Introduction to GTA V's handling physics

Thankfully, Grand Theft Auto V features a surprisingly comprehensive and detailed handling physics system. While the engine simplifies very complex matters into only sets of two or three variables, it does allow to make vehicles behave in a sufficiently complex manner that can mimick IRL to a sufficient degree, allowing for concepts like weight transfer, understeer/oversteer, wheelspin and per-wheel brake luckup to exist within the car's behavior on the road.

V accomplishes this by implementing systems in an appropiate but simplified manner. For example gearing actually modulates the car's acceleration depending on the gear, but torque isn't actually a thing, as its just power scaled up and down by the gears. Wheels actually push (or pull) the car forward, but the rim size on the model does not affect acceleration or speed given the same power.

This guide is appropiate for both vanilla, lore-friendly and real vehicles, because the source of the vehicle model is irrelevant. What's important is the style of handling you want it to have.

Keystones of handling

It is important to understand a few things before embarking on creating/editing handling files to your liking.

• No section works isolated. Every thing you edit will partially affect other characteristics of the car. Think of it as each item being a physical characteristic contributing to the complete physical behavior of the car.
• A good chunk of items do not work the same way their IRL counterparts do, they only approximate.

Center of Mass
vecCentreOfMassOffset

Starts as the model's center, visible with OpenIV's model viewer. vecCentreOfMassOffset offsets it around so you can put the CoM wherever you want.

Must-knows

• Y means ahead/behind, Z means up/down. You don't need to edit X (left/right) most of the time.
• The car will pivot around the CoM while mid-air, but pivot around fRollCentreHeight while on the ground.
• The most balanced car will have a CoM sitting exactly between the axles.
• CoM is directly tied to fRollCentreHeight(Front/Rear) - together they control weight transfer. Think of it as a line drawn from the CoM Z down to the fRollCentre. The longer it is, the more weight transfer. Lowering CoM Z or raising fRollcentre reduces weight transfer, overdo it and the car will never lean.

Where to put it

Usually the CoM can go on the vehicle's center, Y sitting bewteen the axles and Z sitting right below the engine's height.

Do keep in mind the Center of Mass is a great way to define the vehicle's intrinsic stability, as you define how well balanced it is. Think of it as a basic tendency you can put onto the car's chassis.

A vehicle with a raised CoM (Z component) will tend to move around more, specially in landings and mid-air. Here's how it looks:

You can move the CoM forward or back to destabilize the vehicle in a different way, a bit more complex than high-low. Front or Rear biased vehicles can react to this fact in a multitude of ways, in general being more or less prone to understeer or oversteer. A bias front or back bias can also enhance turn-in ability, and take-off and landing behavior.

Moment of Inertia
vecInertiaMultiplier

To be written.

Grip & Traction
fTractionCurveMax, fTractionCurveMin, fTractionCurveLateral, fTractionLossMult

One of the most important aspects of Handling, grip defines how quickly the wheels can make the car accelerate, decelerate, or change direction.

Must-knows

• Grip is measured in G-Forces, remember 1G is 9.8m/s of acceleration.
• The car will pivot around the CoM while mid-air, but pivot around fRollCentreHeight while on the ground.

Interestingly, Grip is measured in G-Forces, which translate to how fast the wheels can accelerate-decelerate the car before starting to skid. This ties in directly with braking and torque, which also are measured in G-Forces. If there's not enough grip to accomodate torque or braking, the wheels are overwhelmed and will lose traction.

• fTractionCurveMax: Defines the base maximum ammount of Gs the vehicle is able to work with.
• fTractionCurveMin: Defines the base minimum ammount of Gs the vehicle has when skidding or losing traction.
• fTractionCurveLateral: In degrees, acts as the tire slip angle. The tires will get the best grip at the top half of this value
• fTractionLossMult: Defines how griploss scales on troublesome surfaces like grass, dirt or snow. Check materials.meta to know how much grip each surface detracts.

Bodyroll & Suspension Pressure effects on Grip

Surprisingly, V simulates grip gains (and loses) from suspension pressure. This means that on whichever tires the car is leaning to, get a significant ammount of traction, while tires on the other side lose traction.

This is best seen on FWD vehicles, which have a real hard time getting traction on launch. If you reduce bodyroll or move the center of gravity forward, the issue is greatly reduced.

Surfaces and fTractionLossMult

Only roads make the most of the vehicles' grip, and other surfaces like grass, dirt,plastic, concrete and snow reduce the overall grip. fTractionLossMult scales how important the effect is.

Braking
fBrakeForce, fBrakeBiasFront, fHandBrakeForce

Must-knows

• fBrakeForce applies per wheel. So don't put it equal to fTractionCurveMax, use about a 1/4 of it. Or 1/X where X is the nº of wheels on your thing.
• fBrakeBiasFront at 0.5 delivers the same brake pressureto all axles. 1.0 only delivers to the front axles, 0.0 only delivers to the rear.
• A well balanced fBrakeBiasFront allows full control of the car under heavy braking, this balance usually is between 0.55 and 0.7. You can produce oversteer by using a lower value, and understeer if using a higher value.

Aero
fInitialDragCoeff, fDownforceModifier

Air Drag is presented as a force that acts against the car's velocity, and tries to slow it down.

In V, air drag scales with the square of speed, but fInitialDragCoeff modifies the scaling so air drag actually scales the way you need it to.

Air Drag can be measured in Gs, which presents how fast you are decelerated at certain speed.

Due to V's vehicle engines outputting "torque" in Gs, both values can be compared easily, which gives us more information about the vehicles' performance. For example, as both the Gs from torque and the Gs from air drag even out, the vehicle stops accelerating - at that speed, the vehicle can't push through the air anymore. You got your real top speed.

This is one of the most arcade systems in V. Downforce translates directly to grip gain on the wheel, and can abide to wildly different rulesets depending on your needs.

The base math goes as follows:

Base fDownforceModifier set to 1 or less
• 0.035Gs, double (0.07Gs) if a Spoiler is installed.

Dynamic fDownforceModifier between 1.01 and 99.99
• 0Gs as the car is stopped, which ramps up to 0.035Gs as the car reaches its top speed. Double that if a Spoiler is installed.
• The value is multiplied by fDownforceModifier, so a modifier of x10 would net you 0.35Gs gain per wheel, double that with a Spoiler.

Vehicles with active Spoilers ignore fDownforceModifier entirely.
There are two types of active spoilers in V:

Raising type (Like the T20):

• No gain when the spoiler is down. As the spoiler raises, you get 0.035Gs. When braking, the spoiler pitches, and you get 0.07Gs.

• 0.035Gs when the spoiler is down. Over certain speed (60mph or so)it raises up to 0.042Gs.
• When cornering or braking, it gets to 0.07Gs.

Power
fInitialDriveForce, nInitialDriveGears, fInitialDriveMaxFlatVel,fClutchChangeRateScaleX

The base power is just a base value on top of which everything else works. Then, gearing modulates that power simply multiplying it up and down (the ratio), which results in the torque (also measured in Gs) sent to the wheels.

While the first and last gear's ratios are always the same (3.33 and 0.9) the gears in between have different ratios depending on how many gears the engine has at its disposal. The more gears it has, the smoother the torque drops from gear to gear - very few gears will result in an incredibly high torque drop on the next gear.

Keep in mind the ratios always have to go from 3.33 to 0.9, so the more steps you give them, the smaller the jump between each gear.

Those gears stretch over the engine's top speed, naturally.

Lastly, while the car is busy switching gears, you have no power. How quick this happens is defined by fClutchChangeRateScaleX.

Suspension & Bodyroll
fSuspensionX, fAntiRollBarX

To be written.

Deformation
damageMapScale, damageOffsetScale, fDeformationDamageMult

There are three main players on deformation.

In vehicles.meta:

In Handling.meta

My Philosophy

If you're not just following IRL stats, specs and quirks (which are fun on their own right if you achieve them)...

To me, a fun handling is a reactive handling. A fun vehicle reacts to the driver and the terrain. This means vehicles should never fully ignore bumps, slides, jumps, overall rough driving. Half the fun in driving is wrestling with the car when pushing it to its limit, or even past it.

Often times, you only need to take care so it sits between the front and rear axles, so the suspension doesn't lean back or ahead. Then adjust rollcentres until you get the desired base bodyroll.

I personally prefer to get it somewhat right and move it up as much as it makes sense for the car. This is a bit harder, but pays off when the car ends up tumbling around (This is GTA) and actually rolls downhill properly. Helps with the personality too.

As hard to work with as it is, vecInertiaMultiplier is very important to make cars heavier or nimbler, and is responsible for the overall stability of the car. Do try to get it right.

Grip numbers are a good base, but vehicle stability is absolutely key to be able to make the vehicle be able to use that grip. Just adjusting bodyroll, suspension strength and rebound you can completely change how much grip the car can actually use.

Downforce is mandatory on almost every vehicle, doesn't matter the type or shape. I would only avoid downforce in things like a semi

This is because, even at very low levels, downforce can be used

Dynamic Downforce (>1.0) is great to make cars specialize on high speed cornering.

Traction loss is more complex than numbers. You can make cars horrible offroading just by making the suspension stiff or have little travel. fTractionLossMult is not that important.