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A PHYSICIST WRITES . . .

(May 2014)

Spring has arrived, and so my afternoon siesta is now sometimes accompanied (for better or worse) by the sound of lawnmowers. Some are noisier than others, making me wonder if they are the sort with a single rotary blade and this has become unbalanced. It happens easily enough, when the blade encounters a stone and gets chipped. Mine is a voice of some experience, from 40 years of owning two small Flymo hover-mowers (in succession).

For most of that time I put up with the sound and vibration of the occasional damaged blade until I was finally driven to replace it. Sometimes I made futile attempts to determine which end of the blade might be filed down a bit to restore the balance: trial and error, removing the blade and examining it closely, trying to balance it on something or suspend it Ė nothing worked. But then a few years ago, I realized that all you need in order to identify the Ďheavyí end is a short length of sticky tape!

If you too have a single-bladed mower that vibrates (particularly when it is starting up), hereís what you do: wind the tape first round one end of the blade, and then round the other. Whichever end makes the vibration worse than it was, as you start up, thatís the end of the blade that you need to file a little. If the vibration becomes worse both times, then use less tape.

Needless to say Ė but I must say it Ė disconnect an electric mower before going anywhere near the blade! Do make sure too that the power is coming through RCD or earth-leakage protection, either within your consumer switch-unit (though this wonít necessarily be protecting every socket in the house) or by plugging the mower into a bought RCD adaptor which can then go into any wall socket. And what does the protection do for you? It cuts the power instantly if thereís the slightest leakage of current to damp grass, etc, through you possibly.

Hereís another tip which I remember reading in the instructions for my first electric mower (I donít think Iíve seen it printed anywhere since, but it works for me): when mowing, loop the cable behind your neck. You should then find it easier to control the run of cable and keep it out of the way of the mower.

But we were discussing balancing of blades: is there a connection between this problem and tyre-balancing? Certainly there is, though Iím not suggesting that if you feel a wheel of your car vibrating, you should try to correct it yourself Ė leave this of course to the professionals! Also, is it surprising that even a brand-new tyre needs to be balanced (before being fitted to the car)? Not really: with all its internal layers, no tyre comes out of the mould perfectly symmetrical. Your metal wheel hub wonít be precisely balanced either, if only because of the valve (and the hole that it occupies). So first the tyre is levered on to the hub and pressurized, and then the assembly is put on to a machine which rotates it at high speed.

There are actually two kinds of balance (or rather, out-of-balance), and the machine should measure both of them. Static balance checks if the weight of the wheel is centred: if not, then it will vibrate up and down at high road speeds. (This is what we are trying to prevent with the lawnmower blade, but horizontally.) Dynamic balance tests whether the weight distribution is tilted at all, which would make the wheel and axle vibrate sideways in a twisting manner. The machine tells the fitter what weight(s) should be attached where (on the wheel hub), for correcting each problem.

Traditionally, lead weights were clamped on to the rims of the old-style wheels. But lead has now given way to zinc or steel, for environmental reasons. Also, alloy wheels arenít designed for being clamped to, so with these adhesive is used. But it may not hold the weight in place for ever: this is one good reason for getting your tyres checked again about halfway through their life Ė another being that ordinary wear of the tread may have introduced an imbalance. And an out-of-balance wheel will not do much good to the suspension, the steering or the tyre itself.

Changing the subject: the door-bell has just rung, to announce delivery of the Haynes Workshop Manual for my (2010) VW Golf, published only last month! I had the manuals for all my previous cars and they were always illuminating, regardless of how much servicing I did myself. Though in fact the amount has decreased steadily over the years, for various reasons.

The main reason is that cars have become more complex and less servicer-friendly. For example, where has all the space gone in the engine compartment? Years ago I could almost climb into it before starting work. Now, I can hardly squeeze a hand in anywhere, let alone see what itís meant to be reaching for. And things seem to be covered over, even the battery. Itís all rather off-putting.

As for the Haynes manual itself, this is heavier than ever Ė nearly twice the weight of the one for Mrs Sís vintage Micra! Mostly itís because the big repairs that I would never attempt need more detailed instructions now. But also, even a simple operation like changing the battery occupies the best part of a page of print with all the precautions you have to take, in order to avoid disabling some of the electrical systems.

Which brings me to all the circuit diagrams in the manual: these used to be a pleasure to study and follow, being clearly labelled and showing the layout of the wiring harness through the car. Now they are abstract, coded and almost identical, like repeating wallpaper. They tell you nothing unless you know it already.

Iím sorry to end this column on such a discontented note. If it has depressed you too, my suggestion is that you do what Iím going to do: go out and mow the lawn!

Peter Soul

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