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Do brake lights always turn on with regen? (incl gap to other cars)
- Thread starter contaygious
- Start date
So that other drivers realize that I'm slowing down and don't run into me.
Agreed; if the rationale for having brake lights is to warn other drivers of one's rapid deceleration, then there's no reason to have them on when you're travelling at a constant speed.
Todd Burch
14-Year Member
Depends on whether they're trying to imitate the behavior of a conventional automatic drivetrain, I guess. Maybe it's all actually even simpler than all of the Tesla reps have indicated. Maybe they were "close" when they said it uses an accelerometer. It would probably be easiest to just look at rate of change of the speedometer and use that. If speed is decreasing at a rate above some threshold, the brake lights come on. (That in addition to the use of the brake pedal).
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Correct.
Correct.
In the extreme case (falling straight down with no air resistance) the accelerometer would register 0 Gs (think astronauts floating around in orbit, or a free-fall ride at a theme park). The reason astronauts float around in orbit is because they are travelling at an orbital speed such that they are continuously "falling" toward the earth (0 G environment)...but are traveling fast enough in a forward direction that this falling results in their orbital path. They are literally in continuous free fall around the earth. (Insert SpaceX reference here).
Regardless of how the car is moving (level on the ground, down a steep hill, or falling straight down) the gravity force itself of course doesn't change. But if the car is accelerating on a steep downward incline, it would register less acceleration (in the forward direction) than if it were traveling at constant speed (or sitting still).
The vector direction of the acceleration is always the same (toward the center of the earth)...but as the incline becomes steeper and steeper, due to the nose down pitch of the car that vector has an increasing forward component...until the car is facing straight down, in which the forward component IS the gravity vector.
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By the way, this is how the accelerometer in the iPhone/iPad determines screen orientation. Even though the iPad isn't moving (relative to us on the Earth), the accelerometer registers the 1G acceleration downward and therefore knows which way is "down".
Raffy.Roma
Rome (Italy)
If the brake lights are driven by an accelerometer I think that there is no risk of having them on at constant speed even though the car is in a regeneration phase.
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In my opinion this is not fair. I hope that in Europe the brake lights will activate also during regen.
This would be bad, and less safe. Hopefully Tesla is trying to get a waiver (or modified legislation) for this.
Raffy.Roma
Rome (Italy)
Raffy.Roma
Rome (Italy)
Thank you. In autumn Tesla will state the features of Model S for EU deliveries. I hope that the brake lights of Model S will work as in the USA.
There's a thread on this here somewhere.
For my first couple of months, the regen brake lights were off. It was very scary. Continually getting tailgated as I slowed down. Tesla, at owners urging, re-evaluated the regs here and determined it was ok to have regen brake lights turned on here. It is just a (protected) software setting. I'm guessing UK is where this came from, as our regs here are based on UK, but not sure.
Raffy.Roma
Rome (Italy)
This is exactly what I wanted to know. So it's only a matter of software. Good to know it.
goyogi
Member
It would be simple to add a little software feature to add to one of the displays to let you know the brake lights are on. Alternatively it would be easy to program in software to regen the max possible without the brake lights coming on. My Volt regens on the go pedal but the brake lights don't come on. It's not as aggressive as the S and I don't think the S is that aggressive. I can live easily if the S slows down like the Volt. I barely touch the brake pedal under normal driving conditions in it as I plan ahead.
I fail to see why you would need to see brake lights on a car going downhill, just because that's what normal ICE cars would do, even if they're going a steady speed or even actually positively accelerating.
I think it makes sense if the brake lights come on whenever you press the brake pedal or whenever the computer thinks it's necessary given the feedback from the accelerometer.
If you are going downhill at a steady speed, and you're not using the brakes, but the regen is holding you down from going faster, I don't think the brake lights should come on in this scenario. They should only come on when you're going downhill if you're actually slowing down, or if you are physically pushing the brake pedal.
I think it makes sense if the brake lights come on whenever you press the brake pedal or whenever the computer thinks it's necessary given the feedback from the accelerometer.
If you are going downhill at a steady speed, and you're not using the brakes, but the regen is holding you down from going faster, I don't think the brake lights should come on in this scenario. They should only come on when you're going downhill if you're actually slowing down, or if you are physically pushing the brake pedal.
Raffy.Roma
Rome (Italy)
It's not how it works in the Roadster. Would be good to verify in the Model S.
I think it's useful for them to show in this scenario. Not all cars coast the same way, and not everyone rides their brakes downhill. I'd rather have the indication. Really, just better safe than sorry. Without the lights, I'm not sure what's going on. With them on, I know what's going on, but I can make a choice about how to process that information.
I think it's useful for them to show in this scenario. Not all cars coast the same way, and not everyone rides their brakes downhill. I'd rather have the indication. Really, just better safe than sorry. Without the lights, I'm not sure what's going on. With them on, I know what's going on, but I can make a choice about how to process that information.
I can't source it at the moment, but Tesla announced early on that the brake lights will work differently on the Model S than the Roadster. It's all about the gyroscope!
Todd Burch
14-Year Member
With all this talk, we're glossing over why there's a need for an accelerometer at all. All of the vehicle speed information is digital already...why would an accelerometer be needed? I wonder why they couldn't just measure the change in the speedometer value over a brief time and use that as an accelerometer. That instantly eliminates a piece of hardware, drops cost, and is more reliable (both mechanically and because you don't have to worry about issues like going down a steep hill in which the forward component of the gravity vector needs to be subtracted out).
Am I missing something obvious? Obviously if your wheels are spinning this isn't an accurate technique, but with the Model S traction control, when are your wheels spinning?
Am I missing something obvious? Obviously if your wheels are spinning this isn't an accurate technique, but with the Model S traction control, when are your wheels spinning?
mnx
2018 M3P
If that brief time is say, 1/2 second then they'd be a 1/2 second lag on your bake lights turning on. It needs to be instantaneous.
I think it is unsafe for the car to light up the brake lights if all you are doing is holding steady speed downhill. If you then suddenly mash on the brakes, no additional lights are show to indicate you started to slow down, and you risk getting hit from behind.
I definately want my brake lights to only light up when I'm slowing down at a significant rate using regen or using the brake pedal.
Todd Burch
14-Year Member
Agree...but it would be easy to check the acceleration every 100 ms, 50 ms, or even faster. Perhaps they don't want such a safety item to be based on software? (This reminds me of my initial concern over fly-by-wire technology, where flight control surfaces are controlled via wiring instead of a physical connection--until you really think about it and recognize that a physical connection is more unreliable).
mnx
2018 M3P
Agree, I guess you could check the speed 100 times or more a second, a 10ms lag wouldn't cause an issue i wouldn't think... (especially since regen isn't that heavy on the Model S to begin with).
