A wheel tractor$-$scraper is operating on a level grade. Assume no power de$-$rating is required for equipment condition, altitude, temperature, etc. Use equipment data from Fig. $10\mathrm{.}9.$

a$.$ Disregarding traction limitations$,$ what is the maximum value of rolling resistance $($in $lb$ per ton$)$ over which the fully loaded unit can maintain a speed of $20mph?$

b$.$ What minimum value of coefficient of traction between the tractor wheels and the traveling surface is needed to satisfy the requirements of part $($a$)?$ For the fully loaded condition, $67\%$ of the weight is distributed to the drive axle. Operating weight of the empty scraper is $70,000lb.$