Pen on Paper Pictures [PoPP 1]- Vehicle body structure
I am creating PoPP because of the risks involved when one posts pictures on blogs (copyright infringement maybe..) and also because it feels good to put Pen on Paper, especially if your an engineer and have to perform calculations on software. Some of these topics might sound complicated and nerdy but most of them comprise of stuff we use in our everyday lives. Understanding them could help you choose a better car or get better grades in University or just pass time. I’m writing today’s blog on vehicle body structures because I recently finished my Thesis in the very same field….so kind-of an expert.
Vehicle body structure and its behaviour during a crash.
Saves passenger’s lives during a crash.
Introduction of the topic
Back in the middle part of the 20th century, an engineer by the name of Béla Barényi started work at Daimler-Benz and invented the new field of ‘Passive safety’. Mr.Barényi was a prolific inventor with more than 2500 patents but the most popular among them were the ‘Safety Cell’ and the ‘Collapsable steering column’. Most engineers like inventing stuff, but to do it with an ideal of ‘saving lives’ is why I always look at Barényi as my hero. When you see a modern vehicle crash, you might observe that everything apart from the passenger cell is crushed to oblivion. This might give you the impression that the vehicle is not safe. But, this is where the paradox lies- The more Kinetic energy you absorb during a crash, the safer are my occupants. Imagine this, your vehicle hits the wall at 50 kph and comes to an abrupt stop. Do you think the passengers would also come to an abrupt stop? No! They keep moving at 50 kph. This is why we wear seat belts. Just saying….. :). So the aim is to slow-down the rate at which the passengers hit the airbag and also make sure the steering column collapses.
Buying a car ? Today’s cars come equipped with ‘Active safety’ systems. Google it and you’ll see that some of really nifty.
Description of the PoPP
Vehicles are usually conceptualised from the inside-out. What do I mean by this? First, I need to decide how many passengers my vehicle would be seating. (Or even weirder, how many wheels would my vehicle need to have?). Where is my engine going to be positioned? Where would I want the trunk/cargo space to lie? So you see, there are a ton of stuff that one needs to decide before it comes to the stage of Art-drawing contests. Automotive engineers need to decide on the passenger space inside the vehicle depending on the size of its occupants. Take a look at the drawing again. See that I’ve labelled the ‘Manikins’ (engineering lingo for dummys) with ‘95%male’ and ‘5% female’. 95% male denotes that the dummy is heavier and taller than 95% of human males. Notice that the rear seats seat only people of size 5% female (roughly 5 feet in height). A common example of this type of ‘package’ is the Audi TT.
Most of the frontal impact energy during a crash is taken-up by the frontal longitudinal members. These are the 2 longitudinal beams in the engine compartment. These beams aren’t desingned to break but are designed to ‘crumple’ (like paper)…well this actually depends on the material, for example, CFRP can absorb energy by breaking. But, you probably get the idea by now. Modern vehicles have such complex frontal crash absorbing structures that it almost looks like a maze. Electric vehicles are easier to design for taking-up a crash, because of the absence of complex mechanical parts in the front. An Internal Combustion Engine on the front is basically a solid block and one wouldn’t want it jutting into the passenger compartment during a crash. So, for an ICE vehicle, the vehicle body looks a bit different.
Again, just a simple googling of the words ‘crumple zone’ will open-up a whole new idea of how vehicles dissipate forces during a crash.
Earlier generation vehicles dissipated frontal crash forces along the side structures, like door beams for example. This was not a good philosophy because it resulted in the A-pillar (see pic) buckling and the roof structure would eventually give-way. So, crash force dissipation became a big topic in the late 20th century. Take a look at some of the youtube crash-test videos and look at how an older vehicle behaves in comparison to a modern vehicle during a crash (https://www.youtube.com/watch?v=joMK1WZjP7g). That didn’t look good, did it? Notice how the A-pillar buckled and the roof followed along with it? That means you have a bad frontal crash structure. But lets go easy on the engineers from the 50s, shall we? Who knows….in 60 years time, the current Tesla Model S might seem unsafe. The Tesla Model S body structure is my most favourite. Its a conventional build but they put-in a few extra beams in the front (mostly for driving dynamics) but some also for crash absorption. The modern Honda cars have re-defined the idea of ‘crash compatibility’ i.e. making sure crashes between vehicles of different sizes dont result in serious injuries. A heavier car certainly has an advantage in terms of safety of its occupants as compared to a smaller one. This is the reason why smaller cars like the Daimler Smart have a super stiff passenger cell. Another example: Mazda 3 uses Skyactiv technology and also has a ‘front crash prevention’ system….and the list of cars goes on.
Once one gets really into this topic, it is funny how fast one forgets that the passengers are the most important part of the vehicle. One starts thinking of ways to protect the passenger cell but then completely forgets about the passengers inside…haha. So, if your an engineer reading this and are part of the crash design team, hats-off to you because your already on the path to making the world safer. 2 billion ICE cars and a few million electric cars…imagine the change you could make as a mere design engineer.
Websites to read further:
1. Book H-point-The Fundamentals of Car Design & Packaging by S.Macey, R.Gilles and G.Wardle.
2. https://derpat.wordpress.com/2013/04/12/kfz-technik-die-moderne-sicherheitskarosserie/ (awesome blog on the evolution of vehicles and their technologies).
3. Wikipedia Béla Barényi for inspiration.
4. Youtube crash test videos. (good passtime too).