CHASSIS MODIFICATIONS

The original Malibu "ladder" frame alone, without body, has virtually no torsional stiffness. In an open roadster, it's really the heavy gauge transmission tunnel, firewall, and the rear bulkhead (behind seats) that give adequate stiffness, so a lot of box steel and sheet metal were required to bring it up to acceptable level. The effect of steel panels, contributing to stiffness, is often overlooked. These panels, sometimes referred as shear panels, will effectively triangulate the structure.

The side rails of the original frame had to be shortened 450 mm to achieve 90" wheelbase, and were boxed. There's also 50x30mm beams welded on top the original rails. You really cannot get enough steel inside the sill in the door area. The side rails not only contribute to overall stiffness, but serve as side impact protection. Some of the kit Cobras utilize a horrible "backbone" frame, where you can find steel only in the trans tunnel, but no structure under the seats or in the sills or doors.

I had to fabricate new engine mounts. Now the engine sits 320 mm further back and 40 mm lower than original. Driveshaft had to be shortened 770 mm !

The GRP door skin  is in fact just "upholstery", carrying no load, since there's a box steel structure inside the door, running from hinges to latches. It is a good practice to mount all hinges, latches, brackets & alike directly to steel, not to fiberglass, as otherwise vibration will crack the body over time. 

It is a wise trick to fill the sills with urethane foam spray. This will not only support the GRP body to minimize the chance of cracking, but will provide a creak and rattle free structure, plus some additional road noise dampening.  This works equally well on any cavities between the body and frame. Be careful when spraying the foam - many foams expand to multiples, and have enough force to break your body!

Firewall and transmission tunnel are made of mild steel, 1.25mm thick, while floorpans are of stainless steel, 1.5 mm thick (I don't like rust repairs).  The floor at the footbox area is dropped 2" below bottom of frame, providing more legroom. Under the floorpan there's a triangulated structure, made of 30x30mm box steel. The lower rear control arm mounting locations were reinforced, too. Fuel tank is made of stainless,  has an aluminum double skin underneath, and is surrounded by heavy gauge box steel for safety. Battery cable, brake lines and fuel lines run inside the side rails. At this stage, there was still something missing, like posts for door hinges and latches. 
There wasn't much room for the radiator between steering components. The "pusher" fan was not a good idea, it's a "puller" nowadays. The original heater was a real bitch to install,  and had to be cut a dozen times to fit in the engine compartment.  All GM G-bodies have steering components mounted ahead of the front axle line. Under cornering forces, as the bushings flex, it will cause the car to understeer, which is better than oversteer for predictability. Note: although the Malibu front "clip" is of heavy structure, it has to be reinforced, too.

Parts Retained From Donor Car (and the corresponding score)
  • Original Frame, shortened 450 mm (10%)
  • Front and Rear Axles and Suspension (20%)
  • Engine + Peripherals (15%)
  • Transmission (9%)
  • Heater and Wiper Systems (some %)
  • Steering Components (3%)
So there's 60% parts left of the original donor car (Malibu), and I could still for instance swap the tranny to a 5-speed manual, leaving me 51% original car.

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