Calculating gear inches for the clueless

Robert said:

I have a Raleigh 3 speed w/ 700 wheels and have gotten the gearing the
way I want for the local terrain. Now , I am setting up a 20" wheeled
bike and would like it to be "geared??" the same. Do I just run a gear
inches program like the one on the Sheldon Brown site till I get the
same numbers or is there more to it than that???

That's the basic purpose of that program. If the two bikes have the
same length cranks, gear inches works fine.

If the crank lengths are different, selecting the (default) Gain Ratio
system will give a bit more precision.

http://sheldonbrown.com/gears

Sheldon "Numbers" Brown
+----------------------------------------------+
| Music expresses that which cannot be said |
| and on which it is impossible to be silent. |
| --Victor Hugo |
+----------------------------------------------+
Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
http://harriscyclery.com
Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com

Robert said:

I have a Raleigh 3 speed w/ 700 wheels and have gotten the gearing the
way I want for the local terrain. Now , I am setting up a 20" wheeled
bike and would like it to be "geared??" the same. Do I just run a gear
inches program like the one on the Sheldon Brown site till I get the
same numbers or is there more to it than that???

Gear inches = chainring teeth/sprocket teeth X 26.4

You'll often see slightly higher answers - some people persist in using
27 inches as the diameter for 700c, but it's a bit smaller.

Duh - should read question in full. For a 20" wheel substitute the
diameter of the inflated tyre instead of 26.4. If you like, you can
knock off 0.2" or so to allow for the flattened contact patch where the
tyre meets the ground.

No need for any fancy measurements of calculations. Since you want to keep
the same gear inches, just increase chainring size by the same ratio as you
decreased tire diameter..

Leo Lichtman said:

No need for any fancy measurements of calculations. Since you want to keep
the same gear inches, just increase chainring size by the same ratio as you
decreased tire diameter..

So where do I find this 70-tooth chainring?

Peter wrote in message ...

Quoted message said:

Leo Lichtman said:

No need for any fancy measurements of calculations. Since you want to

keep

Quoted message said:

Quoted message said:

the same gear inches, just increase chainring size by the same ratio as

you

Quoted message said:

Quoted message said:

decreased tire diameter..

So where do I find this 70-tooth chainring?

Highpath Engineering
Cornant, Cribyn, Lampeter, Wales SA48 7QW

or for those who will not write

www.highpath.net

TJ

Trevor Jeffrey said:

Peter wrote in message ...

Quoted message said:

Leo Lichtman said:

No need for any fancy measurements of calculations. Since you want to

keep

Quoted message said:

Quoted message said:

the same gear inches, just increase chainring size by the same ratio as

you

Quoted message said:

Quoted message said:

decreased tire diameter..

So where do I find this 70-tooth chainring?

Highpath Engineering
Cornant, Cribyn, Lampeter, Wales SA48 7QW

At $400 for the single chainring I think I'd rather do the gear-inch
calculations.

"Peter" wrote: At $400 for the single chainring I think I'd rather do the
gear-inch calculations.
^^^^^^^^^^^^
With all due respect, I fail to see how the gear-inch calculations will
change the size of the chainring you need. My suggested method of
calculation was suggested as a simple way of getting the answer--not a way
of changing the answer.

My Dahon, with 20" wheels has a 52 tooth chainring. How did you arrive at a
figure of 70 teeth? I just went back and checked the OP, and there is not
enough information there to do that calculation.

Since there are lots of bikes around with 20" wheels, there must be
chainrings available to drive them at well under $400.

Leo Lichtman said:

"Peter" wrote: At $400 for the single chainring I think I'd rather do the
gear-inch calculations.
^^^^^^^^^^^^
With all due respect, I fail to see how the gear-inch calculations will
change the size of the chainring you need.

Because instead of changing just the chainring size as you suggested one
can also change the cogs or use a hub with internal gearing. Using the
gear-inch calculations lets you set up the bike with similar gearing but
with more flexibility in how that's achieved.

Quoted message said:

My suggested method of
calculation was suggested as a simple way of getting the answer--not a way
of changing the answer.

My Dahon, with 20" wheels has a 52 tooth chainring. How did you arrive at a
figure of 70 teeth?

My regular road bikes have large chainrings of 52 teeth and 700c/27"
wheels. Taking the ratio as you suggested of 27/20 x 52 gives a
chainring size of 70 teeth, but such chainrings are hard to find and
expensive. The smallest cog on my regular bike is a 13 for a highest
gear of 108". To get about the same gear on my bike with 20" wheels I
use a 60 tooth chainring and an 11 tooth cog for a 109" gear.

Quoted message said:

I just went back and checked the OP, and there is not
enough information there to do that calculation.

Since there are lots of bikes around with 20" wheels, there must be
chainrings available to drive them at well under $400.

Many of them are not intended for aggressive riding where high gears are
needed. Those that do have a high gear are generally equipped with
either small cogs (some Moultons use a 9 tooth) or a hub with internal
gears such as the Sachs 3x8 on many Bike Fridays. I haven't seen any
small-wheeled bikes that get the same gear range as a regular road bike
*solely* through use of an extra large chainring.

Leo Lichtman said:

"Peter" wrote: At $400 for the single chainring I think I'd rather do the
gear-inch calculations.
^^^^^^^^^^^^
With all due respect, I fail to see how the gear-inch calculations will
change the size of the chainring you need. My suggested method of
calculation was suggested as a simple way of getting the answer--not a way
of changing the answer.

Gear calculations will reveal some _combination_ of chainring and rear
sprocket that will do the trick.

It is generally MUCH cheaper and easier to go to a smaller sprocket in
back than to use a bigger one in front, in this sort of situation. We
sell 3-speed sprockets for $11.95, in stock from 14-24 teeth.

Actually, when you know the gear you want, but wish to find different
sprocket combinations that will provide it, a good old fashioned slide
rule is faster than a computer!

See: http://sheldonbrown.com/sliderule.html for illustrated instructions.

Sheldon "Slipstick" Brown
+---------------------------------------------------------------+
| Anyone who can't use a slide rule is a cultural illiterate, |
| and should not be allowed to vote. --Robert A. Heinlein |
+---------------------------------------------------------------+
Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
http://harriscyclery.com
Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com

"Sheldon Brown" wrote: (clip) Actually, when you know the gear you want,
but wish to find different sprocket combinations that will provide it, a
good old fashioned slide rule is faster than a computer!
^^^^^^^^^^^^^
Dear Sheldon, is there a NG for sliderules? (This is an on-topic question,
since they are useful for calculating gearing.) <G>

Peter responded to my (Leo's) earlier post with valid explanations to every
point that I raised, and I concede.

Leo Lichtman said:

Peter responded to my (Leo's) earlier post with valid explanations to every
point that I raised, and I concede.

Dear Leo,

Coward! Quitter! You're just trying to make stubborn fools
like me look bad!

Admiringly,

Carl Fogel