Z - Link

[wally8]

OK, in the interest of adding some tech, here goes.

Is anyone familiar with a Z-link type rear suspension? Don't feel bad if you're not the fellas over at cc.com didn't know what it was either (except for one).



It's basically a Watts link type fore/aft location for the rear axle. There's a lower link that goes to the front of the car (usually on the bottom of the axle) and another link that goes to the rear of the car (from the top of the axle). They pivot on "birdcages" which are link mounts that pivot on the axle housing either by means of a bearing or just a bushing type fit. Here's a link: http://www.speedwaymotors.com/xq/asp...st_display.htm

What's the potential advantage? You can use short links for a packaging advantage and still get very good rear steer performance (better than anything else actually, it's a watts link remember).

A possible disadvantage is that you still need to control axle rotation either by using a top link (same one as you'd use for a 3 link. they're called pull bars in CT) or a torque arm (or lift bar as we call them in CT). A torque arm can be a good packaging solution with good performance.

Other problems: This can really only be used on a circle track style floater rearend. You must have a 3" axle tube and you have to be able to slide the birdcage onto the tube (no axle bearing retainers allowed). I don't see that as a down side since this would be the optimal rearend for a pro-touring app anyway and they're very competitive cost wise. You will take a bit of a weight penalty with the birdcages althought they are available in aluminum to save a little. The last possible disadvantage is that they aren't super cheap. However, it is still much cheaper than what I see other vendors' solutions going for.

If someone wants to volunteer their 1st gen I'd be glad to whip one up for you :-) Just drop it off in Iowa........

Let the discussion begin........


Wally

[Mean 69]

I'm in. But, it'll have to wait a moment, or few hours.... I am a big time Padre's fan, they are battling for first place with the D'backs. Tomorrow. Let the tech begin, I live for this stuff.

M

[wally8]

Too bad. I'm rooting for the D'backs. Of course I'm actually a Dodgers fan when I'm not pulling for the Yankees.

That's right, I admit it, I'm a Yankees fan. I'm even going to Minneapolis next week to see them play the Twinkies.


Wally

[BrianP]

Here’s an idea I’ve been toying with in my mind. What if you used the concept of the birdcage, but keep the leaf springs? The leaf springs would control the position of axle fwd/aft and sideways but the axle would be free to rotate. Then add a torque arm similar to a third gen camaro tied into a crossmember attached to the front subframe.
With this idea the springs would take all of the suspension forces and the torque arm would take the acceleration and braking forces. You wouldn’t have the problem of wheel hop caused by the axle trying to twist up the leave springs. You would not need a panhard bar or watts link to control sideways movement. You also wouldn’t have to move or modify the attach points of the suspension to the unibody structure.

[Matt@RFR]

I am by NO means a suspension expert, but, to me, that sounds like a whole lot of work to still end up with a leaf spring suspension.

With all the talk about truck arms, 3 and 4 links, satchell links, etc., it seems (from my outsiders' view) that any one of those would be a better choice if you're wanting to do all that fabrication anyway.

Comments?

KQM052, after this, if you want to dig deeper into your idea, please start a new thread so we don't muddle up Wally's thread.

[wally8]

That idea has essentially been done already in the CT world (pretty much any rear suspension config you can think of has been tried there).

It was typically done without birdcages though. The springs are light enough that they wrap pretty easily (you're only talking about 6-9 degrees of pinion rotation even in CT which is pretty radical). Rear slider bearings are used for the leaf springs and usually a top link with a panhard bar. The leafs won't quite do the job on locating the car laterally.

In any case, it's been pretty much abandoned by most (everyone?). Most cars run 4 links with birdcages these days because they can dial in the rear steer that they want. Many have a Z-link on the left and a four link (half of a 4 link actually) on the right.

If you're going to go that far, ditch the leaf springs and go with real links and coilovers. Like Matt said, if you want to go over it some more let's start another thread for the sake of keeping the search criteria straight.

The main advantage I see for a Z-link in this app is packaging as a result of the short arms while still maintaining favorable rear steer performance, albeit at the cost of a torque restraining device.


Wally

[wendell]

Wally,

Out side of the LMSCs I'm not real up on my race cars but...

I've only seen Z-links on real light dirt track machines like migets and outlaws. I figured that the birdcages wouldn't be strong enough for a 3300lbs car planting 12 inch tires. Whats the biggest car that a Z link is used on? Are there parts that are tough enough for a street car?

[Steve Chryssos]

I was up in Canada visiting Pete Bicknell last month and saw a z-bar/4 link design on their dirt mods. Most interesting part of the design? No springs!! A torsion rack is mounted at the tail end of the suspension. Basically torsion bars that are mounted perpendicular to the vehicle centerline within two chrome moly tubes. Arms hang of the ends of the torsion bars and connect to the quick change axle. Wana adjust the spring rate? Just slide out a torsion bar or two and slide in another.

Wally,
How far are you from Boone. We should meet up in September.

[wally8]

I wouldn't worry about strength. The biggest cars are in the 2500 to 2700 lb range which is still lighter than our cars but they're used on rough tracks at extremely high speeds. The fastest late model I've heard of on dirt is Bloomquist at 178mph and that's what he's running.

They're set up for 5/8" and/or 3/4" heims which is plenty beefy. I can't offer any actual numbers though. Usually in a crash it's the links that sustain the damage (they get ripped off). The birdcage at most is just bent.

Wally

[Steve Chryssos]

I am by NO means a suspension expert, but, to me, that sounds like a whole lot of work to still end up with a leaf spring suspension.

With all the talk about truck arms, 3 and 4 links, satchell links, etc., it seems (from my outsiders' view) that any one of those would be a better choice if you're wanting to do all that fabrication anyway.
Comments?

The thread is more of an exercise in suspension design. It's not really supposed to be a look at an alternative street car setups, but rather a look at radical suspension design in general. The roundy-round guys--especially dirt--are by far, the most creative at isolating forces SVSA is routinely set at infinity. They've got links going every which way. Just pick any link in the design and say "Okay, what in the world does this link do?" You can look at these designs and work your way backwards to get a better understanding of traditional suspension designs. It's not for everyone. I'm fortunate to have Doug Gore, a renowned racing engineer, in the rental car with me on a regualr basis. We're both bored, and I'm ignorant, so we pass the miles with him explaining (or trying to explain) suspension design to me.

[wally8]

Steve's got it right, the CT guys are way out in left field with what they're doing. Just shows how radical you can get when you're only trying to do one thing.

The 4 links as done on an LM are pretty complex (they use birdcages too). I've been around them for a lot of years and I still find myself asking questions about this or that and many, many times about why. It seems little of the CT evolution has happened as a result of theory. There's a lot of experimentation. When you apply theory to it, many times it makes no sense. Sometimes it's very complex and takes awhile to work throught the theoretical background. PHB jacking was / is a fad right now but some people are figuring out that a car still works best when you use all 4 tires. I've still got a ways to go before I'd be considered an expert in that arena (or any arena for that matter).

I wouldn't want to get into a CT 4 link in this application. It's a little too complex for this and really is doing some things opposite of what you'd want here. There's bar angles and lengths to consider, locking one cage or the other, spring indexing...... It goes on and on.

Steve,

I'm up here where Iowa, South Dakota and Nebraska come together. I'm close enought to meet you there and I could talk a buddy into a road trip to see some racing. Let me know later this summer if it will work out for you and we can see if we can set it up. Maybe I'll pull the camper down and try and find a spot somewhere.

I used to do my National Guard drills in Boone and most of my buddies went to ISU (I'm a Hawkeye, myself).

Let me know. Where are you, BTW? (I should check your profile, I guess). EDIT: I guess that LOCATION: NEW YORK would give it away :-)


Wally

[CoryM]

I looked into Zlinks briefly just out of curiosity. It can be a very tidy package with lateral mounted torsion bars and it certainly looks cool. Most of the sprint car guys here have gone to coilovers instead of torsion bars though. As you said Wally, the only thing I see going for it is the short links, and the ability to crank up the antisquat to pretty extreme amounts. Then again, I'm pretty green on this stuff so I miss lots.
http://68.116.83.184/CP/index.html This guy started a Zlink 2nd gen trans am, hasn't updated for a long time though.

[wally8]

Yeah, the torsion bar thing was interesting but doesn't seem to be the hot ticket these days. I've noticed the same thing. I personally have never used torsion bars so I don't have any real info on them.

I need to go look for some good pics so people can actually see what I'm talking about.


Wally

[David Pozzi]

Here’s an idea I’ve been toying with in my mind. What if you used the concept of the birdcage, but keep the leaf springs? The leaf springs would control the position of axle fwd/aft and sideways but the axle would be free to rotate. Then add a torque arm similar to a third gen camaro tied into a crossmember attached to the front subframe.
With this idea the springs would take all of the suspension forces and the torque arm would take the acceleration and braking forces. You wouldn’t have the problem of wheel hop caused by the axle trying to twist up the leave springs. You would not need a panhard bar or watts link to control sideways movement. You also wouldn’t have to move or modify the attach points of the suspension to the unibody structure.

One of the Ford Trans Am teams, I think it was the Bud Moore team, used a narrow lower leaf, it was boxed in and allowed to slide on it's mount connection to the rear axle, then four links were added and those controlled the axle movement, not the spring.

[David Pozzi]

I'd like to see how a Z link would work hooked directly to the axle without birdcage. The links would have to be pretty close to parallel to the ground.
The advantage is the mounts are allready there!

I've secretly wanted to explore this option but not put much time into the geometry of it. I kinda had a feeling it would be a pretty good setup but need to do the "Homework" on it.

[wally8]

Hey David,

When it was brought up at cc.com that was the interpretation and that was how it was discussed. They didn't realize there were birdcage pivots. Someone posted a pic but if you didn't know what you were looking at it wasn't apparent that it was a pivoting birdcage.

Personally I wouldn't think it would be a good arrangement without the birdcages (which are lockable, BTW). You're going to have pinion rotation because of suspension movement and vice versa. The key is to separate the two interactions. Control the torque individually and control how the axle pushes forward on the car individually (through lower arm angle).

In any case, if you want to see what they came up with for kinematics the cc.com thread is a good read. It didn't fair too well. I think you can get the thread by searching for "z-link".


Wally

[David Pozzi]

Thanks, I'll take a look. I thought that a mount for testing could be made that bolts onto the spring perch and held by the U bolts, it could have a top and bottom mount on it for the Z links and you wouldn't have to weld brackets on the housing. The links could be easily connected up front and rear and only a panhard bar and coil over mounts built.

I suspect not much anti-squat would be available, maybe you could tweak the link angles to improve things.

[Steve Chryssos]

So to recap, the leaf springs are free to control bump and rebound only. Some Comp Eng housing floaters might work well. The Z-Bar links control axle rotation under acceleration/braking. And a Panhard Bar controls lateral movement.

Define which bar controls acceleration or deceleration meaning upper bar (above axle) lower bar (below axle) and fore/aft. I'm guessing the following arrangement:
-Lower/Forward bar: Prevents axle rotation under acceleration
-Upper/Rear bar: Prevents axle rotation under braking

The leaf springs still have some control over fore/aft movement as well. All you really eliminated is axle wrap.

[Mean 69]

There was an example of this shown on cc way back, I don't recall the thread though. It was on a off road truck, used without the birdcages. The pinion angle is fixed throughout the suspension travel, but I really don't recall how it looked from the other aspects of performance. Decoupled suspension systems are really cool, no question that being able to adjust one aspect without affecting others is neat. Just tougher to package, and I'd think that for most applications, probably not worth the extra effort?

M

[wally8]

The true Z-link (with birdcages) is a packaging dream from my point of view. I would think some smart guy could come up with a bolt on kit that uses a bolt on torque arm that would give great performance with the short links that would be required in the stock confines. I don't have an F-body or I'd be taking some measurements.

It is a little complex and there are a lot of pieces and maybe a slight weight penalty but I could see doing this with no floor or back seat cutting and it should work really well.

You probably have a little more insight on this particularly in how it might compare to the 3 link kit you're working on. I'm guessing that you're either having to make a compromise on link length our you're getting into the backseat territory.


Wally