Saturday, May 27, 2017 Detailed Auto Topics
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The frame and unibody structure of a vehicle can be damaged in several ways. Almost all can be properly repaired, if it is properly diagnosed beforehand.

Improper diagnosis and repair of frame and unibody problems is very common. This is where the old adage, "Once the frame is bent it is never the same," comes from. In reality almost all frame and unibody problems can be corrected, if properly addressed.

Proper frame and unibody repair requires skill and knowledge††
Most improper frame and unibody repair starts with improper on no diagnosis of the problem.

There are five primary types of frame and unibody damage


Diamond

†Twist

†Sag

Sway

†Mash

Two of the more common types of frame and unibody damage are twist and sag. Either type can cause the vehicle to lean to one side, wear suspension and drive-line parts and wear tires. The distinction is subtle between sag and twist damage, but critical from a standpoint of repair.

Diagnosing the difference in twist and sag on a bent frame or unibody 

Unfortunately treating sag like twist or vice versa can cause irreparable damage. The symptom of sag in a frame†is most commonly seen as improper gaps in body panels. A gap that is wide at the bottom and narrows toward the top is one visible symptom of sag.

Symptom of unibody with sag at the cowl area

Twist damage to a frame or unibody is much harder to see with the eye. On pickup trucks it is somewhat easier to see.† This is†because of the square lines and separate body panels used in trucks. Severe twist in this truck can clearly be seen looking from the rear.

A pickup truck, seen from the rear and with a severely twisted frame

Twist damage on an SUV or passenger car, or a smaller amount of twist is much harder to detect. One common symptom of a twisted frame is a vehicle that leans down on one side. Leaning is due to the springs not carrying the same weight, because of the twisted chassis. The vehicle may also feel unstable, especially in turns and at higher speeds.

Frame and unibody twist can only exist in the center section of the frame or unibody. The damage may appear to extend throughout the chassis, but it is in the center section. This is imperative to know to make a proper repair.

In correcting twist, opposite low corners, of the center, are blocked up. Using a frame machine, the other opposite high corners are pulled down.† When corrected, the outer frame horns come back into position.† The center section was simply holding the horns out of place.† Trying to pull the frame horns level would cause additional damage and not correct the twist.

Setup to properly correct a twisted frame or unibody vehicle

In correcting sag damage, the low part of the chassis is supported. The rear is held and the high rail is pulled down. This reverses the damage, the same way it was inflicted. The center section is not affected in a proper repair.

proper setup to correct sag on a frame or unibody vehicle 

Of course any combination of the five damage types can exist together in the same vehicle. This makes proper diagnosis even more imperative. A properly repaired frame or unibody is restored to pre-damage condition. Selection of a facility that can properly diagnose the damage is key. Thatís why folks have trusted their frame and unibody damage to AGCO, since 1974. AGCO will set you straight.





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