DOES THE HDK SUSPENSION K-MEMBER HANDLE BETTER THAN A T-BAR SUSPENSION?

Take a look at stock car chassis or other chassis designs that deal with high cornering forces. Cross members are pretty much all much smaller than a stock K because the rest of the chassis is designed properly already.

The idea you initially mentioned seemed to suggest vendors could offer a cf k member. If it has to be altered for each chassis design, there's no chance of making it a standard product is there? I'm pointing out that at the point when it's no longer a stock style K that there's no reason to keep anything resembling a k member at all. A welded in square tube could do the job since the frame horns will be plenty stiff enough already and the only benefit any cross member would be affording is lateral resistance.

Motive force may only push in "one" direction but those loads from the rear end actually react in a number of ways, and not all are linear. Torque is applied in two axes (around the crankshaft axis and the rear axle axis) as well as the longitudinal force developed by the wheels, which also apply additional forces and resultant torques due to rear suspension geometry. This is often manipulated to aid traction by forcing the read end down or lifting the rear up. Front suspension is often designed to develop similar forces under braking or turning. There's a lot to consider and none of it can be isolated nor ignored because a significant portion of the mass happens to be close to some particular parts.

Point being: most folks use a close to stock chassis with some weld in reinforcement as needed and convenient or affordable. The next stage is to custom build to the intended application. Go take a look at chassis designed for drag racing compared to the stock cars mentioned above and the differences are quite obvious. Drag cars don't deal with the same magnitude of cornering forces and so the structure is much lighter. There's no one chassis design solution optimized for both and so the choices made by each builder will be shaped by myriad design constraints, including many defines by class rules which may even be contradictory in some cases. None of those constraints seem to favor a cf cross member. I might even argue that the fact that cf is weakest in compression makes it uniquely unsuited as a cross member since the primary force that will be applied to it will be compression trying to push the front frame mambers together in a corner. The shape and size of the cross member does not add torsional rigidity to the chassis either.

With an adequate frame design, an engine plate would accomplish what's needed for a front cross member because the frame members would be more than strong enough in all degrees of freedom, but "boxing" across the engine bay by stressing the front of the engine through the plate would increase lateral strength and stiffness a whole bunch, much like a typical core support bar but without adding additional frame members and their weight. Point being that a custom front chassis that is properly designed wouldn't need more than a motor plate to deal with the forces present, unless taking crash safety into account or a need for being able to push on the front of the car.

The stock K member exists mostly to support manufacturing. It aids assembly and allows the chassis itself a huge tolerance envelope while isolating the more critical chassis geometry to a smaller, cheaper, and more easily altered and repaired subcomponent. Retaining it in a custom chassis would be fruitless since ease of assembly line operations isn't a consideration for a bespoke chassis and a stronger design can be obtained by eliminating it and fabricating the suspension directly on a properly engineered front clip. One which is most easily fabricated with tube steel weldments. Even the highest performing vehicles outside of F1 still attach a welded sub frame for the front and rear chassis for this reason. The reaction forces can be reduced by resolving them to mostly axial forces over a large bulkhead minimizing the difficulties of integrating attachment points. The cabin and bodywork are typically where composites come into play because it's far less critical.

Establishing fatigue limits for composites is also a nightmare, and for bespoke parts is near impossible except by rough approximation which if sized appropriately often negate any weight savings, and I'd it's not lighter it might as well be steel anyway. After all, there's a billionaire at the bottom of the ocean who guessed a little wrong on his approximation. The consequences at the top end of the drag strip or the end of a long straight are similar, which makes a good case for predicable manufacturing methods like welded steel.