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The concept of combining four-wheel steering with four-wheel drive was a favorite idea in the early days of four-wheeling. Some of the more notable four-wheel-steer 4x4s include the 1900-1902 Cotta Cottamobile, the '04-'07 Four-Wheel-Drive truck, the '06-'12 American 3/4- to 10-ton trucks, the '13-'28 Jefferey (Nash from '16-'28) Quad 3-ton truck, the '14 Golden West truck, and the '15-'17 Beech Creek truck.
The four-wheel-steer rig that made the most lasting impression on the military was the Nash Quad. Not only did it serve with distinction in World War I, but its steering and drive systems were particularly well designed for the day. But then the Quad only had a top speed of 15 mph, so handling oddities weren't a problem. And the Quad's rear steering could be uncoupled with just a few minutes' work. Still, this was a rig that was remembered for many years. Jump ahead to 1940 and the development of the 1/4-ton 4x4. Right from the beginning, four-wheel steering had been considered by those involved with the development of the 1/4-ton 4x4 we now call the Jeep. As history has recorded, the American Bantam Car Company of Butler, Pennsylvania, was first off the line with a Jeep, although it wasn't called "Jeep" at the time. Eight of the first 70 prototype units from Bantam (the Mark II or BRC-60 model) were equipped with four-wheel steering. This was at the specific request of the U.S. Army's Cavalry branch, which was mainly responsible for reconnaissance duties. The Calvary felt the increased maneuverability was an asset, even though testers from other branches of the Army found the four-wheel-steer Bantam lacking in many respects.
The axles were coupled together so that they'd steer in concert, with the front and rear wheels steering opposite to each other. That did offer a very tight turning circle, but there were disadvantages like bizarre handling at higher speeds, the increased complexity of the components, and the extra cost to the Army. Testers found the four-wheel steer rigs more inclined to go rubber side up in tough terrain, and when pulled away from an obstacle, the back end would steer right into it.
It's hard to imagine why the four-wheel steer wasn't made selective, as with the old Quad. Referring again to the old couple-gear trucks, the rear steering could be operated independently and the rig could be made to "walk" sideways. That was useful for the streets of New York, where the couple-gear trucks were often found. It could have been equally useful on the battlefield. The Cavalry branch persisted with it's request for four-wheel steering, and in the spring of 1941, Ford contracted with the government to build 50 four-wheel-steer versions of the GP model for more tests, and Ford delivered the last of those in October of that year.
The GP was a variation of Ford's original Pygmy design, and nearly 4,500 were built. Contrary to popular legend, GP didn't stand for General Purpose. That term was never applied to any WWII vehicle, let alone Jeeps. In fact, the GP was a Ford engineering moniker where G signified a government contract vehicle, and P denoted the 80-inch-wheelbase reconnaissance car. The later Ford GPW used the same two letters, with the W added to signify a Willys pattern vehicle. The Ford was definitely the best of the four-wheel-steer Jeeps to that time. Unlike the Bantam's very touchy steering, the Ford's was progressive. The first quarter-turn of steering input didn't actuate the rear at all. After that point, the rear steering would gradually begin to rack in, increasing with more steering-wheel input. It made the GP much easier to maneuver at higher speeds. The Cavalry guys were pretty happy with it, but the rest of the Army was satisfied with the standard rigs, though they made it clear they didn't begrudge the Cav guys their special rigs. It looked like the Cav was going to get its way.
The bean counters and the highest levels of the Army finally got in the way. The four-wheel-steer program was killed in March of 1942 due to budget and standardization considerations. The special variant needed doubles of certain parts like axle U-joints and tie-rod ends, and those items were in short supply during wartime. Most of the bugs had been worked out of the unit, at least to the satisfaction of the Cavalry, and considering the low-60-mph top speed for the GP, the idea was probably workable.
A long rod transmits the steering input from front to rear. A standard Spicer 25 front axle was used in back, and the tie rod faced the rear. Rear steering was progressive so that during normal driving it wasn't excessively twitchy. The unit actually still drives well, and yes, it turns on a dime. The downfall of the Ford GP model in general was its unreliable and low-power 112ci converted Ford tractor engine.