transmission B mode WHY?

The main reason would be if you were going down a long decline and your HV battery was fully charged, switched out of regen and switched to the friction brakes. The B mode uses the engine as a air pump to help brake the car without having to rely fully on the friction brakes.

 

if you descend long slopes (more than 600 foot drop) you can fill the battery and lose the regenerative braking, this would leave you with only friction braking.

Shifting to B adds engine braking to take some load off the disc brakes. You don't want to overheat your brakes.

 

Using "B" can also reduce 'heat pumping' the traction battery by reducing the charge rate. The battery will still take a charge but testing with my NHW11 shows the traction battery does not heat up as much as descending in "D" and using the brake pedal.

Bob Wilson

 

When you don't want to die if you go down a long steep mountain grade. Note: No Interstate is steep.

A good rule of thumb for braking is that you use your brakes to change speed and your engine braking to maintain speed. Brakes are for short term use.

 

You won't die. The brakes will work just as effective because one the battery is full, the car will brake with the gas engine. The only difference is as Bob pointed out, it will charge the HV battery slower and you'll prolong its life.

 

Yes, some Interstate highway is steep. I-70 through Colorado goes over some serious mountain passes. There are numerous runaway truck ramps located off the shoulder. Even with automobiles, if you ride the brakes down the long descents, you may well overheat your brakes.

 

http://www.oregon.gov/ODOT/MCT/docs/EmigrantHill.pdf?ga=t

 

Fat lot of good the B setting does me here in Florida!...can I get a partial refund if I give back the setting?

I understand Sugarloaf mountain (the highest in Central Florida) has an elevation of....wait for it....300'!

 

Thank you for making my point. That stretch of Interstate may be wet your pants steep for a flat-lander or a heavily loaded truck in poor condition, but at 6% it's not a steep mountain road. BTW, 6% is the legal max limit for Interstate Highways.

Start talking about 15-20% mountain grades that are a mile or more long and then you will be talking about steep mountain grades.

 

I've driven it several times, identify one significant stretch of that highway over 6%. The fact that an incompetent car driver or a heavily loaded truck in poor condition has problems doesn't make it steep. You are in the beautiful state of Colorado, I'm sure you can name some mountain grades that are actually steep and that none of them are Interstates.

 

I-70 Milepost 180 to 189 7% grade up to summit of Vail pass.

I-70 Milepost 205 to 216 7.5% grade west side approach Eisenhower Tunnel.

I-70 Milepost 219 to 226 7.5% grade east side approach Eisenhower Tunnel.

That's around 26 miles of 7%+ grades in a 46 mile stretch of highway.
That's a lot of miles working your brakes.
And you better be watching you mirrors for the runaway trucks so you can get out of the way.

If you don't respect those kind of grades for that many miles that's fine.
I've driven them all my life, and I certainly do respect them.
To each their own.

 

a] this is the biggest lie i've ever heard since the moon landing

b] did it ever occur to anyone that not everyone drives on an interstate? so even if this was true, there are still deep declines elsewhere.

 

Looks like you found some exceptions to the Interstate Highway design requirement of a maximum of a 6% grade. Thank you for that information. I do consider grades like those as grades that shouldn't be descended by loaded trucks in poor condition or incompetent car drivers, but not particularly steep grades.

Since you are a resident of Colorado, I find it difficult to believe that you consider those to be seriously steep mountain grades, unless you always stick to the Interstates. Which should be a crime in a state with back roads like Colorado:cheer2:

You are making the case for my statement that B is essential for descending steep grades, unlike one technically lacking poster here that doesn't seem to have experience with serious grades.

Sometime you should try a real grade like east bound from Sonora Pass in California. A several thousand foot descent from 9600 feet with grades from 26% down to a wimpy 7%. And it's twisty enough that you can't build up enough speed to get maximum engine braking on many auto trans cars. I have also driven less well known roads with smaller drops that were worse. I expect you could find something at least as inmoressive in Colorado.

 

a] Since the moon landing wasn't a lie, I will take that as a complement. Beuregard opened my eyes to the fact that there is one Interstate that exceeds interstate standards and has a grade of up to 7.5%. Based on what you said in b], you don't consider that particularly steep do you?

b] You are making my point, Interstates are no way to judge what is a steep grade.

 

a] haha yeah i know i was just trying to be hilarious

b] wasn't trying to prove you wrong. just wanted to point out that ive had my own fair share of steep roads. I grew up in colorado as well, and when we would go skiing, there were plenty of long roads to drive up and down on.

 

a] You did a good job of getting me to fall for it

 

There's DOT spec for Interstate, and then there's the Colorado exception.
I do still consider the 7% grades steep because it forces you to respect it. A 7% grade drops 7' in 100' of roadway. At 60 mph you're traveling 89' per second. You do the math for over 26 miles and you see how many feet you drop off, and how fast you're dropping.

But on I70 it's not just the grade, It's the snow and ice combined with constant heavy traffic. Sure, it's a safe road, over 10k cars a day get through it without crashing, but you have to respect it.
And then you have falling rocks.

Yes I know the roads you speak of in the Sierra's. That is truly what I'd call a steep road. And yes, you do indeed want to get off the interstate in Colorado if you want to see the back country.

I've only used the "B" gear position once, while in the mountains of Wyoming. It caused the cruise control to do weird things....

 

If you are dead, who will you argue with that it was not THAT steep?

"Emigrant Hill, from mileposts 219 to 228, has historically experienced a plague of crashes. In the five-year period from 2003 through 2007, the area had 51 truck crashes involving a fatality, injury, or disabling damage to any vehicle, including 31 in which a truck was at-fault. The crashes resulted in 21 injuries and 2 deaths."

Motor Carrier Transportation Downhill Speed Information System

Back to the Prius and B mode, you will be on the brakes continuously for about 8 miles and you will lose over 2,000 feet elevation. Taken in D, you will lose regenerative braking less than 1/3 of the way down, in B you will still fill the battery it just takes longer, and when you do, the engine braking is already engaged.

 

For most of us it appears to be a waste of time. I have 8300 miles on my Prius and have fully charged the battery (at least the top bar was full-but it was still charging I think) 6 times only.