Bike Forums > Gearing
 Register All Albums Elite Membership Forum Rules Calendar Search Today's Posts Mark Forums Read

 Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

 09-26-12, 09:08 PM #1 5kdad Senior Member Thread Starter     Join Date: Dec 2006 Location: Northwest Arkansas Bikes: Felt Z100 road bike and Schwinn Frontier Posts: 374 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 11 Post(s) Gearing Living in the hilly Ozarks, I'm thinking of getting my road bike geared down a bit. Someone recently told me that adding one tooth to a rear sprocket is equal to subtracting 3 teeth from a front sprocket. First time I've heard that. Is he correct?
 09-26-12, 09:25 PM #2 FBinNY  Senior Member   Join Date: Apr 2009 Location: New Rochelle, NY Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter Posts: 33,282 Mentioned: 69 Post(s) Tagged: 1 Thread(s) Quoted: 2363 Post(s) Neither right nor wrong, but more wrong than right. It depends on the size of the sprockets. For comparison purposes it's simple division. The front chainring divided by the rear sprocket is the gear ratio, so if you had a 28T granny and a 28t rear sprockets, adding or subtracting 1 tooth form either is very similar. At the opposite end if you had 52/13 or 4:1, 48/12 is the same 4:1 ratio. So as you see, you can't blindly apply some simple rule of thumb. Get a hand calculator and compare ratios. If you want a quick rule of thumb for the effect a 1 tooth change divide the change by the number of teeth there originally, ie. adding 1 tooth to a 25t sprocket is 1/25 or a 4% change, while adding 1 to to a 50t chainring is a 2% change. BTW- if you haven't noticed yet, shrinking a chainring is the same effect as enlarging a rear sprocket, so keep the direction of change in mind. __________________ FB Chain-L site An ounce of diagnosis is worth a pound of cure. “Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin “One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN WARNING, I'm from New York. Thin skinned people should maintain safe distance.
 09-27-12, 03:55 AM #3 Homebrew01 Super Moderator     Join Date: Jul 2004 Location: Ffld Cnty Connecticut Bikes: Old Steelies I made, Old Cannondales Posts: 20,349 Mentioned: 6 Post(s) Tagged: 0 Thread(s) Quoted: 468 Post(s) As suggested above, there are various ways to change your gears. You can go bigger in the back, which may require a different rear derailleur. You can go smaller in the front by getting a compact double crankset, or a triple crankset. Or all the options if you want really low gears ! The best choice is based on what you have now, and how much lower you want to go..... just a notch, or a several notches ? Theoretically you could get smaller chainrings, but double cranksets usually come with the smallest chainring available for that model already installed. __________________ Bikes: Old steel race bikes, old Cannondale race bikes, less old Cannondale race bike, crappy old mtn bike. FYI: http://www.bikeforums.net/forum-sugg...ad-please.html Last edited by Homebrew01; 09-27-12 at 04:01 AM.
 09-27-12, 06:06 AM #4 dsbrantjr Senior Member     Join Date: Sep 2010 Location: Roswell, GA Bikes: '93 Trek 750, '92 Schwinn Crisscross, '93 Mongoose Alta Posts: 5,227 Mentioned: 3 Post(s) Tagged: 0 Thread(s) Quoted: 291 Post(s) Do a Google search on "bicycle gear calculator" Find one you like and play with it to get a feel for what effect various choices of sprockets and chainrings have. This one http://www.kstoerz.com/gearcalc/compare/ lets you compare two drivetrains so that you can, for example, compare your present setup with a proposed different one and see, graphically, the differences.
 09-27-12, 06:55 AM #5 davidad Senior Member   Join Date: Feb 2009 Bikes: Posts: 5,138 Mentioned: 8 Post(s) Tagged: 0 Thread(s) Quoted: 98 Post(s)
 09-27-12, 07:30 AM #6 rdtompki Senior Member     Join Date: Nov 2008 Location: Hollister, CA Bikes: Volagi, daVinci Joint Venture Posts: 3,962 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) As rules of thumb go that one is particularly inaccurate. Hope that didn't come from your LBS! Even if you had the largest small chainring, a 39, 3 teeth would still be 8% give or take and smaller chain rings would produce an even larger change in gearing. Let's say your largest cog in the rear was 25. A 1 tooth change is only 4% with larger cogs resulting in an even smaller gearing change. Conclusion: the "rule" is off by 2:1 even for the most favorable and least likely configuration.
09-27-12, 07:52 AM   #7
OutgunRacing
Owner

Join Date: Jul 2011
Location: Alpharetta, GA
Posts: 18
Mentioned: 0 Post(s)
Quoted: 0 Post(s)
Quote:
 Originally Posted by dsbrantjr Do a Google search on "bicycle gear calculator" Find one you like and play with it to get a feel for what effect various choices of sprockets and chainrings have. This one http://www.kstoerz.com/gearcalc/compare/ lets you compare two drivetrains so that you can, for example, compare your present setup with a proposed different one and see, graphically, the differences.
Neat application. I tried inputting 55/11 and it goes off the chart though.

 09-27-12, 07:58 AM #8 OutgunRacing Owner     Join Date: Jul 2011 Location: Alpharetta, GA Bikes: Road Posts: 18 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) https://docs.google.com/spreadsheet/...Ew0eHhERGJ1cGc I made this spreadsheet long ago. Maybe it will help.
 09-27-12, 08:01 AM #9 OutgunRacing Owner     Join Date: Jul 2011 Location: Alpharetta, GA Bikes: Road Posts: 18 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) I know 10t does not exist. That was put on there just for fun. As you can see, the smaller a cog is, the greater difference it makes to change it by only one tooth.
 09-27-12, 10:57 AM #10 Lazarus Short Senior Member     Join Date: Jan 2010 Location: Independence, MO Bikes: Diamondback Apex/GT fork (modded) Posts: 227 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 2 Post(s) I have lived in the hilly Ozarks, and this is the gearing still on my Peugeot from those days: Front - TA triple - 30-46-50 Rear - Suntour Perfect - 14-17-24-28-38 (but 14-17-22-28-38 fills in a gap the 24T does not, but I have not made the swap yet) Changers - Huret Duopar Eco, actuated with Suntour index bar-end shifters (it works!) Yeah! Click shifting with a Duopar!! That kind of gearing will serve you well on almost any Ozark hill. In my case, I have moved to the flatter desert country of southeast New Mexico, and am considering a much smaller rear cog.