Well-balanced tires minimize the occurrence of vibrations felt in the steering wheel and vehicle, thus reducing driver annoyance and fatigue, and improving road contact, extending tire life and providing a safer vehicle.
Much has been written about tire balancing, but it’s not just about imbalance; it has to be round when rolling on the vehicle, too. Other causes of tire vibration are due to radial run-out and non-uniformity (eccentricity).
Every form of wheel vibration in both static, couple and runout forces creates an upward rise and downward movement of the spindle as the tire is in contact with the road surface. To place it all under one easy to remember acronym, call it RFV (radial force vectors).
RFV balancing is made up of many radial (up and down) forces which often can stack up, increase in magnitude and wreak havoc on ride quality. Those include:
• Wheel eccentricity;
• Balancer centering;
• Hub centering;
• And residual static imbalance.
The goal should be to eliminate vibration created by radial force vectors, since they can stack up, causing annoying vehicle vibration, tire wear and safety-related handling issues.
Why is this important to know? Radial force vectors are all the items that cause tire and wheel vibration. Providing the best possible balance involves an RFV wheel-balancing process during tire service to ensure that the wheels are best balanced and round when rolling when placed on the vehicle.
Most shops have the incorrect assumption that anything more than a simple balance takes too much time. A closer look at correct balancing often takes place only as a problem solver after a problem vehicle comes back with a complaint. The sad fact is most shops do nothing. They lack the simple training, equipment and tools.
New RFV wheel balancers measure RFV eccentricity in seconds. Combine the balance with simple eccentricity reduction processes and it completely allows RFV balancing to be done in the same time as a balance-only process.
Best Practices for RFV Wheel Balancing
1. Clean mounting surfaces. Clean the surfaces of the wheel mounting pad, hub bore and vehicle hub.
2. Mount the wheel on the balancer. Clamp the wheel on the balancer. In general, the best way to mount the wheel centered and protected is with the cone through the backside hub bore and a flange plate with pins from the front of the wheel. Some wheels have hub bore obstructions, as shown, and need short low-taper cones to properly mount the wheel from the backside hub bore.
3. Check centering repeatability. When in doubt, check the centering by recording the imbalance amount and location on the wheel. Loosen the wheel and reposition the hub/shaft and adapter 180 degrees from its original position, retighten the wheel and re-spin the wheel. Weight amounts and weight location on the wheel should be similar between the two spins. Balance and measure RFV eccentricity.
4. Check for residual static imbalance. Balance the wheel and check that, after balancing, the static residual “fine mode” imbalance is lower than the round-off increment of the wheel weights. Some wheel balancers have software that will address this automatically. Some balancers have a third weight display that shows residual static so a third weight can be applied, but most balancers do not.
5. Sensitive vehicle eccentricity RFV should be less than approximately 0.025-in. R1H, less sensitive vehicles up to approximately 0.040-in. If limits are high, remove the assembly, loosen and lubricate the tire, rotate 180 degrees, re-inflate and remeasure.
6. HubMatch mount the wheel on the vehicle. Rotate the wheel until the high point of the assembly eccentricity is at TDC. Mark the high point RFV (R1H eccentricity) of the assembly with a tire crayon across the backside sidewall, rim edge and tire tread.
7. Place the tire assembly high point R1H mark at 12 o’clock on the axle hub. Hand-tighten and step torque lug nuts to spec.
According to Ford TSB 2005-07-2, TSB 2005-26-24, TSB 2005-24-8, TSB 2006-12-2 Vibration, Nibble, Tire & Wheel Runout (F150, Expedition, Navigator, Econoline) and Service Manuals since 2007 – the idea behind the hub pilot is that the hub-to-hub bore fit is supposed to center the wheel. The fact is it is often much “too loose” and the clearances are enough to allow gravity to shift the weight of the wheel to rest only on the upper part of the hub creating a gap in the bottom. Even the smallest gaps result in an egg-shaped motion upon rotation that can cause huge errors in imbalance and run-out eccentricity stacking that causes tire force vibration. This uneven motion also creates a rolling resistance which causes increased fuel consumption and also erratic tire wear. The bottom line is an egg can be balanced, and it won’t roll smoothly.
8. Avoid spirited driving for 500 miles. If a vibration complaint return occurs, always check the crayon mark on the inside sidewall and rim edge for tire to rim slippage.
Why minimize HubMatch Eccentricity?
Consider where your biggest first-time problem complaints are coming from. Unless you’re a deep city shop, it’s the light trucks, especially with aftermarket wheels, right? When aftermarket wheels (often with larger hub bores than the OEM standard of 0.1 mm) are added to larger diameter, tall sidewall, non-speed-rated tires with heavier weight and diameter, the RFV eccentricity becomes extremely critical.
High eccentricity can add to centrifugal force changes in tire shape at highway rotational speeds inducing imbalance changes into the assembly at the high point where the eccentric tire “egg” changes shape. By reducing eccentricity on the vehicle, there is smaller change in shape as the vehicle approaches speeds in excess of 900 rpm at highway speeds.
So, when the wheel is balanced properly, eccentricity is within tolerance, and it is reduced from the balancer to vehicle, the tire assembly is rounder when rolling from low speed to high speed. A superior balance is performed in the same amount of time. Balanced remains balanced. All RFV remains minimized.
David Scribner is a product development manager with deep roots in automotive wheel service equipment, including several patents in iconic wheel balancing equipment.