I am the chassis designer for my college BAJA team, though both the cars are different but the approach will be same. be very thorough with the rule book as the rule book will set the constraints on the dimensions of the vehicle. make the dri. Feb 12, 2009 Another thought about this hot rod cage design is that we might be able to remove a lot of structure surrounding the roof and rely completely on the roll cage to support the car in a rollover. Remove all but the outermost layer of roof metal and even.
![]() 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.
Teacher Notes
Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson. 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.
![]() 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
Step 4: Measure for Height
Two height measurements will be taken, both of the points measured will be needed for the width measurements.
Step 5: Measure for Width
Three width measurements will be needed; we will start from top to bottom.
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.
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.
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
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.
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.
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.
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.
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.
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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How small can the tube or secondary cage be and not start to block light from the primary mirror? The answer depends on the size of the primary, the focal length and the desired field size.
This subject is covered in a couple references but I always found the answer to be a bit simplistic. I wanted a more informative method to allow calculation of the minimal cage ring size and be able to figure the exact tradeoff I face in order to minimize the cage ring size.
The answer turned out to be a somewhat more difficult exercise that I had imagined. I could easily calculate the angles up and down the tube, these were simple trigonometry. But to come to a final answer I needed to find the area of overlap of two uneven sized circles. This took more work and a bit more geometry than I remembered and some reading in the old college textbooks was required. But there is indeed an answer, and when I used the resulting formulas to develop a spreadsheet the answers are indeed correct!
Analysis of my 6' RFT Primero. The focal length of this scope is 777mm, and it is housed in a 8' piece of aluminum irrigation tubing. The distance from the front of the primary to the front of the tube is 31.5'.
I was somewhat concerned because this scope is capable of over a 3° field of view with my 35mm Panoptic eyepiece. The results show that there is no obscuration across a 3° field.
Analysis of my 18' Deep Violet. This telescope is a 18' scope with a 19' I.D. ring located 70' in front of the primary mirror.
The result is a little more interesting showing that the telescope is good for about 1° field of view at best before suffering some roll off. That is fine, as the lowest power, widest field eyepiece I have (35mm Panoptic) results in almost exactly a 1° field.
If I had some hypothetical eyepiece that resulted in a 3° field there would be almost a 10% roll off in brightness, probably objectionable.
Using the Spreadsheet
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