Can someone please explain over/understeer

[greg bradbury]

I have read a lot about torsion bar combinations 22/28 and so on. The overriding theme is that the 911 understeers. That is the front end pushes when you power on in the corner. What I am reading is that to overcome this you increase rear torsion bar size. Wouldn't this actually make the car understeer more. To me it seem that to get rid of understeer you would put heavier torsion bars up front. Or put a stiffer sway bar up front.
What am I missing here?

p.s. I have played around with other cars and suspension and the car oversteered nicely (controlled) with a stiff sway bar up front and a lighter one in the rear.

[coloradoporsche]

Understeer and Oversteer

[2ndTARGA]

It does seem counter intuitive but think in terms of relative roll stiffness (front vs. back), not absolute. Understeer can be addressed by increasing front traction or reducing rear traction. Four wheel drift occurs when both ends break loose at the same time. If one end is stiffer, it will slide first

Here is another explanation from a racing website I found:

Overall, as the body rolls, it changes the suspension geometry allowing the tire to tilt too much and reduce the contact patch size. Too soft a resistance on one end of the car compared to the other can cause the opposite end of the car to lose traction prematurely (also subject to springs, shocks, and tire pressure adjustments).


Ron

[ChrisBennet]

The quick rule of thumb is that making one end of the car stiffer increases traction at the other end of the car. Stiffer rear bar will increase grip at the front.
-Chris

[greg bradbury]

Wait a minute I think I get it. Martin brundle suggests to cure understeer you can soften the front end and thence it can make it more compliant and grip better = less understeer. Too hard and it can bounce. Mmmm.

Still doesn't make sense to me

[rattlsnak]

Some people get so confused over this,.. its simple. Dont think of it backwards... If the front end pushes, you need to LOOSEN the front. If the back end slides, TIGHTEN the rear.

Its saying the same thing as Chris and others. (if you tighten the rear, you are making the rear stiffer than the front, which is the same thing as loosening the front and leaving the rear alone.)

[Eagledriver]

The reason is this: With stiffer bars (torsion, or anti-roll) you transfer more weight to the outside wheel. The grip of a tire is almost proportional to the weight on it but not quite. The end that shares it's weight between the tires the best ends up with the most grip.

Lets say we go into a turn and the front transfers 60 percent of it's weight to the outside tire but the rear transfers 70 percent (stiffer bars). The rear in this case will have less total grip than the front. Therefore a stiffer rear results in oversteer.

-Andy

[greg bradbury]

good explaination Eagledriver the best I've heard so far. It does seem to make sense when you look at it like that.

Therefore the sway bars does play a large part in this. The stiffer bar equals more traction in that end. Heavy bar at the rear = understeer, or heavier bar up front causes oversteer.

Why is the preferred option to use torsion bars to adjust the balance of the car? A lot of people are saying to tune using TBs and fine tine using sway bars, why is that?

[2ndTARGA]

Not to belabor the point.......

This is a pretty thorough explanation. Part way down there is discussion of springs (torsion bars in our world) vs anti-roll bars.

Ron

http://www.grmotorsports.com/news/012005/lean-less-the-inside-scoop-on-anti-roll-bars.php

[Miguel Antonett]

Keep in mind that you have different diameter of sway bars to choose from; BUT in addition, you can also adjust the "radius of torsion" to the bar by adjusting the links distance from/to the bar center line.

[greg bradbury]

Not to belabor the point.......

This is a pretty thorough explanation. Part way down there is discussion of springs (torsion bars in our world) vs anti-roll bars.

Ron

http://www.grmotorsports.com/news/0...i-roll-bars.php

Ron a great article thanks, it makes sense now.

Regards,
Greg.