Build Your Own Dynometer

by Paul Shoemaker

Wouldn’t it be great to be able to analyze, in detail, the motor performance of your top flight slot car, without having to run lap after lap? One to one engine builders use dynometers to rate the horse power their power plants are pushing out, so why not use that same concept to rate and compare our scale machines?

This is how it works: All electric motors are capable of generating current in volts and amperes as well as absorbing them to run. By capturing how much voltage a given motor generates and how much the tested car draws in Amps, we can compare performance and gauge whether changes we make are truly improvements. Now this particular design goes one step further. By capturing the voltage and amperage changes made at the drive wheels we can also gauge gear changes and even tire diameter differences in performance. We do this by driving a roller that in turn drives an electric motor whose performance properties are known. It’s just a matter of recording the differences in performance from car to car, change to change, to see how your cars perform and what changes make the biggest improvements. I’ve included pictures of the unit I have built along with schematics so you can build your own. I used analog gauges (they were cheaper) but digital LED gauges can be used as well.

So let’s get building....

Parts List:

Tools Needed:

The design is very simple. You need a power source and rails or strips to deliver the power to the car’s pick up system. I placed bolts in the casing to clip a variety of power sources to. If you only use one type of power, you can hard wire you power source to the unit in place of using the bolts shown here. A roller machinism, with minimal friction, is placed just above the top surface of the project box so that the tires of the car tested will spin the roller freely. I used a Plastruct 3/16 inch tube for the roller body and slid Tyco standard X-2 rear tires over the tube to add traction. A 1/8th inch drill blank was inserted into the tube and held in place by a brass bracket with bearings placed for the axle to ride in. An old Aurora brass idler gear (large hole) was drilled out to fit on the axle and then soldered into place. A high performance can motor (my own wind of 36 gauge wire, epoxied and balanced) with an Aurora brass pinion gear soldered in place and mounted so the axle gear will smoothly drive the motor. Make sure these components have a minimal amount of fiction as possible. This will make the readings more consistent as the unit gets older and components break in. All of this assembly is held in place by a frame made of bent brass strip (1/32" x 1/2") into the shape illustrated in the pictures below. Solder the motor housing to the brass frame making sure your gear mesh is not binding. Now solder two 12" sections of #16 wire to the motor leads. These will be used later...

Now we will make the power rails for the car to get juice from. Take your one of your project boxes and cut a slot down the center about 4 1/2 inches form one end at least 1/16th of an inch wide or 1/8th inch if you want to use this for larger scales too. (See pictures for example). Cut a slightlty wider notch on each side of the slot to make clearance for the electrical connections. Take two pieces of the brass strip and cut them 4 1/2 inches long. Then drill three holes in each strip and counter sink the holes so the bolts will set flush to the strip’s surface. Using a good 2 part epoxy, glue the brass strips on either side of the slot you cut. After the epoxy sets, drill holes through the project box where each counter sunk hole is in the brass strips. Bolt down the strips.
Let this assembly set up for a couple of hours until the epoxy has had a chance to cure. On the underside, where you made the notches, solder two pieces of # 16 wire to the strips. Be careful to not get the strips too hot and melt your project box! Next we drill two holes for the power bolts and bolt them in place using the 2 nuts and washers to secure them.

Cut your slot for the roller assembly to fit through. Place this cut about 1/8th of an inch from the power strips. Using the roller assembly as a guide, mark where you will need to mount the rollers. Drill the mounting holes in the brass strip and the project box and bolt on the roller assembly. You should have something similar to this.

I cut the second project box at an angle so the gauges would be easier to read. This is not required. Mark and make the cuts required for your meters. Make sure you leave enough clearance for the meters to fit snuggly in the holes Also drill the holes for the on/off and forward/reverse switches. Set the project boxes together as pictures and mark and drill holes to mount the boxes together. Mount each of the components and wire them according to the diagrams below. I split the wiring diagrams into three sections so they will be easier to read.

Diagram 1: Component Location Template

Diagram 2: Wiring DC Motor and Volt Meter

Diagram 3: Wiring Power, Switches and Ammeter

This tool will greatly aide you in analyzing what changes have an effect on the performance of your cars. As you analyze more cars, you will be able to run comparisions of different makes and classes and easily gauge your perfomance. Once you know you have all the power you can squeeze out of the motor, you can place more concentration on handling, set up and driving skills.