Bicycle Mechanics - Gear ratios? Gear Inches?

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View Full Version : Gear ratios? Gear Inches?

oldokie
10-31-06, 09:35 PM
What is the most commonly used method to refer to gearing on bikes when you want to compare different gearing setups? I see references to "gear inches" and "metric development" and other methods. The "gear inches" term seems to pop up in more places but I still see all types of methods used.

What is the best on line calculator to use? (Sheldon Brown?)

LittleGinseng
10-31-06, 10:11 PM
What is the best on line calculator to use? (Sheldon Brown?)

That's the one. Accept no substitute.

sivat
10-31-06, 11:52 PM
With the exception of gain ratio, they all mean about the same thing. Gear ratio works if you are comparing otherwise similar tire sizes. Gear inches and meter development are different ways of expressing the effective size of the gear if one direct drive wheel was used. Gear inches is the diameter of the effective wheel, meter development is the effective circumference (ie. how far the bike will travel for one rotation of the pedals). gain ratio also takes into account crank length as it measures how far the bike will move versus the distance your foot travels.

It seems to me that gear inches is the most popular in the US, and gain ratio is what you use if you are an uber-bike geek.

Retro Grouch
11-01-06, 04:28 AM
I personally use gear inches because it's what I've always done and because I can work them out with a pocket calculator about as fast as I can look them up in a table.

To understand gear inches think of a direct drive kids tricycle. Gear inches is the diameter of the front wheel.

San Rensho
11-01-06, 07:46 AM
I would say that gear inches is the most commonly used method of talking about the overall gearing of a bike. Gear inches is easy to calculate. For a 700c rim, big chainring teeth/cog teeth X 27 is the gear inches.

To get a feel for how gear inches translates to real world riding:

An average gear, used on many single speed bikes, comfortable for most people, is around 70 inches.

Track racers use about a 90 inch gear.

A 52/11, which is a huge gear and very hard to push, is 128 inches.

A 42/28, which is a really easy gear, is 40 inches.

Dave Moulton
11-01-06, 08:02 AM
You can make a handy gear table on your PC with Microsoft Excel. In the A column starting at the top, type the rear sprocket sizes; 12 13 14 15 16 etc down the column.

At the top of B column type the following formula =27/A1*42 (exactly like this starting with equals =)

The formula is 27 (Wheel dia.) divide by the number in A1 (Teeth on sprocket.) times 42 or whatever size chainwheel you have.

Hit enter and you will see the gear in inches for the first sprocket. Click on that cell (B1) then click and hold on the little square in the bottom right of the cell and drag it down the column. You will see the gear in inches for each sprocket size.

In the C column type in =27/A1*52 (The last number is chainwheel teeth.) and repeat as above.

bkaapcke
11-01-06, 08:10 AM
I prefer to use descriptive tems, like; extra hard, hard, midlin, sorta easy, easier and real easy. Try it, it works. bk

Al1943
11-01-06, 08:28 AM
Gear inches just communicates better. We all know that when we are in a gear of 100 or more inches it's high, below 40 is low. Multiply gear inches by pi and you've got development. Using 27 inches for a 700mm wheel diameter or 26 inches for a 650mm wheel diameter is plenty good enough when we are comparing gearing, and you can quickly calculate it on a hand calculator or with a pencil. If you want an accurate development number you'll need an accurate wheel measurement.

Al

oldokie
11-01-06, 05:43 PM
Thanks all. I think that answers my question.
Gear inches on Sheldon will do the trick for what I need.

Olden Crow
11-02-06, 05:55 PM
Can someone tell me what the 3 gear inches were on a typical British three speed touring bike of the 1940s to 1960s? I'm interested in a rough ballpark figure for all three gears. Thnaks! ..

Retro Grouch
11-02-06, 06:53 PM
Can someone tell me what the 3 gear inches were on a typical British three speed touring bike of the 1940s to 1960s? I'm interested in a rough ballpark figure for all three gears. Thnaks! ..

My college beater was a Hercules with a Sturmey Archer hub. Gear inches worked out to 50/70/90.

AndrewP
11-02-06, 08:15 PM
Can someone tell me what the 3 gear inches were on a typical British three speed touring bike of the 1940s to 1960s? I'm interested in a rough ballpark figure for all three gears. Thnaks! ..
The ratio between gears on the SA were 0.75:1 This would give gear inches something like 48, 64, 85.

San Rensho
11-03-06, 10:00 AM
Can someone tell me what the 3 gear inches were on a typical British three speed touring bike of the 1940s to 1960s? I'm interested in a rough ballpark figure for all three gears. Thnaks! ..

The gear inches for a 3 speed are calculted just like any other bike, cr/cog X 27. Thats the gearing for 2nd gear. 1st is 25% less, 3rd is 33% more than second.

Michel Gagnon
11-04-06, 04:48 PM
Gear-inches vs development: it's basically the American vs the continental European way of assessing gears.

Gear inches : (from Sheldon Brown's Glossary (http://sheldonbrown.com/glossary)
"It is the equivalent diameter of the drive wheel on a high-wheel bicycle (http://sheldonbrown.com/gloss_ha-i.html#highwheel). When chain-drive "safety" bikes came in, the same system was used, multiplying the drive wheel diameter by the sprocket ratio. It is very easy to calculate: the diameter of the drive wheel, times the size of the front sprocket divided by the size of the rear sprocket. This gives a convenient two- or three-digit number. The lowest gear on most mountain bikes is around 22-26 inches. The highest gear on road racing bikes is usually around 108-110 inches."

Development:
Development is the distance the bicycle travels for one crank revolution, and is usually measured in meters. Development can be calculated by dividing the chainwheel size by the rear sprocket size, multiplying the result by the wheel diameter and by pi (3.1416).

Both can be compared fairly easily. Development is similar to the "rollout" given by manufacturers of cyclocomputers.

To compare them:
With a 1:1 ratio (ex: 34 chainring and 34 cog) and 700c wheels with 700x30 tires, one either has 27 gear-inches or 2.16 m development.

MikeR
11-04-06, 05:02 PM
I personally use gear inches because it's what I've always done and because I can work them out with a pocket calculator about as fast as I can look them up in a table.

To understand gear inches think of a direct drive kids tricycle. Gear inches is the diameter of the front wheel.
Diameter or circumference?

sivat
11-04-06, 11:28 PM
Diameter. Circumference is referred to as development.