There are a lot of ways to have a dangerous impact in a race car.
1. One is to have a small object flying towards the driver.
...a. If it is very small, a helmet or canopy might stop it.
...b. If it is big, a roll cage might stop it.
2. Another is to collide with something large and heavy enough to slow the car down.
...a. A car with a very rigid frame might keep the object away from the driver.
.......But it will cause very high deceleration rate which may harm the driver.
...b. A car with a crushable frame might reduce the g forces on the driver.
.......But it may allow the intruding object to get to the driver.
At high speeds, you need a lot of distance to slow down or stop at safe levels of deceleration. So a crushable frame is preferred. The frame has to be designed to decelerate the car fairly smoothly. It doesn't do any good to have the first two feet crush with no resistance at all, and then have the last inch take the full brunt of the impact. If the driver decelerates at more than 100 g's, the chances of serious injury are high.
Assume that the vehicle provides fairly constant deceleration in a crash. The crush distance required increases as the square of the impact velocity. If you don't have enough crush distance for a given speed, your deceleration is going to be very high, regardless of the materials and design of the car. All the car can do is absorb energy in a smooth manner so as to give smooth deceleration.
Not showing all the math here.
Remember, this is just to limit the deceleration to 100 g's, which is going to be very painful. If you crash at 20MPH, the safety harness might give you enough crush distance (1.6 inches) to keep you safe, even if the car is completely rigid. At 80MPH, you will need two feet of crush distance. Hopefully, the car has that much, and absorbs energy smoothly over that distance. At 120MPH, you need five feet of crush. You might get that if you hit a tire wall or another car with a crushable frame. But it's getting iffy. Above 120MPH, you really don't want to decelerate to a stop in a crash. For example, at 200MPH, you need 13 feet of smooth deceleration. That is not likely.
What if a rigid part of the car, like the roll bar, strikes a stationary object? It will try to decelerate instantaneously to zero speed (not taking into account that it is slightly elastic). But it is attached to a 1600 pound car that doesn't want to stop instantaneously. The forces could be enough to tear the roll bar out of the car. So placing a roll cage around the driver will help if foreign objects are flying, but it won't help absorb energy in a big collision with a wall or car. Let's say you could build a roll bar that could withstand unlimited forces. The driver might decelerate at 1,000 g's and end up dead anyway. He will be crushed by the safety harness or the seat, or his brain will collide with his skull. You always want to provide some crush distance, no matter what angle the car collides. The problem with very high speeds is that you can't possibly provide enough crush distance.
If your body is being slowed down by the safety harness, you want your head to be slowing down by the same rate, or you could end up with spinal cord injuries. This is what the Hans device does. It stiffens the connection between your head and your torso. http://en.wikipedia.org/wiki/HANS_device But even the Hans device cannot withstand unlimited g forces. Some vendors make outrageous claims, like "Reduces Impact forces by 80%". No, all it does is keep the head aligned with the torso. The only thing that can reduce impact force is more smooth deceleration distance.
If you want to understand helmets, canopies, roll cages, Safer barriers, tire walls, etc., you need to understand crush distance.
Reply With Quote
Originally Posted by ndcrs
Bookmarks