1. Attachments are working again! Check out this thread for more details and to report any other bugs.

Using B for freeway exits

Discussion in 'Gen 2 Prius Technical Discussion' started by dbj, Aug 21, 2007.

  1. dbj

    dbj New Member

    Joined:
    Jul 30, 2007
    9
    0
    0
    Hello all,

    I could not find this particular nuance by 'googling' this site, so here goes:

    Has anyone tried using the 'B gear' when exiting a freeway? It seems like this might be useful, sort of like downshifting a sports car to slow it down without using the brakes. It seems to me that it might be smoother than using the brakes. (Yes, I know that the brakes contribute to the overall operation of the car by providing regenerative power over and above what I would get in 'B'.)

    Out here, we seem to have an abundance of very short straights leading to significantly tight curves as part of the freeway system exits. Posted speed limits go from 70 mph on the freeway to 25mph on the curve a few hundred feet later. My comfort zone lets me take the curve faster than that, but I'd like a less drastic slowdown to my perceived safe speed.

    For that matter, how about 'B' for normal corners and curves as well?

    I know I could just try it, but I'm still a bit intimidated by this very new car, and afraid of doing something to hurt it! Would these actions likely lead to premature wear, somehow?

    I had a friend who loved to downshift his Lotus for any slowdowns or curves, and he explained to me once (with more than a little tongue in cheek wryness) that he did it because "... gears are so much easier and cheaper to replace than brakes ..." :lol:

    Thanks for any experience you may have.

    little greenie
     
  2. qbee42

    qbee42 My other car is a boat

    Joined:
    Mar 2, 2006
    18,058
    3,075
    7
    Location:
    Northern Michigan
    Vehicle:
    2006 Prius
    I can't imaging why you would want to do this. Regen braking and B mode both brake with the front wheels, so there is no gain there. The only difference is that you throw away some power using B, and you have to make the switch to B mode. If your freeway exit goes down a mountain for a couple of miles, then B mode is the way to go, otherwise, just step on the brake.

    Tom
     
  3. efusco

    efusco Moderator Emeritus
    Staff Member

    Joined:
    Nov 26, 2003
    19,891
    1,193
    9
    Location:
    Nixa, MO
    Vehicle:
    2004 Prius
    Model:
    N/A
    Agreed, no benefit at all. Get off the gas as early as is safe. The gently but firmly apply the brakes to get all the regen you can so that you gradually increase brake pedal pressure until you get to your stop....or better yet so that you don't have to stop at all and can maintain your momentum.
     
  4. richard schumacher

    richard schumacher shortbus driver

    Joined:
    Mar 27, 2004
    7,663
    1,041
    0
    Location:
    United States
    Vehicle:
    2004 Prius
    Model:
    N/A
    And if you forget to shift back into "D" you start eating into your fuel economy in a hurry.
     
  5. KD6HDX

    KD6HDX New Member

    Joined:
    Oct 30, 2005
    256
    4
    0
    Location:
    Chino Hills,CA
    Vehicle:
    2005 Prius
    I use "B" mode everyday on a short downhill through the canyon on the way home from work. The posted speed on the last two curves are marked as 25 MPH. The distance on the downhill is about a quarter-mile, so as soon as the road levels out, we switch back to "D". A couple of times we have forgotten to switch back and we quickly noticed the difference in the feel and sound of the acceleration.

    We just use this mode with slight brake pedal pressure to prevent downhill speeds from getting to an unsafe speed for the 25 MPH curves. The transition to and from this mode is smooth and we have not had any problems yet. 67 K miles and counting.

    good luck to you.

    When I asked the dealer about this mode, he just said, "Don't use it". When I asked him why not he said again. just don't use it. Seems strange that they would add this feature if were not supposed to use it. One of my friends who bought and then sold his Prius said that in Colorado, the downhills were so long, that after the battery reached all green bars, there was no braking effect offered by the "B" mode. He sold it and went back to a VW Jetta TDI diesel.

    I am wondering what Evan will say about this mode, and why he does or does not ever use it?
     
  6. Godiva

    Godiva AmeriKan Citizen

    Joined:
    Apr 8, 2005
    10,339
    14
    0
    Location:
    San Diego, CA
    Vehicle:
    2005 Prius
    Unless you have an extended, steep downhill commute forget about the "B". Leave it in "D".

    In this particular case the dealer knows what he's talkinb about. Just don't use "B".
     
  7. efusco

    efusco Moderator Emeritus
    Staff Member

    Joined:
    Nov 26, 2003
    19,891
    1,193
    9
    Location:
    Nixa, MO
    Vehicle:
    2004 Prius
    Model:
    N/A
    <div class='quotetop'>QUOTE(KD6HDX @ Aug 21 2007, 01:52 PM) [snapback]499853[/snapback]</div>
    Hobbit has an Extensive Review of B-mode that goes into all kinds of gory detail about the negatives of B-mode.

    While using B-mode as you describe definately will not hurt the car or cause any problem it's also a waste of gas b/c at any speed over 20mph it forces the ICE to continue to run until you shift out of B-mode again. This burns gas and reduces the amount of regen you can achive b/c some of the potential energy is lost through the engine braking.

    For the short downhill you describe I would go into stealth at the crest of the hill and then use the brakes to regen all the way down keeping the speed below 41mph the whole way so I could get max regen and burn zero fuel.
     
  8. KD6HDX

    KD6HDX New Member

    Joined:
    Oct 30, 2005
    256
    4
    0
    Location:
    Chino Hills,CA
    Vehicle:
    2005 Prius
    OK, I might adapt to not using it, but does it actually hurt anything to use it?

    Thanks in advance for anyone that can tell me. I might read the manual again and look for this info.

    Thanks Evan for the link and the quick response. I will read this when I get home from work.

    Dave
     
  9. efusco

    efusco Moderator Emeritus
    Staff Member

    Joined:
    Nov 26, 2003
    19,891
    1,193
    9
    Location:
    Nixa, MO
    Vehicle:
    2004 Prius
    Model:
    N/A
    Oh, and I didn't say I 'never' use it. I use it occasionally at speeds below 20mph b/c when you coast at those speeds in B mode there is not any/as much battery drain as there is in D and the ICE won't spin up at that speed. I also would use it on a very long steep downhill/mountain if I knew the battery pack would get totally full before I reached the bottom and thus force the use of friction brakes. Even then the benefit is questionable, it's just easier than concentrating on braking for that long distance.

    <div class='quotetop'>QUOTE(KD6HDX @ Aug 21 2007, 02:16 PM) [snapback]499863[/snapback]</div>
    Again, the only thing it "hurts" is your fuel economy.
     
  10. qbee42

    qbee42 My other car is a boat

    Joined:
    Mar 2, 2006
    18,058
    3,075
    7
    Location:
    Northern Michigan
    Vehicle:
    2006 Prius
    <div class='quotetop'>QUOTE(KD6HDX @ Aug 21 2007, 03:16 PM) [snapback]499863[/snapback]</div>
    As Evan points out, it doesn't hurt anything other than your fuel economy, but it also doesn't help anything either. It's pointless to use it in this situation, so why do it?

    Tom
     
  11. dbj

    dbj New Member

    Joined:
    Jul 30, 2007
    9
    0
    0
    Wow, thanks to all. I certainly did not fully understand what 'B' mode does, in spite of reading my brand-spankin'-new '07 owner's manual. Hobbit's document made it a lot clearer.

    Thanks to all who responded with information, and especially to efusco for the link to the explanation.

    little greenie
     
  12. KD6HDX

    KD6HDX New Member

    Joined:
    Oct 30, 2005
    256
    4
    0
    Location:
    Chino Hills,CA
    Vehicle:
    2005 Prius
    From the linked article about "B" mode-

    "But don't get into the habit of using "B"
    to slow down unless you really need it -- that's sometimes hard to get used
    to if you come from ingrained years of "gearing down" in conventional cars
    ."


    This was why I was using it and why I taught my wife to use it too when she drives that stretch of canyon road. I guess old habits are hard to break, but I feel better reading about it and knowing whats going on inside the motor.

    I never used the "B" mode to save gas or charge the battery more.

    Thanks again.
     
  13. SureValla

    SureValla Member

    Joined:
    Jul 13, 2007
    590
    21
    15
    Location:
    Shelton, CT
    Vehicle:
    2007 Prius
    Model:
    III
    <div class='quotetop'>QUOTE(KD6HDX @ Aug 21 2007, 06:54 PM) [snapback]499985[/snapback]</div>
    what about to get better traction in bad weather like standards do by shifting down?
     
  14. efusco

    efusco Moderator Emeritus
    Staff Member

    Joined:
    Nov 26, 2003
    19,891
    1,193
    9
    Location:
    Nixa, MO
    Vehicle:
    2004 Prius
    Model:
    N/A
    <div class='quotetop'>QUOTE(SureValla @ Aug 21 2007, 06:54 PM) [snapback]500012[/snapback]</div>
    While some Prius pundits recommend it I do not. Using B-mode in that fashion is relatively uncontrolled. Your ABS system will do a better job and your VSC (if equipt) better yet. By using the brake you can adjust the amount of pressure precisely to the circumstances. B-mode will give a fixed amount of resistance based upon vehicle speed regardless of the conditions.
     
  15. FireEngineer

    FireEngineer Active Member

    Joined:
    Oct 6, 2004
    1,247
    124
    0
    Location:
    SW-Side of Chicago, IL
    Vehicle:
    2010 Prius
    Model:
    IV
    I just used B for the first time for what it was meant for, slowing down a mountainside.

    6% grade for 6 miles in Arizona mountains. Didn't really slow me down, even with the ICE howling at 4900 RPM and the AC blasting to drain the battery as fast as was filled.

    Going down I-8 in California, 6% grade for 13 miles, didn't use B, just used the brake and modulated it, ICE engaged as it thought was necessary, everyhting was good.

    B mode, why bother.

    Wayne
     
  16. JimboK

    JimboK One owner, low mileage

    Joined:
    May 1, 2006
    2,817
    187
    49
    Location:
    Chesterfield, VA
    Vehicle:
    2005 Prius
    <div class='quotetop'>QUOTE(FireEngineer @ Aug 22 2007, 12:42 AM) [snapback]500121[/snapback]</div>
    I was starting to think the same during my trip to Hybridfest. In my earlier runs through the mountains on the trip I used "B." Then later in the trip I tried leaving the car in "D" and used moderate braking alone, presumably only enough to avoid mechanical braking until the battery was topped out. (It didn't take long; Can-View showed SoC increasing by 1% every few seconds during braking. Within just a couple of minutes of sustained braking the battery hit 80% or more, 82% at one point.) I think the steepest grade I saw (based on roadside signs) was 10%, and the brake was more than enough to slow it. The benefit, as Evan suggests, may simply be that "B" is easier on long downgrades. After the battery fills, it will be forced anyway.
     
  17. qbee42

    qbee42 My other car is a boat

    Joined:
    Mar 2, 2006
    18,058
    3,075
    7
    Location:
    Northern Michigan
    Vehicle:
    2006 Prius
    <div class='quotetop'>QUOTE(SureValla @ Aug 21 2007, 07:54 PM) [snapback]500012[/snapback]</div>
    With a manual transmission you get better traction by shifting up, not down. Down shifting increases the torque at the driving wheels, which increases the chances of wheel spin. With our snowy winters, we used to routinely shift into a high gear and lug up a hill to avoid wheel spin. The only place where I down shifted for traction was with my Jeep C-J5, which had a two speed transfer case. With that, you could shift into low-low and allow the wheels to slip at a controlled rate. There was no chance for spin because they couldn't turn fast enough to spin.

    With the Prius, B mode is not a down shift, it just forces engine braking. It feels like a down shift, but it's not the same at all.

    Tom
     
  18. zzyzx

    zzyzx Junior Member

    Joined:
    Apr 27, 2006
    92
    16
    0
    My rule on using B mode is that if I'm driving down a mountain and the battery is showing a full charge, I switch over. As soon as one bar dissapears I go back to D.
     
  19. Tchou

    Tchou Member

    Joined:
    Sep 3, 2007
    161
    4
    0
    Bizarre I just thought B mode engaged the ICE engine without use of gas exactly the same when you downshift to engine-brake on a normal non automatic car, regarding the weight of the prius and the little size of the motor it may not be as efficient as we could imagine, but in case of full battery there is no more energy recycling so it's the only way without engaging the real brakes.
     
  20. MaxLegroom

    MaxLegroom Junior Member

    Joined:
    Jul 12, 2007
    90
    1
    0
    Location:
    Newport News, VA
    Vehicle:
    2005 Prius
    Well, that certainly is an education. I've tended to use it in certain situations, like when descending to the streets near my home, as both exits from the interstate have steep descents into stoplights, and were always hell on the brakes of my previous car. I've used it a bit on freeway exits, and other situations where I wanted to decelerate faster, or to balance the car in a corner the same way a heel and toe downshift to second did in my previous car. Perhaps using this is why I rarely see the seventh bar, and I should recalibrate what I do...