How to analyze track-day telemetry

Start with the question, not the chart

Telemetry software opens on a wall of traces because it can draw them. The useful question is narrower: where does the time go, and what habit causes it? Everything below serves that question. If a chart doesn't change what you'll do in the next session, close it.

1. Line the laps up by distance, not time

Two laps plotted against elapsed time drift apart as soon as one is quicker, and by the second corner you are comparing different places on the track. Plot every channel against distance along the lap instead, so a braking point on lap 3 sits over the same braking point on lap 5. Most logger software does this when you overlay laps; check that the x-axis says distance.

The catch is that GPS distance varies a few percent from lap to lap (different lines, GPS noise), so a plain distance axis still drifts by a few metres by the end. The fix is to project every sample onto the path of one reference lap and measure along that path. Pitwall does this for every lap automatically, which is why its corners line up foot by foot.

2. Pick the right reference

Compare against a lap by the same driver in the same car on the same day. A pro's lap in a different car tells you the car could go faster, not what you should change. Your own best lap is a good reference. A better one is a theoretical best: your fastest clean time through each corner, stitched into one lap. It only contains things you already did, so every gap it shows is something you can repeat. Pitwall builds it from every lap you load and uses it as the default reference; laps slowed by traffic are detected and left out.

3. Read the delta first

The time delta is the running gap between two laps along the track. Read its slope, not its level: where the line climbs you are losing time, where it falls you are gaining, and the steeper the slope the faster it's happening. Three or four steep climbs on a lap are your corners to open; the flat stretches are fine. Pitwall ranks the corners by exactly this, the time lost in each segment, and lists the biggest three first.

4. The speed trace: three numbers per corner

• Minimum speed (the bottom of the dip) is apex speed. A lower minimum than the reference is over-slowing: braking too early or too long, or turning in too slowly.
• Where the minimum is matters as much as its value. A minimum late in the corner usually means a late apex; early usually means a lazy entry.
• Exit speed, read a few hundred feet past the apex, is the one that costs most, because a deficit there carries all the way down the following straight. A few mph at 300 feet past the apex is tenths at the end of the straight.

5. The braking trace

Brake pressure (or a brake switch, or longitudinal g if that's all you have) gives four readable things. The braking point: how many feet before the reference did you start? The ramp: how long from first touch to peak pressure? A slow build is a long, soft braking zone. The peak: how hard, compared with the hardest stop the car made all day? And the release: did the pressure taper off as the steering went in (trail braking), or did it drop to zero before turn-in? Keeping some brake on into the turn keeps weight on the front tyres so the car turns; dropping it early leaves the car doing neither braking nor cornering for a moment, and the grip circle shows that as a sag toward the centre.

Without a pedal channel, braking can only be estimated from deceleration. Use the estimate for the picture, never for a verdict. Pitwall draws estimated channels with a hollow dot and withholds every pedal call when the measured channel is missing.

6. The throttle trace

Find the pick-up point and compare it with the reference: picking up 30 feet later is one of the most common losses and one of the easiest to fix. Then look at the shape after it. A clean ramp to full throttle is the goal. On, off, on again is a hesitation that costs exit speed and unsettles the car. A lift where the reference stayed flat is a corner that can be flat, or a sign the entry speed was wrong. A gap between brake release and throttle pick-up is coasting, and 50 feet of it is a few hundredths every lap.

7. The grip circle and car balance

Plot lateral g against longitudinal g and the dots form a circle whose radius is the car's grip that day. Dots on the lower edges while turning mean braking and cornering grip are being used together. Corners where your dots stay well inside the ring are corners with speed left in them. If the file has a steering channel and yaw rate, compare them: more steering than the car's turn rate justifies is understeer; rotating faster than the steering asks for while on the power is the rear stepping out. Pitwall calibrates the steering-to-yaw relationship on the car's own data before making either call.

8. From analysis to one thing to practice

Pick one corner or one habit, not five. Write it as a number you can check next time: "brake 20 feet later into T5 and carry 3 mph more to the apex", or "pick up the throttle at the apex marker in T9 every lap". Drive the next session on that alone, then compare the new laps against the same reference and read the same trace. If the number moved, keep it and pick the next one. If it didn't, the drill was wrong, not the driver. Pitwall ends every guided walkthrough with one objective in this form, and the garage keeps both sessions under the same track so the comparison is one tap.

What the data can't tell you

• Conditions. No logger knows the track temperature, the flags or the traffic you could see. A theoretical best from a session where the track changed can be optimistic.
• GPS rate. One fix a second (a phone) puts corner markers within a car-length or so; ten or twenty a second (a logger) puts them within a few feet. Expect tenths from a phone and hundredths from a logger.
• Why you did it. A late throttle pick-up behind a slower car was the right call. The trace shows what happened; you were there.

The full method behind a Pitwall debrief, with the threshold for every call, is on the methodology page. To try it on your own data, drop a session in; for the file your logger needs to produce, see the AiM and Porsche Track Precision pages.