Understeer/oversteer ???

[Powerman]

I know what it is, but how do sway bars help? I understand sway bars keep the tires flat so it increases contact patch. However, how does it work on one end? Meaning, if I increase stiffnes on the rear bar, I get rid of push. But is that because the tail end will step out sooner, or is it because the grip to the front has increased. That is what I'm not getting. How does a rear bar increase front grip (if that is what it is doing).

 

Please excuse my suspension ignorance.

[Powerman]

Anti roll bars provide 2 main functions:

The first is the reduction of body lean. The reduction of body lean is dependent on the total roll stiffness of the vehicle. Increasing the total roll stiffness of a vehicle does not change the steady state total load (weight) transfer from the inside wheels to the outside wheels, it only reduces body lean. The total lateral load transfer is determined by the CG height and track width.

The other function of anti roll bars is to tune the high g / limit understeer behavior of the vehicle. The limit understeer behavior is tuned by changing the proportion of the total roll stiffness that comes from the front and rear axles. Increasing the proportion of roll stiffness at the front will increase the proportion of the total weight transfer that the front axle reacts and decrease the proportion that the rear axle reacts. This will cause the outer front wheel to run at a higher slip angle, and the outer rear wheel to run at a lower slip angle, which is an understeer effect. Increasing the proportion of roll stiffness at the rear axle will have the opposite effect and decrease understeer.

 

 

From Wiki.

[dvdt]

So did you answer your own question?

 

Actually I sort of have the same question.

 

So in adding a stiffer rear bar...

 

Increasing the proportion of roll stiffness at the rear will increase the proportion of the total weight transfer that the rear axle reacts and decrease the proportion that the front axle reacts. This will cause the outer rear wheel to run at a higher slip angle, and the outer front wheel to run at a lower slip angle, which is an oversteer effect.

So theoretically, the total outcome is neutral because rear grip decreases and front grip increases? Is this right?

[Scotty]

In the really basic way of looking at it:

 

Add stiffer front sway bar only, decreases grip at the front end resulting in more understeer.

 

Add stiffer rear sway bar only, decreases grip at the rear end resulting in more oversteer.

[Underdog]

^ The key being, a stiffer swaybar DECREASES traction (all other things being equal).

[dvdt]

So by this logic does adding a stiffer fsb and a stiffer rsb DECREASE traction/grip all around?

 

Or do 2 wrongs make a right?

[LittleBlueGT]

^ The key being, a stiffer swaybar DECREASES traction (all other things being equal).

 

Or it adds more traction to the opposite axle.

[Scotty]

Two wrongs don't make a right. A soft suspension set up will have more grip in a steady state situation, but we don't corner in steady state situations. Soft suspension setups don't react quickly enough from steering inputs. If you run a fully rigid suspension, basically bolt the wheels straight to the chassis, it will be very responsive. But the overall grip will decrease.

 

The right suspension setup is to find a happy medium.

[GoRilla]

This is a tough question. I am going to do my best to explain as I have been taught.

 

By adding a stiffer bar, you are decreasing the steady state grip, but increasing overall cornering ability by decreasing body roll.

 

If keep the proportions the same as stock...but add stiffness to both front and rear...the oversteer and understeer characteristics will be the same...but with less body roll and normally with higher g's.

 

If you add stiffness to just the rear...you will get more oversteer.

 

The way I look at it...add stiffness to both front and rear, but just a tad more to the rear and you should have a nice neutral car.

[emosound]

^ The key being, a stiffer swaybar DECREASES traction (all other things being equal).

 

That assumes that the suspension is perfectly tuned. The stock suspension on the Legacy is, um, not. It's WAY WAY too soft. In this case, the car leans in corners so much that the front tires (on struts) go into positive camber and they lose grip. The rear of the car has a multi-link arrangment so it has some camber gain when the car rolls, so it doesn't lose grip as quickly as the front. The end result is a nice, safe, understeering car.

 

If you're starting with a stock car, or one with mildly lower/stiffer springs, adding bigger swaybars at both ends will increase grip in most conditions, because the car won't roll as much, and the tires will stay square with the road. You can tune the balance with bigger or smaller bars front or rear, but it's hard to go too big, unless you live where the roads are really bumpy. Really big bars don't work so well in the rain, tho.

 

If you have a racecar, with really high spring rates already, addding too much bar front or rear will casue that end to lose traction. Real racecars tend to not use much bar. Road cars can use big bars as a substitute for stiff springs to control roll, because the ride is generally better with bars than springs.

 

-Paul

[RaceComp Engineering]

1. Add a front sway bar and you may lose a little grip from increase weight transfer at that end of the car. However, the reduction in roll/camber loss USUALLY makes up for it, and unless you go too big, you'll have a net increase in front grip.

 

2. For the rear, the shift towards oversteer is more a function of increased front grip due to the change in where the weight transfer occurs. Decreased weight transfer up front improves grip as you have more even tire loads. Again, the increase in weight transfer at the rear competes with the reduced roll/camber loss and you USUALLY end up with a net increase in grip.

 

 

- Andrew