Pedal cadence and motor speed

[jmygann]

Pedal Cadence needed to go 25 MPH with 24" tire and single 16T sprocket in rear ... crank sprocket 64T (just for figuring) .....SRAM DD in Second ..direct drive

or 50 RPM at the crank will give what speed ?

75.384" of travel /1 revolution or rear 24" tire .. 1 crank revolution = 4 times =301.5 " x 50 = 15076" traveled in one minute ... x 60 minutes = 904608"traveled per hour

904608/ 63300 inches in a mile. (5280*12) = 14.3 mph

Is the math close ?

How fast in 3 gear of SRAM DD ? with 50 rpm cadence

Need to change crank sprocket to ?? to reach 25 mph and 50 rpm pedal cadence or change cadence to ??


http://answers.yahoo.com/question/index?qid=20090122212609AAiAdsV

 

[TylerDurden]

3rd gear is 1.36 = 19.44mph (cadence 50)

1.20 * 19.44 = 23.33mph (cadence 60)

23.33 * 1.07 = 24.96mph

64 * 1.07 = ~69t

So, a 69t ring @ 60rpm cadence gets you ~25mph in 3rd gear.

 

[mwkeefer]

Darn that tyler - beat me to the post again = )_

...

jm,

You may find these formulas a bit simpler to work with...

Since your solving for pedal cadence (crank RPM) required for 25mph on a 24" wheel and you know the gear ratio (64t / 16t = 1:4) so lets calculate the RPM needed at the rear wheel for 25mph:

25mph * 336 / Wheel size (24") = 350 rpm requried at rear for 25 mph

Since you have a 1:4 step up in gearing from the crank to the rear, you will divide the rear wheel RPM by your step up:

350 rpm / 4 = 87.5 rpm @ cranks or pedal cadence...

As for the rest of your question... I don't know the ratios on the sram ddrive but with the above math you can calculate them yourself quite easily... just factor the ratio into the 1:4 you already have in theory.

*edit - tyler posts ratio for 3rd gear being 1.36:1 step up so... multiply the 4:1 by 1.36 = 1:5.44 which puts 25mph @ 350 axle rpm at a pedal cadence of (350 / 5.44) = 64.34 RPM at the cranks - so we get different results but within 5 crank rpm of each other, guess we got the math right (more surprised I got the math right) -- shows there's always about 9 ways to skin a cat - tylers math is more elegant (probably because he understands it way better than I).

Hope this helps!

-Mike

 

[TylerDurden]

Results are roughly the same:

64.34rpm @ 64t = 4117tpm

60rpm @ 69t = 4140tpm

about .5% difference

 

[jmygann]

Thanks to all

The issue is ... is the 3 speed internal hub (SRAM DD) with a single 16T sprocket enough gears along with the 1000 watt assist to be a city traffic commuter ?

69T allows me to pedal at city traffic speeds , but will I have a good starting gear ?

On my current bike I just stop pedaling after about 13-14 mph .

 

[JohnH]

Here's perhaps the definitive gear calculator - easy to use, already knows about DualDrive and mony common (and uncommon) cassettes, etc.

http://www.sheldonbrown.com/gears/

Hope this helps,

John

 

[TylerDurden]

The issue is ... is the 3 speed internal hub (SRAM DD) with a single 16T sprocket enough gears along with the 1000 watt assist to be a city traffic commuter ?

69T allows me to pedal at city traffic speeds , but will I have a good starting gear ?

Based on earlier findings...

14.3 * 1.07 = 15.3mph (2nd gear, 50rpm crank, 69t)

1st gear is .73, so 11.17mph @ 50rpm

Starting from a dead stop will have less rpm, so you might be going 3-5 mph to start... your human 200W burst should be able to handle that, even with no motor.

1000W should carry you @ 25mph ok; pedalling will help.

The last question is more about torque; but that's a whole other kettle of fish.

 

[jmygann]

Maybe a contest of ....

0-25 mph elapsed time with and without pedaling

Any figures close to that anyone has done ??

 

[DISH]

Check out GearCalc.

There is a FREE version that can be downloaded from the following website:

http://www.machinehead-software.co.uk/gearpro.html

(I tried posting a direct link to the download but apparently the author has this blocked )

System Requirements

A 386 or better, running Windows 3.1, Windows 3.11, Windows 95, Windows 98, Windows ME, XP

Minimum screen requirement 640 by 480 pixels 16 colours.

I like this because I can run it on my computer when I do not have an internet connection, and most importantly, It gets the job done.