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      12-13-2018, 09:23 AM   #1
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What is the purpose of the seat belt tightening right after we start driving?

Searched the forum but couldn't find an specific answer. Anyone know the engineering or safety purpose of the apparent seat belt pre tensioning that happens as you first start to drive? Some call it a "tug". I know some systems do a special pre-tension upon an impact, but I have not experienced this feature in any other car so far. --Bob
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      12-13-2018, 10:31 AM   #2
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I believe it is a way for the Dynamic Safety system to preset the tension level for use in in the event of an impact.

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Dynamic Safety is the BMW ConnectedDrive preventive passenger protection system. The system is activated after the engine is started and the front seat belts are automatically slightly tensioned when the car starts moving. In a critical driving situation such as emergency braking, or if the vehicle demonstrates excessive understeer or oversteer, the front seat belts are pre-tensioned, the side windows and sun roof (if fitted) are closed and the passenger seat backrest is returned to the upright position (if fitted with electric seats). Reduced belt tensioning provides increased driving comfort. If a crash is unavoidable, the system brakes the car, without requiring intervention by the driver, until the vehicle reaches a standstill. This helps reduce the likelihood of a second or subsequent collision, and or reduces the effects thereof - or in the best case scenario could prevent a further collision altogether. The Attentiveness Assistant also recognises if the driver is displaying signs of increasing inattentiveness or tiredness. This system analyses the steering response and reacts to irregular driving behaviour by issuing a message in the Control Display suggesting that the driver should take a break and emitting an acoustic signal.

Benefits at a glance:
reduces risk of collision
increases safety through preventative measures
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      12-13-2018, 12:00 PM   #3
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Quote:
Originally Posted by Warp Ten View Post
Searched the forum but couldn't find an specific answer. Anyone know the engineering or safety purpose of the apparent seat belt pre tensioning that happens as you first start to drive? Some call it a "tug". I know some systems do a special pre-tension upon an impact, but I have not experienced this feature in any other car so far. --Bob
This „feature“ is annoying. I‘ve had the feature disabled.


Regards, Otis
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      12-13-2018, 06:19 PM   #4
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Quote:
Originally Posted by ksoze View Post
I believe it is a way for the Dynamic Safety system to preset the tension level for use in in the event of an impact.
Exactly. In emergency braking, I have experienced the belts tighten to this preset point, anticipating impact. My 2014 E350 was the first car I had which did this. It is a real safety improvement in an accident, designed to work with the airbags to improve your survivability. Having worked in several ER's, not sure why one would want to disable it.
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      12-13-2018, 10:56 PM   #5
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Originally Posted by hlothery View Post
Having worked in several ER's, not sure why one would want to disable it.
I so agree. The gentle tug as you drive off is reassuring to me. Sort of like the car saying, "I've got ya back" but everyone is different I guess
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      12-14-2018, 09:16 AM   #6
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Originally Posted by Symo View Post
I so agree. The gentle tug as you drive off is reassuring to me. Sort of like the car saying, "I've got ya back" but everyone is different I guess
Exactly.
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      12-14-2018, 10:06 AM   #7
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Thanks to all for the answers. Kind of like I assumed, a pre tensioning to prepare ahead of time for a possible event. I don't think I would want to disable the system but out of curiosity, how does one do it? --Bob
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      12-14-2018, 11:55 AM   #8
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Hi, does the pre-tensioning always adjust at the same level of strength? I noticed on my car, sometimes the pre-tensioning is tight, while at other times it's fairly weak. Just curious. Thanks for any input!
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      12-14-2018, 11:58 AM   #9
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Quote:
Originally Posted by Bill530e View Post
Hi, does the pre-tensioning always adjust at the same level of strength? I noticed on my car, sometimes the pre-tensioning is tight, while at other times it's fairly weak. Just curious. Thanks for any input!
I’ve noticed the same. Mine has been fairly week lately
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      12-14-2018, 12:11 PM   #10
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Quote:
Originally Posted by Bill530e View Post
Hi, does the pre-tensioning always adjust at the same level of strength? I noticed on my car, sometimes the pre-tensioning is tight, while at other times it's fairly weak. Just curious. Thanks for any input!
The car uses BMW's new Adaptive Body Calibration (ABC). It uses GPS data to note which restaurant you are at and adjusts the seat belt tension based on the type of food served. Italian establishments result in less tensioning and a salad place results in more tightening. Also, on the last Thursday of November, U.S. cars use maximum slack.

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      12-14-2018, 12:36 PM   #11
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Quote:
Originally Posted by ksoze View Post
The car uses BMW's new Adaptive Body Calibration (ABC). It uses GPS data to note which restaurant you are at and adjusts the seat belt tension based on the type of food served. Italian establishments result in less tensioning and a salad place results in more tightening. Also, on the last Thursday of November, U.S. cars use maximum slack.


I knew someone was gonna make an "eating behavior" joke! Actually, that thought did cross my mind. D'oh!

Glad I'm not the only one that's experienced this. Yeah, tried just about everything, leaning forward, sitting back, speeding up right away, etc...just never figured out why it feels like squeezing all the air out my lungs while other times it's barely a nudge.

Maybe I will try the salad approach! Thanks all!
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      12-14-2018, 12:36 PM   #12
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Quote:
Originally Posted by ksoze View Post
The car uses BMW's new Adaptive Body Calibration (ABC). It uses GPS data to note which restaurant you are at and adjusts the seat belt tension based on the type of food served. Italian establishments result in less tensioning and a salad place results in more tightening. Also, on the last Thursday of November, U.S. cars use maximum slack.

I heard that BMW was going to do this but I didn't know it was already in place. Must be one of those over the air updates I missed.
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      12-14-2018, 12:51 PM   #13
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Quote:
Originally Posted by ksoze View Post
Quote:
Originally Posted by Bill530e View Post
Hi, does the pre-tensioning always adjust at the same level of strength? I noticed on my car, sometimes the pre-tensioning is tight, while at other times it's fairly weak. Just curious. Thanks for any input!
The car uses BMW's new Adaptive Body Calibration (ABC). It uses GPS data to note which restaurant you are at and adjusts the seat belt tension based on the type of food served. Italian establishments result in less tensioning and a salad place results in more tightening. Also, on the last Thursday of November, U.S. cars use maximum slack.

LOL

The feature doesn't bother me but my wife hates it.

With that said, I'd never disable it because it is a good safety feature.
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      12-14-2018, 06:48 PM   #14
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Quote:
Originally Posted by ksoze View Post
The car uses BMW's new Adaptive Body Calibration (ABC). It uses GPS data to note which restaurant you are at and adjusts the seat belt tension based on the type of food served. Italian establishments result in less tensioning and a salad place results in more tightening. Also, on the last Thursday of November, U.S. cars use maximum slack.

It is not the last Thursday in November. It is the fourth Thursday. The upcoming LCI facelift will also allow breath sensing to adjust tension, and also to squirt out fragrance when needed.
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      12-15-2018, 07:58 AM   #15
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I like it. Kind of feels like my car giving me a hug every morning.
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      12-15-2018, 11:43 AM   #16
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Originally Posted by drjolokia View Post
LOL

The feature doesn't bother me but my wife hates it.

With that said, I'd never disable it because it is a good safety feature.
Yeah mine doesn't like it either. I am kind of indifferent and agree I certainly would not disable it if I even could. --Bob
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      12-17-2018, 02:36 PM   #17
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Its there to totally mess up the shirt / suit that was just pressed!
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      12-17-2018, 05:16 PM   #18
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Its there to totally mess up the shirt / suit that was just pressed!
Hey, man. You need Mizzen and Main.
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      12-17-2018, 06:12 PM   #19
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Hey, man. You need Mizzen and Main.
No kidding!
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      12-24-2018, 03:47 PM   #20
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Quote:
Originally Posted by hlothery View Post
Having worked in several ER's, not sure why one would want to disable it.
At least on my car, this pre-tension feature sometimes doesn't let go (maybe 1 out of 5 times, enough that a dealer will probably tell me it's normal). Then I have to struggle against it before I can move around normally. All of my other past vehicles have worked fine without this feature, so I'm willing to roll the dice if there's some way of stopping this truly obnoxious behavior.

I don't mind the "tug" when it lets go, but I can't stand it when it doesn't.
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      12-26-2018, 11:58 AM   #21
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Quote:
Originally Posted by djrobx View Post
At least on my car, this pre-tension feature sometimes doesn't let go (maybe 1 out of 5 times, enough that a dealer will probably tell me it's normal). Then I have to struggle against it before I can move around normally. All of my other past vehicles have worked fine without this feature, so I'm willing to roll the dice if there's some way of stopping this truly obnoxious behavior.

I don't mind the "tug" when it lets go, but I can't stand it when it doesn't.
Haven't had that experience. Mine just tugs once then the belt is loose. In fact, that is really what prompted me to raise this question in the first place; since it remains loose afterward, what is the point of the initial tug? Other cars don't do the tug yet pass all the same safety mandates. If yours doesn't release on occasion, I would think a dealer look would be a good idea. Even if they say it is normal, good to get your concern on record. It is an important part of the safety system after all and it sounds like it is not working entirely correctly. You could pose the issue by suggesting that if it not working in this regard, who knows if it will work in an accident? --Bob
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      12-26-2018, 12:12 PM   #22
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Quote:
Originally Posted by djrobx View Post
At least on my car, this pre-tension feature sometimes doesn't let go (maybe 1 out of 5 times, enough that a dealer will probably tell me it's normal). Then I have to struggle against it before I can move around normally. All of my other past vehicles have worked fine without this feature, so I'm willing to roll the dice if there's some way of stopping this truly obnoxious behavior.

I don't mind the "tug" when it lets go, but I can't stand it when it doesn't.
Mine, in both my cars (E350 and M550i) only does this when I accidentally tug against it while it is setting the calibration. If I do that, it sometimes does "stick" for a minute, only to release in a few seconds. If I anticipate it and sit still, it has never done that. I now try not to reach for something (or, etc) as I initially drive off. Might try that and see.
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