Can somebody explain the understeer/oversteer concept in relevance to front and rear sway bars? Sorry this may be a dumb question, but I've heard something about having too large a sway bar in the rear causes understeer and too large a front bar caused oversteer or something like that. Can anybody clarify the concept for me? Thank you for your time....

 

Well, you probably will get a lot of people saying "use search, you noobie" and you should use search for basic questions like this, but I will try to answer.

Understeer is when the front tires slide before the back tires - the car turns less than your steering input, hence the term understeer (also known as "push" in NASCAR circles)

Oversteer is when the rear tires give up their grip before the fronts - the car turns more than your steering would indicate, hence the term oversteer (also known as "loose" in NASCAR circles)

Most cars understeer when going in a steady state turn but can oversteer when you give them sudden inputs like suddenly lifting off the gas in the middle of a turn. Understeer is considered safer than oversteer especially for the driver that is not expecting a loss of grip.

Sway bars (also known as anti-roll bars) connect the left side lower suspension with the right. As one side goes up into the body the bar twists. Since the bar is a big hunk of metal it tends to resist the twist. Just like a spring it tends to stop the wheel that's moving up. Sway bars should not be confused with strut bars which go near the top of the suspension and are used to make the chassis stiffer.

What a sway bar does during a turn is effectively increase the spring rate of the outside tire at that end of the car.

A tire's grip increases as force is added but not as quick as the force required to turn that added weight. A tire that is gripping as well as it can in a turn will exceed its limit if it has to carry more weight.

Back to over and understeer - There are a number of factors that determine over/understeer but the one we are interested in here is relative roll stiffness front to rear. If the front of the car is very resistant to roll but the rear is not then when the car rolls more force will be applied to the front than the rear.

We now have all the facts we need to understand why sway bars can effect over/under-steer. If I put a big sway bar in the back of the car and a small one in the front, the relative roll stiffness will be greater in the back. So when I am in a steady state corner the outside back tire will have more force on it than the front and will lose grip sooner and cause oversteer. The reverse is true with a big front bar and a small rear one.

Of course there are lots of other factors that effect under/oversteer - relative spring rates, suspension design, alignment, etc. But the bottom line is that you can make your car more prone to oversteer by putting in a bigger rear sway bar or more prone to understeer by putting in a bigger front sway bar. But I will say that sometimes the opposite is true - a bigger front bar can keep a car more level and allow the tires to have more grip and actually decrease understeer. It's a fine line and basically the only way to know is to experiment yourself or take setup advice from others.

regards,
alan

If I put a big sway bar in the back and a small

 

Thank you very much for the info. I'm currently working on my car's suspension and handling aspects and this info will help me a lot. Much appreciated....