FAQ for R/C electric off-road racing

Contents:


    2.4.2) Tire upgrade

Tires are the biggest factor in how your car or truck handles. If the kit you buy does not include good tires for your track, don't waste your time with the kit tires. Buy tires that will work. Watch what the locals use and follow their lead. On many tracks the best tire will use a soft rubber compound, either the Losi Silver or ProLine XTR-M2.

    2.4.3) Transmitter, receiver, steering servo

The most popular brands of radio gear for off-road racing in the USA are Futaba and Airtronics. Both Futaba and Airtronics make entry-level radios that cost $80 and better radios for around $150.

The radio I quote for the low-priced setup is the JR Python. This is the lowest-priced transmitter that includes a steering rate control. Steering rate control (also called "dual rate") allows you to set how far the front wheels steer when you turn the steering wheel from lock to lock. This allows you to get full steering without performing tedious mechanical adjustments.

The JR Python comes with two basic servos, one for a mechanical speed control and one for steering. It also includes a receiver that incorporates JR's proprietary ABC&W circuitry for increased noise immunity.

I've specified the Airtronics XL2P in the mid-priced setup. This is a popular, high quality AM radio with a full range of adjustments.

The most important property a radio can have is freedom from glitches. Higher grade radios with FM transmission have more noise immunity than entry level radios with AM; the difference is real in competition, with ten radios turned on at one time. The Airtronics CS2P is the lowest priced FM radio available. It is clearly a competition-quality unit based on the number of top racers who use it. For this reason, I recommend the CS2P as a worthwhile improvement over the XL2P and quote it for the high-priced setup.

KO and Futaba also make fine radios. Masami Hirosaka, many-time world champion in off-road and on-road, uses KO. Futaba's entry level radios, bundled with electronic speed controls, are very popular. If one of these brands is popular where you live, you might decide to go with it.

Some receivers come with antennas that are much longer than a normal antenna straw. If you buy such a receiver you'll need to build an antenna loom to to hold all of the antenna wire in excess of what goes up the antenna straw. If you don't build a loom you are likely to experience glitches in competition.

There are two radio bands to choose from: 27 mHz and 75 mHz. The 27 mHz band has only six channels. The 75 mHz band has narrower channels, but fifteen of them, with another fifteen (the odd-numbered channels) now coming into use. Because there are so many more channels, racers on 75 mHz tend to have fewer frequency conflicts when practicing and racing. If seven racers on 27 mHz all qualify for the same main event, the slowest qualifier will not get to run. I have seen this happen. It doesn't happen on 75mHz. A modern radio receiver copes quite nicely with the narrower channels on 75mHz. I recommend that you buy 75mHz equipment. Ask about the status of odd channels in your area before buying.

You will hear people talk about PCM. PCM is a digital encoding technique for communicating control information from the transmitter to the receiver. The primary advantage of PCM is that a PCM receiver can detect errors introduced in the radio transmission; thus a car being controlled with PCM is much less likely to go completely out of control. However, this "fail safe" mode is not recommended (at least by Futaba for their PCM gear) unless you use a separate battery pack to power the receiver. Thus PCM is primarily intended for powerful gas cars, which benefit significantly from the extra safety margin. PCM receivers introduce some extra delay in transmitting your commands to the speed control and steering servo, and they aren't compatible between different manufacturers (for instance you can't use a Futaba PCM receiver with an Airtronics PCM transmitter.) Overall I recommend against investing in PCM for electric-powered racing.


    2.4.4) Transmitter ni-cd, trickle charger

You can run a transmitter on alkaline cells, but in doing so you will quickly spend enough to have bought eight rechargeable Sanyo AA cells and a trickle charger.

    2.4.5) Spare set of crystals

To race you should have at least one alternative to the basic frequency that came with your radio. If you don't have a second frequency, you may be unable to race at all, or you may qualify but be unable to run your main. Buy a set of crystals for a frequency that's next to your basic frequency; that way your equipment won't need re-tuning when you swap crystals.

Crystals are specific to a particular brand of radio and to AM or FM. Futaba AM crystals are the cheapest because they sell in the highest volume.


    2.4.6) Servo upgrade

Just about any standard-size servo will steer a buggy. Your truck will work OK with a standard servo, but won't turn as well as it could. A truck works best with a steering servo the produces at least 50 ounce/inches of torque. The KO PS703 is a good choice at $50; the Airtronics 94151 is a proven performer and slightly faster (according to the published specs) at $75. Faster is better once you've got enough torque.

Most standard servos, including the Airtronics 94102 and Futaba S-148, don't have a ball bearing on the output shaft. When the bushing on this shaft wears, it creates extra slop in your steering. A ball bearing upgrade from LDM for an Airtronics or Futaba servo costs $6.50; well worth the investment if you go with the standard servo.


    2.4.7) Servo saver

The steering servo is designed to turn the steering linkage of your car. When you crash, the front wheels of your car try to turn your steering servo. If you crash really hard the force can be enough to strip the gears inside your servo. This puts you out of action until you can lay your hand on a new set of gears and replace the old stripped gears.

A servo saver is anything that absorbs shocks before they get get to your servo. You want a servo saver for the big shocks, not the little ones, so the best designs pass small forces right through but absorb the larger forces.

Some cars come with built-in servo savers: XX, XX-T, RC-10T2. For the rest, Kimbrough Products makes the industry-standard servo saver. For off-road use you want Kimbrough's large servo saver; the small one is too flexible and thus gives up too much steering.


    2.4.8) Electronic Speed Control

For racing you want a high-frequency, forward-only electronic speed control. In the USA this means a Novak or a Tekin.

A good speed control lasts a very long time; I am happily running a four year old Tekin 411P in my XX. My LX-T runs a Tekin 411G2 that has a couple of summers on it, and my on-road car runs a Novak 410M5. Of these, only the 411G2 is still available for sale, and it may not be around much longer. So my first-hand experience with today's speed controls is very slight indeed. As a result this section will focus on features, then mention one representative model in each price category.

What does "high-frequency" mean? A speed control works by chopping the voltage delivered to the motor. At half throttle, the speed control delivers full voltage to the motor half the time, and no voltage to the motor the other half. The faster the voltage is switched on and off the smoother the motor runs. High-frequency speed controls give you better control, are more efficient at partial throttle (give longer running times), and help the motor last longer. That's why you want one.

Some speed controls offer an adjustable current limiter, sometimes called "torque control." When your car is standing still or moving slowly and you mash on the throttle, the motor draws a huge spike of current. The surge of current generates a lot of torque at the wheels, but the torque may go into wheelspin and be wasted. The current also heats up your motor and drains your battery. So a current limiter is moderately useful in off-road; with proper adjustment it can reduce wheelspin, decrease motor temperature, and increase run time. You can achieve most of the same effect by learning when to use partial throttle.

What about reversing speed controls? Reverse is a great feature to have when you are practicing: When you turn in too soon and run into an obstacle, you can back right out and try again. Reverse is also great for doing stunts when you are playing around. But there are some downsides to adding reverse to a speed control. First, reversing speed controls don't have protection against hooking up the power wires backwards; if you ever make this mistake, you'll usually have to send the speed control in for repair. Second, reverse adds resistance to a speed control. The added resistance of a reversing control will slow you down a little bit, and means that the speed control may overheat if you try to run a low-turn modified motor. (The Tekin 610-G "Titan" is a low-resistance reversing model; they say it handles 11 turn motors!) Third, reverse adds a small amount of weight to the speed control. Finally, it is illegal to use reverse while racing. Some Tekin and Novak reversing speed controls allow you to disable reverse, which makes them legal. If you want reverse and also want to do some stock or mild modified racing, this is the kind of speed control to get.

All of today's Novak speed controls and most of Tekin's feature an on-off switch that allows you to plug in the battery without turning on the car; you flip the switch to turn on the car, flip it back to stop. This is extremely convenient. Beware the inexpensive low-frequency speed controls that are bundled with some radios -- they may lack an on-off switch, making them awkward to use.

The Tekin Formula 10 is the least expensive high-frequency speed control I've seen; it is a new model and I have no word-of-mouth on it. An alternative for $12 more is the Novak Duster, a one-button setup model that's been around for almost a year. Both of these lack a current limiter.

The Tekin 411G2 is a no-holds-barred racing speed control. It has very low resistance, an adjustable current limiter, externally replaceable wires, and strong brakes. It is an older model with neutral and high speed pot adjustments.


    2.4.9) Batteries

The Sanyo 1700 SCRC (black label) cell is almost universally used in off-road racing today. These cells have low internal resistance, high capacity, and are rugged enough to survive being charged more than once per day with no ill effect. They are just wonderful. Buy three packs of Sanyo 1700 SCRC cells so you can be running one, charging one, and resting one (preferably with a fan cooling it).

You will see some racers running Sanyo 1400 SCR cells (red label.) These are fine cells, and cost about $10 per pack less than 1700 SCRCs. If you are on an extremely tight budget, buy Sanyo 1400 SCRs.

Sanyo 1400 SCRs have even lower internal resistance than 1700 SCRCs and therefore deliver higher voltage. But that high voltage drops more quickly due to the lower capacity. In the first minute of a race, the 1400s give slightly more punch than 1700s, but in the last minute the 1700s deliver a lot more punch than the 1400s.

Stay away from packs that are not built with Sanyo SCR or SCRC cells. They don't perform or last like Sanyo SCR and SCRC packs. This advice holds even for other types of Sanyo cells, including the Sanyo SCE cell that once dominated modified off-road racing.

You will hear people talk about matched cells. Cell matching is the process of evaluating a large batch of cells and grouping the most similar ones into packs. Most commonly, cells are grouped according to the number of seconds they take to discharge from fully charged to 0.9 volts under a 20 amp load. A matched pack does not necessarily run longer or make you go faster than an unmatched pack, but a matched pack takes the abuse of full discharging better than an unmatched pack and therefore has a longer lifetime. As a beginner you should prefer packs assembled by Sanyo from unmatched cells. Avoid assembled packs from anybody who does cell matching; you will get their inferior cells. Alternatively, buy inexpensive matched cells from a reputable matcher and assemble the pack yourself. Don't spend a lot of money buying top-grade matched cells; they are overkill for off-road.


    2.4.10) Connectors

To avoid frequent use of a soldering iron, you need two connectors on your speed control: one leading to the battery and one to the motor. You need one connector on each battery and one on each motor.

The battery and the speed control motor connectors should be female; the speed control battery and the motor connectors should be male. So to start with you need four female and two male connectors, plus one extra of each wired up to help with battery charging, motor break-in, etc.

The "Tamiya-style" connectors that come with batteries are terrible. They have high resistance and are unreliable. Replace them with Dean's Ultra Plugs (*not* Dean's 4-pin connector) or with PowerPole type connectors. PowerPole type connectors are sold by Anderson, Sermos, Litespeed, Plus, and others. If you go with PowerPole type connectors, avoid the ones with polycarbonate bodies, since contact with motor spray will wreck them.

You may see some racers "hard wiring", which means soldering connections to their batteries and motors on each run. A good connector introduces very little resistance, is reliable, and is convenient. Get good connectors and you'll be at no disadvantage.

A lot of racers who used to hard-wire have switched to Reedy's new racing connector. Reedy's connector has low resistance and is reliable but is neither polarized nor insulated and thus is only suitable for hand-assembled packs that keep the two battery terminals far apart.


    2.4.11) Battery charger

A low-end battery charger is a timer-controlled current source. If you know that your battery is fully discharged, and you know how long it normally takes to fully charge from the discharged state, you can set the timer and charge the battery. If you miscalculate, you can destroy the battery you wanted to charge. The battery will vent its chemicals, melt its wrapper, possibly even catch fire. You can get a timed charger for about $33 from several companies.

A peak charger operates by sensing the voltage the battery puts out as it is being charged. As the battery reaches full charge, it heats up and its voltage drops. A peak charger is fully automatic; if it makes a mistake, it will stop charging too soon rather than too late. The least expensive competition quality charger that runs off of household current is the Tekin BC 5A at $93. This charger is derived from the DC-only Tekin BC 100L, an old reliable design that may be the most popular competition charger around. I've known several people who tried the Pro Tech peak charger, which is less expensive, but these people have been unhappy due to frequent false peaking. I am completely ignorant about the Astro Flight 111XL peak charger at $84.

$60 is a big difference in price. Because of this, most people start with a timed charger but get a peak charger later when they get serious about racing.

A useful feature in a peak charger is the ability to charge 8 cells, for transmitter packs. You get a fuller charge with a trickle charger, but the trickle charger won't help you if your transmitter pack is dead and you have a race starting in 20 minutes.


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