What about a bigger front bar? Anyone running both front and rear? Any thoughts or feedback?
Vehicle dynamics theory says that the balance of the car in steady-state cornering is a function of the RATIO of front stiffness to rear stiffness (F/R). And obviously the roll angle of the vehicle is a function of the TOTAL roll stiffness (F + R).
So, when you increase the front roll bar diameter, you actually do two things: You increase the overall roll stiffness and, probably more importantly, you've changed the ratio of front-to-rear stiffness. In this example, the vehicle would roll less and understeer more.
So, what is the goal? I think we're talking about migrating toward a more race-worthy setup, right? In that case, we have two goals: First, reduce body roll so that the tires stay flat on the pavement, and second, get rid of the understeer.
We've discussed elsewhere how stiffening the rear will reduce understeer. This is because the F/R stiffness ratio is reduced. However, if we then turn around and stiffen the front, we're back into understeer, although we have reduced body roll.
It is quite possible to achieve both goals, though. All that needs be done is to stiffen front and rear, but stiffen rear more than front.
I once went through an interesting exercise with my old car. I figured out how to measure roll stiffness in terms of pounds per degree of body roll. I did this with a couple of different front bars and with all of the position of my adjustable rear bar. I knew from experience which of those combos I liked at the track, so I knew what the F/R ratio wanted to be. Later, I increased the rate on the coil springs dramatically and remeasured roll stiffness F and R. I then adjusted the bars to get back to my "ideal" F/R ratio (now a very different combo of bars and adjustment settings) and voilà!, the car was nicely balanced right off the trailer, except much stiffer overall.
Interestingly, the optimum F/R ratio was very different for autocross and road course. I have another theory on that, but that's a different thread.
In the real world, we've all seen FWD cars lift the inside rear in a corner. This is often the case when the owner installed a stiff rear bar but nothing else. The experts will tell you that a four-wheel car will corner better than a three wheel car. The "three-wheeled" car in this example would benefit by increasing overall stiffness (to keep all four on the ground), but retaining the F/R ratio that was initially achieved by increasing rear bar diameter.
When the APR guys talk about the benefits of a big front bar, I think they are actually discussing the effect of increasing overall roll stiffness.
Whew. Sorry for that tome.