Educated opinions please: Are seats in rear crumple zones unsafe?

[dotancohen]

I hope that I'm posting in the right forum, if not then I would appreciate if a mod would move the thread. Thanks.

Many minivan-type vehicles have a third row of seats behind the rear wheel. I understand that this is the crumple zone of the vehicle, i.e., this is the area that is specifically designed to deform in a collision. Are these seats unsafe? Where could I find more information?

Thanks!

 

[Mech_Engineer]

Vehicles do have crumple zones in the front and rear, but they don't take up the entire rear of the car behind the axle. Usually a crumple zone is a section of the frame/unibody that is around a foot long which is specifically designed to absorb energy.

 

[dotancohen]

Thanks, Mech. One foot is enough for a crumple zone? I thought that the longer the crumple zone, the more time is bought to decelerate the vehicle. Assuming M=1500 Kg, V=90 Km/s (1 foot~30cm):
F = m*a = 1500*a = 1500*(90^2 / 2*0.3) = 2*10^8 N
That's a lot of force!

 

[Mech_Engineer]

You're right, a crumple zone has to absorb a lot of force and energy. The crumple zone doesn't necessarily absorb all of the force from a collision at highway speeds for example, but it can help reduce energy transferred to the passengers in slow to moderate collisions.

 

[john.phillip]

I am not sure about what you are trying to calculate, but the velocity term should be in [m/s] instead of [km/s] or [kph] to keep units consistent.

 

[dotancohen]

Thanks, John, it's been a while since I've left university!

However, I was not looking to get an exact number but rather the order of magnitude of the force applied a rear-end collision. One third of a meter seems a very short distance to absorb enough of the energy to really make a difference.

 

[AlephZero]

One third of a meter seems a very short distance to absorb enough of the energy to really make a difference.

In a typical rear end collision there are two crumple zones involved, and the front of one vehicle is usually "softer" than the back of the other.

In fact the back of a large vehicle may be designed not to crumple, to stop a small vehicle "submarining" underneath the large one, and the windscreen or the driver's head eventually forming the crumple zone.

 

[AlephZero]

Thanks, Mech. One foot is enough for a crumple zone? I thought that the longer the crumple zone, the more time is bought to decelerate the vehicle. Assuming M=1500 Kg, V=90 Km/s (1 foot~30cm):
F = m*a = 1500*a = 1500*(90^2 / 2*0.3) = 2*10^8 N
That's a lot of force!

My calculator says 2*10^7N. But that is still a very conservative (high) estimate, because real vehicles bounce when they collide with something, so not all the energy is absorbed in the impact.

 

[john.phillip]

dotancohen

I understand, but by working with the wrong units, the calculated force is 1 order of magnitude higher. By the way, even calculating with the wrong units, the answer should be 2x10^7 N instead of 2x10^8 N, which leads to a total error of two orders of magnitude higher.

I see that you are trying to estimate an average crushing force for a vehicle traveling at 90 km/h and coming to a sudden stop in 1 ft. This calculated average force will generate a deceleration of 100 g's, which is well above the maximum allowable deceleration of 40 g's and average of 20 g's on vehicle collisions, in order to protect the occupants. The crumple zone is designed to increase the duration of the collision, thus reducing the impulse. Impulse is Imp = mass x velocity and collision time will be coltime = Imp / Force.

Currently, the required frontal offset impact test is performed at 64 km/h (17.8 m/s) against a deformable barrier simulating a similar vehicle. Thus, considering the energy being dissipated over 1 ft (vehicle) + 1 ft (barrier) (0.6 m) will generate around 27 g's of average deceleration (17.8²/(2*0.6*9.81)).

So far, trying to answer the question "Are seats in rear crumple zones unsafe?", you can say no, if the impact energy is within the required design limits (offset impact at 64 km/h with a vehicle of same size), any other condition above this threshold will make increasingly unsafe to seat close to the crumple zones, but occupants seating on the other vehicle might also experience a deceleration above the allowable and might get severely injured even without being trapped by the collapsing structure, thus making airbags also highly desirable. There is also the probability factor guiding vehicle structural design, as most rear end collisions happen when both vehicles are going in the same direction with a difference in speed below 60 km/h.