FAQ for R/C electric off-road racing


  3.4) Capacitor mounting

The capacitors you use on your motor depend upon your speed control, not upon your motor. So the instructions to your speed control will explain what types of capacitors to use on your motor. In most cases you need three capacitors: one from the positive terminal to the can, one from the negative terminal to the can, and one between the positive and negative terminals.

It is conventional to orient the motor so that the positive terminal (marked on the endbell) is at the rear of the car. (I have no idea why this is, but it is.) Mount the capacitors on top of the motor in this orientation so they are less likely to be scraped off in a collision. Keep the leads short and neat.

Some stock motor cans are finished with a powder coating or with paint. You need to remove this finish in two spots in order to solder the capacitor leads to the can. A small file is the best tool for the job, but a hobby knife will do if you are persistent. You need to expose clean bare metal for the solder to stick.

  3.5) Compatibility of servos, speed controls, and receivers

There is a standard three-wire interface between receivers and servos (also receivers and speed controls): +5 volt power, ground, and signal. This receiver wiring harness originates in the servo (or speed control) and plugs into the receiver. But there is no standard for the colors of the three wires, their arrangement in the plastic connector body that plugs into the receiver, or the shape of the plastic connector body.

Novak and Tekin have dealt with this situation by providing the necessary plastic parts and instructions for converting the receiver wiring harnesses of their speed controls to work with all common receivers (Futaba, Airtronics, JR, and KO). The conversion is simple and takes only a couple of minutes. Similarly, while Novak and Tekin receivers are designed for Futaba connectors, they include plastic parts and instructions for converting the receiver wiring harnesses that plug into the receivers.

The servo manufacturers (all of whom make receivers) aren't so helpful. If you buy a servo from one manufacturer and a receiver from another (other than Novak and Tekin), you'll need help doing the wiring harness conversion.

Here is a key to the wiring of various servos:

        Futaba:     Black (-), Red   (+), White  (signal)
        Airtronics: Red   (+), Black (-), White  (signal)
        JR:         Brown (-), Red   (+)  Orange (signal)
        KO:         Red   (+), Black (-), Blue   (signal)
(JR wins the non-conformist prize for choosing Brown for the ground wire!) Thus you can plug a JR servo directly into a Futaba receiver, but you must reverse the power and ground wires on Airtronics and KO servos before plugging them into a Futaba receiver or you will fry the servo.

  3.6) Servo, speed control, and receiver mounting

Some servos are called "standard" but there is in fact no standard for the size of servos or for the locations of their mounting ears. (Just for fun, servos also differ in the number of splines on their output shafts!)

The Losi XX and XX-T have the best-engineered servo mount around; its only drawback is that to use it you must first grind off the lower mounting ears of your servo. No matter; two mounting ears are plenty.

Associated recommends the use of servo mounting posts. These posts work fine, but you are responsible for drilling holes in the chassis to locate the posts correctly, and for drilling holes in the posts to suit your servo.

When using the large Kimbrough servo saver you will have to either slot the chassis or space the servo up off of the chassis to prevent the servo saver from rubbing on the chassis.

It is quite common to mount the servo using servo tape, a thin foam double-stick tape. Servo tape is not quite strong enough to keep the servo from squirming around while it is in use. This squirming makes steering slower and less precise; in time, the servo will come loose. Therefore it is common to augment the servo tape with a small bead of shoe goo, joining the servo case firmly to the chassis. To remove the servo you cut, chip, or peel off the dried shoe goo. (Not a fun job.) This style of servo mount is used on many Associated cars and trucks.

Receivers and speed controls mount easily to the chassis or shock tower using servo tape. Your kit instructions should give tips on the best mounting locations. Some recommended locations may rely on being able to run the speed control's receiver wiring harness under the battery to reach the receiver. If you are using stick type battery packs (cells oriented with the long axis of the pack instead of running across the pack) then to run wires underneath you will have to raise the pack. One way to do this is with a layer of servo tape, cut out where the wire passes through, and all covered with a layer of duct tape. To avoid this mess you may decide to mount the speed control on the shock tower (recommended on Losi) or the receiver and antenna tube on the shock tower (recommended on Associated). The receiver and speed control are light enough that mounting them on the shock tower doesn't raise the car's center of gravity very much. Never extend the speed control's or servo's receiver wiring harness in order to solve a mounting problem.

Tekin and Novak speed controls have an on-off switch. The standard mounting location for the on-off switch is on the rear bulkhead or transmission case right behind the rear shock tower. This is a good location because the switch is unlikely to be bumped or covered in dirt. If you can't reach this location with your switch, improvise. Tekin switches have ears for mounting with tiny screws; you mount Novak switches with servo tape and shoe goo or with a small cable tie.

  3.7) Body painting

Painting can be more time consuming than all the rest of the assembly. If you want a durable, distinctive, and attractive paint job, it probably will be. Set your expectations accordingly.

Trim the body completely before you paint it. This includes cutting any mounting holes and the hole for the antenna straw. Place the body on the chassis to ensure that you've provided clearance for the shocks, suspension, and steering. If you try to trim after painting you are likely to mess up your paint job.

You can trim the edges of the body most easily with a pair of short, stout scissors. Failing this, use a sharp Xacto knife, but go slowly. Start holes by rotating the point of a sharp Xacto knife, using slight pressure. The best tool for enlarging holes to their final size is a Dremel tool with a tapered steel or abrasive cutter, or a tapered hand reamer. Lacking these, use a set of graduated drill bits, rotating them by hand to keep them from grabbing and tearing the plastic, or use the sharp Xacto knife with great patience.

After trimming, clean inside of the body thoroughly. Though it seems barbaric, I prefer to clean the inside of the body using extra fine (400 grit) wet-or-dry sandpaper, used wet. The scratches produced by the sandpaper are invisible once the body is painted and they help the paint bond to the plastic. Don't sand areas that are to remain clear, e.g. windshields and windows.

Next step is masking. The body may include masks for the windows; apply them first. Then, for a two-color paint job, mask to completely cover the area that will be painted the lighter color. Use a high quality masking tape, such as 3M, and press the edges of the tape down firmly with the back of a fingernail to make it more difficult for paint to seep under the tape.

You can paint with an air brush, a spray can, or a bristle brush. Brushing (air or bristle) gives the widest choice of colors -- you can mix your own. A bristle brush gives a heavier job, which is not an advantage. Most people go with the spray cans.

Some keys to spraying success are:

As noted earlier, the solvents used in paints that stick to polycarbonate are quite bad for you; follow directions by spraying outside or in some other well-ventilated area.

When you are done painting, and the paint has dried thoroughly, reinforce the high-stress areas of the body with RTV adhesive. High-stress areas include the body mounting holes, slots where the shock towers come through the body on a buggy, and sharp corners such as the bottom rear of a truck's cab. A little RTV adhesive applied where the body rubs on the chassis can keep the paint from wearing through in these places.

Chapter 4) Running your car

  4.1) How do I charge my batteries?

If you've bought a peak charger, congratulations! Just follow the directions.

If you've bought a timed charger, you will need to be very systematic about charging and discharging your batteries, or you will ruin them in short order. Here's what to do:

Now you appreciate why peak chargers are worth the extra money.

Don't charge a battery when it is hot from being discharged. Wait for it to cool, using a fan if you want it to cool faster.

  4.2) How do I turn on and off?

If your R/C car is turned on but no transmitter is turned on to tell it what to do, it will go wild (unless it has a PCM receiver with fail-safe turned on.)

Thus to run your car, turn on the transmitter first. Then turn on your car (with the on-off switch or by plugging in the battery.)

To finish a run, turn off the car first (with the on-off switch or by unplugging in the battery.) Then turn off the transmitter.

Don't run your R/C car until the batteries are completely dead and the car stops. This isn't good for the batteries, and you risk losing control toward the end and having the thing run off the track and get run over, or whatever.

When you feel the car lose its snap, it is time to bring it in and shut it off. On that final lap, just nurse it along, don't yank the throttle.

  4.3) How can we run more than one car at a time?

R/C transmitters operate at low power (one watt or less) on assigned frequencies. The range of a transmitter is a few hundred meters at most. Of course, your 1/10 scale car is pretty much invisible at a distance of a hundred meters so this range is ample.

When two transmitters operate in the same vicinity on the same frequency, they interfere with each other. A receiver can't untangle the signal coming from its transmitter from the signal coming from the other transmitter. Result: the car goes wild (unless it has a PCM receiver with fail-safe turned on.)

If you are out running in the street with friends, you prevent this by talking to your friends and making sure that you don't have any frequencies in common. (Actually, it is best to sort this out before your friends buy their radios, so you can all run at once.)

If you are running at an organized track, you use whatever form of frequency control that track employs.

The most common form of frequency control involves clothespins (not very high tech.) The track has one clothespin for each frequency: 1-6 on 27 mHz and 62-90 on 75 mHz. Before running you go to the board with all the clothespins clipped to it, find the pin with your frequency number, attach the pin to your transmitter, and go run. If the pin is gone you go find who has it, wait for this person to finish, and get the pin from him or her. When you are done running you return the pin to the board.

People will get justfiably mad at you if you break the rules, turn on, and wreck somebody else's car. So find out what the rules are and follow them.

  4.4) How do I drive this thing?

This isn't the place for a treatise on R/C race-car driving. But here are a few notes on getting started:

  4.5) What about maintenance?

After each run you should clean the dirt off of the bottom of each shock, where the shaft enters, using an old toothbrush. This will make the shock seals last a lot longer between rebuilds.

If you are running a stock or other bushing motor, put a drop of oil on each bushing before running. If the endbell bushing has accumulated dirt then brush the dirt off before oiling. Let the motor cool down between runs. Put comm drops on the brushes just before the next run.

How clean you keep the car is up to you. If you keep the various suspension hinges clean they will last longer, but if you'd rather spend time driving than cleaning that's your decision. At some point the suspension will start squeaking and feeling sticky, and this will make the car handle worse.

Every so often you should just strip the car down, spray out the ball bearings with motor spray, clean everything else with soap and water, replace the parts that are badly worn, and put the car back together again. This will make the car work like new.

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