Axle wrap---not the proper demon? Ponder this... - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 12-13-2000, 04:17 PM
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Axle wrap---not the proper demon? Ponder this...

I have read many posts demonizing axlewrap as an incurable evil...and many about different ways to cure it. I was just thinking today and thought I'd share another problem to consider. If you've ever seen the suspension on a large tractor, you'd know that there is none. Some smaller tractors are now starting to have very basic front suspensions, but large tractors (tracked tractors excepted here) have no suspension. What they do have is some sort of mechanism that either allows the front axle to pivot independent of the frame or that allows the whole frame to pivot in the middle so that the tractor has some perceived axle articulation. There are no springs, bushings, etc. Everything is very solid. The purpose of the pivots is to allow 4 tires to stay on the ground when you drive one tire over a bump or through a dip. Anyhow, with this setup, you have articulation with absolutely 0 possibility of axlewrap, right? When you have a tractor that is hooking up (pulling) exceptionally hard, where it is reaching maximum tractive effectiveness, sometimes, you experience a really bizarre thing where the whole tractor starts bouncing up and down, very forcefully. It feels as though you've got horrible axlewrap in both axles. This sensation is known as power-hop. The gist of it is that there is a traction imbalance...say the rear tires hook up really well, and start pulling hard...that takes weight off the front tires and overloads the rear tires...the front axle rises slightly off the ground, the rear tires have too much weight on them and their tractive effectiveness changes, and the front axle comes back down and hits the ground. Now the front axle moving down has some momentum which causes the front axle to pull really hard momentarily...as it does this, the rear axle does what the front axle just did...it kind of bounces. Everytime an axle leaves the ground just a bit (really they don't leave the ground, they just reduce the amount of weight that an axle is carrying...the vertical movement is absorbed by the large tire sidewalls), the opposite axle hooks up hard and everytime the axle comes back down, it pulls hard. This is a pretty rudimentary explanation of power hop, or torque-hop as it is alternatively called. Anyhow, even without any sort of springs to flex...with no way for axle wrap to occur...power hop occurs. I was thinking about it, and realized that sometimes, when jeepers are having problems with extreme power application causing hopping, it may be due to something other than spring wrap, especially in 4wd! As you can imagine, power hop only occurs in 4wd tractors. You deal with powerhop in tractors by playing with air pressure in the tires, and by adding/removing/shifting ballast to change the weight distribution. Sometimes, you have to make the tractor less efficient in order to avoid power hop. Just a point to ponder.

Moneyless, Will weld for jeep accessories.
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post #2 of (permalink) Old 12-13-2000, 10:17 PM
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Re: Axle wrap---not the proper demon? Ponder this...

[img]/wwwthreads_images/icons/tongue.gif[/img] Farmjeep, this is one time that I am going to take the opposite view and say that it "ain't the same thing". The power hop that you describe seems vastly different and the root cause seems vastly different than the predictible wheel-hop-due-to-spring-windup that autos have experienced ever since there was suficient engine HP to MAKE it happen. You're a teensy bit too young to remember this but in the thirties, fourties, and into the fifties, Detroit had a slew of torque tube equipped cars. The early Ferds were, of course, and the Chev-Lays and the Boowahs were also. You haven't lived till you jerk the rear end out of a '50 Buick Roadmaster. The axle and the enclosed driveline were one giant "Tee" shape and plugged directly into the rear of the Dynaflow Transmission. Those cars had a noticeable "firmness" to the acceleration, since the rear axle did not wind up at all. They also used sort of an enclosed ball joint to cover the one and only U-joint at the tranny, and of course those joints had to be CV. We always used the solid shaft drivelines out of the Ferds for crowbars, since Henry's meatllurgy was always primo. On one hand, axle wrap due to leaf springs sort of cushions the effect of dumping the clutch, but on the other hand the hop that it can cause can also blow an axle too.[img]/wwwthreads_images/icons/crazy.gif[/img]

CJDave
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post #3 of (permalink) Old 12-13-2000, 11:26 PM
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Re: Axle wrap---not the proper demon? Ponder this...

Farmjeep, Good argument but flawed logic. A jeep will exhibit the exact same wheel hop that the tractor does if you hook a chain to it and the other end to a pullin sled. Most of that wheel hop is induced by the hitch arrangement. Harry Fergusons 3 point hitch was an attempt to correct this by fooling the tractor into thinking the point of attatchment was well forward of the front axle, ie the tractor thinks it is pushing a load instead of pulling it.
A more accurate comparison would be a jeep vs. tractor drag race. lets see.. A spring over V8 YJ up against a 9N with a funk/ford v8 flathead conversion. I'll take the 9N in the left lane, you know that jeeps gonna take a hard right when the left front wheel lifts off the ground.

jjc

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