Fact is, no two makes and models of vehicles are the same, from the floor to the roof, the width of the roof, or in the angles at the roof, the door stills, or A-pillars. Likewise, not everyone strips their car down the same way. And so, whether you’re planning to make a go of it yourself from scratch, or order a custom rollcage from a reputable manufacturer, understanding how to measure the lengths and angles on the vehicle will ensure that the cage is a perfect fit the first time.
Chassis Engineering, one of the leading builders of made-to-order rollbars and rollcages, fields calls daily from customers with a myriad of different vehicles, all with different wants and needs. With the right measurements, their jobs are easy. The challenge is ensuring those measurements are right, because once the bar is bent and it’s sent out the door, it is what it is.
According to Chassis Engineering’s Curt Perry, “When a customer calls and they want to put a rollbar or rollcage into a stock vehicle and retain the factory interior, we generally have those dimensions in our system for a range of popular vehicles. Where that changes is when they call with a specific vehicle, they’re going to skin the car, and they want to build a rollcage or a chassis. By that, we mean they’re taking every piece of non-essential steel out of the car, including the floor, the roof supports, and all.”
Whether you’re putting a cage in your daily driver or stripping the body down (like this Mustang) for a full-on chassis build, procuring proper measurements will ensure a nice, clean fit of the cage inside the car.
What helps Chassis Engineering to fabricate the proper rollbar or cage for the car is then obtaining dimensions for the inside of the vehicle. Because, with everything, including the bulky interior trim pieces removed, the inner dimensions are going to be much different than a stock vehicle — a factory, daily-driven Mustang or Camaro, for example.
The final product of a main hoop in a full chassis car at Chassis Engineering.
While the stock vehicle dimensions could be used, fitment is going to suffer. Roughly, an inch or two of headroom and shoulder room would be lost in doing so. A full-on, skinned chassis build creates a wider and taller cocoon for the driver, and allows for more room should the customer choose to install a Funny Car-style cage, as well.
“Our chassis kit is designed for the specific body that the customer is going to be using. It’s not a mass-produced carbon body, but a vehicle that they’ve skinned that has a unique measurement to it,” Perry explains.
Measuring For The Main Hoop
The supplied cutout from Chassis Engineering (or your protractor) serves as the starting point for the “A” dimension, or the measurement across the top of the headliner. By positioning the cutout at the corner of the roof on both sides in the desired position, you’ll mark the ‘0’ (zero) on the roof, and then measure across, side to side.
For rollers and roll cages, Chassis Engineering starts with what they refer to as the “A” measurement, which is fairly straightforward, but not necessarily easy to determine. It is, in essence, the top, straight section of the rollbar that runs across the roof, from the beginning of the bend on one side, to the beginning of the bend on the other.
With the marks made on the headliner, determining the “A” dimension is a simple matter of measuring from side to side.
To help with this, Chassis Engineering created a basic paper-cutout protractor and includes on their instruction sheets, which one can cut out and utilize to determine the angles within their vehicle. If you want to be a little more accurate, you can also purchase a more specific protractor-like tool that chassis shops use to help determine these angles. Whichever you decide to use, it will be key in determining the “A” measurements, as well as many other key angles within the body.
“We came up with this to help people determine their angles without having to buy a tool. I usually advise people to glue it to a piece of cardboard or something to make it more rigid,” Perry explains.
This is your aim — a nice fit to the body, but not so tight you can’t work between them to weld the chassis.
Perry shares that the “A” measurement can be done a couple of different ways. One, you can use your protractor, and with it set up in the corner, about 1/2-inch from the body, determine where the bend/radius of the bar at the roofline will begin, and make a mark on the roof, or lay a piece of tape at the spot. Then, you’ll do the same on the other side of the car, followed by measuring between them. Second, if you have a helper, you can simplify this by using a pair of protractors on each side, locate the bend location, and measure between them.
As with every step in this measurement process, don’t throw out the idea of simply getting your measurements and mocking it all up with paper, cardboard, or something else to check your work … skilled chassis builders do it all the time.
A diagram showing how the “A” dimension is determined, using the cutouts or a more purpose-made protractor.
Moving down, the “B” measurement is the length of the bar from the roofline to the start of the bend at the shoulder/doorsill, as it angles down toward the floor. To determine the rest of the main hoop, in terms of outside dimensions, you’ll need to get lengths for both height and width.
Fitment is critical to not only a professional appearance, but a car that offers ample room, so you want your bars to be snug to the body, without being so constrictive that you can’t weld the bar. Perry suggests around one inch of gap, perhaps a little more, depending on your desired fit and finish.
Measuring for the main hoop height. This is pretty self explanatory on a car with the stock floor, as that will be your base. On a chassis car, however, you first have to determine where exactly the bottom framerail (your floor, in essence) will be.
To get your height, you’ll first need to measure from the roof down to the floor (the “C” dimension), or the main framerails, depending on whether this is a chassis car or one with the stock floor. Measuring a car with the stock floor is relatively self explanatory; with a chassis car, you’ll generally want the framerail as low on the rocker panel as possible — this will be your “floor” as it pertains to measuring rollbar height.
With a fully skinned body, you’ll measure lengthways across the body at the rocker panels to determine overall width of the rollbar/cage at its base. The fitment here is up to the builder, and in general, a chassis car will allow you to get a little tighter fit than a stock body/floor car.
With that complete, you’ll then measure for the cross section of the main hoop, leaving the desired gap between the bar and the B-pillar. This will be your “B” measurement. Again, whether a chassis car or not, you want a nice fit against the body here. However, if this is a stock body car, you’ll need to leave room in the base of the your main hoop in relation to the body so that you can still get a welder in there. You can put yourself into a pickle pretty easily here, if you get too greedy with the width of your main hoop.
Lastly comes the “E” dimension, which is the cross section of the cage at the doorsill, or the base of the window. This will generally be wider than the “B” dimensions down at the floor, but not always.
With those measurements in hand, all that remains is determining the angles of the bends in the hoop. For that, we turn back to the supplied cutouts that Chassis Engineering has supplied (or your own protractor).
Adding The Rest Of The Cage
A halo bar being mocked up inside a skinned body at Chassis Engineering.
When moving to a full rollcage, there are a few additional measurements that have to be taken. Chassis Engineering, for shipping purposes, builds their rollcages with a halo bar, a U-shaped hoop, for the top of the cage, with a separate pair of bars on each side that drop down the through the dash to the floor. Some builders use a full length bar along the door, from the main hoop all the way through the dash to the floor, with a single bar across the top of the windshield. Chassis Engineering’s design eliminates one measurement — arguably one of the tougher bars to get right — from the process.
Like the main hoop, you need some basic dimensions for the halo bar. The first is the distance down the center from the main hoop to the windshield, along the roof (the “C” dimension of the cage). Then, you need a cross section; to do this, you simply measure the inside of the A-pillar, at the corner of the windshield, across to the same spot on the opposite side (this is the “A” dimension of the halo bar).
To get the “C” dimension of the top hoop, or halo bar, you’ll want to measure along the center of the headliner, from where the main hoop will be located, forward to the windshield.
Then, comes your A-pillar bar, which travels down the through the dash to the floor. Like the halo bar, this can be determined by measuring from the headliner/roof down to the bend in the front door jam. This is the “B” dimension of the cage. Your “C” dimension will then be from the door jam bend down to the floor.
This figure from Chassis Engineering’s interaction guide illustrates how to use a protractor to determine the angle of degrees of the bend, at the doorsill on your main hoop, and on the A-pillar bar at the dash/door jam.
Before doing that, however, you need to decide the base point for the A-pillar bar, at the floor. As we’ve mentioned, this will depend on whether this is a body-off chassis car or not. Regardless, you want to have the bar tucked neatly against the body, forward of the door, so it isn’t out in the open when the car door is open.
You can again use your protractor to get the angle on the bend at the door jam/dash. To do this, you simply place the 45-degree angle quadrant at the center of the bend in the door jam, and follow the arch up as it projects toward the roof, and down toward the floor. Subtract the small number from the later number, and you’ll arrive at the angle of degrees of the bend at the dash/A-pillar.
With these measurements complete, you should be set with your rollcage, whether you’re ordering from a builder like Chassis Engineering, or bending and cutting your own bar stock. As with anything in the construction of a one-off race car, any extra mock-up steps that you can take will ultimately benefit you in the long run, helping to ensure that all of your measurements and angle determinations correct and appropriate for the body
The cutout positioned at the A-pillar bend, with the 45-degree mark at the center.
One thing to keep in mind is that there is no exact science to a rollbar or cage; outside of the NHRA’s requirements for bar diameter and thickness and general placement, where you opt to position the main hoop and the rest of the bars, and the amount of gap between the cage and the body is up to personal preference.
Now, obviously, there’s more to a race car than what we’ve covered here, and especially so if you’re building a complete chassis car, with plenty to be measured both front and rear of the roll cage itself. For the sake of brevity, however, we’ve focused on rollbars and rollcages here. If you’re interested in gaining a further understanding of chassis measurement for the front and rear half of the car, give Chassis Engineering a call and its support team will guide you through those steps.