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View Full Version : Rear seat belts-why no pre-tensioners or load limiters


jatibaik
03-16-2011, 02:27 AM
I was ready to buy a GT and then saw with my family that the rear seats do not have pre-tensioners or load limiters. Other brands which are also promoting safety, normally have pre-tensioners in all three rear postions. A few only have these in the outboard locations with load limiting only for the centre seat.

Can anyone tell me please why BMW do not fit what Mercedes, Volvo, Renault, Volkswagen, Skoda, Citroen and others think and do fit as standard?

Is this cost cutting by BMW?

Thanks,

Semua baik

CC Brown
03-16-2011, 07:44 AM
My rear seat belts are like the front seat belts. Don't know what you saw?

11gt535
03-16-2011, 11:51 AM
Almost all seatbelts in every model car should have inertial reels that lock on sudden forces. This it the effect that you can test yourself. Pre-tensioners are used in airbag installations to buy time for the airbag to deploy fully before you hit it. Load limiters are the opposite, they spool out some slack to decrease deceleration force over time.

Rear seatbelts equipped with pre-tensioners without load limiters, like Volvo has, are a really strange because they may increase deceleration forces (and injury) versus an inertial reel seatbelt if you follow the entire premise of the design. Mercedes has only load limiters, which makes sense since there is no airbag from which to prevent impact and have the potential to decrease deceleration forces.

Whether any of these features are effective is a good question. Studies have consistently had to pick through thousands of cases to show a statistical improvement in injuries in airbag versus no airbag when seatbelts are used. Even the use of child seats for children above 2 has very little statistical backing when you compare injuries, and that's become a legal requirement in many places.

I'm not saying it's right or wrong, good or bad... if you think these features are essential I think you're doing a good job being an informed shopper.

jatibaik
03-16-2011, 01:44 PM
Thanks for the post but I am not sure if I really should be saying that to CCBrown. A very strange post.

The pre-tensioners are used to take up any slack a wearer may have in the inertia reel belt. They also pull the person back into position before the accident is experienced so that there is maximum crush distance for that passenger as the belt stretches and absorbs energy and so if the energy is high the load limiters may release the seat belt without impact into the seat in front. Mercedes does fit pre-tensioners in the rear and I have owned one car with these.

Why some OEMs do not fit into the middle of the rear seat is packaging issues and some have the value equation that the middle rear passenger does not have a seat and head restraint immediately in front of them so there is more crush room. In addition the middle seat is in many cars not used as often. I think one can see the real value an OEM puts on safety when the rear middle seat does have have pre-tensioners.

In the front some manufacturers use two pre-tensioners on each belt. One on the buckle to pull it back and one on the inertia reel to pull out the slack.

So given that major OEMs DO use pre-tensioners and/or load limiters for the rear seatbelts, why does BMW just use interia reel beltswith technology from 20 years ago in the rear? Even cheap little cars have load limiters in the rear.

Cheers

magnumforc
04-20-2011, 08:59 PM
Plus, in the US, the center rear is the standard child/baby seat location where the LATCH anchors are located so that seat position typically has no load limiters. Even our police vehicles had none, and were considered among the safest available. I don't know whether that's any indication of BMW thinking or saving 50 cents per vehicle or ???

chrischeung
12-17-2011, 02:24 PM
BMW fits rear pretensioners on the 7-series with optional rear comfort seats. You have to protect the CEO.