Tech Stuff

NVH revisited: Correcting concerns relating to vibrational issues

The brake alignment shim is placed between the hub and rotor, oriented to correct lateral runout.
<p>The brake alignment shim is placed between the hub and rotor, oriented to correct lateral runout.</p>

Noise, vibration and harshness (NVH) issues are certainly not limited to wheels, tires, brakes and steering/suspension areas. Engine faults (misfires, rotating assembly balance, etc.), worn or damaged engine mounts and driveline issues (driveshaft, transmission and drive axle) are all potential contributors to NVH concerns. In this article we’ll discuss a variety of NVH issues along with tips to avoid and/or cure these ills, with an emphasis on chassis-related areas.

SHIMMY

Wheel shimmy (steering wheel right/left oscillation) can be caused by a number of variables. Follow an organized step-by-step inspection:

1. Inspect the tires for excessive and uneven wear, and inspect for tire structural damage (badly deformed sidewall, etc.). Poorly constructed tires or damaged tires where plies are misaligned or separating can cause a shimmy/vibration. Check the tire and wheel for proper bead seating along the entire bead circumference on both sides. An improperly seated bead will create a radial runout condition.

The brake alignment shims are precision tapered, available in a variety of thicknesses. A small V-notch on the shim indicates the thinnest area of the shim.
<p>The brake alignment shims are precision tapered, available in a variety of thicknesses. A small V-notch on the shim indicates the thinnest area of the shim.</p>

2. Inspect and adjust tire inflation pressure. Low or uneven pressures on the same axle can cause pulls or wander.

3. Check for lateral runout. Using a dial indicator at the wheel rim, slowly rotate the wheel (on the vehicle) a full 360 degrees, noting maximum lateral runout. If excessive lateral runout is found, don’t automatically blame the wheel, since runout may be the result of hub or rotor runout and/or a stack-up of tolerances between the hub, rotor hat and wheel.

4. Before removing the wheel/tire assembly, with the suspension unloaded, check for wheel bearing looseness by rocking the tire inward/outward at the 12- and 6-o’clock positions.

5. Remove the wheel/tire assembly and inspect the wheel fasteners (studs and nuts or bolts, depending on design). Damaged threads can be an indication of abuse/over-tightening which can contribute to wheel/hub distortion.

6. Inspect the mating faces at the rear of the wheel, rotor hat and hub face. A buildup of corrosion can cause a lateral runout condition. Clean all surfaces.

7. Check for lateral runout at the hub itself.

8. Inspect the wheel rims for damage (bent rims, signs of pothole/impact damage).

9. Check/adjust each tire/wheel assembly for dynamic balance.

10. Inspect the entire steering and suspension system. Looseness (caused by wear, damage or improper installation) can contribute to a shimmy, in addition to wander and directional pull.

11. On the alignment rack, measure all angles, paying particular attention to caster angles. An excessively positive caster (caster angle beyond factory specification for the specific vehicle) can promote a shimmy condition.

12. During reinstallation of wheels to the vehicle, always use a torque wrench and follow both the specified torque value and tightening pattern.

Use a dial indicator, mounted to a solid, non-moving component, to check for rotor lateral runout. Secure with lug nuts prior to measuring.
<p>Use a dial indicator, mounted to a solid, non-moving component, to check for rotor lateral runout. Secure with lug nuts prior to measuring.</p>

BALANCING TIPS

Centering of the wheel, both on the balancer and the vehicle hub is critical. If incorrectly mounted on the balancer, a precision balance won’t be possible. By the same token, a wheel that is correctly centered (and balanced) on a dynamic balancing machine won’t be able to duplicate the balance (and road force variation variable) if incorrectly centered onto the vehicle hub.

Most wheels are mounted to a balancing machine with a tapered cone that centers the wheel onto the balancer’s shaft.

Cones are available with different tapers. Cones that feature a low taper are preferable to better center the wheel. Back-cone mounting is preferred (where the cone mounts from the backside of the wheel). However, some wheel designs will not center using back-cone mounting. In these cases, front-cone mounting is required, where the cone centers the wheel from the outer side of the wheel.

When using the back-cone method, once the clamping cup and wing nut secures the wheel, depress the foot pedal to hold the spindle in place, and slowly roll the wheel towards you during initial tightening of the wing nut.

Tags: Mike Mavrigian 
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