Bobs 4th gen CMC build

Building the roll cage

The first step to building the roll cage is reading the rules for the class. CMC requires

1.750 x .120 Seamless Alloy (4130), Seamless mild steel (CDS Mechanical) or DOM (Drawn over Mandrel). This is the requirement for cars over 3000 pounds with out driver. The next step is buying the steel. I bought the steel from a local supplier, Hagerty Steel in East Peoria Il. The price is fluid based on current steel prices. I bought three twenty foot sticks for $110 each and bought two more at $106 each. The sticks being twenty feet long does require a trailer to pick it up, Too long to shove in the back of the Suburban.

Tim Bennett and I built the cage in Tim's CMC car two years ago and we were very happy with the results. Tim put together a program on his computer to figure out the lengths and bends of the tubing. We used a manual tubing bender for this project. It is slow but it is just the right speed for a novice. I am afraid if we used a hydraulic bender it we would bend the tubing too much. Whatever the case this bender works great for us.

Front Plinth

We had the benefit of learning from the mistakes and or improvements from Tim's car so we had a pretty clear direction of how we wanted to do things on this build. For example, on Tim's car I had a heck of a time welding the tube 360° because it was difficult to reach the top due to the roof panel. Rules require all welds be 360° around the tube. This was done but very difficult. On this build to make this easier we made 1 inch tall pedestals for the main hoop and 3 inch tall pedestals for the front down tubes. With these pedestals we are able to weld 360° then lift the main hoop up, slide the pedestals in place and then weld in the pedestals. This adds a little weight to the cage but it is a safer cage in my opinion because the upper welds are of high quality.

Front plinth placement

The main hoop and the front down tubes are the hardest to bend. they have got to be right. The main hoop went pretty smooth and fits very well. Once this was done and the location of the hoop was established we could begin trimming the front down tubes. At this point nothing is tacked into place. We held the main hoop in place with a large welding magnet to the roof. this can be seen in some of the photos. We then trimmed the lower part of the front down tub to set the height then we were able to easily set the for and aft of the front down tubes by trimming the end. Once trimmed we put the end in the tubing notcher and the bar was done. Rinse and repeat for the other side and this porting of the cage is done. We have not welded anything at this point. Next steps are the cross bar in the main hoop and size and fit the rear down tubes to the main hoop. Then the upper and lower cross bars between the front down tubes.

Magnet to hold main hoop

Rear
plinth

in place

I have finished the Front section of the cage. I was able to tack the bars in place then was able to remove the entire front section to weld 360° around the tubes. With it out I masked the areas that need future welding and then painted the whole sub assembly. This is nice to get a smooth even finish around the entire bar.

The rear section / main hoop i have done the same thing. I was able to remove it and sub assemble and then paint. I still need to do the rear down tubes. When that is complete both sections will be fitted in the car and I will only have the welds that unite the front and rear sections to do in the car. I will then bend the side impact bars or more commonly known as the NASCAR bars for the drivers and passenger sides. I am not going to install these bars until after I have wired the car. They are too difficult to climb over when you are wiring a car. that is the plan anyway, we will see if it works.