I would use it!

Diameter vs. length. Example: A 40" inch bar would need to be ____ thicker to have the same stiffness as a 30" bar.

300m vs 4340: strength, stiffness, fatigue.

Practical misalignment degrees of rotation for given thickness, length and material.

Having a constant like Curries formula would be a good base to start from.

Arm lengths would be interesting to add in, too.

 

http://www.buildafastercar.com/tech/Sway-Bar-Rate-Calculator

 

BTW, that calculator is awesome in how it does comparisons.
I just noticed the "Sway bar 2 is xx% stiffer" in small text below the first column. :cool2: Bookmarked!

Changing my arm length from 17 to 12" nearly doubles the rate, which I kind of knew from the digging I did a while back, but didn't feel like finding again.

 

Look at the difference between a .750 bar and a 1.0 bar. It's more than three times stiffer!

The other two diagrams are what it takes to get the 1" bar back to the softness of the 3/4" bar.

 

I can't see it being all that useful for rigs around here. I've played with mine quite a bit by changing arm lengths, and broad changes in settings don't really seem to change the overall road/high speed handling or crawling ability a huge amount. Basically they do what you'd expect- the rig flexes/crawls better without them (or at light settings), and generally handles and sidehills better with them on higher settings. And it does everything fairly well with them connected at a mild setting, which is where they stay most of the time unless I feel like stiffening them up for long road trips.

Knowing the numbers wouldn't really change anything for me except that it might be helpful for comparison to other setups. Knowing what a light or stiff setting for my rig is compared to another might be helpful. A light setting for my top-heavy rig may seem a bit stiff to someone in a light buggy.

So yes, it might be nice as a learning/comparison tool, but I doubt I'd use it a whole lot.
The calculator linked above seems to just about do it all...? (except that it doesn't seem to like firefox).

Even so, the sway bar will work differently for different rigs with different roll center vs. CG characteristics.

 

Are you as smart about these things as you are about links? If so, can you help me figure out why mine keeps bending? Can I pm you? thanks

 

I wouldn't say I know much about links and I bet I know about the same about sway-bars.:homer: I am an engineering consultant for off-road race cars so this is the kind of stuff I do every day. Chances are you bend it because it is too weak:flipoff2::laughing:

 

That is a great calculator. I hadn't seen that one.

So far I have been tossing one together for myself and I haven't even gotten into the stiffness yet. I started with stress analysis. Like if you had a bar that was 36" long and it had 45° angle of twist at full articulation how thick can you make it?

FWIW I got for torsion bar type sway bars so far:

(Max Shear Stress) = D * (angle of twist in radians) * G / (2 * l)
where D = OD
G = Shear Modulus (about 11,600,000 psi)
l = working length of torsion bar
(note that stress is directly proportional to the diameter and inversely proportional to length)

So JR's 1st example (D=.75) at 45° angle of twist has a shear stress of 90ksi and the 2nd (D=1) has a shear stress of 120ksi. The stiffness went up a bunch but the stress didn't change much. Finding actual shear strength numbers isn't easy but you can use:
shear yield strength = tensile yield strength * 0.577
Some people who don't know better try to use tensile numbers. Shear yield strength of 4340 quenched and tempered at 1000°F is 100ksi so as you can see the 1" bar would yield and the 3/4" bar would not. Obviously yield is only part of it because you would want to know fatigue strength to understand how long it would last.

If I was to do it I would put in the arm lengths, load on the end links, and maybe some very basic travel stuff. The next thing to do would be the roll stiffness so you would add springs and roll center and know the roll gradient in g/deg. so you would know your body roll angle in a turn or on a side hill. I guess if I had gone that far calculating wheel loads wouldn't be that far off.

 

To calculate the angle of twist you need:
Working length (doesn't include splineded ends just the length of the bar at the rated diameter)
Material (most likely 4340)
Hardness (they should give this to you if you ask)
Diameter

Get me all those and I'll tell you how much twist it can take.

I posted the equation above and you can solve it for anything you want.
(Max Shear Stress) = D * (angle of twist in radians) * G / (2 * l)

 

I called Schroeder racing and they said its a proprietary steel, but the numbers would be close to 4340 and they said the hardness is about 45.
I'm looking at their 1.25" 49 spline solid front bars 37.5" with a working length of 33.5" diameter is .875

Working length: 33.5
Material: proprietary similar to 4340
Hardness: 45
Diameter: 7/8


I looked at what you posted for JR, but messed up somewhere.
(Max Shear Stress) = D * (angle of twist in radians) * G / (2 * l)
D = Outside Diameter (.75)
G = Shear Modulus (about 11,600,000 psi)
l = working length of torsion bar (36)

D * ( 45 degrees = rad .8754) * 11,600,000 / (2 * l)
D * ( 45 degrees = rad .8754) * 11,600,000 / 72
.75 * ( 45 degrees = rad .8754) * 11,600,000 / 72
.75 * .8754 * 11,600,000 / 72
7,615,980/ 72 = 105,777

OK so you can see I did not go to engineering school as I need to spell it out.

Thanks for the help.

 

I called Schroeder racing and they said its a proprietary steel, but the numbers would be close to 4340 and they said the hardness is about 45.
I'm looking at their 1.25" 49 spline solid front bars 37.5" with a working length of 33.5" diameter is .875

Working length: 33.5
Material: proprietary similar to 4340
Hardness: 45
Diameter: 7/8

Does Schroeder shot peen their bars? They should (and I figured it into the numbers below).

Here is what I come up with


Working Diameter D 0.875 inches
Working Length l 33.50 inches
Angle of Twist Θ 45 degrees
Material 4340 Q&T 800°F
Angle of Twist Θ 0.785 radians
Max Shear Stress τmax 119.0E+3 psi
Shear Ultimate Strength Ssu 185.0E+3 psi
Shear Yield Strength Ssy 130.0E+3 psi
Tinsile Ultimate Strength Sut 217.0E+3
Tinsile Yield Strength Syt 198.0E+3
Rotary-Beam Endurance Limit S'e 89.0E+3
Endurance Limit Se 42.7E+3
Shear Modulus G 11.6E+6 psi
Factor of Safety Ultimate FSsu 1.6
Factor of Safety Yield FSsy 1.1
Factor of Safety Infinate Fatigue FSf 0.36

I looked at what you posted for JR, but messed up somewhere...45 degrees = rad .8754

You crashed the mars lander into the planet right there by swapping the 7&8 (should be 0.7854)

Triaged, do you have the SAE spring design manual? there is some good info on torsion bars and anti roll bars. They have some info about operating shear stresses. You can run torsion bars at higher operating stresses but they are loaded in one direction, anti roll bars are loaded in both directions so you have to reduce the op stress, they recommend 700 MPa or about 100 KSI for 4340. 300M torsion bars can go up to 1250 MPa (180 ksi)

I don't have that book but have thought about buying it a few times. I did the above the with every single modifying factor tossed in. I guess I don't need to be looking at infinite life fatigue but should plan on some finite life...or just design for 100ksi and call it good.

 

JR, the spring rate factors the diameter to the 4th power, so a small increase in diameter is a larger increase in rate. The arm length and bar length is only to the 1st power so it doesnt have as much of an effect on the rate as the diameter.

 

Triaged, do you have the SAE spring design manual? there is some good info on torsion bars and anti roll bars. They have some info about operating shear stresses. You can run torsion bars at higher operating stresses but they are loaded in one direction, anti roll bars are loaded in both directions so you have to reduce the op stress, they recommend 700 MPa or about 100 KSI for 4340. 300M torsion bars can go up to 1250 MPa (180 ksi)

 

I may be missing the obvious but where do you add the calculation for the vehicle's weight and is there a velocity chart for the vehicle's reaction at speed? This would be more for desert racing vehicles but in the era of go fast crawlers...

 

If I'm messing up Triaged thread I will delete this.

Your looking for the roll rate in pounds per inch, but the list of interacting variables seems endless.

Some basic things to keep in mind.
1: The longer the arms the fewer degrees of twist on the torsion bar. Could equal longer life of the sway bar.
2: Longer arms means less force it takes to twist the torsion bar. Think cheater bar, so you may need a stiffer bar.
3: A shorter torsion bar requires more force to twist it than a longer torsion bar of the same size. But the longer torsion bar means the arms are attached farther out on the axle which means less force is applied. Back to the cheater bar analogy.

It looks like most are tying different rates and seeing how it handles, but some would like to understand the math behind it all.

Other side notes: Trophy trucks run longer arms then the rock buggies.

 

Let me see if I can remember some math, Ill delete if it isn't right...

If you use:

(Max Shear Stress) = D * (angle of twist in radians) * G / (2 * L)
where D = OD
G = Shear Modulus
L = working length of torsion bar

then:

(angle of twist in radians) = {(Max Shear Stress) * 2 * L} / { D * Shear Modulus }

This way you can solve for max allowable twist in the bar (in radians).


If you then knew the twist (in radians) and the amount of suspension travel you could find the length of arm needed to not to overtwist the bar:

Arm Length = { 0.5 * Suspension Travel } / tan(twist from ^)

The end link would need to be long enough to reach the mounting location with the sway bar and suspension both halfway through their travel.

The bar length and diameter would be based on the roll rate needed for the vehicle, but that is getting into spring calculations

 

id like to see the "value" of anti sway that Radius arms have and be able to change length, bush spacing etc....is that asking to much? :flipoff2:

Always appreciate your efforts Dan
Serg

 

Generally the longer the radius arm, the lower the roll stiffness and stiffer bushings must be used to compensate.

 

Dan brought up some good points.
1: Rig weight, as weight increases so will the roll rate.
2: Speed, as speed increases so will the roll rate.
We all get the idea is to install a sway bar large enough so that when the truck begins to roll over on one side in a turn, the bar pulls the other side of the suspension up so that the truck both squats down and levels out. The center of gravity is lowered and the ability to take a corner at speed increases.

I'm thinking two calculators?
One would be for the torsion sway bar and the other to help you get in the ballpark for what you need.

Here's a Circle track calculator but would be nice to also know how many degrees of twist the bar can take so we design the arms to the correct length. Looking at KOH many are busting/stripping arms and bars. So we must be exceeding their limits. Kickass offroad offers a 300M bar so the calculator would need to adjust for material used too.
One feature I like about the circle track calculator is arm length. So you know the affect of each position when the arm has several holes.

Things that affect the roll rate are endless, but maybe focus in those components that have the most affect, like spring rate, tire pressure, leaf vs coils?

Edit: Maybe a simple picture icon, once you choose it alters the formula used. This way you could have a buggy, pickup, suv.

 

Could you expand a bit more on this type of measuring. I did some "Google research" and did not enter the correct words as I could not find anything.

If I attached a come-a-long to the top of my cage and measured a 1,000lbs of pull, I would think my tires would be close to comming off the ground on one side.

On a side note, my 7/8 bar arrived, it has a working length of 30" vs 33", but using the circle track calculator it's almost dead on with the manufactures chart. 2lbs off
Using that calculator I see arms with a length of X have Y lbs per inch. I have 3" up and 10 " down travel at the shocks.

12" = 156 lbs per inch at max of 40° would be 1560 lbs total
14" = 115 lbs per inch at max of 36° would be 1150 lbs total
16" = 88 lbs per inch at max of 32° would be 880 lbs total
18" = 69 lbs per inch at max of 29° would be 690 lbs total

I'm looking for better on road performance as mine is a drive to trail type of rig. So I won't be seeing full articulation on the road. I'm leaning toward the 12" arms and running disconnects for off road use.

 

You would want to put the come-a-long at the CG (or have the line of the cable point at the CG). Then you would divide your force by the sprung mass and get your roll gradient in "degrees per g". Some corner weight scales would help get the front/rear distribution but be careful to avoid putting shear loads on the scales.

Google Bill Shope's "Traction Dyno" and think of it sideways. If you take the measurements I'll calculate the results.

 

So the "buildafastercar.com" link earlier in the thread is now defunct. does anyone have an alternative host or another calculator?

 

see post 23, that calc is still up and working.

What I learned. My rig is heavy. i.e. 6K range. The 7/8" dia. rear sway bar with short 12" arms made my road handling a million times better. If I leave it connected on the trail it works fine, but will over flex the front end as it does not have a sway bar. My advise would be to error on the stiffer side.

 

I'll post up what I got done sometime soon. I haven't touched it in a year and it wasn't near finished when I posted it up but it does have some of the stress calculations done. Maybe someone else can put the time into finishing it.

 

Ok...Here you guys go. This is an incomplete beta. It hasn't been checked. Use with caution. Right now there is only some stress equation stuff and nothing on rate. If anyone wants to add some rate info to it go ahead. Just fill in the revision tab and send it to me so I can host it for everyone.

http://mysite.verizon.net/triaged/files/SwayBarV0.1.xlsx

The calculations come from "Mechanical Engineering Design" by Joseph Shigley. I have the 7th Ed. I have no idea what it is up to now. The data came from some AMS datasheets that I got over the years.

I don't like the infinite fatigue approach because I think it would be way over-engineered. Somewhere between infinite life and yield is the right answer. I might put in some equations for finite life or just buy the book Goat mentioned and use some of that data.

Let me know what you think!

 

Crap. Bump.

All the links are dead unfortunately, but I'm trying to figure out the same stuff you guys were talking about. I've got my chassis set up with the option for a front sway bar of 24" working length. I figure it's going to see like 25-30 degrees of rotation max with the configuration of my suspension, and I'm trying to find out the thickest sway bar diameter I can run without expecting failure. Random shot in the dark, like .75" working diameter?