Fabricate a Main Hoop for a Roll Cage

Introduction: Fabricate a Main Hoop for a Roll Cage

We will be fabricating a main hoop for a roll cage using 1 5/8" diameter D.O.M. (drawn over mandrel) tubing with a wall thickness of .120". A main hoop is the foundation for any roll cage build and it is crucial that the fabrication be done with expert craftsmanship if it is to be used for any type of racing environment.

If it is to be used for a racing league be sure to follow the roll cage guidelines they provide.

Step 1: Remove Vehicle Intertior

The vehicle interior should be completely removed of all components leaving only painted metal surfaces. Welding will take place within cab of vehicle and sparks will damage interior parts, and wiring.

Note: Removing the interior is different for all makes and models and comes with its own challenges. Interior removal will not be covered in these instructions.

Step 2: Establish Location of Main Hoop

The main hoop should be located directly above the drivers head or just behind the head of the driver. It could also be placed adjacent the B-pillar of the vehicle.

  • The hoop should be perpendicular to the datum plane (floor of the vehicle).
  • Mark the floor of the vehicle where the legs of the hoop will sit.
    • The marks should be on either side of the vehicle floor near or up to the inner rocker panel.

Step 3: Fabricate Pads for Main Hoop

Note: If fabricating a complete roll cage, using a 1 7/8" hole saw, cut a hole through the floor where the legs will meet the floor (1 13/16" to the inside of rocker panel) before completing this step.

Using 3/16" steel plate fabricate a mounting pad for the legs of the hoop

  • The pad should have a surface area of at least 12".
  • Tack weld (small temporary weld) the pad in a few locations to secure it to the floor.

Step 4: Measure for Height

Two height measurements will be taken, both of the points measured will be needed for the width measurements.

  1. Measure from the mounting pad to the point where the roof meet the side of the vehicle and record.
    • This will be referred to as height #1.
  2. Measure from the mounting pad up to where the B-pillar begins to angle inwards toward the roof and record.
    • This will be referred to as height #2.

Step 5: Measure for Width

Three width measurements will be needed; we will start from top to bottom.

  1. Measure from point used for height #1 to the other side of the vehicle at the same point.
    • This will be referred to as width #1.
    • Subtract 1" and record.
  2. Measure from point used for height #2 to the other side of the vehicle at the same point.
    • This will be referred to as width #2.
    • Subtract 1" and record.
  3. Measure from inner rocker panel to inner rocker panel. The transmission tunnel may be in the way, if so the use of framing squares will help attain the measurement.
    • This will be referred to as width #3
    • Subtract 2" and record.

Step 6: Creating a Substrate for a Template

Using 36" masking paper, cut two strips as long as the car is wide. Lay them side by side and tape the together carefully, this is will be used to mark out a template for the main hoop.

Step 7: Transfer Height Measurements

Using a felt tip marker and a straight edge (long piece of scrap steel) draw a line (datum line) two inches up the masking paper parallel to the edge. This will represent you datum plane, place a mark at the center of the line. The mark will be used to create a center line later.

  • Measure up and mark height #1 on both sides of the masking paper.
  • Draw a line connecting both marks
    • Line should run parallel to the datum line
  • Repeat this same process for height #2.

Step 8: Transfer Width Measurements

Divide width #3 by two which will give the center of width #3. Align the center of width #3 with the center mark of the datum line and place a mark at zero and at width #3. Using a large framing square and and a straight edge, place the framing square on the center mark of the datum line, lay the straight edge up against the framing square so it is perpendicular to all the lines already on the template. Place a mark on the line for height number one, do not draw the center line. Measure diagonally from the mark on height #1 to the marks made from width #3. These measurements should be equal, if not adjust the mark on height #1 so that they are equal. Draw the center line.

  • Divide width #1 by two to find it's center. Align the center of width #1 with the center line at height #1 and mark at zero and at width #1
  • Repeat process for width #2 at height #2 and mark.

Step 9: Finishing the Template

Using a straight edge connect the marks on the left side of the center line, and repeat for the right side of the template.These lines will represent the outer perimeter of the main hoop. Measure the angle of each corner created using a protractor and write them next to their respective corners.

Note: The angles measured should be symmetrical on either side of the center line

  • Place the tube bending die for 1 5/8" tubing at either corner of height #1.
  • Be sure the die makes contact with each line made without going crossing the line and draw the curve.
  • Make a mark at the point where the curve intersect the line drawn for height #1 (closest to the center line of height #1)
  • From this mark measure 1 5/8" toward the center line and draw a mark. This mark will be transferred to the tube later and is known as the offset mark.
  • Repeat for the other corner at height #1
  • For the bends and height #2 follow the same process but the marks will be made on the line going up to height #1.

Step 10: Prepare Tubing

Now that the template is complete measure the outer perimeter of the template and add a foot to this measurement. This will give the total length of tubing needed to create the main hoop.

  • Measure the tubing's total length and mark it's center.
  • Align the center mark with the center line of the template at height #1. The tube should be parallel with height #1.
  • Transfer the offset marks from height #1 to the tubing.

Step 11: Prepare the Mandrel Tubing Bender

A mechanical mandrel tubing bender will be used to create the bends in the tubing. A mechanical mandrel tubing bender is a metal fabrication tool that can bend tubing up to 180 degrees. This tool is operated by hand using the principles of leverage to bend round tube incrementally. A long lever is attached to a ratchet and every time the lever is pulled the tube bends about five to ten degrees. The lever is placed back in its original position and the ratchet is placed in the next notch to allow for another five to ten degrees of bending. This is repeated until the desired angle is achieved. It can be equipped with a variety of tube bending dies to suit different radii of round tubes from a fraction of an inch up to 2". Bending dies for square tubing are also available at a slightly greater cost.

There are many different types of tubing benders but for the purpose of main hoop construction a mandrel tubing bender is superior to all other types. This is due to the fact that each diameter of tubing has it's appropriate bending die that cups the tubing so it does not crush the tubing. Instead a nice steady bend is created with very minimal distortion. If any other style bender were used the tubing would fail at every bend. Also if a roll cage was built with any other style bender the roll cage would not pass an inspection by a race league as it is deemed unsafe.

  • Place the 1 5/8" bending die into the bender
  • Place the matching follower block in correctly and grease it lightly
  • DO NOT grease the bending die.

Step 12: Bend Tubing

Place tubing in between the bending die and follower block. Be sure that the center line mark of the tube is on the ratchet side of the mandrel tubing bender. The bends at height #1 will be performed first, then the bend at height #2.

  • Align the offset mark with the leading edge of the bending die for the first bend at height #1.
  • Secure the tube with the proper U-strap, tighten the bolt on the bender so the tube does not slide out of position.
  • Begin bending the tubing to the desired angle. It is necessary to constantly check progress with your template as you do not want to over bend the tubing.
  • Pull tube out and place the opposite bend in the bender for the second bend at height #2.
  • It is important to keep the tube on a two dimensional plane. To do this use and angle finder and place it on the bending die, read the angle.
  • Place the angle finder on the side of the tube that has been already bent and make sure the angles match.
  • Proceed with bending, using template as a guide.
  • Place the tube back on the template and mark the offset from your template to the tube from height #2 on both side.
  • Bend the tube for both bends at height #2 as instructed for the second bend at height one.

Step 13: Cut Tubing to Length

At this point the tube is almost done. If placed on a level surface there is no rocking in any direction. It was bent perfectly on a two dimensional plane.

  • Place tubing back on the template
  • Mark the legs of the tubing where they intersect the datum line. This will get you the proper height of the main hoop.
  • Cut the excess off where marked. Do not use a cutting torch or plasma cutter as the heat generated can compromise the integrity of the main hoop. A sawzall or band saw should be used.
  • Place main hoop in the vehicle.
  • Looking through the windshield ensure that the main hoop is parallel with the upper opening of the windshield. If not, grind the appropriate leg until the main hoop is parallel to the windshield opening.

Step 14: Weld in Main Hoop

Note: If you are building an entire roll cage do not fully weld the main hoop or mounting pads, a couple of strong tack welds will be sufficient.

This part can be a little difficult and is best done with the aid of another person or two.

  • Center the main hoop from side to side
  • Find symmetrical holes (driver side to passenger side) in the floor of the vehicle, use them to measure the distance from the hole to the legs. This measurement should be equal on both side.
  • Using a framing square be sure that the main hoop is perpendicular to the floor of the vehicle from front to back.
  • Once happy with the placement tack weld the main hoop to the mounting pads.
  • Measure everything again to ensure that the main hoop did not move. If it did, readjust the main hoop to the proper position.
  • Weld the mounting pads in completely.
    • Weld in 3" sections and alternate from one mounting pad to the other until fully welded. Do not weld one at a time.
  • Recheck the fitment of the main hoop.
  • Weld in the main hoop completely.

Step 15: Re-install Interior

Install the interior in reverse order. This concludes the fabrication of a main hoop for a roll cage.

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