# formula for gear rations, speeds

Discussion in 'Cycling Equipment' started by Raymo853, Mar 1, 2004.

1. ### Raymo853 Guest

I got the formula to calculate gear inches (crank teeth count /cog teeth count) * wheel diameter in
inches. But what would it be for say MPH at 100 rmp? For some reason I" am having a bad brain moment
and can't get the conversions correctly.

Tags:

2. ### Cheg Guest

"Raymo853" <[email protected]> wrote in message
news:[email protected]...
> I got the formula to calculate gear inches (crank teeth count /cog
teeth
> count) * wheel diameter in inches. But what would it be for say
MPH at 100
> rmp? For some reason I" am having a bad brain moment and can't get
the
> conversions correctly.
>
>

(Crank Teeth) / (Cog Teeth) * (Wheel diameter in inches) / (12 in per ft) * pi * RPM * (60 min per
hr) / (5280 ft per mi)

3. ### Sheldon Brown Guest

Raymo853 wrote:

> I got the formula to calculate gear inches (crank teeth count /cog teeth count) * wheel diameter
> in inches. But what would it be for say MPH at 100 rmp? For some reason I" am having a bad brain
> moment and can't get the conversions correctly.

MPH @ 100 RPM is one of the modes offered in my online gear calculator:

http://sheldonbrown.com/gears

But the default units are Gain Ratios, which I think are a better system.

http://sheldonbrown.com/gain

Sheldon "Gain Ratio" Brown +--------------------------------------+
| Without deviation from the norm, | progress is not possible | -- Frank Zappa |
+--------------------------------------+ 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

4. ### Raymo853 Guest

Gear ratios calculations are better for some situtations, however, I am concerned about MPH at a
certain RMP while considering cassette choices for an upcoming race.

"Sheldon Brown" <[email protected]> wrote in message
news:[email protected]...
> Raymo853 wrote:
>
> > I got the formula to calculate gear inches (crank teeth count /cog teeth count) * wheel diameter
> > in inches. But what would it be for say MPH at
100
> > rmp? For some reason I" am having a bad brain moment and can't get the conversions correctly.
>
> MPH @ 100 RPM is one of the modes offered in my online gear calculator:
>
> http://sheldonbrown.com/gears
>
> But the default units are Gain Ratios, which I think are a better system.
>
> http://sheldonbrown.com/gain
>
> Sheldon "Gain Ratio" Brown +--------------------------------------+
> | Without deviation from the norm, | progress is not possible | -- Frank Zappa |
> +--------------------------------------+ 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

5. ### Ronald Guest

> Gear ratios calculations are better for some situtations, however, I am concerned about MPH at a
> certain RMP while considering cassette choices
for
> an upcoming race.

You can try EasyGear (http://www.ultimade.nl/bikepages/index.php4?page=easygear) if you're willing
to use metric values.

"Raymo853" <[email protected]> wrote in message news:[email protected]...
> Gear ratios calculations are better for some situtations, however, I am concerned about MPH at a
> certain RMP while considering cassette choices
for
> an upcoming race.
>
>
> "Sheldon Brown" <[email protected]> wrote in message
> news:[email protected]...
> > Raymo853 wrote:
> >
> > > I got the formula to calculate gear inches (crank teeth count /cog
teeth
> > > count) * wheel diameter in inches. But what would it be for say MPH
at
> 100
> > > rmp? For some reason I" am having a bad brain moment and can't get
the
> > > conversions correctly.
> >
> > MPH @ 100 RPM is one of the modes offered in my online gear calculator:
> >
> > http://sheldonbrown.com/gears
> >
> > But the default units are Gain Ratios, which I think are a better
system.
> >
> > http://sheldonbrown.com/gain
> >
> > Sheldon "Gain Ratio" Brown +--------------------------------------+
> > | Without deviation from the norm, | progress is not possible | -- Frank Zappa |
> > +--------------------------------------+ 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
>

6. ### William Holiday Guest

On Tue, 2 Mar 2004 07:38:32 -0500, "Raymo853" <[email protected]> wrote:

<Gear ratios calculations are better for some situtations, however, I am <concerned about MPH at a
certain RMP while considering cassette choices for <an upcoming race.

Why not check out the course and let your experience be your guide?

7. ### Carl Fogel Guest

"Raymo853" <[email protected]> wrote in message news:<[email protected]>...
> I got the formula to calculate gear inches (crank teeth count /cog teeth count) * wheel diameter
> in inches. But what would it be for say MPH at 100 rmp? For some reason I" am having a bad brain
> moment and can't get the conversions correctly.

Dear Raymond,

Here's an ancient spreadsheet that should import into almost any modern spreadsheet:

It lets you set the number of front teeth and the rear wheel diameter and shows RPM versus MPH for a
wide range of rear cogs.

Here's where you can see roughly the same question, with a few pages of output that might be all
that you want:

Good luck,

Carl Fogel

8. ### R15757 Guest

Carl fogeled:

<< Dear Raymond,

Here's an ancient spreadsheet that should import into almost any modern spreadsheet:

It lets you set the number of front teeth and the rear wheel diameter and shows RPM versus MPH for a
wide range of rear cogs.

Here's where you can see roughly the same question, with a few pages of output that might be all
that you want:

Good luck,

Carl Fogel >>

Carl, I'm surprised at you. No mention of crank length? I thought we were all getting behind Sheldon
in the Real Gearing Revolution.

Robert

9. ### S. Anderson Guest

"Carl Fogel" <[email protected]> wrote in message >
> It lets you set the number of front teeth

I'd give anything for my two front teeth...that's all I want for Christmas... ;-)

Cheers,

Scott..

10. ### S o r n i Guest

S. Anderson wrote:
> "Carl Fogel" <[email protected]> wrote in message >
>> It lets you set the number of front teeth
>
> I'd give anything for my two front teeth...that's all I want for Christmas... ;-)

Dating yourself with that one!

Bill "not dinner and a movie, either" S.

11. ### Jeff Wills Guest

"Raymo853" <[email protected]> wrote in message news:<[email protected]>...
> Gear ratios calculations are better for some situtations, however, I am concerned about MPH at a
> certain RMP while considering cassette choices for an upcoming race.
>

Gear inches X Cadence X .003 = speed in MPH

For instance, a 112 inch gear at 90 RPM yields 30.24 MPH .

The constant is actually .0029714... but 3 digits after the decimal point should be good enough.

Jeff "memorized pi to 20 digits when I was in grade school" Wills

12. ### carlfogel New Member

Joined:
Nov 24, 2003
Messages:
241
0
Dear Robert,

You are old, Father William, the young man said . . .

Actually, all that's needed to calculate MPH is crank
RPM, gear ratio, and rear tire size.

That is, for a given tachometer reading, the length of
the crank doesn't affect the speedometer reading.

The original poster wants RPM and MPH in order to
fiddle with his gears, not his crank, so RPM and MPH
are what he gets.

I'm graciously assuming that either the original
poster uses the only correct crank length (175mm,
like mine) or else is willing to risk certain death.

Live and let live,

Carl Fogel

13. ### Bbense+Rec Bicy Guest

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_ I was messing around with gnuplot and came up with this style of plot for doing gear comparisions.

http://www.stanford.edu/~bbense/gear.png

_ It's a plot of MPH vs gain ratio with the error bars representing a range of 60 - 100 rpm. I find
it a very handy way to compare things, but I know from past experience that what makes sense to me
often doesn't for others. So if there was a tool to generate these plots would you use it?

_ Booker C. Bense

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