Mercedes-Benz LP 1923/
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Eylert 38-ton-gross outfit
test by A. J. P. Wilding,
MIMechE, MIRTE
SINCE MAJOR changes in regulations started taking place in August 1964, articulated vehicles have become more and more popular as a means of getting the maximum payload on one set of wheels. Artics have certainly had the biggest share of the cake when it comes to weight increases and against this background it is surprising that no change has been made to the basic position of the tractor-trailer combination. True, the 32 tons rriaximum in this country which has existed for many years can now be carried on a two-axle tractor coupled to a two-axle drawbar trailer instead of there having to be at least one multi-axle part. But the big stumbling block to this type of vehicle is that there still exists the very-much-out-dated need to carry a second man in the cab. There was a suggestion a year or so ago that this requirement would be changed and that the Ministry of Transport would at last acknowledge that the "mate" has nothing to do on a modern outfit apart from sit in his seat. He was put there originally to operate the trailer brakes but this was long ago.
It is certain that within a fairly short time now maximum-gross weights of artics are going to be increased .again and probably to 38 tons but there is still no suggestion of a change in the tractor-trailer laws. Yet there are obvious advantages with the latter type of vehicle and in most European countries these play a very important part in the transport scene. It is true that the artic is gaining ground but on certain classes of work the tractor and independent trailer must give benefits if only because it is easier to manoeuvre when going through built-up areas than is a 49ft artic.
Germany is a country where the lorry and trailer have been and still are very popular and to see just what is involved in driving such an outfit and to get an idea of the type of vehicle needed for 38 ton gross operation, I visited that country at the end of last year to test a Mercedes-Benz LP1923 two-axle machine coupled to an Eylert trailer. Although longer than a maximum length artic by almost 10ft the outfit was much less trouble than a long artic in congested areas and apart from "tugging" of the trailer which is to be expected with this part (22 tons gross as against 16 tons for the tractor), there were no disadvantages at all.
The Mercedes-Benz tractor was an excellent vehicle to drive, requiring little effort and giving a high degree of confidence to the driver through light, precise steering and responsive, efficient brakes. A very high degree of comfort is given to the driver. The seats are comfortable (there were seats for two passengers in the test vehicle), the interior is well trimmed and the heater has a high output. Visibility was found to be very good, with a big area of the screen cleared by the triple wipers; and excellent convex mirrors gave a more-thanadequate view to the rear.
Despite the strangeness of driving on the right in a completely unfamiliar machine I did not find that I needed a settling-in period before driving the vehicle in a similar way to the regular driver. The 1923 was not an ultra-fast machine but the maximum speed of 56 mph is quite fast enough particularly in Germany where the speed limit even on autobahnen is only 50 mph. Even so, sticking roughly to the limit, we completed a 356-mile journey, including travelling through three cities and over a hundred miles of 2nd class road—the rest was autobahn—in only just over 11 hours. What was even more creditable was that the overall fuel consumption was 6.65 mpg which is surprisingly good for a 38 ton gross outfit. Average speed for the total journey was 32.3 mph.
As well as the operational trial, patterned on similar runs made in Britain, the Mercedes-Benz was put through normal performance and braking tests. Acceleration times when towing the trailer were very good indeed and so were the brake stopping distances in spite of the facts that the trailer momentum tended to lift weight off the driving axle on maximum-pressure stops and that the trailer brakes did not appear to be doing their full share of the work. Repeat tests were made with the 1923 solo as the model will often be used in this form, Here the results were remarkably good with the truck behaving on acceleration tests like an empty vehicle with 230 bhp and 16 tons this was expected. Extremely good braking figures were also obtained.
A tractive unit version of the Mercedes-Benz 1923 was shown at the Commercial Motor Show last September and the vehicle tested had basically the same specification. The 1923 is the export version of the concern's 1 623 four-wheeler and is made in right-hand as well as left-hand form. The 1923 can be supplied for operation at 19 tons gross as well as the 16 tons as tested and the only change is that stronger rear springs are fitted.
The engine is the Daimler Benz 0M355, 11.58 litre diesel which gives a maximum net output of 230 bhp (DIN) at 2,200 rpm, the SAE gross rating being 250 bhp. The transmission incorporates a ZF 4K6-80 gearbox with integral twospeed splitter section which gives a stepup on the five lower ratios in the main box to make 11 forward gears in all. The rear axle on the 1923 is to the design used for some years on heavy Mercedes models and has separate load-carrying and driving sections. The drive from the hypoid-bevel primary reduction leaves the gear casing behind the axle beam and is taken through tubes to the hubs where there is secondary reduction through spur gears.
Although complex, the brake system of the Mercedes-Benz chassis is not as complicated as many British three-line systems. The air circuits for the front and rear axles of the 1923 are independent of each other. The two-line connections for the trailer are linked to the front-axle circuit and the tractor also had a circuit and connection for use when coupled to trailers having single-line brakes. The handbrake of the chassis has air assistance and is connected to the trailer connections to apply the brakes on the unit at the same time as those on the driving axle. The trailer brakes are also applied when the exhaust brake on the engine of the 1923 is brought into use. But the design is such that reduced air pressure is fed to the trailer when the exhaust brake is applied-66 p.s.i. as against 88 p.s.i. when the handbrake is applied and 95 p.s.i. for full service-brake application.
Other features of the 1923 specification include integral-hydraulic power steering and suspension which incorporates long semi-elliptic springs with telescopic dampers and torsion bars (to reduce roll) at both axles.
The main part of the test was a 356.5-mile triangular route worked out by Mercedes-Benz to my suggestion for a day's journey that would be similar to CM's operational route for heavies in Britain. Although the country-road section was less arduous than on the latter, the German autobahnen used had steeper gradients than British motorways. The first leg included, in fact, the highest point of the German autobahn system and in 18 miles the road rose some 1,700ft. It will be seen from the results table that the fuel consumption on this uphill stretch was down to 3.3 mpg but the following downward slopes saw an improvement to 11 mpg. This explains the two figures which differ widely from the rest and the majority of the other figures conform to a regular pattern. The reason why it was possible to obtain consumptions from point to point was that the truck was fitted out with metering equipment which gave a continuous reading of fuel usage.
To break down the route under broad headings, the 356.5 miles included 205 miles of autobahn, 123 miles of country roads and 28.5 miles of city roads. Working out the fuel used on the individual sections, the 1923 gave 7.1 mpg at an average speed of 39.7 mph on the autobahn, 6.03 mpg at 28.0 mph average speed on the country roads and 5.95 mpg at 19.5 mph in the cities. These are all very good figures considering the 38 ton gross weight of the combination.
General running on the autobahn was between 80 and 90 kilometers per hour 150 to 55 mph) but on the other roads the speed of other traffic was the limiting factor and in the cities there were two or three congested crossings to navigate as well as a fair amount of heavy traffic.
In spite of the length of the journey and time spent on the road the operational trial was not at all tiring, There were many factors which played a part in this, not least the general quietness in the cab. This, with the very good standard of interior, made driving the 1923 similar in some ways to driving a car,
The steering was extremely good, being very light and perfectly positive and the same could be said of all the controls— clutch, brakes, gear change and accelerator—but not of the handbrake which was rather heavy to apply and not all that well placed; it is an umbrella-type unit mounted in the floor and the effort had to be in a vertical line above it as the handbrake is positioned forward and a little to the right of the driving seat, to get maximum effort I found it almost necessary to stand up (or crouch) close to it: this and the tug from the trailer
were the only bad points of the vehicle.
The tug was worse on uneven surfaces but not at all evident when pulling hard on gradients. There were some very bad surfaces on the autobahn that we covered and on these the suspension showed up very well; there was also minimum roll on corners.
The Mercedes-Benz heavies have a relatively high floor line—and driving position—as the cab more or less slts on top of the engine. This makes access a little more difficult than on a low-set cab but the two steps up to the floor were not
found tooarduous because there are well-placed grab handles. And the advan-. tage of improved visibility with the high driving position far outweighed any difficulties. With an engine output of 230 bhp net at 2,200 rpm, 11-speed gearbox and rear axle ratio of 5.28 to 1, it was not easy to appreciate that the gross weight was 38 tons. The engine hung on well down to 1,500 rpm and acceleration times for the complete combination are comparable with times recorded with much lighter outfits tested previously by CM. Braking and acceleration tests were carried out on the second day on the Daimler Benz test track at Stuttgart with the 1923 solo but tests with the trailer coupled had to be carried out on normal roads. Brake tests with the trailer gave good results in spite of locking of the driving axle due to the "push-up" from the trailer but the stopping distances of 31.6ft from 20 mph and 58.2ft from 30 mph were better than most tests on outfits to the British maximum weight that have been carried out. The difference in the rate of retardation over the stops-13.6ft /sec2 anti 16.5ft/sec2 respectively—indicates the degree of delay in the system which is to be expected on a 59ft combination. The 16.5ft /see shows an overall efficiency of over 50 per cent which compares. with a peak efficiency as recorded by Tapley meter of 62 per cent from both speeds.
Good service brakes Good service brakes
Due to the partial application of the trailer brakes on operation of the handbrake, the good figure of 49 per cent was shown on the Tapley meter for this test. When solo, the handbrake gave an efficiency of 40 per cent—also good—and in this case the service brake tests produced better stopping-distance figures than any 16 ton gross four-wheeler so far tested by CM. The maximum retardation as shown by the Tapley meter on the solo brake stops was 69 per cent from 20 mph and 70 per cent from 30 mph and the stopping distances show overall efficiencies (taking into account lag in application and the build-up in pressure) of around 56 per cent. There were no marks from the tyres on the solo stops whereas the driving axle wheels had been locked for 15ft on the 20 mph stop and 36ft on the 30 mph test on the checks of the complete outfit. As a result of this lack of adhesion at the rear axle there was deviation from the straight line by the rear end of the 1923; but this was very slight and never more than lft 6in. Taken all round this was a very satisfying test not only from the aspect of performance figures but in the way the Mercedes-Benz gave a good account of itself in general handling, ease of driving and comfort. Trucks are usually noteworthy on one or two points and only average on the rest. Most can be criticized for something or other but few that I have driven have shown themselves to be of such high standard and with as little to find fault with as the 1923. In Germany, the model costs something like £5,400 and with high import duties it will not be a serious proposition in this country at present. But when Britain becomes a member of the Common Market, as it surely must one day, the chassis could be a serious competitor to British high-quality heavies.