While throwing numbers at one of the better suspension programs is a good way to model things out, I think a better initial approach is to read up on the physics of what the numbers mean in one of the really good books out there. Herb Adams "Chassis Engineering" is a good one, as is the Milliken's "Race Car Vehicle Dynamics," with the latter being a bit more on the text book, dry reading end of things, but with a HUGE amount of very useful information. "What" you are trying to achieve and what you are willing to compromise is very important in any suspension system, especially so in a T/A arrangement.
Just a couple small points, the additional links (i.e. the locating links, not the T/A itself) are less of an issue in terms of making them as long as possible in a T/A setup than they are in a more traditional three or four link setup. 26" in my opinion is plenty long, even shorter is shown to work very well in other T/A applications.
As for the length of the T/A itself, this is where the fun engineering trade-offs will really show in this setup, and is probably the most important decision that needs to be made on a T/A setup. One one hand, keeping it physically short will help anti-squat for forward bite off the line and out of corners. That's a good thing, as the rear weight bias is low in your application, you'll want all the help you can get. But, the very real downside to this is that along with the lighter rear weight (relative), a shorter arm will increase the potential for rear brake hop, both things are working against you (longitudinal weight transfer forward under braking to an already nose heavy truck, and the downside of a short Side View Swing Arm's influence on this factor). I'd be very wary of this if it were my own project, solid braking performance to me is a LOT more important than forward bite, it's a survival instinct thing.
SO, that might argue that a longer arm would be preferable. But now you have the physics of creating a stiff enough Torque Arm to handle the same amount of rear wheel torque, but with a far longer bending moment, which means it needs to be very stiff in bending. Lower anti-squat resulting from such an approach is interesting, and not ideal, but you have to pick your battles.
The behavior of a Torque Arm application is dominated by the physical arm length itself, obviously things like spring rates, damping, etc are all important factors, but there is only so much tuning around the physical package that you can do. I state this because you noted that you'll make it all adjustable, which is perfect, but the biggest thing you'd probably want to play around with is the length of the T/A itself, and that is pretty much impossible without making several arms and the frame structure to mount the forward pivot(s).
T/A's are awesome, but you really need to have a very good idea of the basic performance goals and understand the trade-offs in order to make a good initial decision. This is one of the important distinctions between a 3-Link (or four link) and a T/A, besides how the torque loads are reacted back into the vehicle, it is a lot easier to build a tunable system using a true link approach. As you pointed out though, it's somewhat more convenient to package a T/A in many cases than a good 3-Link, which is why you see the T/A's on many of the aftermarket suspension supplier's list of products.
Great project, keep us posted on it all so we can all learn from your efforts!
Mark
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