Is the horse pulling the cart or is the cart pushing the horse? Either way, what’s the driver doing?
In all discussions of four-wheel-drive tractors towing heavy trailers, Brian Olson makes it clear that the tractor and driver must remain in control.
The moment your trailer starts pushing the tractor, you’re probably headed for a bad scene. Olson personally experienced this while conducting tests with Prairie Agricultural Machinery Institute to upgrade ASABE S33.8.4 on safety chains.
“I drove the tractor 20 times in the test where we dropped a fully loaded grain cart to recover data from the disconnect on the safety chain. It only takes about 16,000 pounds of draft load to move an 875 bushel cart around a soft field. So any tractor can pull it, even a lightweight 20,000 lb. tractor.
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“The issue is control. We had a 45,000 lb. four-wheel-drive dropping the cart for this test. I could not even feel the disconnect in the cab. That tells us farmers need to ensure they use the right sized tractor for these big carts.”
Braking requirements can be calculated by a formula that considers speed, mass, rolling radius and surface conditions, according to Brad Stage, engineer with Mico, a world leader in building brakes for heavy equipment.
“Most of us are not capable of calculating this, so we react by feel and experience,” said Stage.
“But when you consider that speed exponentially increases the braking force needed, you must understand that high speeds require much more braking force regardless of the mass of the tractor. It’s a simple concept, but one worth repeating. If a trailer is not equipped with brakes, its inertia will keep it moving faster than the tractor that’s doing the braking.”
The same rules of physics apply when pulling NH3 tanks with a pickup. An engineer with years of experience dealing with anhydrous carts made the following comments.
“When you talk about the grain cart pushing the tractor, the same thing happens with NH3 wagons. A large single or double tank unit can easily reach 20,000 to 30,000 lb. loaded. And it’s usually towed by a three-quarter ton or one ton pickup on a gravel road out to the farm. The pickup likely only weighs 6,000 or 7,000 lb.,” said the engineer, who requested anonymity.
“That’s why some of them end up in the ditch with the tank upside down. The other reason is the hitch they use to drag the wagon is often just a light duty hitch two-inch receiver hitch meant to pull a 10,000 lb. camper max. NH3 tanks are so hard on hitches because of the surging of product in the tank pushing and pulling on the hitch. I have seen them rip the hitch right off the frame of the truck.”
