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Old 09-20-2018, 05:13 PM
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Improvements for GM triangulated 4-bar

I have a GM triangulated 4-bar rear suspension from a 95 B-body that I plan to swap into my 1937 Buick (like the one in the picture).
Still considering a torque arm set-up, but also looking at keeping with
Triangulated 4-bar for simplicity and lower cost.
I already bought UMR Performance lower control arms with Roto-joints on the axle end.
The standard upper control arms are much shorter, so I'm wondering if I should try to fit something longer for better ride, or performance.
Suggestions and guidance would be welcome.
Thanks
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Old 09-20-2018, 11:13 PM
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I don't think I would try to second-guess the professional engineers who designed this arrangement in the first place.....not unless I was also a mechanical engineer.
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Old 09-21-2018, 08:35 AM
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The length of the arms and locations of the pivot points control all sorts of handling and braking related issues, including how the suspension moves during cornering, how it dives or squats during braking or acceleration, how the pinion centerline remains parallel with the trans output shaft, how much the body rolls, and how well the axle remains centered under the chassis during turns. Arbitrarily changing the lengths of the arms changes these attributes, usually for the worse. Many aftermarket suspension arm kits are designed to improve one of these attributes, but frequently to the detriment of the others. For example, different length lower arms can improve launch at the drag strip, but usually hurt cornering.
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Old 09-21-2018, 09:35 AM
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Thanks for the feedback. The article I read that said the upper and lower arms should be same length, but my guess is it was related to adjustable height airbag suspension. This would allow the pinion angle to remain constant at any height. I would think the upper triangulated arms would have to be even longer than the lower to keep the geometry correct.
Will probably approximate the mount points and length of the original suspension, but would like to understand if GM made compromises related to space or cost constraints.
Have read articles touting 3 link suspension but recently found this video that shows a triangulated 4-bar to be just as effective:
Note that in this video the upper control arms are shorter than the lower.
I'm building a cruiser for street use only, but would like a smooth ride without a lot of roll.
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Old 09-21-2018, 09:50 AM
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Quote:
Originally Posted by Mutt's37Buick View Post
Thanks for the feedback. The article I read that said the upper and lower arms should be same length,
And yet, GM built tens of millions of cars where they were specifically NOT equal length. Who ya gonna believe?

I'd suggest this book:

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Old 09-21-2018, 11:14 AM
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Quote:
Originally Posted by joe_padavano View Post
And yet, GM built tens of millions of cars where they were specifically NOT equal length. Who ya gonna believe?

I'd suggest this book:

Thanks. I do have that book and I'm starting my second read thru it.
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Old 09-21-2018, 04:46 PM
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Quote:
Originally Posted by joe_padavano View Post
And yet, GM built tens of millions of cars where they were specifically NOT equal length. Who ya gonna believe?
GM wasn't the only one, Ford Motor Company used unequal length 4-link on 1978-2004 Mustang, probably the most commonly used drag, road race, and autocross chassis currently in existence.

Last edited by ericnova72; 09-21-2018 at 04:57 PM.
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Old 09-24-2018, 03:09 AM
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Chassis roll center and link type suspension.

Talk to anybody who has raced or setup a stock car and they will start talking about the geometry of the bars, shorter bars on top center are definitely a compromise for space, equal or close to equal length bars allow the axle to swing on a more even plane. If you draw straight lines off either ends of the link bars you find they run parallel to and sometimes intersec/diverge with 2 lines that run through the center of the car, these two lines are the centerline height and the gravitational centerline, the relationship between the 2 is called the roll couple and how they behave while a car is maneuvering is called roll couple distribution. The dynamic relationship between these 2 centerlines is determined by the geometrics of the suspension system and therefore is beyond critical to building a good handling car. Watch a Nascar car race sometime, the cars come into the pits and they sometimes adjust a 3rd jacking screw on the back of the car that changes the position of the panhard bar, this adjustment changes the roll couple height up or down which puts in or takes out push going into and out of the corners. You could think of the link bars as levers and keep in mind that link bars that run parallel front to back with the chassis at ride height is a good starting point. You have the option of triangulating, using a panhard bar or useing jacobs bars to keep the axle centered in the chassis. Keep your roll couple small and close to the ground for a good handling car.

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Old 09-24-2018, 08:28 AM
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He's not building a race car. He's building a 37 Buick street rod starting with the original frame. Designs dedicated for specific use on a track don't necessarily translate to what you want on the street. As for parallel, equal length rear arms, then you need a Panhard rod as well, so it's a FIVE link rear suspension.
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Old 09-24-2018, 10:01 AM
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Quote:
Originally Posted by Cridder View Post
Talk to anybody who has raced or setup a stock car and they will start talking about the geometry of the bars, shorter bars on top center are definitely a compromise for space, equal or close to equal length bars allow the axle to swing on a more even plane. If you draw straight lines off either ends of the link bars you find they run parallel to and sometimes intersec/diverge with 2 lines that run through the center of the car, these two lines are the centerline height and the gravitational centerline, the relationship between the 2 is called the roll couple and how they behave while a car is maneuvering is called roll couple distribution. The dynamic relationship between these 2 centerlines is determined by the geometrics of the suspension system and therefore is beyond critical to building a good handling car. Watch a Nascar car race sometime, the cars come into the pits and they sometimes adjust a 3rd jacking screw on the back of the car that changes the position of the panhard bar, this adjustment changes the roll couple height up or down which puts in or takes out push going into and out of the corners. You could think of the link bars as levers and keep in mind that link bars that run parallel front to back with the chassis at ride height is a good starting point. You have the option of triangulating, using a panhard bar or using jacobs bars to keep the axle centered in the chassis. Keep your roll couple small and close to the ground for a good handling car.
Thanks for this detailed reply. Although I'm building a street cruiser, I'm trying to understand changes that that will improve handling without a detriment to safety or ride. All I've bought so far is lower bars (UMR Performance lower control arms with roto-joints at axle). According to the UMR website, roto-joints on the upper control arms will complete a triangulated 4-bar without binding inherent with the GM stock components. I also plan on plugging my car data into a 4 Bar calculator called "4_link_calculator, Trianged.xls" to better understand the set up.
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Old 09-24-2018, 10:05 AM
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3 link or 5?

Quote:
Originally Posted by joe_padavano View Post
He's not building a race car. He's building a 37 Buick street rod starting with the original frame. Designs dedicated for specific use on a track don't necessarily translate to what you want on the street. As for parallel, equal length rear arms, then you need a Panhard rod as well, so it's a FIVE link rear suspension.
Okay so by your reasoning the Art Morrison Enterprises' 3 link with Watts linkage is actually a 5 link, apparently even professional chassis builders get the nomenclature wrong?

My bad i thought the OP was building a Hot-rod!
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Old 09-26-2018, 10:25 AM
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Software for 4 link development

Quote:
Originally Posted by Mutt's37Buick View Post
"4_link_calculator, Trianged.xls" to better understand the set up.
Neet the power of Excel, fun with formulas, im working on a (simpler) cam program right now , i couldn't see how to change any of the pivot points in this one, that program looks like it was written for a 4x4 rock crawler. i used to use "Circle track analyzer" (very cool and flexible program front and rear) and then run a simulation. "steve Smith autosports" had some software too. I would think someone has written a software program for street rods by now? i was doing the circle track stuff in the 90's, let me know if you find any other software.
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Old 09-28-2018, 03:53 AM
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Mutt, it seems you may be over-thinking this: the GM B-bodies are big heavy and very comfortable cars, just like your car. They bare also very sturdy. So why try to re-invent the wheel? If you can mount these arms under your Buick replicating the original GM geometry, it should work really well! That aftermarket company tells you about the bind in the original GM design: sure, but has there been a lot of rear suspension failures due to that "bind" on B-bodies over the years?
On the other hand, if you like to design and create stuff, just forget the OEM stuff and just go full aftermarket, which may (or may not) work better than modified OEM.
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Old 09-28-2018, 11:03 AM
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Quote:
Originally Posted by wave1957 View Post
"but has there been a lot of rear suspension failures due to that "bind" on B-bodies over the years?
On the other hand, if you like to design and create stuff, just forget the OEM stuff and just go full aftermarket, which may (or may not) work better than modified OEM.
Thanks wave1957,
Thanks for the feedback. I don't think the products at UMR Performance are focused on avoiding failures due to bind. Their focus is improved performance for racing. I opted for their Roto-Joint that allows 28 degrees of total rotation and unrestricted movement through out the vehicles suspension:
Control Arm Roto-Joint End, ¾”-16 Left Hand Thread- GM B-Body [0020L] - $59.99 : UMI Performance, Inc.
Friends of mine have used "Johnny Joints" and "Ballistic Joints" for four wheeling which are similar in design. For my project, the added benefit of these joints on the lower control arms is that I should be able to use the stock mounts on the rear axle and pivot the other end in slightly to attach points on my narrower 37 Frame. You are correct that I'm probably overthinking, but now is the time before I start modifying the chassis.
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