Casting Bushings

[Soul Shinobi]

In short I'm going to explain everything I've learned, and what I plan on so far, but I've got a million questions too. I was about to buy the materials to cast my own polyurethane 80A hardness bushings (among other things) but I'm still on the fence on hardness, shrinkage, and even material (considering hard rubber like Group N parts). Hopefully this research should also clear up some misconceptions on the function of busings and different materials as it did for me. Please correct me if I'm wrong or forgot something!


Materials and Function

Rubber and Polyurethane--stock parts (suspension, shifter, and steering rack bushings, powertrain mounts, strut tophat surfaces) are made of a relatively soft rubber. Aftermarket Group N parts use stiffer rubber. Most aftermarket parts are made with polyurethane of varying stiffness depending on application and desired response versus NVH (noise, vibration, and harshness).

Most rubber bushings are bound to a metal sleeve in the middle of the bushing, this sleeve is secured rigidly to the chassis, or other suspension parts. Some are also bound on the outside surface, like with the front control arms. Because of this, rubber bushings act like a spring when the suspension moves, it's twisted and tries to return to its original position. You can screw up this effect if you tighten down parts with bushings bound on both surfaces while the car is not at ride height.


Polyurethane on the other hand can't flex and deform like rubber. This in part is why they give better response and consistent suspension geometry, but leads to a downside--polyurethane bushings have to pivot somewhere. Because of this there is a potential for friction, especially with higher hardness bushings. Polyurethane is supposed to be greased about yearly to prevent binding and static friction; this is very important, see sources below. I've just read on Turn In Concepts' site that they have graphite impregnated polyurethane bushings that are essentially self-lubricating, which is brilliant.


Must reads and sources:
Unabomber's Suspension & Driveline FAQ
Tech Topic - Polyurethane Bushings and Friction


Hardness

Aftermarket bushings vary in hardness from 70 to 90 shore A; fresh stock bushings are around 60-70A (maybe less; much less with age), 80 seems common for aftermarket, and 90 is pretty hard (hard like plastic to the touch). Shore A is the typical scale for this application, but sometimes harder polyurethanes are measured on Shore D (not something we'll be using, I imagine). As hardness increases so does response and feedback, however the downside is that NVH also increases, as well as the potential for binding/static friction.

A note on NVH: this drawback is not solely driver perception. NVH that's a result of replacing parts that give can result in more stress on some part, as well as accelerated vibration wear. In extreme cases very harshly setup cars can literally rattle bolts lose, oil filters can work themselves off (it's happened) and the chassis can fatigue causing dull response only curable by stripping it down and having it reinforced one way or another (riveting, spot welding, seam welding, roll cage, etc).




Applications

Rubber parts that don't have to rotate or move much can be replaced with polyurethane with the only downside being NVH. These include:

• steering rack bushings (13)
• rubber in some steering column u-joints (I think there are solid replacements for this, not sure about polyurethane)
• pitch stop bushings
• engine mounts
• transmission mount
• transmission crossmember bushings
• prop shaft carrier bushings
• rear diff and outrigger bushings (11, 8)
• rear subframe bushings
• strut tophat surface (I don't think anyone makes these with polyurethane)
• spring perch to spring separators (I don't think anyone makes these with polyurethane)
• endlink bushings for straight endlinks (front, some rear, 4)
• exhaust hangers
• rear shifter stay bushing

Bushings where rotation and therefor binding/static friction may occur:

• front control arm bushings (1, 2)
• rear trailing arm bushings (6, 7)
• rear lateral link bushings (5, 12)
• swaybar bushings (3, 9)
• endlink bushings for non-straight endlinks (most rear, 10)
• other shifter bushings

Current Plans

Right now I'm likely getting some two-part polyurethane mix with a hardness of 80 shore A. I plan to get grease for it, a mold release agent spray, and maybe coloring and a dry gas blanket to extend the shelf life of opened containers. I'm also considering getting some other kind of molding material of Shore A 20 to 40 hardness for making molds that I can pour the polyurethane into. Currently it looks like I'll be getting most materials from Smooth-On. I feel like there may still be a better source too, to you manufacturers out there, what do you use?


Resources

Information

Custom Bushes article on Autospeed
Hotrodders.com forum discussion of DIY bushings
DIY for making molds for casting polyurethane bushings
Polytek Tips for Success (for polyurethane casting)

Materials
Note that two-part polyurethane mix has a shelf life of about 6 months. Prices exclude shipping.

3M™ OEM Polyurethane Glass Adhesive Sealant (60-65A, no mix, $10.63/10.5 oz = $1.01/oz)
Polyurethane mix at Smooth-On (80A, 2:1 mix, $30.33/48oz = $0.63/oz)
Polyurethane mix at Smooth-On (90A, 2:1 mix, $30.33/48oz = $0.63/oz)
Polyurethane mix at McMaster-Carr (80A, ? mix, $31.24/16oz = $1.95/oz)
Poly PT Flex 70 4lbs at Polytek (70A, 1:1 mix, $60.00/64oz = $0.94/oz)
Poly 75 Series at Polyteck (59-90A)

SO-Strong® Color Tints For Urethane Rubber at Smooth-On
Poly Color Dyes at Polytek
Ease Release® 200 Mold Release Spray at Smooth-On
Pol-Ease 2300 Mold Release Spray at Polytek ($9.95)
Poly Purge Dry Gas Blanket for extending shelf life at Polytek ($12.50)
Mold-Release Lubricant at McMaster-Carr ($10.11)
Multipurpose White Lithium Grease with PTFE at McMaster-Carr (16oz Container, good for lubricating bushings, $5.32)

Questions (I will turn this into a FAQ as answers are provided)

1. Assuming I'm casting bushings inside their suspension components, what kind of shrinkage is acceptable or ideal? Shrinkage per inch varies from 0.0005" to 0.08"; the mix I'm looking at getting is 0.001".
   
2. Thoughts on hardness?
   
3. What materials would be good for making molds? I might have sticking issues with polyurethane on polyurethane.
   
4. How much does liquid cast polyurethane stick when a mold release isn't used? For example, if I'm using it to fill the gaps in a stock transmission mount, how concerned should I be about it separating?
   
5. What should be used to clean and prep surfaces I'm trying to mate with polyurethane?
   
6. What can clean polyurethane off?
   
7. Can I use rubber instead and make Group-N like parts? This is a serious consideration. I have found little on casting or molding rubber however.
   

Dear manufacturers and distributors, sharing any experience and facts would be massively appreciated! I want to know EVERYTHING.

I spent a couple nights writing this, if I forgot something in between or you feel some important aspect is missing, let me know.

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[Yint]

Nick, your crazy

[Soul Shinobi]

Like a fox.

[Turn in Concepts]

Good luck. Been there, done that, won't do it again.

Tony

[tom@kartboy]

dont forget you need a vac chamber to degass all materials before you cast them.
we still will prototype out a small number of bushings here, but all production items are done with a company that specializes in high volume urethane parts.

100% made in the USA.

[Soul Shinobi]

Ah, I recall vacuum casting from videos from Stillen on casting parts for their GT-R they ran in the 2009 Targa Newfoundland rally. http://www.roadandtrack.com/racing/m...and-sept-12-19 (last 4 videos). EDIT: Haha, seems I recalled incorrectly. I was thinking about their pressurized injection. Cool videos anyway, a lot more complicated of a process than I plan on getting into.

The polyurethane I'm looking at seems to expect open air casting, and I don't plan on using it for critical components (starting with endlink bushings and drivetrain mounts). Does all polyurethane benefit from vacuum casting, or are certain kinds intended for it?

[tom@kartboy]

Originally Posted by Soul Shinobi

The polyurethane I'm looking at seems to expect open air casting, and I don't plan on using it for critical components (starting with endlink bushings and drivetrain mounts). Does all polyurethane benefit from vacuum casting, or are certain kinds intended for it?

yes you would almost always need to degass the urethane before casting. you could then pour into an open cast mold.

<20+ years in composites, plastics and the like

[Soul Shinobi]

Reading up on it (link) it seems for my budget I can try to make use of nitrogen gas blanketing when possible, which I'm aware is only a partial solution. Curing parts in a closet with my dehumidifier may also help. That link also had some good mixing tips.

[biggreen96]

Tom- if one was to not degass before pouring into a mold what is the result? Bubbles? Eventual tearing?

I'm really interested in this project, since I'm building a rally Legacy on a budget.

[Power6]

Good thread, great info up top.

I've cast a few things out of urethane over the years, mostly filling up motor mounts where the aftermarket doesn't offer a stiffer part, or making a completely new mount.

Generally I use what is available from MCMaster, they'll have Shore 60A, 80A and 90A. 60A is great for filling in motor mount voids and still absorbs vibration, it feels rubbery. 80 and 90 are pretty stiff, they feel much more like plastic.

I think suspension bushings or any moving joint is going to be tough to cast at home in the open air with good quality. I only ever stuck to non-moving parts. But I only use rubber in moving joints on my cars so I haven't really looked further into it.

[Soul Shinobi]

Thanks for the input. I've put the project off until the spring since the stuff has a shelf life, I want a whole summer to mess around with it. I'll also stick to non-moving parts until I feel I have a really good idea of what I'm doing. I was thinking powerrtrain mounts as well as shifter and steering rack bushings to start. Got any pics of what you've done?

[ovr_bst]

From what I have read it is the best of both worlds, solid but with some give & also does not need lubrication.

[Soul Shinobi]

I don't know anything about delrin, but I've heard polyester is superior in some respects too. I know that the constraint with polyester is cost, but I don't know if that refers to raw materials or cost of forming it into bushings. I imagine delrin likely has similar limitations in terms of ease of manufacturing.

[sniper1rfa]

delrin will provide nearly no give - it's like 80D or so. Good lubricity, though.

[Turn in Concepts]

Originally Posted by biggreen96

Tom- if one was to not degass before pouring into a mold what is the result? Bubbles? Eventual tearing?

I'm really interested in this project, since I'm building a rally Legacy on a budget.

You'll get bubbles, and depending upon use and load could fail due to those.

We did our original prototype linkage bushings by hand. They were full of bubbles, but didn't see much load. As a result they stayed on my own car for something like 40k miles until we had that area torn apart again.

-Clint

[...CHUCKLES...]

I have used nothing, but DELRIN plastic in all of my STI's mounts as well as in my brothers EVO and it is well worth it.

plus far less mess, set up the mill or lathe and let it rock.

you wont be dissipointed.


If you decide to go with the mold then good luck and have fun.

[Soul Shinobi]

Considering getting back on this project. If you want to donate a mill or lathe then delrin would be great.

[XanRules]

I have no idea what's going on.

[Soul Shinobi]

Story of my life.

[Racerfab]

Cheap de-gassing chamber:

Big, orange Gatorade cooler (think construction truck)
Shop vac
PVC reducer/coupling
Silicon/liquid nails

At your local home improvement center, find a pvc section that will fit into/onto a shop vac hose. Using a hole saw of the appropriate size, bore a hole into your cooler and glue one end of the pvc into said hole using silicon or liquid nails. you now have a small degassing chamber.
Here's how you use it:
With your shop vac connected to the pvc, mix your stuff in a bucket that will fit inside the cooler. Put the bucket o' chemistry into the cooler. Put the top on the cooler and turn on the shop vac. You are now de-gassing. PI-YA!

[Soul Shinobi]

Great idea. I wonder how easy a one-way valve would be so you wouldn't need to leave the vacuum on.

[Soul Shinobi]

I'm very close to purchasing the urethane, I've e-mailed the company with a tech question and should order when they get back to me Monday. I also updated the first post with a good article on bushings I found on Autospeed (also available here).

Right now I'm doing more research on types of strength and shrinkage, and I'm have a real hard time deciding on hardness. Does anyone know the durometer of Group N bushings? Stock bushings or any kind? Knowing this would make my life a hell of a lot easier!

[williaty]

If I had a Durometer-meter, I could tell you. I've got both a stock and a Group N rear transverse link bushings sitting here right now.

[A-man07]

Originally Posted by Soul Shinobi

I'm very close to purchasing the urethane, I've e-mailed the company with a tech question and should order when they get back to me Monday. I also updated the first post with a good article on bushings I found on Autospeed (also available here).

Right now I'm doing more research on types of strength and shrinkage, and I'm have a real hard time deciding on hardness. Does anyone know the durometer of Group N bushings? Stock bushings or any kind? Knowing this would make my life a hell of a lot easier!

The concensus is that Group N bushings are 80ish durometer, but I've also read that this varies depending on location on the car and possibly year. Rallispec may have actual data. I've not asked.

A quality durometer tester is pretty expensive and probably beyond the average enthusiast. I've looked into it and just can't justify the cost. Because the aftermarket doesn't make what I want for my LCA bushings, I've contacted a company that I hope will help. If so, I'll post it up but most guys seem to be content with polyurethane.

[fastwrx25]

I remember reading the groupn bushings are ~80 for rules,
sti ~75
wrx~ 65