The following early Z chassis analysis has no analog anywhere, to my knowledge. You may well find it to be rather dry and a bit of a tough go, but hang in there: the conclusions are worth the effort.

The 240Z transitioned the mass sports car market from drafty rag-tops to designs possessing a real metal roof. No longer were designers shackled to the puzzle of creating a rigid structure with a huge hole in the top. Now they could use the roof structure to "mend" the cockpit hole and introduce greater depth„and therefore strength„into the structure.

The early Z was of unit-body construction. Rather than employing a separate, heavy frame to carry all loads, and then attaching a body (i.e.: MGB-GT or Chevy Truck), the body itself provided the necessary rigidity. Where extra strength was needed to mount suspension or powertrain components, local reinforcements were added in the forms of additional layers of steel sheet, three-dimensional sheet metal stampings, box sections, or subframes.

The Z's designers attempted to provide three types strength: torsional rigidity, bending strength, and load point strength. Torsional rigidity is the body's ability to resist twisting. Poor torsional rigidity results in metal fatigue and a short body life, and also prevents the suspension from properly doing its job. Bending strength refers to the body's ability to support a load suspended between two points, in this case, the front and rear tires. Think about two 300 pound passengers, add a G factor of, say, five for a nasty hump in the road and you get the idea. That's a ton and a half trying to bulge out the floor pan, and something has to resist that force. Load point strength refers to local reinforcements that distribute concentrated loads into the frame or body shell. The engine is the clearest example here, with suspension attach points coming in second.

We'll go through the Z's body, top to bottom and tip to tail, and have a look at what does what by starting with the structural components. But first, so we're on the same wavelength, a little nomenclature.

"Forward" means toward the front, or nose, of the car, or ahead of another component.

"Aft" means toward the rear, or tail, of the car, or behind another component.

"Longitudinal" refers to anything running along the length of the car, i.e.: along the axis between the forward and aft extremes of the car; lengthwise.

"Transverse" means perpendicular to the longitudinal axis, or "across" the car.

"Centerline" means a hypothetical line drawn through the center of the length of an object, dividing "left" from "right".

"Right" and "left" as regards car components are viewed as if facing forward sitting in the driver's seat.

"Outboard" refers to being farther away from, rather than closer to, the centerline.

"Vector" refers to a specific direction in three dimensional space and here refers to the direction in which a force is applied.

"Shear web" refers to a (usually) flat sheet of material which absorbs loads in a single plane, i.e.: within its length and width.

"Bulkhead" refers to a relatively flat metal sheet oriented perpendicular to and located either inside a metal enclosure, or as a cap at the end of two or more pieces of parallel material, providing transverse and/or internal support for the combined assembly.

A "box section" is an assembly of sheet metal pieces that, together, forms a rough box shape with length, width, and height dimensions. Box sections may be thought of as conduits for primary load paths, with the surrounding sheet metal being thought of as a secondary load path. The sides of flat box sections act as individual shear webs of the box, held in place by the other parts of the box, providing in-plane strength. Boxes are torsionally stiff and, depending on their shape, can provide strength in multiple directions to absorb multiple load vectors. Box sections need be neither rectangular nor straight and, indeed, come in all kinds of cross sectional shapes. Box sections in the 240 range in size from very small (1-2 square inches in cross section) to very large (50+ square inches).

The Floor Pan
In general, the body is comprised of a floor pan and an upper body assembly spot welded to each other. The floor pan, being fairly flat, provides little torsional rigidity; that will be supplied by the mating of the pan to the upper body. The body pan must provide significant bending strength, however, just to hold its shape and keep the passengers from sinking into the tarmac.

Longitudinal strength for the pan is supplied by the rather large transmission tunnel, and by two outboard box sections comprised of the rocker panel on the body exterior mated to a C-shaped piece on the cockpit interior.

The transmission tunnel, by virtue of its depth and nearly vertical sides, affords a great deal of longitudinal bending strength. However, the tunnel's open bottom prevents it from supplying much of any torsional rigidity. Despite being dimensionally much smaller than the transmission tunnel, the door jamb box sections provide both longitudinal bending strength and torsional strength, while also acting as an outboard crash protection beam.

In addition to the transmission tunnel and outboard box sections, the floor pan also derives additional strength from several smaller features. Extensions of the two engine bay frame rails run aft under the cockpit as box sections. This set of box sections adds bending strength and occupant protection. In addition, a shallow X-shaped indentation in each occupant footwell stiffens the expanse of metal under the occupants lower legs. Lastly, the front and rear seat mounts act as formers (partial bulkheads), connecting and stabilizing the vertical walls of the transmission tunnel, the floor pan, and the inboard faces of the outboard box sections.

The Upper Body Assembly
Until they are mated, neither the floor pan nor the Upper Body Assembly are, by themselves, very strong. Mating adds a third dimension (height) to the completed body, creating a very large box section surrounding the cockpit. The cockpit is, of course, a large hollow space, and the door openings put holes in the sides of the car, so any structure joining the roof to the floor pan must run around the perimeter of the cockpit area and around the doors. Load paths for both bending and torsional forces must therefore run up through the sides of the car.

Datsun provided the necessary strength in the sides of the body by using two vertical sheets of metal and welding them into partial box sections. Forward of the door, the two sheets„as tall as the firewall„tie into the firewall and the forward frame horns (more on those later). At the front edge of the door opening, the aft edges of the two sheets are capped and joined by a metal bulkhead that's often termed the "door post." At the doorpost, the outer of the two sheets is reinforced locally to carry the loads imposed by the door hinges. The tops of the two sheets are spot welded together and continue at their aft upper corner as a very small box section that runs up either side of the windshield and is known as the A-pillar.

Under each door opening, these same double wall sections carry through as the rocker panel box sections mentioned earlier. Aft of the door, the exterior sheet forms the outer fender while the interior sheet is spot welded to the inner fender well to which, in turn, the rear shock tower is spot welded. The inner metal layer continues up through the rear cabin "wings" as a decreasing-area box section tying into the roof and hatch hinge box section frames. At the rear edge of the door opening, the inner and outer metal layers are capped and joined by a vertical bulkhead similar to that of the forward door post area to which mounts the door latch catch. In the rear quarter window openings, the inner and outer sheets come together in two parallel half-inch flanges which are spot welded together.

A torsional load applied to one corner of the body (or a bending load applied to one end of the body) will be dispersed through the body sides, up through the A-pillars and the rear body wings, and into the roof and hatch frames. The roof sheet metal acts as a secondary shear web supporting the roof frame, but the primary load paths are directed through the A-pillar and body wing box sections. Dispersing the load into the entire body structure provides many times more load resistance than the front corner alone could provide.

At the front of the passenger compartment, at the top edge of the firewall, is a major reinforcement in the form of a full width transverse box of triangular cross section which also houses the windshield wiper assembly. The vertical leg of this triangular section is the topmost six inches of the firewall. The bottom leg (hypotenuse) of this triangular section is a steel stamping that is spot welded across the width of the firewall six inches below it's top edge. This piece runs upward and aft at about a 45 degree angle to terminate in the curved edge of the lower windshield frame. On the cockpit face of this bottom steel sheet are welded various tabs and brackets to support the dash panel, heater, and steering wheel column.

The third leg of the triangular box section (normally hidden under the cowl grillework) is a horizontal steel stamping running aft from the top edge of the firewall and terminating in the windshield frame. This top horizontal stamping is lightened considerably by several large holes which provide access to the wiper motor, and also act as air inlets for the heater/vent system which draws cabin ventilation air from the high pressure zone at the base of the windshield. Despite the large number of lightening holes present in the top horizontal leg of this triangular box section, the box adds a great deal of transverse, shoulder-high strength to the front of the passenger compartment due to the curved shape of the windshield frame.

Moving aft of the door opening, a similar full width transverse triangular box section is found on the floor pan at the forward edges of the rear inner fenders, just aft of the seat location. This box is formed by 1) a transverse vertical bulkhead about ten inches tall that defines the front of the cargo area; 2) the forward horizontal floor of the cargo area; and 3) the floor pan sheet metal that curves up to provide suspension and differential clearance. Z owners know the two little nooks found inside this box section, and accessed by the little hatches in the forward cargo floor, as the jack storage location, but the box's primary duty is to stiffen the body between the forward portions of the inner wheel wells, and to provide vertical and lateral support for the forward rear suspension pivot points.

The rear shock towers are conical sheet metal pieces that are spot welded to the inner fender wells. To further stabilize the tops of the towers (which must resist lateral suspension loads) two u-shaped metal channels run from the edge of each tower top downward and inboard, connecting to the cargo floor several inches inboard from the bottoms of the shock towers. The resulting triangle resists lateral movement of the shock tower tops in normal use. The brace bottoms do not connect only to the cargo floor, but more importantly, to a part of the cargo floor that forms the top side of 2" by 3" transverse box section. This box supports the differential and rear, inboard suspension pivot points, and connects with two longitudinal box sections that mimic the purpose of the forward frame rail extensions, supporting the rear cargo deck.

Other than local steel reinforcements for bumper mounts (which steadily evolved in weight and complexity due to escalating federal crash protection regulations through the early ï70's), the only significant feature remaining in the cargo deck is the spare tire recess which, together with the spare tire, forms a crush zone for passenger protection in rear end collisions.

Finally we turn to the body forward of the firewall. Whereas the need to provide space for occupants requires a large and hollow cockpit area aft of the firewall, the loads imposed on the front of the body forward of the firewall are few and concentrated. Load points include: 1) suspension attach points; 2) engine mounts; 3) radiator mounts; 4) bumper mounts; 5) rack and pinion mounts. A cursory inspection of the engine bay clearly reveals most of the primary structural components. Two other major components„the forward frame horns„are visible only with the front fenders removed or by looking inside the fender wells. In order of decreasing importance, these components are the: 1) forward frame rails; 2) forward frame horns; 3) inner fender wells; 4) front crossmember; 5) shock towers; and 6) radiator mount bulkhead. We'll look at each in turn.

The forward frame rails run longitudinally along the bottom of the fender wells providing vertical, lateral, and torsional strength, as well as a mounting location for the front crossmember. Inside the engine bay the frame rails are dimensionally much deeper, and therefore, much stronger in the vertical plane than their extensions which run aft under the cockpit. Several major and minor components mount to the frame rails including (from the front toward the rear) the front bumpers, the radiator mount bulkhead, the front anti-roll bar, the front crossmember, and, near the firewall, the aft end of the suspension's tension/compression rod.

The forward frame horns provide torsional and bending strength along the left and right upper edges of the engine bay, lending strength to the shock towers and providing crash protection for head-on collisions. They project forward from the upper outboard corners of the firewall all the way to the radiator bulkhead as curved box sections of decreasing cross section. Forward of the radiator bulkhead, the frame horns have blended into thin, double walled extensions of the inner fender wells, and act as mounts for the hood hinges. The interiors of the frame horns are also used as secondary air inlets, directing high pressure air from the area in front of the radiator to the individual air vents located near the occupants' knees.

The curved, inner fender wells connect the frame rails to the forward frame horns and provide a large area of support for the conical shock towers which run vertically and are spot welded to the inner wells. The wells extend forward from the firewall to stabilize the radiator mount bulkhead. As a single, curved sheet of metal they provide significant strength only as a connection between the frame horns and the frame rails, though they do add some crumple resistance in a head-on crash scenario.

The front cross member is a transverse steel channel that bolts to the frame rails on either side and directly supports the engine mounts and the rack and pinion mount. As engine and steering loads are significant and concentrated, this channel is thick walled and heavy.

The radiator bulkhead is attached near to the extreme forward ends of the frame horns, the inner fender wells, and the frame rails. This bulkhead not only supports the radiator but connects the left and right sides of the body and provides torsional strength to the engine bay.

Now that you've managed to wade through all the description, here are some observations on how the various parts work, or don't work, together.

Evolution of design
The early 240Z body was found to have at least one major weak point. Torsional forces acting on the body began causing a failure in a sheet metal joint in the box section between the rear hatch opening and the rear window. Reinforcement of this area was introduced in 1972.

Frame rail extensions on the 240Z and early 260Z (until August ï74) extended aft under the cockpit to a point about 18" forward of where the body pan curves up for rear suspension clearance. After August of 1974, these frame rail channels were extended further aft to where the body curves upward and were made slightly deeper in cross section for greater strength. The longer frame rails also provided additional support for the rear roll bar attach point which became standard in 1974. As a matter of interest, only two bodies were made for the early Z's. The 240 and early 260 used the 240 body, while 260's built after August ï74 used the 280 body.

Normal Operation Loads
The 240 body and components were designed to withstand moderate suspension loading, and occupant, fuel, and cargo loads as defined by the owner's manual, body placards, and the fuel tank capacity. While normal passenger and cargo loads were seldom exceeded, the 240's suspension and torsional strength limits were more frequently reached by aggressive driving styles. In practice, however, the post ï71 Z body has been found to be deficient only when saddled with aftermarket suspension parts and full racing suspension loads.

Racing Loads
Full race driving styles, and installation of heavier springs, anti-sway bars, and racing tires will exceed the strength of some early Z body's components, as well as its overall torsional strength. Body racing modifications, therefore, always seek to reinforce these areas or parts. Modifications generally fall into two categories - component upgrades, and body stiffeners or reinforcements - and I'll comment here only on the latter category.

Body deficiencies can be classed as load point deficiencies, shock tower movement, or general torsional weakness. Load point deficiencies include the front sway bar mount and the steering rack mount.

The deficiency of the front sway bar mount is that the frame rail bracket bolts only screw into the bottom sheet of metal in the frame rail box section. This bottom metal sheet is attached to the other three sides of the box only by spot welds and repeated heavy loads on the bracket will break the welds, fatigue the metal locally, and greatly reduce the effectiveness of the sway bar due to bar movement. The modification to counter this deficiency requires that the sway bar mounting bolt extend entirely through the frame rail to access the strength of the top face of the box section, and to better disperse the load into the entirety of the box section. Furthermore it is prudent to also weld a bushing inside box to take the compression loads imposed by tightening the bolt, and to prevent collapse of the box section.

The steering rack mount deficiency, resulting in lateral rack movement, is primarily one of overly soft rubber bushings and can usually be solved by installation of urethane or other high performance bushings. However the rack mounts proper - the two steel channels welded to the forward face of the front crossmember - should be regularly monitored for weld cracking, and preferably should be reinforced to resist lateral loads.

Lateral shock tower movement results from heavy cornering forces, affects suspension settings, and disrupts consistent handling. This problem is countered by bracing the tower tops, the simplest form of which are strut braces which connect the tops of the left and right towers together (both front and rear), allowing the unloaded tower top to support the loaded tower top. A more complex bracing version for the rear towers would be a transverse welded X-brace, connecting each tower top to the cargo floor box section near the bottom of the other tower.

Forward of the firewall, modifications to resist lateral movement of the front shock towers are often combined with modifications to increase the torsional rigidity of the body, particularly in high horsepower or V8 Z's. In addition to a transverse strut brace (to prevent lateral tower movement), additional bracing may also run aft, angling inboard from the tower tops to a rollcage kneebar, or to the firewall at one of the inboard locations where the firewall is supported by longitudinal bulkheads inside the transverse triangular box section.

Additional bracing may also extend forward from the tower tops to connect to the lower and/or upper horizontal boxes of the radiator bulkhead at the car centerline. The radiator bulkhead may be further reinforced by installing diagonal braces from corner to corner.

Current V8 conversions suggest the addition of two longitudinal aluminum or steel channels supporting the forward frame rails in the engine bay, and extending under and bolting to the cockpit body, similar to the stock frame rail extensions.

Abnormal Operation Loads
Abnormal operations are, unfortunately, a statistical reality. Abnormal loads include loads resulting from front, quarter, side, and rear collisions, as well as rollovers. A review of the role of body structure in each is useful.

Body structure resisting a head-on collision concentrates on all structures ahead of the passengers: frame horns, frame rails, inner fender wells, radiator bulkhead, engine, hood, firewall, transmission tunnel, and forward triangular box section at the top of the firewall. Of these components, the greatest strength will be found in the frame horns, frame rails, and inner fender wells, having a total mass of perhaps thirty pounds. This small amount is augmented slightly by the short outboard dogleg Datsun designed into the frame horns near the firewall, which would assist an orderly crushing of the horn. But it is an alarmingly small mass when compared to that of a full sized American sedan, or worse, a laden garbage truck.

If contact is made directly head-on, all the structures mentioned above will come into play, but the chances are much greater that a two car collision would occur off center, on a quartering vector. Concentrating the same amount of load on one side of the body or the other reduces the amount of structure which must intercept the load, and often reduces the amount of distance the body has to decelerate the load. On the other hand, a quartering impact will benefit from the frame horn dogleg, as well as the better impact angle on the triangular transverse box, both of which would slightly improve passenger compartment intrusion.

Side collisions in the early Z are not much fun. Doors are thin, effective door beams were not employed, and the outer box beams are oriented in the wrong direction for strength and are too low to afford much protection. What more can be said? If you want lots of protection in an early Z, install a full roll cage. Rear collisions, despite the short distance between bumper and occupant, are blessed with some effective crush zones. If the passenger compartment does not suffer intrusion in a collision, the leading cause of major injury might well be back injuries resulting from failure of seat reclining mechanisms.

We need to not ignore upside-down excursions in abnormal operations. While the Z body probably possesses the strength to rest on its top without collapse, the chances are that one or more of the few structures that could keep you off your head - the A pillars or rear wings - will suffer lateral damage, and will therefore not be up to full strength (which in the case of the A pillars, isn't that much to begin with). The conclusion on roll-overs could be the same as a breakfast order: over lightly, please.

Body Degeneration
Time changes all things and early Z's faster than many others. Datsun's anti-corrosion program was still in the planning stages, and the Z's sheet metal was thin. So most everything written above now needs to be modified slightly. Early Z's suffer from major corrosion everywhere, but statistically, several areas suffer more corrosion than others. Ordering these corrosion sites in decreasing awfulness, we find: 1) the area around the battery; 2) the area around the brake master cylinder; 3) rocker panels; 4) aft frame rail extensions and cargo area box sections.

The undisputed leader of this sorry bunch has to be the fender well and firewall adjacent to the battery, and the frame rail directly under the battery. Corrosion in this area is so bad in some Z's that the frame rail is nearly severed and the firewall and fender well have significant voids. Now is the time to think about what these pieces of sheet metal do: they hold the occupants and engine off the ground, resist torsional loads originating in the suspension and engine, and protect the occupants in collisions. A Z heavily corroded in this area may suffer from a loss of strength exceeding 75%.

Number two is the mirror location around and under the brake master cylinder. While the problem here is not usually as bad as on the battery side, it can be significant, and combined with corrosion on the other side, you can throw away any idea of upgrading your suspension: the body doesn't have the strength or rigidity to let the suspension properly do its job.

Number three on the bad guy list are the rocker panels. If you have visible rust in your rockers, the strength of the box section has been significantly compromised. Again, remember what this section does: it transmits the loads originating in the suspension and engine up into the roof to take advantage of the entire body structure. If the rockers are rusted, the load path is interrupted, the body is weaker, and you are on the road to localized metal fatigue because a small area must now do major work by itself. Oh, yes: the other thing the rockers do is protect the occupants. Don't want to forget that.

Last on this quick list are the aft frame rail extensions and cargo area box sections, the ones under the cockpit and rear deck that you never see, and in which Datsun drilled holes so the water could get in. These extensions are spot welded to the floor pan, so corrosion has only to degrade the small spot welds in order to cut the strength of the entire rail. And what do they do? They keep the passengers off the ground (remember the two 300 pounders?), transmit engine loads into the body structure, support whatever you toss into the cargo area, and act as mounting points for the rear suspension and differential.

You get the picture. Hidden corrosion affects handling, body integrity, and crash protection, not to mention abnormal loads. You might want to refer back to the corrosion treatment articles run in previous NewZletters at this point.

Conclusion
The early Datsun Z derived some of its performance from its light weight and structural details of the unit-body design were very important to its strength. Examination of the location and size of box sections of a structure will show exactly where and what relative loads the designer envisioned would exist in the working structure. Loads imposed by racing, or by heavy duty, aftermarket suspension components stress body rigidity much more than stock usage and components. Any load path that is interrupted concentrates loads in that area, decreasing overall body strength and increasing metal fatigue. Corrosion decreases strength and can have significant, negative effects on unit-body vehicles.

I guarantee the next time you examine your Z, you'll be looking at it with a different perspective.