2012 Prius V "brake delay" & Toyota recall 18V-684

No change to this transitional effect. No change to drive feel or mpg.

 

Hi ,

You're describing the standard Prius "bump during braking" oddity that every Prius generation has always had, and every new Prius owner has wondered about.

The Prius has two different ways to slow down. It can do that through the power train (regeneration into the battery, engine braking, or a combination of both), and it can use the regular brakes on the four wheels.

The power train braking is good for efficiency (when it captures into the battery energy that would otherwise be wasted), and for saving wear on the brakes. But it is limited in terms of precise control: it only acts on the two front wheels, it has a limited range of torque available, and can only apply that torque through both front wheels together (via the differential, of course).

The regular brakes are just that, regular, modern, ABS brakes. They are on all four wheels, all four independent, and each one can apply braking torque from zero to really strong. This is the system that gives the car the best control under different traction conditions.

When you have "light brake pedal application", the car always prefers to start with the power train braking, which is desirable for efficiency. But any time "tire/wheel perturbation occurs" in that process, the car switches to the regular brakes, which is desirable for control. In the switch, you feel that momentary dip in brake force, as the one system hands off to the other.

Notice this is not quite the same as "activating ABS"; ABS generally activates under more difficult conditions, like if a skid is really starting. Then you hear the clicking and pulsing and all, like any other ABS system.

The transition you're feeling is just a move the car makes under light braking conditions when any little tire/wheel perturbation made it think "hmm, maybe I should be using the regular brakes instead, just so I'll have ABS as an option if this traction gets any weirder."

You'll never notice this when you're braking in a real hurry, because there is a stroke sensor on the brake pedal, and any time it sees you go on the pedal in a real hurry, it goes straight to the regular brakes from the get-go, so there will be no transition.

Just for historical interest, here's the post where qbee42 explained it to me back when I was a first-time Prius owner.