The brakes on your vehicle are not equal, whether you measure their size or the amount of work they do.
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If they were a sports team your fronts would be the star players, and the rears would be the seasoned veterans doing precisely as much work as they have to. No more, no less. And there's a very good reason for that.
It all comes back to the load transfer effect.
To sum it up quickly, the downward pressure on a tyre changes how much grip it has (I've previously used the analogy of having to squeeze a new jar lid tighter to get enough purchase to open it). This downward pressure also changes as you drive, most notably when you brake, accelerate and turn.
When you accelerate, it pushes the rears down a little more and takes a little bit of weight off the fronts. In extreme cases like drag cars it can lift the fronts off the road. When you turn, it pushes down harder on the outside tyres, and reduces the downward pressure on the inside tyres.
When you brake, it pushes the fronts into the road noticeably harder, and also takes a bit of weight off the rears, and this leads us to the exact reason why the brake bias needs to be unequal front to rear.
Except in rare cases where the rear is substantially heavier than the front, pretty much everything you drive will have a brake balance that puts the bias towards the front, meaning the fronts are asked to do more of the work. Even vehicles with the assistance of anti-lock brakes will still have a front bias built into the system.
In road vehicles, this bias will be significant enough that you're far more likely to lock one or both front tyres, and that's done on purpose too. It's far safer to have the fronts lock and the car go straight on, than to lock the rears and start spinning out of control. If you ease off the pedal (or the ABS kicks in for you) you can still steer. Once most drivers start spinning, they've become a mere passenger grimacing before the inevitable crash.
Since most braking systems are hydraulic, and in most cases the compulsory dual-circuit systems separate the front and rears (some old cars split them diagonally), this bias is normally achieved by way of displacing different volumes of fluid in the front and rear systems.
However, the front brake calipers are also usually bigger, meaning they need more fluid to exert the same pressure on the pads (and onto the disc) as a smaller caliper. So they need significantly more fluid than the rears to achieve a front bias. This is worth noting if you think you need a brake upgrade. Installing larger front calipers (larger in terms of the fluid needed inside them to push the piston against the pad), actually shifts the brake balance to the rear (compared to standard, it may still have front bias, just not as much).
In competition vehicles the brake bias is very often adjustable, and usually by the driver. In the 6-cylinder TASCCO Saloon Cars for example, they were all fitted with a particular set of larger front brakes (and standard rears), and also an adjustable valve that could restrict the flow of fluid to the rear brakes. This meant the drivers could counteract the fact that the larger front brakes had shifted the bias towards the rear.
In other categories, endurance racers often want to use as much rear bias as they can though, to reduce the wear on the fronts. That is, unless a pad change is required anyway, in which case conserving the front brakes isn't as important.
The brake bias affects how they drive though, because the more the bias is shifted towards the rear, the less braking force can be carried into the corner when trail-braking. That means braking a bit earlier, and easing off a bit more as they turn in.
Another instance when the bias is adjusted on-board occurs in electric and hybrid vehicles with regenerative braking. Due to the strong retardation effect of regenerative braking, they require computer control of this effect because of its impact on brake bias.
Again, on competition vehicles this will often be adjustable, normally in the form of a preset for how aggressive they want this electric engine braking effect to be.
Sam Hollier is an ACM journalist and a motoring fanatic who builds cars in his shed in his spare time.
