Braking lapse?

<div class='quotetop'>QUOTE(genalex @ Aug 28 2006, 07:42 PM) [snapback]310889[/snapback]</div>

What you are probably feeling is the car momentarily losing traction as it gets light on the suspension, which causes the ABS to momentarily kick in. I'm told that on the Prius the ABS requires a quick transition from regen braking to friction braking, and because of that the transition is more noticable.

 

The stock tires can also accentuate the feeling, as they have minimal grip. This triggers the ABS sooner and more often.
My anti-Prius (Nissan Pathfinder) did this all the time on the stock tires. When I changed them out to Nokian WRs I found out just how bad the stock tires were. I thought the ABS was way too sensitive. With the Nokians, the vehicle just stops, no ABS triggering.

 

That loss of braking is not ABS, it's the traction control kicking in.. Any bump in the road can cause the system to detect slip in the wheels and turn off regen and switch to normal friction brakes and it's the loss of the regen drag that you feel.. (Actually, it doesn't completely switch off regen, but it goes down to minimal levels: you go from a braking current of 60+ Amps down to 10-15 Amps after the bump)

It's a really annoying and disconcerting if you're not expecting it, but its quite normal and you quickly learn the habit of quickly increasing brake pedal pressure to compensate, or if there's lots of room to go, releasing the brake for a second and re-applying to re-engage the full regen mode. Additionally, if you are always driving down the same road that causes the traction control to kick in, you can try shifting to "B", which will cause the system not switch out of regen except in extreme cases of slip; just remember to switch back to "D" after you've stopped..

 

<div class='quotetop'>QUOTE(efusco @ Aug 29 2006, 10:04 AM) [snapback]311075[/snapback]</div>

I was really glad to see these posts! I have noticed this also and found it quite alarming. It feels as if you are loosing control - sort of like the car is jumping forward. My daughter has noticed the same thing on her Prius also. Now at least we know what is causing it and how to compensate.

Thanks to all for sharing this info --

Mary Lou

 

Yup, common problem. Touched upon briefly in the discussion under
http://techno-fandom.org/~hobbit/cars/bp/ and mentioned in many
previous posts. Like the good Doctor, I've learned to try to back
off braking over obvious upcoming bumps and reapply afterward, but
sometimes you don't have much choice when you just need to stop the
car. Anticipatory driving and large following distance is your best
friend here. As far as recovery -- I still haven't figured that out.
It seems to be time based, or possibly resets the next time you
accelerate -- not that you've generally got any leisure to press the
accelerator while coming to a stop! Your best bet is to just whack
the shifter down into "B" which may help recover a little more regen
level, but the system still stays mostly on friction brakes and
you've pretty much just lost it for the duration of that stop.
.
I believe the whole scenario is caused by high regen current peaks
over 100A caused by torque blips over bumpy ground -- for its own
safety, the system just gives up at that point since it can't
anticipate that such high pulses have any likelihood of *subsiding*
again in the near future. Better that than pop the 120A fuse in
the battery-pack service plug, right?
.
_H*

 

This continues to happen to me, and it happened to my girlfriend for the first time a few days ago. She was very scared by it, and I'm now even more motivated to find a solution. I live in DC, where people drive fast, there are lots of hills and poorly-maintained roads, which combine to cause loss of traction at least once every few days. I've moderated my braking when I know a bump is coming, but that can be dangerous when you really do need to stop quickly. I would rather be as sure as I have been in every other car I've ever driven that a small bump won't cause a lapse in braking.

I'm running the stock tires. Is there a good chance I'd see improvement if I switched to better ones? Anything else I should try?

Thanks

 

I agree with Evan, that the momentary loss in braking when going over a bump is due to a corresponding momentary reduction in regeneration. However, could it possibly be something as simple as your foot changing pressure on the brake pedal as you go over the bump? Kind of hard to test, but small changes in the position of the brake pedal will change the amount of regen.

 

Do not know about the 2004/2005 VCS but I see it happening on my 2006 where the yellow sign with a “skidding car†briefly blinks when I run over uneven bumps.

 

<div class='quotetop'>QUOTE(tbullock @ Oct 22 2006, 09:46 AM) [snapback]336405[/snapback]</div>

I think you would notice a large improvement. I haven't had the electronics intercede yet now that I have new Goodyear TripleTreds, but before the switch I could trigger them easily - many times a day if I drove aggressively.

As a temporary fix, I'd try running at the mfg. recommended tire pressures rather than increased tire pressures if you are running higher tire pressures. A more compliant tire sidewall might help the tire maintain better contact with an uneven road surface by damping small movements before the are transmitted to the wheel and suspension.

 

One sure way to test it would be to find a nice, empty (let me stress that again, empty) road with a bump on it that you know during normal, slow braking conditions causes this problem... Then (while the road is still empty), go over it and slam on the brakes - my bet is that it's the regen thats momentarily backing off, and that since slamming on the brakes activates the physical brakes you won't notice anything more than the bump itself.