User's Manual

Because the React 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 (3.14159) 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 React. 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
116/30, so the rollout parameter should be set to 116, and the revolutions parameter
should be set to 30.
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 mark one of your magnets with a colored sharpie
or piece of tape to reduce the risk of incorrect counting. Make sure you are measuring
using the same units (inches or centimetres) that the React is currently set up to use.
Position your car against a wall with the wheels pointed straight ahead. The back end
of the car should be touching the wall.
Carefully watching the magnets on the shaft, roll the car forward until the shaft has
rotated ten or more (more is better) times. The number of rotations will be your
revolutions parameter.
Measure from the wall to the back end of the car. If it should work out to an even
number of inches or CM, that will be your rollout parameter. If your measurement is to
the closest 1/8
th
of an inch, then in order to eliminate the fraction, you must multiply
the revolutions by 8 and convert the length to 1/8
th
inches by multiplying the whole
number of inches by 8 and then adding the number of extra 1/8ths. If you are using
metric measurement, it is simple, just multiply CM length and rollout by ten.