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Core issues

A stiff chassis is the secret behind a strong car

india Updated: Aug 28, 2009 11:29 IST
Grease Monkey

We’ve seen different versions of cars at one point or another. I’m not talking about variants, but major changes in, say, suspension settings or body style. An example close to home would be the Ford Fiesta — they’ve got a petrol variant called the 1.6 S with suspension that’s stiffer than the normal petrol variant. Another car we can relate to is the Honda Civic — the hybrid version is quite different when compared to the other variants. Today, we’ll look at what manufacturers try to achieve when they create different body styles or variants of a single car model.

Similar yet different
First, let’s examine why a manufacturer would put out different models of a single car. There is the obvious — different people will want different things, and developing a new car for each of those requirements is a daunting task. It takes a few years and more than a few million dollars to develop a new car, so car makers prefer to lump new models together under a single ‘platform’. For example, the Indica, Indigo and Indigo

Marina are all variants of the same platform. They have the same suspension components, use similar engines, and share mechanical parts.

However, if you look at the dimensions of the three cars, both internal and external, you’ll notice differences.

There’s also a difference in their weight, so the same suspension settings will not work as well for all variants. The Indica has no boot, so the engineers don’t have to worry about weight at the rear. The Indigo — especially the Marina, which has a lot of space — will expect lots of weight, so the chassis (which you can think of as the car’s skeleton) has to be lengthened and strengthened to cope with the extra forces. It’s the difference between lifting a weight in the palm of your hand, and placing it on the end of a metre-long pole and trying to lift it while keeping the pole horizontal.

The convertible challenge
Another challenge chassis engineers face is the development of a convertible. You see, the roof of a car is strong.

It has got to be strong to prevent the occupant’s heads from being crushed should the car roll over. It also gives the chassis a helping hand — the chassis has got to be stiff in a number of ways; it cannot bend like a snake, else the driver will get a scare in a corner. It can’t bend too much vertically, either, because it’ll then feel extremely uncomfortable over uneven roads. The combination of the vertical and horizontal bending can give the impression that the chassis is twisting, which can be downright scary. To prevent this, the chassis is strengthened in places that undergo a lot of stress, either by using different compounds, or by a technique called ‘spot welding’.

Convertibles usually are a pain in the, well, bottom. Since the roof is lopped off, it can’t contribute towards chassis stiffness. The car is then strengthened in the floor under the passenger compartment to make up for the lack of rigidity. This also adds weight to the car, so if it has the same engine as its sedan or coupe counterpart, it will be slower in a straight line as well. As always, though, there are exceptions to the rule: Honda’s S2000 is one. The other is the world’s best-handling car which is also a convertible — the Porsche Boxster.