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Silent springs

india Updated: Aug 28, 2009 11:27 IST
Grease Monkey

We revisited suspension last week — leaf springs, twist-beam and the like. Today we’ll continue with how one can fine-tune the suspension to suit the application.

Leaf springs are hard to tune. Get it right, though, and you can have a car that will satisfy your objectives. Only, the compromise between ride and handling that other systems offer is much better. Sometimes manufacturers prefer to use a simpler system for other objectives — the old Indica, for example, had a relatively sophisticated rear suspension, but repeated abuse will have the wheels at the rear lose their original settings, compromising grip and thus safety, and putting unnecessary strain on related components.

The right strut
The Indica Vista has moved to a simpler rear suspension that may not be as good for handling, but it liberates so much more interior space, and it isn’t like the average Joe is going to go around corners with the objective of finding the absolute limit of grip.

Most cars today use ‘McPherson struts’ for their front wheels. There are a number of reasons, but the primary ones are its cost-effectiveness and the packaging requirements. McPherson struts are quite a simple design, and then there are economies of scale. The more the number of cars that use this type of suspension that are created and sold, the cheaper the suspension system itself will get. The system also uses very little space, which is another plus point. It means more interior space for the same exterior footprint, and more space for the engine and related components, which are at the front of the car.

Compromising position
There are downsides, though: as the wheel goes up and down, it doesn’t stay truly straight, but follows a slight curve. This can compromise grip while turning. The other downside is, the wheel changes its angle with the vertical when turned. Most of us must have experienced at one point or another the squealing of the outside tyre while accelerating smartly with the wheels turned over as far as they will go. This happens because only a part of the tyre’s contact patch is in contact with the road, thanks to the wheel’s angle with the vertical changing.

If you look at a motorcycle’s rear suspension, you’ll notice springs wound around a tube. If the spring doesn’t have too many coils along this length, it will most probably be a soft suspension. If there are a lot of coils, it will probably be hard. You will also notice that the number of coils along the length increases towards the top. This helps the suspension to avoid bottoming out. The tube inside is the damper. We saw how it works last week, with the valve connecting the two reservoirs arresting the bouncy motion of the spring that absorbs road shock. If the valve is large, the fluid moves easily between the reservoirs, and the damping is soft.

What a damper
The current Mercedes-Benz C-Class has dampers with a mechanical system that recognises bad roads by the frequency of the wheel going up and down, and opens its valve for a more comfortable ride. Similarly, if you drive the car hard, the valve opening gets smaller to stiffen the dampers and offer better handling.

Nitrogen is used in dampers because changes in temperature don’t affect it as much as they do air, so damping is more consistent. Cars with air dampers (like the Volvo buses with ‘air suspension’) monitor the pressure in the dampers and increase or decrease the damping forces by pumping in or releasing air from the dampers.

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