Friction measurement methods and the correlation between road - VTI
Friction measurement methods and the correlation between road - VTI
Friction measurement methods and the correlation between road - VTI
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Some devices use a fixed slip, like <strong>the</strong> Griptester or <strong>the</strong> BV11. There are atleast two reasons for that, <strong>the</strong> slip is chosen so that it will represent <strong>the</strong> optimumbraking friction <strong>and</strong> <strong>the</strong> wear of <strong>the</strong> tyre is limited compared to tests with lockedwheel. One trailer used in <strong>the</strong> Ne<strong>the</strong>rl<strong>and</strong>s, measures with about 86% longitudinalslip [54]. In that case <strong>the</strong> argument is to reduce <strong>the</strong> wear of <strong>the</strong> tyre, <strong>the</strong> frictionvalues achieved correspond more to <strong>the</strong> locked wheel friction than to <strong>the</strong> optimumfriction.In Norway Norsemeter AS 7 has developed a flexible friction <strong>measurement</strong> unitcalled ROAR [48]. In figure 3.9 it can be seen in <strong>the</strong> form of a friction<strong>measurement</strong> trailer, including <strong>the</strong> water supply for wet friction <strong>measurement</strong>s.The measuring wheel has a smooth tyre with an outer diameter of 410 mm,ASTM E1551. It is placed on a separate unit, which include all mechanical partsnecessary for <strong>the</strong> <strong>measurement</strong> so that this small unit can act alone <strong>and</strong> forexample measure dry friction (e.g. winter maintenance evaluation) placed directlyon a <strong>road</strong> maintenance truck. During one <strong>measurement</strong> cycle (about 1 second)ROAR measures <strong>the</strong> complete friction-slip curve, from pure rolling to lockedwheel. The device can operate at speeds <strong>between</strong> 20 <strong>and</strong> 130 km/h.Figure 3.9 Norsemeter Roar friction <strong>measurement</strong> trailer.3.1.2 The use of friction <strong>measurement</strong> in different countriesIt is safe to say that <strong>the</strong> friction of <strong>the</strong> pavement is one of <strong>the</strong> most importantproperties of <strong>the</strong> <strong>road</strong>. Almost every country has some kind of device thatmeasures <strong>road</strong> friction <strong>and</strong> many countries have regulations or guidelinesconcerning acceptable <strong>road</strong> friction levels. The major argument being <strong>the</strong>connection <strong>between</strong> pavement friction <strong>and</strong> accident risks.According to <strong>the</strong> Swedish National Road Administration <strong>the</strong> wet friction of <strong>the</strong>pavement is not a problem in Sweden <strong>and</strong> friction <strong>measurement</strong>s is very seldomdone, even on <strong>the</strong> major <strong>road</strong> network. There exists only very few (about three)7 Norsemeter AS, P.O.Box 42, N-1351 Rud, Norway<strong>VTI</strong> meddel<strong>and</strong>e 911A 19
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