Wheels and NVH -- How to make a vehicle ride like new again
Jacques Gordon has worked in the automotive industry for 40 years as a service technician, lab technician, trainer and technical writer. His began his writing career writing service manuals at Chilton Book Co. He currently holds ASE Master Technician and L1 certifications and has participated in ASE test writing workshops.
When driving a new vehicle, one of the first things you notice is how smoothly it rides. That new-car ride is mostly the result of a tremendous amount of engineering that goes into eliminating noise, vibration and harshness, otherwise known as “NVH.” Of course, as that smoothness gradually fades, most people assume it’s just a consequence of the car’s age and mileage.
But it doesn’t have to be that way, because unless there’s something wrong with the suspension, powertrain or motor mounts, most of a car’s NVH comes from the wheels and tires. With close attention to the details, it’s possible to make almost any car ride like new again, and in the process, give your customer a new appreciation of your professional abilities.
Of course, wheel balance plays a major role in ride quality, but it’s not the only factor. A tire and wheel assembly can be perfectly balanced in two planes, but it will only roll smoothly if it’s also round and perfectly centered on a straight hub. Brake drums and rotors play a role, too. We’ll describe how you can recover that new-car ride by making all of these tolerances stack up or cancel out, starting at the hub and working our way out to the tire.
Hub and brakes
Next time you’re doing a brake job, remove the rotor or drum and turn the hub to see if it’s wobbling. A wobble is typically caused by a binding CV-joint or a bent axle. A wobble of only a few thousandths of an inch might not be obvious unless you’re actually looking for it, but that can cause the tire to squirm on its tread, which increases tire noise. It will make the brake rotor wobble, too, pushing the caliper pistons deeper into the bore when the brake pedal is released, which causes longer pedal travel when the brakes are applied.
Everything that rotates must be balanced. Brake drums have balance weights tack welded to the face of the mounting flange near the outer rim. Brake rotors are balanced with weights in the vents or by grinding material away from the outer circumference. Production tolerances are pretty tight and there’s no reliable way to check drum or rotor balance to the same degree in the shop, so if drum weights have fallen off and if the rotor is worn or heavily rusted, it’s probably not balanced anymore.
You already know it’s important to make sure the hub and rotor/drum are clean where they contact each other. Some manufacturers recommend applying anti-seize compound on that contact surface, but that’s not just to make it easier to disassemble next time. On many OEM rotors, the “hat” section is slightly conical. When it’s installed, the hat section contacts the outer rim of the hub, but not the center of the hub. That little gap at the center is less than two thousandths of an inch, and when the wheel is installed and the wheel fasteners are tightened, the hat section flattens against the hub. This creates a slight bending load that adds strength and rigidity to the whole rotor/hub assembly. Some brake drums are like that, too.
It’s just as important to make sure the wheels are clean and the mating face lubricated with anti-seize, but for different reasons. Several years ago GM issued a service bulletin that said corrosion and grit between the wheel and rotor hub can create uneven clamping loads when tightening the wheel fasteners. Also, because they are dissimilar metals, alloy wheels often corrode heavily where they contact the rotor hat. GM recommends cleaning the hub, rotor hat and wheel with a wire brush, and other manufacturers recommend applying a thin coat of anti-seize before installing the wheel. Take care not to use so much anti-seize that it flings out onto the brakes. A thin film is adequate.
Wheel fastener torque
The importance of proper wheel fastener torque cannot be overstated. If the lug nuts are not torqued evenly, the wheel may not be properly centered on the hub. This alone will mimic a wheel balance problem. If the lugs are over-tightened, even if they’re all torqued evenly, the clamping forces will be uneven. This causes heat to flow unevenly from the brake rotor to the hub. After a few heat cycles, the uneven loading creates a permanent distortion, as much as three-thousandths of an inch, according to GM. Sometimes the rotor will actually warp, but usually this creates lateral run-out of the brake rotor, meaning the rotor face is no longer parallel with the plane of rotation. Even if there is no rotor thickness variation, it will result in brake pedal pulsation.
GM addressed this with TSB 01-05-23-001, which approves the use of a product called Brake Align. These are wedge-shaped shims that fit between the hub and rotor to correct lateral run-out. However, if everything is still straight, it’s easy to avoid this problem by making sure everything is clean and tightening the lug nuts in the proper sequence with a torque wrench... every...single... time.
Several manufacturers have issued TSBs about the proper way to mount a tire onto a wheel. Using too much lube or using it in the wrong place can cause the tire to slip on the wheel in the first few starts and stops. Instructions found in GM service bulletin 12-03-10-001B and Nissan bulletin NTB11-029 describe how to prepare a wheel and tire for mounting.
First clean the bead seat on the wheel to make sure it’s free of grease, rust or corrosion. This makes the tire less likely to leak and easier to seat. Don’t use petroleum-based products or corrosive solvents that might attack alloy wheels. This is the time to examine the wheel to see if it’s damaged or bent. If there’s any doubt about its condition, check it on the wheel balance machine before mounting the tire. We’ll discuss how to do that later.
When lubricating the tire for mounting, don’t use silicone; it can let the tire slip on the wheel under braking. GM and Nissan suggest lubricating the bead seat on the wheel, but other manufacturers say the only thing that needs lubrication is the tire bead where it contacts the tire changer shoe. Either way, make sure to lube only the bead, not the flange or tire sidewall.
Once the tire is on the wheel, pay attention to where you place it before inflating it to seat the bead. Pneumatic tires are springs that absorb bumps and keep the tread pushed against the road. They have a much higher spring rate than the suspension springs, and when properly inflated, that spring rate is determined largely by the stiffness of the tire’s sidewall. However, the sidewall rarely has the exact same stiffness at all points around the tire.
At the factory, sidewall stiffness is measured on a machine that inflates the tire and spins it against a roller under several hundred pounds of force. When the stiffness variations are within tolerance, the stiffest part of the sidewall is marked with a red dot and the tire is approved for sale. When the OEM mounts that tire on a wheel, the dot is aligned with a reference mark indicating a low spot on the wheel, and the production tolerances cancel each other out as the car rolls down the road. This is called match-mounting, and it makes a major contribution to that new-car ride. You can match-mount tires in your shop, too.
There are often two dots on a tire, a red one indicating the sidewall’s stiffest spot and a yellow dot indicating the point of the tire’s lowest weight. For match-mounting on most OEM steel wheels, align the red dot with a white dot on the inside of the wheel. Alloy wheels and GM steel wheels usually don’t have that white dot, so the tire’s red dot should be aligned with the valve stem.
Once the tire is seated on the bead, inflate it to at least 40 psi, and then bleed it down to the correct pressure. Make your own crayon mark on the sidewall next to the valve stem so you can tell if the tire slipped on the rim after test-driving the car.
According to OEMs and wheel balance machine manufacturers, the one detail that techs overlook most often when balancing wheel/tire assemblies is getting the wheel properly mounted on the balancing machine. Sometimes this can be a challenge because it’s not always obvious whether the wheel is hub-centric or lug-centric. In a hub-centric wheel, the hub bore is the center of the wheel. In a lug-centric wheel, the center of the lug pattern is the center of the wheel. With either type, if the wheel is offset on the balancer by just a few thousandths of an inch, it won’t balance correctly.
So how do you tell what kind of wheel you have? A hub-centric wheel has a very close clearance (maybe two thousandths) around the hub when it’s installed on the car. Most OE wheels are hub-centric, but not all. Aftermarket wheels usually have an oversized hub bore so they can fit several different vehicles. They come with a hub spacer ring that makes it easier to install the wheel, but these are always lug-centric wheels.
When mounting some hub-centric wheels on the balancer, a typical steep-taper cone will contact the hub bore on the outside (curb side) instead of the inside, and it might not be obvious. Car manufacturers now require their dealer techs to use Direct Fit collets, also called Precision Fit collets. A collet not only fits the wheel correctly, it’s also less likely to damage the finish on plastic clad or chrome wheels. When it fits correctly, the hub bore will contact the collet near the center of its taper.
Most hub bores are chamfered on the inside. This makes it easier to install the wheel and less likely that the wheel will rust in place. That chamfer is NOT the center of the wheel. When mounting a hub-centric wheel on the balancer, the cone or collet must contact the bore, not the chamfer.
When mounting lug-centric wheels on the balancer, you’ll need an adapter. Some hub-centric wheels have a plastic or chrome cladding that can be damaged by the pressure cup on the balancer’s wing nut. These should also be mounted as lug-centric wheels. Whether you use the adapter that came with the wheel balancer or an aftermarket tool, it’s absolutely critical that this adapter be adjusted to fit the lug pattern correctly.
To achieve that new-car smoothness that we’re after, the wheel must be round and straight. To check this, mount the wheel on the balancer without the tire. The more sophisticated balancing machines have “feelers” connected to sensors that will measure the rim as the wheel turns slowly. If the machine doesn’t have that capability, use a dial indicator.
Place the sensor or dial indicator where the tire bead seats on the inside of the rim and pointing toward the wheel balancer shaft. Turn the wheel slowly to look for radial run-out (out-of-round). Do this on both the inner and outer bead seats. GM allows up to 0.040-inch run-out, and even though that’s enough to feel, the say it can be canceled out by match-mounting the tire as described earlier.
The wheel should also be checked for lateral run-out by measuring the inside of the rims just above the bead seat, parallel to the axis of rotation. Again, 0.040-inch is enough to feel, and that much wobble can make the tire squirm on its tread, which contributes to noise.
One final detail
Often a customer will buy wider aftermarket wheels to enhance the car’s appearance or handling, and then discover the wheels’ offset is different too. Offset is the distance between the vertical face of the hub and the vertical centerline of the wheel. Zero offset means they are the same, and negative offset means the centerline of the wheel (and therefore the tire tread) is outside the hub. Wider-than-stock wheels tend to have more negative offset.
Usually this doesn’t cause any problems, but some cars are notoriously liable to vibrate or shimmy at certain speeds, and it will feel just like a wheel balance problem. Before installing aftermarket wheels, make sure you know if the offset is different from OE, and make sure your customer understands the risk.
If you do nothing more than make sure the wheels are straight and properly mounted on the balancer, you can balance the wheel/tire assemblies and the car will probably roll down the road better than when it came into your shop. But by match-mounting the tires, making sure the hubs, rotors and drums are straight, the wheels are perfectly centered on the hubs and the lugs are properly torqued, you can recapture most of the ride quality the car had when it was new. And your customer will notice. ●