Quote:
So if I am reading the correctly I would need to increase the tooth count of the front sprocket or reduce the tooth count of the rear sprocket if my goal was to reduce engine RPM at highway speeds?
Yes, assuming that the gear number is unchanged, those tooth count changes will reduce engine RPM (at any given speed).
Quote:
I don't think I have read of anyone doing that, can someone tell me how rear sprocket tooth counts relate to engine RPM. For instances if I dropped two teeth on the rear sprocket what kind of RPM reduction could I expect?
From measurement and inspection of the geometry and ratios from the service manual:
(Using left-associative multiply and divide)
Vmph = (Erpm rotations/minute) / Npr / Ntr[gear] * (Nf/Nr) * (Rrear inches) * 2*Pi * (60 minutes/hour) / (5280*12 inches/mile)
where
Vmph := bike speed in miles per hour
Erpm := engine speed in rotations per minute
Npr := primary reduction ratio (71/34 for V-Stroms)
Ntr[1..6] = { 32/13, 32/18, 29/21, 27/24, 25/26, 23/27 } // a tuple of 6 transmission ratio numbers, indexed by ...
gear := an integer between 1 and 6 inclusive representing the selected gear
Nf := tooth count of front sprocket (15 stock)
Nr := tooth count of rear sprocket (47 stock)
Rrear := effective radius of rear tire (distance from rear contact patch to loaded rear axle center) in inches
Combining the constants, including Npr, (and ditching units for checking):
Vmph = Erpm / Ntr[gear] * (Nf/Nr) * Rrear / 351
So, for example, on my bike (as now sprocketed) in 6th gear at 5k RPM, with Rrear = 12.2:
Vmph = 5000 / (23/27) * (16/47) * 12.2 / 351 = 69.5 miles/hour (actual)
Or in 4th gear at 2500 rpm, when it had the stock front sprocket:
Vmph = 2500 / (27/24) * (15/47) * 12.2 / 351 = 24.7 miles/hour (actual)
That lower speed I know well because my GPS would indicate 25 mph at that rpm and gear.
(I often went through a 25 MPH, speed-trapped neighborhood, so learned to keep RPM at 100 desired speed in MPH.)
Another example I remember well from the break-in period:
Vmph = 5000 / (23/27) * (15/47) * 12.2 / 351 = 65.1 miles/hour (actual) or 62.7/0.93 = 70.0 (indicated)
The 0.93 divisor to get from actual to indicated speed reflects the speedometer reading 7 percent low.