Q: I just saw a Mustang going down the freeway with its backend bouncing up and down over bumps like a yo-yo. Then the back tires started doing the yo-yo! What could cause this? Also, I've recently had several drivers flash their lights at me, even though I was on low-beam. Why would they do this?
_Donna P.
A: I'm pretty sure the Mustang's tail-bouncing was the result of really worn-out rear shock absorbers. Shocks and struts dampen suspension spring oscillations, stabilizing the vehicle and keeping tires and wheels planted firmly on the ground for proper traction. Struts incorporate a shock absorber assembly within a tall suspension member typically surrounded by a large coil spring.
Shock absorbers are a tube-shaped component containing hydraulic fluid, a piston, orifice and metering valves. Kinetic energy (suspension travel) is converted into heat as the fluid is forced back and forth between the shock's chambers, through the orifice, by the piston.
Shocks and struts can lose function and become mushy or over-extended. This can happen as the result of faulty valves or worn seals, which can lead to internal or external fluid leakage. This may begin occurring to a noticeable degree at perhaps 40,000-50,000 miles, or sooner, if rough road conditions are frequently encountered. Worn shocks or struts can increase stopping distance and body roll when cornering. They also allow nose dive on braking and acceleration squat, cause tire tread cupping, and accelerate suspension component wear.
Regarding your headlight question, I wonder if you might be seeing the effects of a modern HID/projector headlight cropping over a bumpy road. Many vehicles now use a single element for both high and low beam. The upper edge of the low beam is mechanically cropped by a shade mechanism, providing a much more abrupt beam cut-off than older dual filament halogen lamps. The shade is moved for high beam function. I've witnessed "flashes" from oncoming cars due to their beam cut-off bouncing up and down on bumpy roads as they approach me.
It's also possible your headlights are out of alignment or are projected high because your vehicle is carrying excessive weight in the rear. Some vehicles are equipped with leveling bubbles on the headlight housings. To check them you must first park on a truly level surface with properly inflated tires and typical fuel and luggage loads. If a correction is needed, the method of adjustment can vary a lot between vehicles. A simple-but-primitive way to check vertical aim, if there isn't a bubble level, is to find a level surface in front of a wall or driveway door (check by placing a level on the door sill, door open). Next, measure from the ground up to the exact center of both headlamp lenses (turning them onto low beam helps find the spot) and record the measurement. Mark this same height on the wall with a horizontal strip of tape. With the vehicle parked with the headlights on, 25 feet from the wall on a dark night, check to see if the most intense part of each low beam is about 2-3 inches below the tape line. If not, professional adjustment is recommended. Horizontal adjustment is trickier to check, and could be an issue as well.
