Motorcycle Lean Angle and Cornering Speed Explained

In a steady turn, the lean angle a motorcycle needs depends on just two things: speed and the radius of the curve. The relationship is tan(lean) = speed² ÷ (g × radius), where g is gravity, 9.81 m/s². Because speed is squared, a modest increase in speed demands a lot more lean. A phone can estimate lean from its gyroscope if it’s mounted rigidly, but a GPS speedometer measures speed, not lean. Speed is also the thing you control before the corner, which is why curve warning signs give a speed rather than an angle.

How are lean angle and speed related? #

In a steady turn, the tyres supply the sideways force that keeps you on the curve, and the bike leans until gravity and that cornering force balance. Mass cancels out of the equation, so a heavy tourer and a light sport bike need the same theoretical lean for the same speed and radius.

Look at the 100 m row. Going from 70 to 90 km/h, less than 30% faster, takes the required lean from about 21° to over 32°. Through a tight 50 m bend, 90 km/h would need more than 50°, far beyond sensible road riding.

These are theoretical figures for the combined bike and rider. Wide tyres need slightly more lean than the formula says, and hanging off the inside lets the bike itself stay more upright.

Lean angle and grip #

The tangent of the lean angle equals the sideways acceleration in g, so a 45° lean means 1 g sideways, which takes a lot of grip from both tyres. Dry asphalt and warm, good tyres support high lean; wet roads, painted lines, gravel, leaves and diesel support far less. The lean you need is fixed by speed and radius, while the grip available changes with every surface. That’s why the only real safety margin is entry speed.

Can a phone measure lean angle? #

Yes, with limits. Lean-angle apps use the phone’s gyroscope and accelerometer to track tilt. For that to mean anything, the phone has to be mounted rigidly and square to the bike, because any wobble in the mount shows up as lean. Many modern motorcycles have their own motion sensor (an IMU) and can display or log lean angle directly, which is more dependable than a phone.

Two cautions. Lean data is something to review after the ride, never to watch in a corner. And a bigger number isn’t the goal: good riders use the least lean the corner needs.

Speedometer GPS doesn’t measure lean angle. It shows your GPS speed, averaged over the last five one-second readings, with heading and altitude from the same fix. GPS speed is accurate through a steady bend with a good signal, typically within a couple of km/h, but it lags a second or two when you brake on the approach, so the bike’s own speedometer reacts faster. Why GPS speed lags your gauge explains why.

What are curve advisory speeds, and how are they set? #

In the US, the yellow advisory speed plaque under a curve warning sign comes from an engineering study. The 2009 edition of the federal sign manual, the MUTCD (Section 2C.08), listed accepted methods, including driving the curve with a ball-bank indicator, with criteria of 16° of ball-bank at 20 mph or less, 14° at 25 to 30 mph and 12° at 35 mph and higher. It also noted that research has shown drivers often exceed posted advisory curve speeds by 7 to 10 mph (MUTCD 2009, Section 2C.08). A newer, 11th edition of the MUTCD has since replaced the 2009 text, so check it for current practice.

Other countries use their own methods and signs. For riders, three things follow:

• An advisory speed is set for ordinary vehicles in good conditions, using a comfort criterion rather than a grip limit. Treat it as a strong hint about how tight the curve is, especially on unfamiliar roads.
• A corner you can’t see the end of may tighten, and the sign is often your only warning.
• In the wet, or on a bike you don’t know yet, the advisory speed can still be too fast.

Using speed, not lean, to manage corners #

Rider training courses teach this properly, and they’re worth taking. The general principles:

1. Set your speed before you turn in. Do your braking upright, on the approach, judging it by the sign, the road and the bike’s own speedometer.
2. Look through the corner to where you want to go.
3. Keep inputs smooth once leaned over. Sudden braking or throttle mid-corner upsets grip.
4. Choose an entry speed that leaves margin for a surprise: a tightening radius, gravel, an oncoming car wide on its line.

A GPS speedometer helps most after the ride, not during it. Record the ride, then look at the saved speed chart at home: it shows how fast you arrived at each bend and how much you slowed, which tells you where your entry speeds were higher than you thought.

Setting up a GPS speedometer on a motorcycle #

• Use a solid, vibration-damped mount, or a jacket pocket. Apple advises against mounting iPhones on high-power motorcycles because vibration can damage the camera; see our motorcycle phone mount guide.
• Set it up before you ride. Units, and a speed limit alarm if you want one; Speedometer GPS’s alarm turns the readout red and buzzes and beeps when you pass your limit, so the warning comes to you without reading the screen.
• Don’t touch or read it on the move. Keep your eyes on the road.
• Review afterwards. A saved ride keeps your top speed, average moving speed, the route and a speed chart.

Frequently asked questions #

How do I calculate lean angle? #

For a steady turn, lean angle = arctan(v² ÷ (g × r)), with speed v in metres per second, g = 9.81 m/s² and curve radius r in metres. At 70 km/h (19.4 m/s) on a 100 m radius curve: 19.4² ÷ (9.81 × 100) ≈ 0.39, and arctan(0.39) ≈ 21°.

What lean angle is normal on the street? #

It depends entirely on the speed and the corner. From the table, everyday corners at legal speeds often need somewhere around 10° to 30°. Much more than that on a public road means you’re using a large share of your grip, with little margin for a surprise.

Can my phone measure lean angle accurately? #

It can give a reasonable estimate from its gyroscope if it’s mounted rigidly and square to the bike. Loose or vibrating mounts give misleading numbers, and a motorcycle with its own motion sensor is more reliable. Speedometer GPS measures speed, not lean.

Does a heavier bike need more lean in the same corner? #

No. In the basic physics, mass cancels out: lean depends only on speed and radius. Tyre width, body position and suspension make small differences in practice.

Why do curve advisory speeds feel slow on a motorcycle? #

They’re set for ordinary vehicles in good conditions using a comfort-based method, not a grip limit. That margin is useful: it covers wet roads, debris and corners that tighten. On an unfamiliar road, respecting it is sensible.