Hard braking with TACC in stop-n-go traffic

[sevenout]

Has anyone else experienced hard braking by TACC during stop-n-go traffic? For me, this started with 2017.34 and has persisted with 2017.40.1. As cars begin to slow to a stop, the car will suddenly hard brake and come to a stop. I'm only going maybe 1-2 MPH when this happens but it is abrupt when it does. It does not happen every time either. Half of the time it comes to a nice smooth stop. Also, when it does hard break, I also hear a fairly loud machine gun like sound coming from the front of the car. Could this be ABS? Have never heard that sound before except when this happens. Not sure if I should schedule a service call.

 

[Tam]

...hard breaking by TACC during stop-n-go traffic...

Does this new behavior happens in same familiar old stretches or new roads that it never encounter before?

Would changing the Following Distance to maximum of 7 help?

"Machine gun" sound for braking doesn't sound good. I would have Service Center investigate.

 

[sevenout]

Does this new behavior happens in same familiar old stretches or new roads that it never encounter before?

Would changing the Following Distance to maximum of 7 help?

"Machine gun" sound for braking doesn't sound good. I would have Service Center investigate.

It’s probably been hard breaking for some time. At least since the .34 update. I don’t use TACC that much in town but since I just got the .40.1 update I’ve been playing with it more. I think I’ve started to get more tuned into the hard breaking because of the sound I’m getting when it happens. I noticed the sound the first time a few weeks ago on the freeway in a traffic jam. Both my wife and I asked what that was. We heard it several more times. I think I’ll call and make a SC appt.

 

[sevenout]

I’ve always used a follow distance of 1 since I don’t like people cutting in front of me. I’ll try it on a higher setting and see what happens.

The roads it’s happened on I’ve traveled quite often so nothing new there.

 

[boaterva]

Even 1 leaves a lot of space. I hate to think where 7 or 4 would leave you!

I’ve not had the hard braking except perhaps once. It may have been a timing thing where the car thought it was clear to accelerate and than said, dang, have to stop again!

 

[gregtopf]

I don't get the ABS kicking in, but definitely have noticed some inconsistent braking. I drive home from NYC daily and spend a good 60-90 minutes in stop and go. Used to be very smooth, but about 1/3 of the time the braking is hard enough to send my bag sliding off of the front seat. My following distance is set to 1 car length, but the average speed of traffic is under 5 MPH, so it's not like there is a sharp 60-0 deceleration.

 

[sevenout]

It may not be ABS but it is definitely making some kind of abnormal sound while braking. Weird that it does not happen every time. Tried to get it on video this morning but not enough traffic. Going to schedule a SC appt and see what they can find out.

 

[ucmndd]

I’ve had the same issue, stop/go traffic TACC engaging ABS on occasion in the recent firmware updates. Backing off the following distance even to “2” helped somewhat, “3” more or less made it go away.

 

[JohnnyG]

I can confirm that some time within the last couple firmware revisions, it has started hard braking 30-40% of the time when coming to a complete stop utilizing TACC/AP in stop-n-go traffic.

I'm pretty sensitive to the folks around me, and have found myself not using AP as much in stop-n-go traffic lately, due to the hard braking. I don't want to be conceived as the big ole d-bag that makes everyone behind him slam on their brakes. Or for them to think I'm 'brake checking' them.

I usually leave my following distance set between 1-3. I guess I can try extending it, but anything greater than that tends to cause other drivers to become grumpy.

 

[futurem3owner]

Same for me as well. Got to try TACC in stop and go traffic only a couple of times but during those times, it's the hard braking that annoys me and made me want to turn off the feature fairly quickly. It's not that the car is speeding up quickly and then braking suddenly. It's the opposite where it's slowly coasting to a stop and then the hard brake kicks in. I have my tailing set at 3 car lengths but even at lower distances, it still leaves a ton of gap and another annoying thing about that is other cars cutting in front. Still glad to have the TACC but hoping that future software updates will optimize it to make it more smoother.

 

[boaterva]

On 17.42 I have noticed ‘late braking’ where it seems to sometimes acquire the car in front late and then have to brake late to stop in time. That’s a somewhat different thing from having the target all the time and just deciding to slow at a closer point meaning harder braking. Have to see if 17.42.1 or anything down the road (lol) changes that.

 

[sevenout]

Have any of you also heard a loud noise coming from the front exactly at the time the hard braking happens? Mine sounds like it's coming from the front driver side wheel well. Like it's the brakes. I was able to get it recorded on video. I have a service appt on Wed to check it out.

 

[boaterva]

On the loaner I had, there was the ‘brake actuator’ release of the pedal which was a lot louder than on my car but you’re talking about something on the actual wheel, right?

 

[Cirrus MS100D]

Have any of you also heard a loud noise coming from the front exactly at the time the hard braking happens? Mine sounds like it's coming from the front driver side wheel well. Like it's the brakes. I was able to get it recorded on video. I have a service appt on Wed to check it out.

I’ve experienced exactly what you describe and I think it’s the brake pedal “juddering” (for lack of a better word.) It’s not ABS, but it sort of acts like ABS. Only happens when TACC is slowing a bit more aggressively than normal. Doesn’t happen all the time. I’ve experienced it on previous versions occasionally, but more often on 2017.40 compared to 2017.34

 

[ucmndd]

Have any of you also heard a loud noise coming from the front exactly at the time the hard braking happens? Mine sounds like it's coming from the front driver side wheel well. Like it's the brakes. I was able to get it recorded on video. I have a service appt on Wed to check it out.

Yes. I assumed it was ABS.

 

[R.S]

It may not be ABS but it is definitely making some kind of abnormal sound while braking. Weird that it does not happen every time. Tried to get it on video this morning but not enough traffic. Going to schedule a SC appt and see what they can find out.

How worn down are your tires it might be they lack traction? This sounds suspiciously like ABS and while it could surely also be something else, it might just be a worn down tire. Your tires should be too new to be worn down, but who knows? Some RWD cars had problems with wheel slip, maybe your D has issues on the front, too, or you just got a bad tire.

If you want to test it, do a emergency stop somewhere when nobody is around, does it sound the same? If it does, then check your tires. If those are still good, you might have an ABS sensor issue.

 

[mongo]

Perhaps TACC was updated to use the brakes more to prevent seizing of the slides/ hardware?

 

[sevenout]

On the loaner I had, there was the ‘brake actuator’ release of the pedal which was a lot louder than on my car but you’re talking about something on the actual wheel, right?

It sounds like it's coming from behind the dash down low like it's in the vicinity of the wheel well.

I’ve experienced exactly what you describe and I think it’s the brake pedal “juddering” (for lack of a better word.) It’s not ABS, but it sort of acts like ABS. Only happens when TACC is slowing a bit more aggressively than normal. Doesn’t happen all the time. I’ve experienced it on previous versions occasionally, but more often on 2017.40 compared to 2017.34

Yes, this is what I'm getting too. Can't say it's happening more with .40.1 than .34. I first heard (or became aware of) it when I had .34 and then a few days later got .40.1.

How worn down are your tires it might be they lack traction? This sounds suspiciously like ABS and while it could surely also be something else, it might just be a worn down tire. Your tires should be too new to be worn down, but who knows? Some RWD cars had problems with wheel slip, maybe your D has issues on the front, too, or you just got a bad tire.

If you want to test it, do a emergency stop somewhere when nobody is around, does it sound the same? If it does, then check your tires. If those are still good, you might have an ABS sensor issue.

The tires are brand new. The car is only 2 months old. I did try an emergency stop yesterday on my own and this sound did not occur. I think it's something with TACC specifically.

Perhaps TACC was updated to use the brakes more to prevent seizing of the slides/ hardware?

Maybe. I know it's TACC related since I can't reproduce it when I try an emergency stop on my own. Initially, I had always heard it when both TACC and AP were enabled. The last couple days I have been experimenting with TACC only and it still happens so AP is not needed to make it happen.

 

[JohnnyG]

On the loaner I had, there was the ‘brake actuator’ release of the pedal which was a lot louder than on my car but you’re talking about something on the actual wheel, right?

I’ve experienced exactly what you describe and I think it’s the brake pedal “juddering” (for lack of a better word.) It’s not ABS, but it sort of acts like ABS. Only happens when TACC is slowing a bit more aggressively than normal. Doesn’t happen all the time. I’ve experienced it on previous versions occasionally, but more often on 2017.40 compared to 2017.34

It sounds like it's coming from behind the dash down low like it's in the vicinity of the wheel well. I did try an emergency stop yesterday on my own and this sound did not occur. I think it's something with TACC specifically. I know it's TACC related since I can't reproduce it when I try an emergency stop on my own. Initially, I had always heard it when both TACC and AP were enabled. The last couple days I have been experimenting with TACC only and it still happens so AP is not needed to make it happen.

I'm inclined, based on all the info above, to think it's most likely the physical peddle engaging / disengaging rapidly, which hit's the stop hard when it releases and bounces against it, reverberating. My car will do this exact thing at times, and primarily at times it's moving slow and panic hard braking. It doesn't do it with your foot, because your foot doesn't release from the pedal as rapidly as the brake actuator does.

 

[sevenout]

I'm inclined, based on all the info above, to think it's most likely the physical peddle engaging / disengaging rapidly, which hit's the stop hard when it releases and bounces against it, reverberating. My car will do this exact thing at times, and primarily at times it's moving slow and panic hard braking. It doesn't do it with your foot, because your foot doesn't release from the pedal as rapidly as the brake actuator does.

I think, collectively, we have possibly found the reason for the noise and you have summed it all up nicely. When I'm at the SvC tomorrow I will discuss this theory with them and see what they say.