Quote:
Originally Posted by x-bird
Not sure if it's add or multiply to get to the final ratio. Always thought it was multiply
either way, he was running 21s or 22s on the back instead of the stock 20s (which was the point of his "test") and if you look close, they're aired up pretty hard, likely putting them an inch oversize. This in turn puts his final sprocket gearing at 16-29 or 16-28. That change along with the uni air filter, and custom exhaust and carb rejet could put it into the upper 30s.
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. When more than one pair of gears are used together, that is called a Compound Gear Train. The gear ratios for each individual gear pair are multiplied together to compute the overall compound gear ratio
Using a basic stock Yerf with no reverse as example the basic math formula for getting overall ratio:
Trans gears- 13/40 13(A) , 40(B)
Axle gears 13/31 13(C), 31(D)
Divide A into B (40/13) = 3.08 [trans gear ratio] (E)
Divide C into D (31/13) = 2.38 [axle gears ratio] (F)
Multiply E and F (3.08x 2.38) = 7.33 ( overall drive ratio) (G)
This means for every complete rotation of the axle sprocket the motor turns 7.33 times.
Another way to see it: @7,330 rpm the axle sprocket make approximately 1,000 rotations ( 7.33 to 1)
The higher the final ratio number (G) the more torque and acceleration.
Lower the number (G) = more top speed.
Keep in mind this number (G) is to the axle sprocket. Tire diameter plays a part in this too. Taller tires lower the number. There’s another formula to work out the ratio between tire diameter and sprocket rotation
To figure the true final ratio you will want to use IPR (Inches Per Revolution) which means the number of inches the buggy will travel per each revolution of the motor
To Figure IPR:
Rear tire Circumference (X) 22in. is a popular tire size.
Multiply by ( C) 13, outer trans gear. If with reverse this number would be 16
Then divide by ( D) 31, axle gear. (X*C) / D = IPR
( 22 x 13 ) / 31 = 9.22 . This means the buggy would travel 9.22 inches for each revolution of the motor.
The higher the IPR number the faster you go !
With reverse: (22 x 16) / 31 = 11.31 Which shows with a reverse unit your covering more ground. with the same motor rpm.