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Constraint and contact solver consists of four parts. First, if the number of contacts is low enough, it attempts to use a direct matrix solver. If this doesn't yield a valid solution (which can happen, since it attempts to linearize solve a "linear complimentary" problem - "LCP"), iterative impulse solver is used (in the code it’s referred to as “MC[microcontact] LCP solver”). In most cases this is the "main" solver. Then, if desired accuracy is still not reached, and the number of contacts is not too high, "LCP CG" solver is used. It makes several iterations of preconditioned conjugate gradient to drive velocities to 0 at "sticky" (non-separating) contacts. After each iteration it checks for negative normal velocities or adhesive normal impulses and moves violating contacts to the corresponding groups. The last iteration favors adhesive impulses over negative normal velocities. It treats each contact as having either infinite friction or 0 friction (this keeps the matrix symmetric and positive definite). A contact is assigned one of these types basing on the results of the MC LCP solver (that is, sticky contacts will remain sticky, and sliding contacts will assume that all tangential friction was already applied during the MC LCP stage). LCP CG solver does not try to enforce unprojection velocities for penetrating contacts; the last stage, “consecutive MINRES unprojector” is used for this purpose. It detects conflicting unprojection requests for each rigid body (“unprojection sandwiches”), and then attempts to find an unprojection order that avoid these conflicts. After that rigid bodies are assigned unprojection velocities one by one in that order (using MINRES solver), and these velocities are applied immediately, without affecting global velocities. All solvers are highly customizable in terms of iteration limits and thresholds (mostly via p_max_..., p_accuracy_.. cvars), and can thus be tweaked for different scenarios if necessary.

Wheeled vehicles are built on top of rigid bodies and use wheels to sample environment (by doing wheel geometry casts along suspension directions) and add suspension springs and tire friction impulses directly to the hull. Thus, tires are not modeled as separate rigid bodies. Then wheel contacts are registered for the global solver as special contacts.

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