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A PHYSICIST WRITES . . .
(June 2014)
I do try to round these columns off neatly and avoid leaving any loose ends, when writing them [for the Thames Valley Group Newsletter, ten times a year]. Nevertheless, as I start again for the 120th(!) time I’m wondering if anything from the past 12 years might need adding to. So I have taken a look back through the archive (see www.petersoul.co.uk if you want to do the same) and I’ve found several topics worth expanding upon...
In only my second column, I bravely attempted to explain that when travelling round a roundabout (and whether or not at a steady speed) your car is all the time accelerating sideways towards the centre, which requires a force (in that direction) just as much as when you’re more obviously accelerating – or braking – on the straight. And in each situation, the force is of course the grip between road and tyres. Let’s now go on to consider wider bends and higher speeds.
First I need to refer to the list of Braking Distances (following the Thinking ones) in the Highway Code. What isn’t mentioned there is that all the distances assume a deceleration of just under 15 mph per second: ever since the table first appeared nearly 70 years ago, this has been the ‘official’ rate at which you can reckon to shed your speed in hard braking. In reality, on dry roads you should be able to achieve rather better than 15 mph/sec, while in the wet it could be worse (even with ABS coming to your aid).
But it’s rare for you to have to brake like that – whereas you very often need to take bends and rely on sideways tyre-grip! How much grip is needed? How much is available? Even to give you simple answers, I have to know the radius of particular curves. So I’ve located a couple of bends [local to our Thames Valley Group] that are relatively easy to measure from aerial views on the internet, because each is three-quarters of a circle.
One is in south Bracknell: the exit from Mill Lane to the industrial area. For your benefit I drove round this bend at the highest bearable speed for me, which turned out to be 38 mph. Now to calculate my sideways acceleration (from my measurement of the radius): it was just under 15 mph/sec, or exactly the Highway Code limit for straight-line braking. What this means is that if I take any bend at a speed that’s just tolerable, I will be using the maximum grip that my tyres can officially exert, to keep me from skidding off the road (unless, that is, it’s wet...).
OK then, I don’t think I will try that again! The other long curve is the slip-road off the west-bound M4 at J7 (for Slough West). By calculation I can tell you the speed on this wider bend that corresponds to maximum sideways tyre grip as above: it’s 48 mph. If instead you tried to take the slip-road at 70, the grip would need to be twice as high again (because the continuous sideways acceleration is proportional to speed squared).
Yet as you come off the M4, apart from the big blue sign showing a curve there’s no specific warning to slow down. Well, you’ve been warned now. And do remember: if you also brake on a bend, you are immediately reducing the sideways grip available for holding the car to the curve – not by as much as the braking force being applied, but still by a significant amount (unless you are only braking lightly).
Don’t worry if you still can’t make sense of ‘continuous acceleration towards the centre of a curve’ – it's not an easy concept. Perhaps just imagine the way in which the tyres will distort sideways during rapid cornering, in having to get the vehicle round the bend. Which reminds me of something that’s still a mystery to me (see my June 2004 column): how does a tyre stay attached to the metal wheel-hub under extreme cornering forces, bearing in mind that there’s less than an inch of overlap between their two rims? This is either motoring magic or a wonder of surface physics.
And here’s another adhesion puzzle: most people drive as if there’s 100% grip between their tyres and the road (which is a fair assumption much of the time), but why do so many seem to believe that this grip disappears while they are manoeuvring? Whenever I see a driver power-steering the front wheels on the spot, I can feel the tyres’ pain! The whole steering mechanism must also be suffering. But I partly blame the manufacturers: they could easily install a system to warn you when you’re turning the wheel too rapidly at a low road speed.
A different topic altogether: I’ve written more than once about tail-gaters and how to deal with them. If you can’t shake them off then you must allow them to overtake, but this isn’t always easy or safe. Only recently did it occur to me that the ideal place is at a roundabout: go once-and-a-half round and the problem is solved!
But also recently, I was effectively the cause of the problem, being in a taxi on the M4. The driver persistently switched lanes, exceeded the limit and approached to within half-a-second of the car ahead. If only one could say: in spite of your evident skill, driving so close will not get us there any more quickly; it is probably distracting the driver in front, and anyway it puts us all in unnecessary danger.
Long ago I mentioned the difficulty in seeing a flashing indicator at night when it’s located close to the headlights on an approaching car. I now own a Golf which maybe presents this difficulty all the time: the indicators each lie between a (dipped) headlight and a daytime-running light! But at least they are bright and not too far from the wings. My growing concern is with ones that are placed further in towards the centre line of the car, as on some Ford Focuses. I sense that my brain is slower to react to such indicators, possibly because their ‘directionality’ message (indicating left or right) is weaker.
This fits in with what I’ve often said before: your subconscious brain brings to your conscious attention only the things that it thinks are relatively important, based on long experience and on how much information has been picked up. All you can do to assist it is to look hard everywhere, and at everything...
Peter Soul
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