Barrel roll

Barrel roll is a vehicle state where pitch rotation is unlocked. This allows the vehicle to point directly up/down or even flip completely over.

Overview

Barrel roll is activated by colliding with barrel roll walls (also known as soft walls). On the regular tracks, soft walls are found at any accessible road edge to prevent vehicles from getting stuck on edges. Barrel roll can only be active when

1. the magnitude of vehicle IV is below 15 u/f,
2. airtime is not more than 20 frames, and
3. not all of the vehicle's wheels are touching the ground.

Once active, barrel roll is maintained as long as all three requirements are met, even if the vehicle is no longer colliding with a soft wall. For bikes, this means that doing repeated wheelies to keep the front wheel off the ground allows barrel roll to be carried almost anywhere.

Applications

Barrel roll clips

Wheelies in barrel roll allow bikes to point far higher than usual (even directly up). Since wallclips redirect a vehicle's IV to its facing direction, clipping a wall directly out of barrel roll can allow for much higher clips than normally achievable. This is used on Rainbow Road UR Flap to clip over the high invisible wall next to the finish line, in order to achieve a delayed lap count (DLC).

Wheelchair

Barrel roll allows vehicles to flip completely over; in some cases, vehicles can stably remain upside down. In this upside-down state, known as wheelchair, vehicles rest only on body hitboxes and have no wheels touching the ground.

It is possible for any bike to attain wheelchair with precise wheelies in barrel roll (e.g. by using the falling blocks on SNES Ghost Valley 2), but this is quite fragile, and turning is not possible. Wheelchair is most readily used by Tiny Titan and Classic Dragster, whose unique hitboxes give high stability upside-down and even allow turning by hopping to the left or right during an SSMT boost. Although karts cannot wheelie, they can flip upside down by driving into walls while in barrel roll.

Driving on solid OOB

Wheelchair enables vehicles to drive around on solid out of bounds (OOB), as OOB detection on this collision checks that your wheels are on or very near to the ground. Wheelchair with Tiny Titan/Classic Dragster has been used in Koopa Cape UR Flap, DS Desert Hills UR Flap, and DS Peach Gardens UR Flap to navigate around checkpoints while driving on solid OOB. Wheelchair with Dolphin Dasher is part of the theorized optimal strategy for DS Peach Gardens UR Flap for the same reason.

Speed redirection

While IV is limited in wheelchair, vehicles can gain EV from slopes because the body hitboxes do not have traction with the ground. Separately, when a vehicle flips out of wheelchair, its direction of movement will flip along with it. These two factors together allow a vehicle to redirect instantaneously from moving quickly backwards to moving quickly forwards. This is the optimal strategy for DS Yoshi Falls NU Flap to enter the finish line backwards from the front (after a DLC) and then immediately start traveling forwards.

Horizontal OOB bypass

Horizontal OOB planes count a vehicle out if it travels through the plane in a certain direction. Passing through a horizontal OOB plane while upside down (achievable with barrel roll) allows the player to bypass it without getting counted out. This is used on Rainbow Road UR Flap to complete the full DLC shroomless.

Extended HWG

Colliding with a barrel roll wall sets the "HWG timer" to 10 frames, during which barrel roll can be activated. This timer is so-named because it is also responsible for activating horizontal wall glitch (HWG). HWG usually lasts for 20 frames, but in certain cases, HWG will continue so long as the timer does not count down to 0. This can be exploited by riding a barrel roll wall so that the HWG timer continually resets to 10, thus extending the initial HWG. This is used on N64 DK's Jungle Parkway UR Flap to reach the seam to the left of the finish line.