Kart-to-kart collisions and bumps

When two Karts touch, the Kart Collision settings decide what happens. By default you get an arcade bump in the style of classic kart racers: the Karts bounce apart and lose a little speed, and you can tune every part of it.

The bump runs inside the physics simulation, so it works the same in single player and in multiplayer.

Collision modes

Pick the mode with Kart Collision Mode, in the Kart’s Class Defaults under Kart Base Settings > Kart Collision.

Mode	What it feels like	Bump settings	On Kart Bump event
Arcade Bump (default)	A designed, tweakable bump. The Karts bounce apart with a set push and lose some forward speed. The contact itself never tips or spins a Kart.	All apply	Fires
Physics	Plain rigid body collision: the bodies’ real masses, speeds and physical materials decide. Nothing is scripted.	Ignored	Never fires
Ignore	The Karts drive through each other. For ghost racing, time trials or a party option.	Ignored	Never fires

When two Karts use different modes

Each Kart has its own mode. When two Karts touch:

1. If either Kart is set to Ignore, they pass through each other.
2. Otherwise, if either Kart is set to Physics, they collide as plain rigid bodies.
3. Only when both are set to Arcade Bump do they bump.

Arcade Bump settings

These settings only apply in Arcade Bump mode (the panel greys them out otherwise). Each Kart uses its own values for what it receives, so a Kart with a bigger Bump Push Speed is pushed away harder.

Setting	Default	What it does
Bump Mass	1	Weight class for bumps only. The body’s physical mass (handling) is not involved. See the split below.
Bump Push Speed	400	Push each Kart receives straight away from the other, in cm/s, for two Karts of equal Bump Mass. A lighter Kart receives up to twice this, a heavier one down to nothing.
Bump Elasticity	0.5	How much of the speed the Karts closed in with comes back as separation, on top of Bump Push Speed. 0 = the Karts just stop closing in, 1 = a full elastic rebound.
Bump Speed Retention	0.9	Fraction of its forward speed a Kart keeps when bumped, for equal Bump Mass. 0.9 = lose 10%. The lighter Kart loses more, the heavier less.
Bump Min Closing Speed	100	Karts closing in slower than this (cm/s) only rub along each other: no bump, no event. Keeps a side-by-side race from turning into a bump every frame.
Bump Cooldown	0.35	After a bump, this Kart cannot be bumped again for this long (seconds). Meanwhile the two Karts only push each other like solid bodies.
Bump Upward Speed	0	Small hop the bumped Kart gets, in cm/s upward, for equal Bump Mass. 0 = off.
Bump Yaw Speed	0	Turn away from the other Kart, in rad/s. Strongest for a glancing hit, none for a square one. 0 = off.

A Kart that lands on top of another Kart does not get a bump. The physics engine separates the two bodies instead.

How Bump Mass splits a bump

When two Karts bump, the lighter one takes the larger share of the push and of the speed loss:

Bump Mass (Kart A and Kart B)	Kart A gets	Kart B gets
1 and 1	50%	50%
1 and 3	75%	25%

This lets you make a heavy character hard to push around without changing how the Kart drives.

Suspension and other Karts

Setting	Default	What it does
Suspension Ignores Other Karts	On	The suspension casts and the ground check look through other Karts, so a Kart never climbs onto another Kart’s body. Off: other Karts count as ground for the suspension.

This setting sits in Kart Base Settings > Suspension. Wall Hit settings never apply to another Kart, see Surfaces and walls.

React to a bump

The event On Kart Bump (OnKartBump) fires when this Kart gets bumped by another Kart, in Arcade Bump mode only. It fires on every machine: the server, the owning player’s machine and all other players’ machines. That makes it safe for sounds, particles and animation.

Parameter	What it holds
Other Kart	The Kart that bumped this one.
Push Direction	Horizontal direction pointing from the other Kart toward this one.
Push Speed	The push this Kart received, in cm/s.
Closing Speed	How fast the two Karts were closing in before the bump, in cm/s.

Both Karts of a bump fire their own event (unless one of them is still in its cooldown, or the closing speed is below its Bump Min Closing Speed).

Example: spin out on a hard hit

Combine the Kart Intents StunKart and AddKartAngularImpulse to spin a Kart out when it is hit hard:

Event On Kart Bump
  -> Switch Has Authority
       [Authority] -> Branch (Closing Speed > 1000)
            [True] -> Stun Kart (Duration 0.8)
                   -> Add Kart Angular Impulse (Angular Impulse In Radians 0, 0, 5; Velocity Change ticked)

• Stun Kart ignores steering, throttle and jump/drift presses for the duration and ends an active drift.
• Add Kart Angular Impulse with Velocity Change ticked adds the spin directly, in rad/s around the up axis.
• Max Angular Velocity (Kart Base Settings > Turning, default 5) caps every spin, so raise it if you want a faster one.

The values are a starting point: tune them in PIE. More about these functions in Boosts and items.

Bump values for animation

These read-only variables are handy in your Kart or animation Blueprint, for example to lean the body away from a hit:

Variable	What it holds
Bump Time Remaining (BumpTimeRemaining)	Seconds left of the bump cooldown. Above 0 means “just got bumped”. Counts down on every machine.
Last Bump Push Local (LastBumpPushLocal)	Push of the last bump in the Kart’s local space (X forward, Y right).
Last Bump Other Kart (LastBumpOtherKart)	The Kart that bumped this one last. Can be empty once that Kart is gone.

Multiplayer

Other players’ Karts simulate on your machine too, so a touch between two Karts is predicted for both of them on every machine. The server’s result wins.

Setting	Default	What it does
Predict Bump On Other Karts	On	On: another player’s Kart fires On Kart Bump on your screen the moment the bump is predicted, together with its body’s reaction. Off: other players’ Karts fire it only from the server’s report, a round trip later. Only changes when the event fires, never the simulation.
Bump Prediction Window	0.5	Seconds after a predicted bump on another player’s Kart during which the server’s report of the same bump does not fire On Kart Bump a second time. Editable only while Predict Bump On Other Karts is on.