What causes it?
Vibration can be maddening, especially when you don’t know where it’s coming from or what’s causing it. Typically, vibration is caused by either a tire/wheel issue, or a driveline component. Tire/wheel vibration can be caused by two things: an out of balance condition, and/or an out of round condition. If we suspect that the tire is out of balance, we will put it in our tire balance machine and check it out. It’s important to understand that not all tire machines are created equal. Also, not all tire technicians do things the same way. Variations in the way the wheel is chucked up in the machine, versus the way it mounts on the coach, can cause inaccurate readings. In other words, just because the balance machine says it’s balanced, doesn’t mean it will be on the coach.
To remove as many variables as possible, we use tapered, threaded centering tools that we have for most popular coaches. We put the wheel on with these, first. Then, one by one, remove them and replace with the wheel studs to be absolutely sure the wheel is centered. We’ve also tried several tire balancing machines, and they’re not all accurate. Back in the old days, the best way to determine tire balance was by spinning them up to highway speeds on the vehicle using another machine we have. Now, we have a tire balancing machine that has proven itself to be accurate. When it doubt, we can still spin the tire/wheel assembly on the vehicle as fast as 70-90 mph depending on the size of the assembly. This spinning machine is still very valuable to us when we are trying to diagnose unique problems.
We had a customer come in, a while back, whose coach was vibrating so badly his refrigerator stopped working. We balanced the tire and wheel assemblies several times with no change. It was only until we spun the assembly while mounted on the coach that we found out the drum brake assemblies were out of balance. In that case, we balanced the wheels right on the coach, with our strobe light balancer, to offset the drum imbalance. This process is very time consuming, sometimes taking 45 minutes to balance one tire.
Out of round tires can cause major problems, also. How do you tell the difference between out of balance and out of round vibrations? Generally speaking, with out of balance tires, the faster you go, the worse it shakes. With out of round tires, it will usually shake worse at a certain speed, then you can eventually drive through it. It could be a very high speed before it goes away.
We use a dial indicator to determine the run-out of a tire and check if it's within specs for the given tire and tire diameter. Very seldom are tires perfectly round, so you have to know the specs of the tire you’re working on. If the tire wasn’t mounted properly, it may not be properly seated on the bead, which will cause the tire to be out of round. This is an important thing to take into consideration before “truing” the tires to make them round again. If the tire is not properly seated, and you true it, you’ll end up ruining the tire. Sometimes, it may be necessary to mount and dismount the tire several times to make sure.
Bad wheels can sometimes be the cause of vibration also. If the wheel isn’t round, it’s going to affect the tire and how it works. An alloy wheel, like an Alcoa, is more concentric than a stamped steel wheel. You’ll notice that more expensive cars always have alloy wheels. This is not just for looks. It’s a rounder wheel, so there are less problems with out of round conditions, and less road harshness.
Shimmy or Vibration?
I think it’s important, at this point, to make the distinction between vibration and what we call “shimmy”. If you hit a bump and the steering wheel begins shaking in your hands (sometimes violently), then goes away when you apply the brakes, that’s shimmy. Shimmy not a wheel or tire issue. It means something in the front suspension isn’t right. There’s either a loose tie rod end, loose drag link or a worn-out steering damper. Excess camber can also cause it. You should take your coach to a qualified alignment shop as soon as possible, because something may be coming loose. Interestingly, people often think that their coach is out of alignment because it’s shaking, but that’s usually a tire/wheel problem.
Driveline problems usually come from the center of the vehicle. There are several things you can do to determine if it’s driveline related or not. If the driveline angle is incorrect, you will experience driveline vibration on acceleration or deceleration. Installing or re-installing a drive shaft incorrectly can also cause driveline vibration. The U-joints have to be lined up so they are in phase. If the technician knows what he’s doing, he can look and see that the U-joints are properly phased. A bad U-joint or lower CV joint are other potential causes of vibration. A driveshaft can also be out of round. We’ll check it for run-out with a dial indicator to see if that is the problem.
To diagnose a driveline problem, we put the coach in park and bring the engine up to speed. If there’s vibration, we know it’s absolutely not the tires or the driveshaft. It’s the flywheel, harmonic damper, torque converter, or maybe the engine itself. If there’s no vibration in Park, we can take the wheels off completely, put the coach on a lift, and run it up to 60-70 mph. If we don’t get any vibration there, we know the problem is with the wheels and/or tires.
Like anything else, finding and solving vibration requires a systematic approach. But when you know the root causes, it’s a lot easier.