How -to Examining Your Vehicle’s Frame

March 1, 2010 | By Larry Lyles

Time—and Collisions—Can Make Frames Sag, Twist and Bend. This Will Assess and Address Your Frame’s Condition.

LAST MONTH I addressed some issues pertinent to making sheet metal repairs on unibody vehicles. This time I want to talk about issues relevant to framed vehicles.

In particular, I want to talk about the frame itself.

That’s because when it comes right down to it, as long as the frame underneath your vintage ride is straight enough to be within factory specifications, you can literally cut out and replace just about any part of the body that’s mounted on top of that frame without ever worrying about affecting the structural integrity of the vehicle.

That said, you still need to brace the body just like I showed you in the unibody article before doing any cutting and replacing, and you need to know for certain that the frame underneath that body is straight enough to make those repairs.

Checking the Alignment

It’s The “straight enough to make those repairs” issue I want to go into deeper here. That’s because it can be very difficult to determine just how straight, or in alignment, a frame really is without taking a close look at it. Let’s Face it, with the body hiding most of the frame, you’re left to guess whether the frame is in good alignment, has any twist, or sags in the middle. I’ll start with frame alignment and then move on to twist and sags.

The alignment I’m talking about has nothing to do with the front suspension. This alignment has more to do with collision damage. Your Vintage Ride Hasn’t Been wrecked? That’s great…if you or someone you know purchased the vehicle new and you can be certain about its history.

But most of us have purchased our old rides used and aren’t all that sure about their history. So, drag out the jack stands, a tape measure and a notepad. It is time to verify the condition of your frame.

Before I sit this ride up on jack stands to take measurements, be sure you understand that in this instance I’m working with the frame only. The body has been removed and is currently mounted on a rotisserie for some cosmetic work. Why won’t this work with the body still on the frame? If the body is bolted to the frame that could influence the measurements I want to take. I’ll touch on this influence a little further along.

Placing the Jack Stands

Unlike unibody vehicles, where jack stands would be placed under the torque boxes, framed vehicles are best supported by placing the jack stands under the suspension components. That would be under each lower control arm out near the ball joints in the front and under the axle just inboard of the brake drums at the rear. All that the jack stands are doing is replacing the wheels and tires. That allows the frame to think it is on all fours where it is most comfortable.

So the plan is to leave the suspension on the frame? Yes, the suspension isn’t going to alter any of the measurements I want to take, and some of the measurements can’t be taken without at least the front suspension being intact. Besides, should a problem arise that requires the skills and equipment of a commercial body shop, having the suspension under the frame is necessary to deliver the frame for repairs.

Measuring the Alignment

In case you haven’t been in a body shop lately, I’ll let you know that more than 50 percent of the collisions out there affect the front of a vehicle. Of those collisions, over 50 percent affect the right side. Just thought you should know.

Since front end collisions account for the majority of wrecks, that’s a good place to start checking this frame for misalignment problems.

Notice in Illustration 1 thatI have taken some cross measurements as well as some side measurements on my frame. You’ll also notice in the caption for the illustration that this is a symmetrical frame, meaning, for example, that the measurement taken from points A to G is equal to the measurement taken from points B to F, and so on.

Are all frames symmetrical? Offhand, I can’t think of a single vehicle that shouldn’t be symmetrical when the measurements are taken from underneath the frame. Why take the measurements from underneath? Consider that when body repair technicians perform frame repairs on a crashed vehicle the only view they have of the frame is from underneath the vehicle. Thus the need for measurements taken from that perspective.

Getting back to the illustration, if I take measurement E to G on the right side, which happens to read 24 inches from the edge of the hole in the center of the front cross member to the edge of the indicated hole along the side rail of the frame, I will find a corresponding hole at the exact same location on the left frame rail (F) and a measurement taken from that hole to the hole at the center of the cross member should also read 24 inches. It’s that symmetrical thing; any measurement taken from given points on the right must equal the measurement taken from the same given points on the left.

Does it seem as if I’m calling right left and left right? Well, I’m not. Remember, this view is looking up from underneath the vehicle.

What if the measurements aren’t the same?Can you say frame damage?Let’s look at how this works of measurement E to G reads 25 inches and measurement E to F reads 23 inches. Since these two measurements should be equal, I first need to determine the difference between them. That would be two inches. Now, since this is a triangle I know from geometry class that my frame at the cross member has moved to the left exactly one inch. This Frame has sustained damage and must go see a specialist.

Now, what if the frame measurements are equal but the measurements taken from points C to F read 21 inches and points D to G read 22 inches? In that case I’m going to assume measurement D to G is correct and measurement C to F is off. How can I make that assumption? These measurements are taken from the lower ball joints back to a point on the frame. How many times have you seen a lower control arm bent forward? Me neither. That leaves the left control arm to have sustained damage at some point in which it has been driven back an inch. It needs to be replaced.

Do Try This at Home

Is taking measurements like this something you can do in your garage shop and expect to accurately verify the condition of a frame? Definitely. Even though your measurements will differ from mine, the thing to park in your brain is that the measurements must be equal on both sides of the frame. Of course, should you encounter a problem such as unequal measurements, take the time to write them down on your notepad. That’s because your next stop will be at a body shop for some frame repairs and it is always nice to verify your measurements with the measurements taken by the shop.

Do you need to know the factory specifications for your frame? No, taking the measurements I indicated in Illustration 1 will tell you about the condition of your frame and that is all you need to know.

Anything else you should know about frame alignment measurements?Yes, be sure to notice in the illustration that I have broken the frame down into three separate sections: the front, the centerand the rear. This is how frame measurements are always taken, from section to section.

Frame Twist

The easiest way I know to get twist in a frame is to drive the vehicle. How is that possible? Here’s an exercise for you. Open the hood on your vintage ride, start the engine and rev it up. What did the engine do, just sit there? No, it tried to rock over to the left, correct?

Note: Most engines rotate counterclockwise if you are seated behind the wheel and will have a natural tendency to torque to the left.

That rotational torque equates to automatic frame twist. Certainly this type of twist takes some time to become noticeable, and may never become noticeable until you lift the body off of the frame.

Remember I said I’d touch on why removing the body is important when taking frame measurements. This is where body removal tells the story. Without the body to help hold the frame rigid, the built-up rotational stress in the frame is suddenly released and any twist that has been stressed into the frame by the engine becomes apparent. Quite often what you will find is that the right rear corner of the frame measures higher than the left rear corner when the measurements are taken per Illustration 2.

That’s The Definition Of Frame twist. The vertical frame rail measurements, when taken at the measurement points shown in Illustration 2,will progressively increase as you move down the length of the right frame rail and will stay constant, or may even decrease, when taken on the left rail.

Of course, frame twist can occur due to other reasons, such as collision damage or from having driven the vehicle across a ditch, but the results will be the same. The vertical measurements will change as you measure along the length of each rail.

Is frame twist a serious issue? That depends, a slight twist may only be discovered once the body has been removed and you have taken a few measurements. Generally speaking, bolting the body back down to the frame will remove a slight twist with no noticeable consequences to the fit and alignment of the body components.

Serious Frame twist can render the hood difficult to open and close because it too needs to twist in order to latch. Serious frame twist can make the vehicle appear to lean to one side. That lean is often attributed to a weak spring, and, by the way, the lean won’t necessarily go away if you replace the old, weak spring with a new and stronger one. Serious Frame twist can also cause one or both of the quarter panels to buckle over the rear wheel house.

What’s the difference between minor frame twist and serious frame twist?

Let’s go back to Illustration 2 and take some measurements. If measurements A, B, C, D and E are equal from rail to rail, that is A on the left rail equals A on the right rail and so on, but measurement F on the left rail is off from the F measurement on the right rail then you have minor frame twist.

How much can the measurements at F be off? I’ve seen these two measurements differ by as much as three inches and not produce any ill effects to the body once bolted down. That’s because these rear rails are thin and can easily flex.

By contrast, should the two A measurements differ by more than an inch that would constitute serious frame twist and will require the attention of a professional. Why tolerate so little twist up here? The alignment of the core support, fenders and front bumper rely on these rails being at equal height. Even a slight (half-inch) deviation can ruin the fit of these components.

Note: The Datum line shown in the illustration is an imaginary level line from which all of these vertical measurements are taken. In this case, the Datum line is the shop floor.

What do you do if you find twist in the frame? Again, take notes as you measure the frame and then deliver your notes along with the frame to a qualified body shop. What shops are qualified? Only those that have dedicated frame machines. This will be an elevated driveon unit designed specifically for repairing frames and unibody structures.

Dealing With Frame Sag

This Is a problem that is almost exclusive to vintage convertibles. What happens is that the center section of the frame actually begins to sag downward, giving the frame a slight banana shape. Why does this happen? Without a roof to help horizontally support the body and maintain some semblance of rigidity, gravity takes its toll. That is, gravity and rough roads, bumper jacks and passengers all act to push the center section of the frame down.

Are there any telltale signs of frame sag? One major one is when the doors appear to sag and become hard to open and close. Notice that I said they appear to sag, not that they actually do begin to sag. What has happened is that because the center section of the frame has begun to sag, the top of the door openings have narrowed slightly. That, in turn, pitches the back of the doors downward slightly, making them hard to open and close.

Is there a fix? Take a look at Illustration 3. If you take vertical measurements every six inches along the length of the center section of the frame, you will be able to identify exactly where the frame is sagging and by how much. This information, along with the frame, can be delivered to your local body shop where the frame can be mounted on a frame machine and straightened.

But what if the amount of sag I measured is less than 1/2-inch at the worst point? In that case, I would forgo the trip to the body shop and instead opt for adding body shims between the frame and the body to lift the center section of the body into alignment. You’ll know you have achieved good alignment when the doors open and close with ease and the gaps around the door openings are nice and even all the way around the doors.

A tip: Quite often a gap will be created between the frame and the body at the center section mount locations once the body mount bolts have been removed. This gap is caused by the body desiring to return to its original shape. That would be nice and straight with the doors opening and closing properly. This condition is telling you that you will need to add body shims between those mounts and the body.

Don’t Touch the Sheet Metal… Just Yet

Now that the frame is nice and straight, how about some things you should do before making those needed rust repairs to the body?

Just like when working on a unibody vehicle, you can’t replace any part of the rocker panel without first removing the door or doors on the side of the vehicle where you are working. Why not? Just like working on a unibody vehicle, having a heavy door or doors on the vehicle can cause the body to flex out of shape if you cut the rocker panel apart. How is it possible for a body to flex if it is bolted to a rigid frame?

Look at Illustration 4. This is an exaggeration of what can happen to a body should the rocker panel be cut apart with a heavy door still attached to the cowl. The weight of the door will force the lower part of the cowl to kick forward and the rocker panel to dip down at the cut. If the cowl kicks forward then, obviously, the length of the door opening near the bottom of the door will expand.

Now consider that if you removed a length of rocker with the door still on the car this movement in the cowl could easily go unnoticed. Is the door going to fit once the new rocker panel is installed? Probably not, but even if the movement at the cowl was slight and the door does fit, you could end up with wind noise because the weather stripping no longer follows the contours of the door. Wind noise is expected in a ’30s or ’40s street rod and might be tolerated in ’50s rides, but it’s not acceptable in anything manufactured after 1960.

The best precaution against this movement is to remove the ladder brace from the unibody vehicle you were working on last month and install it in the framed car you are working on now. Weld this brace across the door opening just like you did in the unibody. This will prevent any movement, but just to be on the safe side, don’t make any cuts without first taking some cross measurements of the door opening. Yes, this is just like you did when cutting out the rocker panel on the unibody car. In some worlds all things remain the same.

Then there is the issue of interior floor pan replacement. If you slide under your framed vehicle with a shop light you will find a number of cross braces, brackets, and mounts welded to the underside of the floor pan. All of these, maybe with the exception of the seat belt mounts that are often welded only to the floor pan, must stay in the car when you cut out the floor pan.

The cross braces help maintain the integrity of the body structure and the mounting brackets are usually there to help secure the body to the frame. None of these pieces will be welded solely to the floor pan. All of them will also be welded to the rocker panels or other structural bracing pieces under the car.

What I’m getting at is that you can lay the new floor pan in place on top of the old pan and immediately determine which welds need to be drilled out or cut loose. The same can be said about replacing trunk floor pans. Everything structural is underneath the pan and should not have to be cut out to install the new pan.

Got a question? Send it along.


LPL Body Works, LLC

5815 Contented Lane

Amarillo, TX 79109