by dividing the number of teeth on the cog into the number of teeth on the chain ring. Many fixed gear bicycles come with a 48 tooth chain ring and a 16 tooth cog. Forty-eight divided by 16 is three. This gear combination is expressed as 1:3. This is the same ratio for a 51 tooth chain ring and a 17 tooth cog, 51/17=3. Now that we know the gear ratio number, what does that tell us?
Gear ratios tell us several things. One is the relative difficulty of that particular gear combination. A 1:3 gear ratio takes more force to turn the cranks than a 1:2.53 gear ratio, a 48 tooth chain ring with an 18 tooth cog.
With a fixed gear, proper gear selection is crucial. You can’t shift to an easier gear when it gets hard. You can’t even coast for a second to catch your breath. If your gear is too big, you won’t make it up some of the climbs around Birmingham. If it’s too small, you won’t be able to keep up with the group on Lakeshore Dr. Descending on a small gear is an art unto itself. Since the gear is “fixed”, the faster the bicycle moves, the faster the pedals go round and round, round and round.
Most fixed gear riders use the term gear inches as often as gear ratio. If you know the ratio, multiply that number (3 in the case of our 48x16), by 27 to get 81.0 gear inches. Why 27? Twenty-seven is the “standard” for the diameter in inches of the rear wheel of a modern fixed gear bike. In all actuality, rear wheel diameters vary because of different tire sizes mounted on the rim. If you want the exact value, measure the diameter of your rear wheel and multiply the ratio number by that. Twenty-seven is an easy round number though and easy to use and remember. Comparing various gear inches measurements is the same as comparing gear ratios. Bigger numbers take more “oomph” to pedal, but can move the bike faster on a flat surface.
Multiply your gear inch measurement by pi(3.14) and you’ll have the distance your bicycle will move in one revolution of the cranks. This number will be expressed in inches. Divide it by 12 to find the number of feet you’ll move in that pedal revolution. Dividing by 39 instead of 12 will yield the European standard “metre development.” This measures the same thing, but in metres instead of inches or feet.

2 comments:

Chicken said...

All these numbers make my head hurt!

Sheriff said...

Your headache is not numbers related. Not gear ratio numbers at any rate.