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Discussion Starter #1
When you're driving down out of the mountains, say, the Prius regeneratively brakes, charging the battery. This keeps your brake pads from wear. At some point, on a very prolonged downhill, it would seem like your battery might become fully charged. Then what? Could this put you in a brake wear situation, or not?
 

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Discussion Starter #4
Possible long downhill scenarios

Thanks, Robert, for your many thoughtful posts. You and Graham give such clear technical depth to the discussions for those of us with scientific or engineering bents.

After reading Graham cover to cover, it does appear that the Prius does blend in engine braking, spinning the engine with no gas consumption. At least that is better than wearing your (in the Prius case, light duty) brakepads out. A little like a Jake Brake?

Anything that uses up electricity would help slow you down, too, I assume, such as A/C, radio, whatever that would absorb regenerated energy that exceeds the battery capacity. (Unless, of course, using A/C causes the ICE to fire up, that I don't know).

My main concern is that, since the Prius has reduced the brake system weight/duty due to regenerative braking, if you happen to do a lot of long upgrade, long downgrade driving, such as on the San Francisco to Tahoe run and back, you might wind up less efficient, and more importantly, need to keep an eye on brake wear.

Graham says that the regenerative braking is low efficiency. Any approximate quantification, Robert? Like about what per cent of the energy expended in accelerating is recaptured?
 

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Down-hill braking and fuel efficiency

I'm the guy that keeps talking about vacations in the Colorado Rockies.

On really long extended downgrades, when you use "B", the batteries do get fully charged, but the computer just quits charging them. The ICE is still turning, but no gas is being burned. It is really fun to drive 20-30 miles and get 100 mpg! Too bad all roads aren't all down-hill, we wouldn't need to buy a Prius (but I would anyway!).
 

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Down-hill braking and fuel efficiency

And, the ICE does NOT run louder, and the engine braking does NOT decrease because the batteries are fully charged.

I suppose using the AC would create a little extra drag on the ICE, at least on the classic Prius; because it is connected to engine. However, this probably would not apply to the 2004, because the AC compressor is run by an electric motor.

We drove down some really long down-grades, including Pike's Peak; and at no time did I feel that the brake pads were wearing excessively or even heating up. I really doubt if you could make the rear brakes fade (the fronts are disks), unless you are really inexperienced in mountain driving and rode the brakes continuously without using "B".

Do the 2004's have disks all around?
 

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I am not aware that the Prius hydraulic brake system is any lighter or less capable than other cars in it's size/weight class. Most any car will use the engine as a brake, but the connection is mechanical and direct rather than via a generator and motor. In fact, due to the larger engines and low gear ratios, ordinary cars actually put less strain on the brakes when they are in L/1 and maybe even in 2 than the Prius does in B.

In the 2001-2003 Prius, Toyota's claim is that you can reclaim up to 1/3 of the energy lost due to braking because of the regenerative brakes. I've heard it's higher in the 2004 due to the higher voltage levels, but I'm not sure of the value. In reality, however, for the 2001-2003 Prius, the regenerated energy is usually a lot less than 1/3 because regeneration cuts out if the ABS gets even the slightest bit nervous. So just about any slight bump or pothole kills regen. A scanner showing battery current (available from Graham for example) makes this pretty obvious. I've recently been releasing and reapplying the brakes several times on the hill leading away from my house because its so choppy.

As for brake fade, there's only been one possible time when this happened to Pikachu. I say possible because I wasn't in the car at the time. I drove up Mt. Washington in NH, but a friend drove down (while I took the cog train down). Pikachu had a pretty heavy load with 4 adults and a child. My friend pulled over for a while because he though the brakes might have been overheating. It was about 3 years ago, I forget if he tried B mode or not. I've not been back since. I've driven a lot of hills in PA, but none are anywhere near as long and steep as Mt. Washington. Pikachu has never been west of Ohio, so he's missed out on the whole Rockies experience.
 

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Fading Brakes

I have quite a bit of mountain driving experience, and I think that the "B" setting on the classic Prius is fine for nearly all normal highway driving. The only time that I thought it provided insufficient engine braking was coming down Pike's Peak, and that is really out of the ordinary.

If you use "B", keep off the brakes and use the throttle to keep your speed up (instead of coasting in "D" and using the brakes to slow down), you should be OK.

If you've ever overheated drum brakes to where they fade, it is really obvious. First of all, they smell really bad like they are burning (which is what's happening). Then, when you press on them, it's like they are wet. If you've ever driven with wet drum brakes, they are almost useless. No matter how hard you press, they barely slow the vehicle. On a steep downgrade, they won't even hold the vehicle at a constant speed. You'd best be looking for a level spot with no rocks or trees to run off the road.

And, don't use the runaway truck ramps! That gravel is really deep, and you'll be permanently stuck -- And, you might just get a semi up your back side. Truckers get really upset when they see cars parked in the truck ramps. That's a good way to get killed!

Disc brakes also overheat, but they are less prone to fading. They can get quite hot and still provide sufficient (though slightly diminished) braking. They fade, but they don't fail as quickly as drum brakes.

I have seen cars and trucks in the Rockies with brake drums and rotors that are glowing white hot from the heat. At extreme temperatures like this, the brake fluid can boil and vaporize, which makes the brakes spongy -- another thing to worry about. And, I have seen one instance where the brake lines failed and the brake fluid sprayed onto the hot drums and caught on fire.

Sorry, I have gone off on a tangent; but the Prius brakes are fine for normal driving and for the size car it is. Mountain driving just requires some planning and common sense that applies to any motor vehicle.
 

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I would expect that the Prius has normal brakes for a car it's size, nothing downsized about them. Basically, if you have to climb on the brakes and stop fast, the brakes need to be able to stop you without any regen.

The brakes worked just fine for me last week. I was doing about 65 in heavy traffic when the car in front of me (who was tailgating a pickup, who was tailgating an SUV, who was tailgating ann SUV,...) locked up his brakes. I hit the brakes hard, and although I don't think the ABS engaged, I was able to brake a little faser that everyone in front of me. (I wasn't watching the dash for that little light, but I don't remember the brakes pulsing.) The Prius handled as well as any car I've ever driven, nad better than most. I slowed faster than I had expected, and the car stayed in a straight line. Definitey not under-powered brakes.
 

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In fact, due to the larger engines and low gear ratios, ordinary cars actually put less strain on the brakes when they are in L/1 and maybe even in 2 than the Prius does in B.
You forget to take into account electric motor ("regenerative") braking, that does not exist on ordinary cars and is always active when you press the brake pedal (except when ABS kicks in). Pressing the brake pedal does not always mean engaging conventional brakes on the Prius.
 

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Re: Down-hill braking and fuel efficiency

Phoenix said:
And, the ICE does NOT run louder
Well, something is making an extraordinary noise when using "B" to slow my Prius. At some point (presumably when the battery declines further charging) something is definitely spinning up, which creates a rather loud and somewhat unsettling sound. IIRC, either frenchie or john1701a explained in a previous thread that this was the ICE. However, I am easily confused, so it is quite possible that I am mistaken. :)

Phoenix said:
Do the 2004's have disks all around?
No, the 2004 has disc front brakes and drums in the rear.

Drive happy,
Moo :)
 

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No, the 2004 has disc front brakes and drums in the rear.
Except in Europe, where four discs were standard already for the "classic" Prius. :wink:
Well, something is making an extraordinary noise when using "B" to slow my Prius.
Yes, the engine, and it's normal (the same kind of loise you would have using engine braking with a manual gearbox). But you probably notice it more than on an ordinary car because in "D" the engine is stopped when running downhill, and you can only hear road or wind noise.
 

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Discussion Starter #13
Jake brake option available in 2005?

Thanks to all for the interesting replies.

It's clear that when Toyota offers the true Jake Brake option, which will both help on long downhill runs, plus make that cool BRRRRRRRRRRRRR that drives neighbors crazy, that this point will be moot.

BRRRRRRRRR......
 

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Re: Jake brake option available in 2005?

hinalea said:
Thanks to all for the interesting replies.

It's clear that when Toyota offers the true Jake Brake option, which will both help on long downhill runs, plus make that cool BRRRRRRRRRRRRR that drives neighbors crazy, that this point will be moot.

BRRRRRRRRR......
LOL! I can hear it now, blasting the SUVs as you sail downhill. BRRRRRR!!!! Woohoo!

And, yes, I agree with Frenchie that the spinning ICE sounds strange only because the Prius is so remarkably quiet in most every other circumstance.

Drive happy,
Moo :)
 

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frenchie said:
Well, something is making an extraordinary noise when using "B" to slow my Prius.
Yes, the engine, and it's normal (the same kind of loise you would have using engine braking with a manual gearbox). But you probably notice it more than on an ordinary car because in "D" the engine is stopped when running downhill, and you can only hear road or wind noise.
You will hear sound the ICE kicking in when you continue to drive at downhill and the battery becomes full in "D" mode.

Regards,
Ken
 

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Re: Down-hill braking and fuel efficiency

:!: WARRANTY REPAIR :!:

I too heard this racing noise for months going down long hills, both steep and not so steep. Toyota didn't know what is was.

Two weeks ago, while driving on a flat highway, all the bells and whistles went off - literally! A high pitched tone stayed on constantly, and the "brake" and "vsc" lights came on. The manual said to stop and call Toyota service immediately - which we did.

A week and a half later, I got my 2004 Prius back. I was told it was a VSC computer, then they called back, that that wasn't it. The problem was an actuator. It was covered under warranty.

Funny thing is...mysteriously the racing engine noise while going down hills is now GONE!!!

Keep an eye on this - as I've seen several people complain about the noise - it may become a TSB (technical service bulletin).
 

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Re: Down-hill braking and fuel efficiency

priuschica211 said:
:!: WARRANTY REPAIR :!:

I too heard this racing noise for months going down long hills, both steep and not so steep. Toyota didn't know what is was.

Two weeks ago, while driving on a flat highway, all the bells and whistles went off - literally! A high pitched tone stayed on constantly, and the "brake" and "vsc" lights came on. The manual said to stop and call Toyota service immediately - which we did.

A week and a half later, I got my 2004 Prius back. I was told it was a VSC computer, then they called back, that that wasn't it. The problem was an actuator. It was covered under warranty.

Funny thing is...mysteriously the racing engine noise while going down hills is now GONE!!!

Keep an eye on this - as I've seen several people complain about the noise - it may become a TSB (technical service bulletin).
Interesting. Thanks for posting this information.
Drive happy,
Moo :)
 
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