Section 4.2: Wheel and Tire Balancing and Runout

Section 4.2: Wheel and Tire Balancing and Runout

Principles of Wheel and Tire Balance

Wheel and tire assemblies rotate at high speeds on the highway, often exceeding 800 revolutions per minute. At these speeds, even a minor weight imbalance of a quarter-ounce (7 grams) can generate significant centrifugal force, causing vibrations, premature suspension component wear, and tire degradation. Wheel balancing is the process of neutralizing these heavy spots by adding counterweights to the wheel rim.

Static Balance (Single Plane)

Static balance refers to the equal distribution of weight around the wheel's axis of rotation, concentrated in a single plane.

• The Fault: Static imbalance occurs when there is a single heavy spot on the tire or wheel. When the vehicle is stationary, this heavy spot will always rotate to the bottom of the wheel due to gravity.
• Dynamic Effect (Wheel Tramp): As the wheel rotates at speed, centrifugal force pulls the heavy spot outward. This creates a vertical force that lifts and slams the tire against the road surface once per revolution. This vertical oscillation is called wheel tramp or bouncing.
• Symptoms: The driver feels a vertical bouncing vibration, which is often transmitted through the vehicle floorboards and seats. This vibration is typically speed-dependent, becoming noticeable above 40 mph.
• Correction: A static imbalance is corrected by placing a counterweight on the wheel rim directly opposite the heavy spot (180 degrees away). This can be done in a single plane, usually on the inner lip of the rim or the center drop-well using adhesive weights.

Dynamic Balance (Two Planes)

Dynamic balance refers to the equal distribution of weight along the wheel assembly's centerline, involving two separate planes (inner and outer).

• The Fault: Dynamic imbalance occurs when there are two or more heavy spots offset from the centerline of the wheel, often on opposite sides of the wheel's plane of rotation (for example, a heavy spot on the inner flange at the top, and another on the outer flange at the bottom).
• Dynamic Effect (Wheel Shimmy): When the wheel spins, centrifugal force attempts to align these offset heavy spots with the wheel's centerline. Because they are offset, this creates a couple-force that twists the wheel assembly side-to-side around the steering axis. This lateral oscillation is called wheel shimmy.
• Symptoms: The driver feels a rapid lateral, side-to-side shaking of the steering wheel. This shimmy is highly sensitive to speed, usually peaking in a narrow range between 55 mph and 70 mph.
• Correction: Correcting dynamic imbalance requires a spin balancer that calculates weight discrepancies in two planes (inner and outer). The technician must place correction weights on both the inner and outer flanges of the wheel rim to neutralize the twisting force.

Radial and Lateral Runout

Even a perfectly balanced wheel assembly can cause vibrations if it is not round or does not run true. Runout is the physical deviation of a wheel or tire from its design axis. Runout is measured using a dial indicator with a roller tip.

Radial Runout (Out-of-Round)

Radial runout is the variation in the radius of the tire and wheel assembly from its true center (out-of-roundness).

• Measurement: To measure radial runout of the assembly, the technician raises the vehicle, positions a dial indicator against the center tread rib of the tire, and rotates the wheel slowly by hand. To isolate the wheel, the tire is demounted, and the dial indicator is placed against the wheel's bead seat.
• Specifications: For passenger cars, the maximum allowable radial runout for a complete wheel/tire assembly is typically 0.040 inches (1.0 mm). For the bare wheel rim alone, the limit is 0.030 inches (0.76 mm).
• Dynamic Effect: Severe radial runout causes a vertical hopping vibration that mimics static imbalance, but it cannot be corrected by adding wheel weights.

Lateral Runout (Wobble)

Lateral runout is the side-to-side movement or wobble of the wheel or tire as it rotates.

• Measurement: The technician places the dial indicator against the smooth part of the tire sidewall (avoiding raised lettering) or the outer vertical flange of the wheel rim, then rotates the assembly.
• Specifications: The maximum allowable lateral runout for a wheel/tire assembly is typically 0.045 inches (1.14 mm). For the bare wheel rim, the limit is 0.030 inches (0.76 mm).
• Dynamic Effect: Excessive lateral runout causes a lateral wobble that mimics dynamic imbalance (steering wheel shimmy) and accelerates wear on the wheel bearings and steering linkage.

Match-Mounting Procedures

Match-mounting is a precision assembly technique used to minimize the combined radial runout and radial force variation of the wheel and tire.

• Radial Force Variation (RFV): Tires are not uniform; they have variations in sidewall stiffness (hard and soft spots) created during manufacturing. When rolling under the weight of the vehicle, a hard spot resists flexing and acts like a high spot, generating a vertical force spike once per revolution. This is measured in pounds of force using a road-force balancer.
• Aligining the Dots:
  • Red Dot (High Spot of Runout/RFV): Tire manufacturers stamp a red dot on the sidewall to indicate the tire's maximum radial force variation (the high spot/stiffest point). The technician aligns this red dot with the wheel's lowest spot, which is often marked with a dimple, notch, or white dot by the wheel manufacturer. This cancels out the high and low spots.
  • Yellow Dot (Lightest Balance Spot): If the wheel has no runout marks, the technician aligns the tire's yellow dot (representing the lightest spot of the tire) with the heaviest spot of the wheel, which is the location of the valve stem.
  • Priority Rule: If a tire has both red and yellow dots, the red dot takes priority. Reducing radial force variation (runout) is critical because it cannot be corrected with weights. Once the red dot is matched to the wheel's low spot, any remaining weight imbalance (which the yellow dot would have addressed) is easily resolved using a standard spin balancer.

Technical Scenario: Highway Vibration Diagnosis

A vehicle exhibits a persistent steering wheel shake at 65 mph. The shop has balanced the tires twice on a standard spin balancer, but the shake remains. The diagnostic technician mounts the front assemblies on a road-force balancer. The machine measures the dynamic balance (which is within limits) but registers a radial force variation of 32 lbs on the front-left assembly, which exceeds the passenger car limit of 18 lbs. The machine index-marks the tire and rim. The technician deflates the tire, breaks the beads, and rotates the tire on the rim to align the index marks (matching the high spot of the tire with the low spot of the wheel). Upon re-testing, the radial force variation drops to 12 lbs. The vibration is resolved without replacing any parts.