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Design
    Strategies and Tips


Tips: Chassis


The following tips and information focus on how to optimize a race car chassis, specifically the spaceframe-type chassis. Depending on class rules, these suggestions may or may not be valid. Always check your regulations.

General Spaceframe Chassis Principles

Chassis Design Tips

General Spaceframe Chassis Principals

Spaceframes

The spaceframe chassis is about as old as the motorsport scene. It's construction consists of steel or aluminum tubes placed in a triangulated format, to support the loads from suspension, engine, driver and aerodynamics.

Spaceframes are popular today in amateur motorsport because of their simplicity. Most everyone who has access to a level workshop, a saw, measuring tools, and a welder of some kind can build one.

There are also some inherent advantages to using spaceframes at the amateur level of motorsport as well. Spaceframes, unlike the monocoque chassis used in modern Formula 1 or CART, are easily repaired and inspected for damage.

So how does triangulation work? The diagram below shows a box, with a top, bottom and two sides, but the box is missing the front and back. The box when pushed, collapses easily because there is no support in the front or back.

Of course, race cars need to be supported in order to operate properly, and so we triangulate the box by bracing it diagonally. This effectively adds the front and back which were missing, only instead of using panels, we use tubes to form the brace. See below:

The triangulated box above imparts strength by stressing the green diagonal in Tension. Tension is the force trying to pull at both ends of the diagonal. Another force is called Compression. Compression tries to push at both ends of the diagonal (Shown above in the horizontal yellow tube). In a given size and diameter tube or diagonal, compression will always cause the tube to buckle long before the same force would cause the tube to pull apart in tension. As an experiment, try pulling on the ends of a pop can, one end in each hand. Then, try crushing the can by pushing on both ends. The crushing is much easier, or at least humanly possible, compared to pulling the can apart.

Spaceframes are really all about tubes held together in compression and tension using 3D pyramid-style structures, and diagonally braced tube boxes. A true spaceframe is capable of holding it's shape, even if the joints between the tubes were hinges. In practice, a true spaceframe is not practical, and so many designers "cheat" by using stronger materials to support the open portions of the structure, such as the cockpit opening.

In contrast to spaceframes, the monocoque chassis uses panels, just like the sides of the box pictured above. Instead of small tubes forming the shape of a box, an entire panel provides the strength for a given side.

A common shape for 1960s cars of monocoque construction was the "cigar". The cylindrical shape helped impart something called Tortional rigidity. Tortional rigidity is the amount of twist in the chassis accompanying suspension movement. See the diagram below.

Tortional rigidity applies to spaceframes too, but because a spaceframe isn't made from continuous sheet metal or composite panels, the structure is used to approximate the same result as the difficult to twist "cigar car".

Another reason tortional rigidity is mentioned here is that it greatly affects the suspension performance. The suspension itself is designed to allow the wheels/tires to follow the road's bumps and dips. If the chassis twists when a tire hits a bump, it acts like part of the suspension, meaning that tuning the suspension is difficult or impossible. Ideally, the chassis should be ultra-rigid, and the suspension compliant.

It is important to ensure that the entire chassis supports the loads expected, and does so with very little flex.

Chassis Design Tips

  • Design the chassis after the suspension One of the biggest mistake novices make is to design the chassis before the suspension. It is much easier to design a tentative suspension according to the rules and good geometry, and then build the chassis to conform to suspension mounting points and springs/damper mounts. See our Design Approaches section for more information.
  • Consider the load paths. A chassis is not about "absorbing" energy, but rather about support. When considering placement of tubes, visualize the "load paths". Load paths are defined as the forces resulting from accellerating and decellerating, in the longtitudinal and lateral directions which follow the tubing from member to member. The first forces which come to mind are suspension mounts, but things like the battery and driver place stresses on the spaceframe structure.
  • Maximize CG placement and vehicle balance. Center of gravity affects the race car like a pendulum. The ideal place for the CG is absolutely between the front and rear wheels and the left and right wheels. Placing the CG fore or aft or left or right of this point means that weight transfers unevenly depending on which way the car is turning, and whether it is accellerating or decellerating. The further from this ideal point, the more one end of the car acts like a pendulum, and the more difficult it is to optimize handling. The CG is also height dependant. Placing an engine higher off the ground raises the CG, and forces larger amounts of weight to transfer when cornering, accellerating, or decellerating. The goal of vehicle design is to keep all four wheels planted if possible, to maximize grip, so placing all parts in the car at their lowest possible location will help lower the CG. Of course, in terms of spaceframe design, you have to leave space for each of the parts.
  • Layout the tube members for easy access and maintenance. Maintaining a race car comes after construction. Placing tubes across openings is a natural way of ensuring a rigid chassis. However, in practical terms, you may be making it difficult or impossible to reach the mainenance demanding components. A good chassis design will allow quick and easy access to all components, and will not hamper removal or replacement of any part.
  • Check out cars which are competitive in your class. Cars which are competitive are usually built well, and with appropriate materials and methods. Observe these cars at the track and in the pits, and you can infer a great deal about what makes them winners.
  • Optimize the tubing shape for the job. Square tubing, which is known for it's ease of cutting and joining is better in situations where bending forces occur. However, round tubing is generally stronger in all other cases, albeit at a penalty in the complexity of construction.
  • Optimize the tubing size and gauge for the job. Tubing which is used in tension, can be of a lighter gauge than that used in compression. Keeping this in mind can save considerable weight, although it requires additional joining work and variety of tubing.

Check Back Later For More Chassis Tips.

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Read some good chassis design books...

(c) 1999 Matt Gartner