Weight Transfer Equations - Anti Roll Bars

[rcr286]

I have been working on a simple tool to calculate baseline suspension setups and I would like information on anti roll bar rate calculations. Each time I search, I find a new equation!

Does anyone here have real life experience with anti roll bar production or design? What is the "industry" standard? (I am looking for both solid and hollow bars)

Thanks in advance.

[JohnD]

Ric(?),
Have you read Alan Staniforth's "Sourcebook" or "Competition Car Suspension" book?
He makes the point that each change in design produces a new answer, but that this iterative method will not focus down on the 'right' answer.
While it's good to understand what your suspension is doing, in the end you have to make a subjective choice.
John

[TEAM78]

how are you currently calculating it?

[ubrben]

Originally Posted by JohnD

Ric(?),
Have you read Alan Staniforth's "Sourcebook" or "Competition Car Suspension" book?
He makes the point that each change in design produces a new answer, but that this iterative method will not focus down on the 'right' answer.
While it's good to understand what your suspension is doing, in the end you have to make a subjective choice.
John

There isn't a "right answer" but to say the choice is subjective is not really correct.

What you're doing is making an educated choice.

For weight transfer I'd use Milliken or the OptimumG spreadsheet that Lukin posted on this forum a while back.

Ben

[rcr286]

Thanks Ben...you just took the fun out of my project!

This .xls is VERY similar to what I am building for myself. Actually, this saves me a lot of time. It is not like I was 'bored' and decided to build this to pass the time. As you said, this enables us to make an educated choice. Nothing is really absolute until you test it on the track, and that is only absolute until the next development or theory comes along. So for the 'moment', I will 'roll' with this.

-Roger

[phoenix]

I too have come across more than one formula, but I have found that they all work out to within a couple of percent of each other. As Pilbeam make successful cars, I prefer to use Mike Pilbeam's formula. I also aim to have the lever arms at 90 degrees to the bar axes, that eliminating the variable that angled lever arms have on stiffness.

I can let you have a little spreadsheet that calculates roll stiffness for solid or tubular bars of any dimension and will give figures for lbs/degree and lbs/inch deflection.

As you probably know, you will need to have dimensions for the proposed bar including width, diameter, bore (if a tube) and the length of the 'lever arms'.

If you know the roll stiffness offered by the springs, you can then estimate sensible targets for the stiffness required for the bars 1) to give the total roll resistance you require and 2) to give the weight tranfer you require.

As steel tends to come in stock sizes, it is then a question of playing with diamters, bores, lengths and lever arms lengths to match your requirements, not forgetting where the bar connects to the suspension as this is likely to introduce a motion ratio with respect to wheel travel:bar travel. This motion ratio can be helpful in the design for getting the desired finished roll resistance, however.

[breezeblock]

how much stiffer is a tube over a solid bar is it worth changing to tube for the weight loss

[GordonG]

Torsional stiffness of a bar is proportional to D**4 (Diameter to the power of 4). Torsional stiffness of a tube is proportional to OD**4-ID**4. Play with some numbers in those calculations and you find that you need a tube only very slightly larger in OD than a bar to get the same stiffness.

G

[TEAM78]

Originally Posted by breezeblock

how much stiffer is a tube over a solid bar is it worth changing to tube for the weight loss

Tube isnt stiffer than solid??? the tubular bar is making more efficient use of the material used in the ARB,
Chasing the ideal tube sizes can get very expensive and time consuming, well it was for me anyway, I saved over 1.5kg out my bar by going tube on a saloon car. it depends how serious your formula is, If its F1 then yes, if its a track day car, er no, put it this way I think you will find it very hard to feel the difference.
Also note with tubular bars the production accuracy means the Internal diameter (ID) can vary so beware!
Best to experimentally test the tubular ARB once you have made it to verify the torsional stiffness, do it for solid bars as well if you fancy it

For weight transfer Calcs, Ive used various ones, from Milliken, Optimum G etc. . they all give an answer to within 0.25kg! which is pretty dam good so just use one you feel good with,

[rcr286]

Thank you very much guys, you have been very helpful.

Phoenix, I would like to see the sheet that you use. I believe that my email is available for you to send personal mail. I will take anything I can get. Most of the formulas really are very similar, but the constant (usually Modulus of Elasticity) can vary greatly. With this being said though, I have not seen a difference of only a couple of percent between equations. I guess that is why I am investigating a more definative answer.

I discovered another sheet a while ago while playing Live for Speed. Whenever I play an new video game that allows you to set up cars, I immediately enter a setup that I know works on the car and try it out. Then I enter a completely goofy setup and see if changes are predicatable. This is one of the only games that actually has a grasp on these dynamics. Unfortunately, this and Grand Prix Legends both suffer from crappy tire physics. Anyways, this sheet (download from http://forum.rscnet.org/showthread.php?t=157180 ) has some pretty cool macros and makes it very easy to use and read. It is very similar to the Magic Number, but more graphical...for those visual learners like me. It is very basic, but it is a great foundation for a potentially powerful tool.

Thanks again,

Roger

[idol]

Have a look at this program http://www.bevenyoung.com.au/suswin.htm (free demo version) the help file has a section on Technical and implementation notes with a page on arb calculations. The "constant" E depends on the material used and its treatment.

[Goran Malmberg]

If the car has the same TW as WB we shuld not need any rollbars. The car shuld roll the same deflection as it pitch at the same G-force. So, when the car got a narrower TW than WB, the deflection is going to become greater sideways than in pitch. To get back to the same as the "square" car we must add rollbars of the same dignity as the TW is less than the WB. This is what is the basic balance between the springs and rollbars.

Another thing is to find the motion ratio to calculate the wheelrate from the springrate. AND, the rollbar wheelrate from the rollbar rate. This is no easy task. Moost literature Talk only about the leverage of the A-arm and the mounting place of the spring-rollbar at the A-arm. This is fare from accurate. Scrub distance and upper A-arm angle has a great influence on tha calculation and make this quite complex. In short we could say that a very camber compensative suspension is very dependent on scrub distance for Mr.

Goran Malmberg

[TEAM78]

Originally Posted by Goran Malmberg

I In short we could say that a very camber compensative suspension is very dependent on scrub distance for Mr.
Goran Malmberg

Can you tell us more please Goran, how is a camber compensative suspension dependent upon scrub distance?
im currently in the middle of evaluating camber compensation for a rear suspension on a FS car, I currently have roll camber at 0.4, any more than this sticks the RC up over 2" and provides silly angles on the wishbones, which I really dont want as it will cause undue loadings on everything. There isnt much literature on full camber compensation, and some colleges use it as a bloody buzz word round ere which is annoying as im not that keen on it really hence hope to disprove em.

[Goran Malmberg]

I should use the word "cambergain" is place for "cabercompensation". Neither is "scrub" a proper word to use since it includes the angle of SAI . However, this means that we must concider taking SAI under cocideration even for the rear uppright.

To explain things properly I should need to use a drawing, so I stay with a principal explanation.

As the upper A-arm is at an angle, closer to ground mounting on the car while the lower A-arm is horizontal and longer, the upper spindle balljoint is moving up and in to the car during bump. If the lower spindle balljoint is located at the centre load line of the tire, the tire contact patch will remain at the same height distance to the balljoint, (only a slight movements sideways).

Now, if we draw a vertical line from the lower balljoint to ground, and the configuration is such that we got a distance here to the centre of wheel load line, we is having a lever distance making the contact patch not only moving sideways but also vertically. This distance is not really to be
confused with scrub, since SAI angle is not a vertical line.

So, under bump the wheel is travelling a longer distace than the lower balljoint. And it does so in an increasing rate, progressively reversd to what we likes it to do. If for example the upper A-arm has a static angle of 15 dgr and the "scrub" distance is 150 mm the effect will becom very influencial. A 50% reduction in wheelrate is no fantasy in an extreem case.

We can either use zero scrub OR a horizontal upper A-arm, both action brings the motion ratio back to only depend on the coilover mounting (angle) location on the lower A-arm. Of course we can compensate this by using a higher rate spring , but we still have to deal with the progression in Mr. The best is zero scrub when using exsessive camber copensation.

Compare this to what a slight change in rollbar will do to balance, and it becomes appearent how easy we will get lost in balance if the mentioned things is not taken under cocideration. Long parallel A-arms as used in F1 or le Mans racing prototypes does NOT suffer from these sort of behaviour.

This are things coming from my own studying of my half scale axle model. I must also add that ther might be error in my observation .
Regards
Goran Malmberg

[Goran Malmberg]

Oh, I must add, that it is apparent that the same goes for both front and rear suspension. So, then we must also see to the scrub of the rear suspension even as the rear wheel dosnt steer....

Goran Malmberg

[TEAM78]

Thanks Goran

I now need to get my head round it to fully understand it, I may have a few more questions in the near future, oh the inclination angle of the upper wishbone is currently 18 degrees with the lower one horizontal

[Lukin]

I don't know if I missed the point of the question, but I assume your trying to find the rate (N/mm or such equivalent) for each setting. Im not sure how well the equations using geometry work given deflections and compliance. Perhaps try setting the car on the scales with no load in the bar (very hard I know), with known spring deflection. Put a block under one of the wheels of known height and find the change in load from left to right. You have to subtract the spring load from it, but it might be a good start; for x displacement (relative roll) you have a value for the weight transferred.