Micro Sprint

Page: 1

Sprint (Micro)

Sprint cars began in the USA in the 1930s and 40s as modified Model T racers and evolved into purpose built cars over the 30 years that followed. Their development paralleled the Midget race car, but Sprints were based on full-size cars with a longer wheelbase. The designs evolved to use V8 engines and eventually wings and roll cages. However, the cost to compete in Sprint racing also increased because of the amazing power (700-1000 horsepower) that was eventually produced by their engines.

As a lower cost alternative to full-size Sprint cars, Micro Sprints were introduced which leveraged the same overall car design, but with a shorter wheelbase and 600 cc motorcycle engines. Other types are also produced, with variations in engine size from 270 cc to 1000cc or larger motorcycle engines.

Racing is run in the USA, Canada and Australia on local tracks, with the largest racing participation in the USA.

Power and Weight Stats
Horsepower (Typical Range) 95-125
Race Weight (Typical Range) 340-408 kg
750-900 lb

Design and Construction

Build Your Own Micro Sprint Race Car

Knowledge Level

Due to the scratch-built nature of sprint cars, you should be knowledgeable in handling, chassis, suspension, powertrain, aerodynamic and safety design. These six major areas of the car design work as an integrated unit and the designer must have an understanding of how changes to one area affect the others. Much of the design work is iterative, meaning re-designing areas based on new changes to another area. After the iterations are completed, the design will be complete and optimized.

Understanding the handling and suspension of the live axle configuration and the springing/damping aspects are especially helpful in the design and tuning of sprint race cars. Aerodynamics also play a vital role in producing a fast car.

Design Challenges

Weight Distribution: Because sprint cars run on oval tracks as opposed to road courses, they are generally configured to optimize counter-clockwise or left turns. Therefore weight distribution must be optimized to obtain as much traction force from all four tires as possible while making left turns.

Suspension: Maximizing the compliance with the track is of key importance. Axle, wheel and tire weights (Unsprung weight) affect the compliance of the suspension, which in turn affects handling, so keeping all these components as light as possible is an advantage.

The cars use stagger (larger outside tire) and offset (right side tire farther from the chassis than left side) to manage weight transfer, as well as ride height and springs/shocks. Adjustability in these areas must be designed into the car or tuning options will be very limited. At any given track, at any given race, the surface conditions might dictate tuning of these suspension components to provide more/less "bite" and tightening or loosening the rear of the car (understeer or oversteer).

The suspension link locations impact the chassis design.

Chassis: Providing openings to make internal components accessible for maintenance is also important. Accommodating transverse engine mounting and running the drive chain through the cockpit are requirements factored into the chassis design.

Powertrain: Weight distribution is heavily impacted by engine position. Intake, Cooling and exhaust need to be considered in relation to chassis design, bodywork and aerodynamics. Motorcycle engines are usually stock tuned to provide wide power bands above a certain RPM. If regulations permit modifications, avoid making absolute horsepower at the cost of driveability.

Aerodynamic: Sprint cars usually restrict the dimensions and camber of their wings, as well as end plates and gurney flaps (wickerbill). There may be opportunities to maximize downforce and minimize drag through simulations of various wing shapes and attack angles. Depending on the length of the track, changes to wing shape and angle of attack may have a pronounced effect on downforce/drag. Downforce generated by the front and rear wings will affect front and rear grip levels, and will be impacted by the wake of the cars in front. Designing a large range of adjustability into the wing angle of attack will provide maximum flexibility to tune at the track.

Safety: Modern sprint cars require a full roll structure integral to the chassis, with a racing seat/harness. A fuel safety cell, fire wall, and front/side/rear bumper bars for impacts are also mandatory.

If you intend to race under a sanctioning body, always read and understand the regulations of your chosen racing class before designing or building any race vehicle.

Design Resources

Learn the basics about race cars and race car design from our free online knowledge series

Download our free race car design aids to assist you designing your race vehicle.

In-depth books and learning resources we recommend for micro sprint car design.

Join our forum to ask and find answers to your micro sprint car design/construction questions.

Construction Challenges

Ensuring the chassis is dimensionally accurate and straight is key. The use of a solid, flat and level build space is important. Jigs are often used in this case to ensure that structural tubing stays in alignment during welding/brazing.

The builder should have solid joining and metal working knowledge and skills when fabricating the chassis/rollcage/suspension. While mild steel (1018/1020) is very forgiving, some metals are best welded using a specific method (mig/tig) and some require heat treatments before and following welding to restore their toughness and strength.

Having sufficient space for the chassis and bodywork construction is important, as a cramped workshop can be difficult to work in.

The tools to fabricate the chassis and bodywork can add considerably to the cost of your project if you don't already have a workshop, but borrowing or renting items is also an option.

Build Costs

Many components for an amateur-built micro sprint car can be sourced from micro sprint parts manufacturers. In some cases, where permitted by regulations, parts may be sourced from production cars. This will lower the overall cost.

Components that are specific to racing are also generally what cause the build costs to rise, but many components must be of a racing design, such as a fuel safety cell and therefore care must be taken not to "Go cheap" in the wrong places. If there is no discernible advantage to the construction/materials/weight of a racing part versus an OEM part, then the OEM part may be the way to go.

Build Effort

Because the car is scratch-built, there will be significant effort in design and construction. There is however, an equally great satisfaction and sense of accomplishment at being one of the few people in the world who have built their own race car from the ground up! Many race car manufacturers also started this way, with the development of their own chassis leading them to build cars for others.

Racing Cost

Tires and fuel probably form the largest consumable expenses. Powertrain rebuilds and potentially crash repairs will be periodic expenses. Motorcycle engines tend to be very durable despite their high RPM operation, but they still require rebuilds to remain competitive. If you are able to perform the labour yourself, the cost will be considerably lower.

Transportation and Support Equipment

Trailering required.

Page: 1