MATCHED WHEEL SETS - Derby Tech - June, 1984

by George Brower

Equalizing wheel sets is a difficult task. I state this from my experience in matching sets for derby races, axle testers, and hill experiments. I have also learned that there are definite keys to success in the area of preparing, testing, and grouping wheels.

I'm going to briefly describe some of the elements in each area, with added emphasis on the actual set groupings.


Wheel rolling consistency is the goal to achieve in preparing wheels for testing. To reach this end, the wheels are identified, commonsized, and cleaned.

IDENTIFIED - Each wheel is numbered and given a directional arrow. Once a wheel set has been grouped, the locations on the racer are marked.

CLEANED - Each wheel is simply cleaned all over, including the tires and bearings.

COMMONSIZED - Each wheel is balanced, ground, and equally weighted.

A. Balanced - The balancing prevents the long and short roll out at the end of a test.

B. Ground - The wheel is placed in a machine that grinds off the mold mark (in the center of the tire tread), and gives each wheel the same diameter. The wheels roll smoother and more consistent.

C. Equally weighted - This is the key element in preparing wheels for testing. Each wheel should weigh the same as every other wheel. A merry-go-round tester lets a heavy wheel obtain more velocity, and momentum, allowing it to roll farther than a lighter wheel. This difference is further compounded by the fact that actual racing favors the lighter wheel. On a hill for example, four light wheels that have tested at "6" will easily beat four heavy wheels that have also tested at "6".


It takes time to test wheels. A lot of time. The testing goals are consistency and accuracy. A lot of effort must be expended to achieve these goals. Proper wheel loading, equipment checking, and temperature controlling are the three important aspects of wheel testing.

  1. Proper wheel loading
  2. Equipment checking
  3. Temperature controlling

WHEEL LOADING - The total weight on the tester arm should be 62.5 pounds (senior division), including the axle and wheel. This weight simulates actual racing compression on the tire and loading of the bearings.

EQUIPMENT CHECKING - The testing machine should be periodically checked to see if it is measuring consistently. I remeasure a certain wheel at various times during the testing stage, and compare the results. If they are not consistent, I fix the equipment.

TEMPERATURE CONTROLLING - This key element requires the testing area to have temperature an humidity levels constant, during all of the tests, When it is hotter, for example, wheels run farther than they do when tested at a lower temperature.


The key element in grouping wheel sets is applying the proper equation. I will give you this equation and examples, that show the proper application.

EQUATION - Since all wheels do not run the same speed, an equation is required to match wheel sets. A friend of mine, from the Jet Propulsion Laboratory, developed the following basic equation, based upon the laws of motion, to determine each wheel set's effective distance rating (de):

To state this simply (if mass and acceleration are constant), "the effective distance rating for a wheel set is equal to 4 divided by the sum of one divided by the distance that each wheel traveled." This equation is used because grouping wheels only by their total revolutions can be highly inaccurate.

EXAMPLES - Lets say, for example that 100 wheel sets are to be grouped, an the following occurs:

A. For ease of computation and illustration, the wheel revolutions are shown in even numbers. They range from 2 to 10 revolutions, and average 6 revolutions. The highs (10's) and the lows (2's) are not used.

B. Each wheel set contains a total of 24 revolutions.

C. The wheels are grouped into four categories:

1. As many of the average wheels (6's) as possible are put into wheel sets. They are numbered wheel set #1 through #20.

2. Wheel sets #21 through #39 are made up of two 7's and two 5's.

3. Wheel sets #40 through #69 are made up of two 8's and two 4's.

4. Wheel sets #70 through #100 are made up of two 9's and two 3's.

Now its time to apply the equation to the example. The results of the computation show that category 1 with the four 6's has the best 'de' rating with a 6.00. Following in order are category 2, with a "de' of 5.83, category 3 with a 5.33, and category 4 with a 4.50. The computations and rankings put into chart form show the differences in the four categories as follows:

Wheel Set

Total Set Revolutions

Wheels Used

Effective Distance Rating

Actual Fastest Ranking

Percent of Fastest

Inches Slower than Fastest

#1 - 20







#21 - 39


7's & 5's





#40 - 69


8's & 4's





#70 - 100


9's & 3's





For another example, lets say that I have three wheels that run, 9, 9, and 3, and I want to add the fourth wheel and obtain an effective rating of 6.

The following variation of the basic equation helps provide the answer.

I must add another 9 to achieve an effective rating of 6. To put it simply - a wheel set of 9, 9, 9, and 3 equal a set of 6, 6, 6, and 6.


I want to summarize briefly by stating that the three most important areas of wheel set grouping are the effective distance rating, the wheel weight, and the temperature control. Without the proper attention to any one of these areas, I would just be spinning my wheels.

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