12 BULLETIN 463, U. S. DEPARTMENT OF AGRICULTURE. 



deciding this question, the advantages to be gained by reducing all 

 of the steeper grades on a particular road to a given maximum should 

 be weighed against the additional cost which the reduction involves. 



The following data and suggestions are intended to aid individual 

 judgment, which necessarity must be the prime factor in solving this 

 important problem : 



1. The cost of average pleasure traffic, horse-drawn and motor, is 

 practically unaffected by grades of not more than 6 or 7 per cent (6 or 

 7 feet rise per 100 feet, measured horizontally), pro\ided the con- 

 ditions are such that it is unnecessary to apply the brakes to vehicles 

 when descending the grades. But for traffic where loads are as im- 

 portant as speed, even very light grades may be of considerable 

 disadvantage. 



2. Increasing the steepness of a grade decreases in three distinct 

 ways the load a horse can haul: (a) for the same character of 

 surface, the required tractive effort or pull 1 per ton of load is 

 increased by about 20 pounds for each per cent increase in grade; 

 (5) the possible pull the horse can exert is decreased by an amount 

 equal to the effort required to lift his own weight through the rise; 

 this amount- is approximately equal to one one-hundredth of the 

 horse's weight for each per cent increase in grade; (c) the effective 

 pull of the horse is reduced by the change in the angle at which the 

 pull is applied. 



3. The pull a horse can exert on a level road varies greatly with the 

 individual animal, and is affected by the manner of hitching and the 

 skill of the driver. The character of the road surface also may have 

 an important influence by affecting the security of the horse's foothold. 



Tests 2 made by the Office of Public Roads and Rural Engineering 

 indicate that, on a level road, average farm. horses untrained to the 

 road can exert a steady pull for several consecutive hours equivalent 

 to from 0.08 to 0.10 of their own weight without undue fatigue, and 

 that by resting at intervals of from 500 to 600 feet they can exert a 

 pull equivalent to about 0.25 of their weight, provided the foothold 

 is good. 



4. The tests referred to above also indicate that with a well- 

 constructed wagon the pull -required to move a gross load of 1 ton 

 over a level road varies about as follows : 



Pounds. 



Loose sand road 315 



Average dry earth road (varies greatly) 150 



Firm earth or sand-clay road 105 



Average gravel road 80 



First-class gravel or macadam road 55 



1 Tull is here used to mean the actual strain which a horse exerts against the whiffle- 

 tree, and which might be measured by placing steelyards in the traces. 



- These tests are not yet complete and subsequent results may cause the conclusions to 

 be slightly modified. 



