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Checking bridge and row wear the German way

13th March 1982, Page 54
13th March 1982
Page 54
Page 55
Page 54, 13th March 1982 — Checking bridge and row wear the German way
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Which of the following most accurately describes the problem?

The Germans have been checking the effects of heavier gross vehicle-weights on roads and bridges. Bill Brock sees how they measure up

THERE has been much contention over the intensified wear on roads and bridges that any proposed weight increase may produce.

What is in little doubt on either side of the discussion, however, is the formula used to assess wear. The factor to the fourth power, established some 20 years ago by the Asher report, is as valid today as it was then.

In Britain we have been preoccupied with our own reactions and have not given much thought to the European considerations.

In January 1979 the Commission of the European Economic Community proposed a directive on weights which in the long term will have a greater bearing on the subject than anything resolved at a more local level.

At the top of the scale, the French have a present 13-tonne axle weight, which is far above what the UK is thinking of increasing to, namely 10.5 tonnes. Inevitably there will have to be a compromise, which is likely to be resolved in an agreement at 11 tonnes.

We in the UK may believe that we stand alone in our concern over the implementations of increased weight, but other EEC states are equally interested.

In Germany, joint-evaluation recently carried out by the Ministry of Transport, Daimler-Benz and the country's leading technical universities identified road stress as having two distinct elements — static and dynamic.

Static axle loads depend on the combination of axle loading, tyre type, tyre pressure and axle combination; dynamic stress is related to types of suspension, dampers, vehicle speed and the condition of the road.

To measure how individual factors influence road wear, a strip of road near Munich was strain-gauged. Vehicles with different characteristics were used in a road test.

The tests show that stress expansion of the road surface for twin-wheeled tyre combinations is half of that imposed by the super-single tyre running at the same weight. The theoretical stress curve values are not maintained because of the curvature and camber of the road surface; in practice they work out at 0.54 to 0.9.

The actual road damage resulting from an increase from eight to nine tons using doubletyre configuration is not as high as when single tyres are used on eight-ton trailer axles.

On a drive axle the effect is similar. An increase in the axle weight of one ton from 10 to 11 tons using twin wheels produces less road damage than 10-ton axles fitted with a large single tyre.

Another test over the same stretch of road surface reveals some interesting points about tyre pressures. Lower tyre pressures were shown to reduce road stress, but to take advan tage of this without reducing thE life of the tyre the vehicIE operator would need to fit large' and stronger tyres.

Daimler-Benz suggests tha for a reduction in pressure of " bar (14psi) the same perform ance could be obtained I)) changing the tyre size frorr 12.00-25 to 13.00-22. This however, carried with it othe disadvantages. Higher rolling re sistance would increase fue consumption by as much as fou per cent. As the trend is to harder tyres to reduce rolling re sistance and improve fuel con sumption, we must look to thr suspension for a softer ride Long springs and air suspensior with effective shock absorber: are two of the possibilities, bu rubber is difficult to make softe and larger volumes would la( needed to obtain the same re sults.

Daimler-Benz has used loni leaf spring on models from six tc 38 tons over the past 20 years with different designs to suit th, payload and type of operation.

Steel springs have a small ad vantage when the vehicle has tc match docking heights for un ading, while air is considered atter for distribution vehicles. ew parabolic springs can be lade to give the same quality of de as the standard air package. nly on passenger vehicles is a 3fter air option offered.

The second factor in the eight discussion is the effect

let an increase in overall eight would have on bridges. In the EEC Commission's roposal for standardising eights and other characteriscs of commercial vehicles, iere are suggestions which link -oss vehicle weight and wheelse.

Belgium, Denmark, France id Great Britain each has its Nn bridge formula. Although 3ch is different, they all deand on the distance between le extreme axles of a vehicle determine the permitted -oss weight. None of the other EC member states, that is Gerany, Ireland, Luxembourg, the etherlands and Italy, has a milar bridge formula. Yet the etherlands and Italy permit gher gross weights than that DW being proposed.

The purpose of the proposed 3gulation is to achieve a vourable bridge-load distribuon so as to put the least stress n the bridge. Linking gross Feight to wheelbase is intended reduce load concentration rid effective bending moment. Consideration of bridge :resses has not, however, led to ny technical or scientific justifiation for the introduction of ridge formulae. A report on the ..ibject prepared jointly by the ommittee of Common Market utomobile Constructors, the iaison Committee of the AutoIobile Industry of the countries I the European Communities, rid the Liaison Committee of the Body and Trailer Building Industry claims that stress on longer bridges is determined by the momentary presence of vehicle traffic.

It says that the overall wheelbase of rigid vehicles and vehicle combinations show no uniform tendency in their influence on bridge stress. Longer wheelbase vehicles may cause higher bending moments for a bridge than would shorter wheelbase vehicles. For shorter bridges it is either the single axle, tandem axle or four-axle group that determines the bridge stress.

An example given to me in Germany takes the case of a 12m vehicle where the wheelbase is extended by 1m. This reduces the weight of the axle to about five tons. To maintain the overall weight of the vehicle an increase would have to be allowed on the rear bogie, whch in turn puts up the stress on the road and bridge sections.

However, the theory has little practical application as more than one vehicle may use the bridge at one time. It is the multiloading condition which governs the bridge's strength requirements.

Vehicle designers are opposed to existing bridge formulae. They argue that they will lead to unsafe vehicles. Extreme reduction in the weight of the front axle is likely to lead to steering instability where the weight of the vehicle overrides the grip of the steered wheels. The longer wheelbase reduces manoeuvrability because of the large turning radii.

Access to the cab would require design changes and the wheel would intrude into the driver's cab space. Longer wheelbases would result in the use of heavier chassis and subsequent loss in payload, as well as having an adverse effect on driver comfort.

The EEC report and the CCMCCLCA-CLCCR report both conclude that an increase in axle weight need not impose any additional stress, either to road surfaces or bridge structures than is already being felt.

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