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Why regenerative braking belongs on the brake pedal

Discussion in 'Model 3' started by JeffC, May 25, 2016.

  1. garsh

    garsh Re Member

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    I would say, much more often than the scenarios JeffC presents. The most common safety-related scenario is the panic stop. Who here hasn't had to slam on the brakes at some point?

    There's the "reaction time" between knowing that you need to stop and taking action, and then the "action time" required to perform that action. Heavy on-throttle regen means that you start slowing down immediately after the "reaction time", while your foot is still making it's way over to press down on the brake pedal.
     
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  2. Luke42

    Luke42 Member

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    #102 Luke42, May 26, 2016
    Last edited: May 26, 2016
    Two driven wheels (as opposed to four braked wheels) in most cases.

    Also, in most cars, the driven wheels and the biggest brakes are in the front, which provides more traction and does not encourage oversteer.

    An average driver switching from the throttle to the brakes in a regular FWD car on a snowy/icy surface changes to a much more stable situation with more traction and better stopping power.

    (I've owned and driven AWD, FWD, 4x4, and RWD vehicles in snow/ice -- and deliberately slid then all in a variety of ways under semi-controlled conditions. I drive on ice every year, but have a need for fast driving at the moment - so I'm very interested in marginal traction, but don't care about race driving.)
     
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  3. flar

    flar Member

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    Please read the link that deonb posted back in #86. It's basically a several page report on exactly this phenomenon in the Roadster and how they tuned it out. Some excerpts:
    Basically, they were worried about specifically the problem that you outline, they then went and did actual testing of the actual car and discovered that they could eliminate the problem and have regen on the throttle. Skilled drivers corroborated their conclusions.

    Another thing to consider is that the computer can cancel the regen instantaneously if it detects oversteer conditions, unlike an ICE where you are basically committed once the engine starts braking (you can cancel it out, but the rear end can snap around before the ignition chambers even respond to any inputs). If you are considering naive users then you'd rather have them working with the accelerator where the computer can cancel out any stupid moves they make. If you encourage them to shift to the brake pedal instead (by moving the useful regen braking to it) then you've essentially trained them to put their foot on the "make a bad situation into an unrecoverable major accident" pedal. Anything you can do to keep them on the accelerator encourages them to stay with the control that the computer has the most ability to correct their mistakes...
     
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  4. diamond.g

    diamond.g Active Member

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    Minimizing how often I have to lift my foot of the accelerator sounds fantastic with my commute on I95 near the beltway. I don't see trailing throttle oversteer being an issue there.
    And the stuff @flar said above!
     
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  5. p-f-g

    p-f-g Member

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    The emergency situation is the whole point of this discussion, no? And here, in a curve, the first thing you do is not braking, it's removing the foot from the throttle. And this may lead to a big difference with strong regenerative braking.

    I wouldn't want my e-bike to brake when I stop pedalling. :)
     
  6. diamond.g

    diamond.g Active Member

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    If you have to emergency stop in a curve, regen isn't going to make much of a difference is it? It sounds to me like folks are concerned about going to fast for road conditions and would have an issue in an emergency situation regardless. Or are you guys talking about having to suddenly turn cause say a deer jumps out in front of you?
     
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  7. deonb

    deonb Active Member

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    Why are you removing your foot from the throttle? To press the brake? Then it's really irrelevant whether the regen is on leaving the accelerator or pressing the brake.
     
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  8. jgs

    jgs Active Member

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    Did the person who wrote the "Oversteer vs Understeer" section consider talking with the person who wrote the "What About Rear-Wheel Drive?" section to iron out their disagreements? The former argues (I think, it's not articulated clearly) that it's bad to put regen on the rear axle. The latter makes an almost-explicit case that it doesn't matter ("... or panic stops which are largely handled by the friction brakes.")

    Also, doesn't AWD obviate more than half the points made on the linked page? Pretty much any argument that relies on regen being on the rear axle doesn't apply.

    Finally,

    Without citation of sources, it's opinion as far as the reader is concerned.
     
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  9. ModelNforNerd

    ModelNforNerd Active Member

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    In my (admittedly very very limited) EV driving experience, the regen in a Tesla reminds me slightly of keeping the revs up in a turbo ICE, to stay in the "power band".

    it seemed to me that because the drivetrain remained "engaged", when you bottom out on an "up and down" hill, you have immediate access to your power to make your way back up the crest of the next hill, increasing efficiency.

    not entirely sure if that's 100% technically accurate, but I like it.
     
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  10. plankeye

    plankeye That Vegan EV Guy

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    OMG, you will get that in spades! That was the most surprising thing to me when I drove the P85. The off the line acceleration was amazing, but what was more amazing was the acceleration when you were already going 40 or 45, un-freaking-real! I even broke the tires loose a little while accelerating hard-ish (not full throttle) from 40 or so, with TC on, crazy!

    It's going to kill me to drive the M3 in any kind of sane fashion, I fear. I'm Mr hyper-miler in our Prius C. But I think I'm going to be Mr Hyde in the M3. :D
     
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  11. Zextraterrestrial

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    #111 Zextraterrestrial, May 26, 2016
    Last edited: May 26, 2016
    some engineers should not have feelings or maybe they should not be engineers(were these the ones that made it possible for the front and rear doors of the X to collide?)
    I basically race my S to work and use left foot braking in addition to the accel and trail braking pedal. If it felt more like 1st gear at speed and you force some trail braking oversteer with just regen it would be even better (& potentially dangerous if you don't want/need it)

    regen on brakes sucks -even if it is 'well done' and blended
     
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  12. cantdecide

    cantdecide Member

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    I believe Tesla has it done it right in this respect for real world conditions.

    Safety:
    Most real world emergency situations are:
    • Traffic in front suddenly stops, or this has suddenly become visible
    • Another driver fails to stop at a stop sign or traffic light
    • A child runs out onto the road in front of me
    And not
    • I'm going around a corner too fast
    • Ice... because I'm from California perhaps
    • I'm racing my car
    As such straight line stopping distance matters most for me and I suggest pretty much any non racing driver (as per OP, most drivers don't push their cars to the limit).
    In this circumstance the extra half a second of regeneration caused by regeneration on accelerator is useful without any downside I can see... in both cases the user is braking as hard as they can and trusting the car will do its best to keep itself in line.

    In general even if over braking on a corner were an issue for me I would anticipate a smart Tesla might correct and optimize that?

    In addition having my foot slip while attempting to change pedals is less likely when I change pedals less often.

    Maybe for a racing driver something else is safer, but for me I believe strong regeneration on accelerator is safer.

    Ergonomics:
    We have a Tesla, i3, LEAF, and a Prius.
    Driving down my street in the LEAF and Prius I am constantly changing pedals which is annoying and a repetitive movement. The i3 does it best... in the Tesla I need to switch to the brake to actually stop as regeneration fades at 5 mph.
    If you want constant speed there is cruise control, if nothing you want to speed up and slow down frequently and unpredictably based on traffic. If my computer has 2 mouses... one for moving left and one for moving right... I would not be happy.
    One pedal driving please.

    I'm hoping this will all be moot in 5 years... 0 pedals is the eventual answer once we have self driving better than drivers.
     
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  13. Zoomit

    Zoomit Active Member

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    JeffC--Interesting essay. I appreciate that it made me seriously consider why I find heavy regen so appealing. I think there are two fundamental flaws in your analysis:

    1) Regen does not upset the dynamic response of the car because it is done at relatively low deceleration rates (less than 0.3g). This has been discussed by others in the thread. I agree with your physical description of what could happen with trailing throttle oversteer, but traction control is fundamental to the safety of the vehicle. This is true if the driver lifts quickly and gets regen in a Tesla, lifts quickly at high revs in a manual transmission car, or lifts quickly in any car and impulses the friction brakes. All of these situations are against the "first principles of vehicle dynamics" and all of them are managed my modern traction control. That's especially true with regen, because the regen is so low, vehicle upsets are within the ability of traction control to modulate. Empirical evidence supports this as people aren't suing Tesla for trailing throttle oversteer crashes in poor weather.

    2) The statement that two pedals are needed (one for acceleration, one for deceleration) and other configurations are "incorrect in terms of basic human factor science" is unsupported. The only reason there are two pedals is because in olden times each pedal controlled an independent mechanical system. In any car, we routinely accelerate using the left pedal and decelerate using the right pedal. Whenever we go up or down hills, the desired vehicle behavior can shift to the other pedal. Less braking is used to accelerate down a hill. Slowing down is done using less throttle up a hill. In addition, the vehicle always has deceleration forces therefore on flat ground it always decelerates with reduced throttle, independent of brake pedal use. It's these always changing conditions that make it so much easier to have all the normally-desired deceleration commands coming from the same pedal as the acceleration commands. This is also why the classic two pedal system is flawed and why lifting throttle regen becomes second nature so easily. (BTW, I firmly believe that two pedals will always exist as long as humans are driving, because having that second pedal, for full force braking using a redundant system, is critical for emergency situations.)

    So much driving is done with accelerations on either side of zero, between -0.2g and +0.2g, that it's odd to me that we have used two pedals for so long. Since you invoked human factors engineering, I'll submit (admittedly without evidence) that it is a fundamental flaw in vehicle design to have two pedals perform such similar functions that the driver must routinely decide which pedal to use depending on the specific driving conditions. With heavy regen commanded from the right pedal, the cognitive load is reduced as the right pedal is always used for typical acceleration and deceleration while the left pedal is only used critical stop situations such as emergencies or to keep the car from rolling.

    Tesla was brave to implement the better solution knowing that it would be disruptive to driver habits, but they got it right.
     
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  14. Skotty

    Skotty 2014 S P85 | 2020 3 P19"

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    #114 Skotty, May 26, 2016
    Last edited: May 26, 2016
    Both ways have their pros and cons. I've had it both ways in various cars. My conclusion is that there is nothing wrong with the way Tesla has done it. And there is nothing wrong with the way others have done it either. Comes down to a matter of preference.

    Pros/Cons, as I see it: (RoA = regen on accelerator, RoB = regen on brake)

    Where RoA is better:
    * RoA is much simpler to implement. RoB requires sophisticated controls.
    * RoA provides the driver with precise knowledge of when regen ends and friction braking begins. This is a HUGE advantage for RoA for geeky drivers, but potentially of no clear advantage to people who don't care. RoB can be set up with a screen display to help indicate when regen ends and friction braking begins, but often such a display is not provided, or buried deep in the UI, and can never be as obvious even with a screen display.
    * RoA allows for so called "one pedal driving" which is quite popular.

    Where RoB is better:
    * RoA, while being simpler to implement functionally, does require more complex controls of the brake lights. RoB can just trigger brake lights with any press of the brake pedal, whereas RoA needs additional knowledge like from an accelerometer to trigger the brake lights at the most appropriate times.
    * RoB is arguably simply in that accelerator means go, brake means stop.
    * RoB makes it a little bit easier to coast when desired.

    Other considerations:
    * RoA brings up concerns of it being potentially easier to allow regen on slick surfaces and what that might do. However, I think with proper traction control, this should be a non issue. Does Tesla have "proper" traction control? I don't have first hand knowledge or evidence of this, but I hope they do.
    * RoB behavior is consistent with older gas car behavior, which some may see as an advantage as it eliminates a difference that new EV drivers will need to get accustomed to. However, many think this difference is worth it as it will often be preferred after the initial adjustment period.
     
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  15. scaesare

    scaesare Well-Known Member

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    #115 scaesare, May 26, 2016
    Last edited: May 26, 2016
    An interesting read, and I understand the premise and concern. However, I think you are asserting as an absolute that Tesla's implementation is wrong, when in fact it's a design consideration with trade-offs... as just about all engineering choices are.

    A few points:

    - I'd argue about how heavy the regen on the Model S really is. While it is up to 60KW of force, it's modulated to be what appears to be a constant deceleration. I've heard 0.1g is the number, but I don't know that's been proven out.

    - As has been mentioned, the system as a whole includes traction control that is very well integrated, and I believe is going to prevent the understeer in a vast majority of situations, including the emergency situations you are concerned about. They've had a number of years to improve on the Roadster's already excellent system, talked about here (be sure to watch the videos): Slip-Sliding Away

    - As deonb has pointed out, just because you can lift full-off the throttle and induce max regen in a squirrelly situation doesn't mean you should. There's some driver education here that will help. And I don't believe the "it's not what the driver expects" argument holds water... the same driver who has no idea what understeer is and does a full throttle lift off is the same driver who will also do a full-stab on the brakes in the same panic situation. If you believe it's fair for ABS to bail out the driver in the latter situation, it's fair to allow TC to bail them out in the former.

    - Ultimately regen is something initiated by the driver: either on throttle lift-off, or brake application. In both cases it's up to an educated driver to manage it. Education is needed in both cases. Asserting only one approach is correct because "that's the way we've always done it" is a spurious argument, in my opinion.

    Having driven manuals, autos, and now an EV, and as someone who has practiced deliberately putting every vehicle I've owned in to and past the limits of their handling performance envelope (in a safe area) in order to prepare myself for an emergency situation, I like the off-throttle regen. I also know what it's implications are.
     
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  16. trils0n

    trils0n 2013 P85

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    The regen / one pedal driving is one of my favorite features of the car. It is really intuitive to me and how I think cars should function. The driving experience is so much better putting regen on the accelerator pedal. I drove manual cars all my life, and the transition was really easy, and made me wish my manual cars had more engine braking. Lifting off behaves as a manual car -- the car begins to slow. Less accelerator = less speed. The brake pedal controls the brakes. If Tesla eliminates the one pedal driving by moving the regen to the brake pedal, I will purchase another brand that allows one pedal driving. Don't want to compromise the driving experience.
     
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  17. Zoomit

    Zoomit Active Member

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    Many poe
    Single pedal driving is nothing new to anybody that commutes in slow traffic with a manual transmission. Creeping along at 5-15 mph is so much easier in a manual than an automatic transmission. You have to continuously move your foot between the pedals with the automatic, but the manual will let you adjust your speed (both accelerating and decelerating) all with the right pedal. The brake is only used in an emergency or to stop the car (along with engaging the clutch with the left leg).
     
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  18. trils0n

    trils0n 2013 P85

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    Exactly my experience. The continuous switching of pedals is a chore. The regen on the accelerator eliminates this chore, and improves driving experience. Regen is even easier / more capable than relying on engine braking. It became almost telepathic feeling for me. Very relaxing, but engaging at the same time.
     
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  19. jgs

    jgs Active Member

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    So true.
    I guess I agree with exactly what you've written above, but wow was I glad to get rid of my standard for stop-and-go, because obviously when you're in 0-20 traffic instead of 5-15 you're messing with the clutch quite a bit. And I had a heavy clutch. Other than that, I like a standard a lot, and if I can I rent one while traveling, but overall I miss it a lot less than I expected to.
     
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  20. Skotty

    Skotty 2014 S P85 | 2020 3 P19"

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    I see what you are saying there, but it comes with a counterpoint. A fair amount of driving is "stop and go" as they say, and if you have to come to complete stops or near complete stops, you'll be doing extra work with a manual that you wouldn't have to do with an automatic. This leads the manual driver to try to drive in stop and go traffic in a way where they don't have to actually stop or drop below the minimum speed in 1st gear, which sometimes you can do, but sometimes not.
     

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