Z - Link

[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