Road Access for Disabled Americans


Active Suspensions and Speed Bumps

Active suspensions for automobiles supplement the springs with hydraulic systems that can lift or lower each wheel independently, under control of a computer that in turn has input from a number of sensors monitoring the forces from the road on each wheel, the load, car speed, individual wheel rotation rates, etc. Like anti-lock brakes 15 years ago, they currently are available only on the most expensive cars. Like anti-lock brakes, they have the capability of improving safety, since they can keep the wheels firmly on the road, making braking and steering more uniform and reliable, despite bumps that would otherwise cause the wheels to bounce clear of the road. Unlike anti-lock brakes, they will also improve comfort as well as safety. As a largely unintended side effect, they will make the large cash investment of some cities in speed bumps completely ineffective for all cars with active suspension.

When a car, having only springs and shock absorbers goes over a speed bump at a speed higher than the design speed, the wheels transmit a strong upward force to the springs which in turn push up on the car frame, giving it an upward velocity. By the time the top of the bump is reached, the car has a significant upward velocity, so it does not follow the pavement on the way down, coming down later with a substantial thump. Even below the design speed, the car body goes up and down by an amount comparable to the bump in size in a relatively short time. (The exact amount depends on the length of the bump and the distance between the front and rear wheels.) For some people, such as those with brittle bones or with severe spinal problems, this can be more than just uncomfortable.

On the other hand, if a car with a full active suspension were to hit a bump at a speed that would project a normal car into the air, the upward force and displacement on the front wheels would be quickly measured, and the wheels would be lifted, just keeping pace with the bump profile. Once over the top, the wheels would be actively lowered, again following the profile, while the car body continues on a relatively level course. The action is not unlike that of a roller skater going up a curb, where the front foot and then the rear are lifted, placed on the sidewalk, and then the knees are (relatively) slowly straightened, lifting the body. Now imagine what roller skaters might do if there was a curb going down a few feet later, before they had time to straighten their knees. They could place the front and then the rear foot down on the pavement, straightening their knees as they did so, keeping their upper body level at all times.

There are other advantages of active suspension. For example, with present cars going around a curve, the side closest to the curve center lifts up, exactly the opposite of what would be desired, where one would like the car to be banked as railroad tracks are on curves. Active suspensions will do that. (Active tilting is already in use on European high speed trains such as the Swedish X-2000 and the Italian Pendolino.) Another advantage occurs in emergency braking, where there is a large force slowing the wheels, while the body of the car is still trying to go forward. This causes the body to pivot around the front wheels, lifting the rear wheels up off the road, and reducing their braking ability. Active suspensions, keeping the body level, will reduce that effect. There are enough potential benefits such as these, that when active supsensions become affordable, they are likely to be quite popular. This places in question the long-term wisdom of investments in speed bumps rather than in other safety methods.

Sherwood Parker, 26 Dec. 1999


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