Diagnosing Steering Pulls and Vehicle Vibration

"It pulls" and "it shakes" are two of the hardest complaints in a chassis bay. Both can come from several systems, and both get fixed the wrong way all the time. A car gets an alignment for a pull caused by a tire. It gets four new rotors for a shake caused by a hub. Each wrong guess costs the customer money and costs the shop a comeback.

You already know how to inspect parts and read an alignment printout. This is about the tests that come first, so you know which system to look at before you put the car on the rack.

What You Will Be Able to Do

• Pin down exactly what the customer feels and when it happens.
• Use a road test and a tire swap to find out whether a pull comes from the tires.
• Trace a brake pull to the wheel at fault, and test for a collapsed hose.
• Use road test checks to sort a vibration by speed, engine rpm or braking.
• Work out tire, driveshaft and engine frequency and match them to a reading.

Name the Complaint First

Customers use the same word for different things. Ride with them if you can.

Most roads have a crown, a slope built in so that water drains to the side. A healthy car will drift gently toward the low edge. That is not a fault. Test on the flattest straight road you can safely use, and drive it both ways.

Finding the Cause of a Pull

Work from the cheapest test to the most expensive. Do not start with the alignment rack.

1. Set the tires. Set all four to the placard pressure. Check that each axle has matching size, type and similar wear.
2. Road test. Note the speed, the direction of the pull and how much force it takes to hold the wheel straight.
3. Coast test. At a safe speed on an empty road, lift off the accelerator. If the pull goes away, suspect torque steer. If it stays, keep going. On an automatic, you can shift to neutral briefly.
4. Brake test. Brake moderately on a straight, empty road. A pull that starts or gets worse here points to the brakes or to loose parts.
5. Heat check. After the drive, compare wheel and hub temperatures across each axle with an infrared thermometer. A wheel much hotter than its partner has a dragging brake or a failing bearing.
6. Tire swap. If the car still pulls, swap the front tires side to side and drive it again.
7. Alignment. Only now does the rack make sense.

The Tire Swap Test

Conicity is a fault in how a tire was built. The belt sits slightly off center, so the tire acts like a cone. A cone rolls in a curve, so the tire steers the car even when the alignment is perfect. Balancing does not fix it, and alignment does not fix it either.

Directional tires are built to roll one way, so move them front to rear on the same side for this test instead. On a car with different front and rear tire sizes, you cannot move tires between axles, so a tire shop may need to test the suspect on a force-measuring balancer.

Worked Example: Pull After New Tires

A car pulled right the day it got new tires, and the shop's alignment printout is in spec. Pressure is equal. No wheel runs hot. You swap the front tires side to side, and it now pulls left. The pull followed the tires. You move the left front tire to the left rear, and the pull stops. The tire now on the rear was the cause. It goes back under warranty, and no alignment was needed.

Pulls That Change

A pull that changes direction is not an alignment problem, because alignment angles do not swap sides on their own. Look for something that binds or moves.

• Memory steer. On strut cars, a seized upper strut bearing is a common cause. With the wheels off the ground, turn the steering lock to lock and feel for notchy, sticking movement.
• Pull only on bumps or when you brake. Worn control arm bushings or a loose ball joint let the wheel shift under load.
• Pull only on acceleration. Front-wheel-drive cars with unequal axle lengths have some torque steer by design. Worn engine mounts make it worse, because the engine moves and changes the angle of the axles.

Brake Pulls

A car under braking pulls toward the side doing the most braking. That can mean one side is braking too hard, or the other side is not braking enough. Do not assume the fault is on the side the car pulls toward.

Testing for a Collapsed Hose

A brake hose can break down inside while it looks fine outside. The damaged lining acts like a one-way valve. Pressure gets through to the caliper but cannot come back, so the brake drags. Or it blocks flow, and the brake on that side barely works.

1. Raise the vehicle safely so the suspect wheel can spin.
2. Have a helper press the brake pedal firmly, then release it fully.
3. Try to spin the wheel by hand. If it stays locked or drags, the fault is holding pressure somewhere.
4. Crack the caliper bleeder screw. If fluid spurts out and the wheel turns freely, pressure was trapped. The hose, or something else upstream of the caliper, is at fault.
5. If the wheel still drags with the bleeder open, the fault is at the caliper or pads.

Warning: Do not road test a vehicle with a brake that is locking or dragging hard. An overheated brake can fade, and the fluid can boil. Fix it on the lift first.

Sorting a Vibration

A vibration has three possible sources: something turning at road speed, something turning at engine speed, or something happening only during braking. These tests sort them out.

1. Find the speed. Note the speed and rpm where the vibration is worst.
2. Neutral coast test. Reach that speed on an empty road and shift to neutral. If the vibration stays, it follows road speed, which points to the tires, wheels, hubs or driveline.
3. Engine test. Park, and hold the engine at the same rpm in neutral. If the vibration returns, it follows engine speed, which points to the engine, its accessories or its mounts.
4. Brake test. A shake only under braking points to the rotors, drums or hubs.

Where you feel it helps too. A front tire problem usually shakes the steering wheel. A rear tire or driveshaft problem is usually felt in the seat and floor.

Using Frequency

Frequency is how many times something happens each second, measured in hertz (Hz). Every turning part vibrates at its own speed. If you measure the frequency with a vibration analyzer or a phone app, the math tells you which part to look at.

Order means how many pulses happen in one turn. First order is one pulse per turn, like a single heavy spot. Second order is two pulses per turn, which suggests something bent, off center or at the wrong angle, not just out of balance.

Worked Example: Finding the Source

A rear-wheel-drive truck has a floor vibration at 60 mph. The engine is at 2,200 rpm, the tires are 27 inches tall, and the axle ratio is 3.73. The analyzer shows 46 Hz.

1. Tire circumference: 27 x 3.14 = 84.8 inches.
2. Tire turns in a mile: 63,360 inches / 84.8 = 747.
3. At 60 mph the truck goes one mile each minute, so the tire makes 747 turns a minute: 747 / 60 = 12.4 Hz.
4. Driveshaft: the shaft turns 3.73 times for each tire turn: 12.4 x 3.73 = 46 Hz.
5. Engine: 2,200 / 60 = 36.7 Hz.

The reading of 46 Hz matches driveshaft first order, so you check the shaft for lost balance weights, undercoating or a dent before you balance a single tire. Had it read 92 Hz, you would look at U-joint angles and runout. These numbers point you to a part. Confirm it with a test on that part.

Tire and Wheel Vibration

• Smooth-road shake at highway speed: Balance is the first check, but a good balance does not prove a good tire.
• Shake that remains after balancing: Check radial runout, which is how far the tire is out of round, and lateral runout, which is its side-to-side wobble. A road force balancer presses a roller on the tire to find stiff spots a normal balancer misses.
• Shake after a tire rotation: The vibration moved with the tire. Look for a damaged tire or a bent wheel.
• Wheel seat problems: Rust on the hub or a wheel that is not centered makes a good tire shake.

Brake Vibration

Pedal pulsation or steering shake under braking usually comes from thickness variation in the rotor, as you already know. The harder question is why a rotor got that way.

Thickness variation often starts with runout. A hub face that is rusty, or a rotor that is not seated flat, makes the rotor wobble. It then scrapes the pads on each turn, and the rotor wears unevenly over the next few thousand miles. New rotors on the same hub come back.

1. Clean the hub face fully before you fit any rotor.
2. Measure runout at the hub face with a dial indicator. Limits vary a lot between makers, and many ask for about 0.002 inch or less. Use the spec for the vehicle.
3. Fit the rotor, torque the lug nuts or use conical washers, and measure rotor runout.
4. If rotor runout is high but the hub passes, remove the rotor, turn it to a new position on the hub, and measure again.
5. Tighten lug nuts in a star pattern with a torque wrench. Uneven torque can distort the hub and rotor.

Common Mistakes

Check Your Understanding

1. After a tire swap, a pull to the left becomes a pull to the right. What did you learn? A front tire is the cause. Move one front tire to the rear to find out which.
2. A car pulls left only when braking. The right front wheel is cooler than the left. Which side do you inspect first? The right front. It may not be braking at all, so look for a seized caliper, a restricted hose or contaminated pads.
3. A wheel stays locked after the pedal is released. You crack the bleeder, fluid spurts out, and the wheel spins free. What is the likely fault? Pressure was trapped upstream of the caliper, most likely in a collapsed hose.
4. A vibration at 2,800 rpm appears at 70 mph. It stays when you shift to neutral and coast at 70, but it does not appear at 2,800 rpm parked. What is the source group? It follows road speed, so it is the tires, wheels, hubs or driveline, not the engine.
5. Tire frequency at the complaint speed is 13 Hz, and the axle ratio is 3.42. Your analyzer reads 26 Hz. Is it the driveshaft? Probably not. The driveshaft is about 44 Hz. 26 Hz is second order for the tire, so look for a tire or wheel that is out of round or bent.

Key Takeaways

• Pin down exactly what the driver feels before you choose a test.
• Check tires before alignment. A pull that follows a tire swap is a tire pull.
• A brake pull can mean the far side is not working, so compare both sides.
• Collapsed hoses trap pressure, and the bleeder test proves it.
• Sort vibration by road speed, engine speed or braking first, then use frequency to find the part.
• First-order vibration suggests imbalance. Second order suggests something bent or at the wrong angle.