iPhone G-Force Meter: What Your Phone Can Measure on Track, and What It Can't

The short answer

• Cornering G: an iPhone on a rigid mount measures it well enough to compare corners and laps across a day.
• The mount matters more than the app. A phone that wobbles adds its own movement, and no software can take it back out.
• Peaks are softened. Apps smooth the sensor so the reading is steady, so short hits from kerbs and bumps read lower than they really were.
• PinIt Cam today: the G-meter on your footage shows cornering G. It doesn’t show braking G yet. More on that below.

What a G-force meter measures

One G is the acceleration of gravity, 9.81 m/s². A G-meter shows how hard the car is being pushed sideways, forwards or backwards, compared with that.

• Lateral G is cornering: how hard the tyres are holding the car on its line.
• Longitudinal G is braking and accelerating.
• Vertical G is bumps, crests, compressions and kerbs.

Lateral G follows simple physics: speed squared divided by the radius of the corner. At 60 mph round a 70-metre radius, that works out at just over 1 G. Most road cars on road tyres run out of grip somewhere around 0.8–1 G. Cars on track tyres, slicks or with downforce go well beyond that.

How an iPhone measures it

Every iPhone has an accelerometer that measures acceleration along three axes, and iOS can hand apps a version with gravity already taken out. An app reads that several times a second, works out which axis points where in the car, and draws the result.

The catch is that the phone measures in its own axes, not the car’s. On a windscreen mount, with the screen facing you and the camera looking down the road:

• Across the screen, side to side is the car’s lateral axis: cornering.
• Up and down the screen is the car’s vertical axis: bumps and kerbs.
• Straight out through the screen is the car’s fore-and-aft axis: braking and acceleration.

Turn the phone from portrait to landscape and the first two swap over. An app has to know which way the phone was mounted to put the numbers in the right place. PinIt Cam reads that when you press record, so there’s no setup step.

How accurate is it?

For cornering, accurate enough to be useful. We checked the lateral G that PinIt Cam logged on a real session at Snetterton against cornering forces worked out independently from GPS speed and heading. The two tracked each other closely: a correlation of 0.93.

Three things limit it:

• Sample rate. PinIt Cam reads the sensor about ten times a second. That’s plenty for a corner lasting several seconds, but too coarse for the exact moment you hit a kerb.
• Smoothing. The raw sensor is noisy, so it’s smoothed before it’s shown. Steady cornering G comes through; sharp spikes are flattened.
• The mount. If the phone moves on its mount, that movement is measured too. A phone flexing on a long arm can add more “G” than the car is pulling.

So use it to compare yourself with yourself: the same corner, lap after lap, in the same car with the same mount. Don’t use it to settle an argument with someone else’s data logger.

Mounting the phone for good G readings

• Rigid beats everything. A short, solid windscreen mount or a clamp on the roll cage. Avoid long arms, vent clips and anything on a soft dashboard.
• Close to the middle of the car if you can. A phone well off to one side picks up a little extra movement as the car rotates.
• Upright and square to the car. If the phone points off at an angle, some cornering force leaks into the other axes.
• Tighten it, then shake it. If you can wobble the phone by hand in the paddock, the track will wobble it more.

A good mount also gives you steadier video, so it’s worth getting right whatever you’re recording. There’s more on mounting in our track day camera guide.

Using G-force to drive better

A G number on its own doesn’t tell you much. Next to the video of the corner that produced it, it does:

• Same corner, different laps. If one lap shows noticeably less lateral G through a corner than the others, watch that lap. Did you lift, turn in early, or run wide?
• A smooth build. A good corner usually shows G rising steadily to a peak and holding there. A spike and a quick drop often means you turned in too hard and had to unwind.
• Where the grip runs out. If your peak G is the same in every fast corner, that may be what your tyres have to give. Going faster there is about line and exit speed, not more commitment.

What PinIt Cam’s G-meter shows

PinIt Cam records both iPhone cameras at once and puts a live G-meter on the footage, next to GPS speed, the track map and the name of the corner you’re in. The meter is a dot with a short trail, so you can see how the force built through the corner.

• Side to side on the meter is cornering G, correctly oriented whether you mount the phone in portrait or landscape.
• Braking G isn’t shown yet. On a windscreen mount, braking acts straight out through the screen. The app has logged that axis since August 2026 but doesn’t display it yet, so up-and-down movement on the dot is bumps and kerbs, not braking. Showing braking is on the list, and this page will be updated when it ships.
• Corner names. At the 107 built-in circuits in 22 countries, the HUD names the corner, so you know which bend the G belongs to without counting from the start line.
• Your data comes with you. Long-press a session and choose Share session. You get a .pinitcam file: a zip with both videos plus the GPS and G-force logs as JSON. They’re raw, in the phone’s own axes and ten samples a second, so they’re for people happy to work with JSON, not a ready-made spreadsheet.
• No account, no cloud. Everything stays on your phone unless you share it yourself.

All of this is free. The only limit is exporting video from five sessions a month, and Pro is a one-time purchase that removes it.