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1/24 and 1/25 SCALE MODEL CARS

How often have modeler's admired the detail and realism obtainable with inexpensive 1/24 and 1/25 scale plastic model cars .... and then wished there were some way to motorize the rear wheels in a static display, pose the front wheels at different steering angles, or even provide for full radio control? This paper was prepared to provide information to help the modeler attain these goals. It may also prove helpful in adapting other scale cars to radio control.

These models are lightweight plastic and inexpensive, while still providing wonderfully realistic detail. The cars are small in size. Being only six inches long and two or three inches wide, they are delicate, even fragile, so, rough use is not intended. Do not expect your model to go out clod busting, it won't. These cars are best run on hard, dry smooth surfaces. Bedroom, kitchen, or basement floors are all suitable. The cars offer wonderful slow speed control but are fast enough on hard surfaces to be fun. They will go forward and backward under full control so parking, gymkhana, and table top circle track racing are all possible ... and the finished installation is as "invisible" as possible.

Radio control will require:
       1. Steerable free rolling front wheels
       2. A durable running gear
       3. Proportional steering control
       4. A "Live" rear axle
          a. replaceable gears
          b. available parts
       5. Motor installation
       6. Speed control capable of table-top control
       7. Battery selection
       8. Radio transmitter and receiver
       9. Servo selection and adaptation

How much will it cost?

You should be able to build a MM Scale R/C car for under $50. This would include one small servo, a live rearend, and steering, but without a radio. A suitable radio can be purchased for about $60. With batteries and hardware, you needn't spend more than $130 ... total! A second car would cost only $30-$40! If you already have a two channel R/C radio it will cost very little. If you need to buy a radio the cost will be more, but even then, it will be far less than 1/10 and 1/12 R/C cars. You won't need an expensive speed controller, but, you will need to be aware of certain factors when you order your radio or purchase additional equipment.

A cost breakdown might be something like this:

                      1/24 - 1/25 Scale Car kit        $15.00
                      Hardware                          10.00
                      Rearend                           10.00
                      Radio*                            65.00
                      Servo*                            25.00
                      Receiver*                         30.00 (optional)
                              *first time expenses 

Building a steerable front axle

You will be building a fully functional early Ford style front end assembly from brass. The finished assembly will be capable of proportional steering, carry rolling front wheels, and be durable enough for MM Scale R/C car use.

Bill of Materials:

     2    2-56 Brass tie rod ends  Dubro #306
     1    1/8 square brass tube
     1    1/8  round brass tube
     1    1/32 brass wire

Forming an early FORD style front axle:

You can purchase a ready made front axle, adapt the R&D Unique's unit, or make your own axle from scratch.

The best method to bend the brass tubing to the proper shape is to insert a piece of copper wire into the tube, bend the tube and then pull the wire out. Another way is to just use a solid brass rod for the axle, this can't kink., but is harder to solder. Once the tube or rod is bent to the shape you require it can be cut to the proper width with a cutoff disc.

Next, solder the king pin bushings to each end of the axle tube. You can lay the axle on a sheet of ruled engineering paper as an alignment guide. Presolder the tip of one end of a length of1/16 tubing. Position the tube at an angle of about 3 degrees relative to perpendicular with the axle and solder the two together. Cut off the king pin bushing with the cut off disc. Do the other side the same way. You can clean up the joints with sandpaper, needle files, or emery paper. Your axle is done, now let's make the spindles.


Dubro threaded control rod ends make convenient front axles for steerable front wheels, they also provide for demounting of wheels. The axle nut can be secured with a dot of testors plastic glue or blue Loctite, and will provide nearly trouble free operation.

Cut the rod ends so that 1/8 inch is all that remains of the smooth unthreaded end that is still connected to the threaded portion. The threads are not to be disturbed. Sand the cut end smooth. Insert this into the end of the square tube stock. It should fit nicely. Use a good wet solder technique to secure these two parts. Cut it at 1/4 inch for the passenger side, and 1/2 inch for the drivers side, as you will be using the stock as a steering arm on the drivers side. Use the cutoff disc to fashion the spindle yoke from the square brass tubing. Drill a #72 hole for the king pin. Use pins cut to the correct length for king pins for a nice chrome effect, or use brass wire with a solder blob on one end.

Using 1/16 tube, pinch one end closed with a pliers to form a flat about an 1/8 of an inch long on the end. Drill the flat with a #72 pin drill. Solder this steering arm to bottom of the spindle. Do this for each side. Assemble the spindles to the axle using your king pins. Put on the wheels. Kind-of-a floppy mess huh.. or it should be.

One method of attaching the axle to the car is to solder the axle to an R&D front spring. Brass springs can be cut from shim stock, or a single heavier monoleaf can be cut from heavier gauge brass. Once the axle is secured to the car, the tie rod can be measured and cut. Use the engineering paper to aid alignment. You may need to cut and bend a few before you get the correct spacing of the steering arms. Try again, 'til you get it.

Several servos are adaptable to the steering requirements of MM Scale R/C cars. You should get the smallest you can afford. Space is at a premium in these cars and every opportunity to reduces the size of radio receiver, steering servo, batteries and controller board must be pursued. Futaba S3003, Cirrus Micro Molecular, these are the types you want, don't worry about torque, you don't need much, and speed is not all that important either ... but size is. Use the smallest you can find. You'll need the space.

Preparing the Futaba 3003 servo.

Usually the servo mounting tabs are sawn off to allow flush mounting against firewall, and to gain valuable space. The servo is simply glued to the floor, or frame, or a piece of printed circuit board cut to serve as a mounting plate. The steering control arm is made from 1/16 brass tubing, pinched and drilled at one end. After all but one of the servo controller arms have been removed with the cutoff disc, the remaining arm is relieved to accept the brass steering arm with CA or epoxy. You may wish to shorten the wiring harness by removing the bottom of the servo locating and noting the wire attachment points. Slide the collar toward the connector the appropriate distance. Cut the excess wire from the harness after desoldering at the board. Prepare the wire ends, strip and presolder. And then resolder to the correct locations. Be neat. Pay attention to the tip of your iron at all times. You need only about 1/16 inch of freshly tinned wire to make a quality connection. You can presolder the wire ends and then trim them with a side cutter if you wish, but try keep the joining end small. Once the servo and front end are mounted in location, you can cut the drag link from 1/16 brass wire. Bend a Zee into the tip of one end..the servo end, and an "L" at the wheel end You may require several tries to get the correct length. Solder a blob of solder on to the "L" end when you have it all into position.


A nice chassis can be made from rectangular brass tubing as in the one I have made for my 27 T Coupe. This is particularly applicable to vintage Hot Rods of T or 32 design. I used round tube for the front crosspiece and then rectangular tubing for the remainder. An easy way to build the dogleg for the rear end is to just cut the tubing at an angle and then solder it onto the piece cut off. You will take the dimension from your particular car kit.

For other cars, as in the case of NASCAR stock cars, 40 Fords, or more modern muscle cars the chassis supplied with the kit can often be used. The main consideration is wheel clearance for the rear wheels and steering clearance for the front wheels. I cannot go into all the variations on this theme, but by looking at the examples of my 27 T, and Kinser's sprint car you can get a pretty good idea of what is required.

The NASCAR I built is using the supplied plastic suspension parts along with the LIVE NINE rear end. With this car I used a Speed 280 motor and a Jeti 04 speed controller. This will not allow reverse but offers lotsa GO!! Proof of performance is it's ability to spin the victory doughnuts.

Building the "Ford" 9 inch rear end.

The basic LIVE NINE kit consists of:

               one third member (FRONT COVER)
               one rear end cover

Additional materials are required:

                TOMY/AFX SUPER G Tune Up Kit 
                 (ring and pinion gear set with axle shaft)
                12"  1/16" Steel Rod 
                12"  3/32" Brass tube
                12"  1/8" Brass Tube

                CA cement
                JB Weld KWIK or 5 Minute Epoxy

Be careful not to bend any of the tubes or rods!
Do not apply oil to tubes or rods until assembly is completed.
This will interfere with your ability to glue or solder them.
Rods and tubes may be polished with OOO or OOOO steel wool, but sanding will detract from appearance, and will detract from smooth operation.

Tools required:

                  Eye Protection
                  Dremel tool or equivalent
                  Dremel cut off disc
                  Zona saw or Jeweler's saw
                  Hobby knife
                  400 "wet-or-dry" sandpaper

Parts you will be making:

                  Pinion shaft carrier bushing
                  Pinion shaft
                  left and right axle tubes
                  left and right axles
                  wheel adaptor rods (optional)  
                  drive shaft
                  universal joint

Pinion shaft carrier bushing:

Using a cut off disc in a Dremel tool cut a 1/2" section of tubing from 3/32 stock. Place a piece of 400 sandpaper on a smooth flat surface. Sand the cut off end lightly so that it is smooth, square and burr free. Trial fit the bushing by slipping it over the 1/16" steel rod. It should spin freely with no gritty feel to it. If not, inspect both ends for burrs, and also be sure any visible grit from the cut off disc has been removed.

Pinion shaft:

Use the cut off disc to cut a 3/4" section of rod from the 1/16" steel rod stock. Sand or file the cut end to remove burrs and points. Use a pin to apply a pin point dot of CA to end of the pinion shaft and press the pinion gear onto the pinion shaft. Inspect for any excess glue. Remove excess glue if required.

Pinion shaft Assembly:

After glue has cured lightly oil pinion shaft and insert into pinion carrier bushing. It should spin freely, with no binding or rough spots. If not; clean the pieces, check for burrs, check for straightness, if not straight you may need to make new pieces. Do not glue this piece to third member until final assembly to be able to adjust gear mesh.

Your new third member is ready to use as is. It has been prepared for the axle tubes and carrier bushing as described in this paper. You may polish the exterior of both the third member and rear cover with OOO or OOOO steel wool, also 600 grit paper can be used to bring out details. Needle files can also be used. Be careful not to over do it. The metal is soft, the casting is already quite thin, and be sure to wash your hands after polishing the metal alloy third member. Do not paint the unfinished assembly as this may interfere with fitting the pinion bushing and axle tubes. Paint the finished rear axle assembly with clear to preserve your polished sheen or paint the color of your choice.

Assemble the model car's rearend and suspension assembly as much as possible, without paint or glue, inorder to obtain rear wheel spacing dimension and to become familiar with the suspension parts, some of which will be concerned with mounting your new "LIVE NINE" rearend.

Inspect the model kits rear end assembly and decide how wide the total axle width must be. The dimension will include the brake backing plates, in most cases, and is usually very similar to the car kits rear axle tube. When the kit is supplied with rolling back wheels the jobs half done.

Also, you should decide at this time which method of wheel attachment you are going to use. You have two choices, 3/32 brass tube axle or 1/16 steel adaptor wire. You can prepare the wheels for mounting at this time to gain a clearer picture of your final requirements, but do not cement the wheel assemblies until later.

Preparation of Wheels:

Become familiar with the assembly of the wheels by inspecting the instructions, preparing the parts and trial fitting the pieces. The wheels are attached to the rear axle by either of two methods; 1/16 steel wire adaptor, or directly to 1/8 inch axle.

Prepare all the required kit parts by carefully removing them from the sprue with a modeler's knife, carefully removing all traces of the sprue with either the knife, sand paper, or needle files. Remove molding flash when present. Be sure that tires and wheels can be assembled neatly and roundly to provide smoothest running.

The wheels are usually prepared by drilling the car kits existing inner wheel discs to accept either the wire or the axle. Alternately the car kits wheels can be drilled to accept the HO slot car wheels which can then be drilled to accept the adaptor wire method. Develop a method which will not disturb the outer wheel in any way. Do not drill through the face of those big beautiful chrome reverse wheels. A little careful thinking before hand will allow the wheels to be mounted without detracting from the outward scale appearance in any way. Using the 400 grit sand paper on your work area surface will allow you to "mill" the interiors of the inner and outer wheel assemblies to allow clearance for the HO flanged centers, if required. You should be able to trial assemble the wheels to check fit before glueing. Your axles should fit snugly, the wheel should run true when attached to an axle or adaptor wire and then spun between your thumb and finger. The finished wheel assembly should be well glued as neatly as possible. Use 5-Minute epoxy for permanent strength, or tack it together with CA for additional prefitting. Use a toothpick for tack glueing and by all means keep it off your skin, your eyes, and your work surface. Use a small piece of waxed paper or plastic lid off a disposable butter dish to dispense a drop, use the tooth pick, and throw it away when the building period is done. Keep it neat for better results!

Preparation of H.O. ring gear carrier:

If the rear axle assembly is equipped with wheels, remove them, but do not apply any pressure to ring gear. This is brittle plastic and may break, so ... holding each wheel pull them apart. One will come off. Then, hold the metal axle with a pliers and remove the remaining wheel, don't bend the axle, and do not put any pressure or nicks on the ring gear.

It is neccessary to remove part of the pinion gear shoulder plate inorder to have as small and narrow a third member as possible. Using a razor saw, jeweler's saw, or hobby knife, cut away outer shoulder of pinion guide plate so guide plate width is flush with largest diameter section.

Cut left and right axles from 3/32 stock, these will slip over the axle stubs on the HO ring gear assembly. Place a drop of CA on one of the HO axle stubs being careful not the get any on the guide plate or ring gear, slip the axle over the stub and clean any excess glue with a tooth pick. Do the same for the other side.

Two pictures of Kinsler's Sprint Car during mock-up!

Left and right axle tubes:

By now you should be familiar with the kits wheel spacing so you know the desired final dimension of the total rear axle width. Generally you will be wanting the pinion shaft to be located on center line of frame. You should try to align the finished rear end with the center line of the engine to assure vibration free operation and best performance. By positioning the third member in the desired position and knowing the total width of the finished rear end the axle tubes can be measured for each side. The axle tubes should be cut so that they will enter into the third member but not obstruct the ring gear and carrier. You can assemble the rear drive axle, ring gear and carrier, to determine the amount of engagement. If you are using the adaptor wire method you can "put'em in long and cut'em later".

Attention should be paid to the order of assembly at this point.

1. Insert pinion shaft and bushing into front of third member from the inside out. Pinion gear inside of case, drive shaft yoke outside. Push this full forward for now.
2. Lay the ring and axle assembly into the case. Note that there is a left and right side to the third member case. Position the ring gear on the side with the provision for it.
3. Slide the axle tubes over the axles and position them into the third member tube bosses.
4. Adjust the axle tubes to position the ring gear for engagement with the pinion
5. Slide the pinion bushing back to engage pinion.
6. When smooth operation is obtained, apply a drop of CA with the tooth pick to each axle tube boss and the pinion bushing. Remember, you are gluing the bushing and tubes to the housing not to the pinion shaft or axles. In the event that you have glued the pinion or axles, you will probably need to remove and clean them and then try again in order to assure a clean, free running rearend. Do not sand the tubes as this will raise the metal and interfere with the fit. Use fine steel wool to clean the tubes. Sometimes the glue can be cleanly chipped off after it has set, or cut new tubes. Do not apply pressure to ring gear and do not bend the HO axle stubs. The suspension parts may now be fit to the axle tubes. Use a 3/32 drill to prepare four-bar mounts or spring hangers. Test fit suspension parts to frame before glueing. Keep in mind the suspension mounting locations on the frame. Often times the axle tubes will fit the prepared holes sufficiently well as to preclude gluing. Do not mount the wheels or wire axle adaptors until you have worked out the suspension requirements. The wheels or adaptor wires can be inserted dry for test fitting. Once you have everything worked out mechanically you can paint your new live axle assembly. It is usually the case that all the suspension parts will have been slipped onto the axle tubes at this point and trial fit to the car with wheels temporarily inserted. This is to assure wheel clearance, fender clearance, alignment, spacing, etc., before painting.


Select a servo by removing the back plate and withdrawing the motor. Are there three wires going to the motor? ... No? Reassemble the servo. Choose a different servo. Yes? go on. Remove the motor, circuit board, and potentiometer from the case. Make a written record of the wire colors and locations on the motor. Draw a picture and label it or draw a diagram but be sure you can identify the correct locations from your notes, this will save time in the event you forget.

Desolder each of the three motor wires at the motor. Use a 15-25 watt iron, wait for it to get hot. Test the tip with a bit of fine solder to wet it, clean and rewet if neccessary. Apply a light tension to the wire you wish to remove and touch the wet tip to the solder spot, it should part briefly. Desolder the remaining wires in a similar manner.

You may use the motor direct drive or assemble a simple gear reduction drive using servo parts.

Gear reduction:

Inspect the servo gears which lie before you. Select the one which carries a large toothed wheel which matches the width of the motor gear. This is your new flywheel and gear reduction driven gear. Test fit this over a piece of 1/16 wire it should slide on with a nice fit but free to spin. If not drill the gear with a 1/16 bit. You can usually do this with your fingers or a pin vise. Cut a piece of 1/8 brass tube 1/8 inch long. This is your keeper collar.

Cut a piece of 1/16 wire of the correct length to run from the crank pulley mount to the transmission out put yoke (you can leave it an inch or two too long and cut it off when the engine is complete). This is your crankshaft wire.

Glue the keeper collar flush with one end of the crankshaft wire.

Cut a piece of 1/8 brass tube 5/8 inch long. This is your temporary main bearing support.

Slip the temporary bearing onto the collared crankshaft wire. Position this assembly against the side of the motor parallel with the motor shaft. Often times the motor magnets will assist with the alignment. Position the shaft so the keeper collar assumes the position of the crankshaft pulley. Now slip your selected reduction gear onto the shaft and engage it with the motor gear. You will now need to cut a piece which will fill the gap from the collar to the reduction gear once the temporary carrier is removed. This may require more than one attempt but do not despair you can always try again. Measure the gap accurately. Be sure the reduction gear is clear of the motor case when in mounting position. The Crankshaft carrier bushing will locate the reduction gear in the proper location once the carrier bushing is epoxied to the motor case. (fig.1) Check and deburr all ends recheck fit and adjust if neccessary. Now slip the finished carrier bushing onto the collared crank wire, also slip on the reduction gear. Slide both over to the collar. Hold the carrier sleeve between you fingers does the gear and crank spin freely? It should! Check for burrs, bends, points, or dirt. Remake if neccesary, it only takes a minute or two.

Record the motor wire locations on the controller circuit board. Use written notes, you will save time, later, if you forget.

Desolder the motor wires from the controller board the same way you did the motor wires.
The servo controller board will now become your speed control. Resolder wires of the appropriate length to those positions you have previously noted on your drawing or sketch, and route these to the correct location on the servo motor. When completed, the servo motor will run continuously in whichever direction you have set on your transmitter. Position the throttle in the neutral position and adjust the potentiometer until the motor stops. Now you can go forward or backward using the throttle joystick. This method of using a servo to provide both the speed and direction and to supply the motor for your car provides exquisite slow speed contol, adequate top speed, and all for a very economical price.

Quick Soldering tips:

The iron must be hot, the tip must be clean and just wet with solder, not dripping, Do not over heat the board. It should part cleanly and quickly. If not, check your tip and be sure you are making good contact with your target. Do not heat any other components. Be aware of all sides of the tip, so as not to heat other components. Use care! Pay attention and don't burn yourself. Remember to unplug the iron!! Some builder's rewire the motor directly to the board, others put pin plugs on the wires and pin sockets on the board. The sockets provides mobility of controller board, hard wiring promotes reliability, take your pick.

Hiding or disguising the servo motor can is a great opportunity for you to show some of your creativity. Preparing the motor for the model will require some forethought. Usually it is advantageous to preassemble the key components without paint or glue to check mounting locations, clearances, and alignment of motor shaft with respect to the rearend. Sometimes if the servo motor is small enough and that big block big enuf you can fit the entire servo motor right into the engine block, with trimming.

Otherwise, glue the heads, intake manifold and oil pan directly to the servo motor. It camouflages well. They are often of brushed aluminum. They can also be painted, carefully. Do not get paint on the gears, shaft or air slots, armature or core. Use the "dry brush" technique. Drawn out some of the paint on a scrap piece of cardboard or paper to prevent too much paint being on the brush. Using the oil pan can be advantageous as they often provide mounts, sometimes portions of the block can be glued to the servo to provide detail or engine mounts. Trim the parts with a hobby knife or Dremel or needle files. Use your trusty toothpicks to glue and be neat!

Keep all glue out of motor, crankshaft carrier, and gears!
Epoxy glue offers more control than CA, use it!
Mix small batches and work slowly and neatly!
Don't get glue in the motor, on crank carrier, or on gears!

Introduction to BATTERIES

You're selection of batteries will be determined mainly by how much room you have within the car. AA and AAA NiCd (nickle cadmium) batteries are the obvious choices. Smaller batteries are available when you need to put them into a sprint car or a bucket T. The duration of operation will depend upon the capacity of the battery and the amps used by all the on board equipment.

         Battery capacity = Current draw * time     and,

         Battery capacity/current draw = time


        Suppose your motor, receiver and servo draw 4 amps and 
        you are using batteries with 600 mah (milliamp*hour) capacity.

              600 milliamp hours/1000  = 0.6 amp*hours.

        In this case, your model will operate for

     (.6 amp*hours/4 amps) hours, or 0.15 hours or(*60 min/hr) = 9 minutes.

When you have installed 50 mah batteries into your sprint car, you're not going to be able to run your car for very long before it needs to be recharged, but then, a sprint race isn't very long, and the track is short. In this case .05/4 = .0125 hrs or .75 minutes!!

That's only 45 seconds! not much HUH!!! But, if your circle track is only 10 feet in diameter and it takes you 4 seconds to get around, you'll be good for a 10 lap race! Recharging time for these small batteries is not very long!

Another choice is NiMH (nickle metal hydride) batteries. These usually cost more, but offer increased capacity for a given size battery.

Also, there are Lithium batteries. These cost even more and require a special charger. But may be worth it if you want even longer duration.

Lithium batteries weigh half as much and carry approximately twice the capacity of NiCD batteries of the same physical size.

You also need to be aware of serial and parrallel methods of battery connections. Serial means one after the other, and parallel means side by side.

       [- +}[- +}[- +}[- +} .... this is an example of serial connection.

          The capacity remains the same, but the voltage delivered is
          the sum of all volts of the cells. Here, 4 x 1.2 = 4.8 volts.

        [- +}
        [- +}
        [- +}
        [- +}  this is parallel, the voltage is the same as a single cell,
               but the capacity is now four times an individual cell.

I have never had good luck using battery holders, they cause intermittent connections, and low current flow. It is better to solder wires directly to the battery poles. I use 18 gauge or 20 gauge wire, anything heavier is not needed in these small cars. Usually, stranded wire is easier to work with. The difficulty is in soldering these cells successfully!

The best method I've come up with is to sand or use a motor tool with a cut off disc to scruff up the battery poles and remove a small amount of the plating. My soldering iron does not generate enough heat to successfully complete the solder joint before cooling off from contact with the battery. I have been able to successfully solder to batteries by over-heating my iron with a propane torch, and then soldering quickly. I presolder the wire and the battery terminal and the preheat the tip of the iron and solder the wires on quite easily.

Other topics!

You have a model with a beautiful interior. Make it removeable and you'll be able to install radio control in less than five minutes. Coupes and muscle cars are easy. Pickups can use tonneau covers. Trailers are an option for bucket tees. The steering servo and motor control board and potentiometer can be mounted to a piece of styrene or fiber/epoxy board and then this can be taken as a unit from car to car. The cars operate well on table tops through the virtue of the trim control on the radio transmitter. High speed is obtained by the joystick or throttle trigger.

Copyright Robert Rydman 2002