Importance.:
The chassis (or frame) of the car is more than the sum of its parts. On the surface, the chassis is the single largest part of the car, and really does do much that can be observed other than hold things together. However, its gets more interesting as you start digging into the car.
The chassis first serves as an aero device, both by directing air but also by supporting the deflection of other aero components. It does the same for the power train and suspension: holds the parts together with the ideal balance of stiffness and weight.
The chassis serves as the drivers centre of confidence. If the driver does not feel safe, either due to weak impact zones or flexy suspension feedback, he won't go fast. Arguably more importantly, he may be injured by an unsafe chassis. Finally, the driver must be entirely comfortable and as home in the cockpit.
Perhaps my favourite part of the chassis is that chassis designers are the unsung heroes of race car engineering: they must accommodate all of the other systems in the most harmonious way, and then connect the driver into their creation (or vice versa). Their praises are not sung unless their creation is smashed to prices and the driver walks away: the driver, race engineer, aerodynamicist, engine guy, and suspension engineer get credit for the car's success, in that order. On top of all that, while new aero, power, or grip tunes can be rolled out in the season, you only really get one shot at a chassis. This is regardless of money, as its a man-hour issue.
To answer your question more simply: at best the chassis can only help the aero, suspension, and engine reach their full potential, but at worst can totally destroy any potential in these systems.
Types
There are multiple types of chassis but all of them can be classified into one of two approaches:
- Use lengths of round or square tubing, or other structural metal shapes to form the chassis structure (Space frame, multi-tube, ladder frame)
- Use joined panels to form the chassis structure.
Both approaches can provide a structure capable of mounting other race vehicle components, but each has its own advantages and disadvantages.
Chassis (space frame)
The chassis uses numerous cut and shaped pieces structural metal tubing (usually steel) joined together to form a strong framework. The diagram below from Ron Champion’s book “Build Your Own Sports Car For As Little At £250”, shows an example of a space frame chassis.
The principle of spaceframe design is to use triangulation of the tubes to create a rigid structure. Examples below show how triangulation is used to rigidize a structure:
An un-triangulated box has very little strength. You can see this in action above. As the hand pushes against the corner of the box, the shape warps into a parallelogram.
Now, if we cross-brace or triangulate the box with a tube, the strength is greatly increased:
The tube is being pulled in tension as if the corners of the box to where it is attached were trying to tear it apart. Because of the tube’s strength in tension, the box will not deform into the parallelogram.
Triangulation can also work with tubes in compression. However the ideal design always has the member tubes working in tension which provides far superior strength to tubes working in compression.
The load being applied is now attempting to crush or compress the tube instead of tearing it apart. Because of the reduced strength in compression, buckling can become an issue.
There are numerous examples in this diagram of how open box tube structures have been triangulated to create a much more rigid chassis. The diagram also shows suspension and other mounting brackets.
Spaceframes usually use square or round tubing. Square tube is easier to work with because cutting it involves straight cuts at a particular angle. Round tubing does not butt up against other round tubes well, and therefore requires a special tube notcher to cut round shapes into it.
The key aspect of spaceframe design is to identify and analyze the loads that are to be expected, and design the frame and triangulation to handle those loads in an optimized fashion. As tubing in tension provides higher strength than compression, a lighter gauge tubing may be used in tension loaded areas to save weight. In areas where tubing sees compression loads, a heavier gauge or larger diameter tubing may be better to use.
Chassis (Monocoque & space frame)
The monocoque chassis is technically an improvement over the spaceframe chassis. below shows a simple example of the difference between spaceframe and monocoque design.
The monocoque “Box” on the left uses a panel of material to structurally “complete” the box. When the hand pushes against it in the direction shown by the green arrow, it creates a shear force across the panel. This force is effectively handled the same way a tension load is by the spaceframe triangulated box on the right. However, if the hand were to push from the other side of the box, the spaceframe tube could potentially collapse in compression, whereas the monocoque box would behave the same way it did before.
Both types of chassis can be made just as strong as each other. However, to make an equivalent strength spaceframe generally requires more material and therefore more weight. The materials used make a big difference as well.
both the monocoque “box” on the left and the fully triangulated spaceframe “box” on the right would handle loads in the same manner
Although the monocoque can usually be made lighter and stronger than a spaceframe, it does have some downsides that make it more complicated to design, build and operate.
First, the monocoque requires the structure formed by the panels to be “complete”. If you observe the “box” in diagram that we used to demonstrate the monocoque, imagine one side of it is missing as shown
We can push on the corner of the box where three panels meet (shown on the left) and it won’t warp (much), but push on a corner next to where the missing side should be and the box will buckle (as shown on the right). Where an opening exists, the chassis must handle loads through a supporting sub-structure.
A primary goal in monocoque design is to ensure that there are no unhandled load paths that can cause the monocoque structure to buckle. A buckled monocoque is no better than a buckled spaceframe tube.
In the case of poorly handled load paths, the spaceframe can be more forgiving as the tubing diameter and steel material usually provide a more gradual failure than a monocoque. However, it is better to design the chassis correctly in the first place then to rely upon noticing gradual failures.
This brings us to another key point about the monocoque—If it is damaged, it is difficult to repair compared to spaceframe tubes. It is also difficult to detect damage on a monocoque whereas bent or broken tubing is quite easy to spot.
Rigidity - TORSIONAL
Torsional rigidity is a property of every race vehicle chassis that determines how much twist the chassis will experience when loads are applied through the wheels and suspension. Diagram below shows the principle.
A chassis that has a lot of twist won’t handle as predictably as one which has very little because by twisting, the chassis begins to act like an extension of the suspension. The suspension 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.
Torsional rigidity is measured in lbs-ft/degree or kg-m/degree. One end of the chassis (front or rear) is held stationary and the other end is balanced on a point and twist is applied via a beam.
Key Points in Design
Chassis Modification
When considering modifying a production-based chassis to mount alternate suspension, engines or drivetrain, spend time studying the unibody (newer vehicle) or ladder-frame (older vehicle) structures. The structures formed by the manufacturer’s chassis designers have strong areas intended for loads and weak areas not intended to carry loads. Identifying the correct parts of the chassis structure to cut or modify is critical.
Consider using scale models of the vehicle (if plastic models were made), to mockup the changes, or 3D modeling software to do the same. If the changes involve the suspension, such as lowering the vehicle, model the new suspension first. Sometimes lowering the vehicle while using the same suspension pickup points will create poor handling.
Models building
Modeling a spaceframe chassis with balsa wood sticks enables you to see first hand the differences triangulation makes to the stiffness of a chassis. Herb Adams, in his book “Chassis Engineering” provides a whole chapter on chassis modeling using balsa and paper. His recommendation is for a 1/12 scale model.
Likewise, using cardboard, paper and glue to build model monocoques can be a very rewarding and low cost learning experience as well. The great thing about these materials is that they don’t have a lot of strength and so the deformations that loads create can be easily seen when loads are applied.
1st suspension then Chasis
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.
load paths
A chassis is not about “absorbing” energy, but rather about support. When considering placement of tubes, visualize the “load paths”, and consider using FEA (Finite Element Analysis software) to help analyze load scenarios. Load paths are defined as the forces resulting from accelerating and decelerating, in the longitudinal 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.
Arrangement of CG and 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 accelerating or decelerating. 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, accelerating, or decelerating. 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 height.
Chassis Modeling
Before building your race vehicle, consider building models to aid in visualizing design. Some may prefer to stick with computer tools like FEA (Finite Element Analysis) software, but for sheer hands-on visualization, we recommend building your own balsa spaceframe models or using paper or crafts materials to form a monocoque.
As the photo below from Herb Adam’s book Chassis Engineering shows, these models can provide insight into the design and torsional rigidity of various configurations:
Materials/Shapes
Designing a chassis, be it spaceframe or monocoque requires an understanding of the right materials and their engineering properties. We’ll take a look at some of the basic materials used, but there are whole books available which cover the topic in the detail that a scratch-builder will need.
Structural Materials
Most modern spaceframes are built using steel structural tubing in grades 1018, 1020 or 4130 chromoly. The first two grades are considered mild steel with sufficient strength and toughness for amateur race vehicle builders. These grades are not affected by welding heat and do not require post-welding heat treatment. The 4130 chromoly grade, while stronger than mild steel, does require post-welding heat treatment to restore its mechanical properties.
Steel is very good from a metal fatigue perspective and due to the vibration and oscillating loads encountered in race cars and trucks, it is the preferred material for amateur builders who’s goals are long chassis life and dependable strength.
Aluminum has been used in spaceframe design and can provide weight savings but depending on the grade, it may or may not be weldable. The fatigue life and strength should be understood before selecting aluminum as yourspaceframe material.
For monocoque builders, the materials most often used today is fibre reinforced plastic (FRP). The fibre can be anything from fibreglass to carbon fiber, depending on the requirement to save weight. The plastic is generally 2-part epoxy. Builders make use of composite sandwich structures by laminating FRP over a honeycomb structure. These “Panels” are very rigid in bending. Forbes Airde, in his book “Race Car Chassis Design and Construction” provides a chart showing how composite sandwiches, while almost 4 times as thick as steel weigh only 1/5th the weight of steel for the same bending strength.
Shapes
The selection of structural shapes for spaceframe construction provides the designer with flexibility in how they optimize their chassis designs.
Square and rectangular tubing provides the easiest design and fabrication. As mentioned earlier, cutting and welding/brazing is the easiest with square or rectangular shapes. Rectangular tubing also provides the opportunity to create a “Beam-like” structure where more strength is required on one axis than the other. Square and rectangular 4130 chromoly is also available, but generally at a higher cost than circular.
Circular tubing, while not as easy to fabricate with, is widely available in 4130 chromoly, so to take advantage of the increased strength or reduced weight for the same strength, circular (and oval) may be the shape of choice.
Oval tubing is popular for use in exposed wishbones on open-wheel type race cars. Its shape is more aerodynamic than round or square tubing and therefore it creates less drag and turbulence. The downside is that it is not as structurally strong as round or square tubing in bending.
The I-Beam type shape may be useful where deflection (bending) along the “I” vertical axis needs to be minimized over a span. In long span situations, square tubing will not fare as well as I-beam in terms of deflection.
Key Considerations
- 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 maintenance 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.
- Vehicles which are competitive are usually built well, and with appropriate materials and methods. Observe them at the track and in the pits, and you can infer a great deal about what makes them winners.
- Square tubing is the easiest structural shape to build a chassis from. However, there are circumstances where round tubing can be useful, albeit at a penalty in the complexity of construction. Oval tubing is useful in open wheel race cars for wishbones.
- 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.
- Many race car chassis are built using mild steel in the 1018 or 1020 grade (anything lower than 1018 is not advisable due to its limited strength). There is nothing wrong with using these grades if the chassis is properly designed and weight is not the utmost concern. 1018 and 1020 don’t require heat treatment after welding and can be MIG welded, making fabrication easier.
- The key point to consider is the need for weight saving.
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