Drivetrain

The drivetrain is what turns the driven wheels: a power unit, a clutch, a gearbox with a final drive, and differentials (ADR-0011). Skidpad does not simulate it as a chain of separate bodies passing torque down the line. It is solved as one small rotational system each substep, together with the wheels, so a locked clutch, a spool and a stiff first-gear engine inertia are all unconditionally stable at every substep rate.

One system in the wheel speeds

The unknowns are the wheel speeds and the engine speed. Every shaft between them is a kinematic function of the wheels: the carrier of a differential turns at the mean of its two outputs, a centre differential's carrier at a torque-weighted mean of the two axles, and the gearbox input at the carrier speed times the overall ratio. The inertias of those shafts are reflected onto the wheels through the ratios, which gives a coupled mass matrix, and the tire torques are applied as the boundary condition, exactly as in the single-wheel model (substepping).

Everything that has a limit is a bounded constraint on a relative speed:

Constraint	Drives to zero	Bounded by
Clutch	gearbox input minus engine	the clutch capacity (infinite for a motor)
Centre differential	front carrier minus rear	nothing (open), everything (locked), or the LSD
Axle differential	left wheel minus right	likewise
Brake	the wheel speed	the brake torque

The constraints are solved jointly: all those not at a bound as exact equalities, the ones that overshoot clamped and held, repeated a fixed number of times. A brake within its capacity therefore leaves its wheel at exactly zero, which is what keeps a locked wheel from chattering (validation).

Power units

• Direct is the interim drive of the first milestones: a torque that falls linearly with carrier speed, with no engine state. It is the default and the cheap option for traffic.
• Combustion has a full-throttle torque curve over rpm, engine braking that grows with speed on a closed throttle, an idle governor and a rev limiter. The engine braking is a line from engineBrakingIdle to engineBrakingRedline, or the measured engineBrakingCurve when one is given; at part throttle the torque blends between the two curves. The governor stands in for a starter: the engine never stalls. Its inertia matters: in first gear it is reflected to the wheels at the square of the ratio and can be tens of times a wheel's own, which is why the driven wheels lock last under hard braking.
• Electric gives its peak torque to the base speed and peak power above it, regenerates on a closed throttle, and couples rigidly (no clutch). Reverse is the motor turning backwards.

Transmission

Forward ratios, a reverse ratio, a final drive, a torque interruption while shifting, and a clutch re-engagement time. An automatic shifts on the gearbox input speed at fractions of redline, holds for a moment after each shift, and treats a gear input of zero as drive, so a definition driven with throttle alone goes forward. Its shift points depend on the driver's throttle (ADR-0025). At full throttle it shifts up at shiftUpAt and down at shiftDownAt. On a lighter pedal both points move down toward shiftLightFactor times those values, so a car holding a speed cruises in a higher gear. Pressing the pedal raises the downshift point, which kicks down. An upshift is never so early that the new gear lands near its own downshift point, so the gearbox does not hunt. With the brake applied it uses the full-throttle points, so lifting off to brake does not upshift. While the clutch is open for an upshift the automatic lifts the throttle, so the engine falls toward the new gear's speed. For a downshift it keeps the driver's throttle until the engine has flared up to the lower gear's speed, so the clutch re-engages near sync instead of dragging the engine up with the wheels. A manual follows the gear input exactly: negative is reverse, zero neutral, positive a gear number.

The clutch capacity is the pedal's remainder times an automatic law: the clutch bites between idle and a set speed above idle, so a car launches under throttle and idles with the brakes held, and it opens with the brake once the gearbox input has dropped below idle speed, as a driver stopping would. A manual definition with clutchBiteRpm: 0 is pedal-only.

Differentials

An open differential sends equal torque to both outputs. A locked one (a spool, or the solid rear axle of a kart) ties the outputs together. A clutch-pack limited slip differential transfers at most a preload plus a share of the carrier torque set by the torque bias ratio, with a separate ratio for drive and for coast. The centre differential adds a front torque fraction for the open split.

Telemetry

EngineRpm, EngineTorque, Gear, ClutchSlip, ClutchTorque, DiffLockTorque_F, DiffLockTorque_R, CenterLockTorque and Clutch join the record; DriveTorque_* is the half-shaft torque at each wheel, recovered from its solved acceleration.