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A PHYSICIST WRITES . . .

(April 2004)

Old habits die hard. Every 3000 miles I jack the car up, spin the wheels (watching for any bulges in the tyres) and check for no play in the bearings. Then I take each wheel off, inspect the brakes and dig the little stones out of the tyre tread ó just in case they are the visible bits of bigger ones. Curiously, there are always many more of these little stones in the rear tyres than in the front.

I started doing all this in 1965 when I acquired my first car, a Morris 1000 saloon which was already quite a few years old by then. A couple of years later, though, when I inserted the jack into its hole (which was below the centre door pillar) and turned the handle, the car stayed firmly on the ground!

Corrosion had spread unnoticed, and in sorrow I took her to the breakerís yard where they gave me a fiver in return. I spent this on a little bottle-jack, which I applied directly below each wheel axle of the next car ... and the next and the next (never finding the courage to get out the jack that came with each vehicle) right up to last year.

Thatís when I happened to notice the four jacking points provided on my Corolla, one beside each wheel arch. They ought to be safer than the central points on the old Morris, I thought ó dare I try the proper jack? Well I did, nothing gave way and of course the job was rather easier. One old habit dead!

Up to last year too, I always used just the standard wheel-nut lever to loosen the nuts (before the wheel was jacked off the ground). This job grew steadily harder as the tools that sometimes garage mechanics applied to them became more powerful. Then suddenly my physics education bore fruit and I realized that slipping a tubular plug-spanner over the end of the lever would double its length and halve the effort.

But my aging back still complained at the first pull to loosen each wheel-nut, and again at the last pull to tighten it. Finally something inspired me to reverse the lever and try pushing down on it, instead of pulling upwards. I discovered that this way you can exert just as much leverage and suffer no back strain (as long as you are able to bend down to the job to start with, of course). Old habits can make you feel old before your time.

Letís think now about another cause of strain, namely hauling on the steering wheel to turn into a corner (OK, Iím recalling the time before I acquired the luxury of power steering). Itís not surprising that some effort is required, when you consider the weight of the car that you are guiding round the bend ó and how forcefully the wheel straightens out if you let it slip through your hands afterwards (not that you do, of course).

But hereís a puzzle: on a frosty or icy corner the wheel becomes noticeably easier to turn. Given that the car is entering the curve at much the same speed and isnít skidding, how is it that less effort is needed?

I guess the answer must be that parts of the front treads have to rub across the road surface while you are in the process of rotating the wheel. This sliding is smoother when itís frosty, but adds to your labours when the road is dry. What surprises me is that it happens at all as you steer into a normal corner. I would have thought that the forward motion would give the treads plenty of time and distance to change their angle without rubbing ó but evidently not.

In slow-speed manoeuvring, of course, there is bound to be sliding of rubber across the road surface. And Iíve just realized that all this dragging explains the much smaller number of little stones that I find in the front tyres ó they must get pulled out almost as soon as they get stuck in!

Peter Soul

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