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Need help with gearing(9 posts)

Need help with gearingColorado Ron
Jun 13, 2002 8:17 AM
At 45 I can no longer climb like I used to so I need a little help. I'm thinking of going to a 39/25 from a 39/23 Can somone tell me what is the difference in distance traveled between a 39/23 and a 39/25? Thanks
grade school math....C-40
Jun 13, 2002 8:24 AM
Lets see, 23 divided by 25 is .92. In percentages, that's an 8% reduction in the distance traveled or an 8% increase in available torque.

It can make the difference between standing and sitting on a lot of hills. This is one of the reason that Campy 10 is great. You can have a 12-23, plus the bail-out gear when you need it, without creating large jumps between the cogs.
Hmmmm, sounds too easy.jose_Tex_mex
Jun 13, 2002 8:54 PM
C-40,
I don't mean to argue - especially since I admittedly do not know the answer. But this formula just seems to easy. This problem sounds like one for the Mechanical Engineers. Nonetheless, I would love to see the proof for something like this. I am not trying to be picky but I do love to see the details.

First, how does dividing the number of teeth relate to a linear distance or torque? Just checking the units causes problems here.

Are the number of teeth directly proportional to the resulting circumference of the gear. If so, can we directly applying a circular %differnce to a linear distance. eg. doubling the radius does not double the circ.

Again, I don't know. But if anyone does I would love to hear more.
I am a mechaincal engineer....C-40
Jun 14, 2002 12:53 PM
and it is that simple.
Not an answer but an additional question...HBPat
Jun 13, 2002 8:48 AM
For all my riding and wrenching I have never talked about a "-- inch gear". Lots of (not being insulting) older roadies talk about having a 16" gear or something like that. How exactly do you convert the teeth into inch/gear? And what is the range?

Hope I don't come off as ignorant but I've never needed to know this and I just got really curious.
Doubt we ever talked about a SIXTEEN inch gear! nmscottfree
Jun 13, 2002 8:52 AM
I think I could climb to the moon with a 22/36! nmcollinsc
Jun 13, 2002 8:54 AM
Mr. Brown will explain:scottfree
Jun 13, 2002 8:54 AM
http://www.sheldonbrown.com/gloss_g.html#gearinch
Mechanical Engineer to the rescueAlexx
Jun 14, 2002 4:33 AM
According to Saint Sheldon's gear-inch calculator (much quicker than actually doing the math), you have the following:

39/23:

45.8 gear inches

3.7 meters developement

39/25:

42.1 gear inches

3.4 meters developement

As was said before, this is about 8% loss in developement, increase in gear-inches, and about 8% gain in torque (T=F*d)