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I have a question about the mounting points...the lap part is pretty obvious as to where you stick those, but what about the shoulder part? My car came with a Sabelt setup which bolts behind the rear seat - but then you lose that seat in terms of seating capacity. Is it possible to mount the shoulder bolts closer, like at the back of the rear passenger footwell? This will increase the break angle of the shoulder belts from about 90 degrees to 150-160 degrees. Can't do the physics in my head - will such a mounting position seriously compromise the strength of the rear bolts?
 

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I wouldn't. In a heavy forward impact, with the shoulder belts fastened in the rear either on the C pillar or sometimes at the base of the rear seats then the belts tighten as you fly forward and the primary reaction is an opposing horizontal force that counters your body's forward movement on impact. If you fasten the shoulder belts to the rear footwell when your body flies forward and the shoulder straps go tight they will be effectively pulling downwards rather than horizontally. The immediate impact will send a massive force downwards on the back of the seat through the belt holes, probably crushing the seat and then the same force is applied to your back. Isn't gonna be good for your spine!

If you really want harnesses where you can still use the rear seats then look at Safety Devices - they sell the Scroth inertia reel harness, which I've got in my R32. These are 3 point (two fastenings either end of the lap belt and the shoulder straps join together and fasten to the C pillar. The C pillar fastening point has an electically operated inertia reel, just like a seatbelt, so you can lean forward to change radio etc but when the sensor detects acceleration, deceleration or lateral movement of any kind it locks the inertia reel, keeping your harness nice and tight (just like when you yank a seatbelt forward). Plus the shoulder straps have a detatchable buckle, meaning that you can leave the factory belts fitted and if someone uses the rear seat you can unclip the harness in 2 seconds, the part fastened to the C pillar retracts back into the inertia reel fitting, leaving the rear seat unobstructed for use - you just revert to the factory belts.

If your car has rear seats with belts fitted then the rear belt fitting on the C pillar should always be a safe bet for mounting the harness rear straps. Some cars (but not all) are ok if you fit them to the base of the rear seats (where the lap fastening sits). But not all cars are safe for this so best to check with your harness manufacturer. Even if they ARE safe they will not be as effective as if you use the C pillar. You want to ensure that the webbing that goes into the back of the car from your shoulders is kept as horizontal as possible. This will make the harness most effective in a bad head-on collision. The further away from horizontal those straps are, the less effective the harness IMHO.

Another tip - always tighten the LAP belt before the shoulder straps, and I always keep the lap belt TIGHTER than the shoulder straps. In a bad head-on, you want the lap belt to pull tight first, THEN the shoulder straps. This will mean that as the lap belt goes tight, it pulls the shoulder straps flat against your torso. That's good. The other way round will result in the shoulder straps going tight first, and with any slack in the lap belt the shoulder straps will then pull the lap belt upwards and into your soft abdomen. It is then the lap belt disipating your kinetic energy upwards into your organs (stomach, spleen, etc.) rather than the lap belt holding you in place and the shoulder straps disipating the energy across the much larger, stronger area of your chest. For this reason, serious racing cars always have a crotch strap that goes down through the seat - this completely stops any upwards movement of the lap belt. It's uncomfortable and inconvenient but it's a life-saver in some situations.
 
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