Air Resistance, Tyres and Friction

Topics: Force, Friction, Classical mechanics Pages: 11 (3854 words) Published: October 8, 1999
Dragsters use a combination of large wide tyres or the rear and small narrow tyres on the front this combination is used for the following reasons:
The front wheels:
The front wheels are very narrow. This is so a minimum of air resistance or drag affects the dragster with lower drag better acceleration an in turn a better top speed can be achieved all leading to a better pass (race time).

Now lets try to understand the concept of air resistance and drag. A basic example is placing your hand out the window with your palm facing forwards as you are driving your car along at about sixty kilometres per hour. You will feel a strong force of the wind (air resistance) pushing back at your hand. Now turn your hand side or so that your little finger is facing the front and your thumb is facing the rear at the same speed. The force of air resistance exerted on your hand is greatly reduced. This force is similar as to that exerted on the front wheels of the dragster.

Now dragsters reach speeds of up to five hundred kilometres per hour, imagine the force needed to hold your hand against the wind if your palm was facing the front. It would be much easier to hold your hand side on. The same as it would be much easier for the dragsters engine to push the narrow front wheels compared to large ones.

Air resistance is a form of friction (namely fluid friction) a friction from the air, as we know friction is defined as a force that opposes movement.
The formula used to determine aerodynamic drag is as follows: Drag = 0.5 * rho * Cd * v2 * S

Aerodynamic drag is a function of the following:
rho is the air density, which we cannot change.
v2 is velocity squared which is endeavoured to be maximized for the best time and/or pass. S is the frontal or cross sectional area which we want to minimize. I.e. less frontal area means that a less significant amount of air resistance impedes the top speed and acceleration. Cd is the coefficient of drag, which we want to minimize.

So the two things with which can be worked with or changed, the frontal area and coefficient of drag, both of which need to be to minimized for the best results.
Having very narrow front wheels minimizes the frontal area. This is the main reason why narrow front wheels are used.
If the smaller the wheel the lower the drag, why not have the wheels narrow and very short as well? You ask. Well the reason is that if the wheels were very small they would drop into all the bumps and cause a loss of speed not to mention control. As the wheels would bounce into the depression and then launch up into the air as they come out of the bump. This is extremely dangerous in that the driver can no longer steer the vehicle that is travelling at near five hundred kilometres per hour, the car can also get air flowing underneath the car, with the effect of air resistance the car will lift up of the ground and flip through the air.

Also the rotational force is much harder on the bearings causing more wear and friction meaning slower times.
Large wheels are used because they will skim over the bumps and keep the car moving along a flat plane. They also exert less force on the bearings meaning less friction and better times.
Now if drag cars use narrow front wheels so they can get less air resistance and a better top speed why don't all racing vehicles run narrow front wheels? The answer is friction. The front wheels of drag cars do not have high cornering or driving force travelling trough them. I.e. they are only there to hold the front of the car up and allow it to roll along the road. In conventional racecars high forces are exerted on the tyre in the horizontal plane meaning that they need to have a good tread area so that they grip the road well and hold the car on the track so it does not slide off into the dirt. ...
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