Do I have a valid brake issue in my 2017?

Maybe I didn't explain it very well, the light came on when coming up to a stop sign while braking and at low speed and when trying to accelerate. It wasn't at highway speeds. This new one doesn't seem to be affected at all.

 

In the above scenario, if possible try to momentarily come out of the curve, just as you go over the manhole.

 

The slip the OP mentions usually occurs on regen braking when there is no ABS functioning (via the friction brakes). Imagine regen braking, using one wheel via the differential and that wheel suddenly hits a slippery surface. The wheel skids momentarily, the ABS system sees an imbalance in the rotation of the front wheels and applies the friction brakes.

 

Although from what I have always understood ABS is not looking for imbalance between wheels, it is monitoring individual wheel deceleration rates. Considering that on a slippery surface both wheels on the same axle can lose traction and yet still be moving at the same speed.

During normal braking each wheel will decelerate at a predictable rate. However if one or more wheels loses traction it will begin to decelerate more quickly than it would if it had traction. The computer will instantly detect the increase in wheel deceleration and will reduce hydraulic pressure to that wheel until the deceleration returns to a normal rate, then pressure is reapplied. It can repeat this cycle up to 15 times per second, allowing braking to occur at the maximum amount just below the threshold where traction would be lost. No human driver can brake that effectively, although skilled race drivers could get pretty close. Racers used a pumping action to do this, which led people to think that's the way to effectively brake hard, but without skill that doesn't work, and especially with ABS pumping the brakes is detrimental. With ABS you should always slam the brakes as hard as you can (assuming the goal is to stop as quickly as possible.)

Maybe on some systems it also compares speed between wheels to help determine if a wheel is locking up, although that seems almost superfluous since ABS is able to react very quickly just by looking at deceleration rates for each wheel. But I guess I wouldn't be surprised if some systems take all of this input into consideration.

Of course as discussed, during regen braking even though ABS may detect that a wheel has lost traction it can't reduce hydraulic pressure since there isn't any at that moment. So it has to reduce regen braking, which will affect both wheels even if only one is locked up. Then it switches to friction braking, which also brings the rear wheels into play. By that point presumably any locked wheels are now spinning again so ABS can reapply pressure, but also be ready to instantly reduce pressure to individual wheels if needed. I would assume that friction braking remains in effect for the duration of that braking cycle and it doesn't switch back to regen until the next time you brake, but I'm just guessing on that.

 

Yes, I also believe that's so: the system is capable of blending regen and friction, but once it has decided the situation is exigent, it just stays strictly conventional ABS until the next application of the brakes.

Remember the way very early ABS systems would noticeably pulsate? The actuator could sound like something scraping a washboard. The Prius actuator (probably, most modern ABS actuators these days) has several pressure sensors and a linear-response solenoid, along with the per-wheel SLA/SLR ones. It does a pretty good job of straight-out modulating the pressure it's applying to hang right at the limit of traction it detects, without noticeably pulsing it. I suspect the modern control rate is well above 15 Hz.

-Chap

 

The brakes on the Gen 4 Prius take a little bit of getting used to, but after a few months you'll be dealing with it like second nature.

 

Agreed, particularly if you drive different cars. When I first got my PRIUS, I was driving Mum&Dad's old CAMRY too - which needed quite high pedal pressure, but they were progressive - and going back and forth meant that it took me longer to get used to PRIUS. When we got rid of the CAMRY, I feel quite at home with PRIUS now.

 

That's my advantage - we have the Prius and a hybrid Avalon. Two cars with very similar braking "feels".

 

Some people just cannot get used to prius’ regenerative braking. Add to that the ABS, VPSN, Traction control, various beeping, etc... My sister bought a 2012 and sold it at 60,000 and the brakes were burnt. She routinely stopped 400 feet away from the car ahead after numerous neck snapping jolts. I really tried to teach her how to drive the car, but she never got it.

She now drives a Mazda CX5 and ‘loves it’