How to design a sway bar w/ 3link + watts linkage

[The WidowMaker]

why a sway bar is needed on a 3 Link design?

they arent always needed. but, you may need a sway bar to balance out the front. by adding a rear bar, you are taking some load off the outside front tire in a turn. too tight and you add a rear bar. too loose and you subtract from the rear or add to the front. you can also play around with the rrch, but a bar is good as a tuning aid. thats a very basic explanation, so hopefully the suspension gurus chime in.

a starting point is tough. you might be in a league of your own as far as a setup, but thats the nice part about the splined bars.

[JRouche]

Originally Posted by BANKO

Hey guys, thanks for the responses. I was also looking forward to more of the fundamental suspension design theory on why a sway bar is needed on a 3 Link design? It seems to me that the purpose of a 3 Link + Watts provides the most articulation while keeping both tires in contact with ground. By adding a sway bar, the torsional stiffness of the suspension system is changed, giving it adaptability, but at the expense of articulation. How do you know a baseline for how much torsional rigidity is needed? Got any good books that address these specifics?

Thanks, Josh

As far as fundamental suspension design and operation go I dont have a clue. But that wont stop me from adding my opinion.

I look at the sway bar as the LAST suspension component to be added, after you already have all the other variables and components figured out. And this is from my standpoint. A guy in his home garage working on his car. The aftermarket folks that supply all in one suspensions have it all worked out. But, a lone dude in his garage has to figure it out if he isnt going with a package from a dealer.

Cars dont need sway bars. A properly setup suspension will control the tire just fine, to a point. And the suspension is all about controlling the tire to the ground. Yes, body control is in place, but the objective is still to control the tire to ground. Thats the ONLY objective.

So once you have a suspension that can control the tire there are tuning aids to work with controlling the tire more, making for a more controlled feel or just a more comfortable car or a better preforming suspension for speed.

Different tuning aids provide for various effects.

One of them:
The sway bar. It helps with a few issues VS not having one.

It can help to reduce body roll so you can use a more complient (softer) spring and a softer spring helps with ride feel and control. Now that you get to use a softer spring the shock valving can be modified, and not so much reduced but set to levels where they (rebound and compression) become more active during really hard road cycles.

So you can have softer springs, widen the range of the shock valving for straight sections of road to keep the tire planted then when the excess weight on a turn tries to load up the outside tire it is helped (more spring rate from the bar) so higher wheel springs are not needed in the turn.

Think of them as variable rate helper springs that only come into place when you are turning. You dont want to compromise the straight sections of road travel with stiff wheel springs just so you can make a fast turn, you sacrifice straight road handling. As fun as turns are yer gonna need some control in the straights also.

Now. Articulation losses. I dont look at as a loss, I look at as a determined amount of articulation control. If you do some checking, without the bar in place. Measure the articulation of your suspension and get what degree of articulation you want then you can work up from there to get a bar rate.

Thats what I did. And Im kinda leading into the question about the crap shoot of how you determine the correct bar rate, and it is kinda a crap shoot, thats why so many folks opt for the package suspensions, because ALL the guess work has been taken care of.

But like in my case there is no plug in bar, my suspension is pretty much a one off using aftermarket parts.

I searched high and low for bar rates. They are NOT there. The import car crowd uses bar size as does the hotrod and American car tuners.

Bar size doesnt do squat for me when I was looking for a sway bar. I needed some spring rates. And yes, I was to the point of just getting some similar cars that were close to mine and using those bars.

But nope, I wanted to be able to use bar rates so I could use the bars for what they are, tuning pieces. Just like wheel springs, hell, everyone gives those rates, you have to. Otherwise whats the point. Stuff some springs under the car that suspend it and be done. NO!!! Thats what some folks do. But not what we do here I dont think. Its about tuning. So you need some numbers to beable to tune it up or down.

Well, race care torsion sway bars DO provide bar rates. So I went that route. Still a crap shoot. I had NO idea what rate to start out with. I didnt know what some of the aftermarket or even stock rates were.

But I did know a couple things. I knew the approximate weight of my car. I was lost for wheel spring weights cause I have air springs. I knew what my articulation, min and max of the suspension was, biggy because the rate of the bar is in terms of degrees of rotation. And damm it, there was something else I knew but it eludes me right now LOL It was just here on the tip of my tongue and I lost it.

Anyway. So I picked a specific bar rate for what I knew (it was still a crap shoot). I still had some adjustablity for the rate with the arms I used. They are blank arms and you drill the holes for arm length. I only drilled one hole for a certain bar rate (yeah, optimistic) but there is room for more adjustments and the links will handle about 1-1/2" up or down from my original arms length hole.

Anyway, a summary.....

Articulation:
Dont think of it as a loss. Think of it as controlling the tire in the turn with the ability to use more complient springs for the straights.

Determining bar rate:
Its kinda a crap shoot. Know what wheel spring rates you have, the weight of the car, the needs of the bar and talk with people. OK now I know what the other thing I knew. Rear roll height. Thats a biggy. I have a watts link. My RRC (rear roll center) height is low, so that makes for a "loose" body control during turns. The body has more affect for roll in the turn. So I factored that in and increased the bar rate.

And last, Books!!!

My Bibles for suspension are three.

"Tune to Win" by Carroll Smith

"Chassis Engineering" by Herb Adams

"How to make your car handle" by Fred Phun

JR

[Bryce]

This is a good place to throw out an Idea I had.

If you design/tune a shock for a specific car and a specific spring rate then you add a sway bar, you are changing the spring rate when the sway bar is loaded. This extra load is adding spring to that side, so one should change the shock setting in this situation, that is not possible unless you have a computer controlled shock.


So in a turn where the sway bar is loading the suspension and you hit a bump you have a over-sprung and underdampened setup. I am just thinking outloud.

[JRouche]

Originally Posted by Bryce

This is a good place to throw out an Idea I had.

If you design/tune a shock for a specific car and a specific spring rate then you add a sway bar, you are changing the spring rate when the sway bar is loaded. This extra load is adding spring to that side, so one should change the shock setting in this situation, that is not possible unless you have a computer controlled shock.


So in a turn where the sway bar is loading the suspension and you hit a bump you have a over-sprung and underdampened setup. I am just thinking outloud.

Thats a pretty good thought. Lets think about it....

Springs are not linear or are they, I think of them as being not but I dont know.

Shocks are kinda linear, but not. Maybe more so than a spring, but different speeds of the piston will create a non-linear force due to the fixed orifice and changing piston speeds..

So if you have two components that are non-linear with the force applied they will both add some strange unpredicted handling characteristics during some hard driving.. But still predictable with some use, getting acquainted with the car. All cars have some quirks.

And really, I dont think you can over come that. But thats when the drivers skill comes into play. The driver can react to non-linear suspensions to overcome the imperfection that a suspension has.

We as car modifiers and car builders (the pros) can only take the setup to a certain point. As close to a stable ride as possible.

Then its time for the driver to accommodate and work within the bounds of the car. And some do it better. The guys that know their car and how it handles.

I personally dont think there is a perfect suspension for a car, even the really expensive F1 cars. Thats where the driver comes into play, knowing the car and how it will react.

But off in the not so distant future I see completely active suspensions. Shocks, springs, sway bars, EVERYTHING. All computer controlled.

Active suspensions are around, and they are expensive. But being used. Read magnetorheological damper. They are on many cars and so far so good. Expect to see more of that. Ferrofluid dampers have been around and will be used more.

JR

[funcars]

There are books that have formulas for calculating sway bar rates based on diameter and shape. Then you can calculate wheel rate based on the combination of the springs, locations, angles and sway bar rates. You can start with wheel rates that have an equal percentage of rate front to rear of what you had before you started changing things and go from there. Some of the Steve Smith books have lots of explanation and formulas to help. They are older books but so are most of the cars we are messing with.

An interesting approach to get a good starting point for your combined wheel rate (vs throwing a dart) is to look at the moment-moment method as explained by the articles at circle track. It is based on using combined wheel rates and calculating the overturning moments at front and rear and attempting to get them approximately equal to get a neutral handling starting point. It is a slightly different approach than looking at roll couple lines and easier to calculate - take a look if you are interested.

[JRouche]

Originally Posted by funcars

There are books that have formulas for calculating sway bar rates based on diameter and shape. Then you can calculate wheel rate based on the combination of the springs, locations, angles and sway bar rates. You can start with wheel rates that have an equal percentage of rate front to rear of what you had before you started changing things and go from there. Some of the Steve Smith books have lots of explanation and formulas to help. They are older books but so are most of the cars we are messing with.

An interesting approach to get a good starting point for your combined wheel rate (vs throwing a dart) is to look at the moment-moment method as explained by the articles at circle track. It is based on using combined wheel rates and calculating the overturning moments at front and rear and attempting to get them approximately equal to get a neutral handling starting point. It is a slightly different approach than looking at roll couple lines and easier to calculate - take a look if you are interested.

You are correct. But... When you are talking about formulas, calculations, wheel rates, locations, angles, diameter and SHAPE, moment-moment, overturning moments, roll couple, and the other settings they all translate into numbers.

Numbers some guys might not be able to get, or do and just really dont need that type of detail. There are some numbers through measurements that some guys just cant get, meaning they would have to do some major fact checking to get the raw numbers. More work than its worth.

I have read all the books. And feel like I have a grasp on the relationship between the numbers (measurements). But to understand it doesnt translate into how to pull the numbers out of the geometry of the suspension.

And some folks will get a kinda glazed over look in their eyes when you start talking about ALL the variables.

What I have found is the dart thrown into the board will get you very close if its just a nice street car or even a track car. And unless you are putting in a completely custom bar the extra number gathering is extra.

Weight of the car with a rough estimate and mimicking other cars that are similar.

I dont think there are many engineers looking at this thread. More likely a guy like myself that just wants to get a bar rate as close to usable as possible.

Many car guys that arent hard core racers are on this site. They just want a bar that is well within the range for their car.

Yes, there are some that want to dial it in as close as possible. But Im thinking thats 1%. The other 99% just want to get it pretty darn close..

But great info!!! JR

[funcars]

Yep - I think you are right. These threads seem to be more beneficial to folks who use primarily bolt-on parts that are already designed and analyzed. I'm an engineer myself and designed and fabricated a complete new front end with cup car parts so I had to know enough to get close with all of the changed geometry. That said, the math for sway bars is not that complex and there is a large benefit from having a good feel for how the numbers relate to the change in feel of the car. Almost any car will improve from some chassis tuning to match the driver technique.

Another very helpful tool is to get pictures of your car at the track so you can see obvious issues like an inside rear wheel lifting off, excess body roll, etc.

[JRouche]

Originally Posted by funcars

Another very helpful tool is to get pictures of your car at the track so you can see obvious issues like an inside rear wheel lifting off, excess body roll, etc.

Yes, I agree, pics or a vid. Inside tire lift would be a nice thing to see. And if all the other tires are working hard you may not know what the looseness in the rear is till someone showed you a pic of yer car with it lifting its back leg. Now thats an animal of a car, trying to take a leak while on the run LOL Ever see a dog do that, I have, kinda funny. But really kinda sad. Usually with runners with their dogs. Like dude?? Give yer dog a chance to take a leak or pinch a loaf. Good point.

I dont ever plan to take my car to the track, would love to but money would be the deal breaker. I cant afford it.

But if I did I would love to have a video of the car going around. Vids of any sport, including car racing can show the driver where he might be upsetting the car. See a lil uneasiness in the late braking, maybe a lil smoke from the tires (not good) or a lil wiggle. Or a lil hard on the go pedal on the exit, rear moves out too much, some more smoke from the tires. Smoke from the tires is loss of speed. It wont stop faster with them dragging and it wont accelerate faster with them spinning. Traction is speed. A video might be the only way the driver sees that.

Turning, braking and accelerations should be completely fluid motions. Not jerky and twitchy. Aggressive YES, jerky and snap type driving feels like fast driving on the seat of yer pants. But its slower than controlled driving.

And Im not trying to make any brownie points with the ladies but maybe thats why some of them that want to be involved in the sport are really fast drivers. More control and less aggressiveness. They are great drivers.

A video of yer driving might show some flaws in the driving habits also, besides the unapparent car actions. I like the idea. I SOOO wish I could afford to take a few laps on a track, I think I would be hooked. Thats the cost I cant afford, I would need to go as often as possible. And since time isnt an issue it becomes money. Errrr JR

[funcars]

At least in CA isn't not that bad to get some laps in at a track for a day. A few hundred bucks is worth it to be able to actually use some more of the capability of the car than you ever can on the street and learn a ton about driving too.

You could always autocross your car. Less expense and risk and teaches you tons too.

[JRouche]

Originally Posted by funcars

At least in CA isn't not that bad to get some laps in at a track for a day. A few hundred bucks is worth it to be able to actually use some more of the capability of the car than you ever can on the street and learn a ton about driving too.

You could always autocross your car. Less expense and risk and teaches you tons too.

Yeah, but even a few hundred bucks is more than I have. Its just NOT in the budget. Keeping the house and paying the taxes and ALL of the other expenses is on the top burner right now. Taking my car to the track for fun is a few hundred bucks out of line for me right now, way back burner, on a super slow simmer.

As it is prolly the same for alot of folks these days.

I need to make the extra bucks that are available to treat the entire fam, not just myself. They could care less about me tracking the car. But they would love to take a weekend to San Diego and visit Sea World, still just a few hundred bucks.

Thats the cost of having a family with a limited budget. You need to consider the other peeps.

When you dont you are headed to a single life again and scraping and scrounging cause all the extra money is going to child support and God forbid alimony.

Nope, I have my ducks in a row. I get to play with the car as much as I can. And Im happy. But the family comes FIRST!! Has to, otherwise there is no family and just you. JR