TARA Frame Modification Tech Session Takes Hands-on Approach

Jan. 1, 1999
COMBINING CLASSROOM INSTRUCTION with actual shop demonstrations, the Truck Axle Repair Association's (TARA) frame modification tech session provided an

COMBINING CLASSROOM INSTRUCTION with actual shop demonstrations, the Truck Axle Repair Association's (TARA) frame modification tech session provided an overview of truck frame modification techniques.

Led by TARA administrator Wayne Reich, the two-day course covered the basic fundamentals of frame lengthening and shortening, frame materials, frame strength factors, reinforcing, welding, and the proper use of fasteners. Tom Koedam, brand development manager for Dana aftermarket sales driveshaft division, reviewed basic driveline information.

The first day consisted of classroom instruction, with lectures and several short films. The second day was all hands-on instruction and practice. Class members were given the opportunity to try out plasma cutters, acetylene welders, and grinders in a shop setting.

The opening session covered the three main problems of driveshaft setups: poor lubrication of universal joints, alignment at incorrect operating angles, and over torqueing.

U-joint lubrication problems often result from using incorrect grease, Koedam said. Grease must meet National Lubricant Grease Institute grade-two specifications and be able to withstand temperatures as high as 325 degrees F.

In cold climates it is important to drive the vehicle after greasing the slip yoke, because cold temperatures can cause the grease in front of the spindle to harden.

One of the most recognizable signs of universal joint deterioration is vibration. The three most common causes of driveshaft vibration are driveshaft imbalance, critical speed, and u-joint operating angles.

Driveshaft Balance A driveshaft normally rotates at a rate of speed three to four times the speed of the tires, Koedam said. For this reason, driveshafts also should be balanced.

All driveshafts have a critical speed and a half-critical speed. Critical speed is the point at which a rotating driveshaft begins to bow off its normal rotating centerline. Half-critical speed is one half of critical speed.

When they approach critical speed, driveshafts begin to vibrate. If operated at or near critical speed for an extended period, they are prone to fail. Driveshaft failure can damage vehicles and injure persons nearby.

Driveshafts that are operated at a cruising speed or have a constant running speed that occurs at, or near, half-critical speed may experience a continuous vibration that cannot be fixed by balancing or any of the other common vibration remedies.

Driveshaft fabricators and installers are responsible for checking the critical speed of any driveshaft they fabricate or install, making sure it will not operate at or near critical speed and does not have a cruising or constant operating RPM that is near critical half-speed, Koedam said.

Critical speed of a driveshaft can be altered by changing its tube diameter or by changing the installed center-to-center length of the driveshaft. Changing the installed length of a driveshaft requires the use of multiple driveshafts with center bearings.

U-Joint Operating Angles The angle that occurs at each end of a driveshaft when the output shaft of the transmission and the input shaft of the axle are not in line, is the u-joint operating angle, Koedam said. The connecting driveshaft operates with an angle at each u-joint. Vibrations are created by that angle.

It is possible to reduce vibration with small angles on each end of a driveshaft. To cancel an angle vibration, the u-joint operating angles need to be equal within one degree at each end of a shaft, Koedam said.

The two types of u-joint operating angles are single plane and compound. Single plane angles occur when the transmission and axle components are in line when viewed from either the top or side, but not both.

Compound u-joint operating angles occur when the transmission and axle are not in line when viewed from both the side and the top. Their centerlines, however, are parallel in both views. The angle should be kept at less than three degrees.

Truck Frames The frame is the "backbone" of the truck, said Wayne Reich. The frame supports engine, cab, transmission, fuel tanks, and all other components, including the payload. It is also the bridge that connects the front and rear axles, allowing a useful tool for transportation.

The two basic requirements of a truck frame are sufficient strength to perform the intended job and the rigidity to provide a platform for the truck, said Reich. It should also provide stability, safe operation, and operator comfort.

Truck frame strength is the structure's ability to resist yielding, buckling, fracture, and fatigue.

The section modulus (SM) is an engineering term that indicates the strength of a frame by the shape of its section. The SM in truck frames relates to vertical loads on the frame rail, and is expressed in in3. The SM of a typical frame section is calculated by this formula:

SM=(BxH3)-(bxh3) 6H

SM= section modulus B=outside flange length H=outside web length b=inside flange length h=inside web length

RBM (resisting bending moment) is the product of the material yield strength and the section modulus of one sidemember. RBM is used as a means of comparing the relative strength of sidemembers with different geometry and fabricated from different materials. RBM equals SM multiplied by yield strength of material.

Frame Modifications Frames are modified to change wheelbase, alter the use of the vehicle, and to repair damaged areas, said Reich.

Before modifying a frame, certain facts should be known: what the vehicle will be used for, front and rear axle ratings, wheelbase, CA/CT, frame dimensions, frame specs, frame condition, and whether the frame is straight or tapered, or a single or double. It also must be determined what should be done to make the truck fit its vocation, and how will the modifications affect the truck. Safety factors also must be taken into account.

Before modifying a vehicle, it should be inspected, said Reich. The vehicle should be driven to detect vibrations, handling problems, and whether it turns equally left or right. The alignment of the rear axle or axles should be checked. Inspect the frame to see if it is straight, damaged, level, and assess its overall condition. All gauges and lights should be checked, as well as the brakes, springs, air bags, and whether the vehicle holds air pressure.

Wheelbase Increase/Decrease The load on the frame can increase directly with an increase in wheelbase if there is no change in axle rating or vocation, said Reich. A 10% increase in wheelbase will likely increase the frame loading (bending moment) by the same amount.

Changing a vehicle's vocation must be carefully planned out. If a highway tractor is modified to be a dump truck, remember that loads are applied to the frame on a dump truck in a far different manner than they are on a highway tractor. Crossmembers, suspension brackets, and other components may not be suitable for off-highway service. Most frame shops lack the capability to analyze the loading and determine the frame requirements. Experience, a good track record, and common sense are all beneficial to the successful modification of frames. It always helps to be cautious and ask questions, said Reich. Certification

After modifying a truck frame, certification is required by law, said Reich. The four types of certification are: incomplete vehicle, altered vehicle, intermediate stage, and final stage.

An incomplete vehicle manufacturer manufactures an incomplete vehicle by assembling components none of which, taken separately, constitute an incomplete vehicle. Those components consist of, at a minimum, the frame and chassis structure, powertrain, steering system, suspension system, and braking system.

Anyone who alters a completed vehicle (previously certified in the final stage) prior to the first purchase, is considered a manufacturer of motor vehicles, said Reich.

An intermediate manufacturer is a company, other than the incomplete vehicle manufacturer or the final-stage manufacturer, that performs manufacturing operations on an incomplete vehicle. If truck dealer A, for example, sends a new chassis-cab to truck equipment company B for installation of a lift axle prior to installation of a van body, and final-stage certification by body company C, then truck equipment company B is the intermediate manufacturer, because the manufacturing operation is performed between production of the incomplete vehicle and installation of the van body.

The final-stage manufacturer performs the manufacturing operations on an incomplete vehicle that make it a completed vehicle, said Reich.

The law requires that all vehicles must be certified in the final stage. Any manufacturing operations performed on motor vehicles prior to the first purchase must be certified.

The certification process ends when the vehicle has been certified in the final stage, and the vehicle has been sold, licensed, and titled in some state.