Engineering and Contracting . g about 33 ^S cm.) tread diameter, and 2<i in. () cm.) width of base. Thebrakes were of *he internal expanding type on eavh rear wheel. Typical results for all classes of urban roads tested aresummarized graphically in Fig. 1, and numerically inTable I. It appears that there are three principal elementswhich determine the tractive-resistance-speed curve forunit weight of a given vehicle, within the range of condi-tions covered by this test: (1) A constant resistance, see curve 1, Fig. 2; the mag- 560 ENGINEERINGAND CONTRACTING nitude A of which depends
Engineering and Contracting . g about 33 ^S cm.) tread diameter, and 2<i in. () cm.) width of base. Thebrakes were of *he internal expanding type on eavh rear wheel. Typical results for all classes of urban roads tested aresummarized graphically in Fig. 1, and numerically inTable I. It appears that there are three principal elementswhich determine the tractive-resistance-speed curve forunit weight of a given vehicle, within the range of condi-tions covered by this test: (1) A constant resistance, see curve 1, Fig. 2; the mag- 560 ENGINEERINGAND CONTRACTING nitude A of which depends on the lack of resilience of theroad surface and wheel tire material, i. e., on the energylosses due to displacement of tire material and road-sur-face material. This constant element A would be encoun-tered upon a smooth level road of the particular type con-sidered, in the absence of impact, air, and wind resist-ances. (2) An increasing resistance with increasing speed,due to impact losses (curve 2), which results from lack. 18 20 22 24 SPEED-KM. PER HR8 9 10 11 12 13 14 15 16 SPEED-MILES PER HR Fig. 1—Summary of Traction Resistance Tests. of smoothness of road surface; losses of this nature areusually known to vary approximately as the second powerof the velocity at impact; and (3) An increasing resistance with increased speed, dueto air pressure against the front of the vehicle, curve 3,which resistance is known to depend, roughly, on the sec-end power of the speed. The sum of the three curves foritems 1, 2 and 3, for the case of asphalt roads, results incurve 4. The constant resistance (1) may be briefly calledthe displacement resistance, item 2 the impact resistance,and item 3 the air resistance. The displacement resist-ance is low for hard pavements and high for soft pave-ments (of low resilience). The impact resistance is verymarked in granite-block roads, as already mentioned. Theair resistance, at any definite velocity, is the same for allcurves; because the air-res
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