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No brake llights above 136 kph (85 mph) while using regen braking
- Thread starter slcuervo
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- Driving Dynamics Model S
gelden
Well-Known Member
I generally agree that the brake light is pretty sensitive and probably illuminates in situations where I figure it isn't strictly needed. There are moments when I think the amount of slowdown is much less than a comparable manual gearbox car would experience and the lights probably need not light. This is why the discussion of whether or not the behavior above 85mph is correct or not is crazy talk. The amount of decelleration at that speed is far, far more than the level where brake lights are triggered in other circumstances. I had a chance to drive on an empty stretch of highway this weekend in the wee hours and I'd say that the braking is much more abrubt from 100-85 than it is from 85-70 -- it would make no sense whatsoever to only illuminate the light at 85. I'm convinced this is a software glitch and presumably Tesla will correct it in a future release.
So let me ask you this, you're concerned about being on the autobahn above 85 MPH. Ok, so now let's say you expect traffic in front of you to be cruising along in the left lane at 150 MPH. The car in front of you (and this could be 100+ car lengths) slams on the brakes and decellerates at 1.5+G's. You should do the same, no? OK. Now the car in front of you lets of the accelerator and slows down from 150 MPH to 140 MPH. He's 100+ car lengths away, a distance which it is difficult to determine his actual speed or rate of deceleration. Do you slam on your brakes? Well if it was any normal car (brakes came on), you probably should. If it was a Tesla with your rules, who knows...
The bottom line is brake lights may you "feel" safe, whereas brake lights coming on on the highway when you aren't emergency braking is less safe for _everyone_ behind you and is an anti-social behavior.
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Do you also think that the lights shouldn't come on at 80mph? Because I can't imagine a basis for thinking they should come on at 80mph but not 85mph.
Car lengths was a red herring, it's actually irrelevant if you're maintaining greater than minimum safe distance.
Well, I was not after the threshold kW/speed thing (I don´t think I´m expressing this very scientifically, but I guess I understood you and you understand me).
What I was after is comparing the deceleration levels above and below 85 mph, to try to see if there was a reason there for not having brake lights. And as you can see on the table, deceleration above 85 mph can be as strong as below (example: 150 to 140 kph VS. 100 to 90 kph)...
| Initial Speed (kph) | Final Speed (kph) | Elapsed time (sec) | Deceleration (m/s) |
| 170 | 160 | 2,527 | -1,1 |
| 160 | 150 | 2,551 | -1,1 |
| 150 | 140 | 2,304 | -1,2 |
| 140 | 130 | 2,314 | -1,2 |
| 130 | 120 | 2,862 | -1 |
| 120 | 110 | 2,409 | -1,15 |
| 110 | 100 | 2,001 | -1,39 |
| 100 | 90 | 2,229 | -1,25 |
| 80 | 70 | 2,174 | -1,28 |
I don't think they should come on with small amount of deceleration at highway speeds at all.
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The algorithm is likely using the contribution of regen to deceleration as part of the threshold, at high speeds atmospheric drag should exceed regen, and keep in mind the Model S basically has the lowest drag coefficient of any car on the road.
If you lift off the pedal completely, would you describe the resulting deceleration as a "small amount?"
The faster you go, the less it is. Regen is fixed power, torque is inversely proportional to speed given fixed power, and torque is directly deceleration force.
If you check my chart, you'll see that I have measured very similar deceleration rates at 170 kph and at 100 kph...
Not exactly the same, but very very similar.
We're falling into the technical details again, and in the "do I like brake lights with regen or not" debate again. And that's not really the point here.
I have proven that the Model S deceleration is greater than almost all ICE cars and that it is almost the same at 100 mph than at 60 mph. So to me the picture is very clear: bug, SW glitch, or call it what you want. But something is wrong and needs a fix.
Andyw2100
Well-Known Member
Is the deceleration significantly weaker than Model S vehicles with autopilot that do, in fact, show the brake lights at speeds higher than 85MPH? Assuming the answer is no, and I believe it is, then slcuervo has helped the community, by identifying this issue, and he deserves an answer from Tesla.
CC vs. TACC - 5mph behavior difference. Just another example.
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In a previous (non-Tesla, non-regen) car you've had, when you let off the accelerator a bit -- even more than just to make it "coast" (maintain speed) -- such that it slows down ever so slightly. Would you want that vehicle to put the brake lights on? Probably not.
My interpretation of what some are saying is that they think there is some additional tuning warranted, not that they don't want brake lights to ever come on while regen braking.
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