Soft Does Not Equal Grip - Part 3 - Anti-Roll Bars (ARBs)
Previously we discussed damping and springs, now we move on to the next bit in the suspension; the anti-roll bar (ARB) or sway bar. Much has been written about its functionality so we won’t beleaguer the point here. The goal of this post is to address the approach in balancing the rate of the ARB with the rate of the springs. As with many elements of race car setup, there can be some subjective points applied to the objective. We’ve found that many of our clients benefit from understanding this balance and have found what works for them best because of it.
The role of the ARB is to do what it is named for; control the roll of the vehicle when cornering force is applied. With any racing or performance car, we will normally look for a substantial increase in roll control compared to an average road going setup. When we look at a vehicle in pure roll (this almost never happens by the way) the main springs and ARBs are more or less equivalent. You add the contribution of the springs and the bars to the roll stiffness and viola, that is how much your car will roll per unit of g force. Our goal here is to make sure we keep the floor of the car at an acceptable angle and the tires at an acceptable range of camber.
Throw in Pitch
Roll analysis is easy on paper. We’ve all seen the car roll of the trailer that the engineer proudly says is perfect. This same car has infuriated many a driver. When the car is being driven around a non-perfectly smooth race track there is a constant state of change in roll AND in pitch. Therefore, we must mind our trade-off here carefully. Given a desired amount of roll control, the stiffer your bar is, the less pitch control there is. This means that the front or rear will move up and down more than if you did the job with the main spring. Conversely, with a soft ARB and stiff springs, we have limited pitch movement and limited roll movement.
Just as in the case of pitch, ARBs also present complications with respect to road irregularities, curbs, and general bumps. Since the ARB connects the wheels, the stiffer it is the more connected those wheels are. If you’re running a stiff front bar you may not be able to nail the inside curbs as hard without making some compensation elsewhere. This is not always a bad thing and many creative people have used it to their advantage. The point is that we must keep aware of it so we consider the whole picture when making setup decisions.
The Balancing Act
Getting ARBs right in the big pictures takes time, just like the rest of the setup. We’ve seen big spring soft bar cars win, and others with (what Carroll Smith aptly named) a solid axle conversion kit installed. Much of this comes down to driver preference and style. How each driver approaches the corner, whether they use kerbs, and how they throttle out. As an aside, I have noticed that drivers who come up in dirt cars or sedan classes and drivers who came up in karts are on opposite ends of the spectrum.
We also have the car to consider. Does the front ride height need to be within a small window? Probably can’t use a big front bar. Big V8 with loads of torque? Might not be best to have a big rear bar, but probably loves a stiff front ARB. Front drive touring car? Big rear bar may help you get off the corner very well. The permutations are endless but armed with practical awareness, we can make good decisions.
If you’ve made a bar change that gave positive results in some places and set you back others; look at pitch and bumps. Do your best to isolate each condition and list out the positives and negatives. Talk to your driver and watch what they’re doing in videos. By breaking down the situation into when the bar is working hard and when it is not. Take a lot of notes and be willing to try some counter-intuitive things, you’ll be surprised at what you can learn.