Gear ratio

[nicninco]

Couple questions involving gear ratios. How can gear ratio best be evaluated/measured/calculated? Is there a way to do a simple home calculation to come up with a ratio? How do cruisers of similar proportion (wheelsize/framesize) have completely different drive characteristics? Once measured, how can that ratio be recreated on another frame? Thx.

 

[c.p.odom]

Calculating gear size in inches:
Crank teeth divided by cog teeth multiplied by wheel diameter.
Example:
52/11x27=127.64 gear inches

But you asked for ratio:
Wheel radius in mm / crank length in mm = X (The radius ratio)
X * front chainring teeth / rear cog = Ratio

Example:
A road bike with 170 mm cranks: (The usual generic diameter value for road wheels is 680 mm, so the radius would be 340 mm.)
340 mm / 170 mm = 2.0. (The radius ratio)
2.0 X 53 / 19 = 5.58

The frame is irrelevant.

 

[B607]

Yikes, a math thread! Runnnnnnnnnnnnn! :mrgreen: Gary

 

[Anonymous]

< cant do math...my head hurts.

 

[Korporal]

The math is just a series of conversions. Sheldon Brown's calculator can help you skip the tedium.

http://www.sheldonbrown.com/gears/

 

[Anonymous]

Why do I not care about the numbers ratio on any of my bikes...?

If the gait feels right...its good!

Its all about trial and error.There is no right or wrong.

(been there,done that many times)

 

[Anonymous]

The quick and easy...

Small crank sprocket/large hub cog = low end torque but no top end.

Large crank sprocket/small hub cog=high end speed (you will sweat your buttox off getting there)


Pedal crank arms add into it too...Long arms for low end torque.

 

[ozzmonaut]

I'm the same way. Once it's together, if I can't get it up a hill, I put a smaller front sprocket, unless it's on a cruiser that has the front sprocket that it originally came with. Then I change the rear cog. Fortunately most old single speed cruisers have a comfortable setup. 48-52T front/16-20 rear, unless it's the skiptooths, then 26T/9-10T. My town is near the mountains and pretty hilly, but I can get up almost any of the hills on any of my old cruisers, so I'm content. I might start to sweat, but if you do it enough that will change. Plus I hate having too small of a sprocket up front, then having to pedal twice as much to make it across a flat intersection or stretch of busy road that I'd like to hurry up and get away from.

 

[ozzmonaut]

I'm guessing however, that given the low number of posts from the original poster, that the question is geared more towards race bikes or at least a lightweight bike.

 

[X-RAY]

cruising on my mtbs (21 and 24 speeds) has taught me a few things about gearing bikes. i know its not quite the same, but its easier to shift and see how it feels than to swap out cogs and chainrings. actually, i need to leave the gears alone and lose weight and get into shape! :mrgreen:

 

[deorman]

http://www.machinehead-software.co.uk/g ... ble26.html

 

[socal_jack]

The point of using some quick and easy calculations is that trial and error is not necessary. If there's a certain gearing that you like, it can be replicated or at least closely approximated across any crank-size/wheel-size combo with a cog/sprocket combination first shot.

Why do I not care about the numbers ratio on any of my bikes...?

If the gait feels right...its good!

Its all about trial and error.There is no right or wrong.

(been there,done that many times)

 

[expjawa]

Exactly. It's just math, and there are online calculators if you don't want to work it out yourself. I live on the side of a pretty steep grade, so I have to compromise my single-speed bikes, even if I ride them mostly on the bike path at the bottom of the hill. I've pretty much figured out what I can pull on the hill in terms of crank effort, and I figure out what chainring/cog combination I need to get there for a give tire size. It's just math.