Downforce, How Much You Got?

07/02/02

So, you finally bought that hot new car you’ve been waiting for. You tear it out of the case and throw it on the track to see how it runs. "Hmmm, did those rails just bend," you ask yourself. Yup, this thing is stuck, and the fastest thing on four wheels, but what if you want it to be equal with your other cars? You can spend hours testing or end up pulling the magnets out of everything. Here’s an answer that’ll really let you level the playing field.
Enter the Teamwild downforce scale. The short version goes like this: The car sits on a scale. The scale is then zeroed with the weight of the car on it. The car is then placed on a platform around steel suspended over the scale, the magnets cause the car to press down on the scale. Sounds simple doesn’t it? So here we go with the full version. First the main component is a digital scale. A search on the Internet found
the best solution to be the My Weigh I250. This scale has a max rating of about eight and a half ounces so it does have limits, but the price was right. We’ve since found them selling on Ebay for around fifty bucks. Also it has one hundredth of an ounce resolution, so it’s more than accurate for our needs. The second thing you’ll need is a piece of steel. The best size I’ve found to be is one and a quarter inches wide and a quarter of an inch thick. This is wide enough to cover most magnet locations and thick enough to prevent bending. Cut the steel ten inches long. Now comes the tricky part, unless you’re a machinist or real handy with the Dremel. There needs to be some clearance ten inches long. Now comes the tricky part, unless you’re a machinist or real handy with the Dremel. There needs to be some clearance for the flag so it doesn’t hold the car up off the weighing platform, a three-quarters wide by one inch long or so slot through the steel should do the trick.

With that accomplished we need something for the car to sit on. I decided to use balsa because it would allow for more usable range, also the scale doesn’t seem to like heavier items on it when it’s turned on and gives and error message. So, cut one piece of one-quarter by three inch balsa five inches long as a base. The platform will need to be open in the center for your steel to go through. Our platforms are around seven eights of an inch thick but

yours will vary depending on the height your steel is suspended at so lets do that now.

I’ve been shooting for a finished distance of one hundred thousandths of an inch (.100) from the top of the platform down to the steel. This seems to give a realistic reading. Cut a base seven and a half by ten inches out of plywood, old cabinets, marble or whatever you’ve got. You’ll need two uprights on either side which will suspend your steel over the scale about in the middle of is weighing pad. They will need to be high enough to give clearance for the balsa platform but not so high as to make life difficult. We’ve screwed ours on from the bottom and cut slots in the top for the steel to sit it.

Now, with our steel suspended over the scale we can get back to our balsa platform. Measure from the top of your balsa to the top of your steel and add .100 to the number. If you’re using primitive measuring equipment add 3/32 inch to whatever your ruler says. This will be more critical if you’re doing six scales for local racers and they all need to be the same, if its just for home comparison you don’t need it to be dead on. Next you’ll need to add more balsa to your base to be somewhere

near this measurement. Remember that space in the middle for the steel, plus some clearance so the balsa doesn’t catch on the steel and mess up the reading. Use glue to hold the balsa together. You can also laminate several layers of the one quarter by three and cut clearance in the center on a table saw, just watch that trigger finger.

By now you’re either totally confused or ready to measure some downforce. With just the balsa on the scale turn it on and wait for it to zero out. Place the car on the scale so the magnet is not reacting on the steel this will show the weight of the car. Hit the "tare" button, which will zero the scale to the weight of the car. Place the car on the balsa with the flag in the clearance slot. On cars with magnets in the front make sure the flag is as far back in the slot as it can go without hitting.

The number on the scale is the total magnetic downforce of your car. Note this is not the exact number and different tracks will vary, but it will let you compare all your cars or set maximum downforce rules for racing if you wish. For the finishing touch, glue a round magnet to the center of a flat Popsicle stick. Zero the scale for the weight of the stick and magnet and then place it across the balsa. The old Ninco magnet I used measured 1.50 ounces. Write the number you get on the stick for future setup of the scale or

comparison between scale setups. You can also move the stick around on the balsa to make sure the platform is flat, it usually isn’t. Use tape on the uprights or the bottom of the balsa to tune it to perfection.

That’s about it in a big nutshell. You’ll find many different uses for a scale as you start playing with one. A quick way to compare your magnets is to place a Popsicle stick across the balsa over the steel and place a magnet on the stick. This lets you find out just how good that magnet is before taking a whole car apart to change.

May the force be with you.

Matthew Wilder
E-Mail: mwilder@webryders.net

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