[retromike3]

I'm used to gear inches. If you divide the chainring number by the cog number and then multiply by the diameter of the rear wheel you get how many inches you will go in one revolution of the peddle. I like running a small gear, something like 65 inches. But I'm a fat old guy.


Mike Frye-the bike guy

[Rhythm]

Flip flop running
70 GI on one
74 GI on the other

[Doohickie]

46x18 freewheel. My 700x28c tire measures 27", so I guess that would be 46x27/18 = 69 gear-inches?

[Rhythm]

27" wheels differ from 700c by around 8mm if I remember correctly.

A helpful sight for gear ratio, gear inches, cadence etc is BikeCalc.com - Fixed Gear Calculator

You were close but your at 68.3 GI, not like its a huge difference.

[Doohickie]

Originally Posted by Rhythm

27" wheels differ from 700c by around 8mm if I remember correctly.

Nominally I think you're correct, but I measured my tire with a tape measure and yeah, it was 27" even.

[IanHighfield]

Originally Posted by retromike3

I'm used to gear inches. If you divide the chainring number by the cog number and then multiply by the diameter of the rear wheel you get how many inches you will go in one revolution of the peddle. I like running a small gear, something like 65 inches. But I'm a fat old guy.


Mike Frye-the bike guy

Mike, I like to think in gear inches also, but you're off a bit. That number you come up with after doing your formula represents the old fashion Pennyfarthing big wheel diameter. Example: 50x15 gearing = 90 inch gear if using a 27 inch tyre. That 90 inches is what a Pennyfarthing's wheel diameter would have to be in order to equal a modern 50x15 with 27 inch tyres.

Distance traveled is gear inch (90 in this case) times 3.14. (circumference of wheel) You get 282.6 inches of travel for one turn of the pedals. That's about 23.55 feet per pedal turn.

Think of that one turn of the pedal of your modern bike in 50x15 as equal to one turn of the pedal for a Pennyfarthing with a giant 90 inch front wheel.

Hope that's been made clearer.

Now to further muddy the creek, take that 23.55 feet of travel per pedal stroke and multiply by cadence. Say 90 rpms. 90 x 23.55 = 2119.5 feet of travel per minute. You've gone 2119.5 feet in just one minute. Now multiply that "minute" distance by 60 to get your distance traveled in an hour.

2119.5 x 60 = 127,170 feet traveled in an hour. Divide by 5280, which is a mile, and you have your miles-per hour-figured by cadence and gear.

127,170 divided by 5280 = 24.09 mph. If you can maintain a 90 rpm cadence with a bike wearing 27 tyres (700s) and geared 50x15, you'll be going 24 mph.

Mike, back to your 65" gear. Assuming a 700/27" tyre, you will travel 204.1 inches per pedal cycle. That's 17 feet. Let's say you only turn 60 rpms. That sends you 1020 feet per minute. Times 60 minutes = 61,200 feet per hour. Divide by 5280 (mile) and you are traveling 11.6 easy miles-per-hour. Crank it up and go faster. At 90 rpms, you'll be around 17.4 mph. 65 inches of gearing is a very useful sizing. You can do quite a bit without killing yourself.

[trx1]

i usally run a 39/17 with a 180mm crank on my rides.