The wheels of a car are
slower than the engine
The Sunday Nation
Nairobi,
11 April 2010
I was asked an interesting question at a social gathering recently: What
moves faster, the engine of a car or the wheels? Even though I knew what
was meant, my quick answer was that the two move at the same speed.
After all, both the engine and the wheels are mounted on the body of the
car therefore when one moves (along the road), the others move at the
same speed!
But of course, that was not the meaning of the question. My friend wanted
to know whether the wheels rotate at the same rate as the engine. The
answer to that is not straightforward. It depends on the gear in which
the car is driven.
In lower gears, the engine rotates much faster than the wheels, but does
the situation change in higher gears?
If you wanted to know the relationship between the rotation of the wheels
and that of the engine, then you need to check the revolutions (“rev”)
counter and the speedometer of your car. In addition, you also need to
know the size of wheels you have.
Unfortunately, not all cars have a rev counter, especially in this era of
automatic transmission. However, I know a particular model that does
80km/h at 3,000 revolutions per minute (rpm) when driven in top gear.
The manufacturer recommended size of wheels for this car is 155-60-13.
Now, these size numbers mean that the tyre is 155mm wide; fits onto a
13-inch (diameter) rim; and its side wall is 60 per cent the width; that
is, 93mm.
Thus the total diameter of the wheel is 93mm plus 13inches (330mm) plus
another 93mm. This comes to 516mm. Therefore; the circumference (length
around the “top” part) of the wheel is 1,625mm or 1.625m.
When this car is doing 80km/h it covers 80,000m every hour. One hour is
60 minutes, thus the car is covering 1,333m (80,000 divided by 60) every
minute. The next question then is: how many revolutions does the wheel
make over a distance of 1,333m?
The answer is simply 1,333 divided by 1.625; that is, 820. Thus when in
top gear, this car’s wheels do 820rpm while the engine runs at 3,000rpm.
The wheels are slower!
This calculation assumes that the rotation of the wheels is firmly locked
to that of the engine. This is only so in the case of manual
transmission cars. The clutch ensures a tight connection between the
engine and the wheels.
Automatic transmission cars do not have a clutch – they use oil to
transfer the motion of the engine to the wheels. The oil is pumped to a
“turbine” which in turn rotates the shaft that turns the wheels. In this
design, the engine can run while the wheels are held stationary. This is
what happens when you engage the “drive” gear while stepping on the
breaks – the car doesn’t go anywhere. Indeed,
there is always a “slip” between the two rotations.
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