Progress' 22mm rear antiroll bar added our rear roll stiffness, which helps rotate the car
Another consideration is the type of suspension that's being used. Often times, Japanese and European suspension manufacturers design their suspensions to work with the factory antiroll bars. Damper valving and spring rates are designed to already sufficiently increase roll rates. (This also has a lot to do with the higher spring and damping rates they can run on account of the smoother streets there.) By bolting on a heavier antiroll bar, you could be adding too much roll stiffness and again, not letting suspension do the work it was meant to do.
We opted to only increase the rear roll rate moderately with a Progress Automotive 22mm rear antiroll bar. The rear bar would increase rear roll stiffness and overpower the rear tires so that the rear would lose grip a little sooner than stock. By forcing the rear tires to break loose a little earlier, we've dialed in a little less understeer than what the stock car had. Not going overboard is important. Many companies offer a rear race bar which is much thicker, and will limit rear suspension articulation and pick up the inside rear wheel during hard cornering in order to rotate the car even faster. That's great on a race track with mildly predictable smooth pavement. Not on the streets where you're turning with bumps in the road.
We used an additional washer between the antiroll bar bushing mounting brackets so that th
The bar bolts in simply and has several advantages over the stock bar. Aside from the increase in rear roll rates, the more important aspect is how the bar is connected to the suspension. The stock antiroll bar uses end links with rubber bushings that connect to the rear lower control arm. The Progress bar replaces these end links with heim joints and polyurethane hardware to take the initial slop you feel just as you turn in. That slop is the rubber bushings in the end links deflecting before the bar actually starts twisting to provide roll resistance.
The Progress bar also gets smarter by extending the bar length so that the end links connect directly with the hubs. Because the independent rear suspension has a motion ratio, the further outboard an antiroll bar connects to the suspension, the more resolution there is. So, say the rear suspension has a motion ratio of 2:1 where the stock bar connects, when the wheel moves 2 inches, the antiroll bar end link will only move by 1-inch. A quarter of that motion might be taken up by the slop in the end link bushing. By extending the bar length and mounting the end link outboard you can reduce the motion ratio and gain resolution. If connected to the hub like our Progress bar, you essentially have a motion ratio of 1:1. So, that same 2 inches of wheel travel will move the end link and antiroll bar the same inches making the original quarter inch of bushing slop not nearly as noticeable. Ultimately there will be more feedback in cornering.
Progress supplies reinforced mounting brackets, hardware, and polyurethane bushings. All of this is to further reduce slop in the antiroll bar, and because the antiroll bar bushings experience little load and transfer minimal NVH when compared to a subframe or control arm bushing, we're happy to use the polyurethane. One trick that Di Bella employed, based on years of racing experience, was to put washers between the bushing bracket and subframe so that the antiroll bar bushing isn't completely squashed and adding more twisting resistance. Since we're using a softer 22mm bar instead of the 24mm race bar, we're not too concerned about having too much load on the rear bar to cause the mounting brackets to rip out of the rear subframe, another good reason to keep race hardware on race cars. To better work with the coilover suspension, we left the front antiroll bar stock but also applied the same concepts of heim joints cobbled together from Progress' spare parts bin and Energy Suspension bushings all to minimize slop.