Brake bias, lock-up, and why the front usually goes first

The ideal front/rear brake split changes with how hard you're braking — from 70% at gentle deceleration to 83% at the limit. A fixed hydraulic system can only be right at one point, so which point you choose is the whole design.

Brake harder and the nose dives. That’s not just the springs compressing — it’s load physically moving from the rear axle to the front, and it changes what each end is capable of.

Same equation as chapter 18, rotated 90°:

Track is replaced by wheelbase $L$, lateral acceleration by longitudinal $a_x$. And as before, springs are not in it — anti-dive (chapter 9) changes how much the body moves, not how much load transfers.

The ideal bias is a moving target

The car in the app is 64% front at rest. Under braking:

The ideal brake bias — the split where both axles reach their grip limit simultaneously — is exactly this load distribution, because a tire’s braking capacity is proportional to its load.

So the ideal bias isn’t a number. It’s a curve running from 64% at a standstill to 83% at 1 g.

And your brake system has one fixed number

Chapter 22 measured the installed bias: 75.6% front, set by piston areas and rotor sizes. That’s a constant. Compare it with the curve:

• Below about 0.6 g, the system is sending more to the front than ideal → the fronts run out first → front lock-up.
• Above about 0.6 g, the system is sending less than ideal → the rears are relatively overbraked → rear lock-up.

Which brings us to why the crossing point is placed where it is.

Front lock-up is survivable; rear lock-up is not

A locked front wheel loses steering. The car ploughs straight on, which is alarming and lengthens the stop — but it’s stable, and it stays pointing where it was.

A locked rear wheel loses lateral grip at the rear while the front still has it. Any disturbance rotates the car, and the rotation is self-amplifying: once the rear steps out under braking, the geometry makes it worse. The car spins, backwards, still braking.

That asymmetry is not a matter of taste. Every road car is deliberately biased so the front locks first at every deceleration a driver will reach, accepting a longer stop as the price of not spinning. Regulations require it.

That’s why installed bias sits above the static weight split, and why the crossing point is placed near the top of the usable range rather than in the middle — you want the fronts to be the limiting end everywhere except perhaps at the very limit on perfect tarmac.

What ABS actually does

ABS releases pressure at a wheel that’s about to lock, then reapplies, many times a second. Two things worth being precise about.

It doesn’t fix a bad bias. ABS prevents a locked wheel; it does not make the split ideal. A car with badly rearward bias and ABS is a car whose rear ABS channel is working constantly, having its pressure cut, and therefore contributing less braking than it could. The stop is longer than a correctly biased car’s, and nothing warns the driver.

It’s not always shorter. On loose surfaces — gravel, snow — a locked wheel builds a wedge of material ahead of it that adds retardation. ABS prevents that wedge, so a locked-wheel stop can genuinely be shorter on gravel, at the cost of all steering. Rally drivers on loose surfaces exploit this deliberately; road cars keep ABS because steering matters more than the last few metres.

What ABS does buy, unconditionally, is steering while braking hard, which is worth more than stopping distance in most real emergencies.

Go look at it

Open the strut layout and go to the Brakes tab.

1. Turn ABS off so you can see the raw behavior, and run a stop from 100 km/h on Dry. Note which end locks and the stopping distance.
2. Increase Pedal Force step by step, watching the wheel-lock indicators. Find the pedal force at which the fronts first lock. That’s your bias crossing point, measured.
3. Now bias it rearward — increase the rear caliper Piston Ø — and repeat. The rears now lock first. Watch what the simulation does with the car. This is the configuration road cars are legally required to avoid.
4. Put it back, then switch the surface to Wet and re-run the same pedal force. Less grip means the ideal bias curve is unchanged but the whole thing happens at lower deceleration, so the front-lock margin changes.
5. Turn ABS on and repeat the rear-biased setup. Note that it no longer spins — and that the stopping distance is still worse than the correctly biased car. ABS rescued the stability, not the performance.
6. Finally, tie it back to Module 1: change the front anti-dive in Geometry and re-run. The nose dives less; the load transfer, and therefore the ideal bias, is identical.

What it costs you

Biasing for safety costs stopping distance. A car set to lock its fronts first at every deceleration is, by definition, never using the rear tires to their full capacity. On a 1.0 g stop where the ideal is 83% front and the system delivers 75.6%, the rears are overbraked and become the constraint — so pedal pressure has to stop short of what the fronts could take.

The engineered answers are all about making the fixed number less fixed:

Proportioning valves cap rear pressure above a threshold, bending the fixed line toward the ideal curve. Cheap, common, and a partial fix.

Load-sensing valves measure rear ride height and adjust — which matters enormously on a van, where the rear axle load can triple between empty and laden.

Electronic brake distribution does it properly, in software, per wheel, continuously.

An adjustable bias bar — two master cylinders on a pivoting bar — lets a racing driver move bias by hand for fuel load, tire wear and circuit. It’s the only one of these that puts the decision where the information is.

Rules of thumb

• Installed bias, road car: 60–80% front. Higher for front-heavy and high-CG cars.
• The ideal curve runs from the static weight split at zero deceleration to something near 80–85% at 1 g. Compute both ends.
• Always place the crossing so the front locks first across the range a driver will use.
• Check bias at your worst case, not your best: laden, downhill, wet.
• A high CG steepens the curve, because it transfers more per g — which makes fixed-bias systems worse on tall vehicles and is one more reason vans use load-sensing valves.
• Rally-raid and desert: bias is adjustable from the cockpit and gets used. Surfaces change from tarmac to sand to gravel within a stage, and the ideal split moves with the surface as well as the deceleration. Drivers also brake with the car unsettled or airborne, where a wheel with no load locks instantly — so these systems are set up to be modulated by a human continuously rather than to be right once.

Try this

1. Compute the ideal bias for your own car at 0.3 g and 1.0 g. How wide is the range, and where would you set a fixed system?
2. Find the pedal force at which the rears lock with the bias set rearward. How much margin was there before it happened?
3. On Gravel with ABS off, is the locked-wheel stop shorter or longer than the ABS stop? Does the answer change on Dry?

That’s Module 4 complete. You can follow the force from pedal to contact patch, compute line pressure and clamp force, work out where a brake system’s fixed bias sits against a moving ideal, and explain why every road car is deliberately set to lock its fronts first.

Module 5 is two case studies that use everything so far: lowering a road car without wrecking it, and drawing a front corner from a blank sheet.

Preset values are illustrative starting points, not a measured setup for any particular car. Brake figures use the app’s default brake configuration.