1. Chassis Characteristics
Alex Dardinski
Carbon Fiber Monocoque Chassis for Race Cars
Jon Norcross
Chassis
Sustainability
Advantages
Carbon Fiber in Road Car Chassis
Chassis Characteristics
Importance
High Torsional Rigidity
High performance race cars experience large lateral accelerations when cornering. This puts a high torsional force the car chassis. The carbon fiber monocoque design is very resistant to deflecting when placed in torsion. This allows the suspension to manage the cornering loads effectively for a good handling car. A carbon fiber monocoque can be 3 times stiffer than a steel space frame of the same weight.
Light Weight
The chassis accounts for a significant portion of a car’s weight, so a lightweight chassis makes for a light weight car. Decreased weight improves performance in multiple aspects. A lighter car will accelerate faster, brake harder, and change directions more quickly. A well designed carbon fiber monocoque can be over 30% lighter than a comparable steel space frame chassis while still having greater torsional rigidity.
A chassis is the main structure of the car. It is formed around the driver and provides mounting points for the engine, suspension, and other vehicle components. A race car chassis needs to resist longitudinal torsion applied by the suspension when cornering to avoid deflecting. Deflecting of the chassis causes suspension points, which are designed to stay in the same position relative to each other, to move. A chassis that deflects handles some of the load meant to be managed by the suspension which makes the suspension less effective. This has a major impact on the car’s handling by making it less predictable and adjustable, both of which are vital to racing cars.
Increased Efficiency
Carbon fiber reinforced polymers are 1/5 as dense as steel while remaining stronger. This allows for reduced weight. Less energy is needed to power the vehicle, reducing fuel consumption in combustion vehicles, and extending range in electric or hybrid vehicles .
Carbon Fiber Reinforced Polymer Frames
A carbon fiber reinforced polymer (CFRP) frame is a plastic frame with a layer of carbon fiber on the outside to improve strength. A CFRP frame is stronger than a steel frame and provides increased crash safety.
Feasibility
A CFRP frame is less expensive than a carbon fiber monocoque but works on a similar concept. A CFRP frame is also much easier to mass produce and repair than a carbon fiber monocoque. The BMW i3 utilizes this technology and is available for around $41,000.
BMW i3 with CFRP frame and 124 MPGe
Corporate Automotive Fuel Economy regulations to 2025
The Carbon Fiber Monocoque
Carbon Fiber
Disadvantages
Factor
Importance
Cost
Carbon fiber as a material is much more expensive than steel. Carbon fiber is around $16/pound compared to less than $1/pound for steel. The cost to make a carbon fiber monocoque chassis is at least two to three times more expensive than a comparable steel chassis.
Design
The distribution of force is applied throughout the entire structure which makes design difficult. The process takes more time and increases cost.
Repairs
Repairing carbon fiber after damage is inflicted can be very intensive and most times the entire chassis needs to be replaced.
Maintenance
The monocoque is an entire enclosed tub. This makes accessibility to the components of the car difficult. Technicians’ jobs are more difficult and chassis needs to be design to account for this.
Carbon fiber is made by taking raw carbon material and heating and stretching it until it is long and flat. This process makes the carbon pure and aligns the carbon atoms into a two dimensional hexagonal pattern. This is one of the strongest forms for carbon to exist in.
The carbon fiber can then be manipulated in many ways. For a monocoque chassis, the carbon fiber is cut into long strands and woven into sheets. These sheets are then layered on a mold and cured by applying layers of resin in between the individual sheets. The type of resin used in monocoques is a thermosetting resin. This is a resin that starts liquid then solidifies by chemical reaction, heating, or light application. Once the resin cures it cannot return to its original liquid form.
A carbon fiber monocoque is two layers of carbon fiber with aluminum honeycomb in between. The monocoque is in a tub-like shape in which the driver sits. Due to the light weight of carbon fiber and aluminum honeycomb, and the high stiffness of carbon fiber, a carbon fiber monocoque has a far greater stiffness to weight ratio than a comparable steel chassis. This allows for a carbon fiber chassis to have a greater torsional rigidity while being lighter.
The McLaren MP4/1 (top) with its chassis (bottom) debuted in 1981 as the first Formula 1 car to use a carbon fiber monocoque chassis. By the late 1980s every Formula 1 team was building their car around a carbon fiber monocoque.
Insert actual piece of carbon fiber
Analysis of Forces
Why the Carbon Fiber Monocoque is the Optimal Design for Racing
There are a few main forces which impact a race car under high performance scenarios. These forces are longitudinal torsion, vertical bending, and fatigue.
Longitudinal torsion is a result of applied loads acting on opposed corners of the car. This has a torque on the entire car from corner to corner and can affect the handling and performance of the car. A torque is applied to the front axle that attempts to turn counter-clockwise as the rear axle turns clockwise, or vice versa.
Vertical bending on a chassis happens due to the weight of all the components on top of the chassis. This includes the driver, body, engine, transmission, fuel tank, etc. This force affects the chassis at all times, but can be increased under racing conditions, as in accelerating up a hill.
Fatigue on a chassis happens at key connection points between the chassis and the components of the car. These points are molded or bonded directly into the monocoque structure. The natural oscillations and vibrations of a race car performing at high speeds will potentially deform or compromise them.
Carbon fiber monocoques, although introduced to racing in 1981, continue to be the highest performance design. The combination of light weight and high stiffness has proven to be effective on the track, and the fully enclosed structure has kept drivers safe. There have been no new revolutions in chassis designs, only improvements to carbon fiber monocoques as the complex properties of carbon fiber are better understood.
While the drawbacks to the design are far from ideal, in high performance, highly competitive car racing, winning races and being the fastest on the track is paramount. Expensive sponsorships and revenue from race spectators in high level racing allow the teams to have high budgets. The cost of making a carbon fiber monocoque in this case becomes unimportant. Even though designing and performing analysis on a carbon fiber monocoque is difficult and time consuming, manufacturing is difficult, reparability is low, and access to engines and suspension components is limited, the gain in lap time is worth it.
Carbon fiber monocoques have shown that they are the best chassis design currently available. This has been proven in theory through the molecular structure of carbon fiber and analysis of forces, as well through the dominance of cars that implement this design on the track. The goal in racing is to win, and the advantages of the carbon monocoque outweigh the drawbacks.
“If we construct a rectangular box out of sheets of aluminum (box pictured above) and then try to twist one end of the box relative to the other, the overall torsion gets resolved as shear loads in each of the box’s six surfaces…In the case of the stressed skin structure, the loads are carried by shearing forces in the sheets that form the planes, in the same way that such a flat sheet can substitute for the diagonal tube in one panel of a space frame…” – Forbes Aird