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MG MGF Technical - Wake up mr. Airbag!

I checked the red VVC a bot closer at my BMW dealer today. It looks like a normal VVC besides the detail the front is about 50cm shorter than it should be ;)

Probably ran into a truck, since the lowest part of the spoiler (foglight height) seemed rather intact but the radiator & (V shaped) bonnet are totally wrecked and pushed inside, right headlight is at front wheelhub level.

Now what surprised me is that the airbag(s) did NOT go off!

So my question is, how to check if the airbag really goes off without wrecking your car?!

Or are the airbag ignition sensors pretty low fitted, thus causing them not to go off when you 'shove' under a truck?

It takes a lot to set them off Dirk. Saves them hitting you in the face when not required.
e.g. When I crashed, pretty big initial hit, but on rear left corner, slamming me into seat, airbag did not go off.

As far as I know this applies only to those systems with single occupant air bag system, i.e. those where only one bag is in front of the front seat occupants, no SIPS.

The angle of impact has to be within 15 degrees of an imaginary line throught the vehicles centreline. The system trigger sensor is mounted in the centre of the vehicle, and requires that impact energy is transmitted through the vehicle frame to that point. (Clearly if energy reaches that point in sufficient quantity to trip the sensor then the same energy is likely to do occupant damage.)

Non complete impacts at the front where only part of the front structure is impacted will usually not involve sufficient energy to trip the system. It is quite possible for a vehicle to run under an object so that only a partial upper contact is made, without triggering. eg, impacts that commence half way up the bonnet, which leads to removal of the screen and decapitation of the occupants! Hence the legal requirments for large goods vehicles to have under run protection. (An air bag does little to prevent this!)

Testing of the airbags circuit continuity is possible, but to test the bag would have worked requires actual activation. This is done via a specific test procedure under specific controls. (You should wear ear defenders too!!)

Overall the inactivation of the airbags is a very common sight simply because the specific conditions did not reach the crtieria required. I have seen some spectacular fatal accidents where no airbag deployment has occured, but other parts of the SRS systems have. (belt tensioners)

Roger Parker


As you obviously know a lot about this could you please answer a question I have been having for a while:

Is it possible for the belt tensioners on the F to be activated independently from the airbag in the event of an accident? I am specifically thinking of the car rolling over as the tensioners would be useful in such a case. I remember a few such accidents reported on this BBS (eg Dieter) but there was no mention of the belt tensioners being activated.

Spyros Papageorghiou


recalling that sad night. I had two impacts on the roof, (right top and left top). The car did noz roll like a stunt just easy over bothe doors and the roff but tumbled after the first hit over the front.
One impact with that LHD car to the right top which wasn't so strong or may be the screenframe took most of the energy. But the second ... Oh guys... I hang in the belt just in the cambered direction of the belt and got a full impact on the roof left of my head. In that situation the soft top frame hit the roll bar from the left top side.
I clearly recall a strong 'stitch' in the backbone and thought (true) already .
'OK Dieter we are going to be bound to a wheelchair if that sh*t rolling will ever stop.'

Lucky me cause the guardian angel flew obviously as fast as I drove ;)

Anyway, my head also hang out of the broken side window in the mud. May if if the activated tensioners would have held me more downwards, but I really have no idea when they should have been activated.
Already while spinning on the wet motorway ? At the first 'lift' of the car ?

Surely a difficult situation for measuring whether they should be activated or not. I woun't get pulled back under hard braking.

Related to Dirks doubts on airbag activation. I can confirm the get. I sold the airbags and tensioners of the other MGF wreck to a guy whose got activated.

To get an impression on his accident:
A front impact and he required the chassis fixture of the subframe to be changed. These are the strong triangle parts just below the door hinges and they belong to the security cell of the car...

All around I guess additional to Rogs explanation that the required decelleration to activate the airbag only appears if the strong and hard security cell starts to get bended. So the weak front crunch zone is already fully crunched then.

Recommend on not trying it to see the airbags or belt tensioners work. :(

Dieter Koennecke

Err, forgot the spell check ;). I hope it can be understood though. Just leave out some words and change some letters.
Dieter Koennecke


Thanks for your comments. Actually, it was your description at the time of your unfortunate accident of your head covered in mud that got me worried. As I am a bit tall I would like to think that in the event of such an accident the tensioners will pull me a bit downward. Let see if Rog has any comments.

Spyros Papageorghiou

>> When I crashed, pretty big initial hit, but on rear left corner, slamming me into seat, airbag did not go off. <<

Just to add, Airbags will never get activated if the impact is from the rear, they are designed only to activate in response to a frontal impact.

Steve Childs

when I rolled, I was able after the first revolution to get my hands to my head and curl over (g-force too strong to lift arms first time). Seatbelt had locked, but did not tension. On the first roll, I was forced down into seat, after that I was OK, but had the same thought as Dieter - "when is this going to bloody stop!"
(takes it easier now:-)))

This thread was discussed between 29/06/2001 and 02/07/2001

MG MGF Technical index

This thread is from the archive. The Live MG MGF Technical BBS is active now.