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Discussion Starter · #1 ·
So, if I have a magic gear of 42 x 17, that works perfectly, is there a formula or rule that I can go by to figure out other magic gear combos that will work on this bike?

Does such a thing exist, or is it all just experimentation?

Thanks.
 

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Discussion Starter · #5 ·
magic gear-proper tension

By magic gear, I mean proper tension with vertical dropouts. A 42x17 currently works, but is a bit steep for my cyclocross bike. So, I was wondering if there was a 'rule of thumb' that could be applied here.

i.e. if a 42x17 works on bike A, then a 39x?? will give the also work on bike A, after shortening/lengthening the chain.

I've looked at the gear calculators in the past, but never had much success. I'll take another look. Thanks for the replies!
 

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The program I suggested will give you info like this:


Ring-Cog-Stay length-Chain length-Gear inches
38 13 35.069 83.0 76.5
39 14 35.069 83.5 72.9
40 15 35.069 84.0 69.8
41 16 35.069 84.5 67.0
42 17 35.069 85.0 64.6
43 18 35.069 85.5 62.5
44 19 35.069 86.0 60.6
45 20 35.069 86.5 58.9

Notice these gear combos all produce the same chain stay length, you can then raise or lower your gear inches as needed and stay in your "Magic" stay lenght.
 

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1speedbrian said:
Notice these gear combos all produce the same chain stay length, you can then raise or lower your gear inches as needed and stay in your "Magic" stay lenght.
But doesn't the table also show the chain length changing by 0.5cm at each step? If so, if the chain tension is already correct, an effective need of 0.5 cm more chain (as gear inches increase) will increase the chain tension with the fixed chainstay... If the chain has some slack, that's probably fine, but if the chain is already as tight as it should be, then things could get too tight...

Or am I reading the table incorrectly?
 
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