Trace G-sum around the lap to find grip gaps

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Source path: content/lms/data-interpretation-ii-advanced/03-friction-circle/02-g-sum-tracing.md

Course: Read the data your hands can't feel

Module: Map the grip envelope and find the holes

Estimated duration: 50 minutes

This lesson is about using the G-sum trace as a lap map for unused grip. You are not building the G-G diagram here, and you are not comparing whole-driver grip envelopes. Those are sibling skills. Here, you already have a data channel called G-sum available beside steering, RPM, gear, GPS line, segment reports, fastest rolling, theoretical fastest, total steer angle, throttle histogram, and the other normal channels. Your job is to follow that G-sum trace around the lap and ask a sharper question: where did the car have a chance to use more tire than you actually used?

The important word is chance. A low G-sum number by itself does not prove a mistake. The tire may be cold at the end of a straight before a hard braking zone. The driver may be giving up load because the car is transitioning from brake to turn. The line may be wrong, the steering may be spiky, or the car may be out of balance. The data process in the bonded material repeats the same discipline for lateral G, brake pressure, and other channels: look at the trace, ask why, confirm with other channels, and compare with other laps, other drivers, other cars, or other sessions. That is the whole spirit of this lesson. G-sum is not a verdict. It is a searchlight.

The principle: a useful grip-gap trace is a trace that survives cross-checking. If the G-sum falls away in a place where the car should be loaded, you do not immediately tell yourself to drive harder. You first identify which phase of the corner you are in, then inspect the channels that explain why the load disappeared. Brake pressure tells you whether the initial application, trail, or long tail changed. Lateral G tells you whether the cornering load reached a repeatable peak, whether it sagged, and whether it spiked. Steering, GPS line, throttle, RPM, and gear tell you whether the driver changed the car's path or power state. Segment reports and theoretical fastest tell you whether the change mattered to time. The data skill is not staring at one graph. The data skill is narrowing one question until it becomes a driving action you can test on the next session.

The tire mechanism matters because G-sum is only useful if you understand what the tire is doing. Tire grip is not an on-off switch. The slip-angle material shows that as a tire's slip angle increases, traction rises, then reaches a plateau, and then falls off. That means two opposite-looking problems can both be grip gaps. If you never get near the plateau, the trace may look low and smooth because you are simply below the limit. If you rush past the usable region, the trace may show spikes, corrections, or a falloff because the tire is no longer giving more usable grip for the extra input. Intermediate drivers often need to learn both sides at once: add load where the car is underused, and remove drama where the data shows a spike instead of sustained grip.

The combined-force idea is the second mechanism. A corner is not just lateral G. Braking and acceleration forces matter, and the tire behavior material places longitudinal forces and combined forces in the same discussion. When you brake, weight transfers forward, loading the front tires and giving them more traction. That does not mean you can brake and steer infinitely at the same time. It means the grip picture changes as you transfer load, release the brake, turn in, wait at mid-corner, and feed power. G-sum tracing helps because it lets you see whether the car stayed meaningfully loaded through those phases or whether you created a hole between them.

Temperature is the third mechanism. The tire-development material points out that a tire at the end of a straight just before hard braking into a slow corner is cool compared with the same tire in mid-corner a few seconds later, and that available grip can vary as the tire heats up during each corner. That matters when you read a G-sum gap. A drop early in a session, early in a lap, or at the end of a long straight might not mean the driver suddenly forgot how to use the car. It might be a tire condition effect that has to be calibrated with on-track evidence. The same source also warns that data is only useful after it has been calibrated with on-track tests. For you, that means you pair the trace with what you felt in the car and with repeatable session evidence, not with one heroic interpretation.

Start the analysis by preparing a clean distance-based view of one lap. Put G-sum at the top if your software allows it. Under it, place lateral G, brake pressure, throttle, steering, speed, RPM or gear, and GPS line. Add segment or section times if you have them. If the logger gives fastest rolling or theoretical fastest, include it, but use it carefully. Theoretical fastest can point you toward sections with time available, while the G-sum trace tells you where the car was or was not loaded. You want both views: time result and grip use.

Next, choose a comparison. The weakest comparison is one lap looked at in isolation. A better comparison is your own best lap against a representative lap. Better again is your best sector or theoretical-fastest section against the lap you actually drove. If you have another driver in the same car, same day, and similar conditions, that can be powerful. If you only have another car or another session, still compare, but keep the conclusion smaller. The data process explicitly recommends comparison across laps, other drivers, cars, and sessions, but it also tells you to ask why and confirm with other channels. That is your guardrail against copying a number that does not belong to your car or condition.

Now trace the lap in phases. Do not start with the biggest number. Start with the first braking zone and move forward in the order the car experiences the lap. In a brake zone, the first question is whether the G-sum rises decisively when you apply the brake. Then ask whether it stays useful as you release pressure and approach turn-in. The brake-pressure material tells you to inspect the shape of the trace: initial application, trail, and long tail; inconsistent pressure; light and long versus hard and short. If the G-sum is low and the brake trace is light and long, the gap is probably not a mystery. You did not load the car sharply enough. If the G-sum falls as the brake is trailed, the issue may be a release that leaves a dead zone before lateral load arrives. If the brake pressure has a long tail and the lateral G never builds cleanly, the car may be stuck in an entry compromise where you keep asking the tire for brake but never commit to the turn.

At turn-in, the useful question changes. You are looking for the handoff from longitudinal use to lateral use. Good data does not have to be beautiful, but it should make sense. If brake pressure comes off, steering comes in, lateral G rises, and G-sum stays engaged, the car is doing one continuous job. If brake pressure drops, steering waits, lateral G arrives late, and G-sum has a valley, you have found a grip gap. That gap is not solved by randomly braking later. It is solved by coordinating the release and the turn so the tire remains loaded. For an intermediate driver, this is often the first major gain: the car was not limited by peak grip; it was limited by an empty transition.

At mid-corner, stop asking whether the number is spectacular and start asking whether it is repeatable. The lateral G process says to look at peak g-loads, whether you are consistently using them, whether there are spikes in either direction, and whether the trace is consistent lap to lap. Apply the same discipline to G-sum. A single tall point that appears once and costs exit speed is not a reliable grip gain. A sustained, repeatable level through the portion of the corner where the car should be loaded is more useful. If the car has a plateau of load and the segment time improves, you probably found usable grip. If the trace has a sharp spike followed by a drop, you may have jabbed the tire past the useful region and then waited for the car to recover.

At exit, the question is whether acceleration rebuilt load without opening a new hole. The tire material includes braking and acceleration forces, and the data channel list includes throttle histogram and other channels. When you read exit G-sum, look at throttle and steering together. If steering is still high and throttle comes in abruptly, the tire may be asked to do too much at once. If you unwind steering, add throttle, and G-sum rises or stays engaged without a correction, the exit is more likely using the tire cleanly. If G-sum sags while throttle waits, you may be under-committing. If G-sum spikes and then speed stops improving, you may be over-asking.

The most common wrong use of G-sum is chasing the biggest peak on the page. Peak hunting flatters aggression. Grip-gap tracing rewards continuity. You are trying to find the sections where the car should have been loaded but was not, and the sections where a brief spike was pretending to be grip. That is why segment time and theoretical fastest belong in the view. A higher number that loses time is not the goal. A lower-looking trace that lets you get to throttle earlier and carry more speed may be the faster answer. The Going Faster material describes data acquisition being used to show how the fastest drivers reduce lap times, including an example where the difference between two drivers on the same section comes from one driver slowing too much in the first half of the corner. That is exactly the kind of place where G-sum tracing should send you: not toward drama, but toward the phase of the corner where time was given away.

Use your senses as calibration, not as a substitute for data. On track, you gather visual, kinesthetic, and auditory information. Kinesthetic information includes touch, balance, and sensing g-forces against your body. Better information produces better driving. After the session, the trace helps you name what your body felt. If you remember the car feeling light between brake release and turn-in, look for the G-sum valley and the lateral G delay. If you remember a snap or correction, look for a spike and then a falloff. If you remember waiting forever at mid-corner, look for a long flat or low section with steering held and throttle delayed. The point is not to replace feel. The point is to make feel more honest.

There is a strict order to a good G-sum review. First, find the location. Use the GPS line or distance axis to mark the section. Second, classify the phase: braking, release, turn-in, mid-corner, exit, or fast-corner confidence lift. Third, compare: your good lap, your weaker lap, another session, or another driver if appropriate. Fourth, confirm with channels: lateral G, brake pressure, steering, throttle, speed, RPM or gear, GPS line, segment time. Fifth, turn the finding into one driving instruction that can be tested in the next session. If you cannot write one testable instruction, you do not yet have a finding. You have an observation.

A testable instruction is narrow. It is not drive harder everywhere. It is release the brake later but smoother through the first third of this entry. It is stop the mid-corner throttle lift in this fast corner unless the car actually needs it. It is reduce the long brake tail and let lateral G build earlier. It is carry less unnecessary speed loss in the first half of the corner and check whether the exit improves. The bonded data process supports this because it repeatedly asks you to inspect shape, consistency, comparison, and confirmation. That is also how you stay safe. You change one input in one section, then you re-check the evidence.

For intermediate drivers, the best first targets are not usually the highest-speed commitments. Pick corners where the data shows a clear G-sum valley, where the other channels explain the cause, and where the correction is small enough to practice cleanly. A brake-release dead zone is a good target. A mid-corner wait caused by over-slowing is a good target. A fast-corner confidence lift is a target only if you can confirm it with throttle or brake behavior, lateral G consistency, and line. If the trace is confusing, or if tire condition is changing quickly, pick a simpler section. The goal is not to prove bravery. The goal is to build a repeatable relationship between data evidence and driving action.

When the G-sum trace improves, the lap should become easier to explain. The brake trace should have a more intentional shape. Lateral G should show consistent peak use rather than random spikes. The low sections should either disappear or be explained by a real reason, such as line, traffic, tire condition, or a deliberate safety margin. Segment time should show whether the change mattered. Your body should recognize the same improvement: less floating between phases, fewer surprise corrections, and a clearer sense of the car staying loaded. That combination is the calibration cue. Data alone says something changed. Data plus feel plus repeatability says you learned a skill.

The deeper lesson is that grip gaps are usually phase gaps. The car can be underused under braking, underused while waiting for lateral load, overdriven at slip angle, or unloaded by a confidence lift. The same low spot on the G-sum trace can come from several different causes. That is why this lesson belongs after the G-G diagram lessons. The diagram can show the overall envelope. G-sum tracing tells you where, around the lap, you failed to keep the car working. Once you can trace that, you stop making generic data notes. You start making corner-specific practice plans.

Worked example: hard braking after a straight into a slow corner

Start with the situation described in the tire-development material: the tire at the end of a straight just before hard braking into a slow corner is cooler than the same tire in mid-corner a few seconds later. That detail should make you cautious. If you see a lower G-sum number at the very beginning of the brake zone, do not instantly call it a courage problem. Look at the brake-pressure trace first. Was the initial application decisive or light? Did the pressure ramp cleanly, or did it wander? The brake-process material tells you to look at initial application, trail, long tail, inconsistent pressure, and light-long versus hard-short behavior.

Now move from the brake point to turn-in. If brake pressure rises, then falls completely, and lateral G does not build until later, the G-sum trace may show a valley. That valley is a classic transition gap. The driver stopped asking the tire for braking force before asking it for cornering force. The fix is not simply later braking. The fix is a more connected brake release and turn-in so the car does not coast between jobs. If the brake trace has a long tail and lateral G never builds strongly, the opposite may be true: the driver is dragging the brake in a way that prevents commitment to the corner. Same low G-sum clue, different cause. The confirming channels decide.

Finally, check segment time and exit speed. If a more connected entry raises G-sum but hurts the exit, you may have added load in the wrong place. If the section time improves and the trace becomes more repeatable without spikes, you probably found usable grip. This is the difference between reading a number and reading the corner.

Worked example: a fast corner with a confidence lift

The brake-process material explicitly calls out lifts in fast corners as something to inspect. In a fast corner, a G-sum gap often appears as a sag where the car should be loaded steadily. You do not diagnose it from G-sum alone. Put throttle, brake pressure, lateral G, steering, and GPS line underneath the G-sum trace. If the sag coincides with a throttle lift or a brush of brake, and lateral G falls at the same point, you have evidence that the driver reduced load. If steering also changes abruptly, the lift may have been a reaction to line or commitment rather than a simple throttle habit.

The safe correction is not to promise yourself that you will hold it flat next lap. The correction is to ask why the lift happened. Was the entry speed inconsistent? Was the line different from the better lap? Did lateral G spike before the lift, suggesting the tire was over-asked? Did the GPS line show a later or earlier placement that made the corner look tighter? If the better lap shows the same line, smoother steering, no spike, and no lift, then the practice target is a smaller, earlier commitment change. If the better lap uses a different line, the target is the line, not the throttle.

Success in this example looks boring in the best way. The G-sum sag shrinks, lateral G is more consistent, the throttle trace has less interruption, and the section time improves without a new spike or correction. If the trace becomes taller but messier, you have not proven grip. You have only created a louder problem.

Worked example: two drivers on the same section of track

The Going Faster material describes a data example where the speed difference between two drivers on the same section of track came from one driver slowing too much in the first half of the corner. That is an ideal use case for G-sum tracing. Put the faster trace and your trace on the same distance axis. Do not start by asking who braked later. Ask where the separation begins. If the slower driver loses speed early and the G-sum trace is lower or has a valley before mid-corner, the grip gap may be in the first half of the corner rather than at exit.

Then confirm with brake pressure and lateral G. If the slower trace shows a light-long brake shape and delayed lateral G, the driver may be stretching the brake zone without really loading the car. If it shows a hard initial brake followed by an empty transition, the driver may be releasing too early and waiting. If it shows high steering and low speed at mid-corner, the driver may have overslowed and then held the car in a low-energy state. The faster driver's advantage is not just a number; it is a different sequence of load.

Be careful when the comparison crosses cars. The same source references examples from Formula Dodge and Showroom Stock to Indy Cars, including different tire choices and driving modifications. That is your reminder not to copy absolute G numbers across unlike cars. Use the comparison to learn shape, timing, and phase, not to demand the same magnitude from a different platform.

Common mistakes

Peak chasing is the first mistake. You look for the tallest point on the G-sum trace and decide the lap with the biggest number must be best. Good looks different: you look for repeatable load in the correct phase, then confirm it with segment time and supporting channels.

Single-channel certainty is the second mistake. You see a low G-sum section and label it unused grip. Good looks like asking why, then checking brake pressure, lateral G, steering, throttle, GPS line, speed, RPM or gear, and section time before you decide.

Spike worship is the third mistake. A spike can look impressive, but the lateral G process specifically tells you to look for spikes and consistency. Good looks like a trace that reaches useful load without a random jab followed by a falloff.

Ignoring tire state is the fourth mistake. Available grip can change as the tire heats through the corner, and data needs on-track calibration. Good looks like comparing similar laps and sessions when possible, then treating temperature-sensitive conclusions as provisional until they repeat.

Copying another car's number is the fifth mistake. The data process allows comparisons across cars and sessions, but only as part of an ask-why workflow. Good looks like copying the question, not the number: where did the better trace keep the car loaded, and what channel explains it?

Turning every finding into drive harder is the sixth mistake. Good looks like one narrow instruction tied to one phase of one corner. Change the brake release, the line, the lift, or the throttle timing. Do not change everything at once.

Drill: three-session G-sum grip-gap progression

Use this drill at your next event with two target sections, not the whole lap. The count is three sessions. The review time is ten minutes after each session. The success criterion is a repeatable reduction in one confirmed G-sum gap with no new spike, no loss in the relevant section time, and a driving note you can feel in the car.

Session one is baseline. Drive normally and make no data-driven changes during the session. Afterward, choose two sections where the G-sum trace is clearly lower than expected or has a visible valley. For each section, write the phase: brake, release, turn-in, mid-corner, exit, or fast-corner lift. Then confirm with at least three other channels. For example, brake pressure, lateral G, and GPS line for an entry issue; throttle, steering, and lateral G for a fast-corner lift; speed, segment time, and brake trace for an overslowed first half.

Session two is one-change practice. Pick the easier of the two sections. Write one instruction before you go out. Keep the rest of the lap normal. Run at least three clean laps where you attempt the same change at the same point. Do not chase the number live. Your job in the car is to feel whether the phase connection improved: less empty time after brake release, less mid-corner waiting, or less unnecessary lift.

After session two, compare only that section. Did the G-sum gap shrink? Did lateral G become more consistent? Did the brake or throttle trace show the intended change? Did the segment time improve or at least remain stable? If the number rose but the trace gained a spike, call it unresolved. If the number barely changed but the section time improved and the supporting channels are cleaner, keep the change. The data goal is faster usable grip, not prettier theater.

Session three is confirmation. Repeat the same change for another three clean laps. If it repeats, you have a real skill note. If it does not repeat, downgrade the conclusion and return to the ask-why process. A one-lap improvement is interesting. A repeatable improvement you can describe and feel is learning.

When this principle breaks down

G-sum tracing becomes weak when the supporting evidence is weak. Traffic, missed references, mixed sessions, changing tire condition, and unlike-car comparisons can all make the trace harder to interpret. The tire material is especially important here because it points to thermal sensitivity and the need for on-track calibration. If the tire is changing quickly, your job is to be modest with the conclusion.

The method also breaks down when you use it outside its scope. It does not replace the G-G diagram. It does not by itself explain car balance. It does not tell you whether a damper, pressure, or alignment change is correct. It tells you where the car was not loaded as expected, then sends you to other channels and to your own driving feel. When you respect that boundary, G-sum tracing is one of the cleanest ways to turn data into a practice plan.

Author Review

No quiz questions are attached to this lesson.

Sources

#	Document	Chunk	Pages	Score	Collection
1	Data-for-Drivers-PRINT	c32e7c50-7d5a-817a-2ca4-6ed348f718aa	16	1	uio_books_raw_v1
2	Data-for-Drivers-PRINT	27a86c4f-e78b-a3ac-5011-0bb82408d23d	1	1	uio_books_raw_v1
3	Data-for-Drivers-PRINT	849f6d32-91c8-10c7-d758-d545a8a31713	1	1	uio_books_raw_v1
4	Performance-Driving-Illustrated-Ross-Bentley	9c8404d4-c59f-c510-a38e-61c1fa20498b	16	1	uio_books_raw_v1
5	The Racing and High-Performance Tire Paul Haney	6c84d196-bf72-afa4-53a9-5c12e8e6f58a	204	1	uio_books_raw_v1
6	Performance-Driving-Illustrated-Ross-Bentley	826631be-04bd-43e3-0b83-5a2edd5cf725	9	1	uio_books_raw_v1
7	Performance-Driving-Illustrated-Ross-Bentley	8b67a9e8-4248-eb95-b368-7432c4da6d4b	30	1	uio_books_raw_v1
8	Going Faster Mastering the Art of Race Driving - Carl Lopez	4285b990-c3e7-880e-5596-99af145b469c	300	1	uio_books_raw_v1
9	The Racing and High-Performance Tire Paul Haney	0528b7f6-a4bf-cd9a-4bfb-b305c71aff39	6	1	uio_books_raw_v1
10	The Racing and High-Performance Tire Paul Haney	dd27b82c-8a72-819b-19ef-7b9a54d9a2b4	6	1	uio_books_raw_v1
11	The Racing and High-Performance Tire Paul Haney	ea0cfc03-c9dc-d850-b4f3-4d2d9520f5f3	7	1	uio_books_raw_v1