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How can I calculate gear ratio equivalence?(13 posts)

How can I calculate gear ratio equivalence?Kvonnah
Jul 17, 2003 9:07 AM
I have an Ultegra Triple (should have bought a double) 52, 42 30, and I was wondering how to calculate equivalent gear ratios had a purchased a 53 39. I.e. 52-11 would be equal to 53-? Or if I actually used the 30 what is the smallest gear I could spin without being smaller then 39-23? Any help would be appreciated.
re: How can I calculate gear ratio equivalence?godot
Jul 17, 2003 9:14 AM
this website is pretty easy to use for gear-inch calculations

hope this helps
re: How can I calculate gear ratio equivalence?Kvonnah
Jul 17, 2003 1:12 PM
Great site, Thanks!
Good old Sheldon Brown again...JL
Jul 17, 2003 9:17 AM
Gear Calculator here


Happy riding.

c'mon people, teach a man to fish...Steve_0
Jul 17, 2003 10:04 AM
a ratio is simply the quotient of two expressions.

1. 52/11 = 4.727
2. 53/x = 4.727
3. x = 53/4.727
4. x = 11.21 (11 teeth would be closest you can come).

1. 39/23 = 1.69
2. 30/x = 1.69
3. x = 30/1.69
4. x = 17.75 teeth (18 teeth will exceed 39/23).

Good links, but he didnt ask for gear inches; and even if he did, it wouldnt (directly) provide him the data he's looking for.

Whoever said 'I'll never need this math in the real world!?'
1 tooth in the back...MShaw
Jul 17, 2003 10:18 AM worth 3 in the front. Your 25t cog is going to feel like a 24t with a 39t ring, etc.

Don't quote me 'cause I haven't done it, but I'll bet that the 30x19 (18?) is going to be the smallest gear you can go before it feels smaller than the 39x23.

(sigh). nmSteve_0
Jul 17, 2003 10:23 AM
Don't worry, I'm with you.NatC
Jul 17, 2003 1:32 PM
It's a simple ratio of front divided by rear. How many teeth in front per how many teeth in back. Since you won't be swapping to a different diameter wheel, you don't need to factor in the wheel size (since it remains constant). Gear inches are only necessary if you change to an odd size wheel.

Think through it like this:

"52/11 = 4.7"

"therefore, 53 divided by what gives me 4.7?"

you can write it mathy: 53/X=4.7

Then solve for X:
Gear inchesSpoke Wrench
Jul 17, 2003 5:00 PM
Gear inches = Chainring teeth divided by cog teeth times 27 inches.

With a pocket calculator you can do a whole bike's worth of ratios nearly as fast as you can look them up once you find the chart.
Gear inches in a metric worldchar
Jul 17, 2003 7:08 PM

Close enough for calcuation purposes but not exact enough if you have a pocket calculator or a spreadsheet.

Unless you are still riding that Schwinn Varsity or an English Tourer, most wheelsets sold today are metric; size 700 is slightly smaller than 27 inches in diameter and it is also dependent on the second number listed on the tire, for example, 19c, 23c, 25c, 28c, which is the height, not width. Sheldon's site lists a whole slew of tire sizes and takes care of this, but for gear inches you would need to change 27" to diameter of wheel.

Front teeth divided by rear teeth times wheel diameter.

53/24 x 26.75" = 59 (rounded or 59.0729)
39/17 x 26.75" = 61 (61.3676)

As noted above the question really didn't ask for gear inches (everyone else can just use the metric measurement,)just a ratio so the wheel diameter would be a constant and doesn't need to be used unless you are interested in distance traveled.

I'm just nit pickin' here, close enough for horseshoes, handgrenades and now gear inches.

calculatin" charlie
Like horseshoes and hand grenades, gear inches get close.Dale Brigham
Jul 17, 2003 9:11 PM

You are absolutely correct that 27" is not really the diameter of most 700C rim/tire combos, but I submit that the rough approximation of same works well. The beauty of the gear inches system is that it produces numbers that resonate with us in relation to how hard they are to pedal.

A 100" or higher gear is big, whether it's a 52/14 or a 45/12. A 40" gear is little, whether it's a 39/27 or a 42/28. These are nice, round numbers that are easy to relate to. And, calculating them as you ride is a good way to pass the time!

Why is it y'all are making this...MShaw
Jul 18, 2003 9:11 AM
...more complicated than it needs to be?

Not pointing the finger at any one person, but jeez, who cares how many gear inches a particular gear is and how to calculate it unless you're racing track?

Gear inches have been pretty well useless in the real world since the high wheelers. Shift to the gear that feels good, then ride.

Occam's Razor rules!

Analytic calculatorcoonass
Jul 18, 2003 1:39 PM