By Sidhant Khanna, Batch of 2015
Take a car and strip it right to its knickers, remove everything right from doors, bonnet, seat to every electric component, engine and suspension what you get is the basic skeleton of a car, the chassis (pronounced as ‘shas-ee’) – the part of the automobile onto which everything is bolted and attached. If it wasn’t for chassis, the car would flop about and eventually either crash or fall apart. Let’s take a look at F1 cars, standard commercial cars and cars made by our college team; analyze their chassis and learn about this very fundamental part of the automotive industry.
The spaceframe chassis is about as old as the motorsport scene. Its construction consists of steel or aluminum tubes placed in a triangulated format to support the loads from suspension, engine, driver and aerodynamics.
So how does triangulation work? The figure shows a box, with a top, bottom and two sides, but is missing the front and back. The box when pushed collapses easily because there is no support in the front or back.
The triangulated box beside imparts strength by stressing the green diagonal in Tension. Space frames are 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 its shape, even if the joints between the tubes are hinged.
The space frame chassis which was used in cars from the dominant Ferrari 500 of 1952 and 1953 to the Ferraris, Lotuses and BRMs of the early 1960s, surprisingly began to be considered as old technology by the top echelon of formula racing post the Second World War.
Monocoque is a French word meaning “single shell,” which refers to the process of making the entire body out of a single piece of material. In contrast to spaceframes, the monocoque chassis uses panels, just like the sides of the box pictured below. Instead of small tubes forming the shape of a box, an entire panel provides the strength for a given side.
SPACEFRAME vs. MONOCOQUE
There is one advantage of the spaceframe design – it is easier to repair than a monocoque design, as a piece of tubular frame can be cut out and replaced a lot quicker than having to remove a whole section of body and weld in a replacement in such a way as to retain its structural integrity and strength. But compared to spaceframe, a monocoque has significantly less weight, smoother handling and is much safer due to its integrated body type. Today the Monocoque chassis is universal among F1 cars along with near-universal presence in road cars.
WHAT IS A GOOD CHASSIS ?
Unless you’ve been living under a rock, you know that oil prices have been rising exponentially, and aren’t likely to come down. Cutting a car’s weight makes a lot of sense as when the engine will have less weight to haul around, it will use less energy. But shedding a car’s weight is not as easy as it sounds as they need to safe and durable. Let us evaluate the parameters of a good chassis and how they will improve a car’s performance:
• MINIMAL WEIGHT
Improving fuel economy during the next few years will come down to developing lightweight structures. It will require the use of Carbon fiber, which is a super strong material and is extremely lightweight as well. Engineers and designers love it because it is five times as strong as steel, two times as stiff and yet weighs about two-thirds less!
But if carbon fiber is so great, why isn’t its use widespread in cars? Current production of carbon fibers is slow and expensive. Also, carbon fiber can’t be melted down and it is not easy to recycle. As it stands now, carbon fiber can solve the oil crisis. It is lightweight, durable and safe. But it is also expensive and difficult to recycle.
• WIDER TRACK
As you all would have studied in class XIth Mechanics, the maximum cornering speed (v) of a car (track width 2a, Centre of gravity h) that can maneuver a turn of radius r without toppling is:
• LOWER CENTRE OF GRAVITY(CG)
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. SUVs have gained notoriety in the past few years due to accidents caused by their higher centre of gravity. There were about 90,000 SUV rollovers in 2012, in which an estimated 2500 people died.
• PLENTY OF TRIANGULATION
As seen earlier, spaceframe chassis is strong because of the inherent rigidity of the triangle.
• TORSIONAL RIGIDITY
A common shape for 1960s cars of monocoque construction was the “cigar”. The cylindrical shape helped impart something called Torsional rigidity, which is the amount of twist in the chassis accompanying suspension movement. See the diagram below.
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.