# Recumbent - Gear inches

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View Full Version : Gear inches

oldguy52
01-04-06, 07:08 AM
OK, can somebody explain this gear inch thing??? I was figuring out the gearing on my new Grasshopper and no matter how I add it up I get numbers that aren't even close to what these gear inch calculators give me.

For instance, top gear according to the gear inch program is supposed to be 106 gear inches. When I figure out how far the bike should move using real measurements, 1 turn of the crank in top gear should be about 327 inches. 60 teeth / 11 teeth (5.45) x aprox 60 in. wheel circumference.

Anybody ???

pigasus
01-04-06, 07:18 AM
Gear inch refers to the diameter of the wheel, not the circumference. So if your wheel has a circumference of 60 inches, then it has a diameter of 60/pi = 19.1 inches. So the gear inch value for your top gear becomes: (60/11) x 19.1 = 104 inches.

Sally

oldguy52
01-04-06, 07:24 AM
Thanks Sally,

I got to looking at that after I posted and realised that if I divided my result by PI I got about the same number as the gear inch chart. I guess I don't understand why they wouldn't finish the calulation though and figure how far the bike actually moves for one turn in a given gear.

dogsridewith
01-04-06, 07:57 AM
because "gear inches" was coined to compare bikes with gear trains(chains and sprockets, shafts and gears, etc.) to earlier bikes with a directly cranked drive wheel.

squeaker
01-05-06, 04:28 AM
It's all to do with the good old 'penny farthing' - bigger wheel = faster (provided you have the legs for it). 'How far' in one wheel revolution was just to logical for us Brits, although I think that the continental chappies went that route (as well as inventing recumbents :p )

BlazingPedals
01-05-06, 05:54 AM
Manufacurer gear range numbers are often off by quite a bit from reality because their calculations are based on having, for instance a 26" wheel, when in fact the wheel in question might only be 24.5" in actual diameter. But yeah, what the others said. Gear inch numbers are meant as a means to compare geared bikes to the wheel size on an 'ordinary' (a.k.a. penny farthing.)

lowracer1
01-05-06, 09:19 PM
one of these days I gotta see what my top gear inches are on the vk with the 700c rear with the 58/11 combo. I'm too lazy to search for the gear inch calculator.

meb
01-06-06, 05:48 AM
one of these days I gotta see what my top gear inches are on the vk with the 700c rear with the 58/11 combo. I'm too lazy to search for the gear inch calculator.

Just under 141.

bentcruiser
01-06-06, 07:25 AM
OK, can somebody explain this gear inch thing??? I was figuring out the gearing on my new Grasshopper and no matter how I add it up I get numbers that aren't even close to what these gear inch calculators give me.

Another method of measuring gears is by gain ratios and also meter development.

Sheldon Brown has a good calculator for all three methods:

Gear Calculator (http://sheldonbrown.com/gears/)

BlazingPedals
01-06-06, 07:25 AM
A 700C with a *true* 23C tire would have a diameter of 26.29". That with a 58/11 gear combination would make for a 138.6 inch gear. Carried one step further, if you pedaled that gear at 80 rpm you would be travelling at 32.9 mph. At 90 rpm it would be 37.0 mph, and at 100 rpm it would be 41.2 mph.

meb
01-07-06, 01:17 AM
A 700C with a *true* 23C tire would have a diameter of 26.29". That with a 58/11 gear combination would make for a 138.6 inch gear. Carried one step further, if you pedaled that gear at 80 rpm you would be travelling at 32.9 mph. At 90 rpm it would be 37.0 mph, and at 100 rpm it would be 41.2 mph.

Well Chris could you specify your tire width so we can come to some accord on your top gear inches? :roflmao:

oldguy52
01-07-06, 07:52 AM
Well Chris could you specify your tire width so we can come to some accord on your top gear inches? :roflmao:

Well...... IF you want to get right down to a perfect measurement ........

Knowing what it says on the side of the tire is meaningless and will never be better than approximate. To get it right down perfect, you need to measure the radius from the center of the wheel hub to the ground, WITH the riders weight on the bike (allow for tire squat). This will give you the TRUE rolling radius of the tire. Only in this way will you be able to figure the "exact" gear inch measurement for any given bike.

Rik

BlazingPedals
01-07-06, 10:04 AM
To get it right down perfect, you need to measure the radius from the center of the wheel hub to the ground, WITH the riders weight on the bike (allow for tire squat).

Very true. But there's accuracy and there's accuracy. It's not valid to assume a 23C tire will be an actual 23C, which is why I stated it the way I did. But measuring with weight on the tire IMHO is going farther than you'd have to go for mere gear calculations. Deflection for a properly-inflated tire is 10% according to Michelin. For a 23C tire, that means a 0.09 inch reduction in radius. Since we're only stating gear inches to one decimal place, the difference is not significant.

meb
01-07-06, 01:12 PM
Very true. But there's accuracy and there's accuracy. It's not valid to assume a 23C tire will be an actual 23C, which is why I stated it the way I did. But measuring with weight on the tire IMHO is going farther than you'd have to go for mere gear calculations. Deflection for a properly-inflated tire is 10% according to Michelin. For a 23C tire, that means a 0.09 inch reduction in radius. Since we're only stating gear inches to one decimal place, the difference is not significant.

Since we're turning Chris' gear ratio into a forum winter project, there's no need to limit our precision to useful levels. :rolleyes:

lowracer1
01-08-06, 07:36 AM
my rear tire is a 700c x 21mm width have fun splitting hairs.

maybe you can further tweak the gear inch calculation by taking into account chain stretch , chainring and cassette wear, and road surface which can adversely affect the contact patch area and deflection of the tire squat. Of course the tire squat will change by body weight, so maybe make a chart for my body weight starting at 190 lbs through 200 lbs. You might have to take it out 8 decimal places, but knowing the gear inch at different weights would be cool.

oh don't forget to take into account the riding temperature which would effect tire pressure which would of course affect the tire squat.

Now actually, wouldn't your gear inches actually change during a century event where your body weight will change during the event, plus the temperature usually rises a good 10 to 20 degrees from start to finish.

hmmm, this sounds like you guys have quite a project here. might take until winter of 2009 to finish it.

mattzees
01-15-06, 02:40 PM
Wow, you guys are all nuts, huh?

--M

meb
01-16-06, 07:27 AM
Wow, you guys are all nuts, huh?

--M

Like Waylon-

I've always been crazy but it kept me from going insane.

Carusoswi
09-10-06, 07:33 AM
Ok, I know this is an old thread, but thanks for the link to the gear inch calculator. Now I understand why my cadence is so much slower than what most consider average. If I could maintain 100 RPM in my longest gear, I would be rolling at a sustained speed of 44.8 mph.

I can get there on a downhill, but not close on flat terrain.

I could also print out the calculations and use them in reverse to estimate my cadence based upon my gear and speed. Interesting.

Caruso

BlazingPedals
09-10-06, 05:20 PM
Carusoswi - we could transmogrify this thread into a cadence thread. Whatever speed you are going, you should pick a gear that allows your feet to spin at your ideal cadence. Bigger gears don't necessarily let you go faster; sometimes they move you off your power curve and actually cause you to go slower.

OLD GRANDAD
09-10-06, 05:48 PM
Too Serious For Me. I Just Pedal And Grin.

Dr.Deltron
09-11-06, 05:52 PM
Too Serious For Me. I Just Pedal And Grin.

+3! I ride a recumbent trike!