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 truck’s performance and diff life. With
the truck all prepped as above, install a charged battery and put 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, with no throttle input, 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 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 use
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 a slipping diff. 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.