Chapter 21 Straightening Full-Frame and Unibody Vehicles

Objectives List the types of straightening equipment and explain their operation Describe basic straightening and aligning techniques Identify safety considerations for using straightening equipment Plan and execute repair procedures

Objectives (continued) Identify signs of stress/deformation and make the necessary repairs Determine if a repair or replacement can be done before, during, or after straightening

Introduction Vehicles with major damage must often have their frame, body, or unibody structures straightened Vehicle straightening involves using high- powered hydraulic equipment, mechanical clamps, chains and measuring systems May involve the replacement of welded panels that are damaged

Alignment Basics Straightening refers to using alignment equipment to pull the damaged metal back to its original shape The vehicle is held stationary on the equipment Clamps and chains are attached to damaged area and a hydraulic system pulls out the damage Measurements are made at reference points while pulling to return the vehicle to its original dimensions

Figure Note how to find the general pulling direction. (A) Damage input came from the right front. The damage was from directions Y and Z. (B) Basically, apply straightening equipment force in the direction opposite to the input force. (C) If metal pulls out at different rates, change the pulling direction as needed

Figure If pulling force is divided between two directions (X and Y), the composite, or total, force direction (Z) will change freely with adjustments to force in the two directions. This shows that using two chains to pull allows for better adjustment of pulling direction.

Straightening Equipment Anchoring equipment holds the vehicle stationary while pulling and measuring Pulling equipment uses hydraulic power to force the body structure back into position Hydraulic rams use oil under pressure from a pump to produce a powerful linear motion Pulling posts are used to hold the pulling chains and hydraulic rams If the straightening equipment is straining while pulling, something is wrong – stop pulling

In-Floor Straightening Systems In-floor straightening systems have anchor pots or rails cemented or mounted in the shop floor In-floor systems provide single or multiple pulls, and positive anchoring without sacrificing space Anchor clamps are bolted to the specific points on the vehicle to distribute pulling force to prevent metal tearing Anchor chains are attached from the floor anchors to the clamps attached to the vehicle

Rack Straightening Systems A rack straightening system is a drive-on system with built-in anchoring and pulling mechanism A power winch uses an electric motor and steel cable to pull the damaged vehicle on the rack Rack systems are generally stationary, and can make multiple pulls Pulling towers are attached to the rack, and there may be two or more towers

Bench Straightening Systems Bench system –Portable or stationary steel table for straightening vehicle damage –Hybrid machine that allows quick loading like a rack, but has the other features of a bench Portable puller is a hydraulic ram and post mounted on caster wheels –Often used to pull minor damage –Many units of this type pull in only one direction

Other Straightening Equipment An engine crane can be formed on most racks or benches to raise or remove an engine An engine stand is sometimes needed to hold a power plant after removal from the vehicle An engine holder can be used to support the engine-transmission assembly Portable pulling/pushing arm can be anchored next to the vehicle to remove damage Portable hydraulic rams are small piston and cylinder assemblies for removing minor damage

Measure As You Pull! Vehicle measurement uses specialized tools to measure the location of reference points You must monitor the vehicle reaction to the pulling directions, changing locations as needed Challenges: determine which panels are out of alignment; in which direction; how far they have moved If you accidentally overpull parts, you may have to replace them free of charge

Part Removal Remove only the parts that prevent you from getting to the area of the vehicle being repaired Major straightening operations can be done with major mechanical parts intact Carefully analyze the vehicle and the damage to determine what must be removed In some cases it is best to remove parts before putting the vehicle on the bench Study the engine and transmission, and suspension mounts to see if they are damaged

Planning the Straightening Many times it is best to draw out the repair plan prior to actually straightening the vehicle –Drawing should include OEM and actual dimensions, anchoring, and pulling locations Determine where to straighten from by picturing the damage being pulled with “your bare hands” – Pulling process will work in the same way Straightening is needed whenever damage involves the suspension, steering, or powertrain mounting points

Straightening Safety Inadequate attention to safety can result in part damage and serious injury Use the straightening equipment correctly according to the instruction manual –Never allow unskilled personnel to operate straightening equipment –Never stand in line with pulling chain or clamp –Cover the pulling chain with a heavy blanket –Protect the vehicle and external parts

Straightening Basics Body-over-frame (BOF) vehicles can usually be straightened and realigned with a series of single-direction pulls Most unibody repairs require multiple pulls A single hard pull in one direction can tear the metal before it is straight Multiple pulls spread the force over a large area to minimize tearing Make a light pull first, hold the pressure, then pull on another area

Anchoring Procedures The anchoring points must be able to withstand the total pulling force applied Depending on the angle of the pull, the forces may not be equally distributed, and the anchor points nearest the pull may be loaded more Multiple anchoring uses four pinchweld clamps attached to the body at the front and rear of the rocker panel pinchwelds The bottoms of the clamps are fastened to the bench or rack

Figure When anchoring a vehicle, remember that anchoring strength must be equal to or greater than the pulling force that will be applied.

Anchoring Procedures (continued) Some unibody vehicles do not have pinchwelds, or are too weak to withstand pulling These vehicles use an anchor system that attaches to suspension or mechanical parts If the manufacturer does not have specific instructions for a vehicle, use a creative solution On body-over-frame vehicles, chains are attached in the four torque box areas Anchoring force must be at least as great as pulling force

Wheel Clamps Wheel clamps allow you to access parts behind the wheels and tires –Provide more accurate measurement of body alignment because the suspension is not left hanging unsupported Raise the suspension with a jack and remove the wheel, then bolt the clamp over the lug studs Lower the wheel stand onto the surface of the bench or rack Check anchoring chains or clamps before pulling

Attaching Pulling Chains One end of the pulling chain is fastened to the hydraulic ram, and the other to pulling clamps Do not attach a pull to any suspension or mechanical parts; use only mounting points Set the pulling clamp so that the line extending along the path of pulling force passes through the middle of the teeth of the clamp A welded pull-tab can be used to pull where there is no place to attach a clamp Strut pulling plates bolt to the top of strut towers

Figure When you need to “pull” and there is no place to attach clamps, weld small metal tabs onto the body structure. Then you can attach a clamp and chain to the metal tab.

Executing a Planned Straightening Sequence Progress toward alignment should be monitored with the measuring system during the pull Make the pulls a little at a time, relieve the stress and take a measurement Work from the center section outward Correct the length then the side sway damage, then the height Use a sequence that consists of a pull, hold the pull, more pull, hold and so on

Straightening Directions The pulling arrangement with the vector system is determined with a triangle – change the direction of the pull by altering the triangle shape Stop pulling if the chain between the ram and the anchor goes beyond perpendicular To make a down pull, attach a chain between the vehicle and the floor; pull on the chain bridge The ram is in a vertical position for upward pulls The vector system provides push capability from any angle by means of a triangle setup

Overpulling Overpulling means pulling the damage slightly beyond its original dimension If done in a controlled way, the metal will flex back slightly when the tension is released Overpulling damage results from failing to measure accurately and often When straightening damage, the metal will creep back when the pulling tension is released A slight overpull may be necessary to obtain the desired dimensions

Aligning Front End Damage Pull the side member on the replacement side in the direction opposite to the impact Repair the fender apron and side member on the repair side Repair the front fender apron and side member installation areas on the replacement side If there is severe damage to the side member on the repair side, separate the front crossmember and radiator upper support and repair separately

Aligning Front End Damage (continued) To repair the other front fender apron and side member area, the main repairs are near the instrument panel and the cowl panel If the impact was severe, the damage will extend into the front body pillar During front unibody aligning, confirm pulls by measuring dimensions at reference holes If impact to front side member is severe, height of standard measuring point may be distorted

Figure During a front collision, this frame rail was pushed back and down.

Rear Damage Repairs Rear bumper is usually impacted during rear- end collisions The impact force is propagated through the ends of the rear side members or nearby panels A rear collision damages the kick-up area, and the wheel housings may deform If the impact is severe enough, it will affect the roof, door panels and the center body pillar After pulling, damaged sections requiring replacement can be cut away

Straightening Side Damage If severe impact to the center of the rocker panel, the floor pan will deform To align this type of damage, use a method similar to straightening a piece of bent wire The two ends of the body are pulled apart and the side that is caved in is pulled outward When straightening the side of a vehicle, the center section can be anchored by passing a chain around the pinchweld clamp, and hooking to the edge of the bench

Straightening Sag Blocking under the low area and pulling down on the high end will correct sag The vehicle must also be tied down to the straightening system at the opposite end Sag can also be corrected by anchoring the high spot with chains and pushing up at the low spot Sag can also occur at the front frame crossmember The ends of the crossmember will be closer than normal and the center will be too low

Straightening Diamond Damage To straighten a diamond condition, place a pulling tower or ram on each end of the bench base on opposite sides Adjust chain height and attach chains to the vehicle as for end pull corrections Block or anchor one side of the vehicle to prevent side movement Activate the pull ram while measuring the results

Stress Relieving Stress is internal resistance a material offers to being deformed when subjected to a force Part stress is internal resistance to corrective techniques Mark creases or folds, then pull metal to tension, then loosen it with a hammer and heat if needed If heat is used, allow metal to cool naturally Use a heat crayon to monitor heat applications

Stress Concentrators Stress concentrators are designed into unibody vehicles to: –Control and absorb collision forces –Minimize structural damage –Increase occupant protection Do not remove designed stress concentrators Follow vehicle manufacturer’s recommendations for straightening or replacing parts that have designated stress concentrators

Final Straightening/Alignment Checks Final measurements should be compared to the unibody/frame dimensions book Walk around the vehicle slowly looking for obvious signs of misalignment Check down low at the alignment between the door and rocker sill Check general alignment of all upper body areas Open and close the doors and the deck lid and trial install large bolt-on parts

Summary Vehicle straightening involves the use of high- powered hydraulic equipment, mechanical clamps, chains, and a measuring system to bring the frame back to its original shape Straightening equipment includes anchoring equipment, pulling equipment, hydraulic rams, and pulling posts Stress is internal resistance a material offers to being deformed when subjected to a force

Summary (continued) Stress can be caused by deformation, overheating, or improper welding techniques Indicators of stress include misaligned doors, dents in aprons and rails, cracked paint and undercoating, and split seams and seam sealer Once repairs are complete, do a final alignment check –All measurements must be checked against factory specifications to ensure quality work