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The proportion of total braking force distributed between the front and rear axles. Because braking shifts weight forward, front brakes do the majority of the work. Adjusting brake bias — either via a cockpit adjuster or pad/line changes — allows drivers to tune the car's entry behavior and prevent rear lock-up.
Bentley explains the physics in Ultimate Speed Secrets: "Due to the forward weight transfer under braking, and therefore more front tire traction, most of the braking is handled by the front brakes. So the brake forces will be biased toward the front. This is why all vehicles have more braking power on the front wheels." Going Faster! (Lopez) adds the procedural side: "All real racecars will have a brake bias adjustment on the car. Some are easily driver-adjustable, while others require a stop at the pits. The routine for setting bias is to first run it for a few laps, getting the tires up to temperature." Carroll Smith in Tune to Win describes the physical mechanism of a bias bar: a threaded rod through the brake pedal attachment point whose rotation shifts the master cylinder input left or right, changing front-to-rear bias. Segers in Analysis Techniques adds the diagnostic: "Corner-entry understeer followed by mid-corner understeer combined with low braking Gs indicates too much front brake bias. Too much rear bias leads to corner-entry oversteer." The HPDE guide notes that advanced drivers dial in brake bias for each track, adjusting it between sessions based on whether the front or rear is locking first.