Learn me: Material selection for homemade poly bushings?

Hey y'all,

BABE rally, check, GRM Challenge, check. Onto the 3rd leg of my personal budget motorsports trifecta, a 14-hour road race at Road Atlanta, which brings me to a whole host of projects that need to happen on the Little Lamb-orghini, not the least of which is fabbing and replacing all the crappy 250k mile Lincoln Continental rubber suspension bits with something maybe a little racier, or at minimum, less worn out. I noticed that under hard acceleration it appears that the rear wheels move visibly forward in the wheel wells. That can't be good, and might explain why it can be a bit 'spooky' to drive. Last night I pulled, pressed, or gnawed out with my teeth all the rubber bits and drew up what I think should be appropriate polyurethane (or other material?) replacements. Now to acquire raw material. Jeez there are a lot of choices.

What material spec should I be looking for to replace bushings for control arms? Delrin? Expensive Graphalloy? Hyperlinks would be helpful, as I want to order material ASAP so I can machine them this weekend. Is there a source for off-the-shelf bushings that I can just order them and push them in (I doubt it, not a lot of demand I would think for hi-po Lincoln Continental parts) or maybe would be close enough with minor modifications?

Help greatly appreciated!

I machined suspension bushings out of Delrin for my Challenge car, discovered that it 'pounds out' easily. The car got spooky to drive due to all the slop from the bad bushings. Dunno about homemade poly, IIRC a couple guys on here have done it.

In reply to Curmudgeon:

Pounds out easily?

The bushings were machined to have a tight push fit of the inside sleeves at room temperature and the outside was a snug fit in the control arms. The idea was for the bushing to rotate around the sleeve, the sleeves were lubed with silicone grease.. After ~ 8 months of autocross and then the Challenge, it was spooky to drive at high speeds, I found that I could 'wobble' the rear wheels ~1". The sleeve holes were basically egg shaped and had probably .020 slop, which equated roughly 1" at the far end of the CA. I got a set of poly bushings intended for a Spitfire, those are still in there and snug.

Delrin and Nylon are pretty good materials, but honestly I am not sure how well they would typically work in these applications, as they aren't that strong. I have heard of folks sleeving joint, making the delrin portion like 0.125" thick, which would make it less likely to deform, but I cannot vouch for anything specific. Curmudgeon, how thick were yours?

Plan is to steel sleeve the inside of the bushings, using the stock steel sleeve on the outside for the control arm-to-subframe joint.

McMaster 8579K38 (PRICEY!)
VS the run-of-the mill
McMaster 8576K27
???
OR, better idea?

MrJoshua made some out of bathroom stall partitions... they looked great, I wonder how they're holding up?

Osterkraut wrote: MrJoshua made some out of bathroom stall partitions... they looked great, I wonder how they're holding up?

!?!?
The improvement in ride quality totally worth the awkward moment in the men's room with that heavy-breathing creepy guy at the adjacent urinal.

UHMW polyethylene is what you want. It is engineered as a bearing material (unlike delrin) and is super cheap.

Machinists like to turn bushings out of Delrin because it is so easy to work with. UHMW kinda sucks to turn.

In reply to ditchdigger:

As I am the machinist, I'll suck it up to get the right part. McMaster 5243T21 the stuff?

In reply to tuna55:

IIRC the wall thickness was something like .280. Fairly thick. The temps were not high, maybe a max of 110 degrees. I picked Delrin over nylon because it's not as prone to absorb water. Like ditchdigger says, man it machines nice, like buttah.

sporqster wrote: In reply to ditchdigger: As I am the machinist, I'll suck it up to get the right part. McMaster 5243T21 the stuff?

Yup. That is it. It is also very common so you might be able to find cheap offcuts locally from industrial suppliers.

It gets very stringy when you turn it, Expect to spend some time experiment with tool shapes and feed rates to get a compromise between surface finish and ease of machining.

Made these for the trailing arm mounts for my Fiat

You may find it easier to machine cold. Chill the piece in the freezer before each operation, and then machine it ASAP.

You can also buy T-shaped polyurethane bushings by size from here:

http://polybushings.com/pages/bushingsbysize.html

but yes, you need to freeze them to machine them and then lubricate them appropriately once installed.

Another option for complex bushings is to make a mold and poor polyurethane into the mold, then once set, freeze it and perform the finish machining.

I think Grainger or McMaster Carr might have some poorable Polurethane in various durometer ratings (most bushings are 60 or 80 it seems) or at least some generic polyurethane pieces that can be machined to fit.

Delrin tends to deform and "cold-flow" over time, so you'll have to check them and replace them a bit more often. they also don't flex very much, so for any locations that require movement in more than one direction at the same time, a proper spherical joint, polyurethane or rubber would be a better solution.

Midwest-control.com has some great prices on spherical joints (heim or rose joints, etc) You might find that the added cost would outweigh the effort required to build you're own bushings.

You could use the material that's intended for bushings: http://www.sculpturesupply.com/detail.php?id=700631&sf=searchtext&vl=pmc-780

sporqster wrote:

Osterkraut wrote: MrJoshua made some out of bathroom stall partitions... they looked great, I wonder how they're holding up?

!?!? The improvement in ride quality totally worth the awkward moment in the men's room with that heavy-breathing creepy guy at the adjacent urinal.

Trying to explain that I was drilling holes in bathroom stall walls to make my Miata faster didn't help one bit in convincing the officer that I wasn't a pervert.

On a serious note: the car is now a in house track car at The Firm in Keystone Heights.

I'm using Delrin (POM-H) for the suspension bushings in my GT40 project with 304 stainless steel inserts.
http://www.plastim.co.uk/pomh.htm

I think the 1-meter bar of delrin was $36 and made about 20 complete bushings (so 40 bushing halves) the inserts ran me about $3 a piece (so $60 for the 20 total bushings)

IIRC This is what some of the guys on GT40s.com are using/recommending... Too expensive for my tastes though.

good luck!

I've turned a lot of UHMW.

I use HSS ground with a lot of positive rake and ~razor~ sharp. If the chip - which can be an infinitely long string - wraps around the work, finish the cut the stop and slit the wrap w/ a utility knife, then continue. It can be easier cutting toward the tailstock so any wrap is in front of the tool.