The "B" thing

I'm with Efusco on this one, except to say I think Hobbit's discourse on "B" mode should be mandatory reading.

The only really practical reason to use "B" is to save your friction brakes. The computer in your Prius will protect the battery from over-charge, and, if necessary, it will effectively put the car into "B" to "waste" some energy.

At the same time, if you do live in a mountainous region where you face long, steep decents, this could be just the thing for you. You live in Nice? There are some steep hills there, but not quite long enough to make "B" worth it. Still, I'd love to see pictures of a Prius in Nice.

 

As stated earlier, search out the other threads.

But a quick answer is, while descending long hills (mountain passes for example), due to the Prius' low running friction, the car will "take off" down long or steep hills. Putting the car in "B" means you don't have to rest your food on the brake pedal. Doing this doesn't necessarily drag the friction brakes as the car will use regen. if you're gentle. It -is- easier however to use "B" instead of holding your foot gently over the brake pedal for 10-20 min. If you are descending the mountain and slow down too much in one section of road, depressing the accelerator pedal results in the engine -decreasing- RPM. Very odd, very cool.

 

I use the "B" mode when approaching a stop sign/light on ice/snow, only when I get below 20-25 mph. Works great to slow car down without anti lock brakes coming on.

 

I didn't say that I don't use ABS, I said it doesn't need to come on as often because I have slowed the car down without braking. It's a better controlled slow-down than braking on slick surfaces.

 

The ABS still works when you are in "B" mode.

 

B mode is certainly not a better controlled slow-down than normal braking. Look at the physics behind slowing down your Prius; there are three ways to do this:
1) Use B mode which provides drag through the front wheels.
2) Use regenerative braking which provides drag through the front wheels.
3) Use friction braking which provides drag through the front and rear wheels.

Note that 1 & 2 are identical in terms of drag interacting with the road surface; both apply a resisting force to only the front wheels. Slowing down with B mode has all of the bad weather risks of regenerative braking without the advantage of converting braking energy to electricity for later use. You gain nothing in safety or control while throwing away energy. There is no point in doing it.

Method 3 has the advantage of using all four tires for braking, plus the ABS to help maintain control in slippery conditions. This is why the Prius automatically switches to friction brakes when the front wheels slip or in a panic stop situation. On the down side, all braking energy is wasted.

The best, most controlled, most efficient method of braking in a Prius is to simply press on the brake pedal. The braking system will use regenerative braking as much as possible, but will automatically use the friction brakes when necessary. No fuss, no bother, no shifting gears, no thinking; it can't get much easier than this.

As pointed out on this forum many times, the only good use for B mode is on very long descents to delay the onset of friction braking.

Tom

 

I'm not sure which side of this argument I fall on.

On the one hand, the fact that your ABS has to engage in the first place means you have done something that has put your car out of control and the computer is compensating for that. ABS is reactive, and it only reacts if the driver does something stupid (like using the brakes in snow/ice). Pre-ABS it was common knowledge that hitting the brakes on ice was the last thing you wanted to do, and allowing the transmission to slow you as much as possible was the (vastly) safer option. Transmission-induced deceleration is indeed controlled, and in the most technical terms I would even argue that it is more controlled than ABS controlled deceleration by definition.

However, ABS systems are so advanced nowadays that for all practical purposes if you mash the brakes and the ABS can't stop you, you probably wouldn't have stopped no matter what you did - straight line or otherwise. I would have to be shown actual stopping distance test data of transmission deceleration vs. ABS on a sheet of ice to be absolutely convinced of ABS's superiority though.

Don't get me wrong - I've spent a lot of time on the skid pan and I understand the clear superiority of ABS in almost any conceivable braking situation, but I also happen to be a fan of the idea that if you have to engage the ABS in the first place you are perhaps driving beyond what you should be for the conditions. You should be capable of decelerating to a near-stop using only "B" mode - which, incidentally, I don't see as being any more or any less safe than engaging the ABS as a means of deceleration in snow/ice. If the tires are able to gain enough traction to slow you with ABS then it is a certainty that they will have traction to slow you down with (far less aggressive) transmission deceleration.

 

I use B for the last 200 metres of a 1.5 kilometre 11% downhill. The battery is at full capacity by then and rather than use excess electricity to spin the engine I use the mechanical drive system.

 

Friction is friction. Regardless of whether it is generated by brakes, the transmission, or putting cards in your spokes, the force of friction slowing an automobile is between the tires and the road. The limiting amount of friction is determined by the tires, the road surface, and the number of tires involved in braking. Braking with all four tires instead of two will always produce better braking except in the special cases of near-zero friction on the road surface (wet ice for example) or when the braking forces are able to cause the rear wheels to lift off of the ground (a bicycle for example). Other than these two special cases it is always possible to stop a car faster with four wheels than two. Note that we are assuming that we have working brakes or a transmission that is able to exert the necessary forces needed to brake the wheels. Brake fade is another issue that favors transmission braking over extended periods of braking, but that is not the issue here. In a two-wheel drive car, transmission braking applies the braking force to the road through only two wheels. Friction brakes apply the braking force through all four wheels. Transmission braking with two wheels is never able to exert as much braking force as friction braking with four, assuming we are not dealing with fade or failures.

Another factor in braking is static verses dynamic friction. Static friction is always greater than dynamic friction, except for a controlled slip situation which holds the friction in a transition zone near that of static friction, or in the case of gooey tires that work more like glue than rubber. Tires exert static friction on the road while they are still rolling, not skidding. Once a tire skids, the frictional forces come from dynamic friction, which is lower than that of static. This is why a skidding stop with locked wheels takes longer under normal circumstances than a stop where the brakes are applied just short of skidding. ABS is designed to prevent this locked wheel skid mode; transmission braking does not (except in the case of the Prius which uses the traction control system during braking, and switches to ABS friction brakes if needed).

Looking at these factors as they apply to friction braking verses transmission braking, we see the following:

1) Under all but a few very special circumstances (such as loose gravel), a car will stop in the shortest possible distance if all four wheels are braked just short of the point where they start to skid.

2) Transmission braking, if perfectly applied, can supply optimal braking on only two wheels, and no braking on the others. Furthermore, the transmission supplies no automatic method of getting and holding friction at the optimal level. It is highly unlikely that any driver could hold transmission braking at an optimal level for any length of time, but even if done, the braking force will still fall short of optimal braking on all four wheels. Transmission braking also runs the risk of causing the wheels to skid, inducing a loss of control.

3) ABS works with computer sensors and speed to come close to optimal braking on all four wheels. This happens automatically even for the most unskilled driver, simply by pressing the brake pedal. ABS braking with the friction brakes will always stop the vehicle faster than transmission braking, assuming we are not dealing with fade or system failure. Furthermore, ABS maintains steering control in all but near-zero friction situations. In near-zero friction situations your car becomes ballistic, regardless of the braking system.

Tom

 

I disagree that transmission braking is controlled. This is why late model high powered motorcycles have torque limiting clutches. When downshifting the torque of a big high compression engine can often cause the rear wheel to lock up and skid.
A Prius won't produce those kinds of loads with transmission braking but it does produce a deceleration force that on a slippery surface such as ice may exceed the limit of tyre adhesion. The safest way to stop a Prius is to use the brake pedal. This is the only way you will get the advantage of ABS working for you. The ABS can not control a wheel skid due to transmission braking. It also makes sense that to brake through 4 tyres on a slippery surface has to be better than to brake through 2 tyres out of 4. When braking on ice weight transfer is not significant so the rear wheels have a lot to add to the braking event. After doing slippery surface braking technique training I see no place for transmission braking on slippery surfaces.
Having the ABS engage does not mean a driver has lost control, it can be advantageous to brake hard enough to engage ABS deliberately when you need to stop in a minimum of distance and maintain steering control.

 

Heh...

Didn't stop to consider that the Prius is 2WD. I live near Tahoe and always had a 4WD, which is what I learned to drive in the snow with. I suppose my logic breaks down significantly when you subtract two drive wheels from the equasion.

 

Heh...

Didn't stop to consider that the Prius is 2WD. I live near Tahoe and always had a 4WD, which is what I learned to drive in the snow with. I suppose my logic breaks down significantly when you subtract two drive wheels from the equation.

 

I posted a long reply earlier tonight, detailing the physics behind friction brakes and transmission braking. I see that it's now missing. Assuming Evan didn't pull it (it was kind), it must have been lost in the site slowdown I saw earlier tonight. I don't think I have the patience to recreate it right now. I thought about saving it locally before posting, but that's kind of a pain. Having the forum do things like this makes me not want to post. Bummer.

Tom

 

I posted a long reply earlier tonight, detailing the physics behind friction brakes and transmission braking. I see that it's now missing. Assuming Evan didn't pull it (it was kind), it must have been lost in the site slowdown I saw earlier tonight. I don't think I have the patience to recreate it right now. I thought about saving it locally before posting, but that's kind of a pain. Having the forum do things like this makes me not want to post. Bummer.

Tom

 

Well, there is my post now. I think we have a Twilight Zone thing going with the posting tonight. I'm going to call it quits and see if it smooths out tomorrow.

Tom