Going downhill in b mode, gets fast quickly! Is this usual?

Been there (not the exact place), but I was in front, towing a 34’ trailer, praying that the idiots brakes behind me would hold on long enough for my wife and I to get out of his way. We did, but he didn’t survive the rest of the grade. Neither did his family.

 

He wasn't trying to perform a miracle, he was taking a descent at a rate of gained energy that exceeded B mode's ability to sink it.

You make "decipher the kinetic energy equation" sound like some kind of weird alchemy.

If you teach somebody he could slow down until his rate of energy gain is within the car's capacity to sink it (because the gain is proportional to his speed, there's the whole "cipher"), then you are actually teaching him physics, which will allow him to suit his actions to the tools and conditions at hand, and get the results he wants. ("Teach a man to fish")

"Give a man a fish" would be if you got in his car and drove down the hill at a workable speed in B mode for him, but didn't explain how you did it.

Waving your hands and saying he didn't get the results he wanted because the car's incapable of it, he wanted miracles, he's an idiot and will wreck his car, is taking the fish off his plate and leaving hairballs there, all while mouthing words about the laws of physics.

 

@Doug McC I did say I could sympathize with your frustration...

...but I think he's got you on that one. The fundamental problem isn't that mountain driving is difficult, it's that we can't schedule everyone's first encounter with it at the same time, and preferably in the past. The n00bs need experience, and the rest of us have to be tolerant and vigilant.

 

And I'll confirm from having significant experience with each of the three scenarios listed here.

When I saw and smelled the same situation at the mandatory brake checkpoint on the descent from Pikes Peak, the offender was a Ford pickup with Florida plates. I don't believe anyplace in Florida needs a downshift.

Quite a while back, a new member here in Hawaii had to replace his overheated friction brakes twice before figuring out that 'advice' from a certain few misinformed flatter-land members here was wrong. Brake pedal regen in D mode was not sufficient on his steep slow 3000+ foot descent. Some hills require B mode to prevent overheated brakes. And even that may not be enough on certain select mountain grades.

 

The amount of regen available is a factor of the amount of empty battery capacity available.
A Prime should handle the mountains better. A BZ4X better yet.

 

B mode is not just a function of regenerative braking, per the OM:
■ About engine braking
When shift position B is selected, releasing the accelerator pedal will apply engine braking.
●When the vehicle is driven at high speeds, compared to ordinary gasoline- fueled vehicles, the engine braking deceleration is felt less than that of other vehicles.
●The vehicle can be accelerated even when shift position B is selected.

B Mode effectively (notice the use of the word effectively does not mean the “same as”) changes the gearing to a lower gear, much the same as Tow Mode does in many pickups.
My problem here is people making a rather simple process much more complex than it really is.
In all reality, who really cares how many KW are being generated while descending a steep grade? The simple answer to the OP was the one I gave in describing the safe method of descending such a grade.
Enough said from me.

 

I think he was.

 

I just reread the entire thread and I respectfully disagree. From his description, he actually did. His post was a cry for help.

 

Kilowatts being the SI unit for power, and the whole question being about "will the car slow down, hold speed, or speed up on this downgrade" being a simple matter of comparing one amount of power to another, the answer to "who cares" will be "anyone who'd like to understand why their car behaves three different ways in three different situations, and how to drive accordingly."

The fabulous thing about learning some physics is gaining the ability to understand things like that in real life, and use that knowledge to your benefit.

 

It is even better when you can use that knowledge to provide explanations in simple terms to benefit someone else.

 

If I ask you to carry a bag of roof shingles from here to there, and you find it tough to lift (I do, but then I'm a nerd), you can probably guess that if I show you two bags of shingles, you might prefer to take them one at a time. And if I show you a half-empty bag, you'll think "no sweat".

For this, you don't even need to have a number for the weight of the bag, and it doesn't matter whether you're used to those numbers in pounds, stones, or hundredweight. What matters is you know that:

• there is such a thing as weight
• more of it makes stuff harder to lift
• in fact (strict proportion) twice as much makes stuff twice as hard to lift, half as much makes stuff half as hard to lift

That's it right there for the concepts you need.

Now, when might you like to know an amount (in some familiar unit, pounds maybe)?

Well, if you've already got some idea (from the gym, maybe) what amount you're comfortable lifting, you can make some judgments about this bag of stuff before you walk up to it and heave.

Or, if you've just carried that bag and it felt like work, you can ask "say, what's one of those weigh, anyway?" and remember that, and use it to make judgments about other things later.

If you know you can make it down a certain grade at a certain speed in B mode and not pick up speed, and you come to another hill that's twice the grade, you'll have to drive half the speed (or use the brakes).

Or, if you come back to the same grade but you've loaded your Prius to the gills and made it 20% heavier, you'll have to take the hill that much more slowly (or use the brakes).

We spend our kids' grade school years making sure they will not grow up thinking fractions, multiples, and proportions are scary concepts they can't "decipher".

It does nobody any favors to come back when they are grown up and try to scare them away from the same concepts then.

 

Some of us live in mountainous regions. Brakes matter. I miss the low range in my old Russian jeep. Just have to be careful, go slow. Buy good brakes and rotors