how do i calculate my mid-drive rpm at 43 kph?

[emaayan]

considering that i have a
29er wheel of 2250 mm diameter
alfine 11 (using first gear)
a 20T sprocket on the alfine
a 34T front chainring
an 84T front roller chain ring
a plantery gear with 4:1 ratio on outside ring and 4.5:1 ratio on inside ring.

 

[friendly1uk]

erm.. kph divide by 60 for kpm
kpm divided by wheel circumference for wheel rpm
teeth on rear sprocket divided by teeth on front to give front sprocket rpm
front sprocket rpm times internal reduction ratio

Then look at answer and agree or laugh

edit: It seems you have another layer of gearing. Laugh away!

 

[MitchJi]

Hi,

erm.. kph divide by 60 for kpm
kpm divided by wheel circumference for wheel rpm
teeth on rear sprocket divided by teeth on front to give front sprocket rpm
front sprocket rpm times internal reduction ratio

Too much work!

Use this calculator, be sure to change the "Gear units" pull down to "kph @ 100 rpm". You can use a number other than 100 for rpm, but if you use 100 it is easy to figure out different values:

Sheldon Brown's Gear Calculator

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

EDIT: Use this page for the alfine 11:
http://www.sheldonbrown.com/gears/internal.html

29er wheel of 2250 mm diameter
alfine 11 (using first gear)
a 20T sprocket on the alfine
a 34T front chainring

Gets this:




So spinning the 34t chainring at 100 rpm will get you 42.5 kph in 10th gear. In 11th gear its about 86 rpm for 42.5 kph.

Using the calculator (or the figures above) you should be able to figure this part out yourself:

an 84T front roller chain ring
a plantery gear with 4:1 ratio on outside ring and 4.5:1 ratio on inside ring

 

[emaayan]

thanks!so my motor spins at at 1400 rpm at 42 kph at the lowest gear ..(i have a 29er)