by Jim Kerr
Rough roads, potholes, and curbs all take their toll on your vehicle over the course of a year. Some of the jolts can be hard, and damage to your vehicle can occur quickly. Your first line of defence is your vehicle’s shocks, springs, and tires. Here is how they help soak up the bumps.
As your vehicle drives over a hole, the wheel, tire, and suspension assembly are pushed down into the hole by the vehicle’s springs. Some manufacturers use coil springs, while others use torsion bar springs to support the weight of the vehicle. Regardless of the type of spring, the rapid downward movement of the wheel and suspension is controlled by the shock absorber or strut assembly.
The internal operation of a shock absorber and a strut are the same, but a shock absorber only helps control suspension movement. A strut, by design, also helps hold the suspension and wheel in the proper location. If the shock or strut is working properly, it will slow the downward movement of the suspension into the hole, but quickly allow the suspension to move upwards on the other side. This is accomplished by directing oil inside the shock through a series of one-way valves and orifices.
As the tire hits the exit side of the hole, it must move upward very quickly. This is where damage can occur, both to the tire or wheel and to the suspension and steering. If the impact is severe, the suspension can “bottom out”. This means the suspension hits a rubber stop placed on the vehicle to limit the maximum upward travel of the suspension. A severe bang or jar is felt throughout the vehicle, and then you are on to the next bump or hole.
During the impact, the tire, wheel, springs, suspension and steering may be damaged. Let’s look at what happens to the tire and wheel first.
As the impact of the tire occurs on the exit edge of the hole, the tire pushes in towards the wheel. If tire pressure is low, the tire may be pinched between the edge of the hole and the wheel, cutting the cord material inside the tire. This may not be obvious with an external inspection, but it could result in tire failure at a later date. Low profile performance tires make this problem even worse, because there is less tire sidewall to cushion the blow.
Fortunately, modern tires are extremely tough. If the tires are inflated properly, the chances of damaging a tire on most impacts are small. Check the tire pressure often.
Wheels can be damaged when the tire compresses enough to allow the wheel to hit the edge of the hole. Steel wheels can be bent, while alloy wheels usually are broken, although I have found a couple bent alloy wheels on vehicles. The wheels can be damaged in the lip or bead area or bent in the centre where they bolt to the hub. Damage to the bead area can usually be repaired by speciality wheel shops.
Bent wheels cause vibrations while driving. This can be seen easily when the tire and wheel is spun on tire balancing equipment. A bent or broken lip in the bead area can also cause a slow air leak out of the tire. Check the inside lip of the wheel as wheel as the outside when looking for damage.
Rubber suspension stops are tough, but they can be broken when the suspension hits them during severe bumps. The suspension stops are designed to control only the occasional impact. If they break, then the suspension will have metal to metal contact with the body or frame and any upwards impact can bend or break the rest of the suspension. Note that some cars have the rubber stops built internally into the shock absorbers, so damage to the shock absorbers can also occur.
There are ways of preventing damage to the rubber stops and the rest of the suspension. First, avoid the pothole! This may sound too simple, but if you steer enough to the side of the hole so your tire rolls at least part way on the edge, you have avoided the majority of the impact.
Second, slow down! The higher the speed of your vehicle, the faster and longer your suspension moves upwards on a bump or hole. Driving slowly over a hole should do no damage to your suspension. Just look at the serious off-road 4x4ers and you will see them traversing rough ground very slowly. Always apply the brakes before a bump or hole to slow the vehicle, but release them just before hitting the hole. This allows the tire to roll out of the hole rather than slamming into the far edge of it.
Third, keep good quality shock absorbers on your vehicle. The shocks slow the downward movement of the wheel as it moves into a hole so it does not have to move up as much on the exit side. Then they allow the wheel to move upwards quickly so the impact with the hole and vehicle damage is reduced. The same happens on a bump, but in reverse. First, the wheel moves upward quickly, then the shock slowly lets it back down to contact the road. Good shock absorbers control the action of the suspension to reduce jars and bangs on both the suspension and the body and improve vehicle handling immensely.
Shock absorbers or struts can be checked visually. Wet oil on the outside of the assembly means it needs replacing. A small oil film of oily dirt on the outside does NOT indicate a leaking shock, so if you are shown this on your vehicle and they tell you otherwise, take it elsewhere. Another test for shock absorbers is to bounce the car up and down to test their operation. Push down with your whole body weight on each corner of the vehicle and quickly let the body back up. The vehicle should bounce up, back down, and then up one more time. This shows the valves inside the shock are working properly. If the car bounces more than three times when doing this test, the shocks definitely need replacement.
Finally, any severe impact can slightly move or bend suspension components. This may not show up when driving on slippery roads, but it can quickly wear tires as the pavement dries up. A wheel alignment should be performed after any major impact to prevent unwanted tire wear. As a bonus, most technicians check the vehicle’s suspension, steering and shocks as part of the wheel alignment so you can continue to enjoy safe driving.