Road Force Balancing for Stubborn Vibrations

A standard balancer only checks weight. A tire can be perfectly balanced and still shake if one part of it is stiffer or the wheel is slightly out of round. The road force machine presses a roller into the tire with a set load and measures the variation in force as it turns. Big variation feels like a vibration on the road.

What the machine reports:

• Weight imbalance, the same as a standard balancer, corrected with wheel weights applied in quarter-ounce steps
• Radial force variation, reported in pounds: how much harder the tire pushes back at its stiffest point
• Wheel runout, whether the rim itself is out of round
• Match mount suggestion, where to rotate the tire on the wheel

There is no universal pass mark. The acceptable figure in pounds comes from the wheel and tire maker for your size, and the tech reads your measurement against that, not against a shop rule of thumb.

It makes sense when a vibration stays after a normal balance, when you have low-profile tires on a car that transmits every flaw, or on vehicles known for being sensitive to tire uniformity. It is also useful to find which of four tires is the problem instead of guessing.

Good candidates:

• A shake at freeway speed that came back after one or two standard balances
• New tires that vibrate from day one
• Low-profile or large-diameter wheels. In a size like 225/65R17, the middle number is sidewall height as a percent of section width, so a lower number leaves less rubber to soak up a stiff spot
• Heavy EVs and luxury cars with quiet cabins
• Custom wheels. A replacement wheel has to meet or beat the original load rating, which is stamped into the back of the wheel, and we read that alongside the force numbers. See custom wheels.

If your only issue is a lost weight, a standard balance is usually enough. We tell you which one fits. Read about the regular service on our rotation and balancing page.

Every tire has a high point, where it is slightly stiffer or taller, and every wheel has a low point. Match mounting rotates the tire on the wheel so the tire's high point lines up with the wheel's low point. The two partly cancel each other, which pulls down the force reading in pounds and usually the amount of weight the final balance needs.

How it goes:

1. Measure the assembly on the road force machine.
2. Mark the tire and wheel positions the machine recommends.
3. Break the bead and rotate the tire to the new position.
4. Reinflate and measure again.
5. Balance the final assembly.

If match mounting does not bring the number down enough, the tire or wheel may be the problem. We show you the readings, and if a new tire is under the maker's warranty, see tire warranty inspection.

Road force readings also help decide which tire goes where. The steering wheel is most sensitive to front tires, so the lowest-reading assemblies usually go on the front. The ones with slightly higher readings go on the rear, where they are felt less. We note the positions so future rotations can take that into account.

After placement, we torque the lugs in a star pattern to the foot-pound figure your vehicle specifies, set pressures to the door placard, and road test to confirm the vibration is gone. Any wheel that comes off should be re-torqued after about 50 to 100 miles of driving, and we say so at pickup. If a shake remains, we look at other causes such as brake rotors, wheel bearings or driveline parts rather than balancing again.

A standard balancer spins the wheel, finds heavy spots, and we add weights to cancel them. A road force balancer does that too, but it also presses a roller against the tread to mimic the weight of the car. That roller finds stiff spots and out-of-round tires that a spin-only balance cannot detect.

A wheel can be perfectly balanced and still shake if the tire pushes harder at one spot as it rolls. Weights cancel a heavy spot; nothing you hang on a rim cancels a stiff one. Routine balancing usually rides along with a rotation, which most makers put at every 5,000 to 8,000 miles, and it is covered on our rotation and balancing page.

Then the cause is probably not the tire or the wheel. Road force balancing narrows the list because it rules out the most common culprits. What remains are parts that turn with the wheel or connect it to the car, and we check them in a logical order.

1. Brake rotors: pulsing felt mostly under braking points here, and it usually traces to thickness variation measured in thousandths of an inch with a dial indicator rather than to a warped rotor. See brake service.
2. Wheel bearings: a hum or play in the hub can feel like a vibration.
3. CV axles and driveshafts: a worn joint can shake under acceleration. See CV axle service.
4. Suspension bushings and tie rods: looseness lets a small imbalance feel large.
5. Hub face and wheel seating: rust or debris on the hub can keep a wheel from sitting flat.
6. Engine and transmission mounts: a shake that follows engine speed rather than road speed lives here.

Tell us when the vibration happens: at what speed, while braking, while accelerating or while coasting. That description often points to the cause faster than any machine.