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Arizona ZCar Rear Suspension - Design, Function, Flaw(?)

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I've never really seen a good picture of the Arizona ZCar rear suspension installed.  It seems to me that with three pivot lines in the suspension that the control arm could buckle inward around the main big tube.  With the weight of the car pressing the strut outward, and the lines of force mostly centered during cornering it apparently doesn't happen.  But under the right (wrong) conditions I don't see how it could be avoided.  This would allow the wheel to move inward quite bit.

 

Am I missing something?   Has anyone heard of problems with this system?  I'm pretty sure that I made a similar comment about someone's home design in a past thread.  Didn't realize that AZC was using it.  Attached a picture that I borrowed from Whitehead Performance and a simple drawing to illustrate.

 


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post-8864-0-00023500-1407358727_thumb.png

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It took me a while, but I see where you are coming from. 

 

I don't think what you describe is plausible with a loaded suspension. With the car on the ground, the force vectors would be pointed down and away. The car would want to push towards the earth, and the strut would want to bow outward. So the arm would be "pulled" taught. Even during cornering, the only way for the arm to buckle would be if there was a sudden force perpendicular to the arm towards the differential with the suspension unloaded, and given that the car is still on the ground and the suspension actually being compressed via the weight moving to the outside, then to the back under acceleration, the LCA would still be "pulled" not allowing the arm to buckle.

 

And there is also the half-shaft which would prevent an abrupt shortening.

Edited by seattlejester

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I didn't consider the half-shaft.  Good point.  As far as loads though, the outside wheel will be exerting a load directly across those three pivot points.  The half-shafts and diff may well be taking up the load when buckling occurs.  I wonder if people with the AZC system have extra diff or half-shaft problems.

 

I've got locked in to wrong understanding before.  But I think that the design here lends itself to buckling.  Under normal driving conditions, the shift off center is probably small.  But in a hard corner, with body roll, I think that it has to being trying to buckle.  Once it's off center, there's nothing to stop it.  Except maybe the half-shaft at full compression.

 

 

As noted in another thread, maybe the big tubes don't spin.  The assembly pieces, plastic (end bushings) and various pictures on the web (varying angles of the attachment points) seem to indicate that they do.  Maybe I just don't get it.  Why wouldn't you just attach the heim joints so they have the same pivot center as the original arms?

Edited by NewZed

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My only thought on why they manufactured the piece that way is to allow for either extra adjustment or maybe even to take up slack when the half shaft is indeed compressed, or maybe simply for ease of removal. I still believe given that the strut mount and the LCA mount to fixed locations on the chassis, and the only piece that can compress even a slight amount is the strut the relationship is actually quite rigid. 

 

A more in depth way of looking at it would be a triangle. One side is the strut with the spring, one side is the chassis (strut mount to lca mount) a fixed length, and the last side is this buckling joint. Now if we put forces on the triangle, then the strut will always be counteracting gravity. Given the wheel is mounted on the outer fulcrum it would provide an outer force. The spring compresses. Now for an ideal layout the bottom of the triangle (the buckling point) would be relatively level to the pavement. So the strut is adjusted until lca is level. A sufficiently stiff spring is employed to keep the lca from changing this relative angle too much and cause radical camber changes, so in turn one other side becomes rigid. So now you have two rigid sides with the tire that is pulling up on the corner fulcrum, what happens? The LCA gets pulled out. At up the force vectors and the system cancels out. Car sits as it should.

 

918F2DCB-2CA6-4050-8810-6A71C4A62CDF_zps

 

For the system to buckle, there would have to be a net force that would "compress" the LCA causing it to buckle, not sure where such a force would come from.

 

4C0528D4-63A0-4517-90CF-B8DE087A2BB0_zps

 

I do recall a discussion of the ideal design and some more advanced terms were used to indicate that a fixed point to fixed point would not be an ideal situation (like the stock setup, TTT, or others) as that would introduce bind in the system somewhere. The end result was a separate adjustable link in the middle

 

Found it!

 

IMG_13201.JPG

 

This is amateur bench racing at best. Haven't tested either setup, although I hope to copy myron's in the future.

Edited by seattlejester

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As noted in another thread, maybe the big tubes don't spin.  The assembly pieces, plastic (end bushings) and various pictures on the web (varying angles of the attachment points) seem to indicate that they do.  Maybe I just don't get it.  Why wouldn't you just attach the heim joints so they have the same pivot center as the original arms?

 

I had the same thought. Why didn't they make it square or hexagon, and have fixed points to rotate it to?

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I brought this up when Dave first released it. I think now he suggests using a set screw so that the inner tubes can't rotate. My suggestion to those who use this suspension is to figure out where you want the pivots and then weld a tube across to positively locate the others.

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I brought this up when Dave first released it. I think now he suggests using a set screw so that the inner tubes can't rotate. My suggestion to those who use this suspension is to figure out where you want the pivots and then weld a tube across to positively locate the others.

 

If each of the inner rods had a lever at the end, you could use an adjustable bar with rodends to keep a set distance.

 

Or ever a simple round flange with mutiple holes around the edge, allowing whatever is top most to be bolted to the mount to stop rotation.

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The Arizona Z car stuff shortens the lower control arm significantly, if you lock the tubes in place; the heim joints are your pivot point.

Dave's design would allow you to raise or lower the inner pivot point(to change ROLL CENTER, Camber gain, etc), except that the half-shaft will not allow much upward displacement of the inner pivots.

A better design would use the original inner pivot points, but lower the OUTER pivot points with a cradle or some sort of offset bar across the outer pivots on the castings.

Edited by bjhines

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Silvermine Motors is using the same design.  Looks like they are aware of the roll center change possibility (better geometry, below), but are just using the clamping force of the stock brackets to keep the bars from spinning.  Also looks like they could easily have put the heim ends on the same center line as the original arms.  Interesting how these things come about, I wonder if the original goal was the geometry control or if it was recognized after the fact.  

 

http://www.silverminemotors.com/datsun/datsun-260z/suspension/rear-lower-control-arm-cnc-for-240z-260z-280z

 

"These arms allow for full adjustment of the suspension geometry without having to remove any parts! They allow for on car adjustment of camber, and also the length is adjustable and the pick-up points can be raised for better geometry. Constructed of high grade T6061 aluminum. They can be anodized different colors per customer’s request. These control arms are extremely strong compared to factory parts, and they look BEAUTIFUL! Work great for extreme driving conditions. Please allow 2-3 weeks for delivery."

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Thanks for that.  I looked through the AZC site for instructions or descriptions but didn't find that page.  Funny that the page is called the "lowarms" page, but the picture doesn't show them lowered.  Running them lowered would put a lot more turning torque on the inner tubes.  And still not clear why you'd have plastic sleeves if you're "locking tightly".  The two things don't go together.  The overall path to this point seems cloudy.

 

Still curious if anyone's seen any signs of movement.  But if people are happy with the design, there's no need to change it.

Edited by NewZed

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Lots of people have run them with no problems. gnosez makes a point of pinging me every now and then: "Still haven't broken them." Cobramatt did have a problem. I asked him what happened and didn't get a solid answer. 

 

Agree with bjhines that he should have done it differently. If you're doing the suspension from scratch, why ape the stock crap suspension with aluminum? He could have eliminated the spindle pin, lowered the outer pivots, incorporated bigger brake brackets and a stronger upright, centered the wheel in the well, eliminated the H arm and incorporated an A arm and toe link, etc all in one move.

 

The silvermine parts look even worse from a threads in bending perspective, but at least they aren't relying on "really tight bushings" to prevent movement of the pivots. I'd still put a tube across and weld it with their setup, FWIW.

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I never understood it either.

 

The only benefit I've seen is raising the rear roll center. Changing the inner pivot can raise the roll center but bringing the inner pivot further outboard also negatively affects the camber curve. I think it's a poor design but looks like it's been run "successfully"...

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 It seems to me that with three pivot lines in the suspension that the control arm could buckle inward around the main big tube.

 

Yes.

 

It's typical of many aftermarket offerings:  Wonderful execution of really bad ideas. 

 

It is just irresponsible to put these on the market.  The design flaw is BASIC engineering, and obvious at first glance with minimal understanding of the loads seen by the suspension. 

 

I don't know what is worse this rear design or the AZC front design.  I've ranted on those before, suffice to say they have the same problem, 3 pivot points. 

Edited by rejracer

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The rotation of the inner main tube does happen which plays havoc with the suspension settings. The lower you put the pivot point on the bar (to raise rollcenter) the more problematic it becomes. What also adds to the problem is how the short nose uses the tubes for a front mount. The torque applied to the tubes from the diff movement will rotate the tubes. We pinned the tube with set screws and put flat 1/8" stock between the diff mount flanges to keep then from lifting. No more problems.

 

Joe

Edited by rags

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I don't know what is worse this rear design or the AZC front design.  I've ranted on those before, suffice to say they have the same problem, 3 pivot points. 

Not seeing the problem with the pivots on the fronts. Looks like there is an inner pivot for the LCA, a TC rod pivot, and a ball joint, just like stock. Could you explain?

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The TC rod pivot point on the front control arms is behind the control arm. When braking the compressive loads will want to rotate the control arm. Granted the AZC implementation is the best of these flawed designs because it is such a thick piece. The same basic design could be made much better by moving the pivot point forward to the center of the control arm.

 

Actually my original rant on these is based on ignorant people wanting to put these on daily driven street cars and not inspecting them. The design is good (AZC design only) if kept in good condition. But when the bore that the compression rod bolt goes through wears, and it's just a matter of time, the same twisting will occur under braking.

 

The big problem with a front control arm or compression arm failure is that it usually ends up in a spectacular accident with much damage done to the chassis. With the rear design when it fails at least the strut does not collapse under the car, it's just moved way out of alignment.

 

Once again, my comments concerning this design are in application. The front design is fine for racers who inspect the car after each race.

 

Does that articulate the problem I see in the TC Pivot point location?

Edited by rejracer

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