If that were true then as temperature got colder the bushings would get tighter on the steel control arms and I am seeing the opposite effect. The inner sleeve being steel and the bushing around it, then the bushing would contract more than the steel as it got colder and the subsequent sleeve fit would get tighter. I really think the plastic coefficient is alot less than steel.

Fred:
Think about what you're saying.....that steel is dimensionally less stable than plastic?

...we're not talking the "direction" of the instability...we're talking magnitude of whether plastic changes more ( or less) across a given temperature range than steel. Think of motor bearings and steel body panels and all the other things made of steel. Then think about the temp changes we're talkling about..hi /low ambient conditions vs a much geater temp change for ( say) hot vs cold engine operating temps...

Don't force me to open up text books on thermal coeficients of expansiion for various materails...but I sincerely believe the plastics will change more over a reasonable "ambient" temp range that you quoted.

---Wil Ferch

"Don't force me to open up text books on thermal coeficients of expansiion for various materials...."
- Ah, c'mon - where's your sense of fun?

Actually, I bet there are some plastics with less change with temp. ....

Then if 2 materials that are assembled with say a .001" clearance between the outer diameter of the steel sleeve and the inner diameter of the plastic bushing and that upon cooling down the assembly the clearance increases to .003" (assuming it is that much).....how is it that possible if the plastic had a thermal expansion greater than steel? Would not the inner diameter of the bushing shrink more than the outer diameter of the steel sleeve, thus giving a smaller clearance? Or am I thinking about this wrong?

Not to get too hung up on details, but this is what I do for a living....... Coefficient of linear thermal expansion for plastics is much greater than that of steel (say 5x-8x). Plastics will expand and contract more with changes in temperature in almost every instance. Take a look at the bigger gaps between Saturn body panels and see that the engineers at GM were well aware of this.

The other problem with plastic is that it has cold flow properties when a load is applied for an extended time. A bushing over a fixed diameter shaft will suffer from some degree of stress relaxation. The stress changes (loses its gripping power) while the strain (deformation) remains constant. If a load is applied on the outside of the bushing you have creep problems as well (deformation due to changing strain). A good product designer will take these factors into account with the part design and material selection.

COld flow is common. Can you say that all (that's ALL) plastics do that?

Don't you mean thermoplastic polymer's ;)

All common plastics will see cold flow when under a load for an extended period. I don't know of any polymeric materials that don't do this. I do know enough not to say "all" when dealing with polymers. I will say all plastics that a typical person has come into contact with behave like this.

You can get creep curves from most material manufacturers. It's obviously time / temp / load dependent so plastic product designers commonly use these to make sure their designs don't fail over time, at least before they want them to.

Now if you really want to get me going, I've got plenty of plastics geek stories regarding this stuff :). I've got a friend that works for a refrigerator compressor company in Alabama. I asked him why you see so many 50's refrigerators that have been running nonstop for 50 years but the more recent models have quit and been thrown away. He said "Oh, the old ones were a sealed system. We use O-rings now that we know will fail after x-number of years due to stress relaxation". People get a new fridge rather than replace the compressor. Also, it was stress relaxation of an O-ring that caused the first Space Shuttle disaster.

Sure let's hear somemore plastic geek stories...

BTW, what type of parts do you engineer?

I am fascinated by the things Canon & hp have done with plastics....

I was an injection mold designer, then product designer for about 8 years. I've been teaching in a plastics engineering program at a university in MI for the last seven. What they say about not going into teaching if you want to get rich is true :) I do get 22 weeks a year off which is great since I've got small children.

My "spring break" is coming up and replacing the bushings in my 911 is on my list. I'm not sure if I want to go with plastic or invest in the bronze type. It seems like people have problems getting the plastic ones to fit properly :confused:

The fitting problem is easy to describe...a bit more problematic to do.
For the front A-arms, the steel outer clamshells "compress" the original rubber bushings. For plastic, they alter the diameter of the hole and also make it non-circular. The "fitting" to the proper A-arm shaft clearance is done with the clamshell pieces bolted together around the bushing to mimic the installed condition.
---Wil Ferch

A "friction" fit of the od to the housing is not needed. It is better to have the od fit will no interference and then glue it in place. Then work on the id. All movement and critical fitment is at the id and the od should never move or rotate in the housing.

89911:
Let me understand and restate....
the outer dimensions of the poly bushing ( you're suggesting) should not have a crush-type interference fit with the clamshell housings that hold these in place? Glue instead?

If so ....
1.) what kind of glue do you recommend...and how good of a fix is this? Won't you risk this coming loose?
2.) Have you done this or know others who have..?
3.) Why not slight crush fit on OD and hone the ID with the "crush" in-place?

---Wil

mmmm, i have to go through this soon.

i am not sure about the glue method either wil.

what i intended doing was to clamp the bushing in the housing and by trial and error hone the ID to fit. Then mark the bush in relation to the housing to keep the same position on reassembly.

anything else seems like overkill to me ;)

I agree with Dickster, I would fit the OD so it fits very snugly and then machine the ID after it has already been inserted in the brace. Machine the ID to fit loose enough over the a-arm as to not bind.

When I fitted my rear poly bushings, I used a dremel tool with a sandpaper bit to machine the ID. I simply held it in place and quickly ran the dremel around the id with the bit also rotating. (I hope this makes sense.) The final machined hole may not have been perfectly round but it was close enough without using a lathe.

BTW, I used Elephant for the front so I only had to deal with the poly in the rear.

Noel

I just did this over the weekend and here is what I found... I can't take credit for any of this. All the things I did were taken from a variety of threads here...

I blasted all my parts then powdercoated them all at home. Very fun thing to do.

I sanded then filled the seams on the front clamshells with metal putty and let it set up.
http://forums.pelicanparts.com/uploa...1077551945.jpg

After that I honed all four mounts with a brake caliper hone form craftsman.
http://forums.pelicanparts.com/uploa...1077551998.jpg

Temp in the garage was around 50-55 and the bushing OD fit well. I coated the insides of the mounts with black silicone sealant, (slippery when wet, some adhesion when dry) and was able to slide all the bushings in with just medium hand pressure. No OD modifications where needed in my case. I must have been lucky.

I then sanded and polished the A-arm areas where they would contact the bushing. Arm on left is blasted, Arm on right is polished.
http://forums.pelicanparts.com/uploa...1077552448.jpg

Once polished, I measured the diameters several placed along each surface of the bushing. I wasn't soo lucky here and found that I needed to take off about 1.5mm of ID on the bushings to get a nice fit (not all were 1.5 and not all were the same amount across the whole bushing surface). I made the ID of bushings about .2mm larger than the OD of the arm. This is twice as large as was reccomended (.005") by others but because a majority of my driving will be at 70-90 temps I wanted room for expansion.
With the .2mm clearance, the bushings slide on and off smoothly and turn without binding. They are however snug enough that when dry they don't spin on their own but they will slide off if held at the exact angle. It may be too loose and normal temps, we'll have to see in a few months.

WARNING NOT FOR THE PERFECTIONISTS TO READ: I turned them down by taking an old screwdriver and wrapping the handle with duct tape until it was slightly smaller than the bushing. I then wrapped it in 220 sand paper and put it on my drill. I slowly sanded out the ID, stopping to cool the bushing often in cold water. As the bushing ID grew, I added more tape and new paper. It took about 2 hours to do the first one and an hour to do each after that. I went very slow to avoid taking off too much material.

When I got within .5mm of my desired ID. I drilled the hole for my zirk then used my air die cutter (like a dremel run by air) and cut a circle in the bushing surface, then I used a drywall saw to cut two opposite lines. After I did this I thought that maybe three lines would be better for balancing forces. The channels all connect to the zirk hole to allow grease to move in more freely and evenly. I then finished fitting th ID and installed the zirks that whould extend into the bushing a bit less than 1/8" That left room for the bush to give without the zirk contacting the A-arm, but also will hopefully pin the bushing in place.
http://forums.pelicanparts.com/uploa...1077553266.jpg
http://forums.pelicanparts.com/uploa...1077553292.jpg

They aren't on the car yet, but will be soon :)

Pat

Will I've done it both ways. Because of the large amount of surface area when the two surfaces meet, any type of adhesive (molding adhesive, 3m adhesive, etc.) will more then adequately retain the bushing in the housing. I then used zircs on top of that to further retain it. This technique was used on my brothers. All I'm saying is it is less crictical and should be as close as possible. What you don't want to do is drive this bushing into the housing and have it change the id appreciably, (make out of round). If done correctly, the point where the a arm and bushing meet should be almost frictionless. Call me if have and further questions. I can email you my number.

Thanks... understand.
--Wil