DIY Poly Bushings

Write ups and tutorials for maintenance and modifications.

DIY Poly Bushings

Postby ___Scott___ » Sat Nov 11, 2017 7:33 pm

Let me start by saying that I don't recommend this for anyone who just wants a new set of poly bushings. If you need new bushings, buy them pre-made, shipped to your door and ready to install. This write-up is for DIY'ers who just like to DIY stuff.

For me this was an exercise to learn how to use my 3D printer better and to learn how to create 3D models better. I used FreeCAD because it's free and runs on Linux. Thankfully there are a lot of tutorial videos on YouTube to show how to use just about every feature in the software. I assume there is a Windows version, but if you still run Windows there are better options than FreeCAD. Look at Fusion 360 for example.

I figured that if I could create a mould of the front lower control arm bushings, which needed to be replaced, it would be easy to mix up some poly and pour a new set. It sounded easy enough in my head.

I took a bunch of measurements and drew up the upper half of a mould:
Image

Then I drew up the lower half of the mould:
Image

The two halves snap together:
Image
Note that those models are the final version including tweaks after the successful version of the bushings were installed.

I'm still learning to use my 3D printer so I went through several iterations of printing the moulds. I tried a lot of different settings in the slicer and in the end, to get the pieces to stay stuck to the print bed, I printed each piece one at a time. Here is a pic of my "wishful thinking" all at once approach:
Image

Here is the first version of the moulds all greased up and ready for liquid poly. If you look carefully at the lip where the two halves snap together, you'll notice that initially I didn't run the lip all the way around. I did that to make it easier to snap the two halves together... that was a mistake:
Image

I chose SmoothOn Simpact85 mostly because of price. It's not exactly cheap, but cheaper than the stuff I used for the rear diff mount. It also claimed to be pretty durable and the Shore 85A hardness looked like it would be well suited to this application. I read everything I could find on their website about how to use it, but discovered that they didn't cover all the details. I encountered some surprises. The first surprise was that when it "kicks" it generates a LOT of heat. In hindsight that's not a surprise but they didn't mention it anywhere. Second was that since the working time is 4 minutes, you have to measure/mix/pour very quickly. Quick mixing means you end up with a lot of tiny air bubbles in the mixture. Add heat to that and the mixture expands when it kicks. As a result the PLA moulds softened and deformed at the worst possible time:
Image
Even if that hadn't happened, I typed a dimension in wrong and they are the wrong size anyway. Other than the total failure they came out pretty awesome I thought!

For the next attempt I corrected the dimensional mistake, I ran the lip where the two halves of the mould snap together all the way around, I bulked up the area on the top piece near where it snaps into the bottom piece, and I added a little extra material around the upper edge of both the top piece and the bottom piece to help prevent them from deforming. I printed up a new set of moulds, greased them up (different grease this time), measured/mixed/poured and it all looked great:
Image
The two halves of the mould are a lot trickier to get to snap together with the lip all the way around, but once together they are really hard to get apart. There's no risk of the expanding poly pushing them apart.
Unfortunately, this attempt didn't work either. One mistake may have been the temperature. The instructions say to use at 71F, and this attempt was at 58F (the first attempt was at 68F.) I figured that since the poly generates so much heat anyway, the room temperature wasn't _that_ important. Another factor that may have contributed to the failure was that I may not have mixed it well enough. The first attempt kicked while I was pouring and I didn't want that to happen again so I may have rushed the mix time a little. The thing that probably played the bigger roll in this failure was that I didn't get the mixture exactly right. It's necessary to weigh out the two parts accurately to get the "exact" 0.85:1 mixture. I overshot the weight target of the B component just a little. Instead of starting over, I just added a little extra of the A component. Bad idea. The poly set up with a consistency of cheddar cheese. Obviously that's no good for a control arm bushing.

I printed up another set of moulds and more carefully measured/mixed/poured this third time. Also this third time I printed up a little cone to sit on top of the mould so I could pour more quickly and didn't need to be as careful pouring. They looked like the previous set so I didn't bother with as many pics. Here is one after being pulled from the mould and the grease wiped from it:
Image
Here you can get a better sense of how much air gets mixed into the poly. I'm sure this reduces the hardness below Shore 85A, but they are still pretty solid.
The temperature was 66F for this final set but, like I mentioned above, careful measuring and careful mixing is super important to achieving successful poly projects.

I made the bushings slightly on the big side so they are a tight fit into the control arm. The steel sleeve that slides in the center (not shown) is a tight fit too:
Image

The bushings are only to replace the rubber one that's embedded in the control arm at the front. I haven't attempted to even draw up the bushing at the other end by the sway bar. They've been installed for about a month, so roughly 1000 miles and so far they seem to be holding up. The poly bushings cured a clunk I had in the front end that I thought was coming from the upper strut mounts.

It remains to be seen how long they'll last. The one on the drivers side looks like it did when I put it in but the one on the other side is discolored from being bathed in oil from a leak on that side. There was no mention in the poly claims that it's impervious to motor oil. It may not be, I'll find out.

I should also mention that this wasn't a quick and easy project. It took months of scarce free time. I should just buy a new set to have on hand when these die so I'm not tempted to repeat this project. If only I were that smart :doh:
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Re: DIY Poly Bushings

Postby underscore » Sun Nov 12, 2017 6:59 am

That looks like a lot of work! It's cool to know it's possible though, stuff like this will become very handy as parts become discontinued. Are the molds reusable or are they a one shot thing?
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Re: DIY Poly Bushings

Postby Tecker185 » Sun Nov 12, 2017 12:58 pm

Ive pouring my own rear diff mounts for years. Not as involved as this or as hard but a fun diy. I like projects like this. Its good idea i would have never thought of. Ill keep this technique in mind for when you cant buy off the shelf bushings for something. Good job sir


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Re: DIY Poly Bushings

Postby hoys » Sun Nov 12, 2017 1:54 pm

Great writeup and idea.

Would you provide the .stl for others to try this as well?
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Re: DIY Poly Bushings

Postby ___Scott___ » Sun Nov 12, 2017 2:08 pm

underscore wrote:Are the molds reusable or are they a one shot thing?

I made the moulds with the intent that they be one-time use items.

They are made with cheap PLA plastic. This stuff shrinks when heat cycled, so reusing PLA moulds may procuce unexpected results. Other higher temp plastics like ABS, nylon or polycarbonate will shrink less, so moulds made with one of them may work better when reused.

If I had intended to reuse the moulds, I would have "bulked" them up a lot. To support the poly while it cures, the mould doesn't need to be very strong. To get the part out of the mould requires either a break-away mould (which was my backup plan) or much stronger moulds.
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Re: DIY Poly Bushings

Postby ___Scott___ » Sun Nov 12, 2017 2:14 pm

hoys wrote:Would you provide the .stl for others to try this as well?

Great, now I have to get out of bed. :P

Sure, give me an hour or so.

EDIT: Here are the files (right-click, Save Link As):
Top half, FreeCAD source file.
Top half, STEP format file.
Top half, STL Mesh file.
Bottom half, FreeCAD source file.
Bottom half, STEP format file.
Bottom half, STL Mesh file.

I don't remember all the slicer settings, but I do remember slowing the print speed down to 15mm/s and setting the layer height to 0.2mm. My print nozzle is 0.4mm. I learned on a later project that PLA sticks to the print bed better and is less likely to warp if you set the print bed temperature to just below the glass transition temperature, which is around 60C for the PLA I'm using.

Keep in mind that these are for the ST165. I don't know how they differ from the ST185.
Also keep in mind that I reused the steel sleeve from an old set of SuperPro poly bushings. If you have OEM rubber bushings you'll need to somehow recover that sleeve or source something else that'll work. If you recover the sleeve from the OEM bushing, measure it carefully to be sure it's close enough to the SuperPro dimensions below that it'll work as-is, or adjust the dimensions of the bushing to work with it.

According to my notes, the SuperPro sleeve dimensions are:
OD = 24.15mm
Wall Thickness = 2.94mm
Length = 60.35mm
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Re: DIY Poly Bushings

Postby ___Scott___ » Sun Dec 10, 2017 4:39 pm

In the first post in the thread there was the question of SmoothOn Simpact85 being impervious to motor oil. The answer is no.

The precision that new control arm bushings brought to my steering has gradually gone away and I'm getting close to being back where I was with the old bushings. Visually, the bushings that get soaked in oil appear to be swollen compared to those on the other side that don't. A quick check with calipers shows the oil soaked bushings to be about 0.060" larger diameter than the non-soaked bushings.

I checked Whiteline and it appears they don't have anything for an ST165. GT4-Racing only accepts PayPal, so they're out. Any suggestions where to buy those bushings for an ST165?
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Re: DIY Poly Bushings

Postby fussellbug » Mon Dec 11, 2017 9:57 pm

Have you checked out the SuperPro SPF1044K?

http://superpro.suspension.parts/spf1044k
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Re: DIY Poly Bushings

Postby ___Scott___ » Tue Dec 12, 2017 2:59 am

Thanks for the pointer, I did take a look at those. In fact, that's what my old ones were that I wore out. I prefer my two-piece design, but I haven't ruled those out.

I'm starting to consider making more DIY style, but with different poly this time. I'm looking at poly options at the moment.
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