FAQ for R/C electric off-road racing

Contents:


    2.4.12) Stock motor

The Trinity/Speedworks Green Machine II is a popular ROAR stock motor. As stock motors go it is a good one -- it is powerful yet runs cooler and lasts longer than its predecessor. It comes with good quality brushes and springs already installed.

There are many other stock motors on the market. Some of them are simply Green Machine IIs with racing brushes installed. As a beginner you are better off with the standard Green Machine II, which has brushes that produce a bit less power but are far easier on the commutator and allow the motor to last much longer.

Trinity has just introduced the Midnight stock motor to succeed the GM II. I have yet to see a Midnight on the track but judging by past history it should be a step forward.

You will get longer life from a mild modified motor. You may want to practice with a modified and save the stock motor for races. You'll need a wider range of pinion gears to run different motors (see next topic.)


    2.4.13) Pinion gears

The longer you race the more pinions you will accumulate until you have a full set (ranging from 14 to 25 teeth or so). Start out with one pinion at the recommended gear ratio for your stock motor, and with the pinions with one and two fewer teeth. (For a Losi XX buggy and Green Machine II motor you would buy 23, 22, and 21 tooth pinions; for a Losi XX-T truck and Green Machine II you would buy 22, 21 and 20 tooth pinions.) Start with the smallest pinion gear. After a complete run, press your thumb on the motor. If you can hold it there for five seconds with no discomfort, then it is OK to try the next larger pinion. A larger pinion will give you more speed on long straights but may give less acceleration out of the corners.

If you only buy one pinion, buy one that's two teeth smaller than the recommended one. As a beginning driver you will be carrying less speed through the turns than a more experienced driver, so you will need to run a smaller pinion than the more experienced drivers do.

I like Team Losi pinions because they use a 5-40 set screw. The drawback of the 4-40 set screws used in other US brands is that they are more prone to stripping. It can be hard to remove a pinion whose set screw has stripped!

A convenient feature to look for when choosing a pinion gear is a visible indication of the number of teeth. There are few things so tedious as counting the teeth on a gear.

All the pinion gears I've been talking about are 48 pitch gears, which match the spur gears supplied in competition-level Losi and Associated kits. 64 pitch gears are too easily damaged in off-road use, while 32 pitch gears give away too much efficiency. 48 pitch giears are the right compromise.


    2.4.14) Thin CA glue

You need a small bottle of thin cyanoacrylate ("super") glue for gluing tires to wheel rims.

Since different brands of hobby-quality CA glue seem about the same, I buy based on the quality of the packaging. The glue is pretty useless if it has leaked out, or if the package has become glued shut. My favorite CA glue package is Pacer's, which has a double cap. It travels well.

Resist the temptation to buy the large economy size; you won't be using glue that quickly. A 1 ounce bottle will last you a long time, and even a 1/2 ounce bottle will mount quite a few tires.

Be sure to read and follow the label directions when using CA glues. CA will bond skin to skin, so be careful; you *really* don't want to get any into your eyes. Wear safety glasses. Some individuals display allergic reactions to CA glues. Avoid getting it on your skin (disposable latex gloves ensure this) and avoid breathing the vapors.


    2.4.15) Paint

You will need a couple of spray cans of Pactra R/C Finish for painting the clear polycarbonate body of your racer. (You will hear people talk about Lexan bodies -- Lexan is GE's brand of polycarbonate, the original.) This sort of special paint sticks best. Another reputable brand is Coverite; I have no experience with Coverite paint.

The solvents in paints for R/C car bodies attack the polycarbonate material, giving the paint a better grip. They also attack your nervous system. Be sure to read and follow the label directions when using these paints. Wear glasses and a respirator and do your painting outside if you possibly can.


    2.4.16) RTV adhesive

RTV (room temperature vulcanizing) adhesive is a sticky rubbery glue. It is sold under the Shoe Goo brand name for repairing the soles of tennis shoes; the Goop brand is also widely-distributed. RTV adhesive is perfect for reinforcing the high stress areas of polycarbonate bodies, gear covers, etc. (CA glue attacks polycarbonate, making the material cloudy and brittle.) As a beginner you will be doing a lot of crashing, so reinforce before you begin.

Buy the smallest tube of RTV adhesive you can find; it goes stale in the tube. When the adhesive loses that ultra-sticky consistency coming from the tube, it is time for a new tube.

Be sure to read and follow the label directions when using RTV adhesives. The solvents are really bad for you, so keep the stuff off your hands and use RTV in well-ventilated places only.


    2.4.17) Motor oiler

Bushing motors need a drop of oil on each bushing before each run. The Trinity oiler has a needle tip that's perfect for the job. It is available filled with either bushing or bearing oil; for a stock motor you want the bushing oil.


    2.4.18) Motor spray

Depending upon the track conditions, you should clean your motor every few runs. Buy a motor spray for this purpose. Brake cleaner is cheaper and has a similar list of ingredients but has a different formulation; don't use it.

Motor spray is also good for spraying out wheel bearings as they get gritty; re-oil with one drop of light oil after spraying.

The components of motor spray that are bad for you are absorbed readily through the skin; always wear safety glasses and rubber gloves when using motor spray.


    2.4.19) Comm drops

Comm drops used to be a "speed secret" for stock motors, but they caused accelerated wear of the commutator and were not suitable for modified motors. The current crop of comm drops is different; they should be used in all motors. Comm drops reduce arcing, brush glaze, and comm wear. That they make your motor faster is just a bonus.

Trinity/Revtech drops work, are not horribly expensive, and are widely available. Recent lab tests reported in R/C Car Action show that the more expensive Race Prep and Extreme drops produce more power than the Trinity drops.


    2.4.20) Hardened Allen wrench (pinion set screw)

You want to get that pinion on tight, but you don't want to strip the set screw. To achieve both at once you need an Allen wrench that's a lot harder than the screw, so the edges of the hex end won't round off with use. Many folks make these wrenches. Thorp is the original, and has the best grip; Associated, Trinity, and others make lower-cost versions that have a replaceable tip, an advantage if you drop your tools. Losi pinions use a 1/16 inch wrench; most others (e.g. Robinson, Trinity) use the 0.050 size.

You can live without this wrench for awhile, but once you've had one you'll never go back.


    2.4.21) 2.5 mm ball driver (motor screws)

You need an Allen wrench for tightening the 3mm screws that hold your motor to the chassis. For about $3 you can buy a Bondhus ball driver that makes the job easy by allowing you to drive the screw from an angle. For about $12 you can buy a hardened ball driver from Thorp.

Again, you can make do with a plain Allen wrench. But you will become very familiar with the motor screws as you adjust gear mesh each time the motor goes on after cleaning. You'll be happier with a quality wrench.


    2.4.22) Shock wrench set

Shock wrenches are a wonderful recent invention of RPM. They are for tightening the cartridge on a Losi shock or the cap on an Associated shock (two versions of the same tool.) They are made of plastic to do the job without scratching. They are perfectly adapted to the job. You can do without them but don't mangle your shocks using inferior tools; a shock wrench set costs less than one new shock.


    2.4.23) Tooth brush

After each run you should clean the bottom of the shocks, where the seals are. Dirt that accumulates in this area causes accelerated wear of the seals and slows down the shock action. An old tooth brush is the perfect tool.


    2.4.24) Box

Get a good-sized cardboard box to hold your stuff. Get some smaller cardboard boxes to keep the smaller parts organized. As you accumulate spare parts you will need to get some divided plastic boxes, such as the ones designed for fishing lures.


  2.5) What's the first modified motor I should buy?

You don't need a modified motor right away. But as your driving improves you will get the itch to experiment with more power.

Your first motor for modified racing should be mild, not wild. You want something that's faster than stock, but still easy to drive. Most beginners in modified go slower in modified than in stock, but if you pick the right motor you can go faster right away.

Modified motors are rated by turns and strands. A 14-turn triple has three strands wrapped fourteen times around each pole of the armature.

Other things being equal a motor with fewer turns draws more current, has more no-load RPMs, and produces more power. (This explains why a stock motor is specified to have at least, not at most, 27 turns.) A motor with fewer strands has more "snap" -- it accelerates more quickly from low speeds. A motor with the same number of turns and more strands has more power at the top end.

If you buy a 14-turn triple from Reedy, it won't necessarily perform the same as a 14-turn triple from Peak Performance or Trinity. The variables include the quality of laminations in the armature stack, the strength of the magnets, the wire size and winding pattern used, the diameter of the commutator, and the choice of brushes and springs. But if you stick to one line of motors, you can predict the relative performance of two motors by knowing their winds.

As a beginner in modified you want a motor with a lot of turns and a lot of strands. Especially a lot of strands -- at least four for a buggy, three for a truck. Otherwise you will spend all your time spinning out while you try to develop the sensitive throttle control needed with a big motor.

A mild modified motor that I like a lot is the Reedy Mr. K 17-turn quad. This was the only motor I used in my first year of modified racing. These days I usually run a 12-turn triple in my buggy, but I keep a Mr. K handy for running on very tight tracks and tracks with bad traction. I've worn out a couple of Mr. Ks through hours of use. The Mr. K is well suited to 1400 SCR batteries; a 16-turn triple should perform similarly on 1700 SCRCs.

Of all the makers of off-road racing motors, Reedy offers the widest choice of winds. This is an advantage because if you want to go a little hotter or a little milder, the motor you want is nearly always available. A case can also be made for Trinity motors. Trinity offers a huge variety of winds, but many of them are only available as armature upgrades. Trinity points out that if you want to try a lot of winds it is cheaper to buy armatures than whole motors. Reedy sells only whole motors; his theory is that when the armature is worn out, the magnets have lost enough of their strength that you need a whole new motor.


Chapter 3) Building your car

Building an R/C racer from a kit is mostly a matter of carefully following instructions. Here are a few notes on issues that are often not described well in the instructions provided with kits.


  3.1) Thread locking

If your kit involves any screws that thread into metal parts, you'll need to use a thread lock compound to keep the screws from loosening. The standard thread lock compound is blue Loctite 242.


  3.2) Tire gluing

You must glue your tires to their wheels. Without gluing, you will lose speed as the drive wheels slip within the tires, and you will lose control as the tires come off the wheels due to forces generated in hard cornering.

It pays to do a careful job of the gluing. You want the tire/wheel assembly to run true and you want the job to be strong. The fussy-sounding procedures below are designed to meet these goals.

First, examine the mounting surfaces of the tires and trim away any excess rubber with sharp scissors. This permits the tires to sit down straight and true on the wheels. Next, clean the mounting surfaces of the tires and the wheels with rubbing alcohol. This removes any mold release agents from these surfaces and allows the glue to work its best. Then mount the tires on the wheels, and get them true. One way to get them true is to hold a mounted tire between your hands, press in slightly, and spin the tire back and forth with a rubbing motion. Do this a couple of times, shift the tire 1/4 turn, and repeat four or five times.

Use a thin CA glue (e.g. thin Pacer Zap, Hot Stuff, blue Goldberg Jet). A narrow tip applicator makes the job easier. Place the mounted tire on a table with the outside of the wheel facing up. Pull the tire very slightly away from the wheel so you can apply a drop of glue between the tire and wheel. Work your way around the wheel until the tire is entirely glued, then flip over and glue the other side in the same way.

Now your wheels and tires are ready for use. At some point this set of tires will wear out and you will want to replace it. You can remove the tires from the wheels by boiling the tire/wheel assembly for a minute or so. The easiest and safest procedure is to cut the tread surface from the worn out tire before boiling; this prevents scalding hot water from collecting inside the tire, where it might squirt out and hurt you. After boiling, you should be able to pull the sides of the tire from the wheel without leaving any rubber behind. You will leave some glue behind; clean this off completely with a small file before mounting the next set of tires.


  3.3) Soldering

As a beginner, if you aren't skilled in soldering, you can get someone who is to help you. But even better is to get someone to teach you, and spend some time practicing. You'll be soldering whenver you install new motor brushes or buy a new motor or battery (assuming that you follow my advice and junk the Tamiya battery connector.)

This is not a soldering tutorial, but here are a few basic tips:

Heat-shrink tubing can be applied over solder joints for insulation and finished appearance. Use heat-shrink in preference to electrical tape - it is more permanent. To shrink the tubing, hold a hot soldering iron close below it or a match farther below it.

You will see advertisements for nifty temperature-controlled soldering irons. Unlike regular soldering irons, you can leave a temperature-controlled iron hot all the time while you are racing, so it is ready the moment you need it. (Leaving a regular iron on without using it for long periods of time will greatly shorten its life.) The drawback of the temperature-controlled soldering iron is cost -- roughly $100. And soldering irons don't last forever, especially not when you are carting them back and forth to the race track. You will be better off spending the money on something else -- like high-quality connectors that reduce the need for using a soldering iron.


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