The rally truck in 3D

The engine behind the crew. The Cummins sits turned around behind the seats, flywheel forward, the way some Ultra4 race cars mount theirs. Its automatic gearbox reaches forward between the two seats and the transfer case sits ahead of it, so the crew sits ten inches further forward than before. The rear driveshaft runs back beside the engine in two pieces with a bearing between them. Turning the engine around turns both driveshafts the other way, because the transfer case's outputs turn the same way as its input. The plan is two built Dana 60 axles whose gears still drive on their strong side: a standard-cut centre turned over at the front and a reverse-cut centre at the rear, one of the few made. The gearbox is drawn at the real length of a four-wheel-drive 47RE, about 27 inches — to be checked with a tape on a real one before the tunnel is cut.

Real seats, and a view of the road. Two FIA-rated containment seats with head restraints (Racetech RT4119HRW, chosen because its full dimensions are published) on side brackets and rails, with harnesses from a bar behind them. Two of them beside the transmission tunnel made the cab wider, 1.85 m across the doors. The first drawing put the driver's eye just over the hood, hiding the ground for about 116 m ahead; the seats now stand 200 mm higher, and the ground is hidden only 15.7 m ahead. The steering wheel stays low, under the new line of sight. The pedals are a Tilton adjustable floor assembly on a slider with 177 mm of travel, so a short driver brings the pedals to them instead of sitting on a cushion.

Fuel up front, away from the heat. A 212 litre cell rides over the front axle where the engine used to be, on a steel skid tray behind a full-width bulkhead, filled from outside through the hood. A second cell stays beside the engine on the side away from the exhaust, which now leaves down the left side ahead of the rear tyre, behind a heat shield. About 63 gallons in all; the truck carries its weight about 51 % front and 49 % rear when full.

Radiator and spare. The radiator sits low between the rear frame rails, fed through a sealed duct from a scoop behind the cab, and its four fans blow up and back. The 40 inch spare lies flat on a rack above it, held by a centre post and nut, as its roof: in a rollover the tyre and the cage take the hit, not the radiator. A cage over the bed and a rear bumper close it in.

Why solid axles. The first version was a trophy truck with an independent front end, race axle shafts and a rack. Priced from published parts it came to $110,000–190,000. Solid axles and a built 12-valve come to about $95,000–115,000, which is roughly a new Raptor. The cost is top speed: this is a 70–100 mph desert truck, not a 110–140 mph one. The prices are estimates from a web search, not quotes.

Copied, not invented. The front is a three-link with a track bar, as long-travel solid-axle trucks use; its upper link runs from the top of the differential back between the two footwells, the way a Dana 60 diff truss carries one. The track bar is parallel to the drag link and the same length, so the axle moves up and down without steering itself. The rear uses 60 inch trailing arms with the shocks on the lower links, the trophy-truck layout.

Built to land on its nose and roll. The front of the cage is raked back 20° with a skid plate beneath it, so a nose-first landing glances and slides instead of digging in. A straight tube runs from each corner of the bumper over the front shock towers into the A-pillar, and the lights hang under the front roof bar, so in a rollover the tubes meet the ground before the lights, the shock tops, the radiator or the spare do. Both bumpers reach past the tyres, as the Ultra4 rules ask, so two trucks meeting nose to tail touch bumpers, not tyres. For the driver's view nothing crosses the hood, and the front shocks lean back 20° so their tops stand lower beside it.

The cage and the steering. The main hoop, the A-pillars and the bumper cage are bent on a CNC tube bender; everything else stays straight, so an owner can saw a new piece to fit, and every major joint carries a plate gusset as the race rules require. A steering box, pitman arm and drag link steer it, with a hydraulic ram helping — if a hose bursts, the mechanical link still steers.

The numbers. A 150 inch wheelbase, a 93 inch track and 40 inch tyres, with about 20 inches of travel. The front driveshaft is 28 inches long; with the transfer case clocked up 16° its worst joint bends about 17° at full bump and 13° at full droop. The frame and cage are drawn in 2 × 0.120 inch DOM steel tube for the Ultra4 4400 class, to be checked against the rulebook before any tube is bought. The engine, gearbox and transfer case are boxes sized from published dimensions.

The clearance check ran through the whole steering sweep, full bump, full droop and both diagonal twists, with both footwells and the pedal box kept clear. No part passes through another. What it does not show: strength, spring rates, the brakes, the fire system, and the cage as it would be welded. The figures are estimates from the design, not measurements.