Triangulated 4 bar - what are the basics for installation?

[GUS]

Hello,

What are some of the "rules" for installing a triangulated 4 bar?

Are the upper and lower bars to be parrallel when at ride height?

When angling the top bars from the chassis to the housing, what
are the guidelines for mounting them?

Sorry if these are pretty basic questions but I want to make sure I understand the fundamentals.

BTW, I'm talking about an Art Morrison 4 bar in a tri-five chevy.

Thanks

[silver69camaro]

In a plan view, make the UCAs 35 degrees from axle C/L, and angled down about 6 degrees. Aim the LCA (side view) up on the chassis side, no more than 1 degree, about 1/2 degree is preferred.

[ErikLS2]

In a plan view, make the UCAs 35 degrees from axle C/L, and angled down about 6 degrees. Aim the LCA (side view) up on the chassis side, no more than 1 degree, about 1/2 degree is preferred.

Matt, would this apply to a 1st gen Camaro as well?

[silver69camaro]

It could, depending on packaging. You could reduce the side view UCA angle a tad to lengthen the IC (which reduces roll steer), but you'll probably have to cut into the floor. Or, you could aim it down to about 7 or 7.5 for more bite, but a smidge more roll steer and clear the sheetmetal (ride height dependent, of course). Keep the 35 degree plan view angle for the UCAs, if possible. No less than 30.

For our Camaro rear clip, it seems a 7.5 degree (roughly) angle on the UCA along with a parallel LCA works well.

[Rod]

the answer doesn't get any better than from the AM Engineer

[Rhino]

For our Camaro rear clip, it seems a 7.5 degree (roughly) angle on the UCA along with a parallel LCA works well.

Just for a little clarification, you were meaning the LCA parallel to the ground, and not the UCA correct?

The first time I read that I was scratching my head a little bit. Not sure if anyone else read it that way.

[exwestracer]

It could, depending on packaging. You could reduce the side view UCA angle a tad to lengthen the IC (which reduces roll steer), but you'll probably have to cut into the floor. Or, you could aim it down to about 7 or 7.5 for more bite, but a smidge more roll steer and clear the sheetmetal (ride height dependent, of course). Keep the 35 degree plan view angle for the UCAs, if possible. No less than 30.

For our Camaro rear clip, it seems a 7.5 degree (roughly) angle on the UCA along with a parallel LCA works well.

Matt,
To clarify, I believe you are referring to a minimum 30deg angle from chassis centerline, or 60deg included angle between the links, correct?

Also, the more down angle on the UCA the more likely the car will experience axle hop under heavy braking...nothing's ever easy.

[silver69camaro]

Just for a little clarification, you were meaning the LCA parallel to the ground, and not the UCA correct?

The first time I read that I was scratching my head a little bit. Not sure if anyone else read it that way.

That's correct. The LCA parallel to the ground on a side view.


Matt,
To clarify, I believe you are referring to a minimum 30deg angle from chassis centerline, or 60deg included angle between the links, correct?

Also, the more down angle on the UCA the more likely the car will experience axle hop under heavy braking...nothing's ever easy.

Oops, yep, I should have stated from vehicle C/L is where that angle is referenced from.

That's true about the brake hop. I don't know at what angle that begins to be a problem with that suspension, I've never had the IC short enough to find out. Typically I'll just get the antisquat as high as I can with minimum roll steer.

[Bryce]

Matt,

This seams like a good thread to ask this question in. What is the advantages of a triangulated 4-link versus a parralel 4-link?

[Rhino]

I can't speak for Matt, but the reason I went triangulated over parallel was to negate the need for an additional centering link (Panhard, Watts, ect...)

It also seemed a little easier to package in my car.

[silver69camaro]

I can't speak for Matt, but the reason I went triangulated over parallel was to negate the need for an additional centering link (Panhard, Watts, ect...)

It also seemed a little easier to package in my car.

That's pretty much it, packaging. Lots of lateral stiffness without the need of an extra link.

[Bryce]

Thanks Matt and Rhino.

[ErikLS2]

Matt,
It appears your triang 4 bar kit attaches the upper arm to the front part of the axle tube. How does this compare to say attaching it on top of the axle tube to either side of the center section (in the case of a 12 bolt)? Don't the stresses go down as the axle control arm upper and lower mount points become further apart from one another?

Are you saying the upper control arms forward mounting should be lower than the axle tube side to the tune of ~7 deg.? Most if not all the 4 bar systems I've seen on here are the opposite of that.

[exwestracer]

Matt,
It appears your triang 4 bar kit attaches the upper arm to the front part of the axle tube. How does this compare to say attaching it on top of the axle tube to either side of the center section (in the case of a 12 bolt)? Don't the stresses go down as the axle control arm upper and lower mount points become further apart from one another?

Are you saying the upper control arms forward mounting should be lower than the axle tube side to the tune of ~7 deg.? Most if not all the 4 bar systems I've seen on here are the opposite of that.

Not trying to speak for Matt about the Morrison setup, but I can answer your question in terms of general physics... The higher the upper arms are mounted on the axle housing, the better leverage they have to resist housing rotation from acceleration or braking, regardless of the angle of the actual control link. Also, the closer together the axle ends of the links are, the longer the links can be and still maintain the minimum included angle for stability. This reduces pinion angle change with suspension travel.

The downside for a conversion kit is higher links mean more floor cutting and fabrication to get the front upper link mounts where they need to be. I'd imagine the Morrison system is a well thought out compromise between geometry and practicality of installation.

As far as link angles go, OEM engineering tends toward braking stability rather than anti-squat under acceleration. Pretty much every OEM triangulated 4 link runs the upper links with up angle to the frame at ride height. When we modify those systems for better launch traction, the links should run down angle from axle to frame. Again, too much down angle will cause hop under heavy braking, as the geometry as very little resistance when the axle housing torque is reversed.

[ErikLS2]

Not trying to speak for Matt about the Morrison setup, but I can answer your question in terms of general physics... The higher the upper arms are mounted on the axle housing, the better leverage they have to resist housing rotation from acceleration or braking, regardless of the angle of the actual control link. Also, the closer together the axle ends of the links are, the longer the links can be and still maintain the minimum included angle for stability. This reduces pinion angle change with suspension travel.

The downside for a conversion kit is higher links mean more floor cutting and fabrication to get the front upper link mounts where they need to be. I'd imagine the Morrison system is a well thought out compromise between geometry and practicality of installation.

As far as link angles go, OEM engineering tends toward braking stability rather than anti-squat under acceleration. Pretty much every OEM triangulated 4 link runs the upper links with up angle to the frame at ride height. When we modify those systems for better launch traction, the links should run down angle from axle to frame. Again, too much down angle will cause hop under heavy braking, as the geometry as very little resistance when the axle housing torque is reversed.

OK, so I want to put a triang. 4-bar in my 69 Camaro with a 12 bolt. I would buy AME's universal kit for this and if I can make most of it work I probably will. I think it's designed for a 9 inch though and it doesn't look like the brackets for the chassis side would work as is either.

So, if I made everything myself and used the front leaf spring mount for the LCA's, and would be willing to cut into the floor a bit under the backseat, would it improve the geometry enough to be worth the extra effort? Keep in mind, I still have to have a backseat though.

P.S. Sorry if I'm hijacking here, I believe we're keeping with the theme of the OP.

[silver69camaro]

Erik, Ray's answer to the UCA angle hits the spot.

I was able to maintain good geometry and still retain the rear seat, so you should be able to as well. I would also urge you to reinforce the frame rail where any brackets are attached to - those rails are extremely thin.

[AMC Racer]

Do higher upper links give a higher RRCH and is that a disadvantage?

[Rhino]

It's not only the height of the links, but also their interaction with the lowers as well. Take a look at this thread (http://www.race-dezert.com/forum/showthread.php/59349-Finding-the-Roll-Center-for-a-3-4-Link-Rear). I think after looking at the pics in the first post you should have a basic understanding of where the roll center is located.

The second question is a lot more vague. Yes, it can be a bad thing to have too high of a roll center. You have to determine where you want the roll center, and build the suspension to fit. I've always heard a close approximation on many cars seems to be about axle level.

[exwestracer]

Do higher upper links give a higher RRCH and is that a disadvantage?

I'm oversimplifying, but in general terms, the roll center height on any linkage type suspension is the height of whatever component(s) locate the axle from side to side (panhard and watts link are easy to visualize...). This makes sense, as that is the only point on the suspension that is fixed in space in relation to the chassis. Everything else moves around slightly. In the case of a Tri-4 link, the RCH is the intersection point of the triangulated upper links. The wider apart the links mounts are, the further behind the axle the RCH is located. Also, the more down angle (to the chassis) in the upper links, the higher the projected RCH point is.

I have no hard data to back this observation, but I believe that having the angled links meet at the housing provides a better feel to the driver, as an R/C that is significantly behind the axle will allow the housing to move sideways slightly in roll. It's not that the links aren't doing a good job of controlling the axle, it's just the geometry and the location of the R/C. We do a number of "reversed" Tri-4 links every year for custom air bag suspensions on mini-trucks. With the links meeting just behind the cab, there is visible sideways movement of the axle in roll.

The "correct" height of the rear R/C is a complicated question...The previous poster is correct in that an RCH at axle centerline is a good starting point. However, looking at the case of a Chevelle with a Tri-4 link, the RCH is around the top of the pumpkin, or about 6" above axle centerline. The Chevelle is a naturally understeering car due to poor OEM front geometry, but the high rear R/C offsets this and helps balance the car.

Again, as a general rule, a lower R/C will provide more total grip, as weight transfer does not try to lift the inside tire as much. There will also be more body roll in cornering, which we don't like. Adding a sway bar artificially raises the roll center by forcing the inside tire to lift in roll.

[nprime]

Good stuff!

[GUS]

Wow, Thanks for all the great answers!

I'm learning a ton from this site. :)

[elitecustombody]

here is a bit more basic info as well as comparison between parallel and tri 4 link setup

http://site.streetissuecustoms.com/4linkFAQ.html

[Norm Peterson]

I don't think you want the rear geo roll center to be lots higher than the front, at least not for paved road driving. The car's balance tends to shift toward (more) understeer as roll develops, since most of the spring and bar resistance to roll will be up front.

High mounting the uppers also tends to increase the amount of rear axle roll steer in the direction of heavier vehicle understeer and poorer slalom behavior. You can offset this somewhat by tinkering with the inclinations of the lowers either with axle side "relo brackets" or by raising their chassis side pivots, and by not having the uppers run downhill toward the chassis, but you can't tune much of the roll steer effect out by playing with sane levels of spring/bar stiffness.


Norm

[ErikLS2]

I don't think you want the rear geo roll center to be lots higher than the front, at least not for paved road driving. The car's balance tends to shift toward (more) understeer as roll develops, since most of the spring and bar resistance to roll will be up front.

High mounting the uppers also tends to increase the amount of rear axle roll steer in the direction of heavier vehicle understeer and poorer slalom behavior. You can offset this somewhat by tinkering with the inclinations of the lowers either with axle side "relo brackets" or by raising their chassis side pivots, and by not having the uppers run downhill toward the chassis, but you can't tune much of the roll steer effect out by playing with sane levels of spring/bar stiffness.


Norm

So Norm, in my case my lower link will basically replace the front portion of the leaf spring and therefore will probably have a slightly higher front mouting point. Most of what I've seen favors a lower arm that is level at ride height in side view. Are you saying a slightly upward inclination of the lower arm CAN be overcome with proper angle and positioning of the upper arm?

I'm trying to figure out if I can get away with more involved fabrication/vehicle modification for only one pair of the arms. It would be ideal for me if I can focus that effort on the upper arms and have an effective system.

I'm not looking to set any autocross or lap time records, just a car with predictable handling and good ride quality would satisfy me.

[Norm Peterson]

If you're swapping leaves out for lower links, the lowers would be essentially parallel in plan view and you would want them to be level, maybe just a tiny bit downhill toward the chassis, for minimizing roll steer. This is kind of a special 4-link case where the amount of roll steer follows only the LCA slope, independent of the rest of the arrangement. Advantage - it's relatively easy to get low amounts of roll steer.

If you're using an OE triangulated 4-link arrangement or something similar where the lowers are skewed inward, you'll want the lowers running slightly uphill instead. This is the more general case, where roll steer is determined from the point where the UCAs converge and the point where the LCAs converge. Harder to get low roll steer numbers. Easy to get roll oversteer when shooting for large values of anti-squat.

Either way, rear RC height isn't different.


Norm

[malibu43]

hi guys,very interesting posts.
i run my 80 malibu as a road course,the lower arms are parallel to the ground at ride height,but the confussion are the upper,they has to be parallel also?,mines are lower from the axle side this set up its ok for my kind of race or not,please somebody clarified to me.
thanks

[Norm Peterson]

If your lowers are parallel to the ground as seen in side view, you want the uppers to run slightly downhill from axle to chassis (also as seen in side view).

You want some anti-squat, which your arrangement is giving (geometric construction lines drawn through the uppers and lowers as seen in side view intersect ahead of the rear axle and above the ground).

But if you want a little less understeer coming from axle roll steer, you might be better off with the uppers parallel to the ground and the lowers running slightly uphill from axle to chassis. Whether that can be physically done, done without adversely affecting ride height too much, or done legally for your class are separate questions.


Norm

[ErikLS2]

If your lowers are parallel to the ground as seen in side view, you want the uppers to run slightly downhill from axle to chassis (also as seen in side view).

You want some anti-squat, which your arrangement is giving (geometric construction lines drawn through the uppers and lowers as seen in side view intersect ahead of the rear axle and above the ground).

But if you want a little less understeer coming from axle roll steer, you might be better off with the uppers parallel to the ground and the lowers running slightly uphill from axle to chassis. Whether that can be physically done, done without adversely affecting ride height too much, or done legally for your class are separate questions.


Norm

Is one of the above configurations more desirable for what most of us are trying to accomplish? Since I'm making my own and am willing to cut into the body a little bit, I'm hoping to put together as ideal a design as possible but I'm having a hard time getting my mind around all the different angles at the same time.

[Norm Peterson]

If you're pretty much making your own and none of the pivot locations need be stock, that makes a difference. I'd pick a roll center height based on other things, choose some approximate values for anti-squat (50%?) and roll steer (3%?) and see what it takes to get everything to come together (and still be constructable). Keep in mind that while some ranges of roll center height with a triangulated 4-link are easy to obtain, others are not since the pumpkin occupies the space where you'd like to be putting the UCA axle-side pivots. Work your exhaust around what the suspension ends up being, rather than compromise the suspension around what your initial thoughts of what the exhaust "should look like".

You'd certainly end up with a different solution if you, say, lowered the axle side UCA pivots like AME did (Super Chevy, August 2010). Rear roll center height is lower and roll steer is less in the AME arrangement than with the G-body's OE.

Just looking at the AME, it looks like it would be better at autocross than with where my '79 Malibu is at with its OE (my uppers are level, and the lowers run slightly uphill to the chassis - and it still hates slaloms). With the AME setup, I'd probably need a little more rear bar, though. Slalom behavior may not be quite as important on most road courses, but I don't think I'd want to use that as an excuse for leaving too much roll steer in the setup.


Norm

[malibu43]

NORM,thanks.
But i need to know what will the best arrangement to this kind of race road course,its because theres not rules involved in where to put the uppers and lower rear arms.
The uppers in the pumkin side am trying to leave the way there goes,and the lower ones am trying to leave the way they are from the chassis side.
Any help? i need to get thoses left and right hand turns very fast.
thanks again

[AMC Racer]

Maybe triangulate the lower links instead of the uppers? Easier to get neutral roll or roll understeer with decent anti-squat and a low roll centre ... if that's what you're after.

[Norm Peterson]

If it is necessary to hold the axle side of the uppers and the chassis side of the lowers of a G-body where they are, you can still reduce the roll steer via relo brackets with carefully placed adjustment holes that incline the lowers uphill toward the chassis slightly. I'd still prefer to see the uppers horizontal in side view, with some balance between antisquat and roll steer sought via LCA inclination (axle side relo bracket adjustment hole height).

That won't be able to do much about the sky-high rear roll center, though, since the RC height is determined primarily by the height of the axle side UCA pivots. UCA inclination affects rear RCH slightly, and making them horizontal in side view will drop the roll center slightly if the uppers are currently inclined downhill toward the chassis. This also has a minor effect toward reducing the roll steer.


Norm