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Old 10-13-2004, 09:07 PM
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Triangulated 4 link help?!

Hello! I am new to the board and have a question concerning 4 link (esp triangulated 4 link) suspensions. I am building a 2000# 800hp kit car, and I am needing info on a tri-4link that might be able to handle all the hp and handle decently too. I am very confused (and barely informed) about what angles to put the links on a tri 4 link--basically all the suspension geometry. I have read a lot of the previous threads, but I haven't seen any info (or anywhere else for that matter) on which bars to angle in (I've seen top or bottom--done both ways) or what angles (vertical and horizontal) to put these bars even as a starter for basic adjustments. If I don't get any info soon, I am just going to say screw it and angle the top bars 5 deg up and 45 deg in and call them good. Any suggestions?? thanks a bunch---
Jason

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Old 10-13-2004, 11:10 PM
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4 link

I am using the complete rear clip from a caprice in my latest..I would say we can't do much better than what GM did on this design..The upper links are attached to the center of the axle housing and splayed to the outside at about a 45 degree angle....

I am setting the ride height and chassis so the bottom links are level and the upper links down a bit to get my instant center..the top uppers are adjustable so I can tweak it if neccessary..

using some boxed or tubular links should do the job here..

Note the way chev did the sway bar as it is bolted solid to the lower links..should help with locating the rear as well as being a sway bar..

Since these were originally engineered for a car that is close to 4000 lb gross weight it shoudl be fine in a 2000 lb car and take the horsepower you are proposing..Art morrison does make some adjustable upper links that may work in your app..or you can make your own if needed..

I could get some measurements but it is dark and the shop is locked for the nite..

Hope this helps a bit..
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Old 10-14-2004, 05:02 AM
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Good advice from OMT. I would just add that, when it comes to which pair to splay (upper or lower), the manufacturers have done it either way.

I would also add that, when it comes to the angles (as viewed from the side), pay attention to the OEM setups and NOT to the aftermarket pieces. I saw pictures of one aftermarket arrangement which appeared to have the IC behind the axle, which would mean SEVERE squat on launch. You don't want that. If you want to eliminate squat or rise entirely, arrange the links so that lines through them, as viewed from the side, intersect on a line which passes through the rear tire patch and has a slope equal to the CG height divided by the wheelbase. If the intersection is above this line, the car will rise on launch; if below, squat. It's not necessary for either pair (top or bottom) to be level.
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Old 10-14-2004, 07:16 AM
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Quote:
Originally posted by BillyShope
intersect on a line which passes through the rear tire patch
Hey BillyShope,

What is "rear tire patch"?

Dewey
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Old 10-14-2004, 07:14 PM
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Sorry. I should have been more specific. The tire "patch" is that small area of asphalt actually in contact with the tire at any instant in time. Until you questioned it, it seemed to make sense, but now I don't really know why someone first called it a "patch." I guess it's just shorthand for "tire-to-road-surface contact area."
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Old 10-14-2004, 08:23 PM
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Like most things, once you explained it...it made perfect sense.
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Old 10-15-2004, 09:31 AM
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I just went through the same thing. I wanted to do a 4link triangulated system but there wasnt any info on it. I am building an 87suzuki samurai that weights 2000lbs with me in it, and droped in a motor capable of 750hp with nitrous. What I can tell you is prepare to study and study and study. Because of my unique vehicle and power, I had to design my own suspension from scratch. I had to learn things I didnt really want to learn but had no choice. It's not easy, and all I can tell you is that read as much as you can and take time. I originally planned 3-4 weeks to develop my own rear suspension and it took me 8 months. To answer your question the Triangulated link angles are important. If you dont have enough angle then your putting putting your links under alot of stress due to load angles. Too much angle and your axle is going to twist forward as the axle course moves up. There is no direct answer as each setup is different and the angles at which you put the links effects this too.


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Old 10-15-2004, 09:36 AM
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Nice Job!

That is a good looking chassis setup..
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Old 10-15-2004, 12:22 PM
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Yes, "brainsboy," that's nice fab work. It would appear that your IC is behind the rear axle, which means, as I pointed out earlier, the car will squat on launch. That's not the end of the world, of course. There have been a lot of production cars that almost dragged the rear bumper. It could have been avoided, though.

I like Heim joints, I lost count of the number I used on a sprint car I built a few years ago. But, for something like this, I'd use rubber-bushed sleeves. Even with the triangulated 4link design you're using, there's some bind during cornering. All it takes is a little rubber to free things up.

Am I predicting disaster? No, not at all! But, you show the car with drag slicks mounted, which does worry me a bit. So, I would suggest, if you're driving it both on the street and at the strip, that you check those Heim joints for cracks periodically.
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Old 10-18-2004, 12:24 AM
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I think I have posted this before but here it goes again.

I made up an Excel spreadsheet that will crunch all the #'s for you. Not that you couldn't do all this on a sheet of graph paper but it makes it ALOT quicker to make changes.

I made it for double triangulated 4-bar linkages on 4x4 trucks but I really don't see any difference in the form. I have yet to make my own web site to answer all the questions so for now I will just direct you to that 4x4 site of you can ask here.

This is the link to the 4x4 site:
http://www.pirate4x4.com/forum/showthread.php?t=204893

This is a link to where it is located. There are new versions there that are still in testing so they might not all work properly or give the right answer.
http://home.earthlink.net/~triaged/Files/

At the moment it will give an error if you make the links parallel. To address this I sugest you make them "not parallel" by about 0.1" or so. You will be luckey (or good) to get it that accurate in real life anyways.

As for what you should shoot for...
Slight roll understeer will make it handle more "friendly" at the limit and most triangulated 4-bar's have compliance in the oversteer direction anyways so this will help cancle out some of that.

Most leaf sprung suspensions have the roll center near the tire center. I think that would be a safe place to start.

My one other comment would be to make it adjustable. There is nothing like finding out you don't like the way you made it and having to start over.
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Old 10-18-2004, 07:15 AM
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Triaged has brought up an interesting point in his concern about roll center height. I have mentioned that it appears brainsboy's setup will have the IC behind the axle, resulting in squat during launch. On the positive side, I could have added that, with the upper links angled as shown, the roll center might very well be nearer the "stock" height, resulting in comparable handling.

In changing the angle of the upper links (as viewed from the side) to cancel squat or rise, it's difficult to keep from raising the rear roll center. This will result in a greater portion of the roll couple going to the rear during cornering. In plain English, that means the car will be looser. Is this significant? Well, I don't tend to worry about it with a V-8 car from the pre-IRS era since they normally plowed terribly, in stock form, anyway. Loosening such a car up a bit is an improvement, in my opinion. But, if you want the stock handling, you can install a heavier front sway bar. In short, the average person will never notice the difference and the one who can also knows how to fix it.

Same with roll steer. Roll oversteer can be an irritation, especially if you've never driven even a parking lot slalom, but, unlike the situation where tire cornering loads are actually changed, the introduction of roll steer is not normally dangerous. If I had my "druthers," I'd certainly agree that roll understeer would be desired. Unfortunately, you usually end up with a bit of roll oversteer, but not enough to cancel the overall feeling of understeer.

I'm a bit uncertain as to what Triaged means by "adjustment." I suppose he means different mounting points for the front of the lower links. This might be a good feature for the serious dragracer. But, I see this as more of a "compromise" suspension for the street car that makes an occasional visit to the dragstrip, so I don't think I'd bother. But, if you want to play around with squat and rise and their effects, you definitely will need that adjustment.
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Old 10-18-2004, 02:16 PM
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Quote:
Originally posted by BillyShope
I'm a bit uncertain as to what Triaged means by "adjustment." I suppose he means different mounting points for the front of the lower links. This might be a good feature for the serious dragracer. But, I see this as more of a "compromise" suspension for the street car that makes an occasional visit to the dragstrip, so I don't think I'd bother. But, if you want to play around with squat and rise and their effects, you definitely will need that adjustment.
The CG height is a very "fuzzy" number. I have a friend that measured it on his 8000# truck before he designed his rear suspension. I have only done very small vehicles that the front end could be lifted up by 2 people with someone in the driver's seat. Because it is so difficult to measure most people just take a SWAG (Scientific Wild ***** Guess) at where the CG is. I have heard that camshaft centerline is a good swag but I have no idea how close it would be.

If you do want to find the vehicle CG this a one way to do it
http://www.jeepaholics.com/tech/cog/
Tilting the vehicle up to the balance point and measuring the angle is also fun

All the rest of the #'s are not based on other "fuzzy" numbers so I think you could get away without having them adjustable.
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Old 10-18-2004, 03:32 PM
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Yes, it's difficult to locate the CG height with the degree of accuracy with which we're comfortable when working on other parts of the car, but, fortunately, when it comes to chassis setup, this ain't rocket surgery! When it comes to trying to position the IC on the no squat/no rise line, you can be an inch or two off, either way, on the CG height and the change in link angle is probably within your ability to measure with the tools found in an average shop. And, even if you "miss," you have to be quite a ways off before you can "see" the effect.

(Also, keep in mind that the CG height with which we're interested is the CG height of the car LESS the weight of the rear axle assembly.)

But, real accuracy IS required when attempting to determine the CG height by tilting the vehicle and using wheel scales. If an error analysis is made (i.e., partial differentiation of the measurement parameters), it is quickly seen that very slight measurement errors can result in large errors in CG height. Even a statistical consideration of the possible errors is a bit "scary."

For this reason, I prefer a "tabulation" method for determining CG height. You simply add the products of height and weight for every component you wish, add a final product consisting of the remaining weight and a CG height estimate, and then divide by the total weight. Obviously, this method is only as accurate as the time and effort you wish to expend.

Last edited by BillyShope; 10-18-2004 at 03:41 PM.
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Old 10-18-2004, 05:24 PM
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CG height.if ya really gotts to know

Go here for calculation of CG height..this doc lays out a procedure for doing a CG height test..

http://www.unece.org/trans/main/wp29...SG79_inf11.pdf

It is a PDF file so you will need adobe acrobat reader to see the file..
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Old 10-19-2004, 09:35 AM
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Thanks, OMT. That certainly verifies what I was saying. He's asking that 1000 pounds of loading be measured to plus or minus 2 pounds and that the 15 degree angle indicated is accurate plus or minus 9 minutes!! And then, I couldn't find where he indicates the expected accuracy of the result. When you're employed by someone like the General, of course, you can do that sort of thing, but, that's why, for the average racer, I recommend the tabulation method.

But, as you say, it's only if you really, REALLY gotta know.
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