Temporary brake loss over bumps

I have noticed a similar problem with my 2010 Gen 3

 

This behavior has been the norm in all Prii going back to the Gen2 (2004, The Gen1 may also have this quirk). You may also notice a loss of acceleration under similar conditions (loss of traction at one wheel). Note that the friction brakes do not stop working under this condition, only regenerative braking.

It is indeed part of the mis-named "Traction Control" feature and is there to protect the drive train (which would have been better protected by a pair of torque limiting clutches on the half-shafts like the one in the HSD).

JeffD

 

I've noticed that on our 3rd gen, very rarelym going over a dip or manhole. More frequently, but still quite rare:

The brakes are very touchy, going in reverse just after start up. The ability to modulate brake pressure is gone, they're either off or near-locked. Backing out of our garage for example I'll get this. And it's transitory, even stopping for a few seconds, say to get seat belt on, then continuing in reverse, brakes are back to normal.

First time I noticed this was backing while parallel parked, preparing to leave. I touched the brake and was convinced I'd hit the car behind us, the stop was so sudden.

 

Mendel,

Sudden braking is often caused by rust building up on the rotors. To clean the rotors:

1. Get up to 25-30 MPH in sparse traffic
2. Put your Prius in neutral (turns off regenerative braking)
3. Gently apply the friction brakes
4. Repeat as required to eliminate sticking and braking noise.

Our Prii friction brakes are seldom used so they are prone to rust buildup. If maintained (as above and with periodic lubrication of the slide pins), they last a very long time. My 2004 Prius brakes are original.

JeffD

 

…as mentioned in threads passim, I've noticed a similar phenomenon on my 2009 Gen II, the VSC light will flash momentarily as will the ABS light. It tends to happen when the suspension is being un-weighted (like driving over a hump-backed bridge), or the car is being driven over slippery surfaces (painted lines, manhole covers, steel construction-plates) there is a chattering of the brakes and systems as they try to sort it out, but in less than a second everything is back to normal! - hope this helps - Wil

 

@jdenenberg I don't think it's rotor rust, I'm really "on it" with the brakes, have opened them up and inspected before (lubed pads and pins), and I keep a regular eye on them. And it's not just a cold-start thing either. It's only for a few seconds, after start-up (cold or hot). I really think it's a software issue. Maybe coupled with insufficient brake fluid pressure, at start-up?

(I'm kinda off-topic, btw, apologies.)

 

I agree. Same behavior as my 2008.

 

You've just given a fine description of one of the oldest and most characteristic Prius "features". It goes all the way back to Gen 1. I just recently replaced my Gen 1 with a Gen 3, and felt the old brake bump effect for the first time in that one a couple days ago. That's when I started really feeling it was my car again. It's kind of heartwarming to know it's still there in Gen 4.

It's a really simple consequence of the car having two ways to slow down: one by just repurposing the electric motor as a generator, but that only acts on the front wheels. The other is a full four-wheel conventional brake system with antilock.

The conditions you describe are exactly where the electrical braking is active, and the mechanical braking isn't. The antilock brake computer (in Gen 1 this was separate from the 'traction control' and in later gens both functions are in the same box, but this is mostly about ABS) is always watching wheel speeds for any irregularity that might signal you've lost traction, or could be near to losing it. Hitting a bump the right way makes a wheel speed difference.

When the ABS sees any risk that you might lose traction, it wants to take over the braking. Braking with four wheels is much smarter if there is any question of traction, because it only needs half the grip per wheel, compared to regen using only the fronts. It cancels the regen immediately (because it thought maybe you were losing wheel grip, right?) and brings the friction brakes into the game, which takes a split second because they weren't participating before. During that split second, you're not slowing as strongly, but then you are again, as soon as the friction brakes clamp on.

You don't notice it under harder braking conditions where the car is already combining regen and friction braking, because the friction brakes are already in play, it only has to increase their pressure some.

For a sense of how long this classically Prius issue has been known, here's my post from 2008 when I first bought my Gen 1 and everybody on PriusChat who already knew about it got to explain it to me.

It's odd that, for as old and widely recognized as this Prius quirk is, we somehow don't have enough stickies about it on PriusChat, and fifteen years on newcomers are still discovering it for the first time and being disconcerted by it.

The best advice for a new Prius owner is just to go out on a nice clear day with no traffic and brake over some bumps and learn how it feels. That's just the feeling of how your car does what it does.

-Chap

 

get good snows.

 

As said here it's a Prius thing. Actually it seams much better in my gen 4 than my gen 2 was. All things seam better. But maybe I'm driving the new car a bit easier.

 

Yes!

OK - a bit off topic... as a New-Englander (and engineer) - I always chuckle when I hear people say "I wonder how this or that car does in the snow".

It's (almost) all about the tires. Sure, there's some weight distribution, suspension response, and dynamics effects, but let's start at the beginning: What's different about snow vs no snow? It's the snow. How/why does the snow have an effect? It reduces friction between the tires and road.

So, for a car to "do good in the snow" - it must try to negate the effect of the snow. It must add (or find) friction. How can a car do that? It can't - only the tires have the ability to negate the effect of the snow.

There - rant over. Thanks for listening.

(And, if there are other engineers out there for whom this is there area of expertise -it isn't mine: Feel free to correct me and/or teach me something I've neglected to consider).

 

I had no worries about winter driving in my Gen 1 with just all-seasons (Michelin Defender) in eight Indiana winters. I went out more than once before the street was plowed, got my business done and came back while my neighbors who were trying to get unparked when I left were still there still trying.

A front-wheel-drive car with a crazy 500 pound engine/motor combo spang over the front wheels is not a bad thing.

And that was with my Gen 1, which had only a "traction control" (emphasized quote marks) that was nothing but an MG1 rev limiter given a more appealing name. Every later generation starting with Gen 2 has had actual traction control, able to selectively brake slipping wheels in addition to modulating power output. I'm looking forward to my first chance in snow with my Gen 3.

The Gen 1 output-limiter-only did stop me, one time in eight years, from leaving a parking lot with an uphill exit where I had made a full stop. After a couple tries I just had to back up and use a running start. (edit: faulty memory - it was also a headache on my mom's driveway one time ... that's two incidents in 8 years, not one, sorry. Again, Gen 1 faux-traction-control-only-power-cut-really.)

-Chap

 

which is okay if there isn't a stop sign or traffic at the top.

 

You have just hit one of the two weirdest things about PriusChat. One is how the Gen 1 forum really exists but people fail to find it all the time because it was moved to be a subforum of Gen 2. But the other is that the same software that runs the PriusChat forums actually provides a wiki too (look up! see the link there?), the perfect way to organize information so it's easy to find established knowledge that everyone will want, and it gets, like, nearly absolute zero love. I've never been able to make sense of it.

-Chap

 

I think every exit from a parking lot has an implicit stop sign. I granted myself an implicit exception, and waited for the traffic.

Again, that was in a Gen 1, which only had the reduce-power half of traction control implemented, and even so it was an issue for me exactly once twice (edit: see above).

-Chap

 

gen 2 and 3 have reported inability to move in snow due to loss of traction and self protection system. did i read that ten 4 has traction control off button? i think v got one.

 

There is a button. I'm not sure if it is traction control off or stability control off... I guess I could read the manual... ;-)

 

I've seen plenty of those reports ... and I've also seen the descriptions of the traction control systems starting with the 2004 Gen 2 New Car Features Manual describing how the system works (page TH-47). I've seen various youtube vids of people getting post-gen-1 Prii stuck and blaming the trac, and also a memorable good one shot from outside the car while the driver quite competently took it up a steep slick driveway, with the wheels and car motion looking like an exact interpretive dance of the graph on that NCFM page.

Along with the importance of the tires and the car, driving technique seems to make a difference too. Particular techniques some people seem sort of wedded to (like "spin the wheels until something good happens!") are hard to execute with trac in effect. That's a technique I grew up never using in any car, so in the Prius I never missed it. (On the various occasions where I drive away, come back home, and find neighbors still trying to get their cars out, it seems pretty clear their difficulties are because of that technique.)

-Chap