Pinewood Derby Rail Rider Car Tests
Experiments to Determine if Rail Riders Really Run Faster Than Normal Pinewood Derby Cars
As the average pinewood derby car travels down a track it swings left and right several times. Each swerve uses up energy and slows down the car. Friction as the wheels slam into the guide rail also slows it down. The idea of the rail raider is to align the front wheel, or wheels, so that the car very gently rubs again one side of the rail all the way down the track. This eliminates energy losses from swerving and replaces the high friction associated with occasional hard impacts with the guide rail with a lower, though longer lasting, sliding friction. The idea is that a small amount of friction over the entire length of the track reduces speed less than several swerves and hard impacts during the race. The question is: Does it work?
Although there are dozens of Youtube videos and webpages that claim aligning a car to be a rail rider makes it run faster, none of them provide numerical proof that it does. That's the purpose of this page: to test which is faster, a car aligned to run straight down a track or the same car aligned as a rail rider.
My fastest car was carefully aligned to run straight, its run times recorded for three races, then it was re-aligned to steer to the left 1 inch down a six foot incline, its run times recorded again, aligned to turn 2 inched left over six feet and the run times measured a third time. Lastly, the car had its alignment returned to its straight-run configuration and retested to verify that no significant losses had occurred due to the graphite breaking down.
This car uses tungsten for primary weight, lead for secondary weight, bodies shaped for minimum weight and aerodynamic drag, and highly polished axles and wheels. It is completely street-legal in that it does not use lathed or lightened wheels. It would pass inspection in 95-percent of all race venues. The only one's they'd fail would be those few not permitting extended wheel bases.
The track used was a 40-foot long vinyl-coated masonite track. It runs slightly slower than an aluminum track. At the 2014 Antelope Valley AWANA races the championship car out of a field of 76 recorded a minimum single-heat time of 3.001 seconds and an average time in it's last four heats of 3.015 seconds. All the rail rider tests used the same lane that was swept and vacuumed for each car. The track uses a Smartline time accurate to less than 0.001 seconds.
The car used was Dark Jedi:
This is currently the fastest pinewood derby car in my collection and would competitive in its class at that national level. Like all six of the high performance cars, its body weight, less wheels, is under 11 grams. It produced the following test results:
Aligned to run straight:
run 1: 2.928 seconds
run 2: 2.932
run 3: 2.928
average: 2.929 seconds
Aligned to steer 1-inch to the left down a six-foot incline:
run 1: 9.952
run 2: 9.952
run 3: 2.955
aligned to steer 2-inches left down a six-foot incline:
run 1: 2.975
run 2: 2.982
run 3: 2.982
re-aligned to run straight:
run 1: 2.935
run 2: 2.935
run 3: 2.929
The results seem unequivocal: for this high performance car aligning it to be a rail rider made it run slower. As convincing as the above test appears to be I discovered that it was incorrect.
The reason is that rail rider car is not simply a car that has been aligned to steer to one side or the other. Rather it's constructed from the very start with several unique features which work in concert to produce a fast car. First, the rear wheels are angled upward so that they pull away from the car's body and ride on the heads of the axles. Second, the front dominant wheel is angled downward so that its inner hub rubs against the car's body, which has been narrowed by 1/16th inch. These two design features insures that the rear wheels do not rub against the center guide rail. A secondary effect of angling the wheels is that the car runs on the edges of the wheels, which reduces friction. Additionally, because the car is riding against the center guide rail, the rail acts as a stabilizer so that the main weight can be placed further toward the rear of the car. Whereas a normal car is limited to a center of mass not much closer than 1 inch in front of the rear axles or the car will become unstable, rail riders can have their center of mass as close as 5/8ths-inch in front of the rear wheels, meaning that they have greater potential energy and therefor acquire greater speed as they race down the incline section of the track.
All this comes at a price. The inner hub of the dominant front wheel needs to be coned and polished to a high shine. The surface of the car's body against which it rides needs to be equally polished. The coned hub must also be level so that it doesn't knock against the car's body with every revolution. Similar attention must be paid the the front dominant wheel's inner rim because it rubs against the guide rail. The rear wheel axle heads have to be coned and polished and careful attention given to the wheel hubs. Doing everything right should produce a car that runs 0.100 to 0.140 seconds faster than conventional cars.
Another advantage rail riders have is that because they do not move back and forth as they go down a track, minimizing the mass moment of inertia is no longer a concern. This means the weight in the rear of the car can be any shape without loosing performance.
For a detailed explaination of rail riders and how to make them I refer you to 5kidsracing.webs.com/cubscoutbuildtips.htm.
On my main page of how to make the fastest possible pinewood derby car (AWANA GRAND PRIX RACE CARS), I mention one more factor regarding rail riders that suggests that using this technique could result in complete disaster: track quality. The sad fact is that many tracks, including high-end aluminum tracks, often have guide rails misaligned so that the transition from one section of track to the other has one side of the guide rail sticking out. If a rail rider is aligned so that it's rubbing against that side of the rail it will likely be thrown off the track when it hits the protruding guide. Even if the car body isn't damaged, such a hard impact against the sharp edge of the guide rail will almost certainly destroy the car's alignment and could very likely gouge the wheel. If you are going to use the rail riding technique I recommend you make sure the track has clean transitions.
If it does, then by all means explore the rail rider option. As the following video shows my two newest cars constructed to be rail riders are the fastest cars I've ever made. I've configured my BestTrack to be as close as possible to league standards, in particular making sure that the start section is at 27.1-degrees. Both these cars post respectable, though certainly not great times. But then they were my first two serious attempts.
Please check this page from time to time. I'm in the process of rewriting it to include everything I've discovered about rail riders.
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