Looking through the members cars here I have seen alot of people with bad installs of mulit-point harnesses so I decided to go get a little info for you all to maybe save some people the trouble of learning the hard way.
Harnesses have their own drawbacks. First are roll-over concerns. You should NEVER, EVER use any 4, 5, or 6-point harness in your vehicle without a roll bar. In the event of a rollover a multi-point harness will hold your body upright as the roof collapses on your head. At a minimum you should use a good 4-point roll-cage to help prevent this from happening. The stock 3-point harness, however, does not suffer from this problem. During a rollover accident it will allow your body to move downards with the roof in most cases, helping to prevent crippling neck and spinal injuries.
Secondly multi-point harnesses must be installed correctly in order to be effective. The shoulder harnesses should be attached no lower than six-inches below the shoulder line, and should be at no more than a 45-degree angle from the top of the shoulder to the mounting point. Ideally the belt should be mounted evenly with the top of the shoulder line. Additionally you want some length of seat-belt material between you and the mounting point for adjustability and proper effectiveness. Too much material though can cause other problems.
There are two reasons for these stipulations. First, seatbelts by design are intended to stretch in an impact. This allows your body to slow down gradually in a collision. Too much material between the mounting point can cause an excessive amount of slack after they stretch, allowing your body to escape from the belts. When belts are mounted to the rear seat area of the vehicle, the large amount of material between the mount point and the driver can suffer from this problem. This could allow the body to come into contact with interior parts of the car, like the steering wheel or dash board, or even dislodge completely from the belt.
Secondly by making sure the belts are even with or slightly below the shoulder line ensures proper effectiveness. If the belts are mounted too low they can cause spinal compression injuries in a frontal crash. As the body moves forward the belts pull downward on the shoulders, compressing the spine. This can cause compression fractures in the vertebrae, herniated disks, or even break the spine.
