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The Answer Can be Extracted from This Chart by a Simple Operation Based on the Knowledge of a Few Main Facts Concerning
capable of ascending a slope of about 24 per cent., that is, rather steeper than 1. in 4+.
In preparing this chart, overall transmission efficiencies of 94 per cent. and 86.5 per cent. have been assumed for the high and low gear ratios, respectively, whilst for road resistance, 40 lb. per ton gross weight has been allowed.
With regard to the tyre sizes, the calculations are based on the average radius under load of tyres of the sizes _given which cover most of the wheel equipment in general use.
With regard to low-pressure tyres, a workin approximation is afforded by the following list of the equivalent diameters (as used on the chart) of the various sizes:—
30 6-20; 31-13.5-20; 32-7-20, 9-16, 8.25-18; 33-7.520 ; 34-9-18, 8.25-20, 10.5-16; 35-7.5-22; 36-7.5-24, 9-20, 8.25-22, 9.75-18; 37-7.6-24, 9.75-20; 38-9-22, 8.2524 ; 39-10.5-20, 9.75-22; 40-9-24, 10.5-22, '9.75-24,. 11.2520, 10.5-22; 41-12-20.
Approximation Due to Possible Variations.
The combined effect of the facts that the radii of tyres vary with makes, sectional dimensions and wear, that road resistances are different for the 'many surfacing materials in use, and that the chart has, compulsorily, been compressed into a space that is too small to permit a high degree of accuracy, is to make it inevitable that the information it supplies is only approximate. Moreover, it depends basically upon torque, and this is, itself, virtually a rating which, in the majority of cases, can be obtained only from the manufacturers.
Nevertheless, the interest of the chart is 'considerable and it possesses a definite practical value, both for comparing performances and for drawing up' specifications of chassis required for known purposes.
Not Comparative without Road Speed.
Where comparisons are to be made, however, road speed must be taken into consideration. It can be arrived at from the r.p.m. at which the torque is exerted: If this -factor be disregarded, then the performance of a vehicle, with the biggest engine available installed, may appear to be no better than would be the • case if it were powered by a small motorcycle unit, suitably geared down.
A reliable and readily ascertainable figure for every engine is the piston displacement, and this should appear in all formulm for ascertaining a performance factor for purposes of .comparison. This, however, introduces a timehonoured, highly, controversial and; indeed, probably insoluble problem, which has no appropriate place in a straightforward consideration of calculating, grade-ability from certain simple data for which reliable values are definitely obtainable.