"Winch" bumpers, bull-bars and airbags

[Crispin]

Why do some replacement bumpers explicitly say they are airbag compatible. An airbag gets deployed when the computer decides it's having a crash based on massive deceleration in a specific direction.
If the sensors, which on the 120 are mounted just behind the plastic bumper, were crush sensors, I could understand the statement.

The only thing I can think is that a bull-bar or non-OEM bumper would alter the rate of deceleration for a very short time until the chassis rails hit the object. Would / could this be enough of an alteration? Hitting different objects would cause different rates of deceleration anyway.

 

[Chris]

Are we sure that they rely on being physically crushed or just registering impact through deceleration? I'm not up on these things so it is an innocent question.
If they don't deploy below a certain speed - then surely the speed is a factor? OK let's say you drive into a tree slowly and keep going until the bumper is crushed, would the bag go off? Or do you have to hit it fast and stop with a certain force? I know that they have a system of executive (or is it elective) voting so that they don't go off when you are off road, so that even more reinforces the speed related function.

Long way of asking - decelerometer or crush trigger?

Chris

 

[Rob]

definitely accelerometers as primary sensor, but a crush sensor would assist it probably. i think i read somewhere that certain types of custom made front tow bars could set the airbags off, might of been the elco list

EDIT: when i say crush sensor (no such thing), i mean a sensor which detects buckling, however that's done

 

[Chris]

Oh, like a strain gauge in reverse? Something that registers distortion, perhaps.

Chris

 

[Rob]

yeah something like that, although strain gauges measure strain so could be used indirectly to predict bucking of the beam when you exceed a certain value. you could use a load cell, but that would have to be in series with the structure so not a great idea. otherwise your left with contact sensors like potentiometers and LVDTs or non contact light / radar based sensors. as you can see none of these are ideal so that's why i think accelerometers would be used as the primary data acquisition method.

EDIT: strain gauges would probably experience a sudden drop in strain during buckling so may be difficult to know if the beam has buckled or not it might have experienced an impulse load

 

[Rob]

so back to the original question, if the bumper mounts are too stiff then the vehicle would experience a far greater deceleration earlier on in the collision which would result in the airbags being deployed prematurely, ie you reach the airbag after it deflates

 

[Jon Wildsmith]

AFAIK Crispin the air-bag compatible bumpers have been constructed and then tested to ensure they don't alter the deployment parameters of the SRS i.e. same deceleration / crush characteristics as OEM. I think on the ARB's for example the mounting brackets have a concertina section in them to collapse at the same rate as OEM. That's why they have the winch on a seperate mounting or using the winch would cause problems.

 

[Crispin]

Rob's point about sums it up I guess. As Jon said, as, if they have their own crumple zones in them, it allows the computer to make the correct assumptions.