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· But Mad North-Northwest
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Discussion Starter · #1 ·
When I got my '91 Spider nearly 20 years ago the trailing arm bushings were worn out. My mechanic at the time replaced them with polyurethane in front and HD rubber in the rear of the arm, saying that the stiffer bushings gave better handling.

A few months ago it was time to replace them again, and following some of the advice on the BB I just went with the standard rubber given that the suspension is otherwise basically stock. After the change I started having issues with the car feeling super shaky on any sort of uneven pavement. Like, it was bad enough that I had to check a few times to make sure nothing had gone wrong with my chassis stiffener or the suspension torquing. Just a lot of shake and instability versus what it was before, especially in mid-corner bumps.

This weekend I pulled the stock bushings and installed the HD ones from Classic Alfa. Night and day: the car feels tight again and the shakiness is gone.

Car has stock springs, Koni red shocks set to full soft, and poly cone bushings and flat washers in the trunnion. Based on this result my recommendation is to go with the HD or poly trailing arm bushings as they really seem to tighten up the car.
 

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Very interesting.

The only thing I can think of is the bush bolts we're done up while the suspension was hanging. This would mean the rubber has to twist quite a lot once the car loaded and then the bush would be stiffer than it should be.

Theoretically you want no bush induced resistance so that's a +1 to poly. It would be interesting to do a lap time comparison on a bumpy-ish track.

Maybe in this location poly is superior ... ?
Pete
 

· But Mad North-Northwest
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Discussion Starter · #3 ·
Bolts in all cases were torqued to spec with the suspension fully under load. So I can guarantee that wasn’t an issue.

I even went back and retorqued them after the first change thinking I might have messed something up.
 

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The standard bushing are made by Akron. Same company that makes the round oem motor mounts. I think they use s softer rubber then original. There motor mounts sit lower then then oem once the engine is let back down. My opinion is the hd’s bushings are probably closer to original spec’s.
 

· But Mad North-Northwest
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Discussion Starter · #5 ·
That could make sense. I can't really tell, as I had the uprated bushings on there before the job and it had been almost 20 years since I'd driven the car with the original ones.

The heavy duty ones say they're spec'd at shore 80, which is the same as the Powerflex purple street polyurethane. Stock bushings are generally shore ~70. But who knows what's being made these days? All I know is that it was a big difference on the car, which I was not expecting.

In retrospect I kind of wish I'd gone with the poly, if only for ease of future removal. Getting the stock bushings in is easy but I just hate removing the old ones.
 

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The standard bushing are made by Akron. Same company that makes the round oem motor mounts. I think they use s softer rubber then original. There motor mounts sit lower then then oem once the engine is let back down. My opinion is the hd’s bushings are probably closer to original spec’s.
My experience also tells me that Jim's onto something here. After a motor mount changeout some years back, I also noticed the motor sat just a tad lower, compared to original. And now those aftermarket mounts are nearing replacement, although my original OEM mounts lasted nearly 20 years. Having owned my '91 Spider from new, I know these things to be true. I also had the OEM trailing arm bushings replaced in 2005, with aftermarket parts from a well known US vendor. Those bushings are now squishy, and I have that same set of HD (80 Shore A durometer) bushings from Classic Alfa ready to install. Tom's description of that "dancing" on irregular surfaces is what I now feel in my Spider, and it's not pleasant to deal with. The positive results Tom describes on this changeout to the HD rubber bushings is what I'm looking forward to.
Being an old rubber industry guy, I do know that rubber is not rubber is not rubber. There can be significant differences in how compounds are blended, and differences in basic polymers, oils and carbon blacks. The FI electrical connector boots and steering rack boots on our 164LS years ago, for example, were crap. They literally disintegrated within 4 or 5 years of ownership from new. There's no excuse for that poor a grade of rubber in a modern car.
So I tend to agree here with Jim's premise that the Akron aftermarket rubber parts simply lack the durability and longevity of the OEM mounts. Although their caliper/cylinder seal kits seem to last if properly installed.
 

· But Mad North-Northwest
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Discussion Starter · #7 ·
Good to have someone one here with rubber experience! Closest I got was a summer internship at a local rubber molding company back in college, lol, so I'm not going to be a good source of info on that one.
 

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I would have thought softer rubber would be better in this situation than harder rubber. We want the trailing arms to be able to rotate/twist easily. Harder rubber means more resistance I think.

The ideal rear suspension for best control would be rose joints. In that case the rotate/twist resistance is the absolute minimum. Remember the trailing arms do not control sideways axle movement at all. Softer rubber might mean more torque action movement of the rear axle I guess ...
Pete
 

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Don't know if this is relevant, but @DPeterson3 reported in these threads that "standard" rear trailing arm bushings purchased from Classic Alfa around 2020 were undersized in OD. CA replaced them with "heavy duty," which fit correctly:

(19) NOS parts versus new production parts -- quality | Alfa Romeo Forums (alfabb.com)
(19) CA "Uprated" Rubber Rear Trailing Arm Bushings | Alfa Romeo Forums (alfabb.com)

It seems unlikely that it would have gone unnoticed when installing if they were undersized, but just putting it out there.
 

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Good to have someone one here with rubber experience! Closest I got was a summer internship at a local rubber molding company back in college, lol, so I'm not going to be a good source of info on that one.
I would ask my father in law about rubber. He was a research chemist for goodyear his whole career. Has 3 patents to his name for tire rubber/synthetic compounds. But I wouldn’t understand anything after the first 3 words.
 

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My experience also tells me that Jim's onto something here. After a motor mount changeout some years back, I also noticed the motor sat just a tad lower, compared to original. And now those aftermarket mounts are nearing replacement, although my original OEM mounts lasted nearly 20 years. Having owned my '91 Spider from new, I know these things to be true. I also had the OEM trailing arm bushings replaced in 2005, with aftermarket parts from a well known US vendor. Those bushings are now squishy, and I have that same set of HD (80 Shore A durometer) bushings from Classic Alfa ready to install. Tom's description of that "dancing" on irregular surfaces is what I now feel in my Spider, and it's not pleasant to deal with. The positive results Tom describes on this changeout to the HD rubber bushings is what I'm looking forward to.
Being an old rubber industry guy, I do know that rubber is not rubber is not rubber. There can be significant differences in how compounds are blended, and differences in basic polymers, oils and carbon blacks. The FI electrical connector boots and steering rack boots on our 164LS years ago, for example, were crap. They literally disintegrated within 4 or 5 years of ownership from new. There's no excuse for that poor a grade of rubber in a modern car.
So I tend to agree here with Jim's premise that the Akron aftermarket rubber parts simply lack the durability and longevity of the OEM mounts. Although their caliper/cylinder seal kits seem to last if properly installed.
These are the mounts you want to use. Back in the day these out lasted the oem by 4 to 5 years. Back when these cars were daily drivers.

 
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I would ask my father in law about rubber. He was a research chemist for goodyear his whole career. Has 3 patents to his name for tire rubber/synthetic compounds. But I wouldn’t understand anything after the first 3 words.
Hell, he could sure instruct me alright! The rest of the story is the curing process, which varies greatly by method and parameters.
 

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Hell, he could sure instruct me alright! The rest of the story is the curing process, which varies greatly by method and parameters.
See already over my head. 😁
 
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Pete does touch on a familiar point of design compromise, that is compliance versus accuracy of alignment and geometry. I know there are other experienced Alfisti here with competition backgrounds who advocate for the OEM softer bushings at the trailing arms, presumably to help minimize lifting forces on the inside rear wheel in a corner. That's a valid argument, but then I tend to think more about precise axle location and maintaining the geometry designed into the car. I don't want any rear wheel steering, nor rear axle steering, if I can prevent it. Same argument for alignment of the front end, actually. I want roll control and roll stiffness to come via the springs and antisway bars on my cars, even though yes, I know that the these rubber bushings are clamped and in torsion under roll conditions.
 
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· But Mad North-Northwest
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Discussion Starter · #16 ·
It seems unlikely that it would have gone unnoticed when installing if they were undersized, but just putting it out there.
Can confirm that the original flex ones I got from Classic Alfa were definitely not undersized. Had to put them in with my 12 ton press, and they were the typical PITA to get out. So that wasn't the issue.
 

· But Mad North-Northwest
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Discussion Starter · #17 ·
I would have thought softer rubber would be better in this situation than harder rubber. We want the trailing arms to be able to rotate/twist easily. Harder rubber means more resistance I think.
So that's what I figured as well based on discussion here, but based on my testing that's not correct, at least not for reasonable ranges of rubber flexibility.

Good thing I love doing trailing arm bushings! Oh wait...no, I #!$% hate it :D

Me after the first test drive:

Textile Cartoon Gesture Art Creative arts
 

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Yes I admit to being confused or at least rethinking my assumptions here. It would be interesting to see a photo of your car taking a typical corner to see the body roll, rubber bush versus poly (so can you reinstall rubber and take this photo please ;) :D :D). My point here is that on the road maybe the rear body roll is not that much and therefore the twist component I have always worried about is mute

Pete
 

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FWIW, I went with the "street" poly trunnion bushings and the "regular" poly trailing arm bushings from Performatek here in the US.
Installation is very easy after the old bushings are out. No squeaks or harshness.
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