Anti-Roll Bars? A Good or a Bad Thing?

Uncategorized Aug 04, 2017

Today, I want to show you 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.  Racers know you need them to restrict roll, but many suggest 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 for racing and high performance. 

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.   

I know.  This is a long way away from traditional thinking.  But 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.

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.  

In two wheel bump, both arms move in the same direction, and the ARB has no affect.

Spring Stiffness at the Tyre - The Anti-Roll Bar Acts as a SECOND SPRING, But Only in Roll........

In racing, we want stiff springs for best cornering grip.  This helps keep the rubber at the tyre contact patch keyed into the profile of the road surface.

The anti-roll bar works to resist roll, as its name suggests. 

But the anti-roll bar has another super important role – it acts as a second spring to add spring stiffness at the tyre, but only in cornering.   

If we go too stiff with the springs, grip at the tyres for braking and acceleration will be adversely affected.  The anti-roll bar adds spring stiffness for cornering, while allowing the springs to act alone during braking and acceleration.

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.

Practical Demonstration of Ride and Roll Stiffness -
Springs vs 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 is a torsion bar inside each of the two 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 is very effective in stopping the bar from twisting in the centre.) 

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.  

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…we have the clamped torsion bar working as two springs to support front RH and LH sides in ride.  The unclamped torsion bar acts as a single anti-roll bar.

(For a Formula Vee race car, the bottom clamp is made so that it can rotate and adjust ride height.)

What have we got here?  A front suspension with a single torsion bar spring acting 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 4 spring arrangement.

To state this more generally, as applicable to all regular road and race cars...

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 affect.  We could remove the anti-roll bar, stiffen the springs by the equivalent amount.  There would be no change to the roll angle of the car.

Anti-Roll Bar  - Lack of Independence

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 be acceptable to the owners.

So for us, in road performance and racing, anti-roll bars are only a good thing.  We can have all the stiffness we need in roll, and not exceed reasonable stiffness for the ride mode (and so not degrade braking and acceleration).

What About One Wheel Bump?

Here’s one little extra twist that nobody talks about. 

In one wheel bump, the anti-roll bar gives a big plus, compared to doing the equivalent amount of roll stiffness with springs only. 

Our Veedub example shows this very well.  In the standard car, the stiffness at that wheel is provided by the two half-length torsion bars on that side of the car. 

If we now a have 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.  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 summarise, the anti-roll bar gives us the extra stiffness we need at the tyre contact patch to maximise cornering grip, given that the springs are already at our optimum stiffness - ie maximum stiffness, but not too stiff for braking and acceleration .  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 tyre 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 traction problems under braking or acceleration on bumpy surfaces.    

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 tyres, particularly, the outside front during corner entry. 

We also want to fit adjustable anti-roll bars front and rear, so that we can easily adjust the balance of the car for understeer/oversteer.

There’s a whole lot more to anti-roll bars than we have considered in the past.... 

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