How to read a speed trace
A speed trace is the single most useful picture in racing data, and it is readable in an afternoon. It is just your speed drawn across the lap — but once you know what its shapes mean, it tells you where you are fast, where you are slow, and, laid over someone quicker, exactly where the time is going.
No sensors beyond speed and GPS required. Here is how to read one, and how to compare two.
Speed against distance, not time
The first thing to get right is the horizontal axis. A useful speed trace plots speed against distance around the lap — how far you have traveled — not against time. That is what makes two laps comparable: at the 400-meter mark you are at the same point on the track on every lap, so the traces line up and you can read them against each other.
Plot against time instead and a slower lap simply takes longer, so the same corner lands at a different spot on each lap and the two drift out of register. Distance on the x-axis is what pins the corners together — it is the whole reason the comparison works.
Reading one corner
Every corner is the same three-part shape on the trace, and each part tells you something:
- The falling edge — braking. Speed drops off the straight. How steep the fall is tells you how hard you are braking; where it starts is your brake point.
- The valley — minimum speed. The bottom of the dip is the slowest point in the corner. A sharp V means braking deep, rotating, and firing out (point-and-shoot); a rounded U means carrying speed through a longer, faster corner.
- The rising edge — acceleration. Speed builds toward the next straight. Where the rise begins is where you got back to throttle; how steep it is reflects grip and gear.
Read those three parts and you have read the corner: how you slowed, how much speed you kept, and how early you got back to power.
Comparing two laps
Lay a second trace over the first and the story is in the gaps. Wherever the two lines sit apart, time is being made or lost. Three differences do most of the work:
- Braking later and deeper — the faster lap's falling edge starts further along and drops later. (Only an advantage if you can still make the corner.)
- A higher valley floor — the quicker lap's minimum speed is higher, meaning more speed carried through the middle. This is usually where the real time hides.
- An earlier rising edge — the faster lap gets back to throttle sooner, and that advantage compounds all the way down the straight that follows.
If your tool draws a delta trace — the running time gap between the two laps — read its slope, not its height. A downward slope means you are gaining through that stretch; an upward slope means you are losing. The number at the end of the lap is just the sum of all of it. And watch corner exits: a small loss where the rising edge starts gets multiplied over the whole straight after it, so exits matter more than they look.
Signatures worth recognizing
A few shapes show up again and again:
- Overslowing — the valley sits clearly below a comparable lap's. You scrubbed off speed you did not need and have to build it all back.
- Coasting — a flat shelf between the end of braking and the start of throttle, where you are neither slowing nor accelerating. That flat spot is free time waiting to be closed up.
- A pinched exit — the rising edge comes late or climbs shallowly, usually from getting to power too gently or running out of road. It bleeds speed down the next straight.
- Scrubbed speed versus point-and-shoot — a lower, rounder valley (carrying speed) against a deeper, sharper one (braking hard and rotating). Neither is automatically faster; the trace lets you test which the corner rewards.
A ten-minute workflow in LapLift
- Overlay your fastest lap on a quicker reference — a public lap from another driver on the same track, or your own Theoretical Best.
- Sort the corners by the delta and start with the worst one. That is where the most time is sitting.
- For each, compare the three parts — brake point, valley, throttle point — and check the corner's minimum speed on the track map.
- Fix the biggest gap first. One corner's exit, done right, pays off down the entire straight after it.
Questions
Why distance on the axis and not time?
So corners line up across laps. On a time axis a slower lap stretches out and the same corner lands in a different place each lap; on a distance axis every lap shares the same x-positions, so you can compare them directly.
If I only learn to read one channel, which one?
Speed. It captures the whole rhythm of the lap — brake, minimum, throttle — and most of your lap time is explained by it before you add any other channel.
How many laps should I compare at once?
Two clean ones: your best against a good reference. More than that and the picture turns to spaghetti. Two laps keep the comparison honest and the differences legible.
Do I need throttle, brake and steering traces to get value?
No. Speed plus GPS already tells most of the story — where you are slow and where the time goes. The extra channels explain why, but you can find the where with speed alone.
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