Hi Guys,

I've disassembled the rear of the bumblebee and the control arm pivots and bushings look to be in good shape.

My plan was to blast and powdercoat the control arms, which would require removing the pivots/bushings (due to the heat)...but I hear re-installing these can be a beeotch. And I generally prefer keeping as much OEM as possible.

So...do you think it's worth leaving the bushings and pivots alone and just painitng the arms? I'm aiming for this bumblebee to be a solid #2 car when done.

Thoughts? Thanks!

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Too bad we're 2000 miles apart. We could do a video that would simplify this process for others.

Yes, 100%

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I once did the bushings on my ‘75 years ago and it was a PITA. I had access to a good shop press, but it was getting everything lined up and stable enough to press everything in/out.

I was contemplating the same thing on my current control arms and decided to paint them instead due to the bushings being in good condition. Personally, a good paint job will be almost as good as powder coating and won’t require as much effort, IMO.

914Rubber has the kit now.

Unless bushings are showing signs of dry rot, cracking, or having become delaminated from the trailing arm, I'd leave them OEM for a street car. Photos of your parts look pretty good. I wish I had a set like that! Mine are chunked and cracked. I'd really like to find a decent set of arms rather than rebush mine. We'll see what turns up before I need mine done.

The OEM bushings are vulcanized into the trailing arm and will have a torsional, lateral, and longitudinal spring rate to them. Replacment bushings will never be quite the same and cannot duplicate that rate.

Nothing against aftermarket bushings since in some cases, it simply can't be avoided. Same for track use - there are advantages to changing them.

Once you veer away from OEM, you end up in all the other debates:

Rubber vs. Urethane vs. Delron vs. metal bushings (like self oiling Bronze) vs. needle bearings and/or spherical heim joint solutions all of which have their Pro's and Con's both for ride & handling and NVH.

I think you just tipped the scales, Kent. Part of why I got this BB was to do videos documenting every step of restoration. Thanks for reminding me why I shouldn't skip this. I'm happy to take your reccos, though!

I remember @eric_shea talking about how the bushings need to be jb welded in to the arm. Any other tips?

@Superhawk996 you make valid points. Do you think that the modern replacements are inferior...or just different?

Thanks, everyone for chiming in. And Jeffrrey
@914_7T3 ...wow!

I thought jb weld was for polybronze bushings, not rubber?

Even if they are 100% intact that rubber is still 46+ years old. Rubber ages faster than probably any other material a car. While they may be original bushings guaranteed they do not ride like "OEM" anymore.

I installed the Rebel Racing solid ceramic bushings in the front of one of my cars and despite having a stiffer than stock swaybar and SOLID bushing its still the most comfortable riding 914 i have. 46 year old rubber is trash.

Yes, removing them is a royal PITA and installing (if you stick with rubber) is probably even worse, but if you want your car to be right you have to do it.

I'll start with the fact that I'm biased toward OEM both because I work in the industry and because I used to be involved in chassis tuning. I know how much work goes into engineering bushings and it's not as simple as most think it would be.

Short answer. Because they will be different.

Long answer. Porsche engineers knew what they were doing when it engineered the bushings as vulcanized parts. What might seem like minor differences in bushing design (press fit vs. canned vs. vulcanized to inner/outer cans) can make a huge difference in ride, handling, and NVH.

I'd be a whole lot more inclinded to replace the bushings had they been designed to be replaced. If they were a canned bushing that could simply be pressed in/out of the trailing arm, I think I'd certainly do it. But . . . that is not what they are.

I think there are lots of good reasons to replace a set of obviously worn and/or damaged trailing arm bushings, but, personally, I'd never do it just for the sake of doing it. Though rubber does age, it's a matter of degree and in this case, most of the rubber is vulcanized within the trailing arm and isn't exposed and won't age out as fast as a fully exposed bushing. Yes, the rate will definately be different than it was in 1973 (my car's age). Without data on the installed rate of the OEM bushings vs. new aftermarket bushings of a different design, we are all guessing.

To be completely transparent, it's a hard question to answer the question without data acquisition and some objective evaluation metrics to compare against.

You'd be surprised how evaluation bias (knowing a set of bushings has changed) can over ride subjective comparisons. There have been times when I drove cars blind (not knowing what was changed) and I thought I knew what was changed, made a subjective assessment, only then to find that the objective data didn't match the subjective assessment, and that the parts which were changed were not those that I thought. I.e. I was completely wrong!

Driving parts blind is something that good development engineers do from time to time to keep evaluation skills sharpened and to keep themselves honest. Most of us working in garages on our hobby cars can't do this (we know what was changed) and are inclined to think that we've improved things even if we haven't. Sometimes the metrics change (like wanting improved handling vs. OEM ride or OEM NVH) and that justifies the improvment. Other times, it's just wishful thinking. Been there done that.

@Superhawk996 I like asking dumb question so here is another. Do you think there would be any benefit in coating replacement rubber bushings with cold vulcanizing cement before pressing them in? I know you can't really answer without having data to know what specs we are shooting for but curious if that would help bond the bushings to the tube which seems to be what vulcanizing does.

I don't think I would.

I should be careful with the term vulcanizing before I get eveyone wound up. Vulcanizing isn't really a bonding process.

Vulcanizing is the process of modifying the rubber itself for durability and elasticity.

I belive that the OEM bushings are probably bonded (probably with chemicals & heat - thus the term vulcanized bond) to the trailing arm and the pivot shaft. The other option is that they are press fit so tightly that for all practical purposes, they are mechanically gripping the shaft and trailing arm to the point that they cannot move relative to one another.

So why not bond them with the cold vulcanizing cement?

My reasons would be:
1) Not sure the cement is compatible with the aftermarket rubber and/or could cause degradation to the rubber.

2) If or when the cement bonding gives up and breaks free, it might actually then work against you by allowing easier movement of the parts realtive to one another. Relative movement will lead to rapid wear and won't have bushing rates anywhere near OEM intent rates.

3) From everything I've read (remember my trailing arm bushings are shot), the amount of press work and swearing associated with installation is pretty intense. Having to deal with the possibility of having the cold vulcanizing cement partially curing before I get everything seated home and then inhibiting installation seems like a risk I wouldn't want to take.

Having said all that, I haven't done this job yet myself but the pitfalls are almost enough to make me consider poly-bronze bushings and I know I'd hate those for a street car based on prior experience with them.

I don't wan't to promote myself as some sort of expert on 914 trailing arm bushings. I'm just sharing my rationale on why I'm not inclined to change out a set of trailing arm bushings that aren't otherwise damaged based on my professional experience with the nuances of bushings. There is more to them than meets the eye.

Ask yourself this question, how many vintage cars, trucks, vans, etc. are out there on the road that haven't had all the bushings, trans mounts, and/or engine mounts replaced wholesale? Quite a few, and they still get around just fine. Could they be better? Yup, I'm sure of it in some cases. Could they be worse, yup, lots of cars out there with all sorts of aftermarket bushings & mounts that actually made them worse.

No way to know with certainty without objective data.

Based purely on reported outcomes with the various aftermarket rubber bushings from our usual vendors I think they will be just fine for our hobby cars and I don't want to come off as snubbing them. I'm glad the parts are availble. It's really just the question of effort vs. reward. Each of us will have a different sweet spot.

Thank you sir! As you know, I'm always looking for that extra inch to match OE quality but your answer is pretty much what I expected.

OTOH, I've had a vehicle rendered unsafe to drive due to deteriorating bushings. Old Pathfinders are kind of notorious for it. Wholesale bushing replacement brought the car back to like new.

Unfortunately, the bushings on my 914 were shot and dry rotted so no choice but to replace. I have a set of 914Rubber rubber bushings ready to go in so will be watching for Ian's video

Me too

Will be replacing with rubber and I look forward to Ian's video too!

Yup, me too. Michigan potholed and frost heaved roads destroy bushings fast.

Not sure of Pathfinder bushing design but most modern bushings have way more rubber and voids than a 914 trailing arm bushing which is sort of an apples to oranges comparison.

Modern bushings usually allow for a whole lot more travel (kinematic recession), isolation, and they are used to influence steering behaviors via compliance. All that travel and flexing will lead to them wearing out in different ways vs. 914 trailing arms. Luckily, more often than not, modern bushings are designed to be replaced unlike 914 trailing arms.

I've had cars where the bushings weren't able to be replaced and had to replace the whole control arm.

@914Sixer

I have yet to find the rubber bushing on their site. I ordered the only bushibg they had and they were poly which i will not use.

When i restored my 73 1.7 i went with the rebel racing front and rear set up and i love the way the car feels on the road. Of course it was more money but so easy to install it was so worth it.
And we have some pretty crappy roads here is in northern california too.
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How should the rear trailing arm hang?

Without shocks or the axles hooked up my trailing arms fairly quickly fall till they point close to straight down.
My front A-arms stayed relativity fixed with the old rubber bushings, but now I have McMarks full motion (bearings) bushings they also will hang straight down like the rears.

I'm debating whether to rebuild the trailing arms or not, but I'm thinking seriously about bracing the console as I have 250+hp.
If I let her rip up to 7K rpm I can feel the ass end getting a little squirrelly, mostly from the right rear.

If you decide to paint use Imron. Used on truck frames and trailers, pretty bullet proof.

@bdstone914 ; There you go

I have a set that will go in your rebuilt trailing arms. Not sure yet how I will proceed. Hoping to see a video from Ian

https://914rubber.com/rear-trailing-arm-bus...for-porsche-914

I got the rear bushing kit from Elephant Racing. First one took about 45 minutes and the second one was about 10 minutes. Was easier with a heavy duty bar clamp than my press

@Mark Henry

The bottom line it can hang whereever you want.

But here's the considerations why that is true in your case.

When you migrate to a low friction trailing arm mount like Bronze Poly, Delron, Rebel Racing etc, you esentially have no torsional rate from the bushing and it will hang free as you state. There is no torsional rate contribution to the suspension kinematics so it doesn't matter where you start from.

Why would you want a torsional rate from the trailing arm bushing?

The short answer is that it does two things:

1) As the angle of the torsion arm winds up or down you're effectively adding or subtracting spring rate in addition to what the coil spring provides. Removing this torsional rate means you need a larger spring. Well that doesn't seem like such a big deal. So you add a higher rate spring.

1a. Now that you have a higher rate spring you also need more damping from the damper to keep the wheel end under control. As an example 140 lb/in spring with a OEM damper is going to be underdamped and feel too floaty or bouncy.

2) The inherent nature of rubber is that it also adds some damping. It reduces the tendency of the training arm to oscillate after you hit a bump (i.e. it adds damping). It also adds NVH isolation and reduces transfered noise into the vehicle structure.

2a. Since most of us can't afford to tune our own dampers we take what the market offers and just settle for what we get. Some might use an adjustable damper which is fine but usually that is only adjusting rebound damping and only makes modest adjustements from one position to another. Sure there are better dampers out there (Ohlin, Penske, etc.) that are very adjustable both in compression, rebound, and even at high/medium/low damping rod speeds but that isn't what we're usually talking about for 914's for street use.

OK so now what? Where should my (Mark Henry's) trailing arm hang?

Since you don't have a torsional rate contribution to consider, the short answer is it doesn't matter to you due to the bushings you've chosen.

But what if others have have rubber OEM bushings?

The short answer is that the factory manual says that it shoud basically be horizontal to the outer control arm mount.

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This is the "neutral postion" unloaded (no weigtht on it) position. It's been a while since I installed a set of trailing arms but I recall having to move the trailing arm slighlty with a jack to get the rear damper bottom bolt to line up. When you place the car on the ground the bushing is under some pre-load at curb height.

When you put the trailing arm into compression the bushing is adding torsional rate to the rate provided by the coil spring. As stated earlier the bushing also provides some damping in addtion to the actual damper.

So could it be positioned in other way? Yes.

It could be positioned higher or lower to change the wheel rate curve. Since I don't know anything about the actual rate curve of the OEM trailing arm bushing I can't say how much effect this would have as a percentage of the wheel rate. I know from other vehicles, the bushing contribution can have a sizeable effect depending whether the bushing is tightened in pre-loaded or not.

Many modern cars have a suspension that is tightened down or "neutralized" at curb ride height or even a moderately loaded ride height (say full tank of gas + driver). This is done by leaving the control arm fasteners loose, drawing the body of the car downward to a given ride height by ratchet straps or other means (it's automated in the plant via fixturing) and then tightening the fasteners. When this isn't done properly by the dealerships when a repair is made, the difference is very noticeable and results in a harsh, abrupt ride quality that even average customers will complain about. If the suspension was tightened in full rebuound, the "extra" wind up of various bushings can be excessive at curb height and even worse when fully loaded and leads to premature bushing degradation since the bushing is being cycled though an angular range of motion beyond what it was designed for.

This is a great thread and comments.

I did this replacement many years ago on my car and the bushing replacement is definately a compromise to the stock arm. I ended up customizing the Billy's with progressive valving and upping the spring rate slightly IIRC.

Superhawk; Thank you for this very complete detailed explanation.

So, unless I am mistaking, there is no rubber bushing option available today that is bonded to both the trailing arm and shaft that would recreate the factory setup, and there probably never will be. Which means that we are all stuck with this problem now or in the future, and will have to somehow compensate for this reduction of spring and dampening rate.

To my knowledge the current market offering for these replacement are (...and please add):

PolyBronze: Elephant Racing
Rubber: Elephant Racing
Rubber: 914Rubber
Polyeurethane / Polygraphite: Various
Exotic needle bearing and other mods: Custom

@914_teener What bushings did you end up using?

Yes to all the above....for most people they'll not notice the difference. Not many people are using the stock tires and stock struts ect. anymore.

I used what was available then and the choices were more limited then. Elephant had only a hard polyurethane and PMB had sourced a hard rubber bushing and I talked with Eric several times about setups in changing these. In the end I decided on the harder rubber compound, adjusted the rear spring rate with adjustable perches and helper springs when unloading the rears would occur. I think when I went with progressive valved shocks it made huge difference and now I think you can get progressive springs which in my opinion would be another level of adjustability. Back then you'd cooiuldn't find anyone (at least at a reasonable price) to get progressive springs for a 914. I think Eibach who is here locally in So Cal is doing them.

Anyway...I did a thread on them a while back. I didn't do the progressive shock valving until afterwards. I'll say this: after I did the suspension on the car....front and back it was like night and day and for those who are restoring these cars I'd do suspension first before any engine mods or work. I'd put my money there instead.

Oh...and I talked to several people about powder coating......and mind you....this is not my experience but I do have experience in metalurgy, stress relieving and powdercoating.

The angle(s) on the rear a arm is compound. If you look at their suspension geometry and the degress of freedom you will see why. I never experienced this problem but these are weldments and old ones at that. Check the geometry of the arm and you will need a good surface plate to do this. If these are bent you will never align the rear end of your car if it is bent.

Power coating is dependent on heat to cure the powder and it is baked on with heat. If it were I.......(emphasis on vernacular) I'd paint the surface of the arms with a good quality paint after a through inspection. I'd also check the suspension console ear throughly as well. These two things are probably more important than the type of bushing used in my opinion.

Good Luck and Happy Thanksgiving everyone.

Thanks, but my real question was is it normal for the bare arms to just hang straight down ?
I was expecting it to not fall very far due to the stock (original) rubber bushings.

FYI I'm running, 160lb progressive springs, brand new billys, 205's and a stock rear bar. I realize I'm pushing the limit of the otherwise stock suspention when I let'er rip and I think my rear bushings are OK...but then again I was wrong about the condition of my front bushings.

The left control arm on my car swung down to vertical without much resistance. The right side actually needed to be helped down and never went vertical on its own.

This is a great thread with tons of excellent info. I thought I had resigned myself to replacing the pivot and bushings and doing a video on it to help the community. But destroying a good (albeit old) and irreplaceable factory setup seems like I might be doing this car a disservice. I already have the new pivot and bushings from 914rubber. How about this...if someone is willing to send me a trailing arm (or two) I’ll do the install / video and send back.

Ian - I have a set of trailing arms if you want to do the install video. Mark
Mark@MarkEpstein.net

Powder coating heats the metal to 400 for 20-30 minutes. Enough to melt the plastic powder. It won't warp the suspension pieces on our cars.

@Mark Henry

My mistake in reading your post. I thought you had changed bushings on both front and rear already not just the front.

They should not swing to vertical freely. Probably have lost clamp load between the trailing arm pivot shaft and the body/suspension console ears and it's rotating there.

With the horsepower you're running, I'd agree that console braces are probably in order. You're pushing a lot of force though suspension mounts that were never intended to handle 250HP and modern tires. I'm sure the consoles are flexing with that type of thrust. If the inner console flexes more than the outer, you end up with toe out and that will feel squirely for sure.

In your case, where you're already running high HP, that is the sort of condition where I'd persoanally lean toward the Poly Bronze type bushing solutions that have less compliance when you get hard on the throttle. You will be trading ride and NVH quiteness for handling but when you start going high HP, it has cascading implications to suspension, brakes, chassis. A vehicle is a system of systems. Changes to one system affect others.

My position laid out above is largely for street cars with sub 160 HP. Rember even at 165 HP that is a 50% increase in power vs. OEM /6 that started at 110 HP. And at levels around 200 HP, we already know the the factory 914/6 GT was struggling with handling stability on its race cars thus the chassis stiffening, trailing arm boxing, etc. Modern tire rubber and even more HP would only be worse.

Stress relieving happens at about 500 to 600 degrees. I'm sure the guy running the bake oven uses a templestick to check the temperature. Not. Warping is something different and happens when the steel gets to the transformation phase....so called red hot. So if were I and I was heating up the arm to take out the old bushing and pivot shaft and then heating up again to powder coat it, I'd check it after all that.

It's a lot of work to assemble the bushing, hub and new bearing and CV rehab to find out you can't align the rear end to spec.

I'm not saying it will happen, I'm saying check it, as it is a possibility and I've seen it happen with weldments.

Both our shop and the powder coat shop that I used to go to used an infrared thermometer to check temp on the metal part. They only cost $25 ish so even a diy guy could afford one. Since a piece like a trailing arm takes a while to heat up, I would pre heat the part, blow the powder and then bake the part for 20 minutes when it got up to ~380-400 degrees. The powder coat oven has a max temp of 420 so it's hard to damage even lightweight pieces. I have a feeling that while the surface of the trailing arms were 380-400, the piece never really gets close to that. Sort of like taking a blow torch to creme brulee.
When you heat up the arm to take out the bushing, you heat with a torch until the rubber starts to smoke. Then it's ready to come out. Still not very hot for the metal

Yes do it for a few reasons -
1- you are there and it's almost done- you will hate yourself for not doing it
2- the tube used for the pivot has the ends welded on. You really need to blast it to make sure there are no pinholes that let water in, and yes these will corrode from the inside out.
3- bearing can be pressed out fairly easily, to reinstall put it in the freezer overnight. I have heard youcan put it on dry ice for 30 min, but have yet to do it.

4- we really could use your video and I'm sure more folks will tune it

Yes it is for the metal sleeve that the poly bronze bushing rides on, and Rebel Racings Bushings also benefit form this.

Well...I went ahead and replaced the pivots and bushings. Pleasantly surprised at how easy it went after all I'd heard about discovering new expletives. I used the kit from 914 Rubber (which I believe is poly-graphite?) and a HF shop press. The new bushings seemed to have the same "gription" as the old, when fitting them in the arms. And the pivots seem to be held with about as much force...but of course I have no way of measuring that. Will report back once I've taken a ride.

Here's the video in case anyone wants to see the process.


https://www.youtube.com/watch?v=n37Yx7Nxil8

Ian-

Gave your video

Oscar

WOW! So impressive. Yet another great job, Ian.

Another great video Ian. I'm pretty sure what you installed was 914Rubber's new(ish) rubber bushings. They did sell poly graphite bushings at one time (maybe still do?), but I installed a set of those without any tools at all. Just pushed them in by hand. Now I'm getting ready to replace with the rubber bushings and your video is very timely. Thank you! My only problem is that I don't have room in my overstuffed and cramped garage for a shop press. Will have to improvise.

I am going with the Elephant Racing rubber kit on my reinforced GT trailing arms. They will sell you just the bushes and will use the light weight stock pivot shafts. I posted the weight differences somewhere on here. The stock pivots are way lighter than the solid SS replacements. If this job is too big for you Elephant Racing offers an installation service. I agree with Bruce on the poly bushings. Fit, ride quality and noise are not so great.

Love the vids!

My wife even watches them with me sometimes so she is learning about our little cars.

Man, you are a blessed man indeed.