Slip/Stick
Page 50
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The science of tyre/road adhesion
THE Transport and Road Research Laboratory published a graph in 1973 which showed that the peak braking force coefficient of a good truck tyre on wet mastic asphalt— comparable to a conventional road surface with some degree of polish—at 60km/h
(37.3mph) was around 0.32g. And that of a typical car tyre was approximately 0.62g.
Commenting (CM, August 17, 1973) on the report accompanying the graph I posed the following questions: El Could tyres for heavies be built that would give the same braking force coefficient as car tyres given consistency of braking force 'application? CI And, if so, what would be the penalties in terms of cost, wearing life and reliability ?
I put the same questions recently to Dr A. R. Williams of Dunlop's tyre technical division. Earlier we had discussed the claimed advantages of Dunlop patented Delugrip road surface. Joining in the discussion Mr Barrie Allbert, development manager truck tyres, pointed out that truck/car tyre performance compatibility in the wet would be impossible to achieve in practice. This was mainly because of the much higher laden/unladen ratio of a truck, although near compatibility might be approached if additional axles were fitted to a truck in sufficient number to enable tyre pressures to be reduced from the normal 689kN/sqm (I0Opsi) to the average of car tyres, of say, 207kN/sqm (30psi).
Road surfaces
While the Dunlop 70 Series low-profile tyre (CM, May 9, 1975) has wear-capability, rolling-resistance and weight advantages over conventional counterparts and current research gives promise of improvements in the waterdispersal characteristics of treads, Mr Allbert is not hopeful of a breakthrough in the foreseeable future which would close the gap between the two types by more than 20 per cent.
Closing the gap will, in Dr Williams's view, mainly depend on improving road surfaces (initially of hazardous road sections) because the differential is reduced by an increase in surface friction. Of outstanding practical importance, the coefficient of friction of the most common type of surfacing material—hot rolled asphalt—is reduced by traffic polishing from 0.65 to 0.45 over a relatively short period, notably on sections such as roundabouts where a heavy concentration of traffic is the norm.
Current Dunlop research includes the development of a self-supporting rib system the purpose of which is to enable the number of ribs to be increased without reducing their stiffness, and therefore the stability, of the tyre tread. The truck/car tyre differential is in part explained by the impossibility of reducing the rib width of a conventional truck tyre tread to improve the water-dispersal rate in the absence of a thin-rib pattern which provides the stability of thicker ribs at the high tyre and road-load pressures of heavy trucks.
In the self-supporting tread system two adjacent ribs have a combined resistance to flexing under load which enhances stability and thus provides for an increase in groove width.
Water dispersal
A complex system of circumferential grooves, slots and sipes is incorporated in the tread of a car tyre to increase the drainage area, but a tread pattern of this type would not only affect the stability of the highly loaded ribs of a truck tyre; it would also reduce the road contact area of the rubber to below the minimum required to afford an acceptable braking force coefficient in the dry.
The rate at which it is necessary to disperse water from the tread of a tyre to ensure contact between the rubber and road surface depends on the speed of the vehicle, and if a comparison be made of a roundabout, an urban motorway and a national motorway it is obviously preferable to grade the macrotexture •according to the differences in average or permitted speeds. Design variations of Dunlop Delugrip could include three grades— close, medium and open—to cater respectively for low-speed, medium-speed and high-speed road sections.
While the rate at which the bulk of water on a wetted road surface is dispersed depends on the macro-texture of the aggregate mix (as well as the tyre tread pattern) the microtexture of the mix is all important to the dispersal of the residual film of water between the rubber and the road.
A Delugrip mix comprises two or more types of aggregate each of which has a high resistance to polishing, and the aggregates have differing wear rates. And it is claimed that this combination entirely eliminates traffic polishing of the road surface and closure of the drainage channels. At the same time it promotes surface friction consistency by a continuing process which ensures •that the macro-texture and micro-texture retain their properties throughout the life of •the road surface, which is said to be a minimum of 10 years.
The rate at which conventional surfaces are polished is higher in warmer weather, which softens bitumen; and in winter frost tends to disintegrate the surface and roughen it. Deformation of a surface rather than wear determines the need for resurfacing, and, according to Dunlop, Delugrip is far more resistant to deformation than hot rolled asphalt. Four ruts tend to develop in the surface of the slow lane of a motorway or dual-carriage highway under the impact of traffic, and these are caused by deformation.
Delugrip was developed jointly by Dr Williams and by Dr G. Lees of the Department of Transportation and Environmental Planning, Birmingham University. In addition to the production of a road surface at a normal cost which would provide a consistent coefficient of friction over a greater life span, objectives included a reduction of noise and spray, the latter being cited by Dunlop as a hazard second only to fog in motorway conditions conducive to multiple pile-ups. It has been applied under licence by the leading road-building contractors in the UK to some 50 sites, the longest stretch being about 3.7km (2.5 miles).
Sideways force
Skidding resistance properties of Delugrip may be judged by measurements of sideways force coefficients of the surface of Hammersmith flyover made before and after it was resurfaced with the material, the original hot rolled asphalt having been laid some years previously. An SFC of 72/75 was recorded in tests of Delugrip, which compares with a reading of less than half this figure in the case of the asphalt.
While the improvement has increased the braking force coefficient of car tyres proportionately, it has reduced the truck/car tyre differential from more than 2 to 1 to about 1.3 to 1, the BFC for highway type truck tyres on wet Delugrip being 'approximately 0.5/0.6. When a comparison is made between the two types of surface before the hot rolled asphalt has been polished by traffic, Delugrip is shown to have an SFC of 70/80, whereas the SFC of hot rolled asphalt is 60/70, at speeds up to 80Itm/h (50mph).
Of interest relevant to braking on hot rolled asphalt after some polishing, the differential is very substantially reduced if a comparison be made of the BFCs of cars and lorries when the wheels are locked, the coefficients being then about 0.12 and 0.16 respectively. If a truck were fitted with anti-lacking brakes all round and allowance were made for the fact that wheel locking is a normal phenomenon during emergency stops in the wet, its BFC would be comparable with that of a car.
Noise tests in the vicinity of Hammersmith flyover have shown that Delugrip affords a reduction of tyre noise of 3dBA and that average traffic noise has been reduced by over 2dBA.
Unladen running
As a footnote on possible ways of improving tyre/suspension performance, an automatic inflation/deflation system could be employed, according to Dunlop, to reduce tyre pressures for unladen running. In the case of a maximum-load eight-wheeled rigid, for example, pressures could be reduced from the normal 689kN/sqm (100psi) to 413kN/sqm (60psi). In addition to providing better suspension this would also be favourable to braking performance because it would subdue wheel hop and patter. Currently, this is of academic interest only, the production of a satisfactory long-wearing pressure joint being impossible, it would seem.