Lurchingforward on braking over bump

I think this is a major weakness of The Prius.

If I brake over any instability such as potholes or slick spots, especially in winter, and the traction control activates, it isn't really so much the vehicle lurching forward, as the brakes suddenly cutting power to maintain driving stability.

I personally think The Prius traction control is over sensitive in this regard. Which is why I recommend GREAT snow tires in the winter, and good tires on The Prius all the time.

 

it's been fixed as much as the hybrid synergy drive will allow. is it your 2010? i thought there was a software update?

 

The brake pedal travel is broken down into 2 sections. The first section is approximately the first 25% of the pedal travel. The second section is 25% to 100% pedal travel. The first section is regen braking only. The second section activates the friction braking. The harder you press, the more pressure on the brake pads to slow down the car. 80% of the time, the braking can be done with regen braking only.
When you brake, your upper body is prepared to counter balance the force of slowing down by leaning back. During regen braking, when you hit a significant bump, the computer immediately cuts out all regen braking. Since you are still in the first 20% of your brake pedal travel, you have no regen braking and no friction braking. This means you have no braking at all. At this point, your body is still trying to counter balance the force of slowing down but there is no force of slowing down so you body is pressed to seat back even harder giving you the sensation of lurking forward or unintended acceleration.
Here's another example that's easier to duplicate.
Set your cruise control to 65mph with your foot on the gas pedal and drive like you normally would. A few seconds later, let your body react normally as if you're releasing the gas pedal without cruise control. Your body will instinctively counter balance the normal slowing down associated with releasing the gas pedal. As you release the gas pedal, your body is leaning back to counter the slowing down. This time, the CC takes over and sustains the same speed. Without the slowing down, your upper body is pressed to the back seat giving you the unintended acceleration sensation.

 

there is no fix for this b/c in regen mode you only have 2 front wheel brake, and when one wheel looses traction differential kicks in.

THe software fixes reduced ABS time out, but it is still there.

 

Thanks for the explanation I also always wondered about this.

 

I would add a few more comments to that explanation:
(*) The 'no braking at all' when regen cuts out is momentary, not permanent, until friction braking cuts in. The gap is brief, but very perceptible.
(*) The replacement friction braking force that does cut in is sometimes not perfectly matched to the regen that was cut off. The system isn't seamless, so you will notice when the new braking force is different, especially when it is less.
(*) This scenario happens only from light regen-only braking, which presumably means that you are braking in a non-emergency situation with plenty of margin, and can quickly apply harder braking. It isn't being reported from harder braking where the friction pads are already engaged before the bump.

Numerous owners have complained about this being a safety hazard. It should be a hazard only for those who insist on using light braking to come to a stop just a foot from the obstacle ahead, and freeze up when surprised by the change of brake feel. If they leave more safety margin, or don't freeze, or don't have a mental or physical block against escalating to harder braking, then it should not be a hazard.

 

It's really disconcerting and bugs me every time it happens - but I live with it. I thought the "brake recall" would've fixed it but I felt no difference afterwards. Thanks for the good explanations.

 

One of my very first PriusChat posts was right after I had bought my used Gen 1, and I was lurking on the forum and saw various posts vaguely mentioning some Prius braking thing, so I posted "say, what's the Prius braking thing I see people mentioning?" and from a few responses I got, I just took my new-to-me car out to a nice lightly-traveled bumpy street and practiced stops over bumpy road a bunch of times until I had internalized how the car was going to react and I was completing my stops where I meant to anyway. I'm sure I went through some very similar process all those years ago learning to drive my first car, and it feels very familiar now and really doesn't disconcert me when it happens ... but it probably should be sticky advice for every new-to-Prius owner to go get accustomed to it ... I found that advice from PriusChat very helpful, and I'm sure it saved me thinking "Wth!??!" one or two first times.

-Chap

 

We have definitely experienced this on our 2010 Prius. It results in a sense of losing control, but it has always happened at low speeds. Outside of a little anxiety and concern that there is something wrong, there has been no damage.

 

Wow.. that's great info!

As you press the brake pedal harder, the regen meter increases, indicating more energy is being delivered to the batteries. How does that observation fit in with the 25% / 25%-100% model? Does regen braking increase during the entire range of 0 to 100% pedal travel?

 

the regen meter shows the first 25% of brake travel, from 0-100% regen. when it reaches the left side, the pedal has moved to the 26th percentile.

 

Interesting. Thanks. Our recollection is that the regen meter does not max out until perhaps 75% of brake travel. Good excuse to go for a drive and check it out!

 

i'm just guessing, but it's hard to tell brake travel with your foot.

 

If you think that regen accounts for 75%of the brake travel then you're not pressing the brake hard enough. According to my scangauge, when I brake moderately, it shows friction brake value of around 0.6-0.8 when I stopped and activate the hill start assist, it takes around 1.3 brake value. However, I can press harder and the brake value will increase. According to xgauge chart, the brake value goes up to 4.5.

 

Once I figured out its 'envelope', and that it wasn't occurring during serious braking, it moved from my disconcerting list to my minor annoyance list.

The recall did make it less obtrusive on my early 2010. Some of it remained, and continues on my 2012.

 

Happened on my 2010 and 2015.

 

So ridiculous Toyota hasn't done anything to take care of this. Maybe because no one has died from it yet but its definitely still a big safety issue. I experience it all the time when I hit the brakes on bumpy roads. FU Toyota!

 

I still very much disagree, as explained above.

 

I think, in contrast, I've always been pretty impressed by how well they made the transition between two completely different ways of braking work most of the time ... and considering as soon as I had my car, I found PriusChat was right here to tell me I should go find a bumpy road and get used to braking effectively on it, I've really never had the experience of being surprised by it or thinking it unsafe. I can understand feeling more alarmed if experiencing it for the first time without anyone having mentioned it before. But even then, that initial feeling of alarm can be dispelled pretty quickly just by playing with the car in the right circumstances enough to get the hang of it.

-Chap