sway bar rates

Zambo

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Anybody have a rule of thumb or formula for trying to figure out a sway bar rate/ arm length? Besides trial and error of course. :)Thanks!
 

Wild bill

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Schroeder (sp?) splined my bar. They have a web site and do a lot of nascar bars. They have a chart as a rough starting point.
 

ACID_RAIN28

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These are just what I have come up with over time with personal preference.

Depending on what the vehicle will be doing will determine what direction to go with the bar. Say for instance my TT is going to run the 500, I would want the bar to provide a high roll resistance, to promote over steer, of some where about 50-75% increase of the wheel rate in roll.

If it were going to run Primm or San felipe I would want as little as I could to allow the rear to move around.

For short course it is a completely different game and I would usually have a whole range of bars and arm locations to get anything from 2%-150% increase in wheel rate in roll but that is a whole nother conversation in it of itself.

Also there are some nuances that are not usually seen when it comes to Anti-Roll Bars. If the chassis rolls and the arm link where it attached to the suspension moves up 1 inch it has also moved down 1 inch on the other side for a combined deflection of 2 inches
Follow?
 

atomicjoe23

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I know there is a lot when it comes to suspension design and I obviously have some more to learn. . .

. . .I'm following what you are saying, but I haven't gotten to figuring out wheel rate in roll yet. . .guess it's one more thing to start figuring out. . .
 

ACID_RAIN28

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Wheel rate for any given individual wheel is found by taking the motion ratio of the spring mount to the wheel on a particular the suspension linkage set up, squaring it, then multiplying it by the spring rate for that suspension setup.

WR= (MR)(MR) * (spring rate)
WR= (.6)(.6) * 400lbs/in
WR= (.36)(400)
WR= 144 lbs/in

Wheel rate is only a part of the roll rate. Wheel rate pertains to the stiffness of the movement of the wheel center vertically, and roll rate is the stiffness of the suspension in roll I summarized that original post a great deal
 

atomicjoe23

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And the motion rate is?

I've read "Chassis Engineering" by Herb Adams, but I don't remember that term. . .I know I've probably been told before, but I don't remember.

The other suspension design books are WAY expensive for a college student having to pay over $100/book per class per quarter right now!
 

ACID_RAIN28

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Kinda like choosing suspension ride frequency's, you can choose the Roll Gradient of the vehicle as well. Which is basically choosing the roll rate of a given suspension system.
 

ACID_RAIN28

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"Motion Ratio" or installation ratio is found like this.(also you can go to sway-away.com and go to the tech room and they have online calculators and cool diagrams, a little confusing for the layman I think. Say I have an A-arm front suspension and I am looking at it from the front. For argument sake lets say the A-Arm mounts on the upright at the wheel center line, and the arm length from pivot to pivot is 20". Now lets say that the coil over mounts completely vertical at 12" on the arm as measured from the chassis pivot. All you do is simply divide the length of the coil over mount from the chassis pivot by the total length of the arm. So 12/20 which comes to .6, that is the ratio of spring/shock movement to wheel movement. So lets say the wheel bumps up 1 inch, well the shock will only move .6" inches
 

ACID_RAIN28

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Get some more books. You don't want to take just one road racing book and base your entire build on it.

There is some good info here (that came from an old "Chevy Power Manual")
http://www.pirate4x4.com/forum/gall...=25&orderby=title&direction=ASC&cutoffdate=-1

For a solid axle don't forget that the wheel rate is different depending on 1 wheel bump or 2 wheel bump (as if it wasn't complicated enough already).

What he said exactly, I have gathered a ton of different sources and drawn my own conclusions from them.

Also wheel rate is always expressed as a single wheel movement, hence not wheels rate. Both wheel rates at the front or rear really only come into effect when looking at De/acceleration and pitch formulas but yeah to keep this as simple as possible well stick with 1 wheel bump
 

Scott_F

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"Motion Ratio" or installation ratio is found like this.(also you can go to sway-away.com and go to the tech room and they have online calculators and cool diagrams, a little confusing for the layman I think. Say I have an A-arm front suspension and I am looking at it from the front. For argument sake lets say the A-Arm mounts on the upright at the wheel center line, and the arm length from pivot to pivot is 20". Now lets say that the coil over mounts completely vertical at 12" on the arm as measured from the chassis pivot. All you do is simply divide the length of the coil over mount from the chassis pivot by the total length of the arm. So 12/20 which comes to .6, that is the ratio of spring/shock movement to wheel movement. So lets say the wheel bumps up 1 inch, well the shock will only move .6" inches

Dave, your explanation is close, but it doesn't tell the whole story. Motion ratio can be looked at two ways. The overall motion ratio is the total vertical wheel travel/total shock stroke. However, motion ratio is neither constant nor linear. Motion ratio varies throughout the suspension travel, so at any given point in the travel, you would have an instantaneous motion ratio. Hopefully, that would be a progressive ratio, i.e. 3:1 at full droop and 1.5:1 at full bump. When plotted out, motion ratio should be a progressive curve (rising rate).
 

atomicjoe23

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"Motion Ratio" or installation ratio is found like this.(also you can go to sway-away.com and go to the tech room and they have online calculators and cool diagrams, a little confusing for the layman I think. Say I have an A-arm front suspension and I am looking at it from the front. For argument sake lets say the A-Arm mounts on the upright at the wheel center line, and the arm length from pivot to pivot is 20". Now lets say that the coil over mounts completely vertical at 12" on the arm as measured from the chassis pivot. All you do is simply divide the length of the coil over mount from the chassis pivot by the total length of the arm. So 12/20 which comes to .6, that is the ratio of spring/shock movement to wheel movement. So lets say the wheel bumps up 1 inch, well the shock will only move .6" inches

Cool. . .thanks for that info. . .I knew what it was I just wasn't calling it that. . .we had to figure that stuff out for our buggy to make sure that we got the travel we wanted without tearing the shock apart or wasting shock stroke. . .

Dave, your explanation is close, but it doesn't tell the whole story. Motion ratio can be looked at two ways. The overall motion ratio is the total vertical wheel travel/total shock stroke. However, motion ratio is neither constant nor linear. Motion ratio varies throughout the suspension travel, so at any given point in the travel, you would have an instantaneous motion ratio. Hopefully, that would be a progressive ratio, i.e. 3:1 at full droop and 1.5:1 at full bump. When plotted out, motion ratio should be a progressive curve (rising rate).

If you were to plot the curve at from full bump to full droop then the shape of the curve would look like the right half of the letter "U" right? or would it be the opposite (meaning instead of being concave the curve would be convex. . .like if you flipped the "U" upside down. . .)?

and why do you want a rising rate vs. say a falling rate? Just so I know the reasoning behind it. . .

Thanks!
 

Scott_F

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You would plot it from full droop to full bump, with wheel travel on the X axis and shock shaft travel on the Y axis. The plot line would curve up as it approaches full bump. Rising rate is desirable because for every increment of wheel travel, you get progressively more shock travel, which provides more spring rate and damping per unit of wheel travel, thus more stiffness and bottoming resistance at the end of travel when you need it most. A falling rate gives less spring rate and damping per unit of wheel travel approaching full bump, which is very undesirable.
 

Zambo

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Okay, so lets say I'm building a street legal prerunner truck with a sway bar in the rear. Just to keep the math simple, lets say the springs are halfway down the trailing arm and are 400 lbs. Being as how they're close to perpendicular to the arm, the wheel rate is .5 x .5 x 400, or 100 lbs.

So if I want a truck that has fairly good manners on the street, I'm guessing the rate of the sway bar should be on the high side, which I guess would be about 75% of the spring? So I should look for a setup with 75 lb rate?
 

atomicjoe23

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Zambo. . .what are the two .5's in your equation. . .I get that the 400 is the spring rate, I'm guessing that one of the .5's is the position on the trailing arm in percentage of total length, but is that from the axle or from the chassis side mount?

and what is the other .5 supposed to represent?

Thanks!
 

Halibrand

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.5 is refering to simple math he is using by making the motion ratio 1:2. You see it twice because he is squaring it per the formula given earlier.

This is all GREAT info!
 
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