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 06-18-07, 12:41 PM #1 obstacle - Thread Starter   Join Date: May 2007 Location: Manhattan, NYC Bikes: Posts: 400 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Gear inches/gain ration + rpm = mph/kph I've seen this before but for the life of me can't remember where. I've googled and searched here for it but still have not found it. Can someone please point me in the direction of the said chart? I'm interested in knowing that if I gear my bike at X and am pedalling at X rpm I should be able to go X mph (given the ground is flat and my pace is sustained). I've seen a chart laying this all out. Or is there an equation I could use to figure this generally?
 06-18-07, 01:15 PM #2 dutret Banned   Join Date: May 2005 Location: GA Bikes: Posts: 5,317 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Gi*rpm/1063=mph
06-18-07, 01:27 PM   #3
obstacle
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 Originally Posted by dutret Gi*rpm/1063=mph
What am I doing wrong?

example:
75.5*100/1063 = 6.4

I know when I spin at 100rpm with that gear inch I am going well beyond 6.4mph.

06-18-07, 01:28 PM   #4
piwonka
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Quote:
 Originally Posted by obstacle What am I doing wrong? example: 75.5*100/1063 = 6.4 I know when I spin at 100rpm with that gear inch I am going well beyond 6.4mph.
the only way i could get his formula to make sense was to multiply the GI number by 2.54 to convert to metric...i don't know though, i'm not looking at any formula thangs...just guessing.

06-18-07, 01:38 PM   #5
obstacle
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 Originally Posted by piwonka the only way i could get his formula to make sense was to multiply the GI number by 2.54 to convert to metric...i don't know though, i'm not looking at any formula thangs...just guessing.
That gives me 18.4 as a result, which is interestingly about 6.4 from where my computer says I should be. What's wrong here

Last edited by obstacle; 06-18-07 at 02:27 PM.

 06-18-07, 02:22 PM #6 dutret Banned   Join Date: May 2005 Location: GA Bikes: Posts: 5,317 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) yeah that needs a pi in there sorry.
06-18-07, 02:27 PM   #7
obstacle
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 Originally Posted by dutret yeah that needs a pi in there sorry.

huh?

06-18-07, 02:36 PM   #8
mickster
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Quote:
 Originally Posted by obstacle huh?
He means the formula is Gi*pi*rpm/1063=mph

The gear inch figure (eg 75.5) is the nominal *diameter* of the wheel - need to mulitply by Pi to find the inches travelled in one pedal revolution.

 06-18-07, 03:04 PM #9 piwonka park ranger   Join Date: Aug 2006 Location: mars Bikes: recumbents Posts: 1,794 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) oh yeah...circumference...lol.
06-18-07, 03:41 PM   #10
obstacle
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 Originally Posted by mickster He means the formula is Gi*pi*rpm/1063=mph The gear inch figure (eg 75.5) is the nominal *diameter* of the wheel - need to mulitply by Pi to find the inches travelled in one pedal revolution.

Got it, that does the trick...thanks.

 06-19-07, 12:28 PM #11 Trail_Animal Junior Member   Join Date: May 2007 Bikes: Posts: 6 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Isn't it 1056 and not 1063??? 1/(60min/hr)/(12in/ft)*(5280ft/mile) = 1056 (inch*hr/min*mile) Divide Gi*pi*rpm by this to get mile/hr?!?!
 06-28-07, 06:50 PM #12 CafeRacer Senior Member     Join Date: Sep 2005 Bikes: Posts: 411 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) There used to be a wicked website that had a calculater for that sorta thing. Miles and Kilos.......
 06-28-07, 06:57 PM #13 barba Senior Member     Join Date: Nov 2005 Bikes: Posts: 4,083 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) I made a little Excel chart that calculates rpm-->speed (and the other way around) for any given gear combo. PM me if you want a copy.
 06-29-07, 02:39 PM #14 marcelinyc R900Campagnolo     Join Date: Jun 2005 Location: Brooklyn Bikes: track and road Posts: 884 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s)
 07-03-07, 07:38 PM #15 Gomez308 Senior Member     Join Date: Oct 2004 Location: GA Bikes: Posts: 365 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) sheldon brown has a calculater that will figure this out for you.
 07-08-07, 07:54 PM #16 CafeRacer Senior Member     Join Date: Sep 2005 Bikes: Posts: 411 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) The prob with some of the charts already made is they dont go high enough into the RPMs.......
07-10-07, 03:05 PM   #17
dookie
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 Originally Posted by CafeRacer There used to be a wicked website that had a calculater for that sorta thing. Miles and Kilos.......
maybe this? good to 160rpm.

 07-11-07, 02:02 AM #18 kitten_sandwich ...   Join Date: Apr 2006 Location: MacArthur Park Bikes: http://velospace.org/node/2553 Posts: 57 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) sheldons works well http://www.sheldonbrown.com/gears/ also, if you ever forget, just type in "bicycle gear calculator" into google and its the first one that comes up