Dealing with vehicle shakes

Providing a smooth ride for your customer can be a frustrating process.

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As a technician, you've likely had the opportunity to drive a fair number of different vehicles. Light-duty trucks ride differently than passenger cars, and compacts ride differently than full-size sedans. But all of their owners want a comfortable ride and appear in your service drive when they don't feel that's what they're experiencing. Sometimes the cause of these complaints is mechanical and repairable, and sometimes it's a matter of customer perception. Understanding the factors that influence ride quality can help you solve many of these complaints and explain to your customers why you can't solve them all.

What Defines Ride Comfort?

Manufacturers spend a lot of time and money making sure their products ride and handle well. Several factors impact that quality, including high frequency vibrations, body noise, body roll, pitch characteristics and others. One of the biggest contributors to ride comfort is the car's own natural vertical oscillation characteristics. When an umdamped suspension impacts a road deflection, it will have a tendency to oscillate vertically at its own natural frequency. (Dampeners, or shock absorbers, are designed to quickly dissipate this kinetic energy by converting it to heat, and are used to maintain as constant a pressure as possible on the tire's contact patch.)

Engineer Robert Q. Riley in his article "Automobile Ride, Handling, and Suspension Design" states that "...human sensitivity to ride frequency was believed to be associated with the natural oscillations of an adult human body during a walking gait. An adult walks at the rate of about 70 to 90 steps per minute (frequency), and the torso moves up and down about 2 inches (amplitude) with each step. Early designers therefore attempted to constrain vehicle oscillations to those limits, the ride was indeed comfortable, and the theory was therefore believed to be correct. Today, our information about human sensitivity to vibrations is more sophisticated. We know that amplitude affects human sensitivity to frequency, and that there are some frequencies that are especially uncomfortable."

If a vehicle design oscillates at a frequency outside of the customer's comfort zone, pitches or rolls excessively, or produces a high frequency resonance, the resulting ride will be uncomfortable to the passengers. Engineers can design the vehicle's suspension to keep these frequencies within acceptable limits, but there are some factors outside of their control.

Design Factors

One factor that affects ride comfort is the payload to vehicle weight ratio. Take two cars, one weighing in at 1,500 pounds and the other tipping the scales at 3,000 pounds. Each has a suspension designed to maintain ride height and steering geometry for that weight of car. Now add two passengers, our payload, with a weight of 200 pounds each. Which car will be affected more?

Apply that to the ride quality of a typical light duty pick-up. With nothing in the bed, the truck has some bounce to the back, doesn't it? Remember past instances of some trucks even locking up their rear brakes under a panic stop with no load in the bed? These vehicles were designed for work, and their ride characteristics were designed with the idea there would be stuff loaded in them. How about dealing with abnormal tire wear on a small import, not finding anything mechanically wrong and then meeting the driver? Ever align a car with the driver sitting in it to correct a tire wear complaint? I have.

Another factor is the ratio of unsprung weight to sprung weight. What's that? Sprung weight is the weight of the car supported by the vehicle's springs, while unsprung weight is the weight of the components that are not. The tire, wheel, brake components, hub assembly, even a portion of the steering and suspension components are not supported by the car's springs. Think of a kitchen table with a coil spring between each leg and the table's top. The four legs support the weight of the top and whatever you put on it. The top and its load are sprung while the legs are unsprung.

When the tire/wheel assembly hits a bump, all the weight becomes a mass in vertical motion that must be controlled, and the moving force deflected away from the rest of the car and its passengers, or they'll feel the full effect of the impact. The greater the weight, the greater the effect. Does that explain why you see so many vehicles using cast aluminum wheels, brake and suspension components? And why a customer who traded in those wheels for a set of heavy 20-inch custom rims and tires may have a ride quality problem?

These are just a few examples of situations affecting ride quality that you can do little about, other than educate your customer. Operating their car outside of its design parameters is on them, not you or the manufacturer.

First Contact

The tires are the point of contact between the vehicle and the road. They have characteristics of their own that have a direct impact on how well the vehicle rides and handles and problems with the tire/wheel assembly are common causes of ride quality complaints.

One common cause of ride quality issues is very easy to eliminate. Tires have air in them, and that makes them air springs, doesn't it? Too little air has the same effect as a softer spring would, while too much air is like adding a stiffer spring. I know you've heard those NASCAR crew chiefs instructing their crew to "take a pound out of the right rear." Most of the cars that enter your shop will have tires that are grossly underinflated. As I spoke of last month, performing a "60-second tune-up" and correcting the tire pressures for each and every customer will have them wondering what magic wand you passed over their hood!

The next few causes I'll share all are related to harmonic vibrations. Any factor that creates an imbalance in the rotating tire/wheel assembly creates this vibration. It's called a first harmonic if the cause only occurs once per revolution, a second harmonic if it occurs twice, and so on. The first cause you are most likely familiar with is dynamic imbalance; a problem that hopefully you'll prevent from happening by balancing the assembly on the shop's balancing equipment. And to make sure you do it right, consider these common mistakes:

Incorrect mounting on the machine:

o Some wheels are aligned to the hub using the rim's center, while others are aligned by the stud's position. You have to use the correct adaptor for the type of wheel you are balancing. o On rims that are installed using cones, look for a chamfer on the rim itself, indicating what side of the rim the cone should be installed on.

Incorrect measurement of rim width and size:

o Some cast and closed faced rims cannot be accurately measured with the balancing machines calipers or extensions, and the machine won't accurately balance the assembly if given bad information to begin with. You might have to measure the rim with the tire removed first.

Incorrect bead seating of the installed tire:

o If the tire is not seated properly, it won't spin true and the resulting balance will be inaccurate after a few miles of driving. Properly lube the bead and rim when installing the tire with a product specifically designed for the purpose. In some cases, you might need to inflate the tire to the maximum pressure specified on its sidewall, deflate it and then inflate it again to the car's specification. Never exceed the maximum allowed, and always wear eye protection.

Another cause of harmonic vibration is tire/wheel runout. That's easy enough to understand, isn't it? If the assembly is shaped like an egg, it won't run smoothly down the road. Issues with runout are not limited to just one spot, though. Abnormal tread wear caused by alignment or suspension issues can cause multiple high and low spots around the tire's circumference that will lead to ride quality complaints, even if they aren't severe.

Tread separation can cause vibrations vertically and horizontally, depending on the damage. There are tire runout gauges available, but you can find these problems just as easily by looking closely as you spin the wheel on the car or the balancer. Take the time to run your hands around the circumference and sidewalls of the tire to check for damage and abnormal wear you may not see with your eyes. Just be careful you don't run your hands over any exposed steel.

Radial Force Variation

Remember what I said earlier about tires acting like springs on the car? Not just one spring, but dozens, with each one taking over as the weight of the car moves over it. Air pressure impacts this spring rate, and so does the tire's sidewall construction. If all of these springs are the same, then all is well. If some are stiffer or softer than the others, then we'll have a problem called radial force variation, a problem that the Hunter GSP9700 was designed to help solve.

According to a white paper written by Dan Parker and Dave Scribner of Hunter Engineering, "Tire Uniformity Tester for Automotive Service Industry," "Various manufactures have published limits on tire/wheel assembly free (unloaded) radial runout measured at the center of the tire tread. While this is a measurement that a technician can make with a relatively inexpensive gage, it is not a measurement that has close correlation to vehicle vibration problems. SAE recommended practice J332, which is widely used in the tire industry, describes tire testing equipment to measure tire uniformity. This practice stresses the importance of measuring force variation while the tire is under load and does not acknowledge unloaded free runout measurement. The uniformity of most tires manufactured today is measured with a machine, which conforms, to SAE J332.

"Tires which do not meet uniformity specifications may be brought into specification by grinding small patches of rubber off the sides and footprint of the tread. This grinding procedure is done to improve the radial force variation — but may not make any improvement to the free runout measurement. A tire with large free radial runout may roll vibration free under load, and a tire with low free radial runout may vibrate when rolling under load."

Endorsed and used by several OEMs, the GSP9700 uses a roller assembly against the tire to simulate road load and measure this variation. Of course, it also measures for dynamic imbalance and other causes of harmonic vibration. Often, the tire/wheel assembly can be matched by changing the tire's position relative to the wheel, and according the referenced white paper; can result in a reduction in radial force variation of more than 50 percent.

While problems with the tire/wheel assembly are not the only cause of ride quality complaints, they do make up the majority. Other, less common, causes include mounting of the assembly to the vehicle; runout or imbalance of the hub assembly itself, and others.

Maybe it's a combination of problems, but providing a smooth ride for your customers need not be a bumpy ride for you if you know what you need to be looking for!