Monthly Archives: May 2015
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 dampers and Dr.Amar Bose.
Vehicle dampers and Dr. Amar Bose
Comfort and safety for the passengers.
Introduction of the topic:
Lets assume that a vehicle body is rigidly connected to the wheels (no suspension). If your car travelling at 40 kph encounters a pot-hole, what do you think would happen? As a passenger, you’d feel a disturbance in the form of a upward jerk. If you were driving, your head would most likely dash against the steering wheel. Inorder to avoid this, you want the wheel to have some ‘travel’. With the spring between the wheel and the body, the vehicle is no longer a complete rigid system. The spring absorbs most of the kinetic energy generated as a result of the disturbance.
As suspension springs are designed according for the maximum load case (4 passengers+luggage), our spring absorbs more than enough of the kinetic energy and this usually makes the car bounce up and down.
Look at what happens to this car without a rear damper and see how it bounces up and down. That’s not so comfy and an Automotive engineer would always want the rear stable. https://www.youtube.com/watch?v=5Mr-UgWr8-s
Description of the PoPP:
Dampers are an additional mechanical component that stop the springs from bouncing too much i.e. they perform 1 function and that is absorbing the kinetic energy of the springs. Hence, the damping force is proportional to the vertical velocity of the wheel or the body and this is not so intuitive. Look at the equation I’ve written on paper. Notice that the resisting force provided by the damper depends on two terms, namely a ‘valve constant k’ and the ‘vertical velocity v’.
Vehicles are complicated. They show a particular behaviour when you drive straight (accelerating/braking), and behave differently when turning(left/right/skid). For example, when you brake, the car performs a ‘brake dive’. Brake dives are classified as a low speed disturbance and you would like in such instances that the front dampers avoid ‘pitching of the car‘. A car driving over a series of speed breakers is classified as a high speed disturbance. Unbelievable as it may seem, you want the damper to change behaviour according to the situation and speed of the disturbance (like a chameleon changes colour). This brings me to the great man Dr.Amar Bose. Bose corp., during the mid 80s researched mechanical spring-damper systems and found that such a system would always have its limits to adapt to different road conditions. (fluid flow causes lag)
I have always been fascinated by MIT Professors and their knowledge of applied engineering. But, after reading a short bio of Dr.Amar Bose, I came to the conclusion that all great thinkers have a child-like curiosity.
Such was that extent of Bose’s curiosity, that he invented noise-cancelling headphones during a single flight trip from city A to city B (who cares about airspace…..he did it). Read somewhere that he wrote down the equations during the flight and the moment he landed ordered the engineers to start working on a prototype. Inspiring boss!
The graphs that you see at the bottom of the paper show the difference between passive, semi-active and active dampers. Depending on whether your on ‘Sports Mode’ or ‘Sedan Mode’, the valves in the damper behave a certain way thereby making the damper stiffer or softer. Its most important to remember that dampers behave differently during spring compression than during spring expansion. This is because rebound damping has to just dissipate the stored spring energy. Compression damping has to control spring compression and also at the same time absorb some of the bump force itself. I’m still trying to get to grips with this difference.
Spring-Damper systems are incredibly complicated. For example, as an engineer you even have to take into account the differences in behaviour between your rear susp. system and front susp. system.
This is why most modern cars today use Semi-active and Active damper systems. You want the suspension to react as fast as possible and push against the disturbance.
An ideal car behaves similarly regardless of whether your turning or braking/accelr. Unfortunately, the two actions can’t co-exist with a mechanical suspension. Simply impossible. It took some geniuses at Bose corp. to create something out of nothing- Electromagnetic suspensions. That’s the thing about these eccentric CEOs (Musk,Bose). They somehow have to ability to look beyond the standards that exist. Its like a Mathematician ignoring the axioms that exist and formulating a hypothetical equation just for the fun of it. Let us try to think beyong what exists in the books.
Websites to read further:
1. Dr. Amar Bose bio- http://www.popsci.com/science/article/2013-07/curious-genius-amar-bose
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).