Vehicle Specification and Modification

Frame geometry is defined by four interrelated clearance measurements that determine obstacle-crossing capability.

Ground clearance is the vertical distance from the lowest structural point of the frame to the ground surface, measured perpendicular to the ground plane. Platforms with 200 mm clearance clear most surface irregularities and shallow drainage features. At 300 mm clearance, large rocks, embedded obstacles, and moderate water courses present no structural risk. Clearance is reduced by load mass (see Load rating section), tire deflection under load, and suspension compression when lateral load is applied to frame or cargo mounts.

Approach angle is the angle formed by a line from the front axle center to the lowest structural point of the frame nose section. Platforms with approach angles below 20° catch on obstacles during upslope entry; angles of 30° and above provide clearance for steep lip obstacles. Approach angle is measured with the platform under baseline load, in level-ground configuration, with suspension at rest.

Departure angle is the angle formed by a line from the rear axle center to the lowest structural point of the frame tail section. Departure angle effects are identical to approach angle effects but during downslope exit. Platforms with departure angles below 20° catch on terrain at slope exits; angles of 25° and above clear most slope termination obstacles.

Breakover angle is the angle of the terrain slope at which the frame loses ground contact at its geometric center point (typically the midpoint between axles). This becomes critical on terrain where the front and rear wheels occupy different elevation levels simultaneously. Platforms with 15° breakover angle climb slopes to approximately 30% gradient; 25° breakover platforms climb to approximately 45% gradient. Breakover angle measurement assumes equal wheel diameter front and rear and level frame rake.

Axle configuration and wheel construction directly determine terrain interaction characteristics.

Wheel diameter varies across platforms in service, ranging from 600 mm to 1,200 mm. Larger-diameter wheels (900 mm and above) roll over obstacles more efficiently and reduce the proportion of frame height that sits close to ground. Smaller-diameter wheels (600–750 mm) provide greater ground clearance relative to total frame height but require steeper approach angles and higher breakover angles to achieve equivalent obstacle clearance. Wheel diameter directly affects overall gear ratio and therefore climbing gradient on slopes of equivalent inclination.

Axle track width (the distance between wheel centerlines on the same axle) typically ranges from 1,200 mm to 1,500 mm. Narrower track widths (1,200–1,300 mm) allow passage through confined passages and provide tighter turning radius; wider tracks (1,400–1,500 mm) improve lateral stability on slopes and reduce the likelihood of sideways sliding when cornering on steep terrain with low traction. Track width interacts with load distribution: wider tracks reduce the force concentration per unit ground contact area when lateral load is applied to cargo mounts.

Wheel construction occurs in two primary forms: spoke construction and solid construction. Spoke wheels (laced or spoked design) allow debris to pass through and maintain traction on rocky terrain where debris accumulation would otherwise bind a solid wheel; however, spokes can catch on sharp edges or catch on vegetation. Solid wheels (disc wheels) provide better impact resistance on sharp, angular terrain (basalt, shattered rock) and distribute impact forces over a larger area of the wheel structure, reducing puncture risk in high-vulnerability environments. Platform selection between spoke and solid construction depends on the surface type of the intended route.

Load rating per wheel varies by construction material and wheel diameter. Platforms typically distribute load equally across four wheels (front axle and rear axle, two wheels per axle). A 3,000 kg platform under baseline conditions distributes 750 kg per wheel. Exceeding wheel load rating increases the likelihood of structural failure during shock loading (sudden impacts, pit crossing) and accelerates fatigue failure at the wheel-to-axle connection.

Platforms in service undergo four primary categories of structural modification. Each modification alters measured specifications and must be accounted for in route planning.

Undercarriage armor plating consists of steel or aluminum plates mounted to the underside of the frame, protecting critical components from rock strikes and debris impact. Armor plates (typically 3–5 mm thickness) installed as full undercarriage coverage reduce effective ground clearance by 15–25 mm depending on mounting bracket design and plate thickness. Partial armor coverage (protection of specific components only) reduces clearance proportionally to the protected area. Armor plates also increase total platform mass by 40–80 kg, which translates to reduced wheel load capacity and increased suspension compression. Clearance reduction occurs because armor mounting points sit below the original frame surface.

Suspension travel increase is achieved through longer suspension arms, increased spring preload, or stiffer spring rates to accommodate deeper wheel articulation. Increasing suspension travel by 50 mm reduces measured ground clearance by approximately 25 mm (suspension compression under the platform's own weight accounts for this offset) and increases wheel load capacity by accommodating larger shock loads without reaching suspension bottoming. Travel increase does not affect approach, departure, or breakover angles, but it decreases the likelihood of frame contact on steep slopes where the platform articulates over terrain.

Wheel and axle replacement with larger-diameter wheels (from 700 mm to 900 mm, for example) increases approach and departure angles by approximately 2–3° per 100 mm of diameter increase, increases breakover angle by 3–4° per 100 mm increase, and increases the proportion of total frame height that sits well above ground. Wheel replacement also changes the gear ratio relative to suspension characteristics and increases platform mass. Axle width changes (widening from 1,300 mm to 1,400 mm) improve lateral stability on slopes by 10–15% (measured by maximum angle before sideways sliding occurs) but do not affect clearance specifications.

Frame reinforcement consists of internal bracing, gusseting at structural joints, or addition of reinforcement plates at high-stress points. Frame reinforcement increases fatigue life under shock loading and high-stress terrain (rock gardens, severe washing) but typically increases platform mass by 30–60 kg and may marginally reduce interior clearances (if internal reinforcement is added). Reinforcement does not measurably change clearance, approach angle, departure angle, or breakover angle if reinforcement is mounted internally. External frame gussets can reduce clearance if they extend below the original frame surface.

Additional cargo mass affects multiple performance specifications. All effects are directly proportional to mass added.

Ground clearance reduction occurs when cargo is loaded above the axle centerline, shifting the platform's center of gravity and compressing the suspension under additional mass. Each kilogram of cargo loaded at the geometric center of the platform (mounted at the frame centerline between axles) reduces clearance by approximately 0.5–0.8 mm depending on suspension spring rate and damping. Cargo loaded forward of center reduces front clearance more than rear clearance; cargo loaded aft of center reduces rear clearance more than front clearance. A platform with 250 mm baseline clearance carrying an additional 500 kg cargo at center mount loses 250–400 mm of clearance, reducing effective clearance to 0 mm (frame in contact with ground). Clearance reduction scales linearly with load up to the point where suspension reaches full compression.

Approach and departure angle reduction of 1–2° occurs per 1,000 kg of cargo added at or forward of the mount point, because the suspension compresses and the frame pitches slightly forward. Cargo loaded at the rear reduces departure angle more than approach angle; cargo loaded at the front reduces approach angle more than departure angle.

Breakover angle reduction of 2–4° occurs per 1,000 kg of cargo, measured across the full load range. This reduction reflects both suspension compression and the shifting of the center of gravity, which changes the angle at which the frame loses ground contact at its midpoint.

Wheel load capacity margin reduction scales directly: a platform with 750 kg per-wheel capacity at baseline, carrying an additional 1,000 kg cargo, distributes an additional 250 kg per wheel (250 kg per wheel × 4 wheels = 1,000 kg total), reducing the margin before exceeding the rated load capacity. When per-wheel load exceeds design rating, the likelihood of puncture, sidewall failure, and structural fatigue at the wheel-to-axle mounting point increases sharply.

Center of gravity shift is calculated directly from the mass of cargo added and its vertical and horizontal position relative to the platform centerline. Each kilogram of cargo mounted 500 mm above the axle centerline shifts the center of gravity 500 mm vertically per each kilogram, reducing lateral stability on slopes by a proportional amount. This is the primary reason for load distribution guidelines: concentrating heavy cargo low and centered on the platform minimizes performance degradation.