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Sofa King We Todd Did
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Discussion Starter · #1 ·
OK, here's the deal: one 29er and one 26"-wheeled bike. Here are the parts:
- One 33T chainring
- One 34T chainring
- One 21T cog
- One 22T cog

How do I mix and match these to achieve a relatively similar gear combo? I know that comparing gear ratios isn't enough when comparing bikes of two wheel sizes, but I just can't for the life of me figure out the whole gear inches thing (which is what I think I need to be comparing, right?)

pacman, you're typically a genius with this stuff. Anyone else got a spare second to help a brother out? Both bikes will be sporting 2.3 tires, if that makes any difference.

Thanks a million.
 

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Sofa King We Todd Did
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2,262 Posts
Discussion Starter · #6 ·
Thanks a million, fellas. I do have a 32T chainring, but it means that I'll need to give up my Truvativ Luftalarm cranks, so that's a bit of a deal breaker. I'll run with the recommended gear combos.

Any chance I could trouble you guys for a quick lesson on how you figured it out? I'm in the whole "teach a man to fish" mode, rather than the "give a man a fish" approach.

Cheers.
 

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It's pretty simple:

Chainring divided by rear cog times wheel size equals gear inches

So 33 divided by 22 is 1.5. Multiply that by 29 and you get 43.5 gear inches

And 34 divided by 21 is 1.62. Multiply that by 26 and you get 42.1 gear inches

Have fun playing with ratios
 

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biggieP said:
It's pretty simple:

Chainring divided by rear cog times wheel size equals gear inches

So 33 divided by 22 is 1.5. Multiply that by 29 and you get 43.5 gear inches

And 34 divided by 21 is 1.62. Multiply that by 26 and you get 42.1 gear inches

Have fun playing with ratios
That doesn't work??? 26 and 29 have only a passing relationship to the diameter of the wheel. To calculate the gear development (the distance travelled per crank revolution), you must multiply the gear ratio by the wheel's circumference.
That varies according to the tire you use and the tire pressure you run, so just measure your wheels. Lay out the tape measure on the floor, roll the wheel, with weight on it, one full turn, measure the distance it travelled. Using the valve stem as a pointer is helpful.
Then: chainring / cog x diameter = rollout
Once you have the bike assembled, it's easier, just roll the bike backward one turn of the cranks and measure the distance covered. This was (is?) standard practice for checking compliance with Junior gear restriction rules.
 

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Why overcomplicate things?

Letsinger said:
That doesn't work??? 26 and 29 have only a passing relationship to the diameter of the wheel. To calculate the gear development (the distance travelled per crank revolution), you must multiply the gear ratio by the wheel's circumference.
There's no need to calculate development for this purpose. Gear inches, using the formula shown above by biggieP, is a standard and very widely used method of describing effective gearing, and is perfectly useful for comparing the relative gearing of the two bikes.

The only thing wrong with it is biggieP's math is the wheelsize figures of 29" and 26", which overstates the difference enough to possibly matter. For typical 2.0" mountain bike tires it would be more accurate to use figures of 26" and 28.5".
 
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