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HydroDynamic Rock Buggy

Ken Brubaker
| Senior Editor, Four Wheeler
Posted July 1, 2010

This Is Way Outside The Box

Question: What does a Cal Poly-educated design engineer do in his free time? Answer: If his name is Jeff Friesen, he designs and builds this incredible hydrostatic-powered machine, which he calls the "HydroDynamic Buggy."

Why? "The forest is my favorite place to be. Building is my hobby. So I needed to build something to take me through the forest," Friesen says. "The original idea for the buggy came from working on the American Society of Agricultural and Biological Engineers (ASABE) 1/4-scale tractor design competition when at Cal Poly. Our team built conventional mechanical while the other teams built full hydraulic. I saw the pros and cons of both setups and decided that for a crawler, it would work well."

Friesen says the design started six years ago with research on pumps, motors, engines, and chassis. "The first sketches looked very different from what it has turned out to be, due to design constraints of the suspension and hydraulics," he says. "The suspension was the starting point once I had an idea of hydraulic drivetrain sizes. The hydraulics took priority of the suspension design." For instance, Friesen notes that hydraulic hoses are not as flexible as one would think, nor are they able to withstand rubbing against moving and turning suspensions. Further, hydraulic motors themselves are fine structurally, but are made to be lightweight and could not handle direct contact with a rock or a stump, so they had to be protected. Thus, he designed the suspension to serve two purposes. "The suspension is dual-function as a ride system and a moving guard for the hoses and motors," he says.

Once the design was set and just about every part on the buggy was cut to print, the fabrication started. Friesen says, "The square and rectangle tube allows for precise fitting. Unlike round tube with 0- to 360-degree increments, the square angles must come off at 90-degree increments. This had a large factor in the look and lines of the buggy." He says it also made the buggy easier to build, as the compound angles were minimal. Friesen says he did all the TIG welding, which made for very slow fabrication, but it was very clean and precise and he had a mental image of how strong each weld is on the buggy.

"The flat-plate design and CNC cutting allows for precision parts but also creates a lot of welding as each piece must be welded together. Bending would allow for a much faster build, but bending is not as precise as a welded joint," he says. Safety was a priority, so he joined the tubes together in a way that would reinforce and minimize the potential for a broken tube. He says the roof is also shear plate and secured to the beams to control them in case of a joint failure. The floors are also designed to support the cage across the full area, while keeping a minimal contact area to obstacles under them.

The photos you see here are from the buggy's maiden run in the Mojave Desert. Friesen had this to say after wheeling the HydroDynamic Buggy for the first time: "The hydraulic concept worked very well, and it was fun and easy to drive. The peace of mind of knowing that I couldn't break my drivetrain was relaxing when it was bound up and had all its weight on one corner. The chassis was a perfect platform for the hydraulics. The suspension and steering worked great as well."

So if you're on the trail and you hear something that sounds like a skid steer, it may be Freisen's incredible HydroDynamic Buggy coming your way.

The front and rear suspension (front shown) are basically identical and consist of equal-length A-arms, monotube shocks, limiting straps, and Slam Specialties 8-inch airbags. Up front, custom-valved Fox 2.0 shocks are used, while in the rear, custom-valved Ballistic 2.50 shocks are used. Friesen says the airbags have built-in stops, so they're not damaged when running low pressure, and the bags are rated at 200 psi with a burst rating in the 5,000 psi range. They are fed air from a custom stainless-steel tank, which is filled by a York engine-driven compressor. A Square D 180psi pressure switch automatically cycles while the engine is running. Each airbag has an air line that goes to an up and down valve located in the cab. The valves are controlled manually by pushbuttons on the dash. This system allows Friesen to control ride height for weight balance in technical crawling, as well as to lower the buggy's center of gravity or raise it for clearance. Friesen says this system gives the buggy a maximum two feet of height adjustment.

Driving each wheel is an Eaton drive motor coupled to an Auburn 28:1 planetary. One Sauer Sundstrand hydrostatic M46 pump powers the two front drive motors, while another pump of the same type powers the rear drive motors. They use a series system. In this system, the front pump sends oil into the right front wheel motor, and the oil spins through the motor and is then sent to the left front wheel motor, where it spins that motor before being routed back to the front pump to be recirculated again. The hydrostatic system is a closed-loop style, and the two M46 pumps are bolted together. These hydrostatic pumps are variable displacement, which means that their output can be varied based on the stroke of the piston inside the pump. The stroke is determined by hand levers on the pumps (i.e., levers mounted in the cab). By changing the output of the pump, you can speed up or slow down the motors. Friesen says the system is almost a 1:1 ratio, which means that at full output, the pump and motors will be turning the same rpm. The system also includes a charge pump that does double duty as the filter pump. All of the hydraulics are protected by relief valves, which safeguard the planetaries, steering, and chassis.

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