Airbag deployment

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Tony444
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Airbag deployment

Post by Tony444 » Wed Dec 13, 2017 12:38 pm

Unfortunately I crashed the 240i yesterday into a wall, drivers side front corner took the brunt of the hit and it was a very heavy hit. Myself, the police and the recovery company were all very surprised that none of the airbags went off, like I said was a heavy hit. My head hit the side window and have a slight cut, I would estimate that I hit the wall at 20-25mph. Lost control when accelerating on a patch of black ice, car fished tailed, over corrected and that was it.
Insurance wanted the car at their repairers but the guy from recovery said that he started the car but there was no power to wither rear wheel so I told the insurance that I wanted at BMW if there could be a broken gearbox or prop-shaft etc
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Re: Airbag deployment

Post by godzilla » Thu Dec 14, 2017 3:16 pm

Ouch, sorry to hear that.
Very surprised the airbags didn't deploy too.
Hope you get back on the road soon and you didn't suffer any whiplash etc.
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NISFAN
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Airbag deployment

Post by NISFAN » Thu Dec 14, 2017 3:41 pm

Airbag deployment is a function of G sensor readings. Perhaps hitting at a 45 degree angle meant not enough G in either the X or the Y directions.
For example hit head on at 25mph = 25mph’s amount of force on the unidirectional longitudinal g sensor. Same speed but hitting the front corner at 45 degrees and the g sensor will read 25mph worth of force times 0.5 = 12.5mph worth of force.
Same for the side to side sensor.

When you say doesn’t drive, does it start? There is a battery failsafe that blows in the event of an accident.


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Tony444
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Re: Airbag deployment

Post by Tony444 » Thu Dec 14, 2017 6:56 pm

Well at the moment it’s looking like a write off according to BMW, i hit and cut my head on the B pillar, can’t understand how the car can be a total loss without the airbags firing
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Re: Airbag deployment

Post by markm240i » Thu Dec 14, 2017 7:16 pm

BMW's approach seems to be one of replace everything that could possibly be damaged, irrespective of whether it is.

Your insurance company will want at least 2 estimates and inspections before writing the vehicle off, and I would have thought that for such a large payout they would appoint a loss adjuster to handle the claim.
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Uncle Tupelo
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Re: Airbag deployment

Post by Uncle Tupelo » Thu Dec 14, 2017 7:19 pm

There are various ways a car could be totalled without air bags deploying - low speed, low G crush, fire, etc. Although going by your description, I suspect the explanation might be an airbag deployment fault. Airbags are supposed to deploy at 8-12 mph-ish against a solid object. I'm not quite in agreement with NISFAN's analysis (if I remeber my Applied Mechanics correctly, but it was some time ago. If you were moving at 25mph then stopped against a solid object, you are still going from 25mph to zero practically instantaneously (unless the car spins slightly and continues scraping against the wall), so the G force will be pretty much the same as hitting the wall head on (I think).
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Re: Airbag deployment

Post by Redwoody839 » Thu Dec 14, 2017 8:12 pm

Sorry to hear about your accident, hope you’re alright. Was this under the Manchester airport runway tunnels by any chance?


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Re: Airbag deployment

Post by Redwoody839 » Thu Dec 14, 2017 8:13 pm

Sorry to hear about your accident, hope you’re alright. Was this under the Manchester airport runway tunnels by any chance?


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Airbag deployment

Post by NISFAN » Thu Dec 14, 2017 9:29 pm

Uncle Tupelo wrote:There are various ways a car could be totalled without air bags deploying - low speed, low G crush, fire, etc. Although going by your description, I suspect the explanation might be an airbag deployment fault. Airbags are supposed to deploy at 8-12 mph-ish against a solid object. I'm not quite in agreement with NISFAN's analysis (if I remeber my Applied Mechanics correctly, but it was some time ago. If you were moving at 25mph then stopped against a solid object, you are still going from 25mph to zero practically instantaneously (unless the car spins slightly and continues scraping against the wall), so the G force will be pretty much the same as hitting the wall head on (I think).
The impact force is the same on the car, but the sensors that record impact g are aligned for front or side impacts. A 45 degree impact registers half on each. That make better sense?


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Uncle Tupelo
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Re: Airbag deployment

Post by Uncle Tupelo » Thu Dec 14, 2017 10:12 pm

NISFAN wrote:
Thu Dec 14, 2017 9:29 pm
The impact force is the same on the car, but the sensors that record impact g are aligned for front or side impacts. A 45 degree impact registers half on each. That make better sense?
I understand what you're saying, but I'm not sure it will always work like that. If the car was moving at 45 degrees towards the wall (ie. it was skidding and moving half forwards, half sideways), then your explanation holds. But if the car was heading straight forwards and hit the wall (albeit at an angle of 45 degrees), then all of the motion would be detected by the forward sensing device. It's all about the movement of the car relative to the sensors, not relative to the wall (I think).
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Re: Airbag deployment

Post by markm240i » Thu Dec 14, 2017 10:22 pm

Uncle Tupelo wrote:
Thu Dec 14, 2017 10:12 pm
NISFAN wrote:
Thu Dec 14, 2017 9:29 pm
The impact force is the same on the car, but the sensors that record impact g are aligned for front or side impacts. A 45 degree impact registers half on each. That make better sense?
I understand what you're saying, but I'm not sure it will always work like that. If the car was moving at 45 degrees towards the wall (ie. it was skidding and moving half forwards, half sideways), then your explanation holds. But if the car was heading straight forwards and hit the wall (albeit at an angle of 45 degrees), then all of the motion would be detected by the forward sensing device. It's all about the movement of the car relative to the sensors, not relative to the wall (I think).
I can't contribute at a technical level, but doesn't the fact that the OP states that he sustained a "slight cut to the side of his head" suggest that it wasn't as frontal a collision as maybe suggested?

I'm not doubting the OP, but recollection after a crash and the reality are often very different.
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Uncle Tupelo
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Re: Airbag deployment

Post by Uncle Tupelo » Thu Dec 14, 2017 10:29 pm

If his head hit the side window (as stated in the OP), doesn't that suggest that the car was skidding sideways, in which case, most of the movement would be relative to the side sensor which should have triggered the airbag? We're guessing based on a potentially unreliable recollection I suppose. And on black ice, there won't be tell-tale skidmarks.
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Re: Airbag deployment

Post by markm240i » Thu Dec 14, 2017 11:03 pm

Uncle Tupelo wrote:
Thu Dec 14, 2017 10:29 pm
If his head hit the side window (as stated in the OP), doesn't that suggest that the car was skidding sideways, in which case, most of the movement would be relative to the side sensor which should have triggered the airbag? We're guessing based on a potentially unreliable recollection I suppose. And on black ice, there won't be tell-tale skidmarks.
Yes I agree, and apologies for quoting from memory of the OP. I think the side impact airbag sensors require a bigger hit than a tank slapper impact, but as I said, (personal experience) it's easy to over estimate speed/etc as to what actually happens in an accident.
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