General Cycling Discussion - Calculate ~cadence from speed?

I was wondering if there was an equation or calculator to approximate cadence based on speed and gearing? It would not be exactly accurate because of wind and other factors, but still useful. I dont want to spend money on a cadence sensor and trying to actually count my cadence is a pain.

Thanks,
Brian

Cadence = (Tr/Tc) x (SPD x 1056) / C

Where:

Tr = # chainring teeth
Tc = # cog teeth
C = tyre circumference (inches)
SPD = Speed (MPH)

This is actually how the Shimano FlightDeck computer derives "virtual cadence" without a dedicated cadence crank sensor.

BTW, this does produce an accurate reading as long as the pedals are "loaded" and wind doesn't really have anything to do with it since the effects of wind are already reflective in the speed.

I'm getting that I'll have to have a cadence of over 2000 rpm

*checks his math*

Shouldn't that be

Cadence = SPD / ((Tr / Tc) x (C / 1056))


? I know I got that formula to work but all the distributive properties and stuff are a little early since I havn't had a cup of coffee yet.

Here's what I ended up with

Cadence = SPD x Tr x 1056 / ( Tc * C )

Originally posted by Ritalin
I'm getting that I'll have to have a cadence of over 2000 rpm

*checks his math*

Shouldn't that be

Cadence = SPD / ((Tr / Tc) x (C / 1056))


? I know I got that formula to work but all the distributive properties and stuff are a little early since I havn't had a cup of coffee yet.

Here's what I ended up with

Cadence = SPD x Tr x 1056 / ( Tc * C )
How did your C and SPD get swapped? I'm confused by how u manipulated the equation.

Originally posted by khuon
Cadence = (Tr/Tc) x (SPD x 1056) / C

Where:

Tr = # chainring teeth
Tc = # cog teeth
C = tyre circumference (inches)
SPD = Speed (MPH)

This is actually how the Shimano FlightDeck computer derives "virtual cadence" without a dedicated cadence crank sensor.

BTW, this does produce an accurate reading as long as the pedals are "loaded" and wind doesn't really have anything to do with it since the effects of wind are already reflective in the speed.

I just want to clarify something, for the "C" value, is it the denominator for the WHOLE thing? OR just the (SPD x 1056) I'm guessing it's for (SPD x 1056) / C NOT Cadence = [(Tr/Tc) x (SPD x 1056)] / C

Gonna go calculate it now woo hoo..

If you have a cyclocomputer that displays seconds, it's fairly easy to count your cadence during, say, 15 or 30 seconds and multiply by 4 or 2.

I figured out once that a 100 chain inch gear at 100 RPM was 30 mph. I hope my recollection is correct.

Chain inches can be figured by teeth in chain ring/teeth in rear cog times 27.

So if you have a 70 inch gear, your speed at 100 rpm will be 30 times .7 (70/100) = 21 mph. If your cadence is 90, your speed will be .9 (90/100) times 21 = 18.9 mph.

kuhon's formula is good. Think of it like this:

http://digitallattice.bc.ca/eqn.jpg

but I think the following correction is needed:

Tr = teeth (rear)
Tc = teeth (chairing)

http://www.neci.nec.com/homepages/sandiway/bike/speed.html

or 6 strokes per 5 seconds = 72 rpm
7 per 5 = 84
8 per 5 = 96
9 per 5 = 108
10 per 5 = 120
11 per 5 = 132

Originally posted by F1_Fan
kuhon's formula is good. Think of it like this:

http://digitallattice.bc.ca/eqn.jpg

but I think the following correction is needed:

Tr = teeth (rear)
Tc = teeth (chairing)

I just tried this and it says my normal cadence is around 95rpm. Seems about right to me. I should probably get that up a bit.

If you want I will email you a very nice calculator that answers all these questions.

Terry

Originally posted by F1_Fan
kuhon's formula is good. Think of it like this:

http://digitallattice.bc.ca/eqn.jpg

but I think the following correction is needed:

Tr = teeth (rear)
Tc = teeth (chairing)

Yes... sorry... I confused myself by using too ambiguous variable names.

Originally posted by 1oldRoadie
If you want I will email you a very nice calculator that answers all these questions.

Terry

I sent you a PM with my email address. I would like that calculator please.

BG4533

Did the calculator give you what you needed?

Originally posted by 1oldRoadie
BG4533

Did the calculator give you what you needed?

Hmm...I never recieved it. Did you email it to me? I will send you my address one more time in case I got it wrong the first time.

Thanks,
Brian

Originally posted by AquariaGuy
How did your C and SPD get swapped? I'm confused by how u manipulated the equation.

if you are dividing by a fraction, it's the same as multiplying by the inverse


so 10 / ( 1/10)

is the same as 10 * 10/1

Thanks to 1oldRoadie for the Road Bike Calculator that he sent me. Based on cadence, crank, rear gear and wheel size it will calculate speed, gear inches and what to set your computer at. I have uploaded the calculator to my website so everyone can download it if they want. The program should work on all PCs running the Windows operating system. You can get it at http://www.osupages.com/bikeforums .

Thanks,
Brian

Please note that is is an old VB3 program so it has to have it own VBRUN300.DLL (it's in the zip) in the same directory as RBC.exe

sorry for the inconvenience

Terry