What is an Anti-roll bar?

An anti-roll bar is a tube of metal that works as a spring between left and right wheels. An anti-roll bar connects the vertical motions of the left and right wheels and works in parallel with the coil springs.

During cornering, the outside wheel (be it front or rear) wants to move up in the wheel arch and the inside wheel wants to move downwards. If the two are disconnected then this would take place and the car would experience roll. With the two connected, via an anti-roll bar, the rising wheel is being held down by the falling wheel and vice versa therefore reducing the roll of the car.

The effects of an Anti-roll bar

The main effect of an anti-roll bar from within the car is what the name suggests; it reduces the amount that the body of the car can roll.

Anti-roll bars alter the distribution of lateral load transfer between the front and rear axles of car. This is due to an anti-roll bar increasing the amount of force upon the outside tyre when cornering. Due to this, the stiffer the anti-roll bar, the more lateral load transfer that occurs across it.

The phenomenon is often very misunderstood within motorsport as it is a confusing principle of science.  What it effectively means is that the stiffer end of a car will experience more load transfer across the wheel during cornering which will result in over/understeer at that end.

For example, if a car has a very stiff front anti-roll bar and a soft rear anti-roll bar then the front of the car will tend to understeer during cornering. Alternatively, if a car has stiff rear anti-roll bar and a soft front anti-roll bar then the car will tend to oversteer during cornering.

This key effect of an anti-roll bar is very important to understand when tuning a car on track and adjusting a vehicle based upon driver feedback.

When is an anti-roll bar working?

An anti-roll bar is inactive in a straight line when both wheels on opposite axles are moving up and down the same distance at the same time. This is due to the way in which the roll bar is linked to the suspension system and will just pivot in its mounts in this scenario.

When a car is cornering the roll bar will be forced to twist and becomes active taking effect upon the suspension system and working in parallel with the coil springs to control the car.

An anti-roll bar is most useful during steady state cornering. This is usually mid corner when the car has turned in, body roll has occurred and the car is sat mid corner turning at the point before the corner exit when the car will roll again. At this point, the dampers have stopped working meaning that the only tuneable suspension component working at this point in the corner is the anti-roll bar.

It is very important to remember that mid corner handling is affected most by the anti-roll bar as this can be adjusted based upon driver feedback to solve most mid-corner steering issues.

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2 Comments

  1. Thanks but there is no explanation as to WHY this understeer or oversteer would occur as a result from more downward force.
    Also, in another section under “adjust and Tune, Anti-Roll bars, it says 1 result direction for stiff roll setup and the opposite for sloppy roll setups, and again without any explanation of why either would occur, or why the behaviour would reverse…

    Adding downward force to a toed-in wheel and reducting force on the opposite wheel would increase oversteer for stiff or sloppy suspension roll setups…or is something else happening.

    Dave

    1. Hi Dave,

      Thanks for your comment. This anti-roll bar article has been written to provide an understanding mainly of when an anti-roll bar is working and how it works in terms of generating LLT across an axle and how stiffer roll bars generate more or less LLT.

      Our “Adjust and Tune” article covering roll bars is designed to give a more in depth but practical look at how to adjust and tune your anti-roll bar set up and is designed to be informative and practical so that someone track side could look at the article and have the ability to adjust the anti-roll bars to solve an under/oversteer issue without the information becoming too technical.

      With regards to the soft suspension set ups we are talking about an extreme situation such as a SUV with no anti-roll bars or an American muscle car with no roll bars. The article is written to appeal to all drivers, some of whom will have extremely soft set ups that are not track focused and therefore will be experiencing excessive roll, so stiffer set up would actually decrease outside tyre loading. As mentioned in the article this is few and far between but needs mentioning regardless. The stiffer “more normal” set up alterations apply to the vast majority of our readers where their car already has a roll bar but they would like to alter the handling.

      In light of your comment we will add some further technical information to our article to explain in more detail why the car over/understeers due to increased or decreased LLT across an axle. In short it is due to the increased tyre loading reducing overall axle grip due to multiple tyre characteristics, the main being slip angle data from the tyre.

      Many Thanks
      Suspension Secrets

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