Help- Should I change front bushings for 88 Carrera?

[aj88cab]

Hi folks,
I am suffering with spring allergies and a case of whileIminthereitis and need some help.

I currently have the front end apart doing the following:

Shocks
Rotors
Caliper rebuild
Rubber brake lines
Pads (obviuosly)
Ball Joints
Turbo Tie Rods

So now I'm looking at the A-Arms and sway bars thinking a few more nuts and bolts and I could do the bushings as well. The bushings are all original but do not look to have any major problems.

The car is an '88 and has 112,000 miles. How durable are the original bushings? Is there any way to tell if the bushings really "need" to be replaced? Or at that mileage should I just plan on missing this weekends AutoX and do the bushiings as well.

I have read alot of threads on how and what to expect but didn't really see anything on evaluating the condititon of the current bushings.

There are more AutoX scheduled and hopefully I'll be doing a DE in June (and many more after). So I'd rather do the front end right (and once).

Any insight and advice is, as always, appreciated.

Andrew

[Noel]

You should replace them as part of this project. It is the number of years the weight of the car rests on the bushings that causes them to flow/wear more than mileage. Anything pre 1990 is probably due for replacement at this point. I changed mine to Elephant this winter and like them, although this car is not my daily driver.

Good luck,

[jazzbass]

Oh, what a can of worms this is. I, too, have the same disease and have been going through the same issues. Most likely, the bushings need to be replaced. The rubber will cold flow after a number of year (regardless of miles) and your A arms will no longer be centered. Most will agree that the bushings on a 16 year old car probably need to be replaced. Where the debate comes from is what to replace them with. Your options are:

1. New factory rubber bushings. $400-$500, as there require a special process to vulcainze the bushings to the a-arm. Pros - suspension back to factory spec, rubber will help absorb some of the road harshness. Cons - expensive, rubber will go bad again, loss of some suspension settings when driving at 90%+. If you ever notice this, you're a hell of a lot better driver than me.

2. Poly-graphite bushings. $40. Pros - cheap, hold up better than facotry bushings over time. Cons - PITA to install - lots of custom fitting involved. These have been known to squeek, too, if not lubed properly or not fit 100% correctly. More harsh than rubber bushings - slight degration in street ride quality.

3. Elephant Racing poly-bronze busings. $240. Pros - easy DIY installation, cheaper then new rubber bushings, maintains suspension settings better than rubber, no squeeking. Cons - more expensive than poly-graphite, harsher than rubber.

IMO, if you already have the front suspension apart, replace the bushings. The only decision from there is what to go with.

[ludovicbreger]

I also heard of bearings. Anyone has experience
with those?

[aj88cab]

Noel & Chris,

Thanks for the info. I have ordered the poly-graphite today. After all the other parts purchases that is all that the budget will handle.

When I finish the front end I plan on JUST doing the brakes and shocks on the rear.

Then hopefully it will be nothing but road time (autoX, DEs, N. Georgia mountain runs) until winter.

Andrew

[gduke]

i'm in the process of putting on the Elephant racing bearings on both front And rear. Its also a while your at it upgrade. they're not real hard to install. Good luck.

[Chuck Moreland]

Originally posted by jazzbass

3. Elephant Racing poly-bronze busings. $240. Pros - easy DIY installation, cheaper then new rubber bushings, maintains suspension settings better than rubber, no squeeking. Cons - more expensive than poly-graphite, harsher than rubber.

Jazz

Speculation about Polybronze bearings being harsher than rubber just doesn't prove true.

Polybronze bearings low-friction provides a supple ride similar to rubber despite their resistance to compression and deformation. Before you write that off as a sales pitch, research the comments by the many board members that have actually installed the product.

This is truly a case of "you get what you pay for". The polygraphite wont seem so cheap if you have to repeat the job to fix the friction or squeaks. Been there-done that.

[jazzbass]

Originally posted by Chuck Moreland
Speculation about Polybronze bearings being harsher than rubber just doesn't prove true.

Understand, this is my unbiased opinon, and while I think you make a great product, I stand by it. Here's why.

I am in the process of replacing the front suspension bushings in my car. In this process, I think I've read every bushing thread on this BBS, and talked to many local Porsche experts - guys that have been working on or building these cars since before I was licensed to drive and who's opinion I respect.

In my opinion (as one who did take some ME classes at Virginia Tech) and those of nearly everyone I talked to was that polygraphite or poly-bronze bushings, by the very nature of thier constuction, would transfer more vibrations to the chassis of the vehicle than OEM rubber ones that have the benefit of the dampening effects of the rubber. Someone on this board made the comment that if there was no difference in ride quality, why wouldn't Porsche put solid bushings on all of its cars?

That said, the difference may be minute - the reality is, I don't know. I have read all the threads where people install your bushings and love them. In every case I read where people installed your bushings they talked about how great the ride was now and that it was no harsher than before. In these cases, though, people were almost always doing a full suspension rebuild - replacing worn tbars, worn struts, worn a-arm bushings, worn ball joints, etc. Naturally, a fully refreshed suspension is going to feel great compared to the old, worn out, harsh one. In these cases, too, everone mentions that the suspension is not too harsh "for them". This is a relativistic opinion and hard for anyone to gauge an absolute from - what may be too harsh for me is just fine for you.

What I have not seen is empirical measurements between your bushings and new OEM rubber ones on the same car, with a fresh suspension, in similar circumstances. In reality, I'm not sure how you'd even measure such a vague concept as "harshness", really. But really, you'd have to eliminate all variables except one (the bushings) and then compare apples-apples (i.e. poly bronze vs NEW rubber) for the comparison to have any meaning in this context.

Maybe we could have people take the "poly-bronze challenge". Have two cars with identical set ups and let people drive them and pick the one they like the best. While we're at it, we could put SS brake lines on one and rubber lines on the other and see if anyone could tell the difference there as well.

[Chuck Moreland]

Jazz

I do understand your thought process - It seems intuitively obvious that rubber should ride smoother than a hard compound. But it is simply not the case with Polybronze.

By your own admission the feedback you've seen from those with first hand experience contradicts your speculation.

Intuition and fact don't line up. So what gives?

There is another process going on here that needs understanding; Friction.


Ride harshness is not caused by the hardness of the bushing compoud, but rather by the amount of static friction (stiction) present in the suspension. The suspension doesnt' move until the stiction is overcome.

High stiction bushings need a big bump to start moving, big enough to overcome the stiction. Smaller bumps are passed through and we feel this as vibration or harshness.

Low stiction bushings move easily over smaller bumps and isolate this vibration.

Stock rubber and Polybronze are both low stiction bushings. Polygraphite and Polyurethane are relatively high stiction bushings and hence their reputation for harshness.

So why did Porsche use rubber? Every decision is a compromise; performance, cost, longevity, ride quality.

Following is the text of a tech paper I wrote on the subject:
========================================

Polyurethane bushings are a common replacement for rubber suspension bushings. They reduce suspension deformation under load, providing more precise cornering. They also transmit road vibration and create ride harshness.

Ride harshness is often blamed on the hard-compound of polyurethane bushings. Though partly true, that is not the primary cause of harshness.

In fact, there is another process in play - friction. Friction is the primary cause of ride harshness with polyurethane bushings.

Rubber bushings - how they work

For all their faults, rubber bushings allow suspension movement with very little friction. Rubber bushings do not slide in their mounts, they accommodate movement by deforming in a twisting motion. There is no friction surface hence the friction is very, very low. Rubber bushings resist movement due to the spring rate of the rubber, not friction. Their behavior is similar to a torsion spring though the spring rate is small.


Rubber Bushing Rotates via Deformation


Polyurethane bushings - how they work

Aftermarket polyurethane bushings are completely different than the rubber bushings they replace. Instead of deforming, the polyurethane forms a friction-surface that slides around the steel suspension member or mounting point.


Polyurethane Bushing Rotates on friction surface

Unfortunately the polyurethane-on-steel friction coefficient (up to .7) is significant causing them to "grab" the steel liners. With the weight of a vehicle resting on the polyurethane the friction becomes substantial. The problem is compounded under high speed cornering loads or heavy braking. The problem is further compounded if the polyurethane bushing fitment is not precise or bushing alignment is poor. Grease will help reduce the friction but doesn't last long, as demonstrated by the many cars with squeaky polyurethane bushings.

Friction and damping

Early automobiles actually used friction-type dampers. Their performance is horrible and use was quickly eliminated in favor of hydraulic dampers.

The key problem with friction dampers is static friction and the resulting large force to start the suspension moving. Once moving, the dampers begin absorbing energy with relatively low kinetic friction. Essentially the suspension is locked in position until a large bump creates enough force to overcome the static friction of the suspension. The result is a very harsh ride that is insensitive to small bumps.

Contrast this with a modern hydraulic damper that begins motion with very low force. The damping action increases with the speed of the damper. The suspension responds well to both small and large bumps yielding improved ride quality, superior tire-to-road contact, and road-holding.

Though friction-type dampers are an extreme case, any friction in the suspension causes similar ride harshness.

Stiction

Static friction in suspensions is often called "stiction". The word invokes an appropriate image of a sticking, jerky, binding suspension that does not operate smoothly and only responds to large inputs (bumps).

Unfortunately, some level of stiction is present in all automotive suspensions. Ball joints, shock seals, and bushings all introduce some stiction. Stiction is the enemy of road-holding performance and ride quality. Though it can't be eliminated, all good handling cars take pains to minimize stiction. True race cars use metal heim joints and suspension bearings to minimize friction.

The high-stiction characteristics of polyurethane bushings have created their reputation for harshness.

Stiction and performance

The purpose of a performance suspension is to keep the tire contact patch optimal at all times to maximize grip. For a suspension to work it must move in response to bumps, road contours and driver input. Stiction interferes with movement attempting to lock the suspension in place. Unfortunately, stiction is greatest under high corner and braking loads - just when grip is most critical.

Stiction also makes accurate corner balancing of the vehicle nearly impossible. Stiction creates corner weights that lack repeatability.

Measuring stiction

With the car parked on a level surface, lift one bumper corner by hand extending the suspension as high as possible. Don't simply release, but SLOWLY let the car return to normal height under it's own weight. Do not push down. Measure and record the bumper height.

Next press down on same corner compressing the suspension. SLOWLY allow the car to return to normal height. Measure and record the bumper height.

The difference between the two heights is a measure of the cumulative stiction.

Several factors influence the acceptable range of good values including spring rate and vehicle type. But in general a difference of 1/4 inch would indicate low stiction, a difference of 1 inch or greater would indicate high stiction.

Reducing stiction

Identify and understand the operation of all friction points in your suspension. This includes "A" arm bushings, ball joints, steering tie rods, shock seals, shock mounts, sway bars, linkages and anything that moves with the suspension.

Ensure that all these friction components are in top shape, replace anything that is worn. Ensure that all items requiring lubrication are properly lubricated.

Replace high-friction elements like polyurethane bushings with low friction alternatives. Low friction alternatives include PolyBronze bearings, rubber bushings, and heim joints / monoballs as appropriate.

Ensure that all bushing mounts are properly aligned. Misaligned mounts result in pinching, binding and excessive stiction.

[jazzbass]

Wow! Great response, Chuck. Thanks for taking the time to help clarify this for me. There is no doubt you know way more about this stuff than I. However, let me say that this:

Originally posted by Chuck Moreland
By your own admission the feedback you've seen from those with first hand experience contradicts your speculation.

is not entirely correct. My speculation is that the poly bronze bushings will result in more vibration transferred to the chassis versus the same suspension with new, OEM rubber bushings. I have yet to read an account where this comparison is made, so at this point there is no first hand account that contrdicts my speculation.

Understand this, though - I am not here saying your bushings are not worth the money or are too hard for the street. I have read the many accounts that here that people love your bushings. To me, there's no question they're better superior to poly-graphite ones, esp if you value your time at all (and you should). My point above was that in every single account of poeple who installed your bushings, it always came along with a significant change to their suspension. The net effect is that they all love their new suspension, and don't find it harsh. I don't want to discount that last point to lightly - really, the overall package is what you're after, and if you don't think its too harsh, then its not too harsh. To be honest, this belief is the reason why I'll probably end up with the poly bronze bushings myself.

I think your account above does an excellent job of describing the how the coefficient of static friction is the difference between poly-graphite bushings and yours, and I buy your argument that your bushings would transfer less virbration becuase of that.

The root of my belief that poly-bronze bushings would transfer more vibrations to the chassis than rubber can be shown in the diagrams below:



This shows a simple view of the front suspension with a force, F, being applied to the front wheel. Now, if I remember my vectors and dynamics correctly (and its been a while), this force F is really two forces - Ft acting tangentially to the rotation of the a-arm and a force Fr acting radially on the a-arm.

Clearly Ft is going to be the larger of the two forces, and in fact Fr is somewhat exagerated in the diagram. Ft is what will cause the a-arm to rotate in its bushing. The lowering of the coefficient of static friction, as you descibe above, is what would cause less of this force to be transferred to the chassis and more to the strut as the a-arm rotates.

It is Fr that is the key part of my logic that poly-bronze bushings will transfer more vibration to the chassis. This force will be directed down the radius of the a-arm itself and does not contribute to the rotation of the arm. In a car with rubber bushings, some of this force will get absorbed in the bushing; in a car with poly bronze bushings, all of this force will get transferred to the chassis.

At least, this is how I envision it. I am completely willing to believe that Fr may be very small and contributes little to the overall equation. Finally, its been years since I've done this kind of work, so my logic and FBD may be entirely off - please explain if it is.

[Chuck Moreland]

Your vectors are very much exagerated, in fact at the accepted "Euro Height" the control arm is horizontal or even angled up slightly. That is distorting the magnitude of your force vectors greatly.

But there is another element you left out. The tire is a big rubber bag of air that sits between the road and the suspension, illustrated here as a balloon.



The force vectors work through this "isolator". The tire sidewall flexes laterally in response to movement of the control arm, reducing radial movement at the bushing end.

I submit the tire's abilty to dampen Fr dwarfs the 1/4 inch of rubber in the bushing. Hence the rubber bush absorption of Fr is negligable.

However, I think you draw much better vector diagrams than I do

[jyl]

Interesting discussion. My two cents - I'd guess automakers use rubber bushings because they are cheap, well-understood, and will last 200,000 miles without maintenance, not because they are optimal for ride, handling, or any other characteristic.

[Steve@Rennsport]

Mr. Moreland:

Nicely written Sir, and my compliments to the "Chef",.... I really couldn't have stated it better, myself.

I'll back up every word of what Chuck has said with my own experiences and unequivocably state that the majority of ride quality complaints can be traced to suspension "stiction" both in bushings, shock choices, and design. Up to a point, torsion bars (springs) and swaybars do not make as big a difference.

Lastly, do not overlook the effects of tire pressures, aspect ratio and carcass construction on ride quality perceptions. This is a biggie.

[aj88cab]

Intelligent discourse, informed debates based on sound principles, respect for others opinions.......?

Just WHAT are you guys trying to do!

Could this be the beginning of an era of Pelican Forum enlightenment where the pursuit of knowledge and betterment of all Porsche cars and owners rises above ego, petty differences, and prejudice..............naaaaa


Thank you all again for the wealth of info. I will document my experience (with pictures too) and if my choice of bushing materials comes back to bite me on the ass, I'll be the first to admit the error of my ways and let everybody know.

Andrew

[dickster]

andrew

thanks for the thread, i had bought poly bushes but now i think i'll hold out for poly bronze - i didnt realise there was a second generation, and they look superb, much easier to fit as well. they are more expensive but i only intend doin it once!

shame i already did neatrix in the rear!

[jazzbass]

Just one more thing to add - here is an example of a rubber a-arm bushing that is worn out due to cold flow:



Notice how the part next to the red arrow is much thinner than the part next to the yellow arrow. The red arrow part is where all the weight of the front end rests. My guess is that most of these cars 89 and older with original bushings probably look like this.

Besides throwing the suspension geometry off, the worn bushing allows the front torsion bars to rub on the torsion bar tubes, rubbing the paint off and rusting. Click here to see what my torsion bars looked like

[turbo6bar]

The bushings on my '78 SC looked exactly the same. I already yanked the bushings off, so I can't take any pics.

How did you get your torsion bars out? I drilled a 1/4" hole in the end cap and hit the torsion bar with a punch and big hammer. No give. Add heat and hit again. No give. It is starting to pi$$ me off.

[jazzbass]

I used a pickle fork - the one I have fits around the body of the tbar but not the splines. I used it as a lever against the torsion bar tube and they poped right out. The other end of the t-bar wasn't rusty, so I got lucky.

The pickle fork paid for itself yesterday - seperating the tie rod ends, the ball joints, and getting the torsion bars out of the tubes. Only use it, though, if your replacing said ball joints and tie rods - it tears the boots and generally makes a big damn mess of everything.

[buttjoint]

Pretty interesting read do to the fact that I also recieved my polybronze in the mail today and am currently doing the R/R.