The meaning of tyre
pressure numbers
The Sunday Nation
Nairobi,
08 August 2010
The tyres of most saloon cars are pressurised to 30 units. What is the
meaning of this number? In a nutshell, it is the amount of pressure in
Pounds (of force) per Square Inch, or PSI.
The PSI is one of those Imperial Standard units of measurement that has
refused to go away. Think about it, when was the last time you weighed
anything in pounds? The only pounds we think about these days are those
of the British currency; and those are also likely to disappear as the
Euro takes over…
The pressure of 30PSI means that a force of 30 pounds (lb) is acting on
every square inch of the tyre. The conversion table in my diary says
that one pound is equivalent to about 0.454kg. Therefore, 30lb comes to
about 13.62kg.
Now an interesting question arises can the 13.6kg per square inch
support the weight of a car (about 1,000kg)? The first step in getting
the answer is to convert the kg per square inch to kg per square
centimetre.
Now one inch is equal to 2.54cm; therefore, one square inch is
6.45sq-cm. Thus 13.6kg per square inch comes to 2.1kg per sq-cm.
The second step is to determine the weight carried by each tyre on the
car. Obviously, the front tyres carry significantly more than the rear
ones, but for the sake of simplicity, we can assume equal distribution.
This gives about 250kg per tyre.
So, the question is: at 2.1kg per sq-cm, how much surface area would be
required to carry 250kg? The answer gives us the size of the “footprint”
of the car tyre. It is equal to 250 divided by 2.1, or 119sq-cm.
Now most cars are fitted with 175-70-14 tyres. The first number in the
size (175) is the width of the footprint in millimetres. At 10mm per cm,
the width comes to 17.5cm. Therefore, to make the 119sq-cm, the
footprint would have to be about 7cm long. Is that a reasonable size?
To find out, I pressurised the tyres on my car to 30PSI using one of the
modern digital gauges. Then I measured the length of the footprint (You
should have seen the look on the station attendant’s eyes!).
I got 10cm at the front and, even though it is some 30 per cent higher,
it seemed about right. Reason being that the tyre has grooves etched on
it (the treads), so the whole surface is not in contact with the ground.
*******
At the time of writing this; the referendum results were all but
complete; and the YES side had 67 per cent of the vote against 33 per
cent for NO. Some one said that since the turnout was slightly above 50
percent, then the YES hasn’t really won.
My take: utter nonsense! If you followed the results throughout the
night, you must have noticed that the percentage margin hardly changed
by any significant amount. It started off at 62 – 38 and grew slowly in
favour of YES until the final 67 – 33.
Furthermore,
the last opinion poll had predicted a 70 – 30 win for YES with a 4
percent margin of error. This was from a sample of a few thousand
people. Therefore, it is reasonable to conclude that even if all
registered voters came out, the percentage margin wouldn’t have change
at all.
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