Threshold braking: peak deceleration without locking

In any heavy braking zone, the first task is to slow the car as fast as physics allows. That's threshold braking: finding and holding the brake pressure that gives maximum deceleration without locking the wheels. With ABS, it's the pressure just below where ABS fires. Without ABS, it's the pressure just below where wheels lock.

Threshold braking is the easiest technique to describe and the hardest to execute consistently. The exact pressure that's "just right" varies lap-to-lap with tyre temperature, fuel load, slope, and wind. PaceBoss can't see brake pressure — there's no telemetry. What it can show is the consistency of your braking-zone outcomes.

What it is

A locked wheel has approximately 70% of the friction of a rotating-but-slipping wheel at peak slip ratio. So you want the most braking force you can apply without locking — modern tyres reach peak braking force at around 8–12% slip (slowing faster than rotating, but not by much). Brake harder than that and you cross into lockup; brake softer and you're leaving deceleration on the table.

ABS automates this. When the wheel approaches lockup, ABS releases pressure briefly, lets the wheel rotate, then reapplies — many times per second. The result is near-threshold braking, automatically, but with the small penalty that ABS is conservative about when to intervene.

Bentley's Speed Secrets and Donohue's The Unfair Advantage cover the topic in depth. Donohue's pre-ABS documentation of this discipline is worth reading — when getting it wrong meant you were the only ABS the car had.

ABS on or off: when each is faster

• For amateur and intermediate drivers, ABS-on is faster. Modern ACC ABS is conservative — it gives up a small amount of peak deceleration to maintain steering control — but it eliminates the catastrophic failure mode of a locked-up entry that scrubs 5–10 km/h. Trading a tiny per-lap cost for far less variance is the right trade for most.
• For elite drivers with high-fidelity hardware, ABS-off can be marginally faster. With a load-cell brake pedal and thousands of laps of muscle memory, an experienced driver can modulate to peak slip more precisely than ACC's ABS. The gap is small — typically 50–100ms per heavy braking zone.
• For controllers, ABS-on is almost always faster.

Driver61 has published comparisons with telemetry overlays of ABS-on vs ABS-off in ACC; the conclusion across multiple cars and tracks is consistent with the above.

Why it makes you faster

Shorter braking zones. Peak deceleration is roughly 1.3–1.5g for a GT3 on slicks in dry conditions. If you're getting 1.1g because you're underbraking out of caution, you need 30–40% more distance to shed the same speed — metres of brake zone where you should have been at full throttle.

Earlier turn-in. Reaching the target speed earlier in the braking zone means you can start the rotation phase earlier — earlier apex, earlier throttle application. Threshold braking is the foundation; trail braking is what you do with the saved time.

What PaceBoss can show you

PaceBoss can't see the brake pedal. What it shows when threshold braking is inconsistent:

• Lap-to-lap variance in any sector containing a heavy braking zone. If one sector swings 0.2–0.5s while the others are stable, your braking-zone outcome is varying. That variance could be brake point, threshold pressure, or release rate — PaceBoss can't isolate which. Unlike trail braking, where the signal tends to concentrate in entry-heavy slow-corner sectors, threshold braking variance can appear in any sector where hard braking exists.
• Consistency percentage low while best lap is competitive. One good lap that found threshold, many that didn't.
• Theoretical-best gap concentrated in the braking sector. Your fastest split (one lap that landed the threshold) shows what's available; your typical split is the gap to close.

What you do with the reading

In ACC's replay:

• Watch ABS activation in the HUD. Frequent ABS firing during the heavy-braking phase means you're at or above threshold. That's not always bad — ABS doing its job is fine — but it tells you where the envelope is.
• Note how often ABS doesn't fire. If you're never triggering ABS, you're underbraking. There's pace on the table.
• Compare with a fast ghost. If the ghost's brake bar starts at the same point as yours but reaches max pressure faster, they're applying more aggressive initial pressure — that's threshold-pressure work. If theirs starts later, that's brake-point work.

Common ACC examples

Turn 1 at Spa (La Source). Hairpin after a long downhill — heavy braking from ~280 km/h. The threshold here is grip-limited by the slight downslope; a half-percent more pressure than the tyre can handle locks up cleanly. Brake point and threshold both matter.

Turn 11 at Imola (Tosa). Long braking zone. Straight-line braking into the corner, then trail off into rotation. Underbraking the threshold phase makes the trail phase impossible — you arrive too fast to release smoothly.

The 360 chicane at Mount Panorama. Short brake zone at the top of the mountain. Maximum deceleration in a short distance, no margin for late lifts.

Further reading

• Ross Bentley, Speed Secrets — the braking chapter.
• Mark Donohue, The Unfair Advantage (1975) — pre-ABS threshold discipline.
• Driver61's ABS on/off comparison videos — ACC-specific telemetry overlays.

Related reading

• Trail braking explained — the technique that follows threshold braking through turn-in.
• The grip envelope — why peak deceleration is finite at all.
• Find your pace deficit — workflow for using sector variance to start the diagnosis.