Rallye 3 ASM : The Rallye 3 is the choice restraint for anyone who's car will not work with a Rallye 4. This would be the case if your rear seat was too close to the front seats for optimum geometry. In any harness, you absolutely do not want to have a tailstrap that angles down more than 45 degrees from a line parallel to the ground. Past this angle, the shoulder belts will not work the way they are designed to. In some cases, the Rallye 3 will offer the right solution.
There are two main reasons why the Schroth Belts are the only safe aftermarket harnesses: ASM technology and the ability to install the belts to stock harness mounting locations.
ASM stands for Anti-SubMarining. It's an extra flap of material sewn into the inboard shoulder belt that prevents you from sliding underneath the lap belt. In a 4 point harness made by some other company, the two shoulder straps restrain your upper body equally. They keep your chest from moving forward. However, this also pulls up on the lap belt and allows your accelerating pelvis to slide under the lap belt and cause serious damage to your stomach and intestines. With ASM, one of the shoulder belts will elongate at a different rate which will force your pelvis down into the seat cushion. Upon rebound, you will be placed back in an upright position with the belt correctly placed over your body.
The advantages of being able to mount an aftermarket 4 point harness to stock mounting locations are many.
First and foremost is the knowledge that these points have been load tested. This means that the manufacturer of your vehicle engineered the points to be able to withstand some truly amazing forces for a specified length of time. In plain English this means that the bolts won't pull out of the sheet metal.
It also means (in the case of a Schroth Harness) that you can retain the use of your stock 3-point harness, allowing you the use of the backseat of your car. If you have the need to transport passengers (or cargo) in your backseat, it is possible to disconnect the tailstrap (a feature only Schroth has) and use your normal 3 point harness.
To View the SPS Installation Guide for Schroth Harnesses