My last post was on this subject, back in 2009. Yes, I'm a lurker more than a participant these days as my main project is an SN95 Mustang Autocross / Open Track car.
Anyway, I've watched the 'spaceframe' chassis conversation for years and there has been a lot of mis-information written on it. My first kit car was a Sylva Striker Mk.II, a 'spaceframe' Lotus 7esque machine.
However, it wasn't really a spaceframe, but semi-spaceframe. It is virtually impossible to build a true spaceframe that you can fit a human in.
In a true spaceframe, all the tubes are in tension/compression. Should it be assembled with a ball joint at every tube join, the chassis would still retain it's shape and structural integrity. However, that requires 3-D triangulation of the whole frame, something never achieved in the real world (well, rarely anyway).
In the case of a semi-spaceframe, we rely on sheer panels to transfer some loads in non-triangulated structure and on the beam strength of some tubes where open areas prevent triangulation (cockpits/engine bays etc). In the case of my Sylva or FFR's Mk.IV a substantial backbone structure stiffens up the cockpit section.
All GT40 'spaceframe' chassis are actually semi-spaceframes. Analysing their stiffness without sheer panels attached is largely pointless, the chassis is a system that needs to be analysed in the structural form that will see the actual loads of road/track use.
In the case of my Striker, I bonded the panelling to the frame, in addition to riveting. This resulted in a fairly stiff frame, but likely only in the 1500-2000 ft.lb/deg region. Unpanelled, the same frame is probably only in the 800-1000 ft.lb/deg bracket (estimated).
That may seem fairly 'wet noodle' for a car that dominated 750 Motor Club kit car racing in the UK for over a decade, but absolute stiffness figures don't mean a lot.
What is far more important is the 'Chassis Torsional Stiffness Ratio' (CTSR).
CTSR = 'Chassis Torsional Stiffness' / (Front roll resistance + Rear roll resistance)
Tunable handling comes from minimising the 'sprung mass elastic weight transfer' (SMEWT). As you increase chassis stiffness, the SMEWT reduces as an exponential decay function.
Long story short, with a low CTSR (wet noodle), changing spring and/or anti-roll bar (sway bar) stiffness has little effect on the understeer/oversteer behaviour of a vehicle under lateral G loads. A stiff chassis allows front and rear roll rates to be tuned to produce the handling characteristics desired. A CTSR in excess of 4 is generally considered to be needed for acceptable tunability for competition at the clubman level of motorsport (F1 and comparable standards will be higher) for single seaters.
When using FEA, the problem is that real world CTSR figures for semi-spaceframes tend to be only 70-80% ish of FEA calculated values (the thicker the tubing wall, the closer the FEA figure to the actual, or where square tubing is joined at 90deg in at least one plane). This is because the tubing joints don't occur at a single point, but around the circumference of the welded tube. The material around the weld is not as stiff as a solid joint (and we don't want to incorporate a solid steel block at every tubing joint) so the theoretical stiffness of the tube in compression/tension is not the same as the stiffness of the joint-tube-joint system. Therefore, a CTSR value of 6 should be the goal at the design stage.
Where the single seaters (non-aerodynamic downforce) and Lotus 7 style roadsters have an advantage is that their vertical Centre of Gravity (V.CofG) is only just slightly above their roll centre heights. Therefore, adequate roll control (tyres/springs/shocks/anti-roll bars) and lateral grip is achieved with relatively soft suspension settings. Therefore, it doesn't take a particularly stiff chassis to achieve a CTSR of 4 or more. Hence, you have a Sylva Striker or Caterham 7 that will lap multiple seconds per lap faster (on a track dominated by corners, not straights) than a Cobra with double the chassis stiffness.
Additionally, the vehicle's intended use comes into play. A road car will be set up with a more compliant ride than a track car. Roll stiffness will be less than a track car and in proportion to the car's weight. Therefore, the CTSR of a road setup could be over 4, but the same car set up to run on the track, with stiffer springs/shocks/tyres/anti-roll bars, could end up with a CTSR of less than half that. The chassis stiffness doesn't change, but the roll stiffness does.
The nice handling road car becomes an evil handling pig on the circuit! In some cases of marginal chassis stiffness, the same car will produce faster lap times (due to more predictable handling) with a softer setup because the chassis isn't stiff enough to capitalise on the greater roll stiffness in the suspension.
For a lightweight low vertical CofG car (Lotus 7 style roadster), as a very rough rule-of-thumb, chassis stiffness in ft.lbs/deg should equal the gross vehicle weight in lbs. For a clubman level competition car, it should be double the GVW. My Striker weighed in at a bit over 1100lbs so a stiffness of just under 1500-2000 ft.lbs/deg was adequate for a road/track car.
A 'best practice' design process should follow these steps:
Estimate the GVW
Decide on roll rates/stiffness (road or track, roll centre vs V.CofG, etc)
Determine minimum stiffness to give a CTSR >6 (FEA)
Design chassis to achieve this with minimum weight
Build it and test it
Modify as required
So, back to Mark's original post, if you can't adequately model the panelling, add in triangulation in the areas that will be panelled (simulating a sheer panel for load purposes). See where you are then.
For a replica GT40, 5000 ft.lbs/deg (6779 Nm/deg) would probably be adequate ('69 Lola T70 IIIB hit this figure). Even 3500 ft.lbs/deg would probably suffice for a purely road going build as the Lola T70 Spyder did well enough at 3200 ft.lbs/deg (but then, who really builds a GT40 with that goal?).
For a competition car, I'd want to aim for the 10,000 figure, but not at the expense of too much additional weight.
As a point of reference, Factory Five's Mk.IV chassis delivers somewhere in the region of 3300 ft.lbs/deg (many estimates out there but there have been some good estimations using FEA). Much of the flex in that chassis occurs in the engine bay area. If you don't want doors, their spec racer achieves double that. David Borden's Mk.IV was extensively modified (using Solidworks to drive the modifications, google for several write-ups on the net) to achieve a similar stiffness in a car that has working doors. The FFR spec racer has proven to be a pretty capable track car and I'm sure David's would hold it's own with most modern sports cars in the same weight/hp bracket.
I hope I haven't thrown too many people off with this long post but I've read every 'torsional rigidity' threat on GT40s.com, all the FFR forums, most of the late model Mustang forums and most of corner-carvers.com and, with the exception of the latter, there is much more opinion available than real engineering.