Magnetic Downforce Scale Apparatus

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 exactly like the one shown in the photos. The height of that scale is 17mm, 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. 

Note that 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 at least compare readings on that device with readings on yours, to know what the difference is. 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 necessarily accurate to the real magnetic downforce of the car, or important 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 get a spare piece of the kind of slot car track you have. You're going to 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.

Once you have cut out the rails, you'll need to measure the total length of the rails (remove or cut off any pins at the ends), as well as the width of the piece and the height/thickness of it. This will determine the size of the pockets in the legs that hold the rails in the apparatus as well as the channel in the plate. If you prefer, you can find a piece of flat steel bar that can serve the same purpose as the rails and cut to an appropriate length (about 175mm). These measurements go into the following parameters in Fusion 360: raillength, railheight, and railwidth. 

Once you have your rails (or steel bar), you'll need 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. Finally, scaleheight is the measurement 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.

Overall length is based on raillength plus 10mm, to provide space for the legs. 

Plateheight is based on the railheight, plus 5mm to ensure a stiff/strong plate. 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.

Width is the final parameter to set. This is based on your track lane width, or at least the width of your widest car. This is currently set to 77mm, which is the lane width of Scalextric Sport 1/32 size track.

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.

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.

To increase precision and repeatability, it is advised to secure the scale to the base as well as the plate to the scale pad. Some ideas for this include 3M “Command” strips, regular double-sided tape, or blue painters tape on both surfaces and a drop or two of super glue to bond them together. Preventing the scale from moving from a centered position, and preventing the plate from moving or tilting will help the apparatus provide more consistent readings.

To use the scale, place your car on it's side, OFF the rails, on one side of the plate, and tare your scale. 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. Note that this measurement will likely differ from measurements taken on other devices, even other makes of this apparatus. This tool is to help you compare cars so that you can work to achieve performance parity between multiple cars that are to be raced side by side. Remember, magnetic downforce is only ONE of many factors that impact the performance of your slot cars. Don't ignore the tires!

PS. If you can't do the changes yourself, feel free to message me with the measurements needed, and I'll put up a remix for you.

If you want to get a scale like the one I used, this one appears to be close. Another user has measured it to be 19mm high, so I made a set of legs for it. This is NOT an affiliate link.

https://www.amazon.com/dp/B07F5SCDD3

Update: 9/25 - Increased the fillet size so that the Scalextric Sport version would fit on a Prusa Mini (180x180) printer. Added legs suitable for a 19mm tall scale (base to scale pad). Also changed file names to reflect what track it's made to fit. As people ask for customized versions, I'll add those to this model.

Update: 9/27 - Added models for a larger base and taller legs to accommodate a larger scale (see photos).