Hayden Cobra - Chassis Design

Chapter 2

We continue our story in chapter 2 - with the chassis design and the reason why our Cobra has the best performing chassis on the market.

The chassis of a motorcar can in some ways be likened to the foundations of a house. If the foundations are weak, no matter how much money is put into the rest of the building, one will never have a good house. Similarly, if the chassis of a car is weak it will never be a satisfactory vehicle. The chassis performs an important function in connecting the various suspension components to each other in addition to supporting the engine and transmission, the body and the occupants of the car. The suspension components must be kept correctly aligned so that the suspension movement and geometry which has been so carefully designed in by the engineer remains constant and is not affected by flexing of the chassis under load. A weak chassis which twists and bends excessively under load will lead to a car which can feel unstable to the driver at speed and road-holding can suffer.

The original AC Cobra had a simple ladder frame chassis consisting of two parallel three inch diameter round steel tubes connected by a few welded cross-members. It was probably adequate for the AC Ace on which the Cobra was based and which was powered by a two liter Bristol engine. Carroll Shelby replaced this initially with a 4,2 liter Ford V8 engine, soon increased to 4,7 liters on his early Cobra, and later by the Ford Nascar 7 liter race engine. This was a great deal heavier than the smaller engines and also much more powerful, so Shelby increased the chassis tube diameter from three to four inches on the later model Cobras in order to improve chassis stiffness. However, it remained a rather crude and inadequate structure for a car that could top 165 mph and it required the skills of an exceptional driver to control this beast at speed.

Apart from inadequate stiffness, the chassis design meant that the seats sat rather high on top of the chassis tubes, leading to taller drivers complaining that when seated in the car they found themselves staring directly at the top of the windscreen frame and not through the glass. One even remarked to Les that he felt like a cartoon character when driving his Cobra, with the top half of his body protruding from the car. He was reminded of drawings he had seen of Mickey Mouse and Donald Duck driving their cartoon cars!

Les soon realized that all these problems could be addressed by changing the chassis to a more sophisticated multi-tube structure, properly triangulated to provide greater stiffness and designed to allow a much lower seating position for the taller drivers. He then set about designing this new improved Cobra chassis, and after transferring his original drawings to CAD (Computer Aided Design) he took it to Willem Achenbach, an aeronautical engineer who specializes in Finite Element Method Stress analysis. The FEM program allowed Willem to subject the structure to loads in his computer similar and even greater than those generated at speed and see in detail exactly how it performed under these conditions. It highlighted areas of high stress concentrations in color and also revealed areas where stress concentrations were low and weight could be saved, and clearly indicated the magnitude of chassis deflections under load. The design could thus be fine-tuned on the computer before the first chassis was constructed, and giving Les the peace of mind in knowing exactly how his new chassis would perform in practice.

An improvement of 300% in beam and torsional stiffness was achieved by this method, and Les knew that he was well on the way to improving his Cobra’s high speed handling and safety. Some Cobra replica manufacturers try their best to exactly duplicate the original AC Cobra in every detail, even to the extent of copying the round tube ladder frame chassis. At least one of them even duplicates the curved thin steel tubing sub-frame which supported the flimsy aluminum body of the original car, even though this is now under a much stiffer and stronger composite body and completely unnecessary.

Les considers this to be rather foolish, because to continue to reproduce the faults of the original car when one can vastly improve the handling and safety by installing a more modern chassis under that sexy body seems to be senseless.

A test drive in a new Hayden Cobra built with the improved chassis showed its advantages when Les removed his hands from the steering wheel at over 200 kph and the car tracked perfectly and remained absolutely stable. Hard braking from this speed with his hands still off the wheel had the car pulling up dead straight with no steering input required. However, this car was also fitted with the new compound angle suspension which Les designed for his Cobras.

The story behind that suspension will be revealed in the next chapter.

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