My Prius jumps off the road!

i would'nt consider this fun, my car does this when i drive over a metal sewer cover, or pot holes.. this effect is cause by the traction control being activated.

 

Actually, if you are braking, its not the traction control that causes the "jumping" but it is when the regenerative braking "gives up" or releases so that the ABS can be activated. The ABS can only work with the "regular" brakes, so the regenerative braking cannot be used at the same time as the ABS.

 

<div class='quotetop'>QUOTE(cheminee @ Nov 12 2007, 01:59 AM) [snapback]538251[/snapback]</div>

Normal braking on the Prius is done with only the front wheels. This is because the braking is regenerative, in that the energy is converted into electricity for the battery. If the front tires lose traction, then the control system reverts to hydraulic braking on all four wheels. Hydraulic braking is also engaged when the speed is too low for regenerative actions (about 8 mph), in a panic stop, or any time the battery can no longer accept charge.

Tom

 

<div class='quotetop'>QUOTE(qbee42 @ Nov 12 2007, 11:57 AM) [snapback]538442[/snapback]</div>

Tom, I was actually wondering about this exact idea a few days ago. A question for the Prius gurus around here...

To my knowledge, in many modern cars, 'regular' braking is done by all 4 wheels (with varying degrees) to be more stable as the car decelerates. My understanding of regenerative braking on the Prius is that the friction brakes are not used at all and the electric motor is what slows the car under normal braking.... which means only the front wheels provide braking action.

If this is truly the case, shouldn't the car theoretically tend to "nosedive" more than a regular car using 4-wheel braking? I have not noticed this, but based on my (admittedly limited) understanding of physics, I would think this should be the case...? Or is there actually some braking being performed by the rear drums during regenerative braking?

Hmmm... :huh:

 

<div class='quotetop'>QUOTE(cheminee @ Nov 12 2007, 12:59 AM) [snapback]538251[/snapback]</div>

The Honda Civic Hybrid that I used to have did a similar thing. If you were stopping and went over a bump or something that made one of the front wheels leave the ground for an instant, the regenerative braking cut off and the car seemed to momentarily surge forward until the driver pressed the brake peddle harder. The first time or two caused some concern, after that it was just a normal thing.

 

<div class='quotetop'>QUOTE(Sarge @ Nov 12 2007, 03:39 PM) [snapback]538534[/snapback]</div>

It is true that only the front wheels are used during regenerative braking. Physics shows that a car braking with only the front wheels will show no more tendency to nose dive than one braking with all four, assuming all other factors are equal. What will be different is the absolute limit of braking performance. If the forces on the wheels were all equal, half the number of wheels would equal half the braking force. Of course the forces are not equal, and the front wheels contribute more to forward braking that the rear, but still the absolute braking limit is reduced by using only the front wheels. There is also a limit to how much electrical power the Prius can make and use during braking. This is why the control system switches to friction brakes in the event of a panic stop.

Tom

 

<div class='quotetop'>QUOTE(cheminee @ Nov 11 2007, 08:05 PM) [snapback]538195[/snapback]</div>

I've noticed the same thing, usually in predictable circumstances. I try not to get too hung up on the specifics, but I know that the car is just doing it's thing. Being different is fun!!

 

The acceleration sensation is relative. When the rear brakes engage, the angle at which the car decelerates changes. "jumps off the road"? Please.

 

Never seen the problem you are mentioning, but there is a video of a red Prius jumping off the road and committing tire suicide on a curb during the Darpa Urban Challenge. Something confused the robotic drive system, and the car jumped off the road, and over a curb. Don't let the robot drive! Bad robot!

 

this has happened to every car i've ever owned, it seems that the dampening system (aka shocks) is trying to catch up to the springs and causes this effect on bumpy roads. even tho the shocks/struts are the same unit, they respond at different rates

 

I agree with Tom,
The nosedive when a car brakes is caused by the slowing forces being applied below the center of gravity, not if they come from the front or back. And in all cars using 4 wheel braking, about 2/3 of the braking is applied to the front wheels because they have more weight on them with the nosedive. So if regenerative braking uses only the front wheels, you still have about 2/3 of the normal 4 wheel braking to work with before you need to fall back to friction brakes and ABS.

 

<div class='quotetop'>QUOTE(qbee42 @ Nov 12 2007, 03:57 PM) [snapback]538561[/snapback]</div>

Hmmm, that doesn't seem quite right. Isn't there a rotational torque between the the location of the brakes and the center of mass of the car? A more extreme example being to pull the front brake on a bike and watch yourself rotate about the front axel and land on you head?

Rob

 

<div class='quotetop'>QUOTE(Rob Smith @ Nov 15 2007, 04:37 PM) [snapback]540199[/snapback]</div>

The torque comes from the distance between the braking force and the center of gravity, which in the case of a car or bicycle is the distance from the ground to the center of gravity. The distance is the same for both the front and rear, so there is no greater rotational moment for front braking as opposed to back braking. There is a braking factor that differs between the front and rear, and this factor explains your example of a bicycle flipping over its front wheel: The front brakes are able to develop more braking force than the back, given equal weight distribution. This is because the rotational moment developed from braking causes weight to apparently transfer from the back axle to the front, thereby increasing the amount of available frictional force. In the case of back-only braking, a limiting case occurs when the apparent weight remaining on the back wheel becomes too low to add any additional braking force, at which point the tire skids. Think of it this way: if you are braking with only the back wheel on your bicycle and try to flip, as soon as the back tire begins to lift it can no longer brake. You need more braking force to flip over, but adding braking force just tries to lift the tire and that reduces the braking force. The tire ends up skidding. This doesn't happen with the front tire, where an increase of braking force transfers more weight onto the front wheel and increases the available braking friction.

Tom

 

Thanks for the explanations, Tom. B)

 

mmm. I LOVE physics.

But back on topic: I can make my car roll over and play dead.

 

<div class='quotetop'>QUOTE(darelldd @ Nov 16 2007, 01:12 AM) [snapback]540429[/snapback]</div>

I'm not sure I want to know how but I believe you. Will it sit up pretty on it's hind quarters for a treat, too? I agree the car does "jump" more than any other car/truck I've ever driven, this being my 10th vehicle. I attributed part of that to it's lighter weight than most of my other vehicles.

 

<div class='quotetop'>QUOTE(darelldd @ Nov 16 2007, 01:12 AM) [snapback]540429[/snapback]</div>

can not