Part 4: The chassis essentials
Page 37
Page 38
Page 39
Page 40
If you've noticed an error in this article please click here to report it so we can fix it.
FOR a premium specification 32-ton ractive unit the chassis frame should be 'rom one of the quality makes available with maker's plating in the 38 to 45-ton-gtw -ange, so that there is a good margin )f strength. even if the operator intends o uprate the tractor beyond 32 tons ;tw when this becomes legal.
With a wheelbase of 3.5m (lift 6in.) tnd front overhang not exceeding 0.25m 10in.), it is possible to couple to 12m railers with forward kingpins within 15m 49ft 2--in,) overall length. Substantial louble front bumpers attached to the main rame would help to protect the front if the cab. particularly the headlights, rom the results of minor collisions. Towing iintles should be fitted both front and ear.
iammering Suspension by conventional semi-elliptic ;prings, with shock absorbers, is crude, nit sufficient for the generally good road .urfaces in the UK, but abroad there ire still long stretches of what the French ipdy describe as chaussee deforme or tutobahn which has not been resurfaced ;ince it was built in the 1930s, and or this type of surface something better S required to absorb the hammering. The :hoices are rubber or air suspension, and dthough the main application of both as been to trailers, it is possible to pring front and rear tractive unit ales by air or rubber.
Rubber suspension is not new, and the gorde system, for example, which uses 4etalastik rubber springs, was introduced in the Norde lorry in 1962. The front .xle is attached to parallel trailing links iith a lower link for transverse location, nd the springs are compressed between he links and their mounting brackets. 'he rear drive axle system is similar, xcept that axle location is by an upper bracket, and Norde claims that this uspension gives a good ride without the iaintenance complications of air uspension. However, air suspension offers a number of advantages, the most important of which is that the bounce frequency of air springs is less than that of rubber springs at maximum axle load and the difference increases as axle load decreases, which means much better damping and a better ride on bad surfaces, particularly when running light or part-loaded.
Air suspension is also self-levelling, both transversely and longitudinally, so the tractive unit would remain level with a heavy load at one side of the front of the trailer, and the lights would be at the right setting irrespective of rear axle load. Hands Neway Ltd supplies Neway air-ride drive-axle suspensions developed by the Neway Division of Lear Siegler Inc, for tractive unit rear axles up to 10 tons.
This suspension is of underslung trailing-arm design, the air spring being compressed between a transverse tie bar and the frame. Brake and drive torques are controlled by panhard rods and transverse location by a track bar. This system retains shock absorbers for damping, and the Seale-o-matic inbuilt weighing system is an optional extra. There is no Neway front axle suspension, but the Dunlop Pneuride is an air suspension system for both axles of buses and medium or light-weight goods vehicles and I understand that Dunlop could if necessary develop a heavy-duty version for a 32-ton unit from its Stabilaire trailer air suspension which has a capacity of up to 10 tons per axle. Neither rubber nor air suspensions increase the roll base. To do this would require transverse spring location, or independent suspension — both of which are too expensive for heavy goods vehicles.
Axle requirements The axle requirements for a two-axle premium specification unit depend on the intended gross train weight of the vehicle, which, in turn, depends on the maximum drive-axle weight allowed. Assuming tandem-axle trailers, an artic could gross 36 tons (36.6 tonnes) under the present UK 10-ton limit with a 6-ton front axle and no allowance for uneven loading. With 11-tonne axles the gross figure is 39.5 tonnes with a 6.5-tonne front axle, so to run at 40 tonnes a tri-axle trailer would be required. However, if the regulation limiting gross train weight to 3.5 times drive axle weight is applied, the gross for a four-axle outfit becomes 38.5 tonnes and a tandem-drive tractive unit would be necessary for 40 tonnes.
In this case it is possible that the EEC will decide on an 11.5-tonne axle limit so the rear axle of our premium specification unit will be rated 10 tons or 11.5 tonnes, the make and model depending on the gearbox chosen.
It would be convenient to use a rear axle of the same make as the gearbox, if possible, but a feature of the premiums specification unit is a very high top gear for maximum fuel economy, so the rear axle used must be available with high (numerically low) ratios. Also to keep transmission efficiency as high as possible, single-reduction axles are preferred and the rear axle should have fully floating half-shafts which are weaker than the driving head or propshaft (Hardy Spicer 1700 series), because with a powerful engine and no torque converter, transmission failure is always a possibility, hence the drive line is designed so that the weakest part is the easiest to replace.
Eaton axles meet these requirements. the 19120 series up to 32 tons (32.5 tonnes) and the 19128 series up to 40 tonnes gross train weight. The table in the transmission section shows that with the ratios 3.70, 4.11 and 4.33 and overdrive gearboxes it is possible to obtain engine speeds in the 1500 to 1600 rpm range at 60 mph.
Differential lock Differential lock is another requirement for a premium specification and this is obtainable with Eaton axles by fitting a Detroit Automotive Products "no spin" automatic locking differential. Another advantage of the Eaton axle is that it is particularly easy to fit a Telma retarder to the differential case. On ZP axles, made by Zahnradfabrik Passau GmbH, which is a subsidiary of ZF, the "Lok-o-matic" multiple-disc selflocking differential is optional, and Foden axles can be supplied with a diff lock.
I favour a 6-ton or 6.5-tonne one-piece "I"-section front axle with ZF power steering. Opponents of power steering argue that it removes the "feel" of the road so that it is easier to get into a front-wheel skid on a slippery road but this is not so and, anyway, a much more important advantage is that power steering can prevent catastrophe in the event of front wheel or tyre failure at speed. Such failures are now rare, although there was a wellknown (Continental) 36-tonne tractive unit which, until a modification was introduced about three years ago, used to shed front wheels, but since it had ZF power steering this did not matter much, so the practical advantage of power steering is easier manoeuvring.
ZF power steering is fitted to man) prominent makes of European heavy lorriv including Atkinson, Bernet, DAF, ERF Fiat, MAN, Magirus Deutz, Scania ant Volvo, and because of the feature 01 hydraulic reaction, it is possible to retair "feel" throughout the full operating lock.
Rim effort can be varied to the vehich manufacturer's choice by different torsior bars, so Swedish makes have a very tier rim effort of about 4 kpm while Britisl makes go for higher rim effort. In eacl case castor action is retained and it possible to operate the steering manualll in the event of hydraulic failure. It i; also possible to connect the steering geai to the steering wheel, by means of beve boxes and/or universal shafts, and by thi; means a tractive unit with the Cummin; NTA 400 engine could have right-ham drive. Of leading makes of British 32 ton tractive units, some have 10.00 x 20 tyres and some 11.00 x 22.5, but the majority of maximum-weight trailers have 11.00 x 22.5, so radial tyres of this size on 7.50 x 22.5 wheels would be fitted for a 10-ton axle load. If using Dunlop SP 111 Mark 2 an alternative size is the low profile 11 /70-R-22.5 tyre with a smaller diameter, giving lower centre of gravity and lower platform height, but higher angular velocity. For an 11-tonne axle it is necessary to fit an E20 /11.00-R-20 tyre or its tubeless equivalent E-R-22.5 / 12-R-225. The 12in. tyres have a larger area of contact and lower inflation pressure so would give lower angular velocity, better ride and better braking at a 14.9kg per wheel weight penalty. This size should be fitted for the safety advantage.
Braking system The best braking system obtainable is essential on a premium specification vehicle, but unfortunately the best obtainable system is a compromise. It is technically possible to fit anti-skid brakes to all wheels, but an all-wheel anti-skid system cannot be sold in sufficient numbers to make the price economic.
The brake system is that required to meet future European Standards, which is more complex than a system intended for current British C and U Regulations, because it includes four-system protection valves which ensure that, in the event of a pipe fracture or other serious leakage, the compressor will charge the remainder of the system to the setting of the protection valves. It is then possible to drive the vehicle, but with reduced braking power, as may be necessary if it is in a situation where stopping is illegal.
Continental vehicles usually have only service and emergency connections to the trailer, but a secondary connection could be added, although the trailer brake would still work through a change-over valve and the service line. This is a necessary requirement as it enables the driver, when picking up an unknown trailer, to test the trailer brakes. Another advantage of this arrangement is the separate parking brake valve so there is no combined parking! secondary hand-control valve. These sometimes stick in the "on" position.
A lock actuator should be used for the parking brake. Theoretically, the spring brake is a fail-safe system, but it must be borne in mind that should a pipe break when the vehicle is travelling at high speed the driver has no control over the spring valves, which will automatically come full on. With lock actuators, when parking for a long time or overnight, if the driver suspects an air leak, he can leave the vehicle in gear and use a viteelchuck if on a steep slope.
Automatic drain is taken for granted, and there should be provision for air supply to the brakes from a towing vehicle in the event of engine failure.
Anti-skid devices
Maximum braking efficiency depends on the coefficient of friction between the tyres and road surface, and with highpressure heavy commercial vehicle tyres, this is 0.7 on a good surface, such as dry concrete, so maximum efficiency is 70 per cent. With a damp or slippery surface the coefficient, and hence the efficiency, decreases rapidly and an emergency stop under these conditions will result in a skid and usually a jack-knife crash.
An anti-skid system for the tractive unit rear axle is essential to avoid jack-knifing, and although several companies are working on anti-skid devices, the Dunlop Maxaret is fully developed and can be fitted to new or existing units.
The current Maxaret Mark III is similar in operation to the Mark IIE described in CM for February 11, 1972, but has been simplified to provide even better reliability and to keep manufacturing costs down. It is intended to be an anti-jack-knife device, but on wet surfaces it also provides better deceleration during braking, the improvement being greater the more slippery the surface. When the Maxaret is working on an air-brake system there is no judder at the brake pedal as theri is if used with a hydraulic brake system.
There is no difficulty in making the air brake system powerful enough to lock the wheels at maximum weight, but, apart from the fact that a skidding vehicle is unsteerable, this condition does not give maximum braking efficiency as sliding friction is less than static friction, also rubber from the sliding front wheels would reduce the coefficient of friction under the back wheels. Therefore the premium specification tractive unit brake system should be designed so that it just fails to lock the wheels on a dry surface, thus giving an efficiency of between 65 and 70 per cent, provided the trailer brakes are in good condition.
This is still much less than the 90 to 100 per cent efficiency of most car brakes but, bearing in mind that the lorry driver can usually see potential hazards sooner from his high cab, it should provide reasonable safety. It is a vast improvement on current heavy lorry brakes most of which are dangerously inefficient. Of nine 32-ton outfits road tested by CM in 1971-72, the average braking efficiency was 42.8 per cent. None exceeded 50 per cent and three were below 40 per cent. In terms of stopping from 30 mph these figures mean:— It is not the purpose of this article to comment on the mentality of manufacturers and road transport contractors who Complacently make and operate such vehicles or the DoE which allows them to operate when the legal minimum is 50 per cent efficiency. (These vehicles can pass the DoE test on the basis of a meter peak reading of 50 per cent, so evidently this requirement is completely unsatisfactory.) The figures are quoted only to emphasize
the need for better brakes on heavy lorries. The operator of a heavy vehicle with much better brakes than average must secure loads properly, and it might be a good idea to carry a large warning sign on the back of the trailer.
The other necessary feature of the braking system of a premium specification vehicle is a retarder. The cheapest and most common is a butterfly-valve exhaust brake, but the degree of retardation is not as good as with, for example, a Jacobs brake. The Jacobs engine brake, fully described in CM for August 25 1972, is an effective retarder available on American engines such as the Cummins and a 34kg is not heavy, but because of its very high noise level I find it unacceptable for our premium specification.
The Telma retarder, described in CM for July 28 1972, is an eddy-current brake consisting of a stator attached to the differential or the back of the gearbox in the case of an artic, and a rotor attached to the propshaft. The braking effect is from the opposing magnetic fields produced in rotor and stator when the electromagnet on the stator is energized, an arrangement which has several advantages. Braking is very smooth, four degrees of retardation being available, and the retarder does not fade which is an important safety factor.
Retarder benefits Vehicle speed can be controlled by the retarder on long descents, leaving the vehicle brakes for use in an emergency. The main economic advantage is increased brake lining and drum life, which can be five to 10 times — so, with a high mileage vehicle the retarder soon pays for itself. Secondary advantages are improved tyre life due to the smooth braking — one company reports an increase from 45,000 to 60,000 in tyre mileage — and better running time owing to not having to crawl down hills.
The big disadvantage is the weight penalty of 178kg or not much less than two spare wheels in the case of the Focal 170, which is the appropriate model for a 32-ton artic, but the above advantages outweigh this, so a Thelma retarder is recommended. It must be wired so that it is cut out if the Maxaret works, as on a slippery surface retardation of the tractive unit rear axle is potentially dangerous. As with the Maxaret, it is much cheaper to fit a Telma on the production line than later, and this retarder is not an uncommon fitment on Continental trucks.
It will be obvious that, while a tractive unit with the above braking system is much safer than one without, a really safe artic combination must also have a premium specification trailer with Maxaret, Telma and air springing on the trailer bogie.
An automatic lubrication system would be fitted to the tractive unit to cut down maintenance time. As with every other component on the unit, the system used must be reliable, so the attachment of the oil lines to the lubrication nipples is important.
Twin fuel tanks, very large like the 80gal ERF or 90gal Foden type, would be fitted to enable fuel to be bought where it is cheapest. A vehicle returning from Holland with 800 litres of dery can still show a substantial saving over 1.7K price even though the Dutch price has been raised from 20 cents /litre.
On a unit with twin fuel tanks, the spare wheel has to be carried behind the rear axle on a winch attached to the frame rear cross-member, and the air tanks are mounted inside the frame on either side of the propshaft. The batteries are carried just behind the cab, but on the same side as the starter, not in the middle, to leave room for the corner of a close-coupled trailer. With the suggested 3.5m wheelbase there should be enough room for these items, as well as the Telma retarder and sanding boxes.
Sanding boxes Vehicles intended to operate in Central Europe or Scandinavia in winter should have sanding boxes in front of the rear axle. When using sanding boxes empty them overnight as' if the sand or grit has got damp it will freeze into a solid lump. The Bthderna Forss "Autosound", used on many Swedish vehicles, is suitable. In some areas, such as the French Alps, snow chains are obligatory and operators running there regularly might find it worth while fitting a power-operated jack near the rear axle to facilitate fitting chains.
Jacking the rear wheels up is the quickest way of fitting snow chains, as the approved method of laying the chain out and driving on to it usually results in enough of the sidewall portion of the chain being trapped under the wheel to leave a gap of about an inch between the ends. On a tractive unit with air springs, the jack need only be a lightweight model, sufficient for the weight of the rear axle, as the springs, used in the same way as in the swap-body application, would lift the rest of the weight. The rear mudguards would be rubber to eliminate bump damage, and hinged, or with a gap in the middle, to make fitting snow chains easier.
The custom of supplying new trailers without a fifth-wheel or trailer connections often results in buyers not paying much attention to these items, so unsuitable ones are sometimes fitted. For example, imagine trying to uncouple a trailer when the fifth-wheel is of a type where a small lever or catch on the front of the wheel has to be operated in conjunction with the side lever. The kingpin could be well back, and the distance between the top of the fuel tank and the bottom of the trailer about 6in. To avoid this possibility I would specify a York fifthwheel. Behind it would be sloping guides to facilitate picking up low trailers.
Catwalks Although catwalks are fitted on Ford tractive units some expensive makes still do not fit them, and a driver coupling or uncoupling brake lines on a winter's morning could slip off an ice-covered frame member• or air tank, trapping his leg. That is, assuming he found a way of climbing over the fuel tank. A catwalk with steps at both ends would be fitted, as would blank plugs for parking the brake and electric lines if the tractor is running solo.
Finally, two of the very minor points usually not considered: A tractive unit does not need a rear number plate, but only a number plate holder to remind the driver to take the number plate off a trailer when it is dropped; and, it is easier to check the pressures of the inner rear tyres if it is possible to reach the valves.