Brake Light Actuation
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Thread: Brake Light Actuation

  1. #31
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    Quote Originally Posted by baragona View Post
    Basically two opinions have emerged:

    1. It is the same as "downshifting" in a vehicle with a manual transmission, which has been an issue for 100 years, but not a single auto manufacturer has added it as a feature in a vehicle with a manual transmission, nor has US DOT required it..

    2. It is something GM should add. Can't think of a reason "not" to do it...
    Funny, I find myself disagreeing with both of those conclusions!

    1. It's clearly not "just the same" as downshifting a manual as it requires no affirmative action on the part of the driver, and the rate of decel is more like 2 or even 3 gear downshift, which rather few stick drivers will do with any regularity.

    and 2. I can think of MANY reasons not to have an automatic brake light. The exurb buses in Atlanta have them, and it's a real pain to follow them in the HOV lanes with their blinking lights every time they see the least little bit of decel. (BTW, those are separate from the real brake lights.) Tuned well, I suppose such a system could be okay, but I would expect that if it's added, it would be done badly, just like the forward parking sensors that scream at me as a back out of my garage each morning.

    My suggestion: Just tweak "L" mode a little. Make it max out when the car is not slowing, as when going down a long hill, but modulate it a bit on flat land.

  2. #32
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    Quote Originally Posted by jh26 View Post
    Funny, I find myself disagreeing with both of those conclusions!

    1. It's clearly not "just the same" as downshifting a manual as it requires no affirmative action on the part of the driver, and the rate of decel is more like 2 or even 3 gear downshift, which rather few stick drivers will do with any regularity.

    and 2. I can think of MANY reasons not to have an automatic brake light. The exurb buses in Atlanta have them, and it's a real pain to follow them in the HOV lanes with their blinking lights every time they see the least little bit of decel. (BTW, those are separate from the real brake lights.) Tuned well, I suppose such a system could be okay, but I would expect that if it's added, it would be done badly, just like the forward parking sensors that scream at me as a back out of my garage each morning.

    My suggestion: Just tweak "L" mode a little. Make it max out when the car is not slowing, as when going down a long hill, but modulate it a bit on flat land.
    Isn't the modulation your foot on the pedal?
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  3. #33
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    this thread was helpful because I just got my volt in December and thought the brake lights did come on when my foot was entirely off the pedal in L

    good to know I need to be wary of folks behind me when decel in L

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  6. #34
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    Quote Originally Posted by Zeede View Post
    I drove a stick shift car for almost ten years, with a similar behavior of slowing down w/o the brake light coming on.

    It didn't bother me then, it doesn't bother me now. I'll tap the brake pedal if I think the person is being a moron, but otherwise I don't care.
    It may not bother you, but that's not the point - it bothers others! You say you will tap the brake pedal, but you may not be paying attention that someone is creeping up on you as you let off in L. Someone then slams on their brakes behind you, not having the extra warning time from the brake lights, and before you know it, there's a multi-car chain reaction rear end accident.

    You treat it "if I think the person is being a moron" - FYI, very few rear end crashes are intentional, and even fewer are caused by "morons" - not the right term anyway. They are caused by drivers like you and I, in a weak moment of being in a hurry, distracted, enraged (you don't sound too kind toward those following you), or by unexpected circumstances. If it was as simple as "I'll tap the brake if I think ..." and it always worked, that would be wonderful.

  7. #35
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    Quote Originally Posted by Bob_Livonia View Post
    it bothers others!
    You're missing the point. Haven't you ever been behind someone who hits the brakes repeatedly for no discernible reason, sometimes known as left-foot-braking? It's annoying. It's distracting. And ultimately, it's unsafe, because you never know if this brake light is another meaningless one or a panic stop. Fortunately such drivers are relatively rare, but I will not stay behind one. You're advocating that every Volt in L become one of those unsafe irritations.

    If you don't like the deceleration, don't drive in L. You can get all the regen you want by lightly hitting the brake, and you get the brake lights you want.

  8. #36
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    Quote Originally Posted by davemac616 View Post
    You're missing the point. Haven't you ever been behind someone who hits the brakes repeatedly for no discernible reason, sometimes known as left-foot-braking? It's annoying. It's distracting. And ultimately, it's unsafe, because you never know if this brake light is another meaningless one or a panic stop. Fortunately such drivers are relatively rare, but I will not stay behind one. You're advocating that every Volt in L become one of those unsafe irritations.

    If you don't like the deceleration, don't drive in L. You can get all the regen you want by lightly hitting the brake, and you get the brake lights you want.
    That is my point exactly - I don't drive in L and I do get the regen and brake lights I want using the brake pedal, so why drive in L? (Yes, I know, I tried it, and it is a pain and a distraction, and not all it's cracked up to be). Left footed brakers are another story altogether, and if someone behind them errs on the side of backing off too much, no big deal. I'm not necessarily advocating the automatic brake light, although it can be done - many folks who drive in L actually are decelerating and accelerating way too much, rather than just giving a false signal of it. If one insists in driving in L, they could also put down the window and use hand signals - does anyone remember them from the driver's tests of the days past? But one would need to have a broken arm to do it the way the drawings showed.

  9. #37
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    The reasoning behind having the brake light come on when fully regening in L/Decelerating quickly is quite simply an early warning that you are slowing down quickly. It may be just enough to grab the attention of the person behind you to get them to stop looking at their text messages and look at your slowing down car.

    Have there been reared collisions from people downshifting a standard car over the years? Absolutely. There will be rearended Volts from the driver using L and having no brake lights. Will they be horrible? Most likely not as the vehicles will be going near the same speeds, so it's not like you are coming to a complete stop with no brake lights. But the principle applies. Slowing quickly without warning the driver behind you CAN cause an accident, even if technically it's the following driver's fault.

    When I'm in traffic and the car in front of me puts on their brakes, it's my warning to back off. If you are completely off the gas in L you are slowing down enough in my opinion to justify the brake lights, even though it's not a requirement.

    Yes, someone driving with two feet and flickering the brakes on and off is an annoyance. But how much time do you really spend in L at full regen? That's why I would suggest it's something more G-force or maybe -KWh related. At more than -25KWh in L trigger the brake light, or something like that. That will also eliminate the low speed/low-regen level brake light annoyance.

    The brake lights are there to warn the person behind you that you are slowing down, which is exactly what you are doing with a strong regen in L.

    I recall seeing an early demo of a radar adaptive cruise control system that showed brake lights when it dropped speed to match the leading slower moving car. Not sure if they work this way but it's the same principle.
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  10. #38
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    My work commute is about 14 miles of heavy traffic on a two lane state road with plenty of stop lights. Speed can vary from 0 to 60. Before the Volt, I would drive in L in my Grand Prix in the heaviest stop and go conditions, but of course I had to shift back to D when speeds picked up. Now, I'm always in L. I'm more concerned that the person behind me stops texting.

  11. #39
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    Default Some actual data...

    Since this topic has been talked to death (see here, here, and here. Poll: here), I thought I'd try to make it interesting again by gathering some real data. So yesterday I grabbed an accelerometer from work to measure the Volt's deceleration in L. For fun, we also measured my buddy's 2011 Mustang GT downshifting for comparison. The results may surprise you.

    But before I get to that, I'd like to call everyone's attention to this post from Slapshot28 citing the UNECE specification which states:
    5.2.22. Generation of a braking signal to illuminate stop lamps
    5.2.22.4. Electric regenerative braking systems as defined in paragraph 2.17., which produce a retarding force upon release of the accelerator control, shall generate the signal mentioned above according to the following provisions:
    Vehicle decelerations Signal generation
    ≤ 0.7 m/s The signal shall not be generated
    > 0.7 m/s and ≤ 1.3 m/s The signal may be generated
    > 1.3 m/s The signal shall be generated
    In all cases the signal shall be de-activated at the latest when the deceleration has fallen below 0.7 m/s.
    So the question here is where does the Volt fall in this criterion? It's been suggested several times that the Volt decels at about 0.7 m/s putting it between the"signal shall not be generated" and the "signal may be generated" threshold.

    BTW, per Pete Foss's earlier post I also reviewed the FMVSS regulations here. I am unable to find any standard for regenerative braking stop lamp illumination. In fact the only relevant standard I see is Standard No. 108; Lamps, reflective devices, and associated equipment, which only mandates that the stop lamps illuminate when you press the brake pedal:
    S5.5.4 The stop lamps on each vehicle shall be activated upon application of the service brakes. The high-mounted stop lamp on each vehicle shall be activated only upon application of the service brakes.
    Okay, now to the good stuff. On a nice long straight flat stretch of road (we have lot's of those here in AZ) on a clear 70d day driving in Sport Low (as always), I made a few runs. Here is a run starting from a stop, flooring it all the way to 75mph, then taking my foot completely off the go pedal and regening all the way back down to creep mode of about 3mph.


    As you can see I get a max acceleration of about 0.43Gs briefly then it tapers down. When I go into regen, the max deceleration is about -0.13 to -0.14Gs. 0.1G is equal to about 1 m/s (0.98 technically). So, this actually puts the Volt right at the limit of 1.3 m/s. Any higher would require stop lamp illumination per the above spec. I'm sure this is not a coincidence, GM likely set the regen limit at the max allowed without requiring stop lamp activation.

    I repeated the above run, except for this time I power braked at the launch. It didn't make a noticeable difference, as you see the results are essentially the same:


    Here's a few other runs if you're interested.
    -The 5-75-60 graph shows me accelerating on an freeway onramp from a rolling start to about 75mph, then regening down to about 60 as I merge with traffic
    -The 55-30-45-10-0 shows me cruising at 55mph, regening down to about 30mph, accelerating back to 45, then regening down to 10 before applying the service brakes.
    -The 50-0-50 shows me cruising at 50mph, then regening and slowing down on an incline. I expected to possibly see a higher rate of deceleration here, but didn't. The incline was steep enough to overcome the creep mode, bringing me to a full stop. Then I punched it getting 0.43Gs again
    5-75-60.jpg 55-30-45.jpg 55-0-50.jpg

    So it would seem for those arguing FOR having the stop lamps illuminate, GM is certainly in the window that would allow them to do so. But, for the sake of comparison, let's see what a manual downshift looks like. In the below chart I would guess we were going about 30mph. You see the shift causes almost a -0.5G shock momentarily. Then the engine braking effect approaches -0.3Gs, that's DOUBLE the Volt's L regen decel rate!


    Okay, here's one more. In this one we didn't shift, we just stayed in 2nd gear and accelerated, then let off the gas. In this case we actually exceeded -0.3Gs deceleration just off the engine!


    And lastly, just for fun, here are 2 more showing the Mustang launching through 1st and 2nd from a slow rolling start hitting 0.6Gs and then slowing under relatively heavy braking at about -0.65Gs Good times...
    Mustang Accel.jpg Mustang Braking.jpg
    Last edited by scottf200; 02-16-2013 at 05:16 PM. Reason: Updated first 2 charts for clarity - scottf200 poll link
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  13. #40
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    Quote Originally Posted by Joule Thief View Post
    Since this topic has been talked to death (see here, here, and here), I thought I'd try to make it interesting again by gathering some real data.
    Yes, it seems that whenever there is a newbee on the forum here, this comes up. I'm glad you had the time to gather the data - I am a fan of data (and have a master's degree in applied statistics). But it only proves that GM is smart enough to calibrate to relevant standards to keep themselves out of hot water and potentially having to recall cars and/or be subject to lawsuits.

    On the other hand, simply meeting all applicable FMVSS standards doesn't keep a car from being unsafe in some circumstances, or preventing the driver from doing unsafe or stupid things. You certainly did demonstrate decel, especially on the Mustang downshifting, should there have been any question. And thanks for quantifying it.

    Someone mentioned that rear end collisions are minor property damage nuisances. If that was only the case -- not even addressing those on crowded freeways, with cars following too close, and a car gets clipped in the corner, spins around, gets broadsided by a semi, and you have 40 cars involved.

    If anyone has watched daytime TV, there are plenty of lawyers advertising out there - "if you've been injured, dial 1-800-CALL-SAM". Many people look at being rear ended as hitting the lottery. Whiplash is tough to definitively identify with medical tests (and can be very painful and debilitating). This is made worse in no-fault states like Michigan, where injured people, even if they are uninsured, can get unlimited lifetime medical benefits and damages, and of course, the lawyers get their cut.

    Again, my argument does not go to the legalistic (But I never touched the brake ... but I never exceeded FMVSS limits, etc., etc.), but rather doing what is logical, safe, and the ethical and compassionate thing to do, rather than making excuses ("but smoking never bothers ME") for some unquantified thrill and alleged fuel savings from driving in L that I could never understand.

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