Thread: B13 rear suspension re-assembly problem. Alignment all wrong. (Broadband only!)

I sort of remember the 3 studs on the spring hats only fitting in one way due to the 3 studs not being symmetrical, but I could be wrong. I will definitely check this out ASAP and get back to you. Not sure how these hats being indexed improperly would change anything, as they didn't seem to have any control over the rotation/direction/indexing of the lower assembly, but I'm all ears at this point.

I'm about to go check my rear parallel links to make sure they are the correct part numbers, but I can't find the correct part numbers. I'm searching, but if someone knows could you shoot me the numbers real quick?

I checked the parallel links and found no part numbers at all. It's possible they are aftermarket items and/or the wrong part numbers. I have the ABS mounting holes in all four of my links. I'll post pics if I get around to it. I'm going to look under my brother's B13 SE-R to use as a comparison. I'm hoping to find the correct parts on his car.

Either way, I've got a completely borked rear suspension including massively bent bolts. I decided to do a lot more research on the stock vs. ES bushing argument and have come to a couple conclusions that I'll post next.

B13 Rear Suspension Geometry: Binding With the Best of Them

When I set upon my quest to get rid of the clunking in the rear of my car I decided to freshen up the bushings in the rear. When ever replacing things, I like to replace them with better components if they exist. I figured the 14 year old bushings could be replaced with ES bushings or similar urethane components and I'd be better off. Like a good boy I went over to Shawn B's famous suspension thread to research the option before doing anything suspension related:

http://www.sr20-forum.com/showthread.php?t=1801

Low and behold there is an argument about the merits of replacing the rear suspension bushings with ES units. Wow, this is a first. I kept reading and this is what I found:


Citation: http://www.sr20-forum.com/showpost.php?p=24089&postcount=7

There were links to the discussions provided but I thought the summery was good enough so I didn't keep reading. It seemed to me that the issue was unresolved so I went ahead and ordered and installed the rear ES bushing kit. Boy was that a mistake. After the result I got, I decided to research the topic some more. I'd like to share what I found.

The stock B13 rear suspension geometry binds from the factory. Mike Kojima and Steve Foltz agree on this fact. These facts sort of slipped my mind when reading Shawn's thread. These are very important facts. Since the information is scattered around as to why it binds, how big of a problem this is, and how ES bushings or similar will make this problem worse I've decided to assemble the information here. Hopefully it will make it to the main page of Shawn's thread so no one else makes the same (stupid) mistake I made.


I'd like to start off with a diagram of the ideal suspension geometry as it is likely oriented during rest:



As you can see, the parallel links are at right angles with the chassis mounting points and the hub mounting points. In addition to that the control link is at right angles with the chassis mounting point and the hub mounting point. Each mounting point is essentially (but not effectively; more on this later) a revolute joint providing one degree of motion.



If the joints are treated strictly as revolute joints with one degree of freedom the suspension will not move. This is because the parallel links swing up and down together in the same arc, yet the control link swings up and down in a different arc. The control link's arc effectively shortens and lengthens the link in relation to the parallel links. As the suspension moves up towards the top of the car the control link gets "longer" as it moves towards the maximum of its arc. This effectively lengthens the control link with relation to the parallel links. Watch what happens when the suspension undergoes compression in this manner:



The parallel links deflect from their intended path caused by the control link "extending" due to its arc. As a side effect of the parallel links deflecting, the hub is also brought inboard. This causes the control link to deflect as well (but to a lesser extent than the parallel links).


A similar thing happens during suspension rebound. As the suspension travels away from the top of the car the control link gets "shorter" as it falls in its arc causing the parallel links to once again deflect (this time towards the front of the car) causing the hub to move inboard slightly, also causing the control link to deflect again:



As you can see, if the joints are treated strictly as revolute joints the deflections mentioned won't take place and the suspension won't move through any distance. The ability of the joints to deflect is directly related to how much bind one will experience. Here is a diagram showing the stock suspension bushing:



It shows how the stock units are designed to allow for this deflection by the physical shape of the material and the material properties (rubber being more flexible than urethane). Here is an ES bushing:



As you can see it hinders the deflection in two different ways. The material is much stiffer and the shape makes deflection much less likely.


Once all of this is understood you can see how different combinations of ES bushings in the rear may be acceptable, but having the entire ES bushing set will result in more binding of the rear suspension.
One ES bushing for the hub side of the control link has been suggested as acceptable if necessary as the ES bushing is mounted so that deflection isn't prevented as much as with the chassis mounted bushings. Other than that, I wouldn't recommend ES bushings at all unless you're well aware of the kind of bind you'll be dealing with and have decided it is acceptable.

The joints in reality need two degrees of motion. I've heard mention of spherical joints being used but have no experience with this. Take a look:



These would be perfect, and frankly should have been used from the factory. I'd love to see how people have installed these, or if there are parts to buy that will replace the stock bushings with spherical joints. A spherical joint allowing for both revolution and deflection along the joint would be an ideal replacement for all 6 joints in the suspension. It should literally remove all bind from the suspension and provide the solid feel most are after. I'm looking into this currently. I'll report back with findings.






February 17, 2010 edit: Apparently SoloSol has made some of these exactly as I described. Take a look.



And there is now talk of Blox producing something like it in this thread: http://www.sr20-forum.com/sentra-se-r/28014-any-interest-blox-control-arms.html

actually, quitre a few people have used heims in the rear to replace the stock parallel links. You can buy some threaded aluminum hex or round stock and thread the heim's in and there really would only be an added harshness in the ride I think. Not sure what others think, But I want to do this on my NX.

Brent

Makes me wonder if this might be a good starting point:

http://www.splparts.com/main4/parts/S13/Suspension/Multilink/FrontMultilink.htm#Front%20tension%20rod


I'm certainly leaning towards a custom solution if nothing out there is pre-made...

If only Ingalls Engineering's adjustable parallel links came with spherical joints like pictured above!

wouldn't having a threaded boss for the spherical bearings to sit in on the parallel links also allow for significantly more camber change? ie. longer would push out the bottom of the hub creating more negative camber?

also wouldn't toe be more easily adjusted by a spherical bearing trailing arm?

Yes you're right sqd. Ingalls Engineering sells (or used to sell) an adjustable parallel link pictured in the reply above yours. It allows for camber adjustment when both are adjusted, and toe when adjusted separately as you describe. The stock mounting bolts allow for some toe adjustment. This would allow for more.

hmmm... i wonder how much this would change the handling of the car. i was looking at your illustration of the suspension deflection up above, but i'm not sure if that would help the deflection issue. i have no doubt it would make a significant difference in the suspension bind issue, though. i'm very curious to see how this turns out.

The bind is due to the deflection. The deflection will happen no matter what you do*. If you try to fight it (with stiffer ES bushings) the bind will get worse. If you allow the deflection with no resistance (spherical/heim joints) then you'll remove the binding.



*Okay, okay. If you somehow mount the control link to the chassis with a sliding mount (prismatic joint) then you'll remove the deflection all together.
Hmm...
Now things are getting interesting. See pic below for visual:



Thinking about it, the control link's job is to control caster. Basically prevent the front to back motion of the hub in any uncontrolled manner. Replacing the chassis mounting location with a sliding mount would prevent it from performing its main function. This is not a viable option. Forget I ever suggested it.