De-stiffening the ARB; grinding, filing, sanding

I've been trying to workout what is going on with my anti-roll bar.

I put it onto the front of the car, and it certainly helped to make the car more stable in the corners. But, it gave me dreadful wear on the front outside tyre.

So, I hit the books to try to work out what was wrong.

ARB stiffness is a simple calculation,
Effectively the stiffness depends on the material properties, diameter to the power 4 (for hollow bars you do outer diam^4 - inner diam^4), divided by the length.

Then the effect that this has on the wheels can be determined. This has two parts, one the length of the arms on the ARB, and then the motion of the wheel versus the motion of the end of the arm. By this I mean, if the arm is short then it twists the ARB a lot for a small motion of the suspension. Conversely if the end of the arm is far from the wheel then the ARB will twist less than if the ARB is actually at the wheel.

The above arm and wheel motion ratio effects come in to the second power (once for the leverage effect which gives reduced force, and once for the fact that the angle is wound up less quickly and so it gets stiffer slower).

I'm not sure that explanation will help anyone, but who knows.

My bar is 16mm diameter and 400mm long and steel, with 5inch arms. Various calculations indicated that this was around 3times stiffer than ideal (determining ideal is the most difficult thing, but if you have a bar that is stiffer than everyone elses, and some of the everyone elses are winning races, then it means it is too stiff).

Welding ARB's gets a mixed reception as a method (mine is welded) so I didn't fancy making a new bar (if I did then I would use 2mm thing CDS tube of 1/2inch diameter, and get the ends mandrel bent, maybe bend them myself cold). So I decided to reduce the outer diameter of my bar.

Lots of grinding, filing, and sanding later I went from this red thing;



To this silver thing;


Going for a drive it felt like there was more flex at the front, but this will only come in for "on the limit" handling, which isn't appropriate on the road. The original bar had a stiffness of around 400lbs/in whereas the front springs are rated at 175lbs/inch, so it was like a solid front axle. The revised bar is more like 180lbs/inch. Exact calcs are more difficult, because there is a 70mm section of 16mm, then a 330mm section of bar that is between 12.5 and 13mm. (the calculation is to determine the stiffness of each section and then combine them as (1/((1/Q1) + (1/Q2))) the same as for parallel resistors.

Some software that I received from Tim Hoverd indicates that I should go softer still (maybe 120lbs/inch, although he does have stiffer springs on the front than I do).

Anyway, hopefully it is a move in the right direction.