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Brake Burning Smell after steep downhill

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by HaiImBrian, Dec 23, 2018.

  1. fuzzy1

    fuzzy1 Senior Member

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    This needs some clarification. Used right, regenerative braking should be this high a percentage of your braking use or work.

    But regen is a far smaller portion of your available braking power. Regen can provide only about 26kW of power, which is moderate braking at low speed or light braking at medium speeds. Some drivers can keep most of their ordinary braking within this zone, while others with a heavier foot will regularly exceed this limit, ending up with more friction pad use and less regeneration.

    Hard or emergency braking at highway speed exceeds 500kW in the mechanical friction pad system. But with reasonably cautious and defensive driving practice, this very high power level should be a very small fraction of your overall brake use.

    Though for many people, this may just be an esoteric & semantic engineering/science discussion about work vs power vs usage time ...
     
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  2. bisco

    bisco cookie crumbler

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    if short downhill braking smells, i would be concerned
     
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  3. Grit

    Grit Senior Member

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    When you said "if you pay attention," you didn't mention scan gauge. Made it sound like you can feel temp going down a 6th sense :cautious: :LOL:
     
  4. Mendel Leisk

    Mendel Leisk Senior Member

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    Yeah that's all you've got, stock. For a car that's mpg-centric, engine coolant temp gauge would be nice.
     
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  5. ChapmanF

    ChapmanF Senior Member

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    I find my gen 3 will bring in some engine braking on its own initiative as soon as the battery soc nears 80% as shown on the scangauge, even in D. It seems to me that B mode can be used to more advantage in those situations where you know more about the upcoming grade than the car does.

    Ordinarily, the car is very opportunistic with regen, trying to slurp up every watthour it can whenever you coast or slow down. B mode tells it, hey, this downgrade that's coming is going to be more than you can ever store anyway, so pace yourself.

    That way, it can continue doing some regen, at a lower rate, throughout more of your descent, and postpone the point where you've got nothing left but engine braking and friction. The reduced regen rate heats the battery less, and you still end up collecting as much as you ever could anyway.

    Waiting to shift to B only when the no-other-options point has already arrived would seem to leave some of those benefits on the table.
     
    #25 ChapmanF, Dec 24, 2018
    Last edited: Dec 24, 2018
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  6. sam spade 2

    sam spade 2 Senior Member

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    A brake job might be a good idea because from your description your pads might be "cooked" and permanently glazed.

    But a careful consideration of your braking habits might be a bigger factor.

    Holding the brakes ON too long is not good technique.
    Apply firmly as needed and then release completely.
    If you are in B.....or in cruise where B will be evoked automatically as needed......once the brakes slow you down a bit the electrical and engine drag should keep the speed from shooting back up quickly.
     
  7. sam spade 2

    sam spade 2 Senior Member

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    Absolutely right.
    And if you haven't tried setting your cruise to the desired descent speed at the top and then letting it do most of the work as you go down, you might be pleasantly surprised.
     
  8. Jimi1976

    Jimi1976 Active Member

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    I coast down from Vail pass all the time with the CC set to 70mph.
     
  9. sam spade 2

    sam spade 2 Senior Member

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    Never had an "Aw, s***" moment when you suddenly came up on a semi doing 40 ?? ;)
     
  10. fuzzy1

    fuzzy1 Senior Member

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    Not if he is following proper defensive driving practice.

    And even if he isn't, CC doesn't make it any worse.
     
  11. Jimi1976

    Jimi1976 Active Member

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    Of course.....they’re usually in the far right lane
     
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  12. sam spade 2

    sam spade 2 Senior Member

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    My comment was not intended to be about the use of cruise control.......but about the 70 MPH where he set it.

    I think I usually set mine about 55 and that settles it in around 60........and an occasional tap on the brakes is still necessary.

    And unless you have "adaptive" cruise, it certainly can make a difference in reaction time.......where fractions of a second can make a big difference.
     
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  13. Jimi1976

    Jimi1976 Active Member

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    Weird - when I set my CC to 70....it holds at 70. If I set it at 40 driving around town - it holds at 40. Of course I use the brakes as needed....
    I70 speed limit is 65 in many areas....I normally go 5-10 over without causing traffic flow issues.....but we’ve stray way off OP topic here
     
    #33 Jimi1976, Dec 26, 2018
    Last edited: Dec 26, 2018
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  14. sam spade 2

    sam spade 2 Senior Member

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    I will NOT turn this into an argument.

    In MY experience, the cruise can NOT hold a steady speed when going down a VERY steep hill.
    The speed creeps up a bit and some brake is required.

    Screaming down a steep grade at 70+ when the prevailing speed of other traffic might be closer to 50 average and 40 for trucks and RV's......is just not a wise thing to do. The fact that you haven't crashed doing that yet proves nothing.
     
  15. fuzzy1

    fuzzy1 Senior Member

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    Considering how many drivers dislike others going sub-PSL, and give me flak about it in this forum, I'm not going to tell others to do it.

    55 and 60 were certainly not the prevailing flow-of-traffic speeds any recent time I've traveled that particular road.
    Stated after the fact.
    You don't have the same car as he does, a Gen3 Liftback. Instead, you have something with a smaller engine, less engine braking power.
    Nor is it even close to an accurate description of I-70 across Vail Pass any time I've crossed it.
    And remember, he was talking about this specific road:
    vail pass.JPG
    Unfortunately, Google's Street View right now shows reduced construction PSLs in the other direction, so I can't show the normal speed limit signs.
     
    #35 fuzzy1, Dec 26, 2018
    Last edited: Dec 26, 2018
  16. frodoz737

    frodoz737 Top Wrench

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    @HailmBrian

    I have never used "B" on my 2010 or 2015 to substitute engine braking...and I've done quite a few mountains without brake burn. Check periodically during the day and/or at the bottom of your decent to see if one wheel is hotter than the rest. Not saying anything is wrong...but since "almost" nobody flushes the DOT 3 fluid in their Prius, you could have a caliper beginning to stick. Something else to check anyway.
     
  17. sam spade 2

    sam spade 2 Senior Member

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    Yes but WE were talking about going down steep grades in general.
    Steep grades often include sharp curves and there is a good reason that the posted speed limit going DOWN is often 10 MPH less than the rest of the road.

    Just one of those reasons is: If your brakes overheat and fail, you have a better chance of not flying off a cliff if you are going a bit slower.
     
  18. ChapmanF

    ChapmanF Senior Member

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    The Rockfish Gap Turnpike in VA has a descent where my Gen 3 CC set for 55 is definitely unable to hold 55. (It also doesn't "settle in" some few MPH higher, either; the speed steadily climbs until snubbed by a firm foot on the brake.)

    If fuzzy1's figure of 26 kW powertrain braking capacity (from #21) is right, then the only way to get CC to hold a steady speed on that descent, with no use of the brake, would be to take the descent at some speed lower than 55, where the gravitational potential's rate of liberation would be 26 kW or less. With a topo map and a little figgerin', it'd be possible in principle to calculate that speed. But again, it would be below the posted limit, so I usually just start around 55 and mash the brakes a few times on the way down.

    I'm recently back from one of my occasional chances to play on the Midland Trail in WV. It used to frustrate me that my 2010 doesn't allow the combination of cruise control plus B mode (which was allowed in Gen 1). But then, that combination was necessary in Gen 1 if you wanted the CC to be able to hold speed on descents as well as ascents. The Gen 3 CC does that in normal operation anyway, and also does behave like B when necessary on a steep descent, so it turns out to do everything I want, even if I have to leave the shifter in D rather than B to use it.

    My Midland Trail technique in the 2010 has evolved into: I leave the shifter in D, set the CC at the outset to the lowest allowable speed (25 MPH), and then just drive with my foot on the accelerator as if there were no CC. The cruise control doesn't object to being driven faster than the set speed, but with 25 as the setpoint, just by lifting my foot off the accelerator, I get a much stronger decel than the rate that's programmed into B mode. That allows negotiating nearly all of the quick speed changes and straightaways and hairpins with just one foot on just one pedal. It's a really neat way to be able to control the car.

    It ties in nicely with the other 'feature' I discovered a while back, which is that the skid ECU interprets any use of the brake pedal while CC is engaged as 'urgent', and switches directly to four-wheel friction.

    In most driving, what I do with that knowledge is mostly try to remember to cancel cruise before braking, so it won't be interpreted as urgent. But when playing on a hairpinny road (in December), the typical reason for going on to the brake pedal will be a question like "hmm, is the surface on that curve a little shiny?", and in that case, knowing that CC is on and therefore one press on the brake will immediately bring in four-wheel friction, with ABS and VSC at the ready, is kind of comforting.

    Then, after any use of the brake (which cancels cruise), a quick 'RESUME' tap on the stalk engages the cruise-at-25MPH again.
     
  19. fuzzy1

    fuzzy1 Senior Member

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    That is the traction battery charge rate limit, not the total powertrain braking capacity limit including engine braking.
    Don't forget to gross up that regen/engine braking power to include MG and inverter efficiencies, and include air drag and rolling losses when figgerin' those speeds.

    Looking at a Vail Pass profile, I'm seeing only a single mile segment has 7.1% grade. One more mile is 5.4%, all the rest are 5.0% or lower, down to 2.4%, totaling 2267 feet in 10 miles.
     
    #39 fuzzy1, Dec 26, 2018
    Last edited: Dec 26, 2018
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  20. ChapmanF

    ChapmanF Senior Member

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    I wonder what the total braking capacity is. Probably not simply the total of 26 kW battery charge rate plus the engine braking, because the car doesn't seem to max them both out together, but only to trade out battery charging for engine braking in some proportion.