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How to build a tandem tractor

How the Marshall family came up with their own on-farm engineering 
solutions to create a home-built, articulated tractor

Keith Marshall of Ninga, Manitoba, has had twins in his family since 1967. Twin Minneapolis-Moline U tractors, that is. Back then his father was in the market for a four-wheel drive tractor but wasn’t able to agree on terms with the local dealer. As an alternative, he decided to find a mate to a Minneapolis-moline model U tractor that was on the farm and connect the two together to make his own four-wheel drive.

“So, he had this brainwave,” says Keith. “He said, ‘why not make a tandem’, and I knew where there was another U.”

Keith bought the second tractor. Then, all he and his father had to do was figure out how to put them together. They’d heard of others doing it, but hadn’t seen exactly how it was done. They had no example to work from. In an attempt to find another to look at, they even travelled to North Dakota to see a tandem tractor they’d heard about there, but came back without finding it.

In the end, they had to use their own imagination to get the job done. “It was a total stab in the dark,” Keith adds. With the help of a local blacksmith, the Marshalls designed and built their own pivoting chassis to mate the two Us together into one, higher-horsepower machine.

The process

After both front axles were removed, one half of a two-section frame was attached to each tractor. The frame sections were joined together at a pivot point at the rear of the lead unit. “The pivots are the track rollers off a D2 Cat,” says Keith. They allow the two tractors to articulate, like an ordinary four-wheel drive.

Two hydraulic cylinders control the position of the chassis sections to steer the unit, again just like any other articulated tractor. Converting the standard, factory steering set up to control those cylinders instead of wheels on a front axle was done in an ingeniously simple way.

Instead of the steering wheel rotating up to three or more revolutions to control the angle of turn in the normal way, it now moves only a few inches left or right. The pitman arm on the rear tractor’s factory steering box controls an ordinary hydraulic control valve, which regulates the flow of fluid to the hydraulic steering cylinders.

Turning the steering wheel a couple of inches to the left causes the control valve lever to move, allowing fluid to flow to the steering cylinders pivoting the frame to the left. Turning the steering wheel a couple of inches in the opposite direction moves the control valve lever in the opposite direction to reverse the steering direction. How long the steering wheel is held to its maximum position left or right controls how long the hydraulic valve remains open. That, in turn, determines how much fluid flows to the cylinders and how much the frame articulates.

Because both of the current Us each have hydraulic pumps, the tandem unit still has extra hydraulic capacity to control implements behind it. “The way it is right now, I have overkill of hydraulic capacity,” Keith says.

The two throttle controls are linked together with a push-pull cable to control engine speeds on both tractors. But the tractor engines must be started individually. Gear selection must also be manually changed on each tractor.

The hand-clutch lever on the lead unit is engaged and disengaged by a hydraulic cylinder controlled from the rear unit. Initially, the constant pressure on the lever from the engaging cylinder caused the throw-out bushing on the clutch to wear out. To prevent that problem from reoccurring, Keith discovered that taking the engaging cylinder apart and drilling a 1/8-inch hole in its piston allows the pressure to drain off after the clutch is engaged. The small amount of pressure loss isn’t a problem during engagement because of the relatively light pressure needed to push the hand lever forward.

Both of the original model U tractors the frame was built for have since worn out and been replaced. The current pair were mated only a few years ago as a workshop project. “I put it together (with the new tractors) in 2010,” he says. “I bought one of the tractors and put it together after I retired from farming.”

Keith says the tandem arrangement worked very well for field work on his family’s farm over the course of many years. The tandem tractors actually outperformed another two-wheel drive tractor on the farm that had a higher horsepower rating than the two model Us combined. The current tandem unit, however, doesn’t do anything more strenuous now than march in parades. That’s something the impressive-looking unit is ideally suited for.

Do you have a farm machine you’ve modified or kept working over the year? Send an email to Scott Garvey with a description of your project and we may include it in a future issue. Be sure to include a high-resolution photograph.

About the author

Contributor

Scott Garvey

Scott Garvey is a freelance writer and video producer. He is also the former machinery editor at Grainews.

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