Rebuilding an SCX Rally Car

by
Daniel J. Dyke

A few weeks ago I bought this fine looking SCX rally car.  Now admittedly I knew the dangers involved in buying an SCX car.  They are not as detailed as Scalextric, Fly or some of the others, but they are tough cars and handle well with work.  The biggest fault is that there is a good chance they will have a dog of an engine.  Some are fast and some are not. Such was the case when Brian McNay and I each bought one the same night.  Brian's would fly compared to mine.  Undeterred I set out to make this beast which turned a 15.58 lap out of the box into a top competitor.  Brian was in the 14 bracket.

The first task was to get speed out of the engine. And so I broke the engine and gears in.  The engine was still a dog and would peak five feet into the straight, but the gears meshed nicely.  I tried even the water bath trick on the motor.  This dropped the times to a 14.99, but that was an unrepeatable lap.  The problem was to fix it within the rules and I did. 

The car has now turned a 13.17 lap at the same track. So what did I do? Is it replicable?  The answer to the second question is yes.  I did the following to my car, obviously, and to Ray Torbeck's car.  Ray turned a 13.33 lap and usually turns faster laps than I do.  I should hate myself for telling all my secrets, but we follow a tell all rule.

To The Workshop She Goes

I have a great slotting workshop that is the envy of those who have seen it. Four work areas, power tools, and a test track with computer timing.  I do other things in there, but they are just to justify the slot shop to my wife.

The important objects are numbered.

1) Coffee: the lubricant of the soul.
2) A turned over bottle of Goo Gone to help clean up messes.
3) A heat resistant work pad that has saved my house from burning to the ground.

A Digression to Tools

Three tools are going to be showcased: a setup block, a hobby vise, and a power supply.

1)    The Setup Block:  These are easy to make or they can be purchased.  If you wish to buy one, have your favorite hobby store get it for you with their next order to Professor Motor (LB1020).  Compare their picture with mine and you will see I made some modifications.

I added copper braid and a power supply to the slot so I would have power.  A magnet was glued  to the edge to hold screws.

They can be made by running a wooden block through a table saw set to the depth of a deep slot.  If they are so easy to make then why buy one.  Some people don't have a table saw. And besides that, the one from Professor Motor can be soldered on with no negative results.

2)    A hobby vice is nice.  The jaws are made of plastic and the head can rotated in an infinite number of positions along the x, y, and z axes.  In layman's terms, it frees up a hand.

3)    A power supply.  Use a 6-9 volt wallwart with at least 300ma output and attach alligator clips as connectors.  Clip the wires to the braids of the car and you can again free up a hand.  Isn't it nice to have two hands to use?  If you look at the picture I have used two different clips.  Why? Because I didn't have a matching pair.

As a safety precaution have a place to clip your wires when not in use.  Make sure it is made of a non-conductive material.  If the clips touch for a long enough period, you will ruin the power supply.  Because I have only done this six to ten times I save old wallwarts from gadgets people throw away. The clips on mine are fixed to the plastic base of a clip-on shop lamp.

Attacking the Beast

Here she is before and after surgery.  She does not look a bit different and believe it or not she is within the rules.

The Enemy:

The weakest part in many SCX cars is the motor.  They vary greatly from car to car.  The standard is one is an RX-81.  It was quickly replaced with the Pro Turbo 4x4. There is a rumor that SCX tests the motors and the ones that sound slow are the RX-81s and the fast ones are called turbo motors.  When I changed motors in the car it was as fast as Brian's stock motor.  This change is just a matter of popping out the old and popping in the new.

BREAKING IN THE GEARS AND MOTOR

1)    The first step is to look at the wheels and note how much space is between the wheel and the chassis and place enough washers in this space to remove the slop from the wheels.  To do this you must remove the wheels from the axle, place the washers on, replace the wheels.  If the wheels are loose take them off and put a little super glue or Loctite on the tip of the axle and replace them. In the next illustration I have removed the axle completely after this was done to show the size of the washers I had to use.

2)    Now put oil on the axle bearings and on the ends of the motors where the armature shaft goes through. DO NOT GREASE OR OIL THE GEARS!  If you have dry it up.  Now put a polishing agent such as toothpaste or  rubbing compound on the gears.

3)    Attach the alligator clips from your wallwart to the pickup braids.. 

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4)    Put the chassis in the hobby vice. And let it run for 5-7 minutes Watch the tires and note which ones wobble.  Stop the car remove the tires, refresh the toothpaste, and run the thing for another 5-7 minutes.  This time watch the wheel for wobble.  This can be done by watching the wheel in relation to the chassis in areas one and two of the following picture.

5)    Move the worst wheels to the front and the best to the rear as the rear does the lion's share of the work. My two worst wheels were on the rear.  Do NOT  put the tires back on.

6)     Wash the toothpaste away with water.  Lubricate the motor, bearings, and gears.  A light oil can be used on everything except the gears which should have a good dose of grease.

FIXING THE WHEELS AND TIRES

1)    Fixing the Wheel: The chassis goes back in the vise.  Hook up the wires again like before and carefully hold a razor blade or knife perpendicular to the wheel. I have a carver's  glove I wear to protect the hand as razor blades can break.  Very slowly move the razor blade to the wheel. This should take off the high points.  Warning, go slowly or you can ruin a wheel.

2)    Changing a Flat:  Not really, but at times SCX tires come not very round.  Four wheel drive cars are the hardest to true the tire on. here is a simple procedure I use.

a)    Remove the front axle assembly.  It is best to lift gently until the bearing pops out of the plastic.  Start with side one, which in the following illustration is so numbered.  Side #1 is the side on the backside (the side without the teeth) of the crown gear (#3).  This lessens the likelihood of the pinion (#4) from damaging the teeth on the crown gear. Finally lift on side two until the axle is free.

Place a piece of sandpaper on the setup block (power on) and put the guide in the slot.  Slowly lower the tires to the sandpaper while holding the sandpaper taut.  You will feel the lumps hitting the sandpaper. Slowly they will go away.  When the tire is round, remove the rear axle and replace it with the front axle and sand the tires smooth on it.  Move the front axle back to the front as it has the two worst wheels.  Put the rear axle back in.

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THE LITTLE THINGS THAT YOU DO

1) Remove that resistor as it has been known to short out a motor.

2) On the non-Xenon lights cars (it doesn't say Xenon lights on the box) bend the two tabs for the lighting up slightly as they take a little power. Mine did not work anyway.

3)    Only use the two center screws to hold the body on and turn them out a half to a full turn.

4)    Set the professor motor controller adjustment screws so the front two are down tight and the rest are backed off.  Ray can show you this.

5)    Turn brakes on or off to your liking.  I had them on when for the 13.17 lap, but everyone is different.

BREAK MY RECORD BEFORE RAY DOES