Road and workshop by Handyman
Page 30
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Loads and forces (2)
• From my experience of looking into why vehicles turn over at roundabouts. I would say two regular points can be plotted with some accuracy. It seems that the larger roundabout claims the majority of turn-overs at 90deg from entry, and the smaller island at 180deg. While the reasons for going right over are several, the results are the same; the vehicles crash over onto their nearside at the 90deg point, or onto their offside at the 180deg point, that is when leaving or turning away from the island.
Obviously under some circumstances speed can play a great part in upsetting the balance, but few experienced drivers will knowingly rush a roundabout. Usually, where an error has been made the driver has arrived a little too quickly and is braking heavily while already turning. In that position any one of several factors can turn him over.
In questioning driers I came across a substantial number of cases where each driver sincerely declared that he had made—in his opinion--a normal approach that should have taken him round safely; he had used the minimum of brake power, in many instances borne out by the absence of any tyre marks.
Obviously I looked into what drivers considered a normal approach, discovering that their speed was one that would ordinarily take a loaded sixor eight-wheeler rigid safely round. Tests with a safely loaded eight-wheeler taken round the larger roundabout at 21 mph raised no problems, but the nearside springs were fully depressed and wing to tyre clearance nil.
An eight-wheeled artic tanker was taken round empty at 23 mph and flipped its nearside trailer wheels lft in the air on entry, held them there for some 6yd and thumped them down as the steering was put over to the right. The driver agreed that if loaded he would not have travelled at 17 to 18 mph within the island.
Speeds with the loaded eight-wheeler centred around 20 to 22 mph just on entry into the larger roundabout: these tests were carried out with several drivers using their normal approach, a closed throttle with negligible brake dropping the speed quite quickly as tyre drag set in.
One driver mentioned that artics always cornered more quickly as they were lighter on the steering, unlike the large rigid which made you work much harder and therefore enter more slowly. Discounting the driver who simply miscalculated and arrived far too quickly, the speeds of many artics at 10yd from entry were undoubtedly no less than 25 mph, on occasions well above, and only really savage braking over the last few yards of straight running enabled them to negotiate the island.
Two things stand out so far: (A) the large rigid is safer and less prone to play tricks in the turn; (B) the artics are being taken into the roundabouts more quickly than the big rigids, being easier to turn. Therefore the artic entry speed must come down.
However, what are drivers like at estimating their approach speed? In tests carried out last year the findings were both enlightening and somewhat alarming. Tests were made with one eightwheeled rigid fiat, with a top speed of 36 mph, and one rigid six-wheeled tanker, top speed 41 mph, to establish whether a _driver could estimate a lower speed with any accuracy after he had made a steady run at top speed.
Four drivers were used on the tanker, including its own driver, and five on the rigid, also including the regular driver. In each case the vehicle was cruised at its maximum for at least 2 miles, then the speedometer was covered and the driver told to slow down to an estimated 15 mph. Both drivers normally full-time on the vehicles did this with surprising accuracy—the rigid driver 15,5 mph, the tanker man 16.5 mph; other drivers on the rigid were within 2 mph.
On the slightly faster tanker, however, other drivers were nearer 17 than 15 mph and one man's estimate was 19 mph. However, it was considered that where a driver was on either machine for a day or so, there would be little wrong with his speed estimate. Then a second series of tests were made, this time with plated maximum and medium attics. To avoid any chance of bias, half of the drivers selected were those who dicl the first tests, the other being unaware of the nature of the tests. The larger artic had a top governed speed of 54 mph, and was cruised at this speed by its regular driver for 14 miles of clearway. The speedometer was then covered and he was asked to lose speed and estimate and hold 16 mph, indicating when he was ready for a clock check. On his signal the speedometer was uncovered and read 21.5 mph.
Three other drivers were tried out and in no instance was the estimated 16 mph lower than 20 mph, yet each man had been convinced that his estimate was spot-on. To avoid any doubt about the accuracy of the instrument, the vehicle was timed over a measured half-mile and the electric instrument found to be no more than 0,2 mph slow, which could be accounted for by tyre wear and slight wheel slip empty.
A few days later, the medium artic was taken on test, this time with a fresh crew. The speedometer was checked over a full mile, and with new driving axle tyres was found to be quite accurate; top speed of this outfit was 61 mph. Six drivers were tested separately as before, given two miles at maximum speed then told to select 16 mph and hold it. The lowest result Was 19.5 mph, the estimates climbing to that of 25 mph by two drivers, which was really worrying, and without doubt arose from travelling at a sustained high speed under power.
On the other hand, on three highspeed, dead engine coasting tests carried out later the drivers estimated a lower speed with greater accuracy, although this particular situation would not normally apply.
With a vehicle in the higher speed group the lesson to be learned is that m coming down from its higher cruising speed it is necessary to keep a watchful eye on the speedometer rather than rely on guesswork.