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Old 06-18-07, 12:41 PM   #1
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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?
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Old 06-18-07, 01:15 PM   #2
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Gi*rpm/1063=mph
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Old 06-18-07, 01:27 PM   #3
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Quote:
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.
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Old 06-18-07, 01:28 PM   #4
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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.
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Old 06-18-07, 01:38 PM   #5
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Quote:
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.
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Old 06-18-07, 02:22 PM   #6
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yeah that needs a pi in there sorry.
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Old 06-18-07, 02:27 PM   #7
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Quote:
Originally Posted by dutret
yeah that needs a pi in there sorry.

huh?
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Old 06-18-07, 02:36 PM   #8
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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.
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Old 06-18-07, 03:04 PM   #9
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oh yeah...circumference...lol.
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Old 06-18-07, 03:41 PM   #10
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Quote:
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.
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Old 06-19-07, 12:28 PM   #11
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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?!?!
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Old 06-28-07, 06:50 PM   #12
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There used to be a wicked website that had a calculater for that sorta thing. Miles and Kilos.......
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Old 06-28-07, 06:57 PM   #13
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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.
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Old 06-29-07, 02:39 PM   #14
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http://www.fixedgearfever.com/module...download&cid=1
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Old 07-03-07, 07:38 PM   #15
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sheldon brown has a calculater that will figure this out for you.
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Old 07-08-07, 07:54 PM   #16
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The prob with some of the charts already made is they dont go high enough into the RPMs.......
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Old 07-10-07, 03:05 PM   #17
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Quote:
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.

http://software.bareknucklebrigade.c...it.applet.html
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Old 07-11-07, 02:02 AM   #18
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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
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