Advanced Autocross/Solo (Prepared)

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Autocross/Solo (Top-Level)

Autocross (Also known as "Solo" and "Autoslalom") is a form of time-trial racing run with cars ranging from stock production cars to purpose-built race cars. Within classes that match vehicle type and horsepower to each other, racers negotiate a race course one or two at a time (not side-by-side, but at intervals) while being timed. The emphasis is placed on developing driver skill, having fun and keeping costs low.

Generally speaking, high performance cars and scratch-built cars dominate the top-level category. Still, a wide variety of production vehicle types and configurations can be used as a starting point. Almost any daily-driven vehicle qualifies for autocross with the exception of high roll-center vehicles which can easily roll over in sharp turns.

Top-level Autocross vehicles differentiate themselves from mid-level Autocross by their almost complete removal of stock restrictions and their often non-street legal nature.

Racing is generally on parking lots, kart tracks, road course circuits, and dirt circuits and is widespread throughout the Americas, UK and Australia. There is also a type of off-road racing called Autocross sanctioned by the FIA in Europe.

Power and Weight Stats
Horsepower (Typical Range) 90-600
Race Weight (Typical Range) 590-1816 kg
1300-4000 lb

Design and Construction

Race Car Models of This Type

Production vehicles which are extensively modified with racing/performance parts and may be illegal for street use due to the use of slick tires or track-only car modifications. Purpose-built or scratch-built race cars such as formula cars, sports prototypes. Each vehicle is categorized into its proper class to enable vehicles to race on a reasonably level playing field.

Build Your Own Top-Level Autocross/Solo Car

Knowledge Level

At the top-level, it is useful to have advanced knowledge of all aspects of the vehicle (Handling/operation, suspension, chassis, powertrain, aerodynamics, and safety). Depending on whether you start with a donor production car or start from scratch, your knowledge may need to include designing all aspects from scratch. At a baseline, many more modifications can be made to a donor vehicle and so the greater the knowledge of these systems, the better the choices you can make.

To make the learning curve more manageable, it is recommended that would-be autocrossers select a good base vehicle and work their way through autocross levels using that vehicle as their "development platform". Many of the available books provide basic to reasonably advanced knowledge in the aspects mentioned above, so they can provide a good reference as you advance through each level.

Design Challenges

Suspension: Almost anything goes on a donor or scratch-built vehicle. Basic principles of race car design apply whether you are starting from a donor vehicle or scratch-building:

  1. Maximum track width
  2. Minimum unsprung weight
  3. Maximize tire contact patch and grip
  4. Maximum brake stopping power
  5. Low ride height (with any required changes to the suspension geometry to ensure maximum tire contact patch on the road).
  6. Use appropriate springs, anti-roll bars and damping to manage weight transfer and help the tires follow the surface of the road better.

Chassis: In top-level Autocross, a production chassis may be modified to increase torsional rigidity, lighten the vehicle and increase safety. Changes to the placement of components (i.e. Battery) in order to redistribute weight balance can often go hand-in-hand with lightening.

Powertrain: Powertrain modifications at the top-level can include extensive changes to the engine components and driveline components. The goals of changes to the powertrain should be to increase horsepower and engine efficiency. This can be accomplished through replacing stock components for increased internal flow (Intake/exhaust), improved combustion, reducing friction and removing/replacing power robbing accessories. In terms of final drive, any modifications that increase power delivery to the road should be considered.

Aerodynamic: Aero devices and bodywork changes to enhance aerodynamics may be permitted but unless the top speeds are high enough to generate real downforce, the benefits may not be significant enough to justify costs. That said, there is a benefit to be had if aero devices are large enough and are designed to generate maximum downforce in the speed ranges you will encounter.

Safety: A roll cage provides a measure of safety that a stock vehicle does not provide. Due to the ability to reach high speeds quickly at the top-level, a roll structure is recommended along with other safety items such as a fuel safety cell. Safety features will add a measure of confidence for the driver, who will not be afraid to push as hard as they can in the knowledge they are protected. If a roll structure is not available off-the-shelf, however, it will need to be designed and fabricated, which will require some steel design and welding work (This can be done by speed or metal fabrication shops)

A racing seat with a multi-point harness should also be considered to enable the driver to feel the car more accurately, maintain seating position in cornering and secure the driver fully in the case of a rollover.

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 top-level Autocross car design.

Join our forum to ask and find answers to your top-level Autocross car design/construction questions.

Construction Challenges

If constructing a scratch-built vehicle, then 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.

If constructing from a stock production vehicle, then most components permitted for top-level Autocross are bolt-on type components and therefore are within the mechanical skills of most people. Challenges include the number of components which are changeable, and the difficulty in changing them (i.e. Things like engine removal), which may require specialized workshop tools.

For roll cages that must be custom fabricated, there is the additional requirement of fitting it into a production vehicle and of locating mounting points that do not compromise the uni-body chassis.

Build Costs

If building from scratch, the costs vary depending on the type of race vehicle and the source of the components (Stock vehicle or dedicated racing parts). However, scratch building may be comparable to modifying a high performance production car if you include the cost of the base vehicle.

When building from a stock donor vehicle, the many parts that can be replaced and the new parts that can be added can quite easily run to a significant sum of money. However, with adequate knowledge, it is possible to select areas of modification that provide maximum value and keep costs to a minimum.

Build Effort

For scratch-builds 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!

For a stock donor vehicle build, the effort depends on the number of areas that are modified. If your modifications are performed a little at a time, the effort will also be manageable.

Racing Cost

Consumable costs are relatively low, with the highest expense likely coming from the purchase of racing tires.

Transportation and Support Equipment

The vehicle will likely be trailered to the track unless kept in a street-legal configuration. Support equipment is usually carried in the trailer/towing vehicle.

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