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The physics behind the pandemonium of driving cars November 13, 2006

Posted by Ragesh G R in Physics and Maths, Uncategorized.
Tags: , , , , , , , , , , , ,

You watch a Grand Prix on a Sunday evening, and your mother exclaims, “What pleasure could you possibly derive from watching the same cars go round and round the same track for 2 hours?”.

But there are plenty of things to learn as we watch the cars go by. In fact even without watching it, just a little contemplation on your side is enough to understand the nuances of the dynamics of a vehicle as it goes through sweeping corners at break-neck speeds.

What you see in Grand Prix can be applied in real-life situations. Contrary to popular belief, Grand Prix races do not encourage perilous driving styles. In fact they encourage you to drive safer, better and more efficiently, being cogniscent of what your vehicle is doing.

The secret of the tyres

The silken smooth tyres of the Grand Prix cars and bikes might give an impression that they are very slippery but it’s not so. The friction produced between the tyre and the road is directly proportional to the area of contact, hence the smoother and flatter the tyre, the better the grip. This is because when two very smooth surfaces come into contact, the distance between them is so small that it is comparable to atomic distances and hence nuclear strong forces take over (The same reason you feel grip when you run your finger on a marble floor).

This theory has an exception though, during rains. Then if you use a slick (smooth) tyre, the water that is trapped between the tyre and the tarmac does not have any place to go and formes a layer of accumulated water between the tyre and the road, causing a phenomenon called “Aquaplaning” or “Hydroplaning”, in which the grip reduces drastically and you feel as if your car is skating on ice. Hence during rainy days, it is advisable to change to a tyre that has deep treads that have channels to push away the accumulated water through sides.

Going around corners

Ever thought why the Grand Prix cars seem to be weaving around the track, changing lanes, sticking to a specific line every time? There is a beautiful science behind it. In fact it is very useful in day today life. I use it often. But you must stay within your lane. Every corner has a specific line which will provide you the fastest way around it, called the “Groove” or the “Racing line”.

The simple theory behind it is the less steering input you give, the faster your vehicle can travel, and the more stable it will be. That is because when you turn the wheels, especially the back wheels are not exactly pointing in the direction the car actually goes. Hence every moment the back wheels point along the line of the tangent to the curve that you are describing. Hence there is a constant force or acceleration applied to the vehicle. A component of the force produced by the wheels is expended in over coming the frictional force. Hence the lesser force you have along the direction of the motion of the car.

So the best way to approach a corner is : Enter from the outside, cut through the inside (apex) of the corner and exit through the outside of the corner, as shown in the picture. Our aim is to describe an arc that has the least curvature or most radius of curvature and the least circumference as possible. Also this method puts less stress on your tyres, and less g – force on you, hence, longer life for the tyres and smoother ride for you.

Cutting the apex of the corner

If you follow the inside line through out, then you have to brake and reduce the speed, hence you use more time.

If you follow the outside line, still the curvature is high and the circumference too is high, hence reduced speed, longer distance, hence more time wasted.

The secret is not to perceive the road as a whole and blindly trying to go parallel to the edges of the road. The secret lies in being able to see the straightest path between two points. The curves in the road need not bother you. Why go in a zig-zag way around a series of corners if you can find the straightest path between them.

As the followig diagram depicts, though the road seems to be winding in a zig-zag way, there is a fairly straight path through the series of curves.

Driving Line

The slip-stream

Now, why do Grand Prix cars pull up behind the car in front and sling shot their way past to over take? It is not any ambushing strategy. But it the exploitation of a beautiful phenomenon in fluid dynamics.

When a vehicle moves at speed, it creates turbulence in the air. And the turbulent air tends to stick to the vehicle more. The less aerodynamic the vehicle, the more the turbulence. Now due to this air behind a vehicle is moving and hence when you drive behind it, the relative velocity of the air in front of you w.r.t to you is less. Also since the vehicle in front drags some air with it, there is a low pressure created in its wake. Hence, for you the effect is low drag, and low drag means lesser energy to cut through the air. Hence with the same amount of energy, you can travel faster, travel at the same speed with less amount of energy. Hence you drive more economically. Now this a strategy I use in the high ways. But again be careful, do not go too close to the vehicle in front.

Thus with the help of physics, we can exploit the most out of our cars’ performance. Nature hides many secrets for us to exploit. It is up to science to find it out and explain it and it is up to us to use them to our benefit. I have tried to be as simple and basic as possible. Happy Driving!!

Note: I should clear some things out. When I say cut the corner, I mean in a one way highway with medians (like Old Mahabalipuram road), when u apply the same technique to a two way road like the E.C.R, read road as “lane”. Anyway the techniques are going to be the same, only that you are going to stick to your lane.

So the 2 figures will now look like this :

and this:
Just a little bit of adaptation :).