Braking problems?

The one and only time I encountered a similar problem with the brakes not working as they should was when I first got my Prius. What happened was the floor mat wasn't hooked down in place and had slide up under the pedal.

I hope it's nothing more serious with your car, but to be safe you should get it checked out.

 

For liability reasons, I would have expected the repair shop to simply replace very brake component. I also would think Toyota (who should be paying for all of this, including your damage) would want those parts for examination. I presume you took it to a dealer? What all did they do to the brake system? If nothing, man, I hope their liability insurance is paid up...

 

If there was a failure, it wasn't the computer. The brakes aren't totally controlled by computer, for emergencies the friction brakes are hydraulically controled, just like any other car. Even if the computers are totally fried, the hydraullic system can operate normally. If the hydraullic brakes failed, that should be easier to troubleshoot since it's a mechnanical system.

 

The emergency backup mode of the brakes is not "normal", but will stopthe car in a much greater distance than "normal" from 40 mph. I don't remember the exact distance, but it's a feature required by US cars, as I understand. The description of the accident sounds as though that emergency system might have been working but simply couldn't slow down quickly enough to avoid the accident. It comes into play only with the brake pedal fully depressed, to the floor. In the driver's panic, he might not have detected the emergency backup mode brakeing action.

 

Sorry to hear about your collision.

I'm going to echo what heliotropehead said - floor mats are the most likely cause. They are surprisingly dangerous - 2 inches of forward slip will almost completely prevent the brake from engaging.

 

sorry about your collision.

the carpet hazard has happened to me too many times, even in my other car. floor mats can hinder both braking and accelerating, especially when you need them the most.

 

If there was ANY problem with the brake system, one of the 3 brake malfunction warning lights would have come on.

 

The safety backup system is similar to what cars had before power brakes. My first car was a 66 mustang with no assisted braking and it would still stop on a dime (from 30mph) if you stomped the brakes. It is doubtful that this kind of system would fail. My vote is towards the floormat or something similar.

 

Yes, but if you're about to impact with the car in front of you, who has time to look?

And there's a way to make the brakes "hiccup" (for lack of a better term)... if the road is uneven, lots of bumps, if you panic stop the traction control will engage and it'll feel like you're "sliding forward" even tho you're hitting the brakes. The few times it's happened to me unintentionally (and it's reproducable on the same stretch of road) have really freaked me out.

 

Uh, that shouldn't be. Traction control is for acceleration, when you are NOT using your brakes. ABS is for braking. Think of TC as reverse ABS. Now ABS will let a wheel go if it feels it is locked (not moving or significantly slower than other wheels), but ABS does not let ALL wheels lose.
You can tell when ABS is working by seeing an icon on the instrument panel showing a car with squiggly lines under it. If you also hear a chime/buzzer, then VSC is actively working, which controls wheel braking and steering assist to prevent excessive yaw (doughnuts).

Now ABS can appear to feel like it is "hiccuping" as it tries to prevent you from skidding, but it still applies a brake force probably better than you could.

 

My bad, I'm seeing the light you describe when it happens.. and our other car (with both ABS and traction control) doesn't do the same hiccup on the same bit of road. I can probably do this more than half the time that I hit that stretch.

The stretch of road itself is odd, it's not horrible, but it has like undulations in it, and they're very close together. The one that your right front hits is completely different from the left front. And this hasn't happened in any of the other cars I've driven on that stretch (Grand Am, Jetta)... just the Prius.

 

Something similar happened to me. I was traveling on a road in which work was in progress. It was at night so I couldn't see the road that well. When I applied the brakes, my car initially started to stop and then it felt like the brakes failed and I traveled about six feet and then suddenly the engaged. I assumed that something in the road-possibly gravel- caused a loss of traction causing the so the abs brakes disengaged until I cleared the area. Then they engaged like a panic stop. That was the only time I've had any trouble. It really scared me at the time. It happed so fast I didn't notice any of the lights.

 

i had that problem but in reverse and it was the floor mat also. i am a bit concerned that this has happened to so many people. for me i punched the gas for some reason and the accelerator was stuck in the down position under the corner of the mat which had slid forward. fortunately i had plenty of room so didnt even have a close call, but it was scary for a sec until i realized what it was.

i probably should look for a way to fasten the mat down so it wont move

 

Would the data recorder help in this case? Just a thought.

 

ABS is designed to hold a 10% slip with the surface. That is, if a vehicle is moving at 50 mph and ABS is activated, the instantaneous wheel speed will be around 45 mph. Still rolling and generating max braking force.

If the road surface friction component suddenly falls drastically (say from mu of 0.6 to 0.3 or lower), the wheels will nearly lock up, even tho ABS is responding as quickly as possible. ABS must then allow the wheels to reach the wheel speed at which lock up ocurred, then reapply the brakes and iteratively figure out what braking force is needed to re-establish a 10% slip with the new surface. It's times like that when ABS earns its stripes (or driver generates his). :roll:

Then suddenly the coefficient of friction returns to normal (after the loose gravel patch), and the ABS has to go through the process again by increasing braking force to remain at 10% slip 'tween wheel and road.

Your feeling of traveling 6 feet w/no braking seems to confirm a loose patch of something that was so brief ABS didn't have sufficient time to solve the problem before transitioning to the original surface. Stuff like that happens with patchy ice on runways whilst braking after landing.

 

I think the problem may be that the ABS computer is engaging when it doesn't need to.. I don't understand why it does the "ABS Slide" on my stretch of undulating pavement, when our other car (and previous cars I've owned) have no issues with stopping there.

 

great explanation Denny

 

GM had to recall a huge number of pickups and SUV's to recalibrate the ABS. The ABS algorithm was too sensitive and the diminished braking caused a few crashes.

 

Undulating road can cause reduced level of contact between the road and tire. If the brakes are applied and a wheel loses traction, due to the vertical oscillations, the ABS sees it as a wheel lockup and will release brake pressure to regain proper braking. If 2 wheels are perceived as locking up, all braking "should" be released to regain the proper slip ratio (based on supposition; familiarity with aircraft ABS). In general, all four wheels are averaged to determine road speed. If one wheel starts to lockup, it will be handled separately. If any 2 wheels are seen to be locking up, than all brakes will be released to allow spin-up in the attempt to determine brake pressure needed to prevent lockup.


So, if two wheels had degraded traction over a bump, whilst braking, one could expect sudden release whilst the system sorts things out. If the road conditions are such that the process is repeated randomly, the braking would, indeed, feeling very squirrely!

The Prius' suspension dynamics or ABS effectiveness(as in more advanced speed of ooperation), or both, "may" have been enough to cause the problem. Solution. Slow down when shooting the rapids? :wink: