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

[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.

[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.

[Norm Peterson]

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.

I think that something quite similar has been done at least once. Buried somewhere I've got an article from Hot Rod magazine about John Callies' 303 powered import (something really little, 80" wb IIRC). It did involve some sort of moment decoupling of the leaves from the axle, and it worked well enough for him to trounce the "sports cars" of the day at a hillclimb. I don't remember all of the details, but might be able to drag up the article or at least a xerox copy of it.

Norm

[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

[Mean 69]

Backseat territory, but it really isn't that bad. I'll have to study this one a bit more, look at some of the literature. The torque arm approach is a cool deal too, I tried it on 65 Mustang, but the only way to really make it work without some good hacking is to run the arm under the driveshaft, takes away a lot of the ground clearance.

Maybe it is time to get educated on this deal!
Mark

[David Pozzi]

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.

Steve,
Sorry to mix in the mustang example. That one does not use a Z link, it uses a parallel four link design.

[BrianP]

Sorry Wally, I didn't intend to muddle up your thread. I'm thinking of bolt-on street/strip stuff to get rid of spring wrap-up/wheel hop during launches without compromising handling and your looking for full race 1st setups.

Here's a picture of what I was thinking about. It's a cage assy tied to the lower shock support. The axle housing would float on a delrin bushing. Add some collars onto the axle to limit side motion(or add a panhard bar). Build a crossmember that bolts in between the aft subframe bolts and add a torque arm.

[Steve Chryssos]

Nice. Looks more compact than the Competition Engineering housing floaters.

[MoeBawlz]

can soemone post pics and possibly label the parts that you speak of... Im curious.