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My dear CAE Engineers,
Working many years with seating system and in the field of occupantsafety, I found that modelling of retractor in cae from received testing data, is a big challenge.
With this post, I try to open that black box and explain about that tiny subassembly used in all the cars . Of course, due to word limitation, I cannot explain this completely in a single post, so next few posts would be on this.
Retractor assembly is used in combination with seatbelt to protect the occupant in crash . This is always at one end of the seatbelt. A seatbelt has two ends, like any normal belt. It is routed through several parts, but main positions are D ring position and belt Buckle. The D-ring position is the shoulder upper position of the occupant at B-Pillar of the car. Belt buckle everybody knows. Shown in image.
The two ends of the belt are called, retractor end (of course, the end on which, the retractor is attached) and other end of the belt is called the end point (where belt is connected to the floor of the car ). So pls remember these terminology, used in the field of occupant safety and seat system development..
1. Retractor end
2. D ring
3. Belt Buckle
4. Belt End point
D ring and buckle-latch allow the belt to slip through it, of course with some normal friction coefficient (standard friction coefficient is 0.2 most of the time).
The retractor is fixed on B and C pillar of the car. B pillar fixation is for first row occupants and C pillar fixation is for second row occupants. Yes, for mid occupant in second row, most of the time the retractor is fitted on the back of seat . So principally, this mid dummy is stopped by the seat back, moving forward in the hashtag#frontcrash. But this is another topic to discuss.

This tiny assembly is hidden and coved with the beautiful interiors of the car. This small subassembly is till now purely a mechanical device work in combination with crash sensor. The retractor is designed or optimised for every car, based on front structure, bumper, seat, airbag and steering column designs. One must remember that the seatbelt along with retractor is a very important and crucial device, but alone it cannot save the human life rather kill the person in crash . It works in combination with other restraints systems. That’s why, it must be optimised with several CAE calculations. So, here we are, guys! 😊
This small mechanical device does basically two jobs..
1. Tighten the seatbelt.
2. Releases the seatbelt.
Yes, both functions are opposite to each other. They are carried out in following steps..
1. Locking the belt
2. Pretension
3. Load limiter
4. Spool
Sorry, running out with post limit. These functions will be explained in next posts, and I will also explain about the limitations of CAE solvers, while modelling this system. So, stay tuned for next posts.
Have a good start of the week.

Airbag is a very important safety device. It protects the occupant in all types of crashes like front and side crash. But the car manufacturer and airbag developers have the biggest challenge to control the explosion for protecting the human life.
It is a controlled explosion. If the explosion is not controlled, it can kill the person. A driver airbag releases a force of 7kN to 9 kN and a passenger airbag release the force of approximate 13kN to 15kN at the time of deployment. This force can vary depending on the airbag size and ventholes opening.
This force is sufficient to break the neck of a human if he comes in contact at the time of deployment. Instead of protecting, it can cause a serious head or neck injury and can kill the occupant.
The airbag producers are very careful while designing and developing it and make several tests to check the sensors and algorithms.
Despite of taking extra cares and several testing, the design error occurs. Here one can see a well-recognized, company like Takata must replace their airbags. At the time of deployment, the airbags are exploding when the car is exposed to heat and humidity for a longer time. This causes the casualty instead of saving the life in crash. The actual design problem is still unknown, but National Highway Traffic Safety Administration NHTSA believe a faulty seal on the inflator is causing the explosion uncontrolled and killing the occupants.
Approximate tens of millions of vehicles are under recall list of National Highway Traffic Safety Administration NHTSA. This is a very big set back to Takata as well to OEMs.
Here it is important to know that even, a correctly designed airbag also needs to be optimized by car manufacturer, based on the front and side structural stiffness of the car body. If the airbag is not optimized, even a good airbag cannot protect the occupant. As a occupant safety engineer, we allow the occupant to come in contact to fully deployed airbag at a specific time.
That’s the reason occupant safety has the maximum weightage in all the NCAPs.  
To know more about airbag optimization and BIW stiffness

My dear #cae engineers,
Something new to discuss today…
One of my trainees asked me, what happens in frontal crash, if a car doesn’t have front hood (bonnet) or it has a very small bonnet?
First, we see that how does a bonnet deforms in front crash and absorb the crash energy?
As shown In image1, the car has enough space in front to deform and absorb the KE. The front of the car (bonnet, front cross member, and side member in chassis) is designed in such a way, that they must deform.

In image2, the car doesn’t have any front hood. Take an example of a typical van. In this situation, there is nothing to protect the occupant cabin. In absence of bonnet, the deformations reach easily to occupant. So, no chance of survival of the occupants.

Then question arises, how to design a safe car without any bonnet or front hood? The answer will surprise you.
To answer this, I take an example of a popular car of #germany named #smart (for two) developed by #daimler . This car has a very small bonnet or almost no bonnet. The car is only 9 feet long and weighing approx. 1124 kg.
A car “without bonnet” is designed in such a way, that the opposite car must absorb the energy and make this “without bonnet car” safe. Interesting!
Now I explain you- The car (without bonnet) is designed like a very hard nutshell with high strength steel, so they do not deform in crash (The structure shown with blue color in image4). That’s the reason this small baby of #daimler is costly.
If they do not deform in crash, then who is going to absorb the energy? Yes, the crash object of other side. The #Smartfortwo, designed by #daimler works on this principle. A really challenging design developed by #daimler . Appreciable!
You can check in image3 if a #smart car (for two) crashes with a big car like S-class of Mercedes-Benz AG, the maximum energy is absorbed by the opposite car, not by this baby. You can observe that the deformation in S-class structure is more in compared to Smart. That’s how it has been designed.
But one can ask, what happens when 2 smart cars crash with each other, or this car crashes with a rigid structure? This we will discuss on next week..
till that…Stay tuned! Enjoy the day!

In sequence of our discussion on small cars, I raised a question in my last articles, what happens with the occupants, if the BIW of small car #smart is designed as a hard nut shell?
https://lnkd.in/dXiN8zA9
To answer this, we shall recall some of our basic physics fundamentals learnt in class 12th physics.
Let’s take an example of a car collision with a truck. There are some basic assumptions here..
1.      Mass of the both drivers are same and i.e. “md”
2.      Mass of the truck is M and mass of the car is m. The M > m.
3.      The both vehicles are travelling with same speed (v) opposite to each other.
As both moving bodies are colliding so we apply a Newton’s third law-
Thus both the vehicles are applying same force on each other and that is F.
So                       F (truck) = F (car)   ——— eq(1)
The collision time is Δt . This must be same for both the vehicles.
We can multiply both the side of eq(1) with Δt.
So                     F(truck) . Δt = F (car) . Δt  ————eq(2)
The above eq(2) is called the impulse. The impulse is equal to change in momentum.
So                    M. [Δv (truck)] = m. [Δ v (car)]       —–eq (3)
To balance the above equation (3), the change in velocity of car Δv(car) must be greater than change in velocity of Δv(truck).
So                  [Δv (car)] > [Δv (truck)]           ———- eq (4)
If we multiply both side of the equation (4) with the mass of driver (md) then we get-
                     (md) . [Δv (car)] > (md). [Δv (truck)]  —eq (5)
So we can see from equation (5) clearly that change in momentum of the car driver is more than change in momentum of the truck driver. Thus car driver will be experiencing more force.
This exactly happens in the high stiffness car body, The driver of the high stiffness car body will be experiencing more force than the deformable car structure (with bonnet car).
Actually the hard nut shell car body is not deforming at all, so the change in momentum of the car will be more. Thus driver will be experiencing more force. This could be more challenging, if this hard nutshell collide with a rigid structure. That we have already seen with a test video in my last post.
In this case the restraint system has to work more efficiently to minimize the high force experiencing by the occupants. That’s why the design of the #smart car and minimizing the injury values of occupants were the big challenge for Mercedes-Benz AG
This small baby explains well the engineering and basic physics.
So a stiff car body with unoptimized restraint system is not safe and can never achieve a 5star ratings. Hope the things are clear now.
Wish all of you a happy and prosperous happy new year 2023.
Stay tuned with some more new post….
Automotive Research Association of India (ARAI) SAEINDIA ICAT-International Centre for Automotive Technology