is there a formula for finding rpms?

a.graham52

a.graham52

New Member
if you know the tire size, gearing etc? i dont really want to buy a tach, just want to know my target rpms when im going x mph.
 
a.graham52 said:
if you know the tire size, gearing etc? i dont really want to buy a tach, just want to know my target rpms when im going x mph.
Click to expand...
Best gear ratio calculator!

The program can be downloaded from http://jimsitton.net/ratiocalc/

Just right click on the file gearratio.exe

and select "save link as" and put it somewhere you can find it again.

Once it's saved, double click on the gearratio.exe file and it will run.

The program allows for up to three ratios in series.

If you have fewer just leave the unused ratios set to 1:1.

The program will also calculate speed based on
engine RPM,
drive ratio,and
wheel diameter.

Be sure to measure the outside diameter of your wheel for accurate results.
 
That really is a great program and amazingly accurate. I have emailed him twice over the years to say thank you. It is a valuable tool to the DIYer.
 
well i put 14T in for drive gear, 44 in for driven gear, 27" diameter, put 2000 rpms in and it gave me 51mph... not right
 
Chinagirl engines have a 4.1:1 internal reduction; a 20T crankshaft gear leading to an 82T clutch gear.

So 20-> 82-> 10-> 44 = 18.04:1
29" wheels, you end up with roughly 4.8MPH per 1000RPM

28.7MPH @ 6000RPM
 
heck, never thought of that! if someone had just said "remove the clutch cover and count those gears too" i would be like oh duh. but no... t hey had to be an ass about it.. laff
 
You are quite right! Iv got motorcycles on the brain this week! I kept thinking only about the two drive chain sprockets. TOTALY spaced on the fact that tere were gears inside the clutch cover. Every time "clutch cover" was mentioned, the side cover over the drive chain sprocket came to mind. I'll be glad when this weeks over!
 
knowing i use my bike for commuting. i try to ride an average of 25 mph for about 10 miles a trip, 20 miles a day. what owuld be a good, safe target rpm range?
 
I recommend 20-25, that'll run your 'girl roughly about 4000-5000 with 44T and 29" wheels. Chinagirls make good torque in that range, even stock.

6000+ (29+ MPH for you) is pushing it for the average unmodified chinagirl IMHO. Most of them start vibrating significantly at higher revs, too.
I used chinagirls to commute 12 miles each way to work years ago (34T and 26" wheels, averaged 30MPH) and I had best results not revving em over 5500.
 
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