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U.S. Forest Service
Caring for the land and serving people

United States Department of Agriculture

Forestry Equipment Chassis Configurations


Mechanized forestry equipment, in most cases, evolved from construction equipment. For this reason, many of the machines seen in forestry look and function very similarly to construction equipment. Even today, loggers use construction equipment in forest operations by attaching a forestry attachment to a construction chassis. Loggers may do this in cases where forestry-specific design features are not required or when the purchase of forestry specific equipment is not economical.

Engineers designed forestry-specific carriers to address issues with power requirements, terrain conditions, operating conditions, and general functionality. For example, they often have a higher ground clearance to traverse uneven terrain and a stronger chassis to cope with the demands of pulling loads. The machine forms used in forestry include the articulated chassis, excavator base, skid steer, compact tracked loader, crane, tricycle, and crawler.

Three factors influence the slope on which equipment can operate:

  1. Safety, defined as the safe operating slope at which a piece of equipment can operate without rolling or tipping.
  2. Gradeability, defined as the slope on which a piece of equipment can operate mechanically. Over this slope limit fluids leak out of containers and the engine can no longer pull the load.
  3. Soil disturbance and erosion, which is dependent on soil type, moisture levels and precipitation patterns.


Cabs on forestry equipment must meet stringent guidelines defined by the Occupational Safety and Health Administration (OSHA) for protective structures. In addition to rollover protective structures, they are required to have falling object protective structures. OSHA requires them to have solid or mesh protection around the cab. This provides additional protection for the operator against breaking cables and tree limbs. See OSHA Standard 1910.266 for an explanation of these cab requirements. Operators must modify construction equipment converted for forest-operations use to meet these requirements.

Tracks versus Wheels

The majority of ground-based equipment operates on tracks or wheels. The main differences between the two modes of transport are speed and soil affects. Tracked machines generally are slower than wheeled machines but have greater traction and climbing ability. For extraction equipment, tracked equipment has lower productivity over longer extraction distances.

Soil affects refer to soil displacement and compaction caused during the operation of the equipment. While tracked equipment has more ground contact (spreading the total weight over a larger area) the vehicle weight tends to be much more than comparable wheeled equipment. This translates into wheeled and tracked equipment exerting roughly equal ground pressure. The result is little difference in soil compaction between wheeled and tracked machines. Dual tire or high-flotation tires on wheeled equipment and wider tracked on tracked equipment decreases ground pressure.

The wheels and tracks on a machine transfer the weight of the machine to the ground. The weight of the machine divided by the ground contact surface area is defined as the ground pressure.

There is a larger difference in soil displacement between wheeled and tracked machines. Tracks translate their force to the ground over more surface area resulting in higher traction. This increased traction allows them to operate on looser and wetter soils. The method by which tracked equipment and skid-steer wheel-loaders turn can increase the amount of soil displacement that occurs. This effect is more noticeable in partial cuts where movement is limited and tighter turning is required.

Slope (by itself) limits the equipment’s ability to operate due to its effect on stability. Slope (considered in conjunction with soil conditions) limits the equipment’s ability to operate due to the increased susceptibility to soil displacement and resulting erosion issues, as well as operator safety. In general, land managers suggest a slope of 30 percent as the operating limit for wheeled equipment, whereas they suggest a maximum slope of 40 percent for tracked equipment. These numbers are dependent on the function being performed by the equipment, the configuration of the machinery (e.g., whether it has a self-leveling cab), and the soil and other conditions on the slope.

Swing Machines

The swing machine, or excavator base, is a common machine form that performs felling, processing, extracting, and loading functions in the harvest process. The undercarriage consists of two tracks mounted on a frame. The center of the frame houses a large ring gear and bearing where the chassis is mounted. The chassis contains the engine, cab, and boom. The chassis rotates on the base using the ring gear and a hydraulic motor. A swing chassis turns by slowing, stopping, or reversing the track on one side of the machine.

Excavator-based log loader.
Figure 1 - Excavator-based log loader.

Forestry-specific versions of the swing machine raise the height of the undercarriage frame to accommodate travel over stumps, logging debris, and rough ground. The plates protecting the undercarriage are more substantial, which protects the equipment from damage.

Cabs on excavator-based forestry equipment are located higher and closer to the front to increase visibility around the equipment. The cabs have additional windows on the top and bottom of the cabs to increase visibility to the tops of trees and to the ground. Cabs are required to meet the OSHA guidelines. The engine compartments also receive increased guarding to protect them from falling objects.

Another unique feature—developed for forestry operations—is the concept of zero-tail swing. Zero-tail swing means that the entire rear of the chassis rotates within the dimensions of the tracks. This prevents the rear of the engine compartment from hitting trees while it rotates and allows the machine to operate in partial cuts without damage to residual trees.

Forestry-specific swing machines have booms designed differently from construction booms. Manufacturers place hydraulic hoses and cylinders under the boom to protect those parts from falling objects. The boom geometry produces a motion that keeps the head moving parallel to the ground plane when moving toward and away from the base. Forestry-specific booms do not need as much up and down range of motion as construction booms designed for digging.

Self-leveling cabs allows the cab to remain level, which decreases operator discomfort and shifts the center of gravity when operating on slopes to permit safer handling of trees.

Tracked equipment generally benefits from decreased ground pressure, which minimizes soil compaction. They also have a high breakout force, which means that they can navigate on steeper slopes and looser soils without breaking traction. Another advantage of tracked equipment is that they can reach more areas of the stand without the need to move through those areas. This limits the amount of soil disturbance and compaction that occurs in the stand.

Articulated Chassis

An articulated chassis has a pivot point around which the chassis rotates. Each section of the chassis has at least one set of wheels. Instead of turning the wheels, the two sections pivot to steer instead of the wheels. Four-wheel-drive loaders, wheeled skidders, harvesters, forwarders, wheeled feller-bunchers, and clambunk skidders use this chassis design. This design allows for a tight turning radius and increased maneuverability within the stand.

Articulated harvester.
Figure 2 - Articulated harvester.

The large number of forestry machines that use an articulated chassis indicates that it is a very versatile form. The advantages are a robust design, good maneuverability, and good stability. Most of the designs are wheeled machines that are capable of faster groundspeeds than the tracked machines.

Some machines that use the articulated form, such as forwarders, clambunk skidders, and harvesters, may have more than one set of wheels on each articulation. This permits the use of tracks that slip on over the wheels. These tracks decrease the ground pressure and increase the traction, making them comparable to tracked equipment in their ability to decrease soil disturbance.

Crawler Chassis

The crawler chassis is a tracked configuration that places the cab and engine on the frame with the tracks. Unlike the excavator chassis, the engine and cab do not rotate on the track frame. Tracked skidders, crawler dozers, and purpose-built masticators use this configuration.

Tracked skidder.
Figure 3 - Tracked skidder.

Crawler chassis have low groundspeeds compared to wheeled skidders, and usually are reserved for areas where slope or tree size requires its use. This design is capable of operating on very steep slopes due to its high traction, high flotation, and low center of gravity.

Forest operations use two types of track configurations, rigid-frame tracks and flex tracks. Crawler dozers use rigid-frame tracks. A rigid-frame track rides on bogey wheels with limited suspension and flex. The drivers on flex-track crawlers are suspended and bogey wheels carry the load. The bogey wheels move up and down independently, which permits the track to conform to irregularities in the ground. Flex tracks maintain traction when traversing rough ground thereby limiting slope disturbance and compaction.

Flex-track crawler.
Figure 4 - Flex-track crawler.

Skid Steer

Skid steers (figure 5) are small pieces of equipment with two sets of wheels and a cab and engine compartment placed low between the tires. They turn by reversing or stopping the wheels on one side of the machine, the same as tracked equipment. This turning method permits the machine to have a zero-turn radius. They have a boom mounted in the front that is capable of accepting attachments. Originally, these machines were developed for the agricultural industry for operating in barns but have grown in popularity due to their low cost and high versatility. The most common uses in the forest industry are as feller bunchers, masticators, and brush cutters. Their small size allows them to work among closely-spaced trees, but limits the size of trees they can handle. They tend to have a narrow stance and short wheelbase, which limits their use on slopes due to the danger of rollover. Their small wheels and style of turning also increases their susceptibility to causing soil disturbance. They should not be used in broken, rough terrain due to their short wheelbase.

Skid steer.
Figure 5 - Skid steer.

The compact tracked loader (figure 6) is a skid steer with tracks. Its advantage is higher flotation and increased traction, which lets it operate in loose soil conditions with less chance of soil disturbance. The compact tracked loader has a short track and narrow stance, which limits its use on slopes.

Compact tracked loader.
Figure 6 - Compact tracked loader.


The tricycle configuration has two forward drive wheels and one wheel mounted under the rear of the chassis. It turns by reversing the wheel on one side of the machine. The rear wheel simply rotates 360 degrees in whichever direction the machine is going. These machines are relatively fast and highly maneuverable with low operating costs. They can be used as loaders or feller bunchers. These can be highly productive machines on even ground and low slopes.