VaneG-Force and Lean Angle: What the Numbers Mean
A G-meter turns how hard you brake, accelerate and corner into one number. Knowing what normal looks like makes it a measure of smooth driving, and on two wheels the same number gives your lean angle.
Updated 26 October 2026
What one g is
One g is the acceleration of gravity: 9.81 m/s². If a car brakes at 1 g, it loses about 35 km/h (22 mph) of speed every second, and you are pushed into the belt with a force equal to your own weight. Speeding up, braking and cornering are all measured the same way, so the numbers compare directly.
What the numbers feel like
| g | What it is |
|---|---|
| 0.1 to 0.2 | Gentle, smooth driving; passengers barely notice |
| 0.35 | The braking rate US road design assumes for drivers |
| 0.5 | A firm stop; loose things slide off the seat |
| 0.8 to 0.9 | An emergency stop in a road car on dry tarmac |
| 1.0 and above | Performance tyres, track driving, racing |
The 0.35 g figure is the deceleration AASHTO's road-design guidance uses for stopping distances, chosen because about 90% of drivers brake harder when they have to. Tyre tests on dry roads put road cars' emergency stops at about 0.85 to 0.95 g. On a wet road the tyres give noticeably less.
Cornering
Going round a bend, the sideways acceleration is the speed squared divided by the radius of the turn
(v² / r). Double the speed and it is four times as much, which is why a bend that feels
easy at 30 mph is hard at 60. A smooth driver keeps braking, cornering and accelerating apart, and the
dial shows it: a G-meter is as much a teacher of smoothness as a measure of grip.
Lean angle on a motorcycle
A bike in a steady turn leans until gravity and the cornering force add up to a pull straight down through the tyres. The geometry is simple: the tangent of the lean angle equals the cornering force in g.
lean = atan( v² / (g × r) )
| Cornering | Lean angle |
|---|---|
| 0.2 g | 11° |
| 0.5 g | 27° |
| 0.7 g | 35° |
| 0.8 g | 39° |
| 1.0 g | 45° |
This is the angle the turn needs for bike and rider together. A rider who hangs off the inside lets the bike itself stand up a little, and wide tyres shift the contact patch, so the bike's real lean can differ by a few degrees; a validation study found the simple formula consistently reads slightly low. Ground clearance sets the limit as often as grip does: Cycle World found one bike of its day touching its exhaust down at 37 degrees.
Why tilting the phone does not work
It seems obvious to strap a phone to the bike and read its tilt. But in a balanced turn the combined pull of gravity and cornering points straight down through the seat, exactly as it does when riding upright. The phone's accelerometer, leaning with the bike, sees "down" where it always does, and reports almost no lean. That is why a lean angle has to come from speed and how fast the bike is turning, not from the phone's sense of level.
Sources
- Wikipedia, Stopping sight distance: AASHTO's design deceleration of 11.2 ft/s² (3.4 m/s²), exceeded by about 90% of drivers.
- carsales.com.au, tyre test, dry braking: 0.85 to 0.94 g average from 80 km/h.
- Cycle World (1978), May the force work with you: lateral acceleration in g equals the tangent of the lean angle; ground clearance.
- SAE 2015-01-1422, Validation of equations for motorcycle and rider lean on a curve.
Common questions
What is a normal g-force when braking?
Road designers in the US assume drivers brake at about 0.35 g (3.4 m/s²), and about nine in ten drivers brake harder than that when they must. Most road cars on dry tarmac top out at about 0.8 to 0.9 g in an emergency stop.
How do you calculate a motorcycle's lean angle?
In a steady turn the tangent of the lean angle equals the sideways acceleration in g, which is speed squared divided by g times the turn radius. 0.5 g of cornering is about 27 degrees of lean, 0.8 g about 39 degrees, 1 g is 45 degrees.
Can a phone measure lean angle by its tilt?
Not directly. In a balanced turn the bike, the rider and a phone on the bars all feel the combined pull of gravity and cornering straight down through the seat, so the phone's own tilt sensor sees almost no lean. The lean has to be worked out from speed and how fast the bike is turning.