Sway bars and rates, Some times I over think things a little Options

[914forme]

Okay is just me or do I over think stuff! (IMG:style_emoticons/default/idea.gif)

Anyrate, the Welt 22mm is going going, gone - It is in the FS adds.

Colaman Racing to the rescue. But I needed to know some infor before I started. So thanks to Fred Puhn, I made up an excell spreadsheet to figure out my adjustable rate swaybar and what I had.

Here is the Weltmeister 22mm bar - the conversion was done over to inches. Notice the top number, I was running my bar full hard, about 3 inches from the center of the bar. That produces and effective rate of 150 Ft. Lbs of pressure. With out going into other factors just the most basic physics here.

[914forme]

Okay here is the colman bar I ordered, it is 1 1/4" solid bar. With 14 inch long arms, I am going to cut them down! Way it looks from the current setup, I would run at about 4" out on the arm. And it only goes up from there.

[nine14cats]

Pretty nice....let me get my wife and show her that other folks use spreadsheets too! Oh honey.........come here and look at this! (IMG:style_emoticons/default/laugh.gif)

1.25 inch is ~31mm bar....Trekkor should like this thread. I had a 27mm Smart bar on the 914-6 and on the 911.

Pretty cool!

Bill P.

[914forme]

(IMG:style_emoticons/default/chairfall.gif)

Bill this is for you all things being equal with the smart.

27mm Smart Racing Front Swaybar

[nine14cats]

(IMG:style_emoticons/default/chairfall.gif)

Bill this is for you all things being equal with the smart.

27mm Smart Racing Front Swaybar

Thanks! (IMG:style_emoticons/default/clap56.gif)

You do realize I will use you as an accomplice whenever my wife catches me crunching race car numbers.... (IMG:style_emoticons/default/type.gif)

(IMG:style_emoticons/default/beer.gif)

Bill P.

[J P Stein]

OK...so allow me to overthink this thing.
With say 250 lb springs up front, setting one of these bars to equal 250 lbs of twist would cancel out 1 inch of roll (suspension movement....static) from what ever movement you started out with. Loaded, to around 1 g, it would be twice that....rough numbers....very rough. So, do you know how much weight is on each of your front wheels, what your front spring weights are, how much suspension travel is available?
If you do, those AR bar numbers become useful.....but only to keep the front restricted in movement away from parallel to the ground. This also helps keep the car from running out of suspension travel, but that is the main job of your springs.

So, what do you set an AR bar for.....to keep it flat? Not me, I set it to increase/decrease over/understeer. Sure, it's nice to keep the tire as verticle as possible, but not at the expense of a bad push, for instance. If it pushes when you have the AR bar set, you have 2 options......decrease the front roll stiffness or increase the rear spring rate. If the front AR bar is set at full soft already, your options are very limited.

Things like front (not a problem to me) or rear (a stinker)tire lift enter into your AR & springs rates also. Then there's available grip. A car on street tires will behave one way. Increase your grip by 20-30% with Rspec/slick tires, your roll rate rises and you're readjusting both AR & spring rates (this assumes you have the camber in the tire that it likes)....*to keep the car balanced to whatever handling attributes you are most comfortable with*.

Anybody that has been down this road knows that the above is just a chunk of what it takes to get a car working to it's potential.....something I'm still working on. (IMG:style_emoticons/default/biggrin.gif)

[Borderline]

Your 1.25 dia bar is about 8 times as stiff as my 19mm (.75") bar!! How do you balance out the roll stiffness at the rear to keep the car from plowing out in the corners? Are you going to super stiff springs or are you adding a stiffer bar? Thanks for the spreadsheet. I haven't gotten around to doing my own. I never ran the numbers to see how much difference the lever arm really makes. I am curious as to the fromula you are using. If you have the bar diameter and a lever arm radius, shouldn't the resulting number be in pounds and not in-lbs?? (IMG:style_emoticons/default/confused24.gif)

[914forme]

Okay here goes the answers.

Bill, use me if you want I am know to be a bad influence on others.

J.P. - I was using the calculations to see where to cut the arms off at, they are heavy buggers. So I figure I needed a little less bar for future use, and a lot more bar for current use. I adjust my bar for balance and to correct certain tendencies my current setup has to do.

Borderline - Now I am no excel wizard which you are about to see, but the formula works. BTW, the last column calculates the inch pounds to Foot Pounds.

Here is the formula to calculate the bar Rates!
=SUM(500000*(D19*D19*D19*D19))/((0.4244*(A19*A19)*B19)+(0.2264*(C19*C19*C19)))

This is the conversion to Foot Pounds
=SUM(E19/12)

That one is pretty simple.

In the spreadsheet A is the parallel distance fro the bar to the pickup point. B is the length of the bar, C is the Arm length, D is bar diameter. It only works for bars of a constant diameter. So necked down bars it will be slightly off.

If anybody out there knows how to add the power of to an Excel equation it would clean it up, but the math is correct, logicly.

Now what kind of rear rates do I run. 175lbs springs, and a stock rear bar. Have not done the calculation on the rear bar yet. The rear bar is currently non adjustable. So I have to use only the front. I maxed out the Welt trying to find a good balance, I need a little bit more, so I am moving up large.!!!!

[groot]

Good discussion. Let me fuel the fire a bit.....

You should use your unsprung weights, corner weights, spring rates and motion ratios to figure out what roll rate (degrees/g sometimes called roll gradient) you have with just your springs, then add in the bar to see the change. One rule of thumb is the bar should account for 20-50% of your roll stiffness... I like it on the low end. Adjusting this % allows you to tune the amount of understeer in the car.

Then once you've figured that out you can take it a step further to figure out the amount of roll resistance at each end of the vehicle... this is called roll couple. The % of roll resistance on the front axle should be in the 60% range or so, tune to suit.

These #s are just suggestions that I've found some luck with and can be debated forever...

And of course, none of this matters if you run out of suspension travel too early like most 914s do in the front (because the spring rate goes non-linear).

[grantsfo]

It would seem to me that not all sway bar linkages are the same and would have differing effective rates depnding on how they connect to a given swaybar and suspension. Also how do you account for differing bar shapes and metal characteristics? I have never thought about this, but are all sway bars made from the same type of steel?

Could it be possible that one company's 27 mm could be stiffer than anothers 31 mm just based on steel used and linkage configuration?

It is interesting to see the assummed progression but I'm not sure how to use this data in determining setup?

Kevin brings up points I was thinking of too about preload and non linear aspects of the 914 suspension.

[914forme]

Yes it is possible that due to different metallurgy you can have a different rate off the bar. Some adjustable swaybars use this theory to make it adjustable, by changing the bar arm angle so as it rolls over to the flat side it becomes more springy, thus reducing the arms ability to transfer the swaybars input. None engineers terms here (IMG:style_emoticons/default/biggrin.gif)

So yes in theory the data can be off from this very simple formula that figures all things being equal.

Somebody's 31mm might be weaker also if it is a tubular design also. For a tubular design the Wall thickness ( inner Diameter) determines the sway bar rate. A solid bar has no wall thickness in general theory. In reality they do have a wall, but it is very thin.

So the same bar, 1.25 bar with a wall thickness of .25 would rate out as this. So the inner diameter is 1.00 In this simple example.

I had to modify the formula to work these through.

=SUM(500000*((D47*D47*D47*D47)-(G47*G47*G47*G47)))/((0.4244*(A47*A47)*B47)+(0.2264*(C47*C47*C47)))

[groot]

Actually, a hollow bar's rate is determined my the outer diameter and wall thickness, not just the wall thickness. The inner portion of a solid bar really does very little to increase is stiffness. There are a bunch of simple equations to account for different shapes. Most bars are made of spring steel and should be heat treated to attempt to keep a constant rate.

The linkage of the bar is accounted for in the stablizer bar motion ratio... which is also a very important consideration. Good point, Grant.

[914forme]

Yeah that is correct, I misstated my terms above. I only play an engineer on tv. But the formula for the hollow bars does take into account bar diameter above, and works accordingly.

[J P Stein]

Now what kind of rear rates do I run. 175lbs springs, and a stock rear bar. Have not done the calculation on the rear bar yet. The rear bar is currently non adjustable. So I have to use only the front. I maxed out the Welt trying to find a good balance, I need a little bit more, so I am moving up large.!!!!

I use a 25 mm Tarret (effective 22mm) front AR bar. It provides plenty of roll stiffness.....but I don't use a rear AR bar. Neither the front nor rear inside tires lift off the ground during cornering. I'm presently working on an oversteer condition that (I think) has been made worse by heat cycled-out rear tires. I'm confident that there are enough adjustments to get back to a comfortable amount of oversteer.

[Borderline]

I ran my first AX in the teener last month and the car was suffering from a terminal case of understeer. The car was set up with front 22mm Tbars and a 19mm AR bar of my own design. The rear had only 160# springs. For this weekend I have added a stock rear AR bar and a pair of 175# springs (thanks again, Grant). From what you are saying I'm going to be pretty tailhappy this weekend.

Kevin: your rule of 20-50% AR bar roll stiffness goes against what I have read on other threads here in the past. Others have been proposing keeping the stock Tbars and controlling all the roll with AR bars. I'm not trying to be confrontational. I love this kind of talk! I first AX'd over 30 years ago in college and the theory then was that stock suspensions were so bad that the only way to fix them was to go to super stiff springs & AR bars to limit suspension travel. I've been away from cars for a while and now I'm interested to see if anything has changed over the years.

I sure have been having a lot of fun. the 914 is a great little car! (IMG:style_emoticons/default/mueba.gif)

[914forme]

Boarderline, maybe maybe not, I don't know your combo. The 22mm TBs are large for my conditions. but that combo might be just waht you need.

BTW, did the calculations on a rear bar I have back in storage. The 11 inch arm length means these things should not have the effect you all say they do! I believe that my car rolls so much that I run into a suspension travel issue in the rear. And the Sway bar just adds to minimize the travel.

Why you say, Because the bar rates are so low, they should not matter in this senario.

First one is a 15mm rear bar
Second is a 16mm rear bar

[J P Stein]

Kevin: your rule of 20-50% AR bar roll stiffness goes against what I have read on other threads here in the past. Others have been proposing keeping the stock Tbars and controlling all the roll with AR bars. I'm not trying to be confrontational. I love this kind of talk! I first AX'd over 30 years ago in college and the theory then was that stock suspensions were so bad that the only way to fix them was to go to super stiff springs & AR bars to limit suspension travel. I've been away from cars for a while and now I'm interested to see if anything has changed over the years.

I sure have been having a lot of fun. the 914 is a great little car! (IMG:style_emoticons/default/mueba.gif)

The soft spring/ stiff bar thing goes way back. The proponent (first I read) at the time was Dan Gurney......on infinently adjustable race cars. A truck fulla springs,shox,bar sizes, vairable limited slips, tiar compounds & sizes will help out (IMG:style_emoticons/default/biggrin.gif)
As time progressed, he added wings & things to his bag of tricks. Now Mr. Gurney is atop my short list of American racin heroes, but...these are pure bred race cars on a race track.
Just a smigen below him on this list comes Mark Donahue. Read his book
"The Unfair Advantage" and you will find a set-up guy that will do whatever it takes to make any car....Camero to F1 cars..handle. He gives a wonderful progression in his learning curve over 10-15 years in all kinds of cars.....again, race track cars.

When you get down to 914s, there are some limitations.....starting with a chassis that flexes in both pitch & roll...(prolly yaw also (IMG:style_emoticons/default/biggrin.gif) ) which varies by car in use & abuse. Work at one end does not necessarily transfer to the other like one might think due to this..."feature". It also has a nasty habit of picking up the rear inside tire at some point...the more speed/grip, the earlier it comes.....rear AR bar or no. This is cureable *by working on the end that is fucking up*....be it stiffer springs or an LSD, for instance....or both. (An LSD will not cure a rear wheel lift, but at least the car keeps driving forward....till traction is lost on that lone wheel, then you spin....or take a run at it)
Then you need to work on the front to cure your push or loose condition should this arise. Rear wheel lift is a killer.

My focus is AX...not track racin. The 2 do not correlate set-up wise in my experience. An AX car has to rotate around a tight corner, but not get too loose on a sweeper.....slaloms are sweepers hooked together that keep changing direction.
Understeer in a track car is safe/wise. It is a looser in AX. I do what ever it takes to get a car set up to my liking....with varing degrees of success. (IMG:style_emoticons/default/huh.gif)

Brad pushes the soft spring/stiff bar deal. If you think Brad is the greatest AXer to come down the pike, go with his thinking.

[AndrewBlyholder]

To clean up the Excel formulas, power calculations are done with ^.

3^2 is 3 squared, 3^.5 is the square root of 3.

Good discussion. One of my long term goals has always been to measure all the geometry of the front and rear suspensions so I could figure out the actual wheel rates produced by the springs and anti-roll bars.

I don't know if anyone out that reads Circle Track magazine, but they've run a very good series of articles on suspension analysis based on the moment center concept. The moment center is the geometric point that the car suspension rolls around. It is different for each end of the car, of course, since the suspensions are different. The distance between the moment center and the center of gravity for that end of the car is the moment arm that the car's weight is using to push on the suspension. The complexity here is that the moment center often varies with suspension movement, which means the moment arm length changes, which means your handling can change dramatically with suspension movement. Understanding that situation is the recent advancement that moment center analysis has brought to many forms of racing in the last few years. There are several touring lecturers in vehicle dynamics for racing cars. This is the analysis method I believe they are using in those seminars.

What all this means is that the car has two types of roll stiffness: a constant stiffness created by the springs and anti-roll bars, and a suspension geometry created stiffness based on the moment center moment arm length, which can vary. You can't really predict what a car will do unless you know both parts.

That's a summary of my limited understanding. Some day I hope to get around to figuring out the 914 suspension geometry so that I can try putting it to use. I throw it out here just to pique other's curiousity. Maybe someone else will pick up the ball and run with I further than I've been able to.

Andrew

[914forme]

Thanks Andrew, I now see that ^ (shift 6) Doah!

What intrigues me the most is how this post progressed from my simple calculation to see where the I should cut a sway bar arm, became a huge discussion of suspension and frame dynamics, theories, and general idiosyncrasies of the 914. And how I plan things way to much.

Take care, Stephen

[914forme]

Here is the formula to calculate the bar Rates!
=SUM(500000*(D19*D19*D19*D19))/((0.4244*(A19*A19)*B19)+(0.2264*(C19*C19*C19)))

This is the cleaned up version (IMG:style_emoticons/default/biggrin.gif)

=SUM(500000*(D19^4))/((0.4244*(A19^2)*B19)+(0.2264*(C19^3)))