TO TREE OR NOT TO TREE - Derby Tech - July, 1984

by George Brower


An axle testing machine can measure more than relative axle speed. I have participated in numerous experiments with different ideas including axle tree comparisons, axle doubling, four point suspension stabilizing bars, and axle stressing. The test results will be shared with you as will my opinion of these figures. Similar tests allowed Shelly Brower to win both the 1977 and 1979 NDR National Championships. In 1977 tests showed that the unbalanced loading of axles to be quite an advantage. In 1979 the tester clearly showed that stressing the axles with cables made them slower, and the more the cables were tightened the slower the performance. And yet, cars with cable stressed axles were much faster than cars with bare axles. The deduction was then made that the cables reduced a drag to a large degree, and could be made more streamlined with elongated tubing.

The machine tests were performed according to the guidelines discussed the April (1984) issue of Derby Tech. The difference this time was that the medium speed axles being tested were run against #1 axle, then they were modified and tested against #1 again for comparisons. The five modifications were:

1. Axle Trees - The trees were made from pine, fir, oak, polyurethane foam and thin wall aluminum. They were streamlined to a similar size and shap and attached flush against the axle. The first modification tests were run with plastic tape holding the trees to the axles. The second tests we conducted with the trees glued to the axles, using 5 minute epoxy, and the third set of tests were done with the axle trees fiberg1assed into place using one layer of 4 ounce (3.74) cloth and one coat of laminating polyester resin covered with one coat of polyester finishing resin.

2 . Axle Doubling - A,double axle is, as the name implies, two axles, with one having the spindles cut off. The axle fastening was done with four bolts equally spaced. The holes were drilled before testing. The spindles were rechecked for 0 - 0.

3 . Axle Stressing - A hole was drilled near the axle square stock end before testing. The modification was done by Putting cable through th holes and then tightened under a lot of pressure. The spindles were re-checked for 0 - 0.

4 . Four Point Suspension - Two 3/4" , 1 1/4" pieces of oak were bolted to the axle with a 3/16" bolt 1/2" from each end of the square stock.

5. Stabilizing Bars - Two stabilizing bars were bolted 2" from each end of the square stock, and the other end to each other to form a triangle, using 3/16" bolts. A third bar went from this junction to the king pin hole using a 1/4" bolt. The spindles were rechecked for 0 - 0.

The figures in the results charts represent only one axle, so they should be doubled. The numbers are useful and tell a good story, but not the whole story. The tester was not set up to measure the effects of streamlining. .

AXLE TREE RESULTS CHART

 

Column A Column B Column C Column D Column E Column F
Material Held on With Average % of #1 Before Modification Average % of #1 After Modification Average Slower (C-D) Slower in Track Inches (.61E) - see note below
Polyurethane Foam Tape

Glue

Fiberglass

108.7

102.3

110.1

104.4

96.5

102.4

4.3

5.8

7.9

2.6

3.5

4.8

Aluminum Tape

Glue

Fiberglass

108.7

103.8

106.6

102.4

96.7

98.2

6.3

7.1

8.4

3.8

4.3

5.1

Fir Tape

Glue

Fiberglass

108.7

108.6

101.7

102.3

100.7

93.0

6.4

7.9

8.7

3.9

4.8

5.3

Pine Tape

Glue

Fiberglass

108.7

107.2

105.5

101.6

99.0

96.2

7.1

8.2

9.3

4.3

5.0

5.7

Oak Tape

Glue

Fiberglass

108.7

105.9

102.4

100.6

96.8

92.5

8.1

9.1

9.9

4.9

5.6

6.0

Notes:

  1. The .61 conversion factor explanation is in the April issue of Derby Tech.
  2. The same axle was used in all Tape tests.

Before we get to streamlining, and trade-offs involved in derby racing, I want to list some of the charts "pure" findings.

  1. The unmodified axle is best, and the double axle is worst.
  2. The fastest axle trees are taped on, and the slowest are fiberglassed.
  3. Polyurethane foam is the fastest axle tree material.
  4. Stressing, four-point, and stabilizing bars all hinder performance.

The unmodified axle going down the hill is faster than being treed in every respect except one - streamlining. This aspect easily overcompensates all the others. Two axles taped with fir axle trees are slowed by 7.8 inches, but on a hill they help well over a car length in streamlining. A net trade off benefit of plenty. Obviously, the axles should be streamlined. Working from this statement and using the results chart, it is clear that the taped polyurethane foam trees are the fastest. But are they the best? They have negative trade-offs that may make you select a different type of attachment. Taped trees are easily damaged, especially polyurethane foam. They do not remain lined up perfectly with the axle, and enlarge the streamlined area. Tape stretches and bubbles, again increasing the area.

AXLE ASSEMBLY RESULTS CHART

ASSEMBLY MATERIAL Average % of #1 Before Modification Average % of #1 After Modification Average Slower (C-D) Slower in Track Inches (.61E)
 DOUBLE AXLE BOLTS 101.2 78.0 23.2 14.1
STRESSED WITH CABLES 103.5 95.5 7.6 4.6
FOUR POINT BOLTS & TWO 3/4" X 1 1/2" OAK 108.7 92.1 16.6 10.1
STABLIZING BARS 1/8" X 1" METAL STRAPS & BOLTS 106.0 94.5 11.5 7.0

 In conclusion, axle trees should be used, and with as little extraneous attachments as possible. The, selection of the tree material and fastening must be weighed and decided upon based.on ease of construction, durability and speed. I prefer glassed polyurethane foam or glassed fir trees on a car that is going to be in a number of different races. For pure speed at single important race, taped polyurethane foam is my choice.


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