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Derby Math and Science and Such

Masters of Gravity

Masters of Gravity
Teacher Guide

Physics involved in
Soap Box Derby Racing

Instructional television programs and classroom activities using soap box derby concepts to help students learn.

Teacher Guide for Masters of Gravity.

154 pages, .pdf file.

Physics of Soap Box Derby Racing. Energy, Acceleration...
53 pages, .pdf file

Soap Box Derby
Senior Design Project

AutoZine article
on Aerodynamics

Red Bull Racing’s ultimate soapbox-building guide

University of Akron Mechanical Engineering senior design project to build a soap box racer, .pdf file

Aerodynamics article from AutoZine. Drag and Lift, Aerodynamic Aid, etc.

Tips and hints on building a soap box racer, from the fine people at Red Bull Racing.

The Surprising Complexity
of Soapbox Derby Racing

Article by a guy who built a soapbox racer with his son.

Seating Dimensions for different ages
11.02.23

What derby car dimensions are comfortable for your kids? I'd really like to know so I can design cars to fit.

My daughter had one of the fastest cars in the race, but it was too small to fit anyone older than about 10 or 11. At any given time I have between two and four cars that I can loan out for the race, and I'd like to build cars that could be used by people of any age.

I’ve designed a pretty slick mechanism that will make the brakes adjustable on a derby car. I think I can get away with just three positions on them, but I can put as many as I want.

But that may be more adjustment even than I need.

My idea is that the brakes and the steering wheel can be fixed, and then the seat can move to accommodate different sizes. The premise is that the person with longer legs for the brakes will also have longer arms for the steering wheel.

 

So it would be interesting if you'd set your kids on the floor, knees up just to a comfortable position to push on brakes, and let me know what dimensions work best for them from the seatback to the brakes and to the steering wheel, and the height of the steering wheel.

Obviously, I don’t need names, but ages might be helpful. You can put the gender if you want, but I just need a sampling to know how much adjustment to put into the design.

 

While you’re having fun doing that, you might as well lay out a large piece of cardboard and draw the outline of a derby car they would like to build. It’s not too early to get them excited and drawing designs for cars for the derby next August.

Timing advantage from car length
11.02.23

Since the starting bar indexes off the nose of the car and the finish line stops the timer when the front wheels hit, there's always some discussion about how much advantage a shorter nose might give you.

This same effect comes into play with the Colt class, who are started from their back wheels and are also timed by when the front wheels cross the tube.

Well, here's your answer.

Not much.

Bearing Lubricant Test
11.02.23

I conducted a set of tests to determine what lubricant is most effective at reducing friction on a derby car wheel. This is the 13” Harbor Freight internal bearing wheel specified in the rules.

I tested five lubricants —Cooking oil, UltraTech spray lubricant, WD40, motor oil, and graphite—against a baseline of a dry bearing.

Test procedure

We mounted the wheel on a stand and attached a strip of reflective tape to its periphery. We spun the wheel up to an angular velocity higher than a pre-determined initial velocity, as determined by an optical tachometer. We started the time started at the predetermined upper RPM and stopped when the wheel had slowed to the lower RPM.

Between each test the bearing was washed out with solvent and water, then blown dry before the next lubricant was applied. After the six lubricants were all tested the series was repeated two more times in a different random ordered determined by a roll of dice.

Results

The wheel maintained its speed longest with the graphite lubricant. The particular motor oil used in the test performed worse than the spray lubricant, WD40, and cooking oil. The dry bearing, as expected, lost speed most quickly.

Note: This wheel was not under load and the results may not definitively represent how the lubricants would perform carrying the weight of the car.

Here is a .pdf file of this test.

Weighing a Derby Car on the Ramp
11.01.23

For some time I've been messing with how to weigh the cars. Back in the Salem Days days I used a giant ramp that I built. It worked pretty well, but it was heavy and hard to move around.

For 2024 I am building a svelte scale that can be easily used and moved around.

But quite a while ago I came up with the idea of just weighing the car on the ramp. My original idea was to use the starting posts themselves as the scale, but every car touches the post at a different height, so the idea of using that moment had some issues that would have had to been worked out.

But the idea is solid. Basically you attach a cord to the car, pull it back parallel to the ramp, and read the weight on a strain gauge. Once you know the angle of the ramp you scale the indicated weight to give you the weight of the car.

And Bob's your uncle.

You can print off this .pdf and make a weight calculator of your own, but I can't imagine why anyone would want to.

 

   

  Scaling a Derby Car
  10.10.23

If you're building a car based on a full-sized vehicle you can use the tires for a scale factor. Just divide the 13" diameter of the derby tire by the diameter of the full-sized tire. Use this factor to scale things like the wheelbase, length, and width.

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This table shows the derby car wheelbase based on the full-scale tire size and wheelbase. The outlined cells correspond to the cars in the list below this table.

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This is a table of derby car dimensions for some actual vehicles.
The lines around the cars correspond to the lines in the cells of the table above.
Click on the thumbnail for the full-size image.

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This spreadsheet has all this happy horse puckey in a form you can manipulate yourself.

Our Gravity Gran Prix has no affiliation with The All-American Soap Box Derby