RLCA Help: Spherical Bearing vs. Poly Bushing

[KellTrac]

....and let's not forget deflection under acceleration AND braking loads. (Fore and aft) Probably one of the worst flaws to a rubber bushing in this application.

Geometry and Alignment changes are rarely a good thing when tuning /dialing in your suspension system.

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[fatbillybob]

There is simply nothing good about having that bushing there except good NVH and cost. For performance and ride, it is hot garbage, and Ford should be embarrassed that they "engineered" that.]

I wish I did more research before I decided to race this chassis. Brian is right.

From a purely simplistic way of looking at it, the oem bush an unpredictable spring that changes with time and wear. What’s predictable is a coil spring and sphericals. None of the options is carefree. The rubber bush is not forever. In fact people leave them unattended for more time and miles than they should. The fall off in performance is so slow people don’t perceive performance degrading.

 

[Brian@BMVK]

I wish I did more research before I decided to race this chassis. Brian is right.

From a purely simplistic way of looking at it, the oem bush an unpredictable spring that changes with time and wear. What’s predictable is a coil spring and sphericals. None of the options is carefree. The rubber bush is not forever. In fact people leave them unattended for more time and miles than they should. The fall off in performance is so slow people don’t perceive performance degrading.

Even when new it's got no business being on a car like this.

 

[Brian@BMVK]

....and let's not forget deflection under acceleration AND braking loads. (Fore and aft) Probably one of the worst flaws to a rubber bushing in this application.

Geometry and Alignment changes are rarely a good thing when tuning /dialing in your suspension system.

Yup. That's immediately felt as soon as you power down out of a corner with a spherical in its place.

 

[Norm Peterson]

The off-axis quality of the motion probably makes the spring rate non-linear, but that's just a guess.

Off-axis rotation requires physical distortion of the bushing material, which in turn induces forces and moments in directions other than about the axis of the sleeve running through the bushing material proper. Generally, these induced forces aren't desirable or wanted.

The stiffer the bushing material, the greater these forces and moments become, and the end result tends to be some untunable amount of roll resistance. This is separate from the resistance to pure rotation as shown in SuperPro's video for pure "on-axis" arm rotation of the rubber- and poly-bushed arms. OEMs use relatively soft rubber at least in part to reduce the magnitudes of the off-axis effects, and sometimes even their rubber bushings are "voided" to reduce this effect still further. What the OEMs are attempting to do is approximating spherical joints with cylindricals, giving up a little suspension accuracy for cost and reduced maintenance advantages with only a minor downside in off-axis effects (that's negligible as far as most customers are concerned).

Polyurethane improves accuracy at the expense of increasing off-axis effects.


Norm

 

[fatbillybob]

Generally, these induced forces aren't desirable or wanted.



Norm

...nor predictable...

 

[fatbillybob]

I have a different kind of problem. My race class allows delrin or oem rubber. It really sucks! I have always lathe cut and hand fit my own delrin bushings for standard A-arm wishbone suspensions. Can you guys envision what a "delrin" solution would look like that could provide the on and off axis movement of a spherical?

 

[Brian@BMVK]

I have a different kind of problem. My race class allows delrin or oem rubber. It really sucks! I have always lathe cut and hand fit my own delrin bushings for standard A-arm wishbone suspensions. Can you guys envision what a "delrin" solution would look like that could provide the on and off axis movement of a spherical?

Not at all...

Your best bet to minimize the twisting portion of the bind is see if you can bore out the center and get a crush sleeve in there that the bolts clamp onto, rather than the indexing fingers that force the twist.

 

[TeeLew]

I have a different kind of problem. My race class allows delrin or oem rubber. It really sucks! I have always lathe cut and hand fit my own delrin bushings for standard A-arm wishbone suspensions. Can you guys envision what a "delrin" solution would look like that could provide the on and off axis movement of a spherical?

Yes, I'm busy right now, but I'll get back to you .

 

[Norm Peterson]

I have a different kind of problem. My race class allows delrin or oem rubber.

Any chance they'd allow a combination of Delrin and (mostly) OEM rubber?

I have a different kind of problem. My race class allows delrin or oem rubber. It really sucks! I have always lathe cut and hand fit my own delrin bushings for standard A-arm wishbone suspensions. Can you guys envision what a "delrin" solution would look like that could provide the on and off axis movement of a spherical?

If you only need a little off-axis flexibility, maybe take a drill and make your own "voided Delrin". You'd need to leave the middle solid Delrin to maintain radial location accuracy, but you ought to be able to create a little flexibility further out. It may take a bit of experimentation as to hole size and depth and the number of holes. The result won't be all the way to spherical off-axis flexibility but it ought to be better than solid Delrin.

Something like this, which I did to polyurethane for a previous car to minimize these same off-axis effects. I think I drilled about a quarter of the way through from each end, but I suspect you might want to go just a little deeper with Delrin.

Norm

 

[fatbillybob]

Any chance they'd allow a combination of Delrin and (mostly) OEM rubber?

If you only need a little off-axis flexibility, maybe take a drill and make your own "voided Delrin". You'd need to leave the middle solid Delrin to maintain radial location accuracy, but you ought to be able to create a little flexibility further out. It may take a bit of experimentation as to hole size and depth and the number of holes. The result won't be all the way to spherical off-axis flexibility but it ought to be better than solid Delrin.

Something like this, which I did to polyurethane for a previous car to minimize these same off-axis effects. I think I drilled about a quarter of the way through from each end, but I suspect you might want to go just a little deeper with Delrin.

Norm

Interesting idea. So the holes in delrin would allow the delrin to sort of yield and free up for the off axis. I wonder how many bend cycles the delrin could take? I think if delrin as pretty rigid vs poly u which is more elastic.

 

[fatbillybob]

Was this funny design just in the eco and gt? Did Ford at least improve the design for the gt350 and above?

 

[Brian@BMVK]

Was this funny design just in the eco and gt? Did Ford at least improve the design for the gt350 and above?

Nope...

 

[TeeLew]

This is something along the lines of what I'm thinking. The two outer races would stay fixed in their position. The 'ball' has a through hole with a curved profile which pressed into the tension link & accommodates a couple degrees of off-axis motion.

The inner sleeve would be steel and sized to accept the compression load of the bolt clamping load. An alternative approach would be to have the through hole with straight sides and machine the compression sleeve to accept the off-axis motion.

For what it's worth, I have a local machinist who could produce this, but low numbers are always expensive. I'm a little concerned about the ability of the Delrin to accept the bearing loads, so we'd want to limit the amount of curve on the hole & it would probably have some sort of interference fit.

Keep in mind that this is what popped into my mind when you posed the question. I'm sure there are many other ways of doing it.

 

[KellTrac]

Damn Ford Edge IRS.....

Drives me crazy.

I'm very hopeful Ford will catch up in the IRS game and make multi link IRS on the new S650

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