Wednesday, June 17, 2009

The anti-roll bar...

What keeps you from toppling when you take a curve at over 60 kilometers an hour in your hyperactive hatchback sporting the world’s best independent suspensions ???

The answer is the stabiliser bar aka anti-roll bar aka sway-bar aka anti-sway-bar (courtesy Wikipedia). You would find this when you bend down the front of your car and feel your hand behind the plastic bumpers. This is an inch fat bar in the form of ‘U’ connecting both front or/and both rear wheels.

This of course is not put to keep the wheels from running away.

Imagine what happens to you during a tight right turn in a car. You would be thrown to your left. The same thing happens to all parts of the car. The body, the engine, the chassis and the tyres. Why do modern cars have an independent front suspension? The answer is to have a comfortable ride and not letting everyone in the car know of the bump you just drove the right front wheel of your car on. Now during a turn, the independent suspensions would be free to act independently and that could be particularly dangerous in a curve. When you turn hard, the suspensions on the wheel to the outside of the curve (left ones during a right turn) collapse and the ones to the inside stretch. This stresses the outer wheels heavily making the steering behave more maniacal and can cause the inner ones to leave the ground. And when done harshly, it may result in a car topple!

So what does the bar do?

The antiroll bar is connected in such a way that the base of the ‘U’ is clamped to the body to behave like a pivot and the limbs are connected to the control arms (lower portions of the steering knuckle) on both wheels. The result now during a turn would be, when the outer suspension collapses, it drags one of the limbs of the ‘U’ with it and the other stretching suspension drags the second limb in an opposite direction. It is something like twisting the base of the ‘U’ with a crowbar between the limbs. The antiroll bar is made with tempered spring steel to resist this twist and return back to its normal position and to continuously do so over three million times. The result now is that there is some resistance provided to the suspensions to behave independently during a turn and prevents the car from rolling too much.

How is it made…

The grade is spring steel with the thickness of approximately an inch for a small car. The bars are cut to required lengths and the ends are forged or turned and threaded to fasten to the control arms. This bar with the ends prepared, is now fed into a Radiant Tube type LPG furnace. This is a simple furnace controlled by temperature controllers which regulate the air supply valve and hence the fuel supply to the furnace constantly maintaining 950 degrees Celcius. At the heart of the furnace is a burner where LPG mixes with air and fires up. The rod is heated here for approximately twenty minutes. It then comes out like a glowing orange fluorescent tubelight, onto a module. A module is a set of fixtures that bend the bar to the shape you want powered by a hydraulic power pack. The hydraulic cylinders on all fixtures on the module have pre-programmed working lengths which at the end of the working cycle would have formed the bar in the shape you want resembling a ‘U’. The bar now at about a nine hundred degrees and still glowing is quenched in a seven thousand litre oil sump at room temperature for five minutes. The bar would be brittle when it comes out. It then is passed on the LPG fired conveyorised mesh furnace for ninety minutes, where LPG is burned to continuously heat the bar to 450 degrees throughout the time. It then is allowed to cool to room temperature by when the bar would have accumulated all properties of a spring by aligning the grain structures in the way metallurgists wanted it. The bar is now stored for a while before being led to shot peening in a Hanger Type Shot Peening machine. Shot peening is an operation where metal balls, millions of them, (approximately 0.9mm dia) are shot at very high velocity on the bar continuously for about half an hour. This continuously rams the surface to make it tough and eliminate all surface cracks. This also prevents surface cracks from occurring when the twisting happens on the bar in the car. The absence surface cracks can be seen through a magnaflux gauge. Here the whole bar is magnetized and when you spray oil filled with ferro particles, they settle all over the bar and concentrates on surface cracks. When seen through ultra-violet light, the crack reflects as a fluorescent yellow line. As long as you don’t find one, you can continue to be happy.

The stabiliser bar is now ready to function. The next process on the line is zinc phosphating and powder coating to provide an aesthetic appearance and prevent rust. Two aluminium rings are crimped on the base of the ‘U’ to act as stoppers for fitment on the car. This ready bar is inspected for surface finish and bound in bubble wrap before sending it to the carmakers with a fat bill.


prajwala said...

good one!!...."the anti-roll bar"

Adithya said...

Hey Subba,

Nice write up... Lotsa detail, I'm guessing you did a process walk at caparo or something..

Nice to read some really technical stuff after a while, but these anti roll bars are so damn important.. Especially in tall boy designs such as wagon R and Santro. But, there's another thing.. The anti-roll bar, deprives the wheels of weight transfer, therefore reducing the grip. The original Ikon for example didn't have an anti-roll bar. And man, that car could grip..

Another thing is around the humps and bumps. When one of the suspensions is compressing and the one on the oppsosite side expanding. The anti roll bar, stops this, thus making for a thudding sound and a hard ride. Especially seen in wagon R which comes with a very stiff Anti-roll bar.

An even more stiff version of the anti roll bar is something called rigid axle, found on the 4wd scorpio, perhaps you should research that. you will find it very interesting..

Sub said...


It was a PCPA at SSS :P

Gecko said...

And there's something called as a Kinetic (regd) suspension technology, on the world championship winning Citroens Xsara machines and the Mitsubishi Rally raid Pajero machines which connect the front and the rear stabilizers in a passive way so as to improve handling, mostly in extreme offroad conditions!