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2010 2011 BMW 5 Series Forum F10 BMW 5-Series (F10) Forums General 5-Series Sedan and Wagon (F10 / F11) Forum ACC Braking Question
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      06-21-2014, 08:51 PM   #1
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ACC Braking Question

Has anyone with ACC ever seen their car (using this feature) from behind during operation? I'm curious how braking is handled.

For instance, do the brake lights always light up under ALL braking conditions? I would imagine the answer is yes (for safety reasons), but I'm curious if anyone knows for sure. It made me wonder since sometimes the car is only slowing down 1 or 2 miles per hour to match the car in front, and it may need to do this off and on in traffic. Normal drivers may just lift off the gas and coast a bit, letting the distance between the car in front become slightly variable, but it avoids jumping on and off the brakes. Since ACC seems to more strictly maintain that distance, it uses the brakes often, even to compensate for 1 or 2 mph. If the brake lights are constantly on and off for these small adjustments, I wonder if it drives the party behind a bit nuts... as you'd probably appear to be a terrible driver off and on the brakes constantly.

Anyone with experience riding BEHIND their F10 with ACC?
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      06-22-2014, 01:34 AM   #2
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Good question
Problem is here in the uk i personally have not even-noticed another car with acc (other than my own) so it appears to be a rare option over here.
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      06-23-2014, 09:25 AM   #3
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It may call for an experiment, AP. I may see if I can get someone to drive the car with ACC on and ride behind them to check it out. It's more of a curiosity thing, but I know I get annoyed when I drive behind someone constantly tapping their brakes on the highway (for minor speed variations) - usually a sign they're either (a) following the car in front too closely, and/or (b) just a poor driver.
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      06-24-2014, 03:31 AM   #4
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I have ACC and have tested this with a follow-car and cellphone

The car acts just like it would if you operated the brakes. As soon as you feel the car brake the lights come on.

I've also been checking for my self when driving at night. You can often see the reflection of your brake light light up road signs behind the car and from my experience the brakes is activated almost every time you see the engine go into "regen" mode. (Blue efficient dynamics bar in the cluster)

So to sum it up... when you feel the car brake the lights come on. So in some situations the following car will see your brake lights a little more often than if you operated the car yourself. And yes... cars with ACC will eat the brakepads a little faster than cars without ACC.
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      06-24-2014, 06:26 PM   #5
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Quote:
Originally Posted by torcar View Post
I have ACC and have tested this with a follow-car and cellphone

The car acts just like it would if you operated the brakes. As soon as you feel the car brake the lights come on.

I've also been checking for my self when driving at night. You can often see the reflection of your brake light light up road signs behind the car and from my experience the brakes is activated almost every time you see the engine go into "regen" mode. (Blue efficient dynamics bar in the cluster)

So to sum it up... when you feel the car brake the lights come on. So in some situations the following car will see your brake lights a little more often than if you operated the car yourself. And yes... cars with ACC will eat the brakepads a little faster than cars without ACC.
This is great! Thank you for confirming how this works. I had a suspicion it had to work this way for safety reasons, if nothing else. I'm hesitant to use it in heavier traffic since I think it may drive the people behind me a bit crazy. Like I said, it taps the brakes quite frequently, even if just to slow a single mph or two. That must be pretty annoying from behind.
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      06-24-2014, 08:28 PM   #6
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I asked BMW about this.
The reply I got was that when the car regulates the speed by lifting the throttle, with the gears or coasting nothing happens.
But when the car applies the brakes, the brake lights come on as if you was pressing the brake pedal.
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      06-24-2014, 09:11 PM   #7
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Quote:
Originally Posted by Noggie View Post
I asked BMW about this.
The reply I got was that when the car regulates the speed by lifting the throttle, with the gears or coasting nothing happens.
But when the car applies the brakes, the brake lights come on as if you was pressing the brake pedal.
Hmm... interesting. I guess the car DOES coast sometimes to regulate speed, but it doesn't appear to really happen in traffic but rather during "wide open road" use where going down a slight incline or something with no obstacle ahead.
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      06-25-2014, 02:27 AM   #8
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Quote:
Originally Posted by ezmaass View Post
Hmm... interesting. I guess the car DOES coast sometimes to regulate speed, but it doesn't appear to really happen in traffic but rather during "wide open road" use where going down a slight incline or something with no obstacle ahead.
I use it pretty much all the time. from country roads to motorways. when my car has locked on to the car in front I feel it is using the throttle to regulate, and only the brakes if letting off the throttle is not enough to maintain the distance.
in slow traffic I guess the brakes are used a lot more, so I guess you are right about your assumption.

I asked BMW this question because before I had ACC, when I was coasting, or braking using a gear down an incline, I would tap the brake, or just put my foot on the brake to get the brake lights on without actually braking to inform others that I was slowing down.
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      06-25-2014, 05:58 AM   #9
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as we're talking about ACC , those of you who have this know that you can set 4 levels of distance from teh car in front. On my car it defaults to 3.
I've thought about this and on a dry road at say 60-70mph, I feel 4 bars is the closest I want to be to the car in front.

If the car in front were to suddenly 'slam' on their brakes, could you stop your car in time. I think yes but Its pretty close.

Anyone else had thoughts on this?

Always thought eh rule of thumb was (at least for a dry road) , "only a fool breaks teh 2 second rule. namely allow 2 seconds between your car and the one in front. ACC set to 4 bars seems to be just about that, 3 seems a bit close.

Asusming acc has the car in front 'radar locked' so to speak, if it did stop suddenly, I assume emergency break assist would kick in, not sure if it would be enough though. I think the 2014 models are improved in this respect.

It can be quite frightening when a car in front stops suddenly, the distance between you and it decreases very rapidly indeed.
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      06-25-2014, 09:52 AM   #10
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Quote:
Originally Posted by AP View Post
as we're talking about ACC , those of you who have this know that you can set 4 levels of distance from teh car in front. On my car it defaults to 3.
I've thought about this and on a dry road at say 60-70mph, I feel 4 bars is the closest I want to be to the car in front.

If the car in front were to suddenly 'slam' on their brakes, could you stop your car in time. I think yes but Its pretty close.

Anyone else had thoughts on this?

Always thought eh rule of thumb was (at least for a dry road) , "only a fool breaks teh 2 second rule. namely allow 2 seconds between your car and the one in front. ACC set to 4 bars seems to be just about that, 3 seems a bit close.

Asusming acc has the car in front 'radar locked' so to speak, if it did stop suddenly, I assume emergency break assist would kick in, not sure if it would be enough though. I think the 2014 models are improved in this respect.

It can be quite frightening when a car in front stops suddenly, the distance between you and it decreases very rapidly indeed.
Mine also defaults to 3 bars, my impression is that each bar is one second, at least on my car.
On motorways at 60mph I use 2 bars as with 3 the gap is so large that other cars constantly cut in. At 40-50 mph I use 3 or 4 bars.
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      06-25-2014, 10:56 AM   #11
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I was going to raise the point on the number of bars, as well, as this may heavily play into when coasting is used versus braking to regulate the speed. Perhaps the more bars (further distance) you're using, coasting is a more acceptable option for regulation of speed. The closer you are, perhaps braking is more necessary?

I tend to use only 1 or 2 bars. My car defaults to 4 bars, and the distance on the highway is tremendous. Even at 1 bar, it appears to be at least 2 seconds behind the car in-front. At 2 - 3 bars, it encourages a lot of people to cut-in... but this is in heavier traffic.

At 2 or more bars, I've never experienced an issue with emergency braking. The car seems to respond well and can bring the car to a complete stop - and take off again when the car in-front begins to move. However, with only 1 bar, I've experienced the collision warning system kick-in when using ACC under emergency braking at higher speeds. My understanding is that ACC can only apply "some portion" of the car's braking force - and it's quite substantial from what I've seen. However, in true emergency braking situations, under very heavy braking of the car in-front, the collision warning system has blinked and beeped at me, and I've responded by applying the brakes myself. Would ACC have stopped the car regardless? I don't know - and I didn't want to find out.

After re-reading the manual on the above subject, it appears that if the collision warning kicks on, you need to manually press the brake pedal - the car may not be able to apply enough braking force alone via ACC. I've had this scenario happen a handful of times, and each time I was able to safely stop without using all of the cars braking force - no squealing tires or ABS kicking-in, etc.

So, I'd probably conclude that more bars may allow the car to regulate speed more frequently via coasting versus braking, but less bars may cause the brakes to be used more heavily to more strictly maintain the smaller distance. Obviously, I don't know this for certain, but I may experiment with it to see. I also believe 1 - 4 bars are all completely safe, but clearly the 1 bar setting may result in situations as I've explained above, where collision warning kicks on and causes you to intervene.
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