1995 BMW M3 Lightweight Weld-in Roll Cage Installation

(or, how to make your M3 Lightweight heavier)

Copyright 2000 Andrew E. Kalman aek@pumpkininc.com

 

Here are some pictures of a weld-in cage I designed and had installed at McGee Motorsports at Sears Point Raceway in Sonoma, California in the spring of 1998. The cage is made from 1.75” x 0.120” mild steel DOM (drawn over mandrel) continuous tubing. This heavy-gauge tubing was required to satisfy the safety requirements for a 3,000+ lb car (wet, including driver) in the High Performance Showroom Stock class at the Pikes Peak International Hill Climb. In this class, the cage can only be tied into the floor of the vehicle, the main hoop must be made from a single piece of tubing, and the four corners of the main cage must be gussetted.

I designed the 6-point cage after reviewing a variety of cage designs. The strongest influences were the Porsche factory (i.e. Matter) 911 cages, as well as the cage from the Mercedes ITC car and a few others. We were going for maximum safety and stiffness, in that order, using a minimum number of tubes. Oh, and the complete interior had to go back into the car when we were done. One other requirement was that the stock seats fit with the door beams out.

I gutted and later reassembled the car with some assistance. Alan McConnachie did the fabrication and welding of all the sheet metal and tubing. Most of the welding was TIG welding, though MIG welding was used in a few spots where it was more appropriate. I fabricated the steel bosses (all 16 of them) that enabled the use of removable door beams.


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Picture 1: Boss for removable door beam

Picture 1 shows a removable door beam boss permanently attached to the cage. This is probably the passenger-side, front, upper boss looking into the car. There are four such bosses per side. The hole is a through hole for an M10 Allen-head bolt. Each boss is machined from solid steel and is designed to slip inside the tubing to a depth of about 1”. Each of the 4 rosette welds (one such weld is clearly visible) on a single boss has of the area of the weld where the horizontal tubing meets the vertical tubing – hence the rosette welds are as strong as the nearby weld tubing-to-tubing weld. The four corners of a removable door beam also have bosses, the only difference being that the bosses are rotated 180 degrees about the tubing axis, and the holes in the door beam bosses are threaded for M10x1.5 bolts. The bosses are laid out so that the M10 bolts contribute only to keeping the door beams in place fore-and-aft and up-and-down. If the cage sustains an impact from outside of the car (e.g. the car is T-boned), the solid boss withstands the force due to its construction. With the door beams in place and bolted down, the door beams blend smoothly into the rest of the cage. There are 16 bosses (8 each of two types) in the car.

 

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Picture 2: Middle passenger’s weld-in point

 

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Picture 3: Same weld-in point, top view

Picture 2 shows the “box” that Alan constructed for the main hoop to tie into the floor. This is the passenger’s side, looking to the rear, and you can see how the box is tied into the M3’s side rails as well as the floor. The box also conceals the experienced cage-maker’s secret – namely that the cage is dropped through the floor during construction in order to be able to get at the cage’s top corners and fully weld all the tubing. After that’s done, the cage is lifted back up “into” the car, material (either a plate or a box) is slid under the tubing to cover the whole, and then it’s all welded up. I believe the plates are 3/16” thick. The wires (thin black ones are ABS sensors, thick red ones are for the battery) had to be moved to the other side of the tubing after the fact – that was especially difficult with the ABS wires!

Picture 3 is a view of the same area, but from above. Here you can also see a door beam gusset, and the counterbore for the Allen-head bolt. The diagonal going off to the left goes to the upper driver’s side corner of the main hoop.

 

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Picture 4: Top corner, passenger’s side

 

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Picture 5: Same shot, from inside

Picture 4 is a shot of the busiest corner of the cage, right above the passenger’s right shoulder. It looks like 6 tubes meet here, but the main hoop is continuous, so there are really five separate tubes, plus the gusset! In the picture you can see (clockwise) the main hoop, the down tube to the rear passenger’s wheel well, the cross tube to the rear driver’s wheel well, and the tube that goes alongside the top of the passenger’s door to the windshield. The tubes you can’t see are the continuation of the main hoop, and the roof diagonal that goes from this corner to the upper driver’s corner of the windshield. You can see them in Picture 5.

 

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Picture 6: Tie-in at passenger’s wheel well

Picture 6 is a shot of the two tubes that meet at the passenger’s rear wheel well. The upper tube comes down from the main hoop (and can be seen in Picture 4), and the lower tube is nearly horizontal, coming from the main hoop at the height of the top of the door beam. You can see two very different kinds of welds here – MIG welding was used to attach the 3/16” plate to the (thin) body sheet metal, and then the tubes were TIG welded to the plate and to each other.

 

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Picture 7: Front passenger weld-in point

Picture 7 shows the front lower corner of the cage on the passenger’s side. Unfortunately the picture isn’t sharp, but you can probably still make out the base plate and how it’s attached to the floor and the side sill. Also, the upper door bosses are horizontal, but the lower ones are installed at an angle.

 

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Picture 8: Anti-submarine belt mounting point from underneath car

 

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Picture 9: Same mount from inside car (driver’s side)

 

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Picture 10: Lap belt mounting point on passenger side of center tunnel

Picture 8, Picture 9 and Picture 10 illustrate how we chose to mount the lower 3 points of the 5-point belts. M12 flange nuts were welded to a 2” diameter ” thick steel plate with a hole in the center. A hole large enough to accommodate the nut was drilled in the body, and then the plate was welded, nut-side-out, onto the body. Sometimes the 2” plate had to be modified somewhat to fit the contours of the body. In Picture 8 you can see how the heat of the welding destroyed the underbody coating – I cut it away, and then had those areas (as well as the wheel wells) re-primed, re-painted and re-coated.

 

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Picture 11: Main hoop

In Picture 11 you can see the main hoop in all its glory. There are three diagonals in the cage, and you can see two of them in this picture – the one that’s in the main hoop, and the one that ties the hoop to the driver’s rear wheel well. The third diagonal is under the roof. Before re-installing the interior all of the welded areas inside the car were re-primed and repainted in white, and then the cage was painted in black.

 

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Picture 12: Fully assembled, passenger’s side

 

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Picture 13: Fully assembled, driver's side

In Picture 12 you can see the result, taken from the passenger side. The one-piece, removable door beams are in place, and the stock interior has been reinstalled. In Picture 13 you can also see the NASCAR-style quick-release window net.