The SL-C’s aluminum semi-monocoque chassis is a work of art, but the nose support structure is an after thought. There are two vertical 0.19” aluminum supports, each bolted to the chassis with two 1/4” bolts which are vertically separated by only 2”. It works, but it allows the nose to move around more than one would want and it would absorb very little energy before before a front impact reached the monocoque.
Speed has never killed anyone, suddenly becoming stationary… that’s what gets you.
— Jeremy Clarkson
That’s why modern cars have crumple zones to absorb the energy from an impact during a collision by controlled deformation. While I’m not capable of designing and building an engineered crumple zone, I’m going to build a tube structure which will absorb some energy before the the stout monocoque is reached. I have the following objectives:
Stiffen the nose and splitter
Absorb some energy in a collision without being too stiff
Provide a towing/recovery point at the front of the car
Provide a solid mounting plate for the nose hinge
Isolate the radiator which is wedged between the support verticals which move around
Provide additional support for the intercooler’s two heat exchangers mounted on the splitter
The first challenge was that the floor of the nose was sloped down towards the front-left side because the bottom of the extended foot box was not properly fixed when it was welded. As can be seen in the picture below the footbox extension is pushing the floor down. This required a massive amount of grinding (see the green arrow pointing to the black sharpie line), but I was able to get it to a good place without causing a structural problem.
Each side of tube structure will be mounted to the nose box via two 3/16” steel plates, each with four 1/4” bolts. This results in 4x the bolts and slightly more than 4x the vertical separation between the top and bottom mounting points vs. the stock solution. The top plates ties into the 1/2” upper-control-arm bolts and the lower plate wraps around the corner of the chassis. To create the bend in the lower plate, we clamped the plate and a steel rod which matched the radius on the corner of the monocoque in a vice. We didn’t have an acetylene torch, so we heated the metal with a propane torch. This took a while and the metal never got red. We bent the metal about 30 degrees and noticed that the bend was occurring above the steel rod (i.e., higher than desired). We opened the vice, dropped the hot rod on the floor and re-positioned the bend where it should have been with respect to the steel rod and bent it to ninety degrees… basically a game of hot potato with profanity. The result was a near perfect bend.
The first picture shows the top plate and a modified version of the vertical support mounted on top of the bottom plate. We cut 3/16” off the back of the vertical support to account for the thickness of the lower plate and welded it back together. This allowed us to use the vertical support as a surface on which to mock the tube structure. The stock solution is just the two 1/4” bolts spread 2” apart on the lower plate.
The frame will be made from 1” x 0.095” chrome molly tube. I choose 1” tubing for two reasons; I didn’t want the tube structure to absorb too much energy before crushing (I hope) and there is only about 1” between the vertical support and the radius on the monocoque and I wanted the tube to T-bone the chassis on a flat surface. The picture below shows a mockup of the bend angles using pieces of DOM tubing. The side tube will be made in two pieces.
After bending the side tubes we machined slugs out of 1” solid stock. I learned about the importance of drilling a small hole (i.e. 1/16”) so that hot air can escape the tube while welding. In this case I drilled the holes in the mounting plates. I also learned a way to quickly dress up the chrome molly tube. The side tubes are fully welded and tacked to the mounting plates. The tube that crosses in front of the radiator will be 0.095” chrome molly. I now need to decide if I should use 1” or 1-1/4”. I will mount a towing point to the center of the cross bar so strength is important. The 1” tube can be bent and coped into the side supports and it will look at lot better. The only question is, “is it strong enough?”
