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Even more low-level than the car's basic physics is the way Racer interprets these.

[This is an OLD text and may not apply anymore to current Racer versions]

Underneath the hood, there are some things which you can edit easily in Racer that really change behavior at a much lower level than any of the general physics (car.ini files).

COLLISION DETECTION

In debug.ini, there are a couple of tweakable options. Note that the bounciness of the car chassis is set in the car.ini files itself (per car). Note these are used for ODE (the rigid body library).

• collision.mu; the friction coefficient for car-track collisions. Try 0.1 for a reasonable default value. The more friction, the more the car will slow down when touching things. May be set per surface in the future though.
• collision.cfm; this is Constraint Force Mixing. Valid values range from 0..1. Do NOT use negative values. This can be used to make track polygons softer. A default value of about 0.0001 will give quite hard surface. Try 0.1 and see how spungy the surfaces become (note your tires may cross the polygons and have a hard time dealing with outer space, and how you can drive THROUGH polygons at high speed even). This parameter is used mostly in case the car 'explodes', meaning it begins to rotate at high speeds, due to overly high corrective constraint forces.
• collision.erp; this is the Error Reduction Parameter. As a car enters track objects, some interpenetration is allowed. The car is, however, pushed back in these situations to move out of the track object(s). The speed at which this is done is defined by the ERP; at 0, no corrective action is taken (bad), at 1, the complete error will be fixed in a single simulation step. Although the latter sounds perfect, in practice it is best to use a value well below 1 (values between 0.1 and 0.8 are recommended, with a default value of 0.2) for better numerical stability (less explosive behavior).

TIRE PHYSICS

In debug.ini, you can find many options. Most of them I use while developing to be able to do slomo's to really get a good look at the numbers. There are some fun settings though you might want to experiment with.

• car.apply_friction_circle; if set to 1 (by default it is), the will apply a circle to the forces that the tire generates. A tire is limited to a maximum force, which is independent of direction. Still, most tire models (like Pacejka, which I'm using) prefer to handle longitudinal and lateral forces apart, and do some afterthought-like combining later on. This is to avoid exceeding the tire's friction circle, which is a concept that at maximum lateral force for example, you can't at the same time have maximum longitudinal force.
  With this setting you can turn it off. Just for testing, I would never do it for real racing.
• car.friction_circle_method; an important one. This decides exactly HOW the tire forces should be combined. Method 0 is from Giancarlo Genta's book (see the Credits & Refs page), which uses a simple elliptic approach. It tops off lateral forces when the longitudinal forces get too large, basically. So it prefers longitudinal forces, not lateral ones. Results in perfect ellipses, but not too realistic at the limit.
  Method 1 is a vector trimming operation; when the vector length of the lat+long forces exceed the maximum tire force, both forces are reduced in sync to fit the circle. Gives somewhat better grip, and you might like it.
  Method 2 is a null operation; just leaves the forces as they are. Good for comparison with the other methods, so you can feel what it is like if no circle is applied. May be the same as setting 'car.apply_friction_circle' to 0.
  Method 3 was invented by Gregor Veble and Brian Beckman, both independently, so there must be something to it! ;-) This method also modfies the input to the Pacejka formulae, and then changes these according to the ratio of slip in longitudinal and lateral directions. It is found to feel much better, and is the default in Racer v0.4.7.
• car.wheel_lock_adjust; if 1, this will gradually change the tire force direction to -slipVelocity when the wheel gets locked. This is done to simulate the fact that with locking front wheels you can't steer. Set it to 0 and you WILL be able to steer somewhat.
• car.use_slip2fc; if set to 1, this will reduce the friction based on a file (slip2fc.crv), which in turn is based on slip velocity. The idea is, the more slip velocity, the worse the friction. This was built in to get the car to do doughnuts, but other bugs were found and this became obsolete (but is still mostly in there). The tire forces now take care of this effect.
• car.max_force/torque; these two define the maximum forces and torques that will be applied to the car. Previously, some bad situations could make the car go very wild and the numbers would go to infinity and the program would grind to a halt. So these max figures keep the car from doing all too wild stuff (hopefully).

Check out the other options in debug.ini; most are mainly of interest while debugging though.

(last updated February 28, 2013 )