Some people call it as "Stabilizer bar" or "Sway bar". As its names indicate, anti-roll bar is used to reduce body roll. It is a torsion bar fixed to the body of car and is attached to the lower control arms of both right and left wheels.
Deal with body roll due to road irregularities
When both wheels are on the same level, the anti-roll bar does not affect the suspension motion. When one wheel is riding on a hump, the anti-roll bar will be twisted and transfer the force to another wheel to push it down, so the car body will remain balanced. The thicker the bar, the more anti-roll ability it has.
Deal with body roll due to cornering force
Based on the above description, you might be surprised to find out that anti-roll bar can introduce even more roll when the car is subjected to cornering force. Nevertheless, such impression is actually incorrect.
When body roll occurs due to cornering, the outside wheels are compressed, hence the anti-roll bars will push the inside wheels away from the body. Does this action result in even more body roll ? no. Because even without the anti-roll bar, the inside wheels will also keep in contact with the ground (due to weight of the car and the expansion force from springs), thus are pushed far away from the body (don't expect them to hang in the air !). In other words, the anti-roll bar doesn't alter the natural movement of the inside wheels.
On the contrary, the twist torque of the anti-roll bar always tries to fight against the compression in outside suspension. Therefore it also reduces body roll.
The advantage of anti-roll bar is very clear - it suppresses body roll on bump or during cornering, but does not deteriorate ride comfort in straight line.
No, these aren't the things that are bolted inside the car in case you turn it over - those are rollover cages. Anti-roll bars do precisely what their name implies - they combat the roll of a car on it's suspension as it corners. They're also known as sway-bars or anti-sway-bars. Almost all cars have them fitted as standard, and if you're a boy-racer, all have scope for improvement. From the factory they are biased towards ride comfort. Stiffer aftermarket items will increase the road holding but you'll get reduced comfort because of it. It's a catch-22 situation. Fiddling with your roll stiffness distribution can make a car uncomfortable to ride in and extremely hard to handle if you get it wrong. The anti-roll bar is usually connected to the front, lower edge of the bottom suspension joint. It passes through two pivot points under the chassis, usually on the sub frame and is attached to the same point on the opposite suspension setup. Effectively, it joins the bottom of the suspension parts together. When you head into a corner, the car begins to roll out of the corner. For example, if you're cornering to the left, the car body rolls to the right. In doing this, it's compressing the suspension on the right hand side. With a good anti-roll bar, as the lower part of the suspension moves upward relative to the car chassis, it transfers some of that movement to the same component on the other side. In effect, it tries to lift the left suspension component by the same amount. In doing this, it's actually compressing the suspension on that side which basically counters some of the roll in the chassis by lowering that side of the car.
If you're loaded, you can buy cars with active anti-roll technology now. These sense the roll of the car into a corner and deflate the relevant suspension leg accordingly by pumping fluid in and out of the shock absorber. It's a high-tech, super expensive version of the good old mechanical anti-roll bar. You can buy anti-roll bars as an aftermarket add-on. They're relatively easy to fit because most cars have anti-roll bars already. Take the old one off and fit the new one. In the case of rear suspension, the fittings will probably already be there even if the anti-roll bar isn't.
Typical anti-roll bar (sway bar) kits include the up rated bar, a set of new mounting clamps with polyurethane bushes, rose joints for the ends which connect to the suspension components, and all the bolts etc that will be needed.
Still not convinced - no neither am I, the great LKJ thought Anti-roll bars to be an abomination, and I can see his point. First you build expensive independent suspension, and then you join it together, effectively making back into a live axle.
The whole thing beats me, all I can say is it seems to work as every one and their dog uses anti-roll bars to control body roll. Perhaps one day a Manufacturer can come up with a design with a CoG low enough not to need them, until then they seem like a bit of a kludge.
explanation 1 above expresses the view that the Anti-Roll bar doesn't affect ride, whilst explanation 2 suggest the stiffer the bar the worse the ride. If you have a better explanation then please let us know.
In the mean time here's the most plausible Explanation from man who knows a lot of stuff
"The anti-roll bar is a torsion bar (torsion spring) connecting the suspension systems on each side of the vehicle in such a way as to allow both wheels to respond unhindered to two-wheel bumps, such as a ridge across the road."
"But if the wheels try to move independently, as with a single-wheel bump, or in opposite directions when the car rolls then the anti-roll bar resists this tendency. Roll is reduced as intended but comfort suffers as the effective spring rate of each wheel is increased in the individual single-wheel bumps, although the combined spring rate of the two wheels is unchanged over joint disturbances."
"compromise must be reached between the requirements of minimum roll and good response to road shocks. Roll bars, unlike the springs, are undamped (theoretically damping could be incorporated, although the manufacturers have generally concluded that it is not worthwhile)"
So there Anti roll bars leave your suspension settings unchanged, if both wheels rise over the obstacle. ( like a sleeping policeman), but increases the effective spring rate, if only one wheel rises over the obstacle or it is forced down by cornering.