Regenerative Versus Friction Braking

Hi and congratulations on owning such a smart, efficient car. I love mine too.

There's no way, without further instrumentation, to know when you're using your friction brakes; it's seamless. In general, friction braking only occurs at really low speed (under about 8 mph), in panic stops, and if the battery is as charged as it should be. There's not enough regenerative braking power at low speed. If you jam on the brake, both regen and friction try to stop you. When the battery shouldn't receive any more juice (going down that long hill and not using 'B') the friction brakes burn the excess kinetic energy as heat.

Prius brakes last a really long time. More people have problems with grabby brakes because of rust on the rotors than with eating brake pads/shoes.

If you really MUST know when your frictions brakes engage, search for CAN VIEW and for posts by Hobbit (I think). The CAN VIEW reveals all kinds of vehicle inner workings, and I believe Hobbit built a simple LED indicator to let him know what sort of braking he was doing.

Have fun with your Prius.

 

The can view is well worth adding if you can and will help you tell when you are in regen braking and how much regen current you are generating.

However you can tell a lot by feel and sound. Open the windows and you should be able to hear the disks making some noise when they engage. At least for medium to slow speeds.

By feel, regen is normally used for the part of the brake travel where the motion affects the breaking. Where the pedal stops and your foot pressure controls the brakes is where the hydraulics take over. This is not true always but most of the time.

For example if you brake too quickly the regen goes up to some current limit and then shuts down early and you are using all disk braking.

Also think of the rate the car can accelerate. If you slow down much faster than that rate you will have to use the disk brakes.

Then of course as posted many times going slow below around 8 mph you are on the regular brakes, at least in drive.

Try some stops in neutral and you can be sure you are braking with no regen, so this can help you learn to feel the difference.

 

<div class='quotetop'>QUOTE(jmccord @ Apr 23 2006, 12:32 AM) [snapback]243857[/snapback]</div>

Talking about the 325i, I have never seen more brake dust on any car than my 740iL!

 

There's one thing I've always wondered about with regen braking.

As you brake harder (before the pads kick in), how does the Prius apply more "frictiion" to the brakes?

Thanks
Bill

 

One quirk I've noticed with my new (2000 miles) '06 Prius is when I slow to a stop. Just as it's coming to a complete stop there's a bit of a clunking, like the system can't decide properly when to grab the discs.

Is this normal? Common? Something needs adjustment?

Thanks,
Erik Tkal

.

 

<div class='quotetop'>QUOTE(jmccord @ Apr 22 2006, 09:32 PM) [snapback]243857[/snapback]</div>

The CAN-View, mentioned in an earlier post, can tell you the amperage, whereas the native MFD display only shows the direction, of the current flow. When you step on the brake the amperage goes positive. (It also is positive when the engine is charging the battery.) The harder you step on the brake, the higher the positive current flow. When you go below the speed at which regen is used, suddenly the amperage goes back to its idle value of around 1 or 1.5 amps negative. I've seen 100 amps very briefly during hard braking. Around 60 or 70 is more common for me.

If you love knowing what the car is doing on the inside, the CAN-View is a great little gadget. It provides something like one or two dozen parameters (I forget exactly how many) which you can display 4 at a time on graphic screens, or 7 at a time on numeric screens.

Norm's web site is here. I think he's working on a version for the 2006. The '04 and '05 CAN-View won't work on the '06. But with luck an '06 will become available. Norm builds these things one at a time, to order, and I think there's only 50-odd of them in the wild at present.

 

<div class='quotetop'>QUOTE(billf @ Apr 23 2006, 02:11 PM) [snapback]244033[/snapback]</div>

MG2, the main drive motor, becomes a generator for regen braking. It is possible for the computer to increase or decrease the load on MG2 (or MG1, for that matter) to control the amount of drag it applies to the wheels. The detailed electronics of how it does this is beyond me.

 

see http://techno-fandom.org/~hobbit/cars/cours-section6.pdf
.
_H*

 

<div class='quotetop'>QUOTE(efusco @ Apr 23 2006, 07:47 PM) [snapback]244074[/snapback]</div>

This is as I come to a complete stop, more like 1-2mph.

Erik

(edit)
What I'm describing seems to be the topic of another thread - "Brake Grabbing" at http://priuschat.com/index.php?showtopic=11858&hl=

Erik

 

Sometimes when I'm braking on a long downhill I will notice a sudden change in the braking behavior of the car. I think what is happening is that the regen turns off either because the battery is full or because the road becomes too steep for regen only. When the friction brakes grab I can fell the level of the car change.

The regen brakes are on the front only, while the friction brakes are on all four wheels. So heavy regen braking will tend to make the car dive forward a little, while friction braking keeps the car more level.

 

<div class='quotetop'>QUOTE(tmorrowus @ Apr 24 2006, 03:34 PM) [snapback]244409[/snapback]</div>

Are you sure about that? I thought the regen braking was simply drag that was created by MG1 and/or MG2 converting shaft motion into electrical power and that there really is no such thing as any other brake besides the brake pads.

 

<div class='quotetop'>QUOTE(Erik Tkal @ Apr 23 2006, 04:59 PM) [snapback]244042[/snapback]</div>

Doesn't sound normal to me. On mine if I pay close attention to it I can feel the transition to the friction brakes at 8mph. But there is no sound and when having passengers in the car they can't feel the change at all.

 

<div class='quotetop'>QUOTE(kirbinster @ Apr 24 2006, 12:42 PM) [snapback]244416[/snapback]</div>

Both correct: MG2 is connected directly to the front wheels. With no drive train connected to the rear wheels, regen braking must be front-wheels only. The friction brakes, as on any other car, are on all 4 wheels.

Braking simply means slowing. So you are correct that the brake pads are the only conventional brakes. But braking is accomplished by MG2 putting a generator drag on the front wheels. This, however, will not bring the car to a full stop, so the friction brakes do that.

 

OK, I did a search on "can view" and oila!, the very same post that directed me to do the search, popped up, plus a number of other posts with the word "can" in them. So, what is Can View and is it available to the general public or is this a gadget invented in Los Angeles by Prius Techies?
Bob

 

<div class='quotetop'>QUOTE(tomdeimos @ Apr 23 2006, 08:28 AM) [snapback]243949[/snapback]</div>

When the brake pedal is pressed, the pads WILL be in contact with the rotor even if not really used for friction braking. They have to be - all slack must be initially taken up so the seamless transition to the pads can happen. I'm not sure you'll be able to tell the difference by listening in this way. There is always some friction going on - just not very much at low pedal pressures.

Assuming that the battery isn't "full," then the regen doesn't terminate just because the friction brakes are added - they work together as much as possible. There is an upper limit to the regen, certainly, but when that is reached, the system does not "shut down" it just adds as much friction braking as is needed, and both systems slow you down.

 

I experience a low speed very light "clunk-clunk" under braking as I near a stop in my '06 - it's similar to the sound of the ABS system resetting as you start forward in some ABS vehicles after backing out of a driveway, but also seems to have a very mild pedal pulse associated. I've always assumed that this was part of the transition from regen to full friction braking since I don't remember it under heavy braking when I assume I'm already on the friction brakes at higher speed (Hmm = now I'll have to really pay attention during a harder stop to be sure... ). I've perceived it to be noticeable but mild in nature, and frankly I'm surprised that the transitions are so seamless.

One point regarding the nosedive under friction braking - the front wheels do the vast majority of the braking , especially given the significant front weight bias of any front-driver. Under any kind of braking, the car will experience forward weight transfer - at higher braking decelerations/harder brake applications you're likely to notice the nosedive that results from this weight transfer. The weight transfer from the rears limits the amount of braking force they can apply through the tire contact patch before they lose traction and skid, while the opposite effect occurs up front.

I believe that the same amount of braking deceleration will result in the same amount of weight transfer, regardless of which system is braking you - the regen system just doesn't generate high enough deceleration to create the obvious nosedive. I don't think it's about regen on the front wheels only.

Jim

 

<div class='quotetop'>QUOTE(darelldd @ Apr 24 2006, 06:47 PM) [snapback]244521[/snapback]</div>

This is not correct. The sound is obvious and the brakes do take over completely from the regen. This is also obvious if you have a can view and can see the 80 amperes of regen drop immediately to near 0.

Regarding the latter it is quite annoying and I have to release the brake pedal completely and then brake again in order to regain some regen braking.

Note I am not saying this happens all the time, just enough to bother me. Regen and friction can work together also. And it is clearly related to the battery current limits, so happens more in cold weather.