anti-sway bars

My latest experiment is a strategy from racing that is coming over to street cars -- small spring/big bar.  This means going with lighter than typical springs, but the largest sway bars.  This allows for flat cornering (an anti-sway acts effectively adds spring rate to the outside tire), while still maintaining a reasonably smooth ride.

But in any event, bigger anti-sway bars will add cornering power with little comfort decrease in any car, stiffly sprung or loosely sprung.

The bottom line:

According to my calculations, the Hotchkis bars are substantially stiffer than stock, and the Hellwig bars are way stiffer than stock:

Hotchkis:

For anti-sway bars, Hotchkis is the most popular choice.  The Hotchkis rear bar is only marginally thicker than stock (1" versus .9").  The front, however, is 1.5" (hollow) versus the stock 1.2" (solid) bar.

Note, however, that the stiffness of the bar is proportional to the diameter of the bar raised to the 4th power, so a small increase in diameter yields a large increase in stiffness.  On the other hand, the nominal gains are less than that, because solid bars are, other things being equal, stiffer than hollow bars.  It's really confusing.  See the tech stuff below for a full comparison of Hotchkis and Hellwig stiffness and weight.

After lowering the rear 3", I faced serious clearance issues with the frame and Mag-Hytec diff cover.  The Hotchkis bar was hitting where the blue tape is shown above.  So I was forced to switch to a Hellwig rear bar

Hellwig:

NOTE (Oct. 2006):  Hellwig has upgraded the fit and finish of their bars.  Some of the below comments apply to the old bars.  Please read carefully.

Hellwig F150 anti-sways are big, thick, solid bars, meaning that they are much stiffer than the hollow Hotchkis bars -- with a corresponding weight penalty.  The front bar is, gulp, 29 lbs.  They are less expensive than the Hotchkis bars ($300-350/pr. vs. $450/pr).   The part numbers are 7646 front (all F150 2WD) and 7821 rear (Lightning-specific).

The Hellwig rear bar is completely different from the stock or Hotchkis bars, mounting with the arms facing forward.  These photos are not my truck.

Some have mounted the rear bar under the rear axle.  That should handle just about any drop you can throw at it.

(Nov. 2006)  I just received a new front and rear bar from Hellwig.  Man, what a difference a couple of years makes.  My first Hellwig rear bar had a terrible fit and finish, but the new ones have a smooth, sexy silver-veined powder coat finish.  They went from this:

to this:

The finish on the mounting hardware is also much improved.  All of the supplied hardware is super-heavy duty, fine-thread, Grade 8 stuff.  Hellwig is a major supplier to the bus and off-road markets, so they build things tough.

installation:

The front install is a no-brainer.  The rear, however, is a little tricky.

The rear instructions seem clear to someone who has already put one on, but really aren't that clear beforehand.  The best thing to do is to loosely preassemble both frame mount/end link assemblies before you get under the truck.  Once you are under there, it will begin to make sense.

I had significant installation difficulties with the (old) rear bar on the truck now.  I ended up scrapping the frame brackets and mounted the end links directly to the bottom of the frame rails.

Hellwig states that the brackets are different now, so I have deleted most of my previous comments.  I will re-post installation notes on the new bars.

Bottom line for rear bars:

Hotchkis:  Great fit and finish.  Painless 15-minute installation.  Not able to deal with extreme lowering. 

Hellwig:  Very stiff.  Work with extreme lowering.  ?? installation difficulty.

tech stuff:

From Neuspeed's Web site:

The stiffness of any torsion bar (and that's what an anti-roll bar is) can be approximated using this equation:

K = 1,178,000 x (D4 / LA2)

Where K = bar rate in lbs/inch D4 = diameter of the bar, raised to the 4th power, measured in inches L = center length of the bar, measured in inches A2 = lever arm length, squared, measured in inches and 1,178,000 is the rigidity modulus constant

When you are working with solid bars, D= the outside diameter (O.D.) of the bar. When you are working with hollow bars, D = the wall thickness of the tubing, NOT the O.D. of the bar.

So, if company "A" offers a 28mm. hollow bar for your car, and NEUSPEED offers a 28mm solid bar for your car, the solid bar will have the higher rate if the lever arm and center lengths are held constant. Do the math before purchasing a hollow bar. It may be lighter, but it's not as stiff as a solid bar if the O.D.'s are the same.

The stiffness of an anti-roll bar may also be calculated based on the torque force required to deflect the lever arm by 1 degree. The mathematical formula is different, and the unit of measure is Inch-pounds per Degree.

Now, similar wisdom from the Mirrocerros Web site:

A number of factors determine the stiffness of a sway bar, including the bar's thickness and length, the length of the swing arm or lever, the bar material, and the bar's attachment method. But if two sway bars differ only in their diameters, all you need to know is that their relative stiffness (torsional rigidity) is proportional to the 4th power of their diameters. A small increase in diameter results in a large increase in sway bar stiffness. You can calculate the actual percent difference in the stiffness using the following formula:

larger bar diameter4/smaller bar diameter4 * 100 = percent change in stiffness.

What if one or both of the bars is hollow? That adds one more step to the calculations. After you have calculated the rate of the hollow bar based on its outside diameter (o.d.), you have to subtract the rate of a hypothetical bar that matches the inside diameter (i.d.) of the space within. To do that, you need to know the wall thickness of the hollow bar.

I'll illustrate the above by comparing three stock Mustang rear sway bars: a 21mm solid bar (1996 V6 Mustang), a 24mm hollow bar with wall thickness of 3.84mm (1998 GT Mustang) and a 27mm hollow bar with wall thickness of 5.4mm (1998 Cobra). Here are the calculations:

The V6's 21mm solid bar has a relative rate of (21^4) or 194,481

The GT's 24mm hollow bar has an i.d. of 24 - 3.84*2 = 16.32mm. and a relative rate of (24^4)-(16.32^4) = 260,838

The Cobra's 27mm hollow bar has an i.d. of 27 - 5.4*2 = 16.20mm. and a relative rate of (27^4)-(16.20^4) = 462,566 Conclusions: The 27mm Cobra bar sway bar has about 2.4 times (462,566/194,481 * 100 = 238%) the stiffness of a 21mm V6 sway bar and about 1.8 times (462,566/260,838 * 100 = 177%) the stiffness of a 24mm GT sway bar.

Okay.  Does your brain hurt yet?  Using this info, and converting the listed bar diameters to millimeters, then using the power of 4 function described above, I come up with the following stiffness per inch for the competing bars:

15678131

1 = The front Hotchkis bar has an estimated 5.5 mm wall thickness, so the effective stiffness = 38.1^4 - 27.1^4 (38.1mm-11mm). 

2 = The rear Hotchkis has estimated 4.5mm wall thickness, so the effective stiffness = 22.86^4 - 13.86^4 (22.86-9mm).

3 = The Hellwig hardware adds another 10 lbs or so versus stock.

The problem with comparing the Hellwig rear to the others is that its stiffness is a little less than this oversimplification would indicate because the Hellwig arms look to be a little longer.