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gear ratio question(9 posts)

gear ratio questionballbite
Jun 1, 2003 4:56 AM
At least once a year I like to do some mountainous rides in New Hampshire. These usually involve stretches of up to 5 miles at a 9% grade. My older road bike has a triple with the lowest gear ratio being 30/25. I found I had to use this gear on a lot of the climb because of its length.

My new road bike has a double. I had it equipped with a 27 tooth large ring in the back so my lowest gear ratio is 39/27.

Since I don't have long stretches of 9% grade where I live, I don't know how this ratio is going to be if I try those New Hampshire rides again. Any ideas if this is going to be low enough for me to do these climbs?
Try this:Spoke Wrench
Jun 1, 2003 5:19 AM
Gear inches equals chainring teeth divided by rear cog teeth multiplyed by 27. That gives you a number that you can compare and it's a common bike term so most bike mechanics will know what you are talking about.

Your old bike had a 32 inch low gear. New bike has 38 inch low gear. That's quite a bit of difference. The cheap, dirty solution is to install a mountain bike cassette, rear derailleur and new longer chain. That can give you a low gear as low as 30 gear inches, will work fine with the shifters you have, and you can take it off after your tour is finished.
Swapping the chainring would also work [nm]bent_spoke
Jun 1, 2003 7:43 AM
38 is usually the smallest you can go...CurtSD
Jun 2, 2003 7:30 AM
and that won't be enough. You could swap in a different crank, like a 110 bcd TA Zephyr (http://www.peterwhitecycles.com/zephyr.asp) and put something like 50/36 rings on it, but if you're doing that you might as well just go back to the triple.
No, it won't beAlexx
Jun 1, 2003 8:53 AM
30/25 = 32.4 gear inches
39/27 = 39.0 gear inches.
You would need a 33 tooth cog to give you nearly the same gear with a 30 tooth ring. The other idea of switching rings is silly, since most road bikes can't take smaller than a 39 tooth small ring. Chances are pretty good that the 27 tooth is the biggest you can use with a short cage rear derailleur.

http://www.sheldonbrown.com/gears/
Not enoughLC
Jun 1, 2003 9:11 AM
If you really needed the 30x25 then you are going to need a 12-32 cassette and a MTB rear deraileur to be able to get nearly the same gears. Even that is not quite as low as you had before, but your new bike is probally a little lighter and you should be able to make it up the hill in 39x32.
re: gear ratio questionfirstrax
Jun 1, 2003 10:41 AM
Exactly where in NH?
re: gear ratio questionballbite
Jun 1, 2003 5:28 PM
Kancamagus highway and from North Conway up to Pinkham Notch are a couple examples of 9% climbs I've ridden.
you can get a 38 tooth ring; will help a little nmDougSloan
Jun 2, 2003 6:32 AM