Anti-Roll Bar Information

Anti-roll bars can be made of solid bar stock or tubing, bent into the proper shape with arms at approximately right angles to the chassis-mounted section of the bar.

Roll couple distribution is the combined resistance to body roll provided by the springs and anti-roll bars at the front versus the rear.  Even though anti-roll bars distribute transferred weight differently than springs, the effect is the same.  If you stiffen the front anti-roll bar, the understeer is increased (or oversteer decreased).  If you stiffen the rear anti-roll bar, the overstreer is increased (or understeer decreased).  Softening a bar has the opposite effect.

If a car oversteers during steady state cornering, a softer rear anti-roll bar should help reduce or eliminate the condition.  Which bar to change depends on several factors. 

There are situations where increasing the stiffness of an anti-roll bar will have the opposite effect.  Most stock vehicles have excessive understeer because it is easier to control and provides more stability for the average driver than a vehicle that oversteers.  A big part of this comes from excessive body roll, which induces too much camber change, and a good portion of the front tire contact patch loses contact with the road. 

In this instance, adding a stiffer front anti-roll bar which would typically increase the extreme understeer, actually reduces the understreer by reducing the body roll induced camber change.  The front tires now stay in better contact with the road surface, creating more traction and reducing understeer.  This is an old trick in SCCA auto-crossing in stock classes where the few modifications permitted include changing the front anti-roll bar.

On a front drive car with a front weight in excess of 59 percent, the rear bar will be quite stiff.  For competition, the bar will be stiff enough to lift in the inside rear tire contact patch off the ground slightly in a corner at the limits of traction.  For the highway, the bar should be softer for improved stability. 


Anti-Roll Bar Rates

Four factors determine the rate of an anti-roll bar: the diameter of the bar along its active length, the length of the anti-roll bar arms, the active length of the anti-roll bar, and the modulus of elasticity (strength) of the bar material.  Stiffness increases at a rate of four times the increases to the bar’s diameter, thus doubling the bar diameter makes the bar eight times stiffer.  The active length of the roll bar and the length of the arms operate in linear fashion, with shorter length increasing stiffness and longer length reducing it in both cases.  The modulus of the material, which is nearly always high tensile strength steel, is considered a constant. 

Because of multiple bends in a stock replacement anti-roll bar, it is difficult to calculate the rates.  Most manufactures calculate the rates then check the actual rate of the bar mechanically.  Adjustable anti-roll bars have multiple mounts on the arms to change the arm length and therefore the bar rate. 

***The above information is straight from High-Performance Handling Handbook used with permission from the author, Don Alexander.  You can purchase the book in its entirety here:  High Performance Handling Handbook.

Original Diameter	New Diameter
	16mm	17mm	18mm	19mm	20mm	21mm	22mm	23mm	24mm	25mm	26mm	27mm	28mm	29mm	30mm
14mm	71%	117%	173%	239%	316%	406%	510%	628%	764%	917%	-	-	-	-	-
15mm	29%	65%	107%	157%	216%	284%	363%	453%	555%	672%	803%	950%	-	-	-
16mm	-	27%	60%	99%	144%	197%	257%	327%	406%	496%	597%	711%	838%	979%	-
17mm	-	-	26%	56%	92%	133%	180%	235%	297%	368%	447%	536%	636%	747%	870%
18mm	-	-	-	24%	52%	85%	123%	167%	216%	272%	335%	406%	486%	574%	672%
19mm	-	-	-	-	23%	49%	80%	115%	155%	200%	251%	308%	372%	443%	522%
20mm	-	-	-	-	-	22%	46%	75%	107%	144%	186%	232%	284%	342%	406%
21mm	-	-	-	-	-	-	20%	44%	71%	101%	135%	173%	216%	264%	316%
22mm	-	-	-	-	-	-	-	19%	42%	67%	95%	127%	162%	202%	246%
23mm	-	-	-	-	-	-	-	-	19%	40%	63%	90%	120%	153%	189%
24mm	-	-	-	-	-	-	-	-	-	18%	38%	60%	85%	113%	144%
25mm	-	-	-	-	-	-	-	-	-	-	17%	36%	57%	81%	107%
26mm	-	-	-	-	-	-	-	-	-	-	-	16%	35%	55%	77%
27mm	-	-	-	-	-	-	-	-	-	-	-	-	16%	33%	52%
28mm	-	-	-	-	-	-	-	-	-	-	-	-	-	15%	32%

Anti-Sway Bar Diameter/Stiffness Formula
When you upgrade to larger sway bars, what is the net effect on stiffness? When sway bar diameter is increased, stiffness increases to the 4th power.

ie:

= 1.24
This example shows an increase in stiffness of 24% for a 1 mm increase in diameter. Note that this is for the bar only, and does not account for adjustment holes, end link length or tension, bushing material, or differences in metallurgy. Tubular anti-sway bars are not covered by this formula.