Your Vehicle's Frame & Undercarriage

Most people rarely, if ever, look underneath their vehicle. The undercarriage of a vehicle often tells a story about the life the vehicle has lived. The story may consist of the climate the vehicle is operated in, the amount of pampering it has received, or whether the vehicle is parked in a garage or outdoors. The story may consist of a major wreck, abuse, improper repairs, bent components, or major rust. All of these things can be concealed or hidden by a nice paint job, undercoating, and new attaching components.

The traditional method of assembling a car or truck is called body-on-frame construction. Essentially, this process starts with an underlying frame, then the vehicle’s body goes on top of that. Nearly all modern passenger vehicles, except for trucks, and some vans and SUVs, have migrated from body-on-frame to unibody construction. The term unibody is short for unitized body or unit-body, which refers to the body and frame constructed as one unit. Body-on-frame and unibody are the two possible types of vehicle structure.

The unibody design offers the advantage of a lowered center of gravity and is generally lighter than a vehicle having a separate body and frame. The lighter unibody allows greater fuel efficiency and is cheaper to manufacture. It also offers better handling performance and ride comfort, and is safer overall since the entire body can absorb the energy forces in a crash.

Some modern vehicle manufacturers still use body-on-frame construction especially for vehicles that tow or haul heavy loads. The frame generally features two long rails of high-strength steel that are connected by shorter steel cross-members. As a result, these are often called ladder-type frames. This solid foundation is important for towing and hauling and is better able to withstand extreme twisting forces. This is a major advantage for off-roaders travelling over mountain roads, river beds, large rocks, logs, and other uneven terrain. Body-on-frame vehicles are less likely to suffer structural damage or fatigue from corrosion over time, because it takes longer for rust to eat through a heavy steel frame rail than through the sheet metal used on a unibody. On the road, however, body-on-frame vehicles are heavier, which means reduced fuel economy. Also, the same rigidity that is so helpful on the trail creates a noticeably harsher and less forgiving ride on the pavement, especially over bumps and potholes.

Unibody vehicles rely on the combined strength of sheet metal stampings and welded joints for their structural rigidity. The vehicle's roof, floor, A, B, and C pillars, firewall, and door sills are all key components to the unibody vehicle's skeleton. Many unibody vehicles incorporate front and rear sub frames that provide a place to hang the vehicle's suspension parts. Repairing a unibody vehicle after an accident can involve major sheet metal surgery that must be precise to ensure correct vehicle tracking. Major damage after an accident often results in the vehicle being unrepairable.

A unibody is designed to protect passengers by crumpling upon impact. Although a unibody frame absorbs impact energy of a collision better than a body-on-frame constructed vehicle, the damage to the unibody frame is permanent and can compromise the structural safety of the vehicle. Unibody frame damage can also cause vehicle mechanical problems. It is critical a unibody is repaired correctly.

A used vehicle inspection should always include the undercarriage and structure of the vehicle, which will either be unibody or body-on-frame. The inspection is best accomplished with the use of a hoist since this allows a comfortable viewing position underneath the vehicle. A high-quality flashlight should be used to visually inspect the floor pans, sub frame, wheel wells, and entire undercarriage.

Look for rust, and for marked differences in the condition of different sections. One pristine or freshly painted section in an otherwise moderately rusty vehicle is a reliable indication of a repair. Undercoating is often sprayed under vehicles to conceal fresh repairs, poorly welded joints, and hide rust. Inspect for bent, damaged, or misaligned sections. Bent undercarriage components will often exhibit popped paint with freshly exposed or rusted metal. Gobs of body sealer are often used to conceal wide gaps and improper repairs. Wheels and tires should sit in similar positions, as far as distance to wheel openings, on the front and rear of the vehicle.

Other symptoms that may suggest frame or unibody damage include:

• Excessive leaning from one side to the other
• Persistent tire wear and pulling to the left or right, even after replacing tires and performing an alignment
• Being told the vehicle cannot be aligned
• Excessive or unexplained wear to suspension parts or breaking of suspension components

The undercarriage of your vehicle should be inspected annually by an ASE certified technician, and the vehicle should be test driven to make sure it is tracking correctly. These steps should also be taken anytime you purchase a used vehicle. It is important to not only pay attention to the things you can see, but to also pay attention to vehicle components that are hidden and not seen so easily.