Tuesday, May 27, 2014
Bad Wheel, No Bounce!
So there you are, driving along the road, and a pothole or bump jumps out in front of you too late to miss. After the bump, the shudder and wobble continues for a hundred feet after the bump. Or instead, you hit the brakes hard and swerve around the bump; the car nose- dives and sways, then bounces back and forth a few times afterward. Or, you glance under the car at the shocks and see they are coated in oily dust. Guess what?
Any or all of these symptoms point to a likely failure of your shock absorbers or struts- they are ruined and need replacing. Blown shocks can cause all sorts of problems with handling, cornering and stopping. Lets take a few minutes and explore this a bit more for safety's sake.
Springs bounce. Thats the nature of springs. They flex, they bend, they move, but the tradeoff is that springs also bounce. We need the wheels on our vehicles to move and follow the terrain its rolling over, otherwise the wheel would often not be touching the ground at all. Without constant contact with the ground, the wheels cannot transfer forces of braking, steering or acceleration and are simple floating in the air uselessly. When a bump initiates a bounce, that elastic behavior will continue and oscillate the vehicle, or the wheels or both. When the wheel is bouncing, it cannot help to control the vehicle according to the drivers wishes.
Long ago humans discovered springs and other elastic material which allows one part to move relative to another. Carts and coaches of the 1500's and later used a single or double steel spring to help keep the ride smooth hundreds of years ago. Nowadays, its just a higher tech improvement of the old systems. The issue then was, and still is, undamped swaying or oscillation, rocking and heaving back and forth. The violence of these motions is directly related to speed and will get worse and rapidly more dangerous as speed increases because the impulse energy imparted to the wheel at greater speeds is greater as well.
At those higher speeds, a small bump can become a great impulse to the suspension system of a vehicle. Those greater energy impacts are shocks to the system which must be absorbed so as not to be allowed to become oscillation or vibration. Those vibrations can ruin other parts such as bearings, tie rod ends, ball joints, CV joints and especially tires if allowed to continue. A shock absorber unit maintains constant pressure downward on the wheel, is difficult to compress or move up, yet is easy to move back down, but will not easily move faster downward than it will by itself.
The viscosity of the fluid inside, the size of orifices that oil flows through, the valves that open or close in whichever direction, and the overall physical piston size inside the unit all contribute to the behavior and damping characteristics of the shock absorber. Larger vehicles need physically larger shock absorber units. A motor-scooter may only have a few ounces of fluid total in the shocks, while a large truck may move many times that amount of fluid every time it hits a bump. Every bump and pothole allow or force the wheel to move up or down at the rate determined by the springs, and controlled by the design of the shock absorber.
Inside, each bump moves fluid easily in one direction, and restricts fluid flow in the other direction. The valves and orifices inside restrict oil flow in order to control rate and speed of wheel movement. In doing its job to control wheel motion, the kinetic energy that gets absorbed gets turned into heat inside the shock absorber unit. The size and reservoir capacity of excess oil help control temperatures by providing storage and transferal to the air around it. Some high performance shock absorber systems use a remote reservoir and cooling system to control that heat buildup.
Struts are also shock absorber units, but along with just vertical energy absorption, the strut also acts as the upper steering linkage joint and rotating bearing as well as attachment point to the frame of the car. A strut is a glorified, multi- purpose shock absorber designed for a specific application and requires several special tools to change out. Most shock absorbers can be changed easily with no special tools.
In nearly all cases of moderate to extreme driving, a better, higher quality shock or strut is an upgrade and is noticeable in the difference in handling of a vehicle. If you have the option to get a better performing shock or strut unit, please do. A better quality unit will likely last longer, provide better ride and handling, and will tolerate more abuse and heat before being damaged. Consider an upgrade the next time you swap shocks.
Absorbing, controlling and containing the shocks and bumps of the road as well as preventing any vibration or bounce is an absolute requirement for safety. Keeping the tires in contact with the road or trail is imperative for control of the vehicle. Loss of contact of tire to terrain means loss of control forces directed at the ground. Without solid contact with the ground, there can be no steering, braking, or stability of the vehicle. Its not just shocks, its your safety involved. If they are leaky, bouncy, or mushy, replace them.