Occasional Issue when braking over bumps

yes, standard prius fare. it occurs because the region is slowing and the friction is increasing, causing an abrupt lurch due to traction control. get used to pressing harder on the brake when you feel it, and leave plenty of room between you and the car in front of you.

 

All part and parcel of the wonderful DrivingThePriusExperience! You just gotta get used to it!

 

As others have said, traction control. My Prius c also did that... it also had traction control and the traction control active indicator would flash.

 

Traction control? Or a forced transition from 2-wheel-regen braking to 4-wheel-friction-only braking, triggered by ABS?

When regen braking with just 2 wheels, hitting a bump, bouncing slightly and locking up one wheel due to lack of traction, ABS should terminate the regen and switch to all-wheel friction braking. But the pause while traction is lost and ABS engages can be quite disconcerting.

The 2010s had a recall over this, reducing but not eliminating the pause. But even prior Prii had some of this too.

Non-regen cars don't have this 2-wheel regen braking mode, but use the more sure-footed all-wheel friction braking mode from the very start.

 

Yeah this seems to happen to me. The fraction control light turns on so I can only guess it's Abs

Posted via the PriusChat mobile app.

 

Any vehicle with anti-lock brakes does it.

 

When starting from light regen braking, Prius definitely does it different than my non-hybrids with ABS.

 

People find it more noticeable in the Prius, because the Prius is doing something most vehicles with anti-lock brakes don't.

When you first begin gently-to-moderately braking, the Prius is slowing you down without using the brakes. It is just doing electrical regeneration, which is done with only the two front wheels, and the actual brakes have not been applied yet. That's a difference from most vehicles with ABS.

If one of those wheels hits a bump or hole, it travels a longer distance than the other wheel on flat pavement. To the car's wheel speed sensors, that looks like a speed difference between wheels. That could mean that one tire is losing grip. (In this case it doesn't—all that happened was one rolled a shorter distance than the other—but the skid computer has no way to know that.)

If the skid computer thinks the grip might be failing while slowing the car with only two wheels, the best thing it can do is switch to the real friction brakes, which act on all four wheels, so only needs roughly half the grip per tire. That should give a nice increase in safety margin. But it can't do that quite instantly, because the friction brakes aren't applied yet. It can bring those in as fast as possible, but it doesn't wait to back off the regen braking of the front wheels. It wants those to keep rolling, not sliding, so you keep directional control.

So there's this little split second where your rate of braking decreases (which feels like being goosed from behind) until the four-wheel friction brakes are fully in play. Push a little harder on the pedal to make sure you'll end up stopping where you originally meant to.

That need to suddenly switch from two-wheel slowing without the brakes to four-wheel slowing using the brakes really isn't something conventional cars have to deal with.

 

Is this from some Toyota bulletin,or just a theory? Cause normally when a wheel hit a bump or pothole while braking, it loses traction, locks, and ABS kicks in. Nothing to do with longer distance. In Prius, with its skinny low resistance tires, I suspect the grip is lost easier.

 

We're both theorizing. Your assumption about the wheel losing traction and locking doesn't fit some of the parameters of the situation: this is often experienced in Prii under quite light to only moderate braking, on perfectly dry pavement, well below the limit of traction even for two skinny tires.

But what you are right about is the ABS kicking in. What triggers ABS to kick in? Mismatched signals from the wheel speed sensors, which can indicate an actual loss of grip by one wheel. You get the same mismatched signals if neither wheel has lost grip, but one has rolled a flat path and the other has rolled a longer path over a sharp bump or down a pothole. The ECU can interpret the signals as meaning that the tire on the shorter path lost grip.

The skid computer ends up reacting the same way whether it was an actual loss of grip or just a signal that looked like one. It reacts by going to friction, four-wheel braking, immediately, and further on to modulated ABS braking if it has to (which is only if the tires really are losing grip and continue losing it even after all four are participating).

But the unusual step that has to happen in a Prius, and not in other cars, is the switch from slowing electrically without using the brakes at all, and only on two tires, to bringing the brakes into the game at all four wheels. That's the feeling that spooks a lot of new Prius drivers, because it feels like a more noticeable momentary pause in braking than you feel in a conventional car where the brakes are already in the game and it merely starts modulating them with ABS.

There would not, otherwise, be so many new owners posting on PriusChat, people with plenty of experience driving other ABS cars of the last 30 years, saying "wow! this feels different!".

 

I'm not. The pros and cons of ABS have been discussed and documented since its appearance.

 

(…reaches for the pop-corn!) Anyone care to join me? Feel free to help yourself! (…just put those bottles in the fridge, and grab a clean glass!)

 

You're not guessing about how ABS works, no, you're just guessing about what is happening between the road and the tires of a Prius at the times this hybrid-specific behavior happens to be triggered. You seem to have convinced yourself it makes no difference that a Prius has a whole mode of operation (braking on two wheels only with the friction brakes unapplied) that doesn't exist at all in the cars you're basing your guess on. But it would take more work to square your explanation with what Prius drivers experience about the conditions when this behavior crops up.

 

The difference in the Prime is that if it's regeneratively braking with the front wheels, the friction brakes are often not engaged at all. So if one of the front wheels loses traction even slightly, it switches to friction brakes. While it's switching, ALL BRAKING STOPS for a moment, since it takes time for the friction brakes to react. There's no bulletin, but you can feel it, and it can be disconcerting. It works the same way as ABS in a normal car, but it seems to be more sensitive because if traction is actually limited it needs to switch to friction braking right away.

In a normal car the ABS can ignore one wheel briefly hitting a small bump so that the ABS isn't running all the time, but since the Prius has to switch braking modes it seems to be set to switch over at the slightest loss of traction. That is speculation, but it's pretty easy to confirm if you pay attention to how it reacts while you're driving.

That raises the question of why there is a delay while it switches to friction braking, and I think we can only speculate there. It seems like it could be possible to keep the regenerative brakes engaged until the friction brakes are also engaged, but they must have a good reason not to. Regenerative braking is through the differential, which is interesting.

 

I'm going to disagree with this statement.
Whatever combination of regenerative braking, and hybrid traction control might be in play, it's definitely different with a Prius than with a regular anti-lock vehicle. It's a unique hybrid phenomenon.

 

IF.....braking was active, it would be ABS.
If accelerating, TC.
I think it most likely was ABS. Personhole covers and wide painted lines actually are quite slick, especially when wet.

This kind of a response is NOT unique to the Prius......or even to Toyota. It is pretty common these days but might feel somewhat different depending on the amount of regen braking at the time.

The first time a driver experiences ABS activation, they usually swear that the wheels are about to fall off.

 

A 'normal car' also has a whole 'nuther axle of wheels that continue braking right on through any ABS-driven release of a wheel over a bump, so the car as a whole should not be experiencing a 'braking pause'.

And a Prius is operating from 'normal car' mode during hard braking. Note that we never hear about this complaint during hard braking.

I suspect that the immediate release of regeneration is likely related to shock-loading protection built into the propulsion system. This shock-loading protection also seems to the 'pig' that was given the 'lipstick' name of Traction Control on early Prius generations. It took several revisions before it actually performed an adequate Traction Control job.

 

I still think the sensitivity of the ABS for one wheel slipping is higher on a Prius than a normal car. Since the regenerative braking has less traction (only two wheels) it has to switch to friction braking right away just in case it's needed. On a normal car they could tune the ABS to ignore small bumps, since they are common and most of the time ABS isn't necessary. For example they could set a slightly higher threshold for wheel speed difference or time the wheel is slipping on a normal car, to avoid activating all the time. Of course there are still situations where it will activate in normal cars when one wheel hits a bump, it's just not as common as in the Prius.

It is plausible that it could be to avoid shock loading, but they don't always avoid shock loading. For example the car seems to allow an excessive amount of spin on the front wheels. If a wheel starts spinning, then either regains traction or the brakes kick in to slow it down, that can be a pretty big shock (usually a loud bang inside the car). I'm pretty sure they could change how they control MG2 to prevent wheel spin if they wanted to.

Also, I know this is a totally different vehicle, but a lot of the 4WD electronic aids in the 4runner and other 4WD Toyotas work mainly by pulsing the brakes, which creates big shock loads. For example ATRAC (or Auto LSD on 2WD trucks) will send power to wheels with traction by braking the slipping wheels. It can get the car moving even if two wheels are in the air (which isn't normally possible with open differentials). It is best used by holding the engine at around 2000 RPM and letting the brakes bang away for a few seconds.

Traction Control or TRAC in the Prius works similarly, but it seems to be much weaker. I know the drivetrain is weaker than a 4WD vehicle, but the car is also lighter and the power output is pretty weak. The electric motor(s) have a lot of torque from a stop, but I suspect still less than a 4WD vehicle with low range gearing. I'll have to get one of the front wheels in the air and see how the Prius reacts.