TWO SPEED SCORES ON THE UPS AND I OWNS
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by A. J. P. Wilding,
MIMechE, MIRTE WHAT DIFFERENCE would a two-speed axle make? Could there he better performance and /or fuel consumption or would overall results be the same? These are often questions that come to mind when testing goods vehicles and while guesses can often be made at the probable answers it is rarely possible to form a definite conclusion.
One vehicle tested where the use of a two-speed axle was the subject of speculation was the Dennis Pax V 15-ton-gross four-wheeler which had a single-speed axle. But in this case a definite answer can now be given. Since the original test, an Eaton two-speed axle has been made the standard with the Dennis single-speed the option and I have just been able to repeat the operational trial with a Pax V identical to that tested last year (CM November 15 1968) except for the driving axle.
In brief, the fitting of the two-speed axle gave a worthwhile improvement in the average speed and fuel consumption over the arduous section of the route and gave a marginal improvement in overall consumption too. But there was a reduction in the overall average speed, although this result cannot be attributed to the different transmission. High ratio in the axle was exactly the same as the final drive ratio in the single-speed unit tested and average speeds on the majority of the run were therefore dependent on the speed which the driver chose. This was certainly the case on the motorway and trunk-road sections.
The only two-speed axle offered by Dennis in the Pax V is the one as fitted in the test vehicle which has ratios of 6.14 and 8.38 to 1. Offering a higher pair of ratios such as 5.57/7.60 has been considered by Dennis but it was felt that the higher maxi
mum speed—around 65 mph would be unacceptable. In the circumstances, provision of the same "high" ratio with a second lower ratio did not prove of much benefit With a bottom gear in the Dennis gearbo) of 8.056 to 1, the Pax V with single-speec axle can climb any hill likely to be met ir Great Britain so bottom /low was not needei on my run.
An important advantage of a two-speec axle is that the number of ratios available tc the driver can be effectively doubled bu there was only a real benefit with the Denni! in second and third. Low-axle ratio in these gears was nicely placed between the ratio! with high engaged but fourth low was very close to third /high and fifth /low was close to fourth /high. Although this gave a sligh advantage on some gradients there wa! obviously less than if there had been a moil even spacing between the upper ratios. This point is illustrated by the maximum speeds in the gears. Starting in first /low, the progression through the box and splitting each gear gave 3, 6, 10, 14, 18, 25, 29, 39, 43 and 59 mph.
With a 5.57/7.60 axle ratio the "steps" would have been unchanged but there would have been an improvement by having the four highest overall ratios giving maximum speeds of 31, 43, 48 and 65 mph. There would still have been a more-than-adequate gradient ability and I would say that there could have been an overall fuel consumption of the order of 0.8 mpg if the same average speed had been aimed at by keeping below 60 mph or conversely there would have been some improvement in the overall average speed. All this illustrates the importance of correct selection when an operator has a choice of two-speed ratios although there is an obvious problem if the one considered most suitable gives too high a maximum speed. This problem would be eliminated if an efficient maximum-speed governor were available but such a device is not obtainable.
The overall results obtained with the current Pax V of 10.8 mpg at a 36.3 mph average speed compare with 10.7 mpg at 38.0 mph for the original test. On these operational trials efforts are made to eliminate variables in order to make comparisons practical. One way in which this is done is by always carrying the tests out on the same days of the week. This usually ensures that there are similar traffic conditions but unfortunately there were big hold-ups on the latest run between the end of M6 and the top of Shap through a succession of accidents and road works sites. As a result, this 24-mile stretch of A6 took 44min longer than on the original test.
To get a better comparison of the comparative performances of the two versions of the Pax V it is necessary to exclude the 84 miles between the fill-up points at Fourton service area and Gretna. If this is done the results from the two-speed-axle tests are 10.8 mpg at 38.3 mph as compared with 10.6 mpg at 38.8 mph for the test of the Pax V with single-speed axle.
The test was carried out at exactly the same gross weight as before-15 tons but with a 2cwt-less payload due to the extra weight of the Eaton axle-and as far as the rest of the vehicle's characteristics were concerned the same remarks made in the 1968 test report apply. The steering was perfect, the brakes were responsive and gave a good efficiency and the suspension was very good. The level of noise in the cab was still on the high side even though a padded cover was fitted over the engine cowl this time-most of the noise seemed to be coming through the area towards the windscreen-and the seat became rather uncomfortable after four or five hours at the wheel.
The Dennis U-type gearbox was again found to be not one of the easiest for making clean changes but it did not take too long to find the right technique. Getting the right technique for making satisfactory changes in the rear axle also took a little time but the electric change was found to be more consistent than air or vacuum types.
Timed climbs of the usual four hills on the circuit were planned-Shap, Carter Bar, Riding Mill and Castleside-but difficulty was experienced on the last two in changing from second to bottom gear which caused "unscheduled" stops. The traffic on Shap hampered the climb which prevents a worthwhile comparison also with the single-speed Pax V. The 4.25-mile climb from the Jungle Cafe to the summit took 12min 14sec, over 1min longer than on the previous test. Traffic on the approach meant that the speed at the bottom was only 23 mph and while the minimum speed on the 1 in 9 part was 5 mph, general speed on the steady 1 in 11/12 gradient was between 8 and 10 mph. Second gear with high and low axle ratios was employed for the second stage of Shap and upward changes on the easy stretch to the summit brought the road speed there to 31 mph.
The climb of Carter Bar was straightforward and so the figures can be compared with the earlier test. The 1.85-mile climb started at 40 mph and was made in 6min 51sec as compared with 7min 4sec before. Third /low and high were adequate for the 1 in 13/14 gradient but second /high was needed for the 1 in 9 section at the top where the speed dropped to 6 mph as against 15 to 18 mph for most of the climb. Speed at the top was 18 mph. After the unscheduled stops the checks on Riding Mill and Castleside were abandoned.
Basic list price of the Pax V with either the two-speed or single-speed axle is £2,790. The chassis gives only a little less payload than most 16-ton-gross chassis as a result of its lightweight construction but the design appears sufficiently robust that the model will give reliable service. With its advantageous features from the driver's point of view the model tested appears to represent good value for money.