Eight small wheels, a lot of physics.

Skate wheels are made of polyurethane. Where the wheel meets the ground it flattens slightly, then springs back as it rolls on, but not perfectly: some energy is lost as heat (materials scientists call it hysteresis). Bumps and grit add small impacts. Together they create rolling resistance, a nearly constant force: F = Crr × weight, where Crr is the rolling-resistance coefficient.

Bearings: the bit everyone over-thinks

Almost all inline and most quad wheels use the same bearing size, the 608: 8 mm bore, 22 mm outside diameter, 7 mm wide. Each wheel takes two, with a spacer between, so a pair of 4-wheel inline skates or quads carries 16 bearings.

The ABEC number (1, 3, 5, 7, 9) printed on many bearings is a scale of manufacturing tolerance: how precisely the parts are made. It is not a speed rating. Wheel rpm on skates is modest (Chapter 2), and at those speeds a clean, lightly lubricated bearing loses far less energy than the wheel's own rolling resistance. Dirt, sand and water are what really slow bearings down, and they ruin them.

Durometer: how hard the urethane is

Wheel hardness is measured on the Shore A durometer scale, printed as a number and the letter A (e.g. 78A, 85A). Lower is softer. Soft wheels (roughly the high 70s A) deform around pebbles and cracks: better grip, a smoother ride, a little more rolling resistance and faster wear. Harder wheels (mid-80s A and up) deform less on smooth floors: faster, more willing to slide, but harsh and skittish on rough tarmac. Outdoor recreational wheels sit at the soft end; rink and slide wheels at the hard end.

Diameter: big rolls easier, small turns quicker

A bigger wheel meets each bump at a gentler angle and spreads its contact patch over a longer, narrower area, so it typically carries speed better. That's why speed and marathon skaters use large wheels (often 100–125 mm). Smaller wheels (around 72–80 mm) sit lower and accelerate and turn more nimbly, favoured for hockey, freestyle slalom and learning.

Quad vs inline

Quad skates put two wheels side by side at the front and back, giving a wider side-to-side base and a short front-to-back one. They're stable standing still and agile for dance and rink skating, and they usually have a toe stop for braking. Inline skates line three to five wheels in a single row: narrow side-to-side (you balance like on a bicycle, steering under yourself) but long front-to-back, which makes them stable at speed and efficient for distance. Most inline skates for learners carry a heel brake.

Standing on skates, your body behaves like an inverted pendulum: the centre of mass (roughly at your navel) sits about a metre above a very small support base. If your centre of mass drifts outside that base, gravity starts toppling you, and the further it drifts the faster you go. Balancing on skates means keeping it over the wheels, by moving the wheels back under you (steering) or by shifting your body.

Why the "ready position" works

Coaches teach a stance with knees bent, hips back, weight over the middle-to-front of the foot and hands forward. Three physics gains: the centre of mass is lower, so small tilts take longer to grow; bent legs act as springs, soaking up bumps; and weight forward stops the classic beginner fall, where the wheels roll ahead and you sit down hard behind them.

Leaning into turns

Just like on ice or a bicycle, a turn needs a force toward the centre, m × v² ÷ r, supplied by wheel grip, so you must lean in: tan θ = v² ÷ (g × r).

A wheel rolls easily along its direction and grips hard sideways. Push straight back and the skate just rolls back. So skaters angle the pushing skate outward and drive it sideways. The ground pushes back perpendicular to the wheels, and the forward share of that push propels you, while the gliding skate carries you on. Good skaters make each push long, low and sideways, then bring the leg back under the body.

The cost of going faster

Two forces resist you on the flat: rolling resistance (constant, Chapter 1) and air drag, ½ × ρ × v² × CdA, which grows with the square of speed. Power is force × speed.

Every stop turns your kinetic energy, ½mv², into heat, either in a rubber brake pad or in wheels scrubbing sideways across the ground. The common methods, roughly in the order they're taught:

• Heel brake (inline) or toe stop (quad): scissor one foot forward and press the pad down. Simple and controllable; the first stop most instructors teach.
• Plough / A-frame and spin stops: steer the skates to scrub speed or turn it into a circle.
• T-stop: drag the back skate at right angles behind the front one. Effective, but wears wheels and needs balance.
• Powerslide and hockey-style stops: advanced. The wheels are deliberately slid sideways.
• Grass stop: roll off onto grass and let the high rolling resistance slow you, with weight back and knees bent.

Ramps: height becomes speed

Rolling down from rest, potential energy (mgh) becomes kinetic energy (½mv²), so v = √(2gh). Mass cancels: heavy and light skaters reach the same speed, if we ignore friction and air.

Long hills: the terminal speed

On a long slope, you speed up until air drag plus rolling resistance balance the downhill pull of gravity. For our 70 kg upright skater (CdA 0.45, Crr 0.01), the speed you'd settle at with no braking is:

A 5% slope barely looks like a hill, but left alone it takes you to 36 km/h, faster than most recreational skaters can stop from safely. Heavier skaters go faster still, because drag is a smaller share of their weight. Control speed on hills from the top (slaloming turns, plough or brake), and walk down anything you're unsure of.

Falling well

From a standing centre-of-mass height of about 1 m, a fall reaches ≈ 4.4 m/s, before adding any forward speed. Instructors teach falling forward and low onto knee pads, then elbows, then wrist guards, rather than backwards onto the tailbone or head. Protective gear works by sliding and spreading the impact: a longer stopping time means a smaller peak force for the same change in momentum. Helmet, wrist guards, elbow and knee pads are standard requirements in instructor programmes and many clubs.