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Bouncing back

We’ve had a look at suspension many months ago, and you learned that its primary job is to keep the tyre in contact with the road.

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

We’ve had a look at suspension many months ago, and you learned that its primary job is to keep the tyre in contact with the road. Today, the priority is rapidly changing to the comfort of the vehicle’s occupants, but handling is not allowed to suffer too much — and these two objectives are always at odds with each other.

Up to a point, a stiff suspension will make a tyre stay in contact with the road much longer than a soft suspension, which makes the tyre bounce. This will not be a visible bounce, but the less weight on the wheel while it is coming upward means less grip as well, which compromises the handling and at some level, the safety.

You would always want the ability to steer out of a tight spot, wouldn’t you?

Suspension tactics
We’ve discussed leaf springs, the oldest and most rugged (and coincidentally, the cheapest as well) form of suspension. Our buses and trucks use them, as do American muscle cars and off-roaders like the Ford Endeavour and Mahindra Bolero. They’re hard to tune for a good ride/handling compromise, so they’re best left for load-bearing vehicles. Today’s cars use more sophisticated suspension — but that term is relative.

For example, most small hatchback cars use a ‘twist beam’ or ‘torsion beam’ rear suspension. This is nothing but a beam that runs across the car with an arm that goes to the axle, along with a spring that absorbs the energy that comes to it. Every time the wheel goes up (like when the car goes over a bump) the spring absorbs the shock, and the beam twists, damping the motion of the spring. This suspension is used because it is cheap and liberates a lot of space in the car.

Bouncer on a pogo stick
Each wheel has a spring and a damper to suspend it. The spring absorbs the shock of the wheel moving up and down, and the damper arrests the rebound motion. If it weren’t for the damper, the car would bounce around much like a pogo stick. The spring usually has a load rating for it that increases in a fixed pattern. If you look at the rear suspension of most Indian motorcycles, you’ll notice that there are more turns near the top compared to the bottom because the more the spring compresses, the more the load on it, and you never want your suspension to ‘bottom out’. For one, the occupants will get a bone-jarring shock, and secondly, it will really harm your wheel and suspension — don’t be surprised to see prematurely worn out bushings or bent wheel rims if you bottom your suspension out regularly. Some machinery like the Yamaha YZF-R15 have suspension at the rear that firms up as more load is applied on it, and prevents it from ever bottoming out.

Dampers also can be tuned to change the car’s suspension characteristics. They are usually filled with a fluid substance. It is similar to a syringe in the sense that there is a plunger of sorts, and a valve instead of a needle to let out the fluid slowly. When the suspension is compressed, the fluid leaks out through the valve, and is sucked back in on the rebound. Since the valve’s small opening makes it difficult for the ‘plunger’ to go up and down easily, the spring’s bouncy motion is arrested and the wheel stops moving up and down over and over again.

We’ll discuss different methods of suspension and damping next week — from McPherson struts and multi-link systems to electronic systems that change the damping from hard to soft at the touch of a button.