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Discussion Starter #1
Hi all!

I'm very new to the motorsports/modding world and I've been doing alot of reading about suspensions lately. I have never had a car w/ a rear sway bar before, so I'm trying to gain some insight on what goes on.
So from my understanding, (please correct me if I am wrong) sway bars help increase the contact patches on the tires during turns and other lateral stress.
So that means sway bars help keep one wheel from moving up too much more than the other one right? If that is the case, then that means any shock/force applied to one, will be applied to the other as well right? Since they are now connected by a bar. This would then mean that more or less the wheels will move up and down in unison?
Now what if, you were on a track and ran up a rumble strip? The bump that pushes up on the side thats on the rumble strip will be applied, in a lower amount, to the side that is still on the road... so will this make the car jump?
If so, then wouldn't it make running over rumble strips very dangerous?

Again, I have never had a rear sway bar before, my 2.5i is my first car ever and I'm very new to all this.
I guess its not just rumble strips but all bumps/roadkill etc.

Thanks for any/all help! :D
 

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haha no. what i think would happen is that as your one side hits the bump, your sway bar will try to pull the chassis even with the wheels again, making your car line up quickly with the new "ground". basically a swaybar would probably make your car react quicker to the bump and end up more tilted.

the swaybar tries to limit, in essence, the car being misaligned with the wheels, which is also why its flatter on turns.
 

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Principles
A sway bar is usually a torsion spring that resists body roll motions. It is usually constructed out of a U-shaped piece of steel that connects to the body at two points, and at the left and right sides of the suspension. If the left and right wheels move together, the bar just rotates about its mounting points and does not bend. If the wheels move relative to each other, the bar is subjected to torsion and forced to twist.

The bar resists the torsion through its stiffness. The stiffness of an anti-roll bar is based on the fourth power of its diameter, the stiffness of the material, the inverse of the length of the lever arms (i.e., the shorter the lever arm, the stiffer the bar), the geometry of the mounting points, and the rigidity of the bar's mounting points. Some anti-roll bars, particularly those intended for use in auto racing, are adjustable, allowing their stiffness to be altered by increasing or reducing the length of the lever arms. This permits the roll stiffness to be tuned for different situations without replacing the entire bar. The stiffer the bar, the more force required to move the left and right wheels relative to each other. This increases the amount of force required to make the body roll.

In a turn, the sprung mass of the vehicle's body produces a lateral force at the center of mass, proportional to lateral acceleration. Because the cg is usually not on the roll axis, the lateral force creates a moment about the roll axis that tends to roll the body. (The roll axis is a line that joins the front and rear roll centers (SAEJ670e). The moment is called the roll couple.

Roll couple is resisted by the suspension roll stiffness, which is a function of the spring rate of the vehicle's springs and of the anti-roll bars, if any. The use of anti-roll bars allows designers to reduce roll without making the suspension's springs stiffer in the vertical plane, which allows improved body control with less compromise of ride quality.

One effect of body (frame) lean, for typical suspension geometry, is positive camber of the wheels on the outside of the turn and negative on the inside, which reduces their cornering grip (especially with cross ply tires).


Anti-roll bars provide two main functions:
The first function 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-force / 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.


Drawbacks
Because an anti-roll bar connects wheels on the opposite sides of the vehicle together, the bar will transmit the force of one-wheel bumps to the opposite wheel. On rough or broken pavement, anti-roll bars can produce jarring, side-to-side body motions (a "waddling" sensation), which increase in severity with the diameter and stiffness of the sway bars. Excessive roll stiffness, typically achieved by configuring an anti-roll bar too aggressively, will cause the inside wheels to lift off the ground during very hard cornering. This, of course, is only possible if the regular spring rate actually allows the outside wheels to handle the much increased load. This can be used to advantage, in fact many front wheel drive production cars will lift a wheel when cornering hard, in order to overload the other wheel on the axle, so limiting understeer.


^^^^Answer to question 2

Some high-priced cars, such as the Range Rover Sport and BMW 7-series, have begun to use "active" anti-roll bars that can be proportionally controlled automatically by a suspension-control computer, reducing body lean in turns while improving rough-road ride quality. The first[citation needed] to use this was the Citroen Xantia Activa, a medium sized sedan sold in Europe. The Activa system was an antiroll bar that could be stiffened under the command of the suspension ECU during hard cornering. The car rolled at any time at most 2 degrees. Mercedes S-class ABC system uses another approach, the computer uses sensors to detect lateral load, lateral force, height difference in the suspension strut and uses hydraulic pressure to raise or lower the spring to counter roll. This system removes the antiroll bar. Most active roll control systems allow a small degree of roll to give a more natural feel.
 

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Discussion Starter #4
ah! thank you for all the replies!

hmmm, so that means the amount of force from one wheel effected on another is dependent on how fat the sway bar is.
I guess i should take things in moderation then hahaha :)
 

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rally (gravel) cars run w/ no or tiny sway bars- obviously they have a great need to maintain as much suspension independence as possible

road going cars generally can greatly benefit from the relatively easy and cheap roll resistance sway bars provide

as even a road going car can benefit from suspension independence- it's beneficial to have all the sway bar you need, but no more

this takes a bit of experimentation (and maybe some luck :)) I ran no less than 15 different combos of sway bars on my last two Imprezas

I think it is better to work your way upward w/ sway bars, the trend however seems to be buy the biggest bar out there (which may possibly be exactly what you need, but certainly not always)
 

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road going cars generally can greatly benefit from the relatively easy and cheap roll resistance sway bars provide

as even a road going car can benefit from suspension independence- it's beneficial to have all the sway bar you need, but no more

this takes a bit of experimentation (and maybe some luck :)) I ran no less than 15 different combos of sway bars on my last two Imprezas

I think it is better to work your way upward w/ sway bars, the trend however seems to be buy the biggest bar out there (which may possibly be exactly what you need, but certainly not always)
I can't agree with this more. Start smaller and go bigger if you need to. This is a good reason why adjustable bars are a good option for many people. The trend to get the biggest bar available is exactly the wrong trend to follow, and I try to emphasize this whenever possible.
 

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Discussion Starter #7
well, my first impression when i was looking into all this was that having a stiff car would be dope.... buuut then i started reading more on what the sway bars actually do and i started to question how big of a change these big bars will have on my car... thus this thread :D

I think I will start by adding a STi rear sway to my car... this will make it a 19mm Front and 18mm Back. Since my car does not have a back sway to begin with, I'm thinking this will make a big big change to how the car drives. Hopefully this does not make me loose too much suspension independence as the track/roads here are anything but smooth

what do you guys think?
 

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yes going from no sway bar to a 18mm will make a significant change in handling

as there are already oe platforms that run the same sway combo it's not going to be "too much" IMO
 

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Do yourself a favor and do both front and rear swaybars. Peglegging (lifting a rear wheel in a corner) is cool on a FWD car.. but means that you can't put any power down in an AWD car like our Subarus. Just FYI, the fastest Subarus in autocross use the stock rear bar, a large front bar and coilovers with high rate springs in the rear. Just throwing on a rear swaybar is one of the most common noob mistakes made by new Subaru owners.

The front swaybar will actually kill more roll since the majority of the car's weight is on the front axle. It will also help the car to react much faster to steering inputs. I'm currently running a 25mm FSB, 22mm RSB, SPT pink springs. The car rides very similarly to stock. Uneven bumps (of the larger variety) do cause the car to snap from side to side (body roll matches the bump.. car tracks straight just fine) more sharply. You have to be a little more careful turning into driveways and such.. especially when you have a passenger onboard who isn't expecting the car to follow the lay of the land so closely. ;D Handling-wise, the car is worlds better than it was stock.
 
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