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A PHYSICIST WRITES . . .

(February 2010)

In a department store the other day, I almost fell over as I stepped on to a stationary escalator.  I could see it wasnít moving, but I was unable to shake off the expectation (deeply ingrained in my brain, no doubt) that my foot would be carried forward. And then the climb up the steep escalator seemed so much more taxing than going up an ordinary flight of stairs would have been!

This has set me thinking of other expectations and habits that might trip us up. Perhaps because of the snowy January weather as I write, the one that occurs to me first is the assumption that if you hit the brakes, your vehicle will stop.  I can only recall two occasions, up to 2006, when this didnít happen for me.  Luckily no damage resulted. You instinctively feel, though, that itís the brakes that are failing rather than the tyre grip, hence all you can do is press harder still ó which wonít improve your chances of recovering control!

But why 2006, you ask. That was the year I went on a skid-pan course (as I reported here). It was a morning of sliding on the straight and on bends, losing grip at the front and at the rear (or both), all great fun but with a serious purpose: to learn what to do and not do in a skid.  But four years later, can I still expect to get it right if I face a sudden slide? Maybe itís time for another visit to the pan...

Hereís a habit I observed while staying with a friend last year and being driven by him around the countryside: on gentle downhill stretches of road, he would slip out of gear and coast. This was in a nearly new car that had replaced a much older one.

Cautiously I raised the subject. My friend said he had long been doing it, as he found it relaxing and it saved fuel. I said: OK, in neutral the petrol is only being consumed at Ďtick-over flow-rateí, but if you simply lift your foot in gear, I believe modern engines cut the fuel off completely.  But I wasnít certain and his driverís manual didnít say, so I fear I didnít convince him.

I am better informed now: in nearly all fuel-injected engines, petrol is indeed cut off Ďon the overruní (ie, in gear with your foot off the accelerator), above a certain engine speed at least. So coasting out of gear is wasteful. There are safety reasons for not doing it, too: an emergency could call for sudden acceleration ó and while re-engaging gear, you would only have one hand on the wheel. Alternatively, you might need to steer and brake heavily simultaneously, and as both systems are servo-assisted by the engine, I wonder if it would fully cope with the demand when running only at tick-over speed? Anyhow, you canít claim to be in full control of the vehicle when coasting, as required by the law (also, you would fail any driving test).

But why didnít my friendís manual refer to the cut-off feature?  My informant in the industry says it is just one of many clever things built into cars now, and owners wouldnít want to read about all their complexities.  Also, manufacturers tend to be cautious with their advice on how to drive, in case it isnít applied sensibly.

Several of those clever features are there to help keep you safe, in one way or another. But letís suppose that youíve exceeded their capabilities and a crash is inevitable.  In the couple of seconds before it happens, you think to yourself: well, in the old days I would soon be flying towards the steering-wheel and the windscreen ó at least now I can expect my seat-belt and air-bag to protect me.

But only up to a point.  Remember that you still have to be slowed down by something, and a locked-up seatbelt is designed not to possess much Ďgiveí (correctly so: itís this, plus the concertinaing of the car, plus the softness of your body, that allow you yourself the maximum time for deceleration).  So in a sudden impact, donít hope to escape bruising or even fractures. Or burst ear-drums from the air-bag.

I remember reading somewhere that a driver had come to grief (or maybe just nearly so) by misjudging the distance of a pair of oncoming headlights at night. He assumed they belonged to a car, but in fact they were close-spaced and on a motor-bike.  This seems an unlikely mistake to make, unless the lamps on the bike were exceptionally small.  However, something similar happened to me once, at a dark roundabout: headlights coming from the right seemed too far apart to be on a car, and so my instant assumption was that something much bigger was approaching from further away, giving me time and space to move. But I was wrong ó though I realized my mistake before it was too late, fortunately.

Planning to hire a car in New Zealand? They drive on the left there, but even so Iím told you need to break one habit: if youíre turning left, and an oncoming vehicle is turning right into the same road, itís you that has to give way!

Finally, Iíve just been reading up on tyres. I didnít fully realize that they come in distinct summer, winter and all-season types; Richard Porter (our Thames Valley Group Newsletter Editor) tells me he has a set of winter ones that are serving him well right now ó though he wouldnít want to drive long distances on them.  But what was quite unexpected from my research was that the difference between types isnít just a matter of the tread: summer tyres (which mine are, I guess) actually start losing flexibility when the temperature drops below about 7 ļC.  I think Iíll stay right off this snow, then.

Peter Soul

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