Budget grip for load-sensitive tires
Generated from
content/lms/vehicle-dynamics-ii-theory/01-tire-models-and-the-grip-budget/03-load-sensitivity-and-grip-budget.md; edit the source file, not this page.
Source path: content/lms/vehicle-dynamics-ii-theory/01-tire-models-and-the-grip-budget/03-load-sensitivity-and-grip-budget.md
Course: Read the forces that steer the car
Module: Decode the tire's force language
Estimated duration: 55 minutes
Purpose and boundary
You are learning how to budget grip when tire load changes. That is a different skill from memorizing a friction number. A tire that is carrying more vertical load can usually produce more absolute force than it did at a lower load, but it does not gain that force in a straight-line proportion. Equal additions of load return smaller additions of grip. That is the central load-sensitivity problem, and it is why a car can feel like it has less total cornering capacity right when the corner loads the outside tires hardest.
This lesson sits beside the Magic Formula, pure-slip, combined-slip, friction-ellipse, camber-thrust, and self-aligning-torque lessons. Those lessons explain other pieces of tire force. Here you are learning the budgeting habit: before you ask for brake, steering, or throttle, you ask what each tire is already carrying, which tire is becoming inefficient, and which tire is being unloaded so much that it cannot contribute much. If you only think about the total car, you miss the four separate budgets that actually make the corner possible.
The clean rule
Treat grip as four tire budgets, not one car budget. When load transfers, the tire that gains load gains less grip than the tire that loses load gives away. So your job is not simply to load the outside tire and hope for the best. Your job is to make the unavoidable load transfer happen in a controlled way, then ask each tire for a force it can still produce at its new load and slip state.
Haney gives the load-sensitivity shape directly in the tire-performance data. In his example, lateral force is compared at 200 lb, 400 lb, and 700 lb loads. As load rises, lateral force rises too, but each added block of load buys a smaller block of added force. That is why the effective grip per pound of load gets worse as load climbs. Bentley states the same driver-facing consequence from the four-tire view: the tires that receive extra load do not gain as much grip as the unweighted tires lose. The result is uncomfortable but useful: in the middle of a corner, when you need traction most, the loaded car may have less total available traction than the same four tires had before the load transfer.
That does not mean load is bad. You cannot corner, brake, or accelerate without load moving through the chassis. It means load is expensive. You should spend it only when it gives you something back: rotation, braking, drive, or stability. A sudden input spends load quickly and often asks for peak force at the same moment the tire is becoming less efficient. A measured input lets the tire build slip angle, build force, and stay closer to its useful plateau.
Why load sensitivity happens
The useful mental model is tire efficiency. A loaded tire has a larger job, and the contact patch changes under that job. Haney describes one important mechanism: as load increases, the contact patch lengthens. At the same load and slip angle, a wider and shorter contact patch gives more grip because less of the patch is sliding. When added load lengthens the patch, more of the patch is in the sliding portion, and grip efficiency drops. The loaded tire is still doing more absolute work, but it is not doing proportionally more work.
That mechanism connects directly to slip angle. Bentley shows that as slip angle increases, tire traction rises, then reaches a plateau, then falls away. Racing tires and street tires can have different curve shapes, but the driver lesson is the same: more slip is useful only until the tire reaches the useful part of the curve. Past that, you are not buying more cornering force. You are adding scrub, heat, wear, and delay. Load sensitivity makes this more important because the heavily loaded outside tire is already less efficient. If you also push it past the useful slip-angle range, you have stacked two losses on the same tire.
This is why a car that feels powerful and planted in the paddock can feel strangely narrow on grip at turn-in. The tire with the biggest force demand is often the tire whose efficiency is getting worse. Meanwhile the lightly loaded tire on the other side of the axle has lost useful contribution. The car can still be fast, but the fast version uses smooth force timing rather than big isolated commands.
The driver principle
Budgeting for load sensitivity starts before the mistake. You do not wait until the front washes wide or the rear steps out. You predict which tire you are about to load, then you shape the next input so that the loaded tire can accept the request without being shoved past its useful slip range.
On entry, that means you avoid asking the front tires for an abrupt steering force at the same instant the car is still settling from braking. The front outside tire may accept a large force, but it is less efficient under the extra load, and the inside front has lost contribution. If you turn too sharply, the outside front becomes the whole plan. If it cannot make the force, the car understeers and you often add even more steering, which only adds slip angle after the useful part of the curve.
At mid-corner, budgeting means listening for the tire to settle on the plateau rather than chasing more steering angle. If the car has taken a set and is near the limit, more steering is not automatically more rotation. Sometimes it is only more sliding in the contact patch. Your better tool is to adjust the request: a small release of speed demand, a cleaner maintenance throttle, or a line that asks for less radius. The goal is not to drive timidly. The goal is to keep the tire in the part of the curve where extra input still returns useful force.
On exit, budgeting means adding power in a way the rear tires can afford. Bentley describes throttle modulation as a way to manage oversteer: you can use throttle to transfer weight rearward, or reduce power to the rear wheels. Those are different effects, and a good driver separates them. If the rear tire needs more vertical support, a smooth throttle application can help. If the rear tire is already beyond its lateral budget, adding power may only increase the combined demand. A precise driver uses the throttle as a load and force control, not just as a speed request.
The four-tire budget
A useful budget has three parts. First, identify the tire that is gaining load. Second, identify the tire that is losing load. Third, reduce your expectation for the pair because the loaded tire does not pay back everything the unloaded tire lost.
That pair thinking matters because the biggest tire is not always the fastest plan. The outside front in a corner may be carrying the heroic number, but the axle total is what turns the car. If the inside front has been unloaded and the outside front has been over-slipped, you have turned two tires into one tired tire. The same idea applies at the rear. A car may feel dramatic when it leans onto an outside rear and powers out, but if the rear axle is being asked for lateral force and drive force at the same time, the load-sensitive budget can disappear quickly.
Think of the car as making a sequence of requests. Brake force is one request. Steering force is another. Throttle force is another. The combined-forces lesson covers the friction-ellipse version of this, but load sensitivity adds a prior question: how much useful grip did each tire have after the load moved? A tire with less vertical load may need a smaller force request. A tire with more vertical load may accept a larger force request, but not a proportionally larger one.
This is why smoothness matters for reasons deeper than style. Smoothness is not a moral virtue. It is a way of keeping the budget legible. Abrupt inputs make the tire load change quickly, the slip angle change quickly, and the driver feedback arrive late. A progressive input lets you feel whether the tire is gaining useful force or only gaining slip. The good intermediate driver is not slow with the controls. The good intermediate driver is progressive enough that the tire can answer before the next demand arrives.
Sub-skill one: load-transfer awareness
Before each major input, name the load move. If you brake, the car changes the vertical work the tires are doing. If you turn, the outside tires gain load and the inside tires lose it. If you add throttle, Bentley gives you the important rearward effect: throttle can transfer weight to the rear. You do not need a full engineering model in the cockpit. You need the habit of asking which tires are about to become expensive.
The simplest cockpit version is this: entry spends front capacity, cornering spends outside capacity, and exit spends rear capacity. That sentence is not a full vehicle-dynamics model, but it is a useful prompt. When you are about to combine two of those phases, get more careful. Turning while still asking for major deceleration is a bigger front-tire budget problem than turning after the car has settled. Adding throttle while the rear tires are still busy with lateral force is a bigger rear-tire budget problem than adding throttle as the wheel opens.
Sub-skill two: slip-angle discipline
Slip angle is not automatically bad. Bentley shows the curve rising as slip angle increases, then plateauing, then falling. Your job is to use slip angle, not avoid it. The mistake is treating more steering angle as the answer after the tire has stopped returning more force.
In practice, this means your hands should be diagnostic. When you add steering and the car responds with more rotation, the tire is still paying you back. When you add steering and the car only scrubs wider, the tire has probably moved past the useful part of the curve for that load and surface. Load sensitivity makes that expensive because the outside tire was already less efficient. The correction is not to keep winding in steering. The correction is to reduce the demand or change the timing: release some brake demand, wait for the car to accept the set, unwind slightly, or open the radius if the track allows it.
Sub-skill three: axle-pair thinking
A single loaded tire can feel impressive. It can make noise, load the wheel, and give the driver the sensation of commitment. But the car turns and accelerates through axle pairs. When one tire gains load and the other loses load, the pair can lose total capability even though the outside tire is working harder. That is the Bentley four-tire consequence, and it is the practical heart of this lesson.
Use this when diagnosing balance. Understeer is not only a front-tire problem. It can be a front-axle budget problem: the outside front is overloaded and over-slipped, while the inside front has lost contribution. Oversteer is not only a rear-tire problem. It can be a rear-axle budget problem: the outside rear has a large lateral job and the driver adds power before the rear pair can afford it. The correction is often to reduce the peak demand and improve timing rather than to add a bigger input.
Sub-skill four: contact-patch preservation
Haney links load to contact-patch length and sliding portion. That gives you a concrete image. You are not managing an abstract number. You are trying to keep the useful part of the contact patch doing work instead of dragging more rubber through the sliding portion.
This is one reason repeated abuse carries forward. Haney describes graining as a pattern that, once worn into the surface, is difficult to wear away. The ridges continue as wear progresses, the tread is no longer evenly loaded, and the tire loses grip. In lesson terms, a poor grip budget can become a tire-condition problem. If you repeatedly overload and slide the same tire, you may not get a fresh tire back on the next lap. Your bad budget can write itself into the tread.
Sub-skill five: force sequencing
Force sequencing is the habit of not peaking every request at once. You still drive fast. You still use the tire. But you arrange the requests so that the tire can trade one job for another.
On entry, the sequence is brake demand, then steering demand, with any overlap kept deliberate. At the apex, the sequence is lateral support first, then throttle demand as the steering opens. On exit, the sequence is stabilize the rear pair, then add power in a ramp the tires can accept. This connects to the combined-slip sibling lesson, but load sensitivity tells you why the timing margin is smaller when the car is heavily loaded on one side. The loaded tire has more absolute capacity but worse efficiency, and the unloaded tire has less to contribute.
Calibration cues
The first cue is response per input. A well-budgeted input gives you a clean response. The car takes a set, the tire builds force, and the next small input still changes the car. A poor budget gives you a dead input. You add steering and the car does not tighten its line. You add throttle and the rear correction appears before the car accelerates cleanly. You hold brake too long into the steering request and the front tire feels busy but the car does not rotate the way you expected.
The second cue is correction count. If the car needs several quick corrections at the same corner every lap, the tire budget is probably being exceeded in the same phase every lap. Corrections are not proof of bravery. They are evidence that the requests are arriving faster or larger than the tires can accept. A cleaner budget usually reduces the number and size of corrections even when speed rises.
The third cue is tire condition. The bonded tire material does not give you a target temperature table, so do not invent one. It does support using the tire surface as evidence. If a graining pattern appears and then keeps reinforcing itself, Haney explains why the tread may stop loading evenly and lose grip. That is a sign that the driver, setup, or both have been asking the tire to slide in a damaging way. For this lesson, treat graining as a budget warning: you may be spending the same tire too hard, too often.
The fourth cue is phase repeatability. Pick one corner and ask whether the entry, middle, and exit happen in the same order each lap. If your turn-in point is the same but the car alternates between understeer and oversteer, you may be varying how you transfer load before the tire sees the steering or throttle request. Load sensitivity punishes that inconsistency because the tire budget is different each time.
How this changes your driving decisions
When the front washes wide, do not make the first correction more steering. Ask whether the outside front is already over its useful slip range. If it is, more steering only increases the sliding part of the contact patch. A better correction is to reduce the request: slightly unwind, reduce speed demand, or use a wider radius if available. Then rebuild the steering request progressively.
When the rear starts to rotate on exit, do not treat the throttle as only an on-off switch. Bentley gives two throttle-related tools for oversteer management: rearward load transfer and power reduction. If the rear tires need support and still have lateral capacity, a smooth throttle can help the car take a rear set. If the rear tires are being overpowered while already cornering, reducing power is the useful move. The skill is knowing which situation you created.
When the car feels slow but tidy, do not assume the budget is too conservative. Check whether you are leaving the tires on the far-left side of the slip-angle curve. Bentley describes a driver who spends most of the time far left of the curve as driving below the limit. Load sensitivity is not an excuse to avoid load or slip. It is a reason to build force deliberately until the tire reaches the useful region, then stop adding demand that does not pay back.
When the car feels fast but ragged, do not assume the budget is heroic. You may be living beyond the plateau, especially on the loaded tire. A tire past the useful slip range can feel dramatic and still be slow. The loaded tire may be making noise and effort while returning less force per pound of load. The faster lap often comes from slightly less peak input, better timing, and a tire that stays in its useful operating range longer.
What to carry into the next lesson
Load sensitivity is the vertical-load side of the grip budget. The combined-slip lesson will ask how lateral and longitudinal requests share a tire limit. The Magic Formula lessons will give a modeling language for force curves. The camber-thrust and self-aligning-torque lesson will add other ways the tire produces and communicates force. Keep this lesson as the checkpoint before all of them: before asking how much combined force a tire can make, ask what load it is carrying and how efficient that load has become.
The practical summary
A tire carrying more load can make more absolute force, but it becomes less efficient as load rises. Across the car, the tires that gain load do not repay everything the tires that lose load give away. The contact patch changes, more of it can slide at the same slip angle, and the slip-angle curve still has a plateau and falloff. Your technique is to make load transfer progressive, sequence force requests, stop adding inputs after the tire stops paying back, and use tire condition as evidence. That is how you budget grip around load sensitivity instead of being surprised by it.
Worked example: the 200, 400, and 700 lb load curve
Use Haney's tire-data example as the cleanest mental picture. The tire is tested at increasing vertical loads: 200 lb, 400 lb, and 700 lb. The important lesson is not an exact number from the table, because the bonded chunk does not provide the values. The important lesson is the shape. The force curve rises as load rises, but it rises in diminishing increments. The jump from the lower load to the next load buys a bigger gain than the later jump to the higher load.
Now translate that into the cockpit. Suppose a corner loads one tire heavily and unloads its partner. The loaded tire is not useless. It may be the tire doing the most visible work. But every additional pound on it is buying less force than the earlier pounds did. At the same time, the lighter tire has lost contribution. The pair can therefore lose total useful capacity even while the outside tire feels heavily planted.
The driving answer is to stop treating the outside tire as an unlimited account. At turn-in, do not shock it with a steering demand that forces it immediately to the falling side of the slip-angle curve. In the middle, do not keep adding steering after the car stops rotating. On exit, do not assume the loaded rear tire can accept a full power request just because the car feels squatted and committed. The data example teaches a habit: when the load is high, the next unit of demand is expensive.
Worked example: the four-tire cornering situation
Bentley's four-tire explanation is the driver version of the same math. In a corner, some tires gain load and some tires lose load. The extra load helps the loaded tires, but not enough to offset what the unloaded tires give away. That is why a car can have less total traction in the exact phase where you most want more.
Picture a familiar medium-speed corner. You arrive with speed, begin to brake, release into the steering request, and feel the car roll onto the outside tires. If you make a quick steering input, the outside front receives a large lateral job while it is already carrying extra load. Because of load sensitivity, it does not become proportionally better. If the inside front is too light to help much, the front axle budget narrows. The car pushes, and the tempting novice correction is to add steering. The better intermediate correction is to recognize the budget failure: you asked the loaded tire for too much too quickly while its partner was not contributing enough.
Run the same corner with a more useful sequence. Finish the largest braking demand earlier, release enough that the front tire can accept steering, build steering progressively, then hold the tire near the useful slip-angle plateau. The car may feel less dramatic, but the response per input is better. You are not avoiding load transfer. You are shaping it so that the loaded tire gains force without being shoved into inefficient sliding.
Worked example: exit oversteer and throttle modulation
The bonded Bentley chunk gives a precise exit lesson: throttle can be used to transfer weight rearward, and throttle can also be reduced to manage power at the rear wheels. Load sensitivity explains why those two tools must not be confused.
On exit, the rear tires may still be producing lateral force while you begin asking for drive. If the rear pair has enough lateral budget but needs support, a smooth throttle application can help by moving load rearward. The key word is smooth in the technique, because a sudden power request can add longitudinal demand faster than the rear tires can accept it. If the rear tires are already sliding laterally, adding power may not rescue the car. It may push them farther past the useful region. In that case, reducing power is the budget repair.
A good exit therefore has a clear order. You feel the rear take a set, you begin opening the wheel, and you add throttle in a ramp. If the rear rotates more than intended, you ask what the tires need. If they need load and still have grip, keep the throttle change calm and let the car settle. If they are overpowered, reduce the power request. Both actions are throttle modulation, but only one is right for each budget state.
Common mistakes
The first mistake is the constant-friction shortcut. You assume that twice the load means twice the grip, so you drive as if the outside tire can pay back every load transfer. Haney's load-sensitivity data rejects that shortcut. Good looks like expecting the loaded tire to make more force, but not proportionally more force.
The second mistake is outside-tire worship. You feel the car lean on the outside tire and interpret that load as free grip. Good looks like thinking in axle pairs. The outside tire matters, but the inside tire has lost contribution, and the pair may have less total budget than your hands suggest.
The third mistake is steering after the plateau. Bentley's slip-angle curve rises, plateaus, and falls. If you add steering and the car does not tighten its path, more steering is likely adding slip rather than force. Good looks like reducing demand, slightly unwinding if needed, or changing the timing so the tire can return to the useful region.
The fourth mistake is overlapping force requests without noticing the load state. You brake, turn, and later add throttle as if each request has its own separate tire. The combined-forces sibling lesson handles the shared-force model, but load sensitivity adds that the tire may already be less efficient because of vertical load. Good looks like sequencing requests so the tire trades one job for the next.
The fifth mistake is using throttle as one tool instead of two. On exit, throttle can help move load rearward, but too much power can also overwhelm rear tires that are still cornering. Good looks like deciding whether the rear needs support or less power, then modulating accordingly.
The sixth mistake is ignoring tire-surface evidence. Haney describes graining as a pattern that can persist and reduce grip because the tread is not loaded evenly afterward. Good looks like treating recurring graining as evidence that the tire has been sliding or loaded poorly, not merely as cosmetic wear.
Drill: three-session grip-budget audit
Do this at your next HPDE or test day on one corner you know well. Use the same corner for all three sessions so the tire-budget change is easier to feel. The drill is not about setting a personal best. It is about proving that smoother load timing creates more response per input.
Session one is the baseline. For six clean laps, drive the corner normally and note the first phase that needs a correction: entry understeer, mid-corner push, exit oversteer, or late throttle hesitation. Do not fix it yet. Your success criterion is a clear diagnosis written in your notes after the session: which phase exceeded the budget first, and which tire pair was likely involved.
Session two is the entry and middle audit. For six more laps, keep the same braking reference and turn-in reference, but change the rate of the brake release and steering build. The goal is to avoid loading the outside front abruptly and then asking it for a large slip-angle jump. Your success criterion is that the car accepts the steering with fewer mid-corner corrections, and adding steering still produces rotation rather than scrub.
Session three is the exit audit. For six laps, begin throttle at the same place each lap, but change the ramp so the rear tires can accept load and drive in sequence. If the rear moves, decide whether it needs rearward load support or less power. Your success criterion is a cleaner throttle trace by feel: fewer lifts, less countersteer, and a more repeatable track-out point.
After the drill, inspect the tire surface if you can. Do not invent a temperature target from this lesson, because the provided corpus does not give one. Look for obvious signs that one tire is being abused repeatedly, especially graining that begins to form a persistent pattern. That evidence tells you whether the budget mistake is being written into the rubber.
When the principle changes shape
The principle does not disappear, but its expression changes with tire type, tire condition, and the part of the slip-angle curve you are using. Bentley's graph separates racing tire and street tire behavior, and both rise, peak or plateau, and fall. That means you should not copy a slip-angle feel from one tire to another without recalibration. A race tire and a street tire may both be load sensitive, but the useful region can feel different.
The bonded chunks also do not provide exact force numbers for your car, tire, pressure, camber, or surface. So the correct use of this lesson is qualitative unless you have your own tire data. You can know that added load returns diminishing grip. You can know that unloading one tire can cost more than the loaded tire gains. You can know that contact-patch shape and sliding portion matter. But you should not invent a pounds-of-force budget for your car from the excerpts provided here.
The practical exception is not an exception to physics; it is an exception to overconfidence. If the car is far below the limit, adding load or steering may still produce a clean response because the tire has not reached the demanding part of the curve. Bentley's slip-angle graph includes that early rising region. The point of load-sensitivity budgeting is to become more precise as you approach the limit, not to tiptoe everywhere.
Author Review
No quiz questions are attached to this lesson.
Sources
| # | Document | Chunk | Pages | Score | Collection |
|---|---|---|---|---|---|
| 1 | The Racing and High-Performance Tire Paul Haney | c79d98da-73bb-1216-af29-4ea8a3abd1b9 | 198 | 1 | uio_books_raw_v1 |
| 2 | The Racing and High-Performance Tire Paul Haney | 6e391a37-bc0a-06ba-630e-4fc12256bf36 | 117 | 1 | uio_books_raw_v1 |
| 3 | Performance-Driving-Illustrated-Ross-Bentley | 60f27f4d-c68a-17eb-61a1-62a0ad51f68e | 10 | 1 | uio_books_raw_v1 |
| 4 | Performance-Driving-Illustrated-Ross-Bentley | 9c8404d4-c59f-c510-a38e-61c1fa20498b | 16 | 1 | uio_books_raw_v1 |
| 5 | Performance-Driving-Illustrated-Ross-Bentley | f417489a-badf-465f-c02e-cce0f4cbe388 | 16 | 1 | uio_books_raw_v1 |
| 6 | The Racing and High-Performance Tire Paul Haney | 776a98f3-6670-c34c-4fab-6db2c55377fd | 145 | 1 | uio_books_raw_v1 |
| 7 | The Racing and High-Performance Tire Paul Haney | 0528b7f6-a4bf-cd9a-4bfb-b305c71aff39 | 6 | 1 | uio_books_raw_v1 |
| 8 | The Racing and High-Performance Tire Paul Haney | ea0cfc03-c9dc-d850-b4f3-4d2d9520f5f3 | 7 | 1 | uio_books_raw_v1 |
| 9 | The Racing and High-Performance Tire Paul Haney | dd27b82c-8a72-819b-19ef-7b9a54d9a2b4 | 6 | 1 | uio_books_raw_v1 |
| 10 | The Racing and High-Performance Tire Paul Haney | b7f57ad2-d935-17b9-144f-3fb747b5236e | 290 | 1 | uio_books_raw_v1 |
| 11 | The Racing and High-Performance Tire Paul Haney | b4546caa-ae28-00f3-a7c4-2d428aaf445b | 7 | 1 | uio_books_raw_v1 |
| 12 | The Racing and High-Performance Tire Paul Haney | 221f3474-bd81-b046-beff-b7986be85df8 | 8 | 1 | uio_books_raw_v1 |