Ever since radio control systems have been available commercially, modelers have gone to sometimes extreme lengths to retrofit them into items not intended necessarily for R/C. GI Joes have been given servo-controlled arms to maneuver an R/C released parachute after being dropped from an R/C airplane or helicopter. Stuffed animals and dolls have been fitted with motors and controls to make them walk and move their arms. Cheap Styrofoam free flight gliders from the toy department of Walmart have had 2- or 3-channel R/C airborne systems installed to turn the $10 models into respectable thermaling machines. You can buy micro R/C systems and motor propulsion for installing on paper airplanes nowadays. This article reports on an effort to convert free running model cars into radio controlled vehicles - back when R/C systems were anything but small and light-weight.
How to add Radio Controls to Your Scale Model Auto
Here's what happened when we presented Howard McEntee with one of ITC's Mercer auto kits ... he soon had a remotely-controlled radio-operated car!
Staff members were so impressed by the Raceabout's lively performance and pin-point control we figgered our readers would like to get in on the fun, too.
Those realistic scale model autos in plastic kits make fine radio control subjects. Our R/C installation is easy to duplicate. To get a good turn of speed, a larger motor now drives our car; proportional control permits precise steering for obstacle races and model "gymkana's".
How could a dyed-in-the-wool R/Cer look at a speedster like ITC's 1911 Mercer Raceabout without getting ideas? (This comes with a pre-set control unit that steers it, flashes lights and switches the drive motor.) The ITC "gear box" was set aside, its .place taken by simple friction drive on one rear wheel, using a much larger motor than supplied in the kit. Otherwise, car was built about as described in ITC's instruction book.
Our assembly work started with Step 7. To beef up the two steering knuckles (parts 40 and 43) 13/16" lengths of 9/32" OD brass tubing were slotted at one end to slide over vertical portions of knuckles, unslotted portion acts as a new stub axle. Original axles are 1/4" dia. so tubing is a snug fit. Smear axle and tubing inside with plastic cement, slide tube in place, add plastic spacer (cut from unused kit parts) and bend slotted tube tips around knuckles, cementing brass well. Aurora cement sets rapidly, but takes time to really harden ... give it all it needs. Complete front axle assembly per instructions, cement to front springs.
Drill out front wheels to fit enlarged axles. We did this with a letter J drill, then ran a letter K through. Plastic drills cleanly if you go slow. - Drill a little, then withdraw drill to clean off chips and give plastic time to cool.
Rear wheels must turn freely so a new axle of 1/8" music wire was made. To give it rigidity, an "axle housing" of 1/8" ID brass tubing is run through plastic shaft hangers under car; this 5 3/4" long tubing must be well cemented. The pillow blocks (part 65) were not used, but the axles were cemented to rear springs with scrap plastic fillers. Also, 1/4" dia. pieces of red plastic "tree" clipped off and cemented against springs and axle supports beef-up assembly. Rear wheels' square hole is opened up with a #30 drill for the new shaft. To the 7 5/8" axle cement a brass washer at one end. Axle and wheels are held via model plane landing gear wheel collar (with setscrew) at other end.
Our substitute motor, G.M. Hobbies' Super-Monoperm, drives right rear wheel by friction. It must have a pivoted mounting so a spring will press it against the wheel. The broad brass "U" pivot attaches to motor base by 2-56 bolts. A sturdy aluminum or brass "axle" supports assembly; it bolts at one end to car's plastic underside, is braced at outer end by aluminum strip cemented to right rear fender. Holes in area allow cement to really bind tight. File rear spring on this side to clear motor mount. Fit these parts so none snag rear wheel or its brake drum, spacing must be close.
Motor has a short 3/32"-dia shaft which is extended and enlarged in diameter. Allow enough room between tubing and motor case to grip shaft with needle-nose pliers ... this to keep heat from soft plastic motor case. To gain more speed, we later soldered 3/8" length of 1/8" ID tubing over smaller tube. This holds larger size fuel tubing which contacts the rear wheel.
Lights, horn, pumps and spare tire could be damaged in handling so don't mount till R/C and drive mechanism is complete. Then add all the details you desire.
A geared-down small motor is required for proportional steering. Our unit is from Citizen-Ship Actuator kit parts (or C-S type HRH, 2R2H, 1R3H or 3R servos - but no switching circuits are needed). Basically a double-geared Mighty Midget motor, this is much like servos for proportional plane steering. We omitted contact strip and switching discs; you could probably utilize some of these parts to switch on car lights when wheels turn to extremes. Before motor is fastened in case, tin stub end of shaft (don't get solder on pinion teeth). Attach a hook to shaft for centering rubber band; ours is 3/4" length of 1/16" ID brass tubing, with one end flattened and bent to hook shape. Heat end red hot then let it cool before you flatten it. Open hole in case end to clear hook shank, clean inside of tubing and tin it, install hook after motor is in place. The kit's 30 ohm resistor soldered across motor's terminals acts as arc suppressor to keep relay contacts clean. This resistor, somewhat lower than necessary for our purpose, may be raised to 100 ohms to lessen overall current drain of servo. As added insurance against commutator sparking bothering receiver, wire 0.001 mf ceramic capacitor across terminals.
Servo is on ply mount cemented to upper surfaces of front springs and under side of plastic Rear Gear Box Support.
First the servo was centered (as shown) but later it occurred to us that if offset to left side of chassis, steering wheel could be linked to the front wheels - as kit's designers intended. Thus steering wheel turns as servo swings front wheels - an action that sure intrigues spectators! Lower end of Metal Steering Rod (part 96) was clipped short since servo and mount are in the way.
A short 1/16" soft iron wire link runs from servo crank (which projects down toward ground) to hole in plastic tie rod. Four wood screws hold servo.
Center area of chassis under seats holds receiver, its batteries, all power switches. All plastic projections here are cut off, 1/8" ply 2 1/2 x 3 1/2" is fitted to forward half of area. To this fasten battery case, angles to hold receiver power cable socket and vertical aluminum strip for rear support of centering rubber. Bolt holds latter strip and a wire hook to which one end of receiver retaining rubber band goes - other to hook cemented to chassis. Cement in 1/4" foam rubber to protect receiver from jolts - like when you goof and ram car into a table leg or wall!
Radio installation with servos and receiver (front & rear underside).
All switches mount on 1/8" ply strip 5 x 1 1/8 ", cemented inside chassis on left side. Rather large receiver cells are used since there is no need to skimp on size or weight; E93 Eveready alkaline dry cells should give at least 100 hours of service! Average receiver current drain is about 30-ma (5-ma with no tone, 60-ma with tone on). We had intended to have a superhet in the car; a sharply selective receiver would allow several vehicles to run at once on the 27 mc R/C spot frequencies. CG's Jupiter superhet worked fine - until the servo was turned on. Much time was spent trying to eliminate servo motor "electrical noise", but to no avail. While the servo motor made the receiver unhappy, drive motor knocked it out completely! It might be possible to use a superhet with careful shielding of all leads and perhaps RF chokes on the offending motors, but it didn't look like a job the newer RK'er could handle, so a CG Saturn was plugged in - and our troubles ended. This receiver comes with plug on power cable and socket connections as shown.
It was our intention to run antenna under fenders out of sight; this worked up to 30' range. Farther from transmitter dead spots were encountered when car turned in certain positions relative to the transmitter. A wire all around car, under fenders and chassis, didn't help much. So - while original Mercer Raceabout had no radio antenna, ours does! It's a 16" length of 1/32" music wire raked rearward along right rear fender; small spring fastener at lower end allows easy removal. Antenna which permits 70' reliable operating range could be shorter if you don't want to run the car that far away.
The same cells are used for drive and steering motors. Since average current drain when running straight ahead on a smooth surface is 425-ma, rechargeable cells seemed the answer so four nickel-cad units are carried under the rear deck. Four 1 AH cells taped into a black cylinder are button-style Eveready N4OT's. The "T" means they have solder tabs which makes for easy connections. Since cells never require servicing they may be soldered into circuit. To make charging easy install 2-pin connector in an inconspicuous spot so charger may be connected without removal of rear deck. Cell pack is kept from rattling by foam rubber cemented in car "fuel tank". Tank and rear deck snap firmly into place, may be removed any time.
N4OT cells, new to Eveready line, may not be widely available, though any distributor can obtain them for you. We did quite a bit of car testing with four Eveready N52 cells, long thin cylinders of pen cell diameter about twice as long; they are rated at 900-maH capacity. Lower in cost they may be easier to get. Stocked by mail order radio houses such as Lafayette, they also have solder tabs. N52 has less capacity and it is not intended for such heavy duty as the N4OT.
This completes the car, except for such plastic scale details you may want to add and, of course, coloring.
Now about the transmitter? Short range and average power are all we need, so let's employ one of the all-transistor transmitters. We picked Babcock's BCT-16 (though tiny, this has ample power to operate model planes at standard flight distances).
For proportional operation we need a pulser, again transistors do the job - via a C&S Model CS-504 Pulsi-Tran. The BCT-16 comes with a keying button in case front, but has no external on-off switch. Actually, a switch inside changes output from keyed CW to keyed CW and tone. When this switch is in position #1, no current flows unless key is depressed. This transmitter comes with a red plug in its side; an accessory is a green plug - when latter is in use, and internal switch in position #2, transmitter sends out constant CW signal, and tone is keyed on and off. In view of electrical noise of two motors in car, we feel latter mode of operation preferable and advise use of green plug. (You can make one using connections shown.)
With green plug, current drain on 9-volt battery is constant so a larger battery than Eveready #216 which fits inside case is dictated. Two of these may be paralleled, or you can tape a larger #246 battery to case bottom as we did. We have a push-pull wire coming out case side so transmitter internal switch can be moved to "off" #1 position when not in use. (We hear later BCT-16 models will have external on-off switch, also room for a #246 battery inside.
To make a single handy unit, we attached C&S Pulsi-Tran to BCT-16 case front after removing key switch. Two leads from pulser connect in place of key.
What about operation? We suggest you keep car speed low till you get knack of steering. To do this, use only 3/32" ID brass tubing on drive motor shaft, and very thin rubber or plastic tubing over it (fuel tubing works fine). Later you can speed up the auto by rubber or plastic of 1/4" diameter or more - this will put more load on drive motor so you'll have to recharge batteries more often. Motor must press against left rear wheel with moderate tension; a coil spring will do the job, run from one motor base hole back to rear axle housing, or to spring shackle. Use as little tension as possible, but get positive friction drive without slipping.
If steering motor turns wheels opposite to direction you move pulser control knob, reverse connections to motor. If drive motor moves car in reverse, shift built-in reversing switch on motor. A single rubber band 1/8" wide, 3 1/2" long centered the steering servo. Too much rubber won't permit sharp turns - too little will result in a "wandering neutral". C&S pulser is ideal for this since it has a trim control which allows you to set car for straight running, with steering knob centered. Also, its pulse rate is high enough so front wheels have very little wiggle. Our car travels a smooth path, either straight or in turns of any radius.
While most would consider this a simple R/C installation, you can add to it to obtain drive motor control, perhaps forward and reverse, even change of speed, all on the same single channel radio equipment. A "pulse omission detector" (POD) can be used directly to start and stop the drive motor, or it could be linked to another servo which will allow the more complex forward-stop-reverse action. Car is large enough to take all this equipment under its hood, chassis and rear deck so appearance won't be spoiled. We've given you a basic racer - now let's see what more you can do with it!
Parts Required: ITC Mercer Raceabout kit; CG Saturn 3 volt super-regen receiver; Citizen-Ship Actuator kit; Super-Monoperm electric drive motor (GM Hobby Spec.); four Eveready N4OT nickel-cad cells (or N52 - see text); two Eveready E93 C-size dry cells; Acme battery holder; three SPST slide switches.
Also: Brass tubing, rod, cement, plywood, other small parts; Babcock BCT-16 all-transistor tone transmitter; C&S Pulsi-Tran proportional pulser; Eveready #246 9-volt battery for transmitter; three Eveready pencells for pulser; one green plug for transmitter (or 9-pin plug to make your own).
This advertisement for a 16" Motorized Giant Bobtail model car appeared on a page adjacent to the article. It undoubtedly was meant to be an enticement to modelers thinking about making the kind of R/C conversion detailed in the article. Amazingly, you can still buy that kit from Lindberg Products. According to the BLS's Inflation Calculator, the $9.98 price in 1962 is the equivalent to $78.23 in 2014 money. The current Amazon price for the Lindberg Bobtail is $75.28 - amazing!
Posted February 21, 2015