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Swaybar settings, weird to me!

19K views 31 replies 16 participants last post by  Alex323Ci 
#1 ·
How is it that you get more understear if you set the front (aftermarket) swaybar to a stiffer setting?? To me, that means that if you set it to be stiffer and reduce body roll, you loose traction! :hmm:

Same thing with the rear... You get more overstear if you set the rear sway to a stiffer setting...

Why even change to stiffer sways then?

I know, this one might be an easy one but I'm ready to be taught!! :craig:
 
#2 ·
In simple terms you aren't trying to increase either over or understeer, you are trying to balance the car.

If you have too much understeer (most cars stock) then adding a stiffer rear bar (in relation to the front bar) you get a balanced car for the corners.

Make sense?

Tim
 
#3 ·
Yeah, in a way, but I still don't understand the underlaying theory that give such outcome..

To build on what you said: Too much understrear -> stiffer rear bar or softer front bar -> reduced understear. Why is that?
To me, that means that a stiffer sway bar reduces the traction and a softer bar increases the traction. But at the same time, we all do it because it gives the car better traction since you reduce the lateral movement of the car in corners...

Just seems backwards to me.. I would have thought you would get better grip if you stiffen the sway bar. Which would mean that a stiffer front bar would reduce understear, but according to all info I've found, it doesn't, the opposite is true.

:hmm: :hmm: :hmm:
Anyone?
 
#5 ·
Nice article, thanks for sharing!!

The covered my question perfectly by:
"In fact, given the above information, one might even assume that a firmer anti-sway bar, which leads to better camber control, would lead to better traction. So if we add a firmer anti-sway bar to the front, traction loss diminishes, so understeer is reduced, right? Wrong!"

This is what I get out of the article:
Two things act together with a swaybar:
- Stiffer bar = recuced body roll = increased amount of rubber to the ground = improved grip
and
- Stiffer bar = faster rate of lateral load gets transferred to the pair of wheels = additional demand on the tire = reduced grip

The very interesting point (and I must admit still a little confusing to me) is how they seems to counter each other. The first one is straight forward.. reduced rubber = less grip but the second one is a little bit more intangible. And together... a little weird still.. :confused:

But, I take it has there must be a balance between front and rear when picking the stiffness of sways and I now have the acronym "TLLTD" to throw out amongst friends! :)

Thanks again for taking the time sharing this article.
 
#6 ·
My pleasure - that's exactly the way I learned about many of things - one question leads to another and more terms to google.
The key to you tire grip question lies in the relationship of tire loading to slip angles. While "grip" increases as load increases, it does so at a slightly lesser rate. I'll try to dig up some good links for you, but in the meantime google "slip angle" "traction budget" "tire loading" and should find some gems.
 
#7 ·
It's all a balancing act. The stock suspension has a front bias = understeer. In other words as the car wants to roll the front is taking more load than the rear so that at the limit it's the front tires that give up first. Adding a stiffer rear bar allows the larger staggered rear tires on my sport suspension to take a larger share of the load. Get the balance just right and you have a neutral handling car. Even if not at the limit, the car will feel better during all phases of driving. So you can see that people who go out and by brand name sways with no regard to the relative sizes front to rear have no idea what they are doing. All they know is roll is reduced. Keep asking questions and know what sizes you need to get the results you want BEFORE you buy.
 
#9 · (Edited)
Another good way to look at it! Thanks!

I already bought and installed my UUC sways. I'm really happy with the performance, the car feels more solid and rigid!
I set them up as medium soft in the front and firm in the back.

Some users have issues with rubbing and hitting the ctrl arms or axels but I think if you fine tune it right (with loads on the wheels) you should eventually be ok. The instruction from UUC doesn't help much though so it's mostly up to you to tweak it. (Personally I can think of some minor design changes that would make them easier to install and perform) But, overall great performance once they're in correctly.
 
#8 ·
As a follow on to hummer's note, the balancing act is more than a single component, and it's not all equal all the time.

The balance of your car is affected not only by the sway bars, but springs, shocks and tires. Then, you have to consider what you are trying to do and where you are trying to do it.

First, your tires have to stay on the pavement, and to do that, you have to have some reasonable degree of softness to your over all suspension - our day to day driving is not on the super smooth surface of the new F1 tracks.

Then, you have to determine where you need to improve the handling - corner entry, corner exit, neutral throttle cornering, high speed or low speed corners???? My car will occasionally lift the inside front tire coming off / full power on of turns 5 and 7 at Road Atlanta with a little bit of understeer - which would lead you to think I need to either adjust the rebound of the front shocks and / or stiffen the rear bar. However, I absolutly do not want my car to over steer in either of those corners, and my car is extrememly neutral in turns 1, 4, the esses, and turn 12 which are the smoother fast parts of the track. So, over all I am very pleased with the balance of the car and have made no adjustments to compensate for the generally irrelevant understeer off of 5 & 7.

However, that is not to say that my car is set up firm enough for the track - I am getting way too much roll, but to take it to the next level will make it too uncomfortable on the street, and I'm not willing to spend the $$$ for double adjustable shocks so I can adjust the rebound independently of the damping.

All that said, for street use, most of you just want to limit body roll because it feels more responsive which it will be - however, there are very few occasions on the street (I hope) that you will get to the limits of adhesion and actually have to deal with the inherant understeer of your car, nor is it a good idea to purposely create a car that oversteers as a natural road going characteristic.
 
#10 · (Edited)
Further thought on Sways-Rob Levinson your input on my thoughts would be appreciated

I was thinking about this last night, and can't quite figure out what hummer is saying, although as I noted, I totally agree that it's about balance. I am certainly not a suspension expert, but this is my understanding - if I am out of line here, somebody please jump in and straighten me out (Rob Levinson, are you out there???!!!)

A stiffer front sway bar keeps weight from transferring to the outside front tire during cornering, and hense keeps the inside rear wheel with more weight (and traction) on it and therefore reduces oversteer. While a softer front sway bar allows more weight to transfer to the outside front and off the inside rear helping the car to rotate "loosing" it up as it may be. In general, my thought is that the front bar is the key to corner entry balance, as well as mid corner roll.

A stiffer rear bar reduces roll to the outside rear, keeping more weight and therefore traction on the inside front, hence reducing understeer. This is key for our cars on corner exit with power on (as I alluded to in my first post) as well as mid corner roll.

In all cases, the stiffer bars reduce the tendancy to roll, and keeps weight in the inside tires during cornering improving the feel and responsiveness, while it is the realitive difference between the bars that help you balance the car. The advantage of sway bars is that you can control weight transition without affecting the spring rates. This is really the main benefit of stiffer sway bars in street driving as you are seldom if ever (I hope) feeling the real effcts of understeer of oversteer.

If your car is totally loose or tight throughout the 3 sequenses of cornering, you probably have a weight distribution problem that needs to be addressed with your spring rates and probably your sway bars as well once you get your springs re-sorted.

You control the speed of weight transfer with your shocks. You control the rate of front dive (under braking) with rear rebound and rear squat rate (power on) with front rebound. While damping rate is to used more to control compression due to road surface irregularites (including how your car reacts to curbs on a road course.)

Front drive cars provide a totally differenct senario. With thier tendancy to understeer dramatically, you'll see them with very soft front sway bars allowing the car rotate around the outside front tire while they enter a corner, while as your power is on the front tires, you do not wany any rear weight transfer during power on corner exit, and hence you have a very stiff rear bar. That is why you see all these front wheel drive cars lifting thier inside rear tire when they autoX as well as in road racing to a somewhat less degree.
 
#11 ·
Wow. Titanic sig, there, kanaljen...

The thing to remember also about suspension design is that there is no answer that's right all the time. What works in steady state cornering may hurt the transitions, and vice versa. Also, it's never a linear process.

Increasing roll stiffness may improve traction, but that graph is not a straight line. Starting soft with low traction, increasing stiffness will improve traction up to a point, but after that point, traction will begin to fall off again as the suspension becomes too stiff.

The reason cars can be balanced with swaybar settings has to do with the concept of wedge. Wedge is a measure of the relative amounts of lateral weight transfer measured between the front and rear axles.

Positive wedge means that the inside rear tire is carrying a greater percentage of the total rear axle load compared to the percentage of front axle load carried by the inside front tire. Negative wedge means the opposite.

In other words, POSITIVE wedge means that the load is distributed MORE equally across the rear axle than it is in the front. NEGATIVE wedge means that the load is distributed LESS equally across the rear axle than it is in the front.

Increasing wedge tends to generate more understeer and decreasing wedge tends to generate more oversteer.

That's about as clearly as I can put it unfortunately.
 
#13 ·
The thing to remember also about suspension design is that there is no answer that's right all the time. What works in steady state cornering may hurt the transitions, and vice versa. Also, it's never a linear process.
I thought I will never see someone else using these words on this forum...

steady state
never a linear process

You an engineer, Duke W?

Yes, suspension tuning should be done in a highly dynamic simulated environment if possible. Often we "freeze" a frame and tune from there. IMO, the most challenging to tune for is the slip angle of the tires. There are so many other variables (weather, track surface conditions) that can affect the slip angle already.

There's another other layman way to look at over- and understeer correction with stiffening and softening of sway bars, even springs or shocks. Think of stiffening as putting various components tighter to the chassis. I use the word tighter a bit loosely here. Suppose you hold one end of a spring and your buddy holds the other end. If the spring is, say 1000lb/in, then I move you an inch to the left, chances are your buddy will move almost immediately in the same direction, because the spring is so stiff, it acts like a metal bar. Now if the spring is 0.001lb/in, and I move you in the same manner, the force will be transfer to stretching the low-rated spring first before actually pulling your buddy. So the question comes down to how soon do you want your buddy to follow you when you experience a force vector change. This two scenarios are pretty basic thus far.

Now, imagine if you are the chassis and your buddy is the tire/wheel combination. Again, a very simple model. If the chassis experiences a force in a certain direction, the spring rate (or sway bar setting or shock rate) will determine how soon to pull the tire/wheel along with it. So almost always whenever you stiffen up the rear springs/shocks/swaybars, you loose rear traction earlier and hence you will oversteer. Think of that your buddy not able to stick to the road any longer.

There is a balance between setting to not too stiff and not too soft. Reason being the car goes beyond just a simple mass-spring-dashpot model. It's a multi-dimensional system. Yes you want the car to go around the track as if it's a single mass unit. But you want to avoid the car being to twitchy to handle at every corner cos whenever your tail breaks loose, you will have to waste time correcting it. This is why some race cars are prepared to be a winner and others aren't. Of course, the skills of driver is just as important as well.
 
#12 · (Edited)
I'll give the explanation a shot, trusting others will correct my mistakes as well -
A stiffer swaybar actually increases the rate of load transfer by more or less directly connecting the wheels together. The reduction in roll in comes from having proportionally more of the load transfer throught the swaybar than through the springs (and body).
The increased rate of load transfer is what is responsible for the improvement in transient response. Since you can shift the loading faster, you can change between cornering states faster.
The understeer/oversteer phenomenon comes in to it as since load is transfered faster, the tires will saturate faster. The the balance of relative stiffness front and rear affects the proportion of load each end will take The faster/stiffer end will take proportionally more load, the tires will saturate and lose traction on that end first.
At corner entry, stiffer bars help the transient response, but there isn't enough load transfer yet for them to make much of an effect understeer/oversteer. The damper settings and spring rates will make more of a difference at this point. (ignoring all other non-spring/swaybar/damper influences) To address a corner entry issue, you typically want to make the adjustment on the front end.
At mid corner (or steady state skid pad) understeer/oversteer is mostly influenced by the relative roll resistance, which for a given set of springs will essentially be determined by the swaybar settings (again ignoring all other non-spring/swaybar/damper influences)
At corner exit, it is again all about transient response just like the entry, and typically you want to make the adjustments at the back end.
Here's a few more links -
http://www.whiteline.com.au/default.asp?page=/faqswaybars.htm
http://www.stockcarproducts.com/tech1j.htm
http://www.automotivearticles.com/123/Anti-sway_bars.shtml
 
#16 ·
The missing conceptual piece might be "tire load sensitivity -
Here's a quick overview -
We can see that as the number of pounds of weight the tire supports increases, the units of traction do not increase at the same rate. The dashed line would represent a linear equal increase in traction to the increase in weight supported by the tire. In reality, the solid line more closely represents the true picture. At 300 pounds of load, the units of traction are 2.4. If we double the load to 600 pounds, the units of traction only increase to 4.4, instead of double, which would be 4.8.
http://www.circletrack.com/techarticles/139_0310_traction_tire_setup_optimization/index.html


and a more detailed discussion is here -
 

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#17 ·
Here we go again! This ought to start a fire storm.
In a right hand corner the drivers side fender wants to go down. To limit this downward movement, some force must be applied up on the left side. As the right side wheel is unloaded and pushed down by the right spring. The swaybar left end is thus forced down. The reaction to which is an upward force on the left side.
 
#19 · (Edited)
#20 ·
jpr - thanks, but I guess I'll have to wait until I'm the last one in the office on night so I can print out 112 pages :eek: :yikes: !!!!!!!!!!!!!!!!!!!!!!!!

Good discussion!
 
#23 ·
I read like half of the first page so far cause theres a **** load of reading. Anyway when i take medium fast turns my car reachs its max then the back kicks out and then its like the whole car starts sliding to the outside. I wanna say it understeers but its not like the understeer i know. Same thing though.

And thats with DSC completely off. That might have been because of my shitty continetals. Ive got dunlop direzza going on tomorrow. so ill let you guys no whats up then. But i think i need a rear bar on the stiffer side and a medium front

Is that correct?



Whatd you get for the ticket in your sig?
 
#27 · (Edited)
I finally registered here so I can actually use some of the info I have recently learned. I studied vehicle dynamics in school. Duke W and MrPaddleShift have made great points so far. Something no one has seemed to mention is roll steer and chamber change as a result of roll.

First, roll steer. Simply put, when your car leans into a turn (rolls) the suspension geometry changes. Sometimes the kinematics work out so that the inside or outside tire or both may turn a bit adding or subtracting steering angle from the wheels. Most setups try to minimize the roll steer, but if you have any rolls steer I think you would want positive. That is having the wheels add steering angle into the turn under roll. So, if your suspension exhibits roll steer, then a softer anti-sway bar will allow more roll and thus more steer, which makes the car feel less understeer at the steering wheel (notice I say at the wheel, the tire will still be understeering, but is automatically adding steering angle that you can't feel). For a stiffer roll bar the roll steer could be lessened thus seemingly added understeer (at the steering wheel).

Second, camber angle is important. Radial tires are affected less by camber angle than biasply so this effect may be negligible. In a turn the tires also tend to lean in or out, adding or subracting camber (more body roll will create a larger effect, not a linear relationship on most any suspension). In a turn the inside tire will lean in and the outer tire will lean out. This lean creates lateral force in addition to the slip angle of the tire. If you turn your wheels to the limit while stationary and get out, you will see that the tires are leaning. BMW has designed the suspension geometry so that under high lateral acceleration, the tire will still lay flat on the road. The body roll tilts the tire a few degrees the opposite way and it is standing straight up again. By increasing the siffness of the front roll bar you decrease the effect of BMWs steering geometry and remove the lateral force added by the camber change. This will induce understeer.

These are not the largest factors, but just two I saw weren't mentioned. Suspensions are so complex that to tell anything you really have to just test in the real world and at minimum have a full kinematic model of the system. Some of this may be unclear. If it is just ask and I'll try to elaborate.

For those interested, I use Gillespie "Fundamentals of Vehicle Dynamics" for my everyday references. You can find a copy online somewhere for free if you really want one, but I think linking to it would be a violation of rules here. And yes I know this thread is old as balls, but thought I'd add to the forums already huge knowledge base.
 
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