Pen on Paper Pictures[PoPP 2]-Dampers
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