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A PHYSICIST WRITES . . .

(March 2003)

Did I perhaps get a bit too worked up last month over vehicle lighting? Letís look at what the Highway Code has to say on the subject (simplifying slightly):

You must ensure that side, rear and number-plate lights are lit at night ... you should use dipped headlights at night in built-up areas ... you must not use fog lights unless visibility is seriously reduced ... take special care that all your lights work ... lights must be adjusted so as to prevent dazzling other road users.

Well, as I drove across Reading at twilight recently it looked as if about one in every six vehicles was falling foul of the above rules ó and thatís from just a passing glance, not an all-round lighting examination! But then what real incentive is there for drivers to check their lights, put them right, and then use them correctly?

I mentioned last month that part of the risk in having a burnt-out bulb is the chance of the other one of the pair failing. Afterwards one evening I came up behind a car which I could see had lights at the front ó but none facing me. Eventually I succeeded in indicating to the driver that there was a problem. When he stopped and I told him what it was, at least he seemed grateful.

I also mentioned last time what happens to coloured indicator bulbs behind clear lenses: they fade with age. I never noticed this until the garage decided to change a set for me at MOT time. Now, looking around, I see pale indicators everywhere! But the real danger, I think, is from the ones you donít see because they are located close to an over-bright headlight.

And of course the other driver is quite unaware of the situation. This could be the explanation for what happened one evening at a mini-roundabout, which I approached looking hard at an oncoming car for any sign that it might be about to turn right ... I saw nothing, but the car crossed behind me with a blast of the horn.

Back to physics: older cars can suffer from a problem which looks like failed bulbs but isnít. Sometimes called an earthing fault, it happens when a junction in the wiring becomes loose or corroded and therefore offers a resistance to the flow of current.

The junction may still be good enough to pass the current from a rear light back through the chassis to the battery, but the larger current from the brake light or the indicator is too much for it. What you then see from behind, when the driver applies the brakes or tries to indicate, is that the rear light simply blinks out.

But this hardly begins to explain a bizarre display of lighting that I saw on the North Circular Road (just before Christmas, appropriately). The car in front seemed to be showing its rear lights properly, but then they suddenly went out and both indicators came on briefly. This happened a couple more times and I realized that the driver was applying the brake pedal occasionally. But how could this be bringing the indicators on, even if there was a loose junction knocking out both the brake and the rear lights?

And what would happen when the driver tried to indicate left? I soon found out: the indicator winked a few times, together with the left rear light winking off. But then the right indicator and rear light took over ó and then it was back to the left again! This switch-over must have been governed by the brake pedal too, as the car slowed down on the slip road.

I have been trying since to work out how the lights on this car can possibly have been wired up so as to cause such an extraordinary performance. And if the front and rear indicators were connected together on each side (as they should be, of course) then pedestrians and oncoming traffic must have been just as confused, if not in some danger. It certainly left me with a headache.

Peter Soul

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