My Prius 2015 slips when braking

The manhole cover or pothole causes one of the wheels to spin faster than the other. Then the anti-lock braking system (ABS) senses this speed difference and releases the braking on the other wheel so it can catch up. This creates the unnerving feeling of loss of braking (which it is) for a moment before braking is re-applied. There is no solution, unfortunately. All you can do is be aware of this behavior and keep your foot on the brake pedal. If you release the pedal, it only lengthens the time for braking to be re-applied.

 

Assuming that it never does that except where there is something slippery on the road surface......including painted lines.......you just need to get familiar/comfortable with what the ABS feels like when activated. It often is accompanied by a momentary grinding noise.

 

Consider making it a habit to do your braking early and release the pedal before going over significant bumps, including manhole covers, RR tracks, potholes etc. This has always been the advice to lessen the occurrence of warped brake rotors and damaged suspension parts in conventional cars. If it avoids ABS activation, that’s one more good reason to brake early and then glide over the bumps.

 

My 2008 Prius did it all the time but the 2016 doesn't seem to be so sensitive. My 2008 didn't do it as often after I but different tires on it. Did you change your tires?

 

welcome! make sure you have all the software updates.

 

That's the part of the story that's the same for any car with ABS, but there's also a part of the story on a Prius that explains why you notice it more.

In the Prius, as you were gently slowing before you hit that cover or pothole, the brakes were not in use at all. The car was using electric regeneration to slow you down, and that happens through the drive train, so it only uses the two front wheels.

To slow down at the same rate using only two wheels instead of four means you are demanding twice as much of the traction from the two lonely contact patches where those two tires touch the road. So when the ABS senses that you might be at the traction limit there, its first priority is to let those two wheels go so you don't lose your traction and steering, while it brings in the all-four-wheel friction brakes as fast as it can. It only takes a split second, but it's long enough to notice, and startling if you're not used to it.

This has been a Prius "feature" from the earliest days. I was advised right here on PriusChat to just go take my first one, a 2001, out on a road with some rough spots, and make it happen until I had learned what it's like, and that's still the best advice out there.

In case you're worried the same thing might happen in a panic stop, it won't, because when the ECU detects a panic stop (recognized by the pedal angle sensor in Gen 3 and how fast you stomp the pedal) it skips regen and goes straight to four-wheel braking so there's no transition involved.

So this is strictly a gentle/moderate braking phenomenon that millions of Prius drivers get used to while managing not to drive into things. You can just react by pushing the pedal a little harder to bring your stopping point back to where you wanted it.

-Chap

 

That is a good explanation Chap, I didn't know that.

 

Another obscure point I eventually noticed by watching the ScanGauge: the ECU also considers as an exigent stop, and skips regen to go directly to braking, any time you have cruise engaged and cancel it by using the brake (no matter how gently). So if I have cruise engaged and just need to slow or stop routinely, I now always cancel the cruise using the wheel stalk before I touch the brake.

-Chap