The "B" thing

[mandl]

Does anyone use the "B" transmission setting to brake going down hills ?
I tried it , and it seemed kind of scary to have to shift thru "N"to "D" at
speed! - Also , if putting your foot on the brake causes " regenerative braking" slowing the car , and charging the battery at the same time, why
the need for the "B" ?- I don't get it.http://priuschat.com/forums/images/smilies/confused.gif

[efusco]

Originally Posted by mandl

Does anyone use the "B" transmission setting to brake going down hills ?
I tried it , and it seemed kind of scary to have to shift thru "N"to "D" at
speed! - Also , if putting your foot on the brake causes " regenerative braking" slowing the car , and charging the battery at the same time, why
the need for the "B" ?- I don't get it.http://priuschat.com/forums/images/smilies/confused.gif

Do a search for "B-mode", you'll find about 100 posts very similar to yours.
Or you can get more information than you can absorb in one sitting by reading Hobbit's excellent article on the subject:
http://techno-fandom.org/~hobbit/cars/b-mode.html
OR
Just don't bother with B-mode at all...there's very little reason to use it and in no situation does it really provide any benefit other than reducing how much pressure you need to apply to the brake pedal.

[a priori]

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.

[David Beale]

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.

[craigk]

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.

[efusco]

Originally Posted by craigk

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 do not support this recommendation. Antilock brakes and designed exactly for slowing down on wet/ice/snow without locking or inducing a spin. B-mode is a completely uncontrolled situation...while it will effectively slow you down it does not have the computer controls that ABS does.
Use the right tool for the right job...the job of ABS is to slow you down in slick situations without loss of control. B-mode is designed to control your speed by using engine braking on long steep hills.

[craigk]

Originally Posted by efusco

I do not support this recommendation. Antilock brakes and designed exactly for slowing down on wet/ice/snow without locking or inducing a spin. B-mode is a completely uncontrolled situation...while it will effectively slow you down it does not have the computer controls that ABS does.
Use the right tool for the right job...the job of ABS is to slow you down in slick situations without loss of control. B-mode is designed to control your speed by using engine braking on long steep hills.

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.

[efusco]

Originally Posted by craigk

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.

This is exactly what I don't agree with or support or suggest. My entire point is that B-mode is an UNCONTROLLED slow down and ABS is controlled. I feel that using the brakes and brakes alone is a safer and more controlled way to slow down. ABS coming on less is not a good thing. ABS is doing what it's supposed to do...increasing the driver's control over the vehicle while it slows.

It also offers the advantage that the ICE can shut off as it's supposed to if you stay in D. If you drop into B at any speed with the ICE running it will continue to run, if you drop into B above 20mph the ICE will start and stay on. Only if you're already in stealth AND going less than 20 will the ICE stay off when you go into B-mode.

I realize that John1701a has long supported using B-mode as you describe and I can't say that there have been any untoward consequences from doing so, but, IMO, it removes one element of safety and control and adds nothing. ABS alone or ABS + VSC for those equipt while still in D offers the best means of slowing and stopping on slick roads...or dry ones for that matter.

[craigk]

Originally Posted by efusco

My entire point is that B-mode is an UNCONTROLLED slow down and ABS is controlled...it removes one element of safety and control and adds nothing.

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

[qbee42]

Originally Posted by craigk

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.

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