gearing

[dan rider g]

hello i was hoping some one could help me. i am currently building a 26" bike i have a chrisking hub wich has a 15 tooth surly cog on it i also have middleburn cranks and a middleburn pro trials bash gaurd 5 bolt. i would like the ratio to be good for street and pure trialsy riding kind of in the middle i was wondering what options i have thanks.

[Danny00135]

Normal stock ratio is 18-15

I'd suggest you use this ratio and use short cranks,

less leaverage=harder pedalling more appropriate for street, yet normal ratio great for pure trials.

[ooo]

if you have a 22t chainring, then the factory hex uses 22t-18t (22/18 is almost equal to 18/15).

sheldonbrown.com/gears is also quite useful if you want to compare gear ratios across wheel sizes and crank lengths.

Edited March 4, 2013 by ooo

[dan rider g]

i do not understand one bit how to use that website. thank you anyway and so will i deffinatley need to change the cog on the back ? to say an 18 so i can run 22-18

[albert_03]

This is tangential to the original poster's question, but relevant. Bikers have historically talked about gear ratios, but that's only part of the equation when you're considering how "hard" or "easy" a bike is to pedal. There has been a move by some (reference given to Sheldon Brown's website above) to talk instead about Gain Ratios, instead of gear ratios. Gain Ratio is a preferred measure in my view because it is a (unitless) number that takes wheel size AND crank length into consideration, as well as gear ratio. I wish use of this measure would catch on faster. With skinny road bike tires, it didn't make as much of a difference because all the wheel diameters were very close and varying crankarm lengths was not really common. So many old school roadies just talked gear ratios. (I'm kinda guessing this is how it happened, but bear with me.) Now there are a lot more common wheel sizes (20, 24, 26, 650B, 29er), and tire width and effective wheel diameters can be all over the map. (E.g., the Surly Krampus.)

Here's my explanation of the Gain Ratio (just skip to the Bold if you're bored):

Gain Ratio = Bike Travel / Pedal Travel (think arc lengths, s=r*θ)

= radius_wheel*θ_wheel / radius_crank*θ_crank.

Since the angles through which the crank and wheel rotate are related by the gear ratio (number of teeth), substitute using

θ_wheel = (front chainring teeth/rear cog teeth)*θ_crank.

So,

Gain Ratio = (radius_wheel/ radius_crank)*(front chainring teeth/rear cog teeth), or

Gain Ratio= Radius_ratio * (Teeth_chainring/Teeth_rearCog)

This is a unitless number that takes crankarm length AND tire size into account.

If you do a couple of calculations for different bikes, especially stocks vs mods (use data from Inspired's website, for instance), you may gain some insight into why certain gear ratios /gain ratios are preferred, which are equivalent, and how much the gain ratio can change when you make seemingly minor gear or cranklength changes, or when you put on a really fat rear tire.

NB: You should try to use the effective radius of the installed tire in these calcs; measure vertically from ground to center of rear axle.

[Blake]

Mah brain...