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Re: Re: Re: John T
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Posted by John T's Engineering Ramblings on February 12, 1999 at 20:08:15:
In Reply to: Re: Re: John T posted by al on February 12, 1999 at 15:42:27:
Hi again al. Lets go with the drawbar up closer the better question first and this time Ill explain it different. (Closer is better!!!!!) Physics tell us for every force there is an opposite and equal force, right?? When you stand on the floor you exert a force on it and it exerts an equal but opposite force against your feet. When the tractor pulls a force against the sled, that pull chain is also pulling back on the sled the same amount. BUT the direction of that force determines how much that force is straight down, which adds to traction, and how much is straight back which hurts traction. To get the most added traction by that sled pulling down on your wheels, you want that force vector to be straight down as much as you can get it as opposed to straight back against the tractor. If you understand force vector diagrams, you understand the geometric sum of the backwards and downwards forces equals the total vector sum of the force the tractor is exerting on the sled (some of its back and some is down but sum equals total). If that drawbar is way back behind the tractors axles, the force pulling back on that chain is more horizontal than vertical and its pulling the tractor back as compared to pulling the tractor straight down (what you want) as would occur if the chain was close to and high up near the axles. The bottom line is you want that force youre puttin on the sled to be pulling its reaction force straight down on your axles (more traction) as opposed to straight back horizontal which is what you get more of the farther back the drawbar. I think youre confusing the farther back and longer lever arm thing (although its true) the more it would tip the tractor up, transferring tractor weight off the front and on rear. The force were talkin about here though, is the force on that pull chain (nothin to do with tractor weight) and Im sure you understand it helps you more if its down on your axles (short drawbar) as compared with back (horizontal) towards the sled which doesnt help traction. NOW belly bars versus behind tractor weight question. In a static setting non pulling condition, the more the weights towards the back, the more force is on the driving axles and less on front which (other than enought to steer) is all wasted. Again then even when pulling, the max traction is achieved when you just float front end. But, the inherent front weight and COG and balance of tractor does not allow all the weight in rear, because the front lifts too easily. So, whether to use belly bars or behind seat weights and where to put them is dependent on tractor itself, but if each track had same identical bite, one setting would always remain the best which is the one that floats front end slightly. You just cant say therefore what location is always best because it depends on the bite of the track. The only engineering thing for certain I can say is the weights best the farther back it is just so long as the added reactive force of the pull chain doesnt lift the front end too high. Believe me, if anyone knew any best way to arrange the weight, they could sell the secret for big bucks, but theres no right answer for each tractor and each track. Start with weight as far back and if the front gets too light, move it more forward like belly bars etc. See, if you could hook to a point just very slightly in front of and above rear axles (shorter higher drawbar) then you would really have it cause the front end would never lift at all. To win in antique classes with a JD though, you need to figure a way to scream off the line and develop some serious early ground speed (building early kinetic energy momentum) like those fast out of the hole Ollies do and enhance it with the JD's ability to continue to develop almost full hp even when revs drop down. Then youre a sure winner, trouble is, getting those big heavy long stroked two cylinder engines to the high rpm's without shakin them apart. Although you could static balance them perfect, their great mass and physical size causes harmonic vibrations way worse than the 6 cylinder short stroke lighter machines. Enough already John T in Indiana
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