Posted by LenNH on February 03, 2010 at 14:34:19 from (24.60.236.239):
If you are an armchair engineer (I confess I am), you might like to search the web with terms like "tractor plowing speeds." I found the complete text of an old IHC booklet on plowing (but no pictures). There are summaries of some modern research on plowing done at universities, but the academic jargon is about as understandable as some of the stuff written by computer programmers. I have a copy of an article published in 1920 by Oliver Zimmerman (apparently of IHC) called "Tractor Plowing Speeds." It is fairly long and goes into a lot of different conditions, but one of the graphs clearly shows why steel-wheeled tractors could pull their "rated load" at about 3 mph, but not much faster. As speed increases (a straight line on the graph), plow draft rises faster than the speed (a curved line going upward on the graph). A plow that takes 4.5 hp at 2 mph requires about 12 hp at 4 mph. At 4.5 mph, the plow needs almost 15 hp!!It is easy to see that if your tractor is rated at 10 drawbar horsepower (which might put out 14 or 15 maximum hp), it will run out of steam around 3.5 mph (plow requires almost 10 hp at that speed). Anybody who has ever worked with steel-wheels will remember that the lugs really rob the power, and that increasing speed takes a lot away from the drawbar hp. I remember very well that our 10-20 would pull through anything in second gear, but would pull almost nothing in third. An F-20 that I used to plow with sometimes had been converted to 36" rubber. It would plow in second at close to 4 mph (h.p. of the 10-20 and F-20 are close enough to make a reasonable comparison), sometimes in third (estimate 4.5 mph) and even in 4th (over 5 mph) in very light soil. If you are an avid reader of Nebraska tractor tests, you can see the loss of power to the wheels with steel-wheeled tractors (I'd put it at about 35% loss on average; this helps explain the old ratings like 10-20, with the higher number for belt hp). Today, four-wheel-drive tractors are probably putting around 85-90% of the engine power to the ground. I'm sure modern plow bottoms are shaped for the new speeds, too. In "my day," I believe the moldboards were actually designed to do a good job of turning over the soil at about 3 mph. In the old tractor brochures from the 20s and 30s, they often talked about "plowing speed," which was usually in the neighborhood of 3 mph.
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