Speed Secrets: Anti-Roll Bars? A Good or Bad Thing?
By Ross Bentley
December 13, 2016
I've always thought that one of the things that make performance and race driving so challenging, so addictive, and so much fun is that there's no one-size-fits-all solution to pretty much anything we do. And just when you think you've figured it all out, someone comes along with a different opinion...
Take car set-up for example. A few issues ago Jeff Braun wrote about how to tune shock absorbers, and he couldn't say, "If the car understeers, always make this adjustment."
The same can be said about tuning the anti-roll bars on your car. And that's why Dale Thompson is going to share his latest thinking about them this week. Dale, if you recall from his previous articles here, has a prep shop in Australia, and has created an online training program: Suspension Tuning 101 and 102 (you can learn more by going to www.suspensionsetup.info)
Enjoy! – Ross
Let’s look at our very latest thinking on anti-roll bars (ARBs - also known as sway bars or stabilizer bars.)
Anti-roll bars are often characterized as a necessary evil – you need them to restrict roll, but the interconnection between wheels on the same axle causes “bad things” to happen. Proponents of this view say things like “the suspension ceases to be truly independent.” On the surface, this might appear to make sense.
The reality is very different. The interconnection between wheels on the same axle has a negligible effect. Follow along with the rest of this article, and check out the true benefits of anti-roll bars when you are upgrading the handling of your car.
In fact, for racing and road performance, the anti-roll bar is a “good thing” and there are no downsides – only limitations as to how stiff you can go, just the same as there are limitations as to how stiff you can go with the suspension springs in your car.
Let’s start with the important components of the anti-roll bar (ARB).
The ARB has two lever arms that twist the “torsion bar” in roll only. When cornering, weight transfer compresses the suspension on the outside wheels and unloads the suspension on the inside wheels. The suspension of the car pushes and pulls the lever arms in opposite directions.
In two-wheel bump, both arms move in the same direction, and the ARB has no effect.
Spring Stiffness at the Tire
The anti-roll bar works to resist roll, as its name suggests. But it has one other super-important role – it acts as a second spring to add spring stiffness at the tire, but only in cornering.
In racing, we want stiff springs that will help us maintain maximum grip at the tire contact patch for cornering. But if we go too stiff with the springs, grip at the tires for braking and acceleration will be adversely affected. This is where the anti-roll bar helps – to add spring stiffness at the tire to aid grip in cornering, while allowing the lesser spring stiffness needed for braking and acceleration.
The importance of the anti-roll bar is to separate ride and roll modes. In racing, the springs affect the tire grip you have for the ride mode - braking and acceleration phases (plus ride comfort if you drive on the road), and the springs and anti-roll bars together affect the grip you have for the roll mode – the all-important cornering phase. For road performance, all the same criteria apply, except we run softer springs (but still as stiff as possible) so as to maintain acceptable ride on the road.
And how important is grip in cornering? To call it an obsession wouldn't be an overstatement. At the 2016 World Time Attack at Sydney Motorsport Park, the top cars were running so much aerodynamic downforce (no rule restrictions on aero), that even though they gave away 20 – 30 kph on the straights, they did their best times because of the huge cornering grip available.
Roll Stiffness from the Springs vs Roll Stiffness from the Anti-Roll Bars
Check out this front suspension for a VW Beetle. The point of this demonstration is to show you that roll stiffness from the anti-roll bar is the same as roll stiffness from the springs - in fact, that spring roll stiffness and ARB roll stiffness are interchangeable.
There are two torsion bars inside the cross tubes. Both of the torsion bars have lever arms attached to each end. The lever arms make up the upper and lower suspension trailing arms, as shown.
Note that the torsion bars are clamped in the centre. (The torsion bars are flat, laminated springs, rather than a round bar, so the centre clamp was pretty easy for VW to engineer.) Each half of the torsion bar acts as a spring for the side that is twisting it. So we now have two separate springs each side – two springs for the RH wheel and two springs for the LH wheel.
Note particularly, that each pair of upper and lower torsion bars are providing both ride and roll stiffness, just the same as a regular car without an anti-roll bar. (In this case, there is only one spring at each wheel. The spring is active in both ride and roll modes.)
Now let’s turn one of the torsion bars into an anti-roll bar.
Say for instance, if we removed the top centre clamp. Now the top torsion bar would act as an anti-roll bar.
So in two-wheel bump the suspension will be a lot softer – only working on the bottom tube. But in roll, we still have both top and bottom torsion bars working.
Then, hey - presto!
Our Modified Front Suspension Works on a Formula Vee Race Car...
What have we got here? A front suspension with a single torsion bar spring each side, and a single anti-roll bar.
In pure roll mode, our new set up with two springs and one anti-roll bar is providing exactly the same roll stiffness as it did in the case of the unmodified four-spring arrangement.
To state this more generally, in roll mode, anti-roll stiffness provided by the anti-roll bar is exactly the same as stiffness provided by the springs. We can replace one with the other and still have the same anti-roll effect; eg. we could remove the anti-roll bar, stiffen the springs by the equivalent amount. No change to the roll angle of the car.
What About One-Wheel Bump?
Now’s about the right time to deal with the counter argument that “the anti-roll bar results in lack of independence of the suspension across the axle – a deflection of the wheel at one side of the car is reacted at the other side of the car via the ARB.”
For vehicle manufacturers, it is an issue of “refinement” and ride quality. The engineers are able to identify what they call “lateral head toss.” This happens as a result of too much anti-roll bar stiffness as a proportion of ride stiffness.
But for the manufacturers of true performance cars, “lateral head toss” does not seem to get a look in. For decades, Corvettes (independent rear suspension) and Mustangs (live rear axle up to 2015) have run some very serious anti-roll bars front and rear, with only good acceptance from the buying public. The “lateral head toss” may exist, but must at a level acceptable to the owners.
So for us, in road performance and racing, anti-roll bars are only a good thing. In fact, with stiff racing springs, a deflection of one wheel is reacted at all three other wheels via the chassis, not just the one on the other side of the car. Comfortable ride is not an issue in racing – the driver accepts the poor ride. The main aim of the set-up with the stiff springs and anti-roll bars is to optimize the grip potential of the tire.
Here’s one little extra twist that nobody talks about. The one-wheel bump situation is an example where running an anti-roll bar gives a big plus compared to doing all roll stiffness with springs only. Our Veedub example shows this very well. If we have no anti-roll in one-wheel bump, the stiffness at that wheel is provided by the two half-length torsion bars on that side of the car. If we had the top torsion bar working as an anti-roll bar, the roll stiffness is the same, yet in one-wheel bump suspension stiffness will be less, i.e. better able to absorb the bump. In one-wheel bump, the anti-roll bar is twice the length of the half-length torsion bar, and is therefore only half as stiff in twist.
To summarize, the anti-roll bar gives us the extra roll stiffness we need, given that the springs are already at our optimum stiffness. The anti-roll bar gives us softer response in one wheel bump than the equivalent roll stiffness from springs alone.
So let’s get back to something that looks a bit like your car:
The anti-roll bar is in red. And just to keep the torsion bar theme going, our springs are torsion bars each side of the car, rather than regular coil springs.
Everything we described for the Veedub still applies.
Most importantly, the tire does not know where the stiffness comes from - whether it is anti-roll bar or spring.
When we are setting up the suspension for racing, we choose spring stiffness first, the maximum stiffness we can have without causing the wheels to unload under braking or acceleration on bumpy surfaces. This “sweet spot” is where the Contact Patch Load (CPL) variation is at a minimum, and therefore grip is enhanced. (The CPL variation is a number that they get from seven-post rig testing in professional racing. Because of the cost of running these tests, you and I are never going to see any CPL results. So for us, it’s just a handy concept to help us with our thinking about grip.)
The next step is to add the anti-roll bar stiffness we need for cornering. We need extra stiffness for the more highly-loaded outside tires, particularly, the outside front during corner entry.
We want to fit adjustable anti-roll bars front and rear, so that we can easily adjust the balance of the car for understeer/oversteer.
Hopefully, this article has sparked your interest in anti-roll bars. There’s a lot more to anti-roll bars than we have considered in the past.