Magnetic Downforce Scale Apparatus - Adjustable

This is a simple concept that has been used by many over the years, usually using various bits of wood and other hardware. I designed this to be as simple as possible to make using your printer and minimal tools and hardware. Using the Fusion 360 (f3d) file, you can easily change the parameters to change the models for your own scale and slot car track rails. The provided STL files will only work properly for Scalextric Sport (½ straight) track rails and an electronic scale like the one shown in the photos. The height of that scale is 30mm, so if yours is the same height from the table to the top of the scale pad, then these models should work with your scale. 

While this version allows you to tune the readings by adjusting the height of the rails, the numbers you get from this apparatus are for comparative purposes only. If you race in a club with a downforce limit, you will want to adjust your MDSA to match the readings on their device as closely as possible, and then note the difference that can't be eliminated. This is not meant for club use, unless this IS the club scale. This is meant for the average user to have a simple device with which to compare his cars for racing on his track under his own rules. The number itself is not meaningful in any way other than to see which cars have more or less downforce, and to hopefully group cars in a way that encourages even and fair racing. Good? OK, then let's go…

The first step of the process is to figure out how large the plate that rests on the scale needs to be. Find your longest car, and your widest car, and measure them in millimeters. Enter those numbers into the parameters for MaxCarLength and MaxCarWidth. 

Next, decide how much extra rail you want beyond that length to go into the rail carriers. The default value of 10mm is usually enough, but you can make them go deeper if you want. That parameter is called CarrierDepth.

Refer to the value shown for RailLength, and get a piece of spare/scrap track that is at least that long. Cut away the track surface, keeping only the rails and the plastic that holds the rails in place, including the slot between them. Most track can either be cut with a sharp knife (aka Stanley knife), or scored and then snapped apart at the score line. See photo for example using Scalextric Sport track. Remove any pins on the ends of the rails, if possible, or cut them off or bend them inward so that they don't protrude. Then cut the rail to the length determined earlier.

Once you have cut out the rails and trimmed to length, you'll need to measure the width (RailWidth) of the piece and the height/thickness (RailHeight) of it. This will determine the size of the pockets that hold the rails in the apparatus as well as the channel in the plate. Enter those values into those parameters in Fusion.

Get a small scale such as for baking or postage (See image) and grab measurements from it. Scalepadlength and scalepadwidth are for the actual scale pad surface itself. This shape is centered under the floating plate the car rests on to pull against the rails and therefore press against the scale. We want this to be as centered as possible, so only measure the actual pad of the scale. Scalepadextension is how far the scale extends from the pad, where the buttons and screen usually are. ScaleCaseLength is the total length of the scale from front to back, including buttons/display/case. ScaleCaseWidth is the same from side to side. Finally, scaleheight is the measurement of the top of the scale pad off the surface the scale sits on. Be sure to account for any feet that may be on the bottom of the scale.

Plateheight is based on the railheight and raildown parameters, plus 3mm to ensure a stiff/strong plate. Raildown is how much downward motion you think is necessary for adequate adjustability. The default of 2mm should be sufficient for most users. There is an intentional gap of 2mm between the bottom of the rails and the top of the channel in the plate. This is to ensure that the car wheels are the only thing pressing down on the plate.

There is also an intentional 1mm gap between the legs and the plate, as well as between the rails in the sides of the channel in the plate. This is to ensure that the plate floats freely, so that all the magnetic pull is transferred to the scale.

After you've made your edits, export the models as STL files, or the whole thing as a STEP file, and then import that into your preferred slicer program to print. Note that the legs are designed with support bridging and should be printed upside-down with the rail carrier channel nearer the build plate. The STL in this print is provided in the appropriate print orientation. 

The legs are held to the base using 4 m3x10mm socket head cap screws, but whatever works will do. The rails are held in by friction, but you could always slather some glue in the pockets if you felt the need.

The rail carriers are adjusted using an M6x1 40mm socket head cap screw in each leg, held in place on top with an M6x1 nylock nut. You will need to insert the screw from the bottom of the leg, then insert the rail carrier and thread the screw through it and through the top of the leg. Secure with the nylock nut and tighten down to the top of the leg. It should be just loose enough to allow free rotation of the screw and not deform the top of the leg. You should be able to adjust the rail carrier by tuning the screw or by turning the nut (which is locked to the screw).

The precision and repeatability of the MDSA will depend a lot on the quality of the rails you use (new versus used and all bent up) as well as the quality of the scale you use. A scale that has a loose/wobbly pad won't get repeatable readings compared to a scale with a fairly solid pad. Don't skimp too much on the scale if you have to buy a new one. Sometimes a larger scale is better than the tiny ones lots of slot car guys use for things. Also, the guide resting on the rails can affect the reading, especially if it has magnets in the center or near the front of the chassis. If possible, removing the guide or braids can help alleviate that for a better reading.

To use the MDSA, place your car on it's side, OFF the rails, on one side of the plate, and tare your scale to eliminate the weight of the plate and the car. Then, place the car on it's wheels above the rails. This will provide a measurement of how many grams/ounces of downforce is being created by that car over those rails. If you want the readings to match those of another device, turn the screws or nuts as needed to raise or lower the rails. Raising the rails will increase the reading, and lowering the rails will decrease them. Change a small amount at a time and test the scale by starting from zero again. 

Remember, magnetic downforce is only ONE of many factors that impact the performance of your slot cars. Don't ignore the tires!

If you can't do the changes yourself, feel free to message me with the measurements needed, and I'll upload the models for you. The Fusion 360 file currently has parameters to accommodate a 1/32 scale modern F1 car and Carrera 132/124 rails.

Pretty much a complete redesign for parametric editing so that it doesn't mess up when you enter certain values in some parameters.