Belts and Harnesses
The expected restraining function of any seat belt or racing harness can only be achieved by optimized strap routing around and from the wearer’s body as well as optimized anchor point locations.
An occupant can be effectively restrained ONLY by load transfer through the hard points of the occupant’s body. The only accessible hard points are the following:
Thorax [chest] to a limited level only
Therefore, it is essential that strap routing be optimized and mounting points be in the correct location to keep the straps in place.
WHAT HAPPENS DURING A FRONTAL IMPACT
This data is based on an optimized installation with an upright seating position during dynamic testing. It simulates a 90° head on collision, utilizing a165 lb mass dummy, an impact velocity of 30 mph and a stopping distance of approximately 16 inches. Giving a maximum deceleration of 30 G’s.
The pelvic load, is expected to surpass 3,100 lb at each side. This will elongate the lap belt and compress the tissue on the pelvis. The pelvis will slide forward by 3”-4”.
The upper torso load is expected to surpass 1,550 lb on each strap. This will elongate the shoulder belts. The upper body will roll in, and the adjusters will move up the chest by approximately 8”.
Forward head trajectory will be up to 16”.
The pelvic movement combined with the shoulder belt forceswill load the anti-sub straps to more than 1,320 lb each in a 5- or 6-pointracing harness and can be intentionally higher in a F-type model where theanti-sub straps are routed rearwards.
In more reclined seating positions, the pelvic load will be reduced to approximately 2,000 lb since the seat pan, designed as a ramp, will take some of the load.
Therefore it is essential the seat pan is strong enough not to bend or even collapse under extreme loads. In any case, seats are a significant component of the safety system!
Lets go back and look at those numbers on the lap belt. In a 30mph impact with a 165 pound driver,the pelvic load on the lap belt is expected to surpass 3,100 pounds on EACH side! That’s a total of 6,200 pounds of pressure being put on the pelvic area of your body. And this is with a PROPERLY fitted and mounted 5 or 6 point harness!
If that lap belt is allowed to ride up off of the pelvic area, your internal organs are going to take the blunt of that 6,200 pound impact. They can NOT withstand that sort of punishment!
This is why properly fitted and mounted belts are so important! And it is also why that 5thor 6th point crotch strap is so important!
And the load on that anti-sub crotch strap is going to exceed 1,320 pounds of force simply trying to keep the lap belt in place.
Even with a properly fitted and mounted harness, the shoulder straps are going to elongate enough that your head is going to move forward by16 inches. This is NOT a defect in the design of the belts! In fact it is part of the safety features built into the belts! That stretching of the belts helps to take some of that deceleration load off the upper body. By manufacturing the straps out of materials that allow some stretching in the event of an impact,that 1,550 pound load being put on EACH of the shoulder straps allows the body to decelerate a little slower and with less force.
Lets just go back and look at these numbers one more time.
6,200 pounds on the lap belt. 3,100 pounds on the shoulder straps. And 1,320 pounds on the sub strap. That’s a total force of 10,720 pounds in a30mph impact on your 5 point harness!
Those numbers should point out the importance of the harness being properly and securely fastened to the chassis. If any one of those straps is allowed to move out of place on your body, the loads going into the soft tissue will be far higher then your body can withstand.
One common mistake in shoulder belt routing is to have the mounting point too low below the shoulders. This causes the belts to go up over the shoulders and then down to their mounting point.
This compresses the spine of the occupant and because of the extra length of the belts can allow that occupant to pendulum from side to side. So the driver will fall over to the side in place of staying centered and secure in the seat.
If the driver is allowed to move side to side in the seat they run the risk of taking much of the loads being put on the belts by contacting hard surfaces in the cockpit of the vehicle. That 3,100 pound load on the shoulder straps can easily become a 3,100pound blow to the side of the head if the driver is allowed to pendulum out of place.
Your helmet, and more importantly your brain can not withstand that sort of force to the head.