AHM motor rpm & gear ratio

[the nitro man]

does anyone know what the gear ratio in an AHM challenger is? also any idea of how much rpm the AHM motor will put out on wide open throttle. it's a 1978 year model i believe. it's a grey 3975 or something like that.

[Jim Banner]

If you have the model, why not just measure the gear ratio?  Mark or flag with bits of tape both the motor shaft and the driven axle.  Then turn the motor shaft, counting the turns, until the axle makes one turn.  That gives you the gear ratio of motor turns : axle turns.  For more accuracy, turn the motor shaft until the axle turns 10 times then divide by 10 to get the gear ratio.

To calculate motor rpm at full throttle, measure how many actual inches the model will travel in one minute with the throttle wide open.  Divide the inches per minute by the driver circumference.  (The driver circumference is the diameter of the driver in inches multiplied by 3.14.)  That will give you driver or drive axle rpm.  Multiple drive axle rpm by the gear ratio to get motor rpm.

For example, suppose you have to turn the motor shaft 32 times to turn the drive axle once.  Then the gear ratio is 32:1.  Then suppose your locomotive runs 90 actual feet in one minute at full throttle (this is about 90 miles per hour in H0 scale.)  90 feet x 12 = 1080 inches in one minute.  Suppose the drivers are 7/8 inch in diameter.  Then their circumference is 7/8 x 3.14 =   2.75 inches.  Dividing the 1080 inches per minute by 2.75 would give 393 rpm at the axle.  Multiplying by a gear ratio of 32 would give a motor speed of 393 x 32 = 12,567 rpm.

Your answer will depend on the gear ratio you measure, the distance your locomotive travels in one minute at full throttle on your power pack, and the diameter of your model's wheels.  And you thought you would never ever have to use your grade 4 math!

Jim 

[the nitro man]

thanks