PART 1 OF THIS FEATURE, PUBLISHED ON APRIL 18, DEALT WITH THE STABILITY OF SINGLE-AXLE OUTFITS.
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EARLIER this year I carried out a series of practical tests to evaluate the relative stability characteristics of a conventional leaf-sprung semi-trailer and a similar but independently sprung semi-trailer under certain cornering conditions. The results were published in CM on April 18, and showed convincingly that the independent unit could be cornered faster without becoming unstable—though the safe cornering speeds for high-loaded artics, especially through an S-bend, were shown to be low in both cases,
The manufacturer of the independent suspension, the Peak Trailer and Chassis Co,, Ltd., of Stockport, has recently introduced a
tandem-axled unit employing basically the same independent suspension as is used on the single-axle version. Thanks to Peak's ready co-operation I have now been able to conduct a second set of trials, using the tandem-axle trailer, with a normal beam-axled Peak tandem for comparison. And while the independent unit did not prove capable of safely outspeeding its leaf-sprung counterpart on the curves used for the test, it did demonstrate the potential ability to do so, and showed a far greater degree of general stability and controllability. The tests also showed the tandems to be much more inherently stable than the single-axle types tested previously.
To achieve a reasonably high centre of gravity, as in the previous test, I loaded the independently sprung trailer on to the beam-axled unit: the upper trailer was carrying 15 tons of concrete blocks (we had used 11 tons of steel blocks for the single-axle tests). With securing chains and bolsters the total payload was 19 tons 9cwt and the gross weight of the outfit, including the driver, was 29 tons 12cwt, Seddon had once again kindly lent us a 32:four tractive unit with Rolls-Royce Eagle engine, and the centre of gravity of the whole outfit when continued on page 49 loaded was estimated to be just over 10ft above ground level.
I first checked the speed at which the beam-axled unit would raise its inner wheels from the ground when driven in a continuous circle. The single-axle trailers had required a 53ft-radius circle for the test but the smaller cut-in of the tandems permitted a circle of only 41ft radius which should, had all things been equal, have produced the onset of instability at considerably lower speeds than achieved with the single-axle types.
Circulating at 6 mph and then building the speed by 1 mph per lap, I reached 11 mph before the inner trailer wheels suddenly left the ground. I was able to feel the balance of the trailer so finely that I could continue circling with the wheels just clear of the ground, and even raising the speed to 12 mph did not result in the outfit becoming unhandleable.
While this set the maximum safe circulating speed at around 11 mph, I elected to try a higher speed for the S-bend manoeuvre designed to simulate the critical left and right curves at the entry to a roundabout. Going through these two 30ft-radius bends (leading into the full roundabout) the outfit proved stable at speeds up to 14 mph but it was most noticeable that, even with power steering, it was virtually impossible to make any alteration to the steering once the vehicle reached a critical angle. Here, perhaps, is the clue to those accidents of which it is sometimes said that the driver could have avoided the collision or roll-over if only he had done so and so. My guess is that such drivers found, as I did, that the steering just locked up on them.
I took the vehicle through the S-bend at speeds up to 17 mph, but at 15/16 mph the _inner wheels of the trailer suddenly lifted a couple of inches and remained clear of the ground through about 30 to 40 degrees of the circle. I could retain the balance of the vehicle at this speed, but an increase to 17 mph brought sudden and uncontrollable breakaway. Because accelerating at the point in the S-bend where direction is changed is thought to he one of the causes of roll-over, I simulated this. Before starting the tests I measured the acceleration of the outfit, getting 0 to 10 mph in 5.8sec in second gear and 10 to 15 mph in 10.1 sec in third gear. Using an approach speed of 13 mph, the vehicle remained stable under acceleration at the direction-change point, though at speeds up to 13 mph the load set up a marked pendulum action, so on each attempt I kept the power full on until the swinging stopped and the vehicle lay at a steady angle. At 13 mph the outfit became stable in this condition with its inner trailer wheels clear of the ground, but repeating the manoeuvre at 14 mph caused a sudden rearing up of the bogie and complete locking of the steering. I can't now recall how I unravelled that one.
We then changed the trailers over, putting the beam-axle model on top and adjusting the load so that distribution over the independently sprung model was identical with the situation in the previous test.
Circulating the 41ft-radius circle with the independent model, results were at first identical with those for the conventional trailer—but with the difference that I could not detect the lifting point of the inner wheels. But after a •few laps with the bogie wheels lifting and falling I got the feel of the outfit and was able to circulate at a constant 15 mph a 25 per cent improvement over the conventional vehicle. However, pushing the speed up to 16 mph brought a sudden and uncontrollable wheel-lift and the now-familiar solid steering. I was glad at that point that we were not on a public road, but using the roadways of the test station at Bredbury which the MoT had kindly put at our disposal.
I was puzzled at the way in which the suspension suddenly "went solid", because, up to breakaway point, progress was considerably more stable than with the conventional outfit. I suspected that the coils of the independent suspension were perhaps becoming bound under deflection, and investigation using some of photographer Harry Roberts' plasticine showed this to be the case.
More roll stiffness
We nevertheless continued the full test sequence and throughout the S-bend manoeuvres the independent proved to have considerably more roll stiffness than the conventional trailer. It was significant that I could go through the S-bend 1 mph faster than it was possible to complete the full roundabout circle. Once again, however, as soon as the coil-binding point was reached the machine threatened to take charge. It was completely stable up to 16 mph through the S. but raising this to 17 mph caused immediate and forceful breakaway.
When I tried accelerating through the second curve of the S. I found that the maximum safe speed was 11 mph 13 mph slower than with the beam axle). With slight lifting of the inner bogie wheels I was able to raise the speed to 12 mph, but circulating steadily at this speed I could not get the wheels to touch down again.
The photographs of the tests confirmed my driving impressions. The independent suspension gave clearly better results up to the point where the coils of the springs met and deprived the suspension of further deflection. Balancing the other debits and credits, the Peak independent suspension rides very much more smoothly on bad ground and has absolutely no reaction to brake torque: but it is rather heavy in its present form and has a few more working parts than its counterpart. But for any operator with high loads or delicate cargoes, the credits vastly outweigh the debits.
Peak's design department was quick to take a look at the coil springs, and when I left the works on the final day of the test the suppliers were already investigating the possibility of greater clearance between the coils. I am convinced that there is a potential gain to be achieved here, and at a later date hope to prove the point with a further test.
There should be no doubt in reader's minds, however, that both the trailers tested were subjected to cornering far more severe than is ever likely to be encountered in normal use. Just how severe was evident from the savagely worn tyres at the end of the tests, despite the relatively small mileage.
I was also interested to discover, under such demanding conditions, how much more stable the tandem-axled semi-trailer is than its single-axle brother, regardless of the type of suspension used.