Don Kulha

Above: PV panel angle adjustability aids collection of maximum energy.

elcome to Home Power's new series on Solar Sprint racing. Solar Sprint is an opportunity for young and old to have fun with solar energy, learn some useful lessons, and introduce new folks to renewable energy. We'll be promoting the sport, sharing vehicle construction tips, resource info and report on race events.

Below: Adjustable mount attaches panel to chassis and allows easy and accurate angle changes. Note line guide below rear axle.

Though fairly new to SS racing, my experience in big car racing helped win the inaugural Senior SS race at MREF this year. Many of the same principles apply directly to SS in terms of efficiency, chassis design, gearing and testing to name just a few. It worked for me and I'll share what I've learned.

The Rules of Solar Sprint Racing

The official rules for Junior Solar Sprint (also used for the Senior edition) are posted on NREL's SprintWeb site and in a sidebar within this article. The race consists of a twenty meter drag race down a tightly stretched guide line using a specified PV panel and motor for propulsion. Two or more cars compete at a time in an elimination tournament until a winner is declared. It is a test of ingenuity, attention to detail, and teamwork. Contestants all use the same specification motor and PV panel which must be used unmodified. This levels the playing field and puts the emphasis on design, planning, and construction.

Any number and type of wheels, transmission (usually belt, friction, or gear), chassis material and design, bearing type, or line guide strategy can be used. The choices are numerous and all yours to make. That's the challenge, to research possible designs, make the best

Above: Long wheelbase aids directional stability and aerodynamics with little weight penalty.

choices and work together as a team to build, tune, and race your creation. Along the way lies the opportunity to learn a great deal, develop new skills, and have a lot of fun. Not everyone competing will win that final race but the path to it can yield substantial rewards.

Solar Sprint Hardware

Currently Pitsco is the prime supplier for SS kits. They offer a basic kit consisting of the regulation panel, motor and instructions for $29.95 and a deluxe kit including axles, wheels, gears and a few other parts for $10 more. My recommendation is to get the basic kit and hunt up the drive components on your own (better components are available for those willing to hunt for them). The PV panel provides about 3 volts open circuit, 1.6 short circuit amps and is 4.75 inches wide and 13 inches long. The spec motor is a Mabuchi model 280 plain bearing, metal brush motor with a 0.078 inch diameter output shaft. Tests on a batch of motors yielded about a 2% variation in power tested against a "prony" brake (another motor used as a load) using the PV panel to power the motors being tested. While this is a pretty tight spread for a batch of mass produced motors it points out one area for optimization. 2% over a 65 foot course is over a foot, a possible margin of victory (more on motor testing soon).

Parts for your racer can be had from many sources. Radio controlled (R/C) vehicle and slot car shops can yield a variety of useful items as can surplus electronics outlets. Gears, bushings, and shafts can be recycled from old computer printers and VCRs. Tires and wheels can be had from R/C planes, toys, or made from plastic lids, CDs or other items. I've listed some online resources for information and parts in the Access section at the end of this article.


I'm going to use this section to provide random notes and food for thought in the form of brief bits of info...

1. Reshaped R/C airplane tire; 2. Precision 64 pitch nylon spur gear; 3. Pivoting front line guide; 4. Shielded ball bearings (0.187 inch I.D.); 5. 0.04 inch fiberglass sheet mounts electrical connections (0.10 inch square pin) and front PV panel pivot

Solar Sprint

Junior Solar Sprint Race Rules and Vehicle Specifications

Official JSS Rules


1. The motor and solar panel must be used without any modification.

2. The remainder of the vehicle must be your own design and can be made from any other material.

Vehicle Specifications:

1. The vehicle must be safe to contestants and spectators, e.g., no sharp edges, projectiles, etc.

2. The vehicle must fit the following dimensions: 30 cm. by 60 cm. by 30 cm.

3. Decals of the sponsor organizations (provided by JSS) must be visible from the side on the body of the car. A 3 cm. by 3 cm. space must be left for the assigned car number.

4. The sun's light is the only energy source that may be used to power the vehicle. No other batteries or energy storage devices are permitted.

5. Any energy-enhancing devices, like mirrors, must be attached to the vehicle.

6. The vehicle must be steered by the guide wire using one or more eyelets affixed to the vehicle. The vehicle must be easily removable from the guide wire, without disconnecting the guide wire.

7. The body of the car must be three dimensional. Teams will NOT be allowed to bolt the axles and wheels to the solar cell. The solar cell cannot be used as the body of the car.

• Start a project log book to include notes, sketches, specs, resource info, test results, ideas, etc. Folks in professional racing do this for good reason. Try to identify possible mentors, resources, and sponsors. The more brains you apply to the task the better your results are likely to be.

• Aerodynamic resistance equals your vehicle's frontal area times it's drag coefficient (slipperiness) times the velocity squared. You control the first two elements of the equation.

• Weight is critical - shave every gram you can without weakening your vehicle. Multiple races will be required in the process of eliminations. You must finish to win.

• Vehicle mounted reflectors improve performance by increasing panel amperage and motor torque output. More torque allows higher gearing hence more speed. The downside is increased aero drag and sensitivity to crosswinds.

• Spur gear systems using properly designed and profiled gears can be 98% efficient at power transmission. Profile refers to the shape of the contacting gear faces. Gears designed for R/C racing are designed for maximum efficiency and come in different tooth counts allowing gear ratio tuning.

• Pinion gears used on slot car motors fit the 0.078 inch shaft of the Mabuchi motor. I attach mine to the motor shaft with a good grade of CA glue allowing them to be changed easily.

• Races in any league are rarely won by applying one major tweak to the car. It's the team that best optimizes the most factors in their overall design that usually wins. When competition is close this attention to detail often provides the winning edge.

Okay, start your logbooks, get your team together, identify potential resources (people, information, parts sources, sponsors) and start planning.


Don Kulha, PO Box 7518, Santa Rosa CA 95407, [email protected]

NREL Education Office, Event host technical assistance, 1-800-NEW-ENGY.

Builder and Host info at: www.nrel.gov/business/education/SprintWeb

Solar Sprint kits: Pitsco, 1-800-835-0686, www.pitsco.com

Some Parts Suppliers on the World Wide Web

Sheldon's Hobbies: www.btown.com/sheldon.html

Tower Hobbies : www.towerhobbies.com/index.html

Kimborough Racing Products (R/C racing gears): kimbrough-products.com

Aerospace Composite Products: ^


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