Why Does A Single Wet Towel Shake Your Washing Machine, But A Full Load Spins Smoothly?

A washing machine shakes when its load is bunched on one side of the drum, because an off-center mass pulls outward as it spins, and that pull grows with the square of the spin speed: a 0.7 kg wet lump at the drum wall pulls with about 6 newtons during a 60 rpm wash tumble but about 3,300 newtons (roughly 337 kilograms-force) at 1,400 rpm. A full load spreads its weight all the way around the drum, so the outward pulls on opposite sides cancel and the drum runs true. The worst banging happens partway through spin-up, when the drum speed passes through the natural bounce rate of the spring-mounted tub, which is why machines carry concrete counterweights, shock absorbers, balance rings and imbalance sensors to get through that moment.

Six bath towels go in on Sunday morning. The machine fills, tumbles, spins at full speed and hums like a fridge.

One bath mat goes in on Sunday afternoon, on the same program. At the start of the spin it begins to thump, then bang, then take a few steps toward the door, as if it had somewhere better to be.

Every instinct says this should be the other way around. More laundry is more weight, and more weight should mean more shaking. The answer lies in the physics of anything that spins, and it has little to do with how much you washed. It is about where the weight sits.

What Happens When You Spin A Weight In A Circle?

Tie a shoe to a string and swing it around your head. The string tugs outward on your hand the whole time. Let go, and the shoe does not fly outward. It flies off in a straight line, in whatever direction it was heading at that instant.

That tug is the whole story of a washing machine. A moving object wants to keep going straight. To drag it around a circle, something has to keep pulling it toward the center, and for the shoe that is the string. Physicists call this inward pull a centripetal force. OpenStax defines it as “any net force causing uniform circular motion.” The string pulls the shoe in, and the shoe pulls back on the string just as hard. That pull back is what your hand feels.

The outward pull has a popular name, centrifugal force, and OpenStax is blunt about it: “centrifugal force is a fiction.” Nothing pushes the shoe outward; it tries to go straight, and the string stops it.

Now swap the string for a drum. The towel is the shoe and the drum wall is the string. Every scrap of wet cloth is trying to fly off straight, and the wall yanks it around the circle instead. In return the cloth pulls back on the wall, and on the axle that holds it.

Why Does A Washing Machine Shake Violently During The Spin Cycle?

With a full drum, those pulls come from every direction at once. A sock on the left pulls the axle left while a shirt on the right pulls it right. Add them up around the circle and they cancel. The axle feels plenty of pulling but almost no net pull in any one direction, so the drum spins true.

Now put one soaked bath mat in on its own. It cannot spread itself around the drum. It sits in one lump, on one side, and its outward pull has nothing opposite to cancel it. The axle gets dragged toward the lump instead. A quarter-turn later the lump is somewhere else, and the axle gets dragged that way instead. The whole tub is swung in a circle once per revolution, and the cabinet shakes with it.

A Whirlpool patent on front-loader control agrees. The lopsided pull shows up “precisely when the laundry loads are small.” A small load slides to the bottom of the drum and collects in one place. The amount of laundry was never the problem. The lopsidedness is.

How Much Force Does One Wet Towel Make At 1,400 RPM?

The pull from an off-center lump has a tidy formula. The force needed to hold a mass on a circle is the mass, times the radius of the circle, times the square of the spin rate. OpenStax writes it as F = m r ω². Cleveland State's vibration notes call this the rotating unbalance force. Here F is the force, m is the mass of the lump and r is its distance from the spin axis. The symbol ω (omega, a Greek letter) is the spin rate in radians per second. One full turn is 2π radians, so ω = 2π × rpm ÷ 60.

The squared term is the part that matters. Double the speed and the pull quadruples.

Take a front-loader at 1,400 rpm, which the Whirlpool patent uses as a typical top speed for everyday cottons. For the lump, assume a soaked bath towel with 0.7 kg of its mass against the drum wall, 0.22 m (about 9 in) from the center. These are round numbers for a worked example, not a spec for any one machine.

1. Convert the speed: ω = 2π × 1,400 ÷ 60 ≈ 146.6 radians per second.
2. Square it: 146.6² ≈ 21,490.
3. Multiply through: F = 0.7 × 0.22 × 21,490 ≈ 3,310 newtons.
4. Put that in human units: 3,310 N ÷ 9.81 ≈ 337 kilograms-force, or about 744 pounds-force.

So one modest towel, at the wrong spot, tugs the drum with the weight of four adults. That tug sweeps around the axle more than 23 times a second. During the wash tumble at 60 rpm the same towel pulls with about 6 newtons, the weight of a small apple.

Why Does A Wet Towel Make It Worse Than A Dry One?

Water is heavy, and the kilogram was first set as the mass of “1000 mL of water.” Every liter a towel holds adds a kilogram to the lump. Dry, a towel is light cotton; out of the rinse it carries its own weight in water, and all of that goes into the m in the formula.

Wet cloth also stops moving once the spin begins. During the wash the drum turns at a crawl and the laundry tumbles over itself. Speed it up and the outward pull on the cloth soon matches its weight, so it stays pinned to the wall through the top of the turn. A Chalmers University study puts that sticking speed at 60 to 80 rpm. That is for household drums 30 to 50 cm across. Above it, the laundry is a fixed part of the spinning drum. Wherever the towel stuck is where it stays, all the way up to 1,400 rpm. A full load sticks too, but all around the circle, so the pulls cancel. (Why machine-dried towels feel stiff is a different story.)

Why Is The Shaking Worst Partway Through Spin-Up?

If the pull grows with speed squared, the shaking should be worst at top speed. Yet the loudest banging often comes partway through spin-up, then eases as the drum reaches full speed. That is resonance, the same effect that lets marching soldiers bounce a bridge.

The tub hangs on springs and shock absorbers, so it has a natural rate at which it bounces if you nudge it. Physicists call that rate a resonant frequency. Georgia State's physics pages call it “a natural frequency of vibration.” Push a child on a swing in time with that rate and each push adds to the last.

The off-center towel is a push, delivered once per revolution. As the drum speeds up, the pushes come faster. At some point they arrive at the tub's own bounce rate, and the tub swings further with every turn. The Cleveland State notes warn that here “the amplitudes can reach dangerously large values.” Then the drum keeps accelerating, the pushes outrun the bounce, and the swing settles to a steady wobble.

So a well-designed front-loader does not spin at its resonance. It passes through its resonance and keeps going. The Chalmers team tells designers to “pass by these frequencies quickly.” Never park a spin speed near one. Full speed sits above the danger zone, in what engineers call the supercritical range. One wrinkle: the Chalmers test machine's feet felt a force spike near 800 rpm. The team traced it to the cabinet flexing, not the tub swinging. A cabinet has bounce rates of its own, and so does the floor.

What Stops A Washing Machine From Shaking Itself Apart?

Open up a front-loader and the first surprise is a lump of concrete. An Electrolux patent calls tub counterweights ballast against the stresses of high-speed spin. It adds that “traditionally, they are formed by concrete blocks.” The same 3,310 newtons of towel-pull now has to move a tub that weighs far more, so it moves it less. The concrete makes the lopsided load a smaller fraction of the thing it is trying to swing.

The second surprise is that the tub is not bolted to the cabinet. It hangs on springs and leans on shock absorbers. That lets it swing a little without handing the motion to the cabinet. The Chalmers test machine used four struts between base and tub. Each had “a spring and a friction damper integrated into one unit.” Many machines hang the tub from coil springs at the top and prop it on dampers at the bottom. Either way, the springs let the tub swing and the dampers soak up the energy of the swing.

How Do Balance Rings And Steel Balls Fix An Unbalanced Load?

Concrete and springs only soften the problem. The clever fix cancels the problem instead. Around the rim of many drums runs a sealed hollow ring. Inside it is something free to move: a few liters of liquid, or a track of loose steel balls.

Spin a ring of free balls fast enough and they do not stay where gravity put them. They roll to the spot where they are flung outward hardest, and that spot is opposite the heavy lump. A 2024 study in the journal Symmetry simulated the mechanism. The authors' summary: “the balls settle in the opposite position to an unbalanced mass.” The towel pulls one way, the balls pull the other, and the axle feels the difference instead of the sum.

The liquid version works the same way. A Maytag patent describes one: “a balancing fluid partially fills the circular chamber.” Once the tub spins fast, the fluid “is free to move circumferentially to whatever position necessary.” It even names the villain: “These towels create an unbalance.”

There is a catch, and it is timing. The Chalmers paper says the balls only settle “at a steady state overcritical speed.” In plain terms, that means above the resonance. Below it they are not yet where they need to be. The ring fixes the cruise but not the climb.

How Do You Stop A Washing Machine From Shaking Violently?

Modern machines try to catch the problem before the spin. The Whirlpool patent describes “an imbalance sensor that changes the rotation of the drum.” In practice the machine tumbles, measures and tumbles again. If it cannot get the load even enough, it caps the spin speed or stops. LG's front-loaders flash a UE code for this. The company says it “occurs when the laundry is piled up on one side of the drum.” That is not the machine failing. It is the machine declining to do the thing you just read about.

The rest is up to you, and the physics tells you what to do:

1. Never spin one heavy item alone. Add a couple of towels so there is weight on the other side to cancel it. LG's advice for laundry nets applies to any lone item: “wash two or three nets at a time along with some clothes.” The same page suggests filling the drum to about two-thirds.
2. Mix the load. Whirlpool says to put “heavier articles around lighter ones.” Do not pack the drum tight, so items can spread around it.
3. Level the feet. Whirlpool's test: “grip the washer from the top and rock back and forth.” Then twist the feet until all four sit firm and the rocking stops. A foot in the air is a cabinet with a built-in wobble.
4. Check for shipping bolts. New front-loaders ship with bolts that lock the tub for transport. Leave them in and the tub cannot swing on its springs at all.
5. Mind the floor. On a floor that gives, Whirlpool suggests “reinforcing the area or using a washer vibration pad.” Walking, the Chalmers authors note, stays a small problem given “high enough friction.” The feet hold with static friction, and once they break loose they have less grip to recover it.

So, Why Does One Wet Towel Shake The Machine But A Full Load Doesn't?

Because the drum does not care how much mass it spins. It cares about where that mass sits. Everything in the drum is dragged around a circle, and everything drags back on the axle in return. A full load drags back from every direction, and the drags cancel. A lone wet towel drags back from one direction, and nothing cancels it. That leftover pull grows with the square of the spin speed. It goes from the weight of an apple during the wash to the weight of four adults at 1,400 rpm. And it sweeps around the axle more than twenty times a second.

The violence you hear is that pull meeting a tub on springs. On the way up to full speed, the drum passes through the tub's natural bounce rate. For a few seconds, every turn adds to the last. Past that speed the shaking settles, which is why the banging often fades before the spin ends. Concrete, springs, dampers, balance rings and sensors each fight one stage of that story, and none of them is perfect, which is why machines still walk. A foot a few millimeters off the floor can undo a block of concrete. The fix that costs nothing is the one the physics points at first: put a second towel in, on the other side.