:mad: VSC got me in an accident today

Here is the technical explanation:

During regenerative braking, braking force is generated by MG2 and applied to the front wheels through a normal differential (transaxle). Since the differential is not limited slip, loss of traction with either front tire results in a loss of all braking force. In this mode, the Prius has no means to control the braking force at individual wheels. When a loss of traction is detected by the traction control system, the Prius immediately stops regenerative braking and engages all four friction brakes. There is no delay testing for a return of traction: the transition is immediate and friction braking continues until the driver stops braking. The transition time is the sum of the time required to detect the loss of traction plus the time to pull in the friction brakes. It is a very short but finite amount of time, which feels longer relative to the nearness of adjacent traffic.

Tom

 

The memory in the data recorders is not infinite. The info from the collision was probably overwritten within 5 minutes of leaving the scene presuming you drove away. I'm sure we all drive distracted at one time or another so my only advice is "stop eating in the car".

 

I have experienced several episodes where the car seemed to accelerate when I was applying the brakes. In all cases, I was using light to moderate brake pressure. I drive rather slowly and don't usually have a need to use heavy or emergency braking. In the instances that I recall these episodes, slight pavement irregularities caused the sensation; not pronounced washboard surfaces or major potholes, just a slight rise or dip in the pavement.

For example, a couple of days ago, I was slowing for a turn onto a perpendicular street. As I gently applied the brakes I could see a slight dip in the pavement - no more than 2", maybe lasting for 18"). I was traveling at 25 MPH. While steadily holding the brake pedal down, I felt a sudden surge of acceleration, lasting less than 1/2 second. This caused me to apply slightly more braking power - without an immediate response.

I find this rather disconcerting. I imagine that is the same sort of reaction the OP experienced; however, in an emergency situation, it would be even more disconcerting.

 

Just FYI for those reading this thread:

Often, a common side effect of ABS is actually longer stopping distances. The #1 goal of ABS is to keep the wheels from locking up so you remain in control of the vehicle and can steer. With the wheels locked up, you are only going to go into a straight line.

Whether or not ABS actually improves stopping distances or makes them worse depends on on road/traction conditions.

One common situation where braking distances will be increased with ABS is on gravel and snow.

 

That's all it takes. You don't need a major pothole to lose traction, especially with the crappy OEM tires that came with the Gen II. Hopefully the Gen III tires are better.

Tom

 

Hi All,

Lets not forget the "Law of Prius Braking Relativity" - The switch from electric to friction brakes is a momentary accelleration, the car is still slowing down with the friction brakes after the switch, but not nearly as quickly. The friction brakes are the same brakes all the other cars have, and slow the car down just like other cars too.

Somebody needs to do a little You-Tube Einstein skit to explain this!

 

You (and you're far from alone) simple don't understand how ABS brakes work.

The bottom line is that your braking distance would have be LONGER (and you would have lost your ability to steer!) sans the ABS brakes.

Do some research, and try to understand just what the ABS braking system does.

... Brad

 

You couldn't be any more wrong with your last statement. Yes, in theory one could beat the ABS brakes, but not in practice.

You are, however, very correct in terms of the ABS's real gem ..... steering while applying the brakes at 100%.

... Brad

 

Surfaces that allow material to plow up in front of the tires will stop faster without ABS. Harder surfaces more closely follow the ideal rules of static and dynamic friction, in which case ABS stops more quickly.

Tom

 

A minor quibble: acceleration is greatest during braking. During the short transition between regenerative braking and friction braking the acceleration approaches zero. The problem here is that the term acceleration is commonly used only for forward acceleration, while from a physics standpoint it is any change in velocity, including accelerating around a corner while maintaining constant speed.

Your statement is correct. I am just pointing out a small technical detail in terminology.

Tom

 

Yes yes yes... i know exactly what you mean. There's a bump right before a store I go into all the time and traffic is really fast there so you have to kinda roll in to the store parking lot fast or you'll get r-ended. First time or two I hit that bump while rolling in and as you say all braking goes away for a second. Man its terrifying if you don't know about it the first time it happens at speed and you expect a fast pull down and instead....you got nothing!!!!
I'm not voicing any opinion on exactly what your situation was as all the posters here have excellent points but I do know what you mean.

 

on loose gravel?

sofar i understand is
the wheels then block and you glide over the loose gravel.
with abs the car will prevent blocking and give you still some grip for turning the wheel to prevent hitting something.

abs is not for stopping a car faster its for keeping some controll over the car and be able to still turn the car away form a collision

 

Agreed. The transition to friction braking is very fast. It feels scary, but it doesn't take anywhere near two seconds. In your situation the problem was almost certainly the road surface combined with tires and suspension, which combined to produce a lack of braking. It happens. I've had it happen in my old Jeep, which has the most primitive and un-computerized brakes that you can imagine. Once tires start bouncing and sliding, you don't get much friction.

Of course we can't completely discount the possibility of a bug or design error in the new Prius braking system, but it is unlikely given the safety and fail-safe nature of this system. Generally the simple answer is the correct answer, which in this case is the road surface. If it is a design error, only additional incidents will prove it.

Tom

 

Absolutely you will stop faster on a gravel road, if you can lock up the brakes. At 50 km/h on the gravel road to the hobby farm, I could stop up to 2 car lengths sooner

The vehicle can "dig in" a bit to stop faster. With ABS, the loose stones on top act like ball bearings, and the ABS pulsing dramatically extends the stopping distance

In normal real world driving conditions, I much prefer having modern equipment like ABS and VSC

 

ok

but is abs not made just to have some steering controll when braking and not for stopping distance.
in a car without abs and locking of the wheels during hard braking will give you no control over steering direction with abs it will and thats what its for.

 

Yes, the primary purpose of ABS is to allow you to remain in control while braking, not necessarily to shorten stopping distances

Without ABS, as highway speeds if you have to suddenly stop, most likely the car will go sideways, possibly spin out