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/dept_id.L2~737/dept_name_p.Race+Products/dept_id_p.2/deptSearch_id.2/qx/prod_list_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. :screwy: 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?" :hmm: 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

[MoeBawlz]

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

[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.

[Mean 69]

I did a little more looking into this, and also read some bits in Milliken's book about it. It is a pretty slick little setup, I must say that there is merit in looking into applying to a PT car. One question, how do you mount the brakes onto the axle? It looks as if they float relative to the housing, at least in Milliken's example. I couldn't open the link for some reason on the parts you linked up. You could do a short torque arm, and get huge anti-squat numbers without the downside of expected brake hop, if you can address the brake "geometry." Do you have a pic on a car?

Mark

By the way, I made a mistake in the post regarding the truck setup previously, please disregard the comment about pinion angle being fixed.

[wally8]

You can do the brakes either way. You can clamp / weld the bracket to the housing (inside of the birdcage of course) or you can use a brake floater and run another link to the front to control the caliper brake torque reaction. A brake floater is a mini birdcage that has an attachment point at the top for a link (usually no bearing on these).

That, of course, gives you the opportunity completely control your braking reaction separately from your acceleration reaction. It used to be fashionable in the CT world but not so much anymore.

Just go to speedwaymotors.com and look under rear suspension for birdcages.


Wally

[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

[Norm Peterson]

Is anyone familiar with a Z-link type rear suspension?On a theoretical level from reading here and there and trying to picture the motions that it permits. Some of the reading goes back a ways, as I think there was an attempt by one of the Chrysler Corp Trans-Am teams back in the day to run this arrangement (IIRC, they never did quite get it sorted and bailed back to something more conventional for the era).

It seems that you get the ability to a scary amount of asymmetrical tuning at the expense of added complexity. Even a symmetrical setup for road course/high speed road timing/auto-x looks like it offers benefits in being able to tune one parameter without screwing anything else up big time. If I had to position this arrangement relative to others for kinematics and tuning options, it would probably look something like:

3-link
decoupled 3-link
Z-link with separate brake and traction pinion angle control

For street use (dare I suggest daily driving?), I'm curious about the long-term durability of the birdcages, specifically the surfaces that permit the relative rotation between cage and the axle. Or can this issue be solved by a method of simple (and inexpensive) refurbishment of the bearing surfaces?


Norm

[wally8]

Hey Norm,

I was wondering when you'd chime in on this.

I would agree with your ranking given an open design platform. I like the 3 link if there's enough room for a good length on the links. It's a very elegant solution as we say.

For the confines of a stock vehicle is where I see the advantage of the Z. I also see the same advantage as you in that you can tune individual things without upsetting other things so much.

Durability is actually pretty good. The good quality pieces use a sealed roller bearing that holds up really well. The cheap way is to use the axle tube as the bearing surface for the birdcage. I wouldn't go that route on the street.

You're right on about the asymetrical tuning. This is exactly the reason they've been used in CT. My main focus here is good rear steer performance with short links in a symetrical design.

Wally

[Mean 69]

That, of course, gives you the opportunity completely control your braking reaction separately from your acceleration reaction. It used to be fashionable in the CT world but not so much anymore.

The ability to decouple braking from the other aspect of positive torque reaction is really important. I stated you can get a killer amount of A/S by using a short T/A, for instance, but not taking advantage of the braking decouple, in this example, would tend towards brake hop, potentially (likely?). As an example, I know of an individual on another board that is going to a decoupled T/A on his fourth gen Camarao track car, that is very well sorted and documented, because he is now experiencing some brake hop under certain track conditions. I believe the SVSA on the stock Camaro T/A is of the order of 60"!!!

Anyway, using the Z bar in this config, would seem to me to "only" address roll steer, which is clearly important, but you can mitigate this by clever placement of the lower links on a three linker.

Mark

[brianj5600]

A recent article in one of the CT mags said it behaves like a standard four link. Moving the rear arm, IIRC, has the opposite affect on the movement of the axle. The article made it seem like there was no difference in the direction of the arm(s). The difference came in tuning changes are just opposite. They also said rear steer is good for a loose DT, but the better the traction, the less you want. How does all this transfer to a street car? I have no idea, but packaging w/o seriously modifying a chassis could be an advantage.

[Norm Peterson]

Rear steer is something that you can't avoid almost no matter what kind of stick axle suspension linkage or leaf arrangement you've got. It's just that the dirt track branch of motorsports has different requirements for gaining traction off the corners than you normally have for asphalt or concrete. And I think the range of how much steer you want is also wider on dirt, because conditions vary more widely, even over the course of a single event. That's why it's more of a discussion topic for dirt racing.

This effect normally goes under the term 'roll steer' when it's being discussed by OE suspension engineers, road racers, sports car tuners, and those whose entry to the technical side of automotive competition is more related to one of those than to any form of circle track racing. But it's really just a difference in terminology. FWIW, it wouldn't surprise me at all if some road racers use a somewhat different roll steer percentage as part of their setup for a wet race than they do for a dry event.

Norm