Excellent, I love to see actual scientific testing done. Very cool test rig.
Any feedback on how this bushing's spring rate compares to others from similar applications? Or some idea how that number relates to things overall (what does 6 kN/mm really mean here)?
I had the same thought as
@kmead; I assume that new bush was a aftermarket part and wonder how it compares to an original one.
I've noticed the rubber in old original bushings (that I remove from X arms) have become very hard and therefore would have a much higher spring rate? Can you test one of your old ones to compare with the new one?
Answers below!
Q1)
Any feedback on how this bushing's spring rate compares to others from similar applications?
A1) So most modern vehicles utilize an isolated subframe in the rear suspension. This allows for the subframe bushings (which isolate the structure that most of the rear suspension arms/links attach to) to be independently tuned from the primary suspension bushings in the rear arms/links. My observation/experience is that in most modern vehicles with multi-link rear suspensions, the primary suspension bushings are quite stiff radially (15 to 30 kN/mm on center) and the subframe bushings are one order of magnitude softer (maybe 1.5 to 3 kN/mm) [generalities only, there are many, many unique cases]. So this would lead to the thought that the X1/9 bushings, at a radial rate somewhere between those two, were specified to provide a good balance between isolation and handling. But this observation could be utter hogwash; I suspect the people who actually knew may sadly not be with us anymore!
Modern three point control arms that are not connected via an isolated subframe (mostly front lower arms but sometimes rear upper arms as well) generally have one soft (ride) bushing and one stiff (handling) bushing, with the position of each determined by the arm geometry in the vehicle. I'd guess that we don't see that here due to the economical nature of the X1/9 platform; the complexity was likely just not worth it.
Q2)
Or some idea how that number relates to things overall (what does 6 kN/mm really mean here)?
A2) What I'm reporting is the pseudo-static (low speed articulation) spring rate. This is the radial spring rate that the bushing is presenting to the vehicle during gross motion at relatively low translation rates (think of handling maneuvers). When I say "6 kN/mm", this means that the slope of the force-displacement curve is at or around 6 kN/mm in the linear portion of the bushing's travel around the zero load point. For every additional mm traveled, the bushing is presenting ~6 kN additonal reaction force. You can see on the graphs in the original post that the slope is beginning to change at higher load; the spring rate will eventually go non-linear after a certain amount of travel and the bushing will present a stiffer spring rate to limit motion (this is due to the changing confinement geometry of the rubber section being compressed). A bushing also has three other important static rate directions that are considered during the design process. These are axial (deflecting the inner sleeve along its long axis relative to the bushing), torsional (twisting the inner sleeve about its long axis), and conical/cardanic (twisting the inner sleeve perpendicular to its long axis).
Natural rubber (and other polymers) are viscoelastic materials, this is why I stated that this was a "static" rate. The dynamic rates of this (and any) bushing will be very different depending on load application speed and amplitude. These higher-frequency characteristics become very important when considering isolation!
Natural rubber (which the vast majority of all OEM suspension bushings are made from) also has a remarkable characteristic that is called strain crystallization. When subjected to an impact/high speed load, the rubber changes structure, briefly forgetting to act like a fluid and restructuring itself into a crystalline solid, protecting itself from damage/crack growth. Hard to believe that tree sap is such an important part of modern society!
Q3)
I assume that new bush was a aftermarket part and wonder how it compares to an original one.
A3) This I don't know; I got the new bushings from Matt. If they are aftermarket, they are very well constructed. It is absolutely possible that the rate I'm measuring does not match the Fiat original specification. I would LOVE to see the orginal Fiat technical drawing for PN 4290107...
Q4)
I've noticed the rubber in old original bushings (that I remove from X arms) have become very hard and therefore would have a much higher spring rate?
A4) This is correct, rubber does age harden and presents higher spring rate over time. Especially when pushing 50 years old!
Q5)
Can you test one of your old ones to compare with the new one?
A5) Unfortunately, my old bushings were in a pretty sad state; I did not test them. I'll see if maybe I can find an OEM one somewhere. It is worth noting for posterity, though, that the large cracks that present themselves at the end of old bushings do not necessarily mean that function is greatly degraded. Natural rubber, epecially when used in a bushing like these, usually has a low loading of anti-ozonant protection (which takes the form of a wax mixed in with the rubber and would make the part more likely to come apart in the application). The ozone cleaves the sulfur cross links in the rubber (resulting in the cracks), but it cannot continue this cleaving action into the highly compressed center section of the bushing.
Joe
