On a quest to find a cheap source of additional horsepower, Norfolk farmer Sam Hill came up with the idea of building a wheeled power pack to sit between a tractor and its implement.

To test his “Twin Pull” concept, he bought two old Sands self-propelled sprayers – based on 64hp David Brown 955 skid units – for £500 apiece and, with the help of engineering contractor Justin Dennis, set about marrying them together.

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The process involved building a frame to mount the skid units side by side, with a drawbar at the front attaching them to the lead tractor and a hitch at the rear to connect the trailed implement.

But the most complex job was rigging up a way of activating the clutches and throttles, allowing the driver to engage and disengage the drive, as well as matching the speed to the tractor up front.

To do this, they fitted linkages and hydraulic rams to each function, which are operated by two spool valves. There’s no way of shifting gears on the move though, so the most appropriate ratio has to be selected before work commences.

Telescopic drawbar

As an added safety precaution, the pair fitted a telescopic drawbar that releases tension on the throttle cables if the power unit starts to catch up with the tractor, causing it to slow down and prevent jack-knifing.

Total build cost was £1,500, which works out at less than £12/hp.

If he had a bigger farm, Sam says he’d consider building a more powerful setup with two 100hp tractor units which, when combined with a 200hp tractor up front, would provide 400hp for a relatively modest outlay.