The Tachometer Test
by Daniel J. Dyke
(ddyke)

This test has not been refined and so it is best to view the results as tentative.  Basically what is done is that a rear wheel is removed from the car and is replaced by a short propeller.  The car is run at full throttle and the rpms are measured by aiming an RC airplane tachometer at the spinning prop.  This gives the maximum rpms delivered at the rear wheels at the voltage supplied.  If the results are multiplied by the gear ratio the number of rpms of the motor can be determined.

Why not attach the propeller to the shaft of the motor?  There are several reasons, but the most telling is that the tach only goes to 30,000 rpms and some slot car motors are rated above this.  The second reason is that the gears cause drag and slow the engine down.  One can tell how much improvement a new set of gears makes to a car's performance by running this test before and after a new set of gears is installed.  Also some motors are hard to remove either because of the design of the engine bay or they have been glued in by the user.

Where does one get an engine tachometer? Tower Hobbies lists three.  Their own brand is exactly the same tach as the Hobbico one, but cheaper.  Where does one get the propeller? I made mine by drilling a hole through a wheel and inserting a piece of piano wire through it and the putting duct tape around it to increase the diameter.  Pictures will be posted later when I get the thing perfected.

My tests are run with a 12 volt power supply that is rated at 1.5 amps.