Chassis (re)construction, Radically increasing torsional stiffness with a minimum of weight Options

[Hydra.]

Okay everyone, so my 914 tub was stripped out and sandblasted. Removing the passenger-side rocker cover revealed a hellhole of biblical proportions, but I was neither surprised nor terribly disappointed, as my plans were to chop up the car anyway. My plans for this car are to increase its torsional stiffness by almost an order of magnitude (looking for a target stiffness of ~5-6000lbft/deg) , without really adding any appreciable weight to the chassis. Tubular reinforcements are out, as they provide nowhere near the necessary structural efficiency.. I should add that the car is to be a speedster, with no targa bar, windshield frame, or functional doors.


First order of business is to replace the rotted factory longs, one at a time, with custom sheetmetal longs approximately the same width as the original pieces, but significantly higher, reaching up to the top of the inner door post. These would tie in to the inner door post and the rear of the front wheel housing up front, and to the rear frame rails out back. The idea is that the greatly increased cross-sectional area, combined with the diagonal sheetmetal X-bracing within the new longs should greatly stiffen the chassis where it is most flexible.

A very helpful, albeit simplified way of estimating the deflection of a frame is to figure out the stiffness of all its longitudinal and lateral elements (the same way you calculate the equivalent resistance of parallel resistors in a circuit). This means that if our longs are super-stiff and our-lateral connections aren't up to snuff, then that's where most of the deflection will take place. So the next step will be to completely box in the rear shock tower area. I will be cutting out the rear trunk floor, so I will probably
be using that sheetmetal to do the job. Much attention will be paid to the connection between the frame rails and the shock tower area, as it is a bit of a weak spot the way it is.

The front-end is also due to receive some attention, specifically where the bulkhead meets the longs, but I still need to figure out the details..

Of course, I will also be adding localized reinforcements as-needed (suspension ears, front trunk, etc..), as well as a roll-hoop behind the driver.


I'd like to hear your thoughts on this, specifically on the best way to remove the rotted-out longs and replace them with my "superlongs" without mangling the chassis. It is a given that I will be installing an adjustable bolt-on brace from the seatbelt mount on the targa bar to the upper door hinge mounting, but I was also thinking of installing another bolt-on brace from the 3-bolt outer suspension ear mounting to the lower door hinge mounting, then add another brace between the upper and lower door hinge mounts. Should this be enough to keep the chassis from moving when the longs are removed (one at a time of course)?