Hi,
You have the first part of the equation right by dividing the size of the sprocket (chainring) by
the rear cog. The second part is multiplying that result by the diameter, not the circunference of
the tire. Also, the 406 20 inch tire is not nominally 20 inches exact. It is more like 18.7-18.8
inches. So using the result from the first part of the mathmatical operation which is 1.85 and
multiplying by 18.7 (the true diameter) will give you 34.59 gear inches. Very close to the 34.5 from
the online calculator. Remember there is another variable and that's crank length. This is just a
"rough" way to quickly find out an approximate gear ratio given specific values such as chainring,
cog and tire size.
Remember that I did observe how much I rolled ahead with each crank revolution last night on my
Revive and it seemed like 3 feet or so...less than the length of the bike. A ratio of 2 meters
developed or so is quite high for a first gear though some of those single speed beach cruisers are
close. Maybe the DX is geared that way, I don't know. I can tell you how my bike is geared;-) I do
believe that there is an explanation of gain ratios, gear inches, etc. on Sheldon's site.
I didn't know that the chainring is riveted to the crank assembly. Tomorrow morning I am going to
take a close look at mine.
Happy trails, Edward Wong Orlando, FL
"Offsite" <[email hidden]> wrote in message "]news:[email hidden]...
Quoted message said:Hi Ed,
Would you be willing to help me understand why simple rules of mechanics
don't
Quoted message said:seem to apply to your model of the wonderful, innovative (in my opinion,
anyway)
Quoted message said:Revive bike... <grin>
If your front (chain ring) sprocket has 48 teeth, and your rear (cassette) sprocket has 26 teeth,
and you rotate the chain ring one full revolution,
the
Quoted message said:rear sprocket will rotate almost two times (48/26 = 1.85).
And since the rear wheel is directly attached to the rear sprocket it must
also
Quoted message said:rotate almost two times for each chain ring (crank) revolution.
So if your tire is nominally 20" in diameter, the bike should roll almost
63"
Quoted message said:for every single revolution of the tire... and for almost two revolutions
of the
Quoted message said:tire, the bike should roll almost 10 ft! ( 9.67 ft. to be exact)... just
like
Quoted message said:the DX we measured in Dave's shop yesterday.
So how is it that your bike only rolls 34" for every rotation of the chain
ring
Quoted message said:(crank)... did you *actually* try this experiment? <grin>
[14 friends and neighbors are on line and eagerly awaiting the answer to
this
Quoted message said:mystery]... 🙂)
PS: when Dave (shop owner here) called Giant tech services to ask about
the
Quoted message said:smaller chain ring, the person he reached said there were none available
since
Quoted message said:the chain ring was riveted to the crank assembly??... and since we
*really* want
Quoted message said:to buy and enjoy these two DXs, we are hoping somebody here at a/r/b/r has
a
Quoted message said:better connection to Giant engineering and can assure us a fix is in the
works
Quoted message said:for this otherwise splendid machine.