User's Manual

66
Tach
The tach, or speed sensor requires several
measurements to be made in order to provide you
with accurate speed readout. This is where you
enter those measurements.
The Sensor must be programmed with distance in
inches traveled per number of revolutions. The
sensor then “does the math” in real time to
provide you with a speedometer function that can
be used for alerts.
The two parameters that are used to make the
calculations are rollout and revolutions.
Because the Sensor cannot use fractions, it is necessary to find two whole numbers that
represent shaft turns and wheel turns. There are two ways to do this. One involves math and
the other involves physically measuring the car’s rollout.
Calculation method
The rollout can be calculated if you know the diameter of your tire and the gear reduction
ratios between tachometer installation point and the wheels. If you are not sure of these
ratios, you should probably use the Measurement method.
• Multiply tire diameter by PI (3.1415) to get length of one tire revolution.
• Multiply that by the gear reduction of your differential or transmission. The result is the
number of inches your model will travel for each tick of the tach sensor.
• Convert that number to a fraction that can be input into the Sensor. You can do this by
multiplying the number by 200 or more and then rounding to the nearest whole
number. So if your model travels 4.1242 inches per tach and you multiply by 500, the
“rollout” number will be 2062.1, which rounds down to 2062, and the “revolutions”
number will be 500.
An Example: Your Model has 3.2” diameter tires. Multiplying by pi results in 10.053”
circumference. Your Tach is mounted in front of a 13:5 gear reduction. So, the model travels
10.053 x
5
/
13
= 3.8666 inches per tachometer. This is approximately equal to 1160/300, so
the rollout parameter should be set to 1160, and the revolutions parameter should be set to
300.
Measurement method
In this method, you simply measure the distance traveled by your model over a short length
and count the revolutions of your driveshaft. Due to the possibility of measurement errors, this
method works best if you measure at least ten shaft revolutions. We recommend that you