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Old 11-29-2004, 07:42 AM
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4-bar axle mounting points

I've seen some pics here and there that appear to have both rear housing mounting points of 4-bar system at or below the center line (the horizontal line) of the axle. Seems like this would be a no-no but I'm curious if it can be done that way. Granted, most every setup you see has the top mount above the housing and and the lower mount below the housing but it would solve some space problems on my new project if it could be done with both mounting points below the centerline of the axle (but still separated by about 5").

Dewey

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Old 11-29-2004, 09:09 AM
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The nice thing about Morrison's 4bar is that you can mount the front bracket anywhere you like and the bars remain parallel, meaning you won't have any binding while cornering.

I might be mistaken, but I think Morrison recommends the bars be parallel to the road surface. I don't agree with this, as the result will be a lot of squat on launch. If you angle them up with an angle having a tangent (rise over run) equal to the center of gravity height divided by the wheelbase, the car will launch with no squat or rise. Unfortunately, this is not what you want to hear.

Further, if you want to cancel the tendency of the driveshaft torque to unload the right rear, you can increase the right side angle by about 15 degrees and decrease the left side by the same amount.
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Old 11-29-2004, 11:53 AM
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Quote:
Originally posted by BillyShope
The nice thing about Morrison's 4bar is that you can mount the front bracket anywhere you like and the bars remain parallel, meaning you won't have any binding while cornering.

I might be mistaken, but I think Morrison recommends the bars be parallel to the road surface. I don't agree with this, as the result will be a lot of squat on launch. If you angle them up with an angle having a tangent (rise over run) equal to the center of gravity height divided by the wheelbase, the car will launch with no squat or rise. Unfortunately, this is not what you want to hear.

Further, if you want to cancel the tendency of the driveshaft torque to unload the right rear, you can increase the right side angle by about 15 degrees and decrease the left side by the same amount.
As usual, Billyshope, you are "da man" when it comes to suspension questions. But I'm still not quite certain from you answer, whether it's a "yes" or a "no" to my original question. Can the top bar on a 4-bar suspension be anchored to the bottom (or a point below the centerline of the axle shaft) rather than the top of the axle housing? This assumes, of course that there is room to also hang the second bar parallel to and 5-6" below the upper bar.

I also want to make sure I understand how you would find the optimal angle for the rear bars. Let's assume the center of gravity height is 18" and the wheelbase is 106" and the 4-bar bars are 20" (eye center to eye center). Would the calculation be 18/106 X 20? This calculation would predict the optimal rise of 3.4" from the mounting point on the frame to the mounting point on the axle housing. Am I figuring this correctly?

If those numbers are right, it would really help my setup in terms of the front mount placement - which interferes with the interior/seat placement in the car if I run the bars parallel to the ground.

Of course that still leaves the question - how do you accurately find the center of gravity height (which I think you've discussed on some other threads).

BTW, I really appreciate your taking the time to walk through this. I'm in the shop right now mocking up this rear suspension so it's "show time" for me.

Dewey
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Old 11-29-2004, 01:45 PM
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First, let's be certain there's no misunderstandings. I'm assuming that each side...right and left...has 2 bars and 2 brackets. The bars pivot on the brackets and those pivot points remain fixed on the brackets. So, when you move one bracket relative to the other, the bars pivot, but remain always parallel to one another. One bracket attaches to the axle housing and the other to the frame. Actual means of attachment is unimportant. You can weld them in place or use 2 or 20 bolts. The actual attachment point, at the housing, is also unimportant. It can be in front of, below, or above the housing. (Obviously, the strongest point is directly in front of the housing. But, if this is not convenient, don't worry about mounting it above or below.) The important parameter is the angle.

The angle determination is not affected by the lengths of the bars. The rise over run is the CG height over the wheelbase. Don't be concerned about the accuracy of the CG height. A reasonable guess will get you close enough.

Obviously, the angle will change as the car squats or rises. Longer bars minimize this change, but, again, don't worry about it. The 20 inch bars are adequate.
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Old 11-29-2004, 08:32 PM
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Hey BillyShope,

I haven't run across that particular formula. Where did you get it? Looks good from an intuitive standpoint.

I'm used to working with CG height in relation to the roll axis. CG Height from the ground minus the height of the roll axis at the location of the CG along that axis. I assume that's the measurement you mean for this calculation too?

This would affect your numbers, CBOY.

In my experience 15 degrees would be a lot of angle for the lower bar, but in reference to your calc's I'm working with cars that have low CG heights.

The only way I know of to get CG height is to use a set of scales and use a formula to calculate the height based on the different weights at different elevations of one end of the car. Let me know if you want the formula or have an easier way of doing it.

Interesting stuff.

Later....

Wally
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Old 11-29-2004, 09:16 PM
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Quote:
Originally posted by BillyShope

The angle determination is not affected by the lengths of the bars. The rise over run is the CG height over the wheelbase.

Was my calculation in error then? Maybe if you just ran through the calculation using the "givens" that I used in my example:

CG height = 18"
wheelbase = 106"
Bar length = 20"

What is the optimal angle going to be?
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Old 11-30-2004, 12:24 AM
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Quote:
Originally posted by wally8
I assume that's the measurement you mean for this calculation too?
No, that's not what I meant, Wally. The measurement to which you're referring is related to an evaluation of that which is called the "roll couple" and is of particular interest to the roundy-round boys.

Cboy, the rise over run would be 18 over 106 or an angle of 9.64 degrees.
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Old 11-30-2004, 08:39 AM
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No offense to Billyshope, he has provided a great explanation of optimal bar positioning here. Unfortunately, I've forgotten WAY too much of my high school algebra so I had to visit a couple of web sites to dust off the old brain cells. If you totally understand how Billyshope arrived at the optimal angle above, then you can skip the rest of this post. But if your brain functions at the same snails pace as mine, let me pass along what I picked up from my web search.

As I understand it Billyshope is actually dealing with two factors in determining how to position mounting points of a simple 4-bar system (this does NOT deal at all with triangulation etc - just your basic parallel bars.)

SLOPE - The slope, in mathematical language is Rise over Run, just like Billyshope explained. It is expressed as a simple number or fraction. In my example the estimated Center of Gravity is 18 inches - thus the Rise is 18. The wheelbase is 106 inches - thus the run is 106. We divide the Rise by the Run and we get .1698. For me, it is easiest to view this number as the RATIO of rise to run. And we can apply this ratio directly to the mounting of our bars to get their optimal height positions. If we already have our bars and know their "Run", or length, we can easily determine the relative position of the "eyes" at each end.

In my example I said the bar was 20" long, eye to eye. Thus the run is 20. To get the rise we simply multiply 20 times the slope - or the ratio of our CG/wheelbase calculation. Using this method we get a rise of 3.4 inches [20 x .1698 = 3.3962]. From this calculation we know that the optimal position of the axle mounting point (the eye) will be 3.4" above the mounting point on the frame. The second bar in the system will have the exact same relative position, just 5 or more inches above or below the first bar.

ANGLE - If we don't have our bars we can use the ANGLE calculation not only to determine the optimal mounting points but the length of the bars as well. To find the angle you need to apply one of those old, dusty, algebraic formulas (or better yet use your handy dandy pocket calculator). First you calculate the Slope, just as we did earlier. Then we can determine the angle by using the Tan-1 (tangent -1) function on the calculator. In our example the Slope was .1698. Entering that number and applying the tan-1 function we get an Angle of 9.6 degrees.

So now, by using a magnetic angle finder, we can mock up the bar placement simply by keeping our hypothetical bar at 9.6 degrees and experimenting with the length and placement on the frame and axle of the two ends.

Hope that helps - and Billyshope, my apologies if I've muddied up the water with that lengthy explanation.
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Old 11-30-2004, 09:20 AM
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Quote:
Originally posted by cboy
From this calculation we know that the optimal position of the axle mounting point (the eye) will be 3.4" above the mounting point on the frame.
Good work, Cboy! The only problem is that you have the bars angled in the wrong direction. In the quote above, it should be "...will be 3.4" BELOW the mounting ...." The bars should be angled UP from their rear mounting points.

Visualize some invisible giant pushing forward, with a big stick, on the car at the rear tire patch. If he pushes forward horizontally, he tends to push the rear axle assembly out from under the car and the rear of the car will squat down. If he angles the stick so that it pushes almost vertically, he'll jack the rear of the car up as he pushes. The angle which I've defined (angled up from the rear) will cause the bodywork to neither squat nor rise. (This is really a crude attempt to explain it, but perhaps it will help.)
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Old 11-30-2004, 10:56 AM
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Whoa, okay, that's why it's good they don't let me build these things without adult supervision. I did have it backwards for sure. And really raises havoc with my interior. A pox on you Billyshope.

Well, back to the drawing board.
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Old 11-30-2004, 11:47 AM
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Yeah, you're right. I am one of those roundy round guys :-)

So for straight line stuff it's strictly CG height from the ground, right? This all applies to roundy rounders too but I've spent a lot more time on making a car turn than hooking it up. We've always had pretty good luck there.

Good formula, I'm going to have to add it to my spreadsheet.

Again, just for clarification, you're referring to the lower bar angle here, right? How about upper bar angle? I understand the relationship of instant center to CG and how adjusting the uppers changes the application of the force. Any formulas or guides to start off with?

Sorry, cboy, I'm not trying to hijack your thread, just trying to understand as much as possible.

Later...
Wally

Yeah, you're right. I am one of those roundy round guys :-)

So for straight line stuff it's strictly CG height from the ground, right? This all applies to roundy rounders too but I've spent a lot more time on making a car turn than hooking it up. We've always had pretty good luck there.

Good formula, I'm going to have to add it to my spreadsheet.

Again, just for clarification, you're referring to the lower bar angle here, right? How about upper bar angle? I understand the relationship of instant center to CG and how adjusting the uppers changes the application of the force. Any formulas or guides to start off with?

Sorry, cboy, I'm not trying to hijack your thread, just trying to understand as much as possible.

Later...
Wally

Last edited by wally8; 11-30-2004 at 11:47 AM.
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Old 11-30-2004, 12:18 PM
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Wally, this thread specifically addresses the installation of a 4BAR setup, not a 4LINK. With a 4bar, the upper and lower links are "locked" in parallel. The only choice you have is where to mount the end brackets.

It's a different story with a 4LINK. You indicate that you understand the concept of an "instant center," so I won't get into that. To achieve the same effect with a 4link, you'd place the instant center on a line, with the same slope as we've been discussing, which passes through the rear tire patch.
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Old 11-30-2004, 01:16 PM
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Oh yeah, I just realized that shortly after I posted. You're right, I was thinking 4 link not 4 bar.

Sorry about that.


Wally
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