# A Method to Compare Pinewood Derby Wheel-Axle Frictions

## By Stan Pope

### Summary

The purpose of this document is to provide a sensitive method for comparing various Pinewood Derby wheel and axle treatments and lubricants. The method isolates wheel-axle friction from all of the other influences normally encountered by a pinewood car in it's trip down the track and from variations in unrelated car preparation that are usually present when comparing wheel-axle treatments by comparing car run times.

The method creates an analogy for all pertinent forces and measures and produce measurements which should be possible with commonly available hand-measurements.

### The Analogies

• Load - The weight of a ring and wheel sitting on an axle replaces the weight of a car body and axle pressing on a wheel.
• Inertia - The angular inertia of the ring and wheel replaces the linear inertia of the car body.
• Gravity Effects - Gravity pulling a weight on a thread replaces gravity pulling on the car. The thread released from the pin on the ring equates to the car reaching the bottom of the incline.
• Run Time - Total Spin Time of the ring and wheel replaces Elapsed time of a car from starting gate to finish line.

### Equipment

Form a "wheel" by fabricating a "ring" from wood.
The OD is about 2".
The ID is about 1/32" larger than a typical PWD wheel, allowing a press fit when shimmed with three strips of rubber band. (Using four strips of rubber band centers the wheel more reliably. The strips should be diametrically opposed and spaced approximately 90 degrees apart.)
The thickness of the ring should be about the same as a PWD wheel hub.
The total mass of the wheel assembly is about 2 ounces. The mass of the ring dominates. (With most wood, it will be necessary to augment the weight of the wood with diametrically opposed lead slugs. Others have formed the ring from steel or aluminum.)
The rim of the ring should allow attachment of a thread such that the thread may be secure tangentially but loose radially. A headless pin radially into the rim should work for this.

Construct a support which can hold the axle about 50 inches above the floor. The support should be adjustable so that various axle alignment modes can be compared. An eyescrew should also be attached to the support so that it is below and slightly to the outside of the edge of the ring. The support for the eyescres should be integrated with the support for the axle so that the eye of the eyescrew is in the plane of the spinning headless pin regardless of how the axle support is adjusted.

### Experiment setup

Mount an axle support about 50 inches above the floor so that a pwd axle can be inserted into the support horizontally, adjustable to level or inclined.

Mount a small eye screw on a support so that the eye lies in the plane of rotation and so that the thread makes a slight (5 degree) bend as it comes off the wheel, through the eye, and toward the floor. (The purpose of the eye screw is to assure that the thread comes off the wheel in its plane of rotation even if the axle is sloped off horizontal.)

Attach a weight of about 1/4 ounces onto a 4 foot(+) thread. Loop the other end. (The weight is chosen to induce the approximate the angular velocity of an actual wheel on a car at the bottom of the slope.)

Attach the loop to the headless pin in the rim and wind the length of thread around the rim.

The mass and moment of inertia of the wheel assembly, the amount of thread wound onto the ring, and the angle of thread deflection must remain constant.

### Experiment Measures

Measure and record the time elapsed from (1) release of the weight until (2) the wheel stops turning. Since these times will typically be in the tens of seconds, a common stopwatch provides adequate accuracy.

If the study includes lubricant lifetime, repeat trials without relubricating.

Alternative Measure 1: Measure and record the number of rotations completed by the wheel. Measured by LED + Photodiode across the wheel. Drill 1 hole to measure total rotations, drill N equally spaced holes to measure in units of 1/N rotations. Use off-the-shelf pulse counter. This is more complicated but possibly more accurate than hand timing.

Alternative Measure 2: Measure and record the time required for the wheel to make the number of rotations, R, that equates to the length of a track (typically 28 feet). Measured by LED + Photodiode across the wheel. Drill 1 hole to measure total rotations, drill N equally spaced holes to measure in units of 1/N rotations. First pulse starts the clockcounter. R time N + 1 pulse stops the clock. This is also more complicated, but may equate more exactly to times of a real car running on a real track.

### Treatments to Evaluate

Question 1: What treatment produces the lowest turning resistance under pwd loads?

1. Aligned level polished (baseline)
2. Reversed direction of rotation
• control: rotation in polished direction
• rotation in opposite direction
3. Sloped axle
• flat head
• tapered head
4. Notched axle
• control: polished axle
• single wide notch, polished as above
• multiple, narrow notches, polished as above
5. Reduced diameter axle (hub diameter unchanged, moly graphite lube)
• -0.005"
• -0.010"
• -0.015"
6. Threaded hub on polished shaft
7. Lubrication:
• unlubricated
• Lemon Pledge
• same, wiped off carefully
• graphite, regular
• graphite, moly
• white lube
• same, wiped off carefully
• petroleum jelly film
• salad oil film
• silicon spray lube
• same, wiped off carefully
• 3in1 Oil
• cumulative effects of graphite lubrication
• new vs. spun in graphite
• "Rain Shield"
• white lube, washed
• Nyoil
8. Axle shaft polishing - (head treated and polished. shaft bumps removed):
• untreated - control
• polished - toothpaste (Pearl Drops)
• polished - jeweler's rouge
• polished - polishing compound
• polished - polishing compound + jeweler's rouge
• polished - rubbing compound + polishing compound
• polished - crocus cloth + rubbing compound + polishing compound
• polished - emery cloth + crocus cloth + rubbing compound + polishing compound
• bleached, not polished
• polished and bleached
• polished and bleached and polished
• polished opposite direction - rubbing compound + polishing compound
• polished opposite direction - crocus cloth + rubbing compound + polishing compound
• polished with Mother's Chrome Polish

Question 2: What is an effective method for removing the various lubricants?

1. Chemical:
• detergent
• mineral spirits
• paint thinner
• brush cleaner
2. Mechanical:
• plastic scrubber
• steel wool

### Things to think about:

Review the sequence to minimize the needed number of trials.

Carefully evaluate the sequence to minimize the effect of prior treatments on the materials.

Rerun "baseline" at strategic points to identify the effect of prior treatments.
Original: 12/2/2003
Latest update: 6/12/2011
Copyright 2003, 2011 © by Stan Pope, All rights reserved.