K021 Instructions v1.0
Race Prep
Page
71
BREAK IN THE DIFF
We’re ready to run now, right?! Not Quite. The next thing to do is break in the
differential. This is exceedingly important to the performance of your car and life of your
diff. With the car all prepped as above, and a battery in and ready to go, get the car
back on its stand, with the rear wheels off the ground. Turn on the transmitter and car,
then adjust the trim on the throttle so that the motor begins running at a slow constant
speed. You want this to be low enough that the tires don’t expand at all. Using a block,
the car stand, or something similar, stop the rotation of one of the rear tires. The other
tire should continue to turn with the motor, nice and easy. Leave the car like this for two
or three minutes.
By holding one outdrive/diff ring is held still, you force the balls in the diff gear to roll,
slowly flattening the microscopic surface features of the diff ring and creating a smooth
‘polish’ where they will operate for the life of the diff. Too much throttle would cause the
balls to just slip along the surface, grinding and wreaking havoc on the diff rings. Letting
them run in slowly creates a nice mirror finish. After two or three minutes, switch the
block from the one rear tire to the other, and let the motor run for another few minutes.
Doing this procedure on the bench creates a much smoother and more consistent feel
than trying to break the diff in on the track. Bring the throttle trim back to neutral to stop
the rear wheels.
Now we’ll set a starting point for diff adjustment. Turn everything off, hold the spur gear
with your left hand, and give the right tire a light flick of the wrist. The tire will probably
rotate 1.5 or 2 turns; you’ll want to tighten the diff so with a similar input it only rotates
once. Pop off the right rear turnbuckle, swing the hub and tire out of your way, and grab
your 5/64” hex driver. The diff screw is on the right side of the car; slip the wrench into
the head of the screw to hold the screw steady. Rotate the left rear wheel backwards
slightly to tighten the diff. Pop the right side back together and ‘feel’ the diff again;
adjust it in small increments so as not to over-tighten.
NEVER run the car with the diff that’s slipping. If you hear the diff ‘bark’ or ‘chirp’ – kind
of a screech sound usually out of corners or landing jumps – immediately stop running
and tighten your diff (check your slipper setting too, but we’ll get to that). Even in a
race, is finishing a run really worth replacing the diff balls and rings before the next one?
It’s a quick adjustment now rather than a lengthy and expensive tear-down
