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Gentlemen, I have just finished reading a number of magnificent threads on the use of poly bushings in the trailing arms. A few mentioned the trailing arms becoming torsion bars. It made sense when I first read it, but now I beg to differ.

My father-in-law (a nuclear physicist for the Savannah River Facility) and I, re-examined the topic and found a number of calculations and explinations for the difference rubber vs. poly bushings would make.

The post suggests that the stiffer bushings make the arms into torsion bars and increase their risk for fracture. The increased risk for fracture is not only TRUE, but can be calculated. The Torsion bar conversion however, is wrong.

A torsion bar is a substitute for shocks and springs. Its functionality is based upon its material properties for RESISTING torsion, NOT twisting. Simultaneously, since all materials will flex to some degree, the Rockwell value of the bars must be low so as not to break when they eventually do FLEX. This is much like a sway bar. A sway bar must resist the twisting motion inflicted upon it, so that it can maintain a downward force on the compressing axle. Since it will eventually flex, one doesn't want it to be brittle.

The trailing arms of the Alfa are NOT designed to flex. They have a higher Rockwell number than Torsion bars. If they were to be subjected to greater amounts of rotational stress, they would eventually break.

It was this logic that makes it seem plausable that stiffer bushings would transmit greater torsional forces to the trailing arms. And that's the mistake.

A rubber bushing compresses more than a poly. In order for it to compress however, it must be both acted upon by a force and RESISTED by a equal and oposite force. (Otherwise is would move in space instead of compressing)

If the rubber bushings in a stock trailing arm are being compressed by rotational forces, then the trailing arms must be adequately resisting these forces or they would rotate themselves. The rubber transmits ALL the force. The fact that it compresses DOESN'T decrease the the TOTAL amount of force transmitted.

Poly bushings are less compliant and therefore compress less. Substituting poly for rubber doesn't make the car exert any more rotational force on the trailing arm. The arm still has to resist the same force.

What changes is the SPEED, and FREQUENCY of the rotational stress. Imagine if you will, that you jump down onto a concrete floor with hard leather shoes. You might hurt your feet. Now jump onto the same floor, but have on softer rubber shoes. Hurts less. Why? The surface area is the same, your weight didn't change, the force of impact was the same. (F=MA) And most importantly, the floor still resisted the same amount of force. (Otherwise you'd deform it or push it into the earth below.)

The rubber delays and seperates the frequencies of impact. This results in less stress to the concrete (and your bones). Poly bushings transmit these frequencies more like dense matter, with all wavelengths of impact arriving closer to the same time. It's this activity that causes undue stress to the trailing arms, not the rotational forces. Given that the bars ARE brittle (a higher Rockwell number) they are more subject to damage from these frequencies.

I hope I did the explination justice.

Here's a bonus topic for thought: What difference does this make on the bolts themselves?

Hint: EL-AL airlines had a plane crash in Germany many years ago.
 

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The post suggests that the stiffer bushings make the arms into torsion bars and increase their risk for fracture. The increased risk for fracture is not only TRUE, but can be calculated. The Torsion bar conversion however, is wrong.

++++++++++++

Poly bushings transmit these frequencies more like dense matter, with all wavelengths of impact arriving closer to the same time. It's this activity that causes undue stress to the trailing arms, not the rotational forces. Given that the bars ARE brittle (a higher Rockwell number) they are more subject to damage from these frequencies.

Since it's unlikely that we'll have access to a lab test of a trailing arm equipped with poly bushings, the next best thing would be accounts of actual failure. Has anyone witnessed a trainling arm w/poly bushings develop cracks or other failures? While I understand your analogy of the dynamics of rubber vrs. poly bushes, I wonder---and I'm just speculating here---if the failure threshold of the trailing arms is high enough that problems develop only under extreme (i.e., racing) circumstances? I ask this because, although I've been around Alfas for a long time, I've never actually seen a cracked trailing arm. I don't say they don't exist. I've just never seen one. :)
 

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what about stiff spring rates wouldn't that have the same affect, albeit indirectly?
 

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..yup..:).....
 

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I've heard of cracked trailing arms but never actually seen one ... close up. I did see a car not finish a race, and then spoke to "my boss" at the time and he told me the trailing arm had separated and he welded it up.

Well written 180OUT, except that anything that resists twisting but does actually twist is a torsion bar. Not a very good one yes and it will eventually fail, but while it is hanging together to is acting in some way like a torsion bar. Just like early inboard F1 suspension wishbones acted partly like a spring because they flexed. A proper torsion bar ofcourse is designed to twist and return to normal ... as you say.
Pete
 

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The trailing arms should locate the axle fore and aft, not side to side (that’s the torque tee’s function, or sliding block, or panhard rod/watts linkage). Nor should the trailing arms control the vertical motion, that’s the spring’s job. You want each element of the suspension to perform its role, and not confuse itself or chassis behavior trying to do double duty.
For example, when installing a panhard rod, the torque tee is removed and a short rod with heim joints is substituted to control the rotation of the axle about its long axis, but not resist the panhard rod’s function to locate the axle side to side.
The shape of the trailing arm, with overly stiff bushings, results in a very poor torsion bar (too stiff and non-linear), and I suggest that the more linear and compliant locating device (ie the springs) will govern vertical deflection until the torsion bar (with non-compliant bushings) binds and you get a sudden and very unwelcome increase in spring rate. The compliance in the stock bushings prevents this from happening. However, the stock bushings provide overly compliant fore and aft control.
When racing, Alfa recognized this problem as they replaced the bushings with spherical bearings (and in some cases installed a thin bushing slice on each side to cover/hide the modification). The spherical bearing has no fore and aft compliance (good), yet provides no resistance in the other planes of motion (also good). This is not suggested as a street modification (harsh and subject to dirt/wear), but rather to illustrate the ideal way to isolate the specific suspension component’s role/function.
If the trailing arm is bushed too stiffly, and the arm tries to act as a lateral or vertical restraint, then the cracking discussed in above posts is more likely to occur.
 

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I agree with Wil. The trailing arm does bend slightly given that most metal is flexible by nature. But the arm won't act as a torsion bar (i.e flex to regulate/absorb impact) Unless the springs are stiffer than the arm itself, or they compress fully, with ample energy left over to bend the arm. That would cause a crack or failure, and seems to indicate an improper/inadequate setup altogether. I also agree that the reason bushing upgrades are done, are to give closer tolerances than those at the factory (as they were designed for a variety of conditions, wheres a race car is designed for a select few) Such is the problem of a road/race car. That is also why most poly bushes have a shorter lifespan, because in a racecar it is assumed that everything will be replaced sooner, as they are used harder, and require a precision that can only be achieved with shorter service intervals.
 

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trailing arms vs bushings

One issue that is missing from this thread is the type of car we are talking about.

Giuliettas had tubular trailing arms; this type of cross section has a high torsional rigidity.

105's on the other hand have ''hat'' section trailing arms with little torsional rigidity. They can actually be flexed by hand, which comes in handy when putting the suspension back together.

Now, let`s remember that all cars roll when cornering, and stock Alfas roll more than other sports cars. When a car rolls, the body rotates along a longitudinal axis parallel to the ground while the rear axle wants to stay parallel to the ground. This means that both ends of the trailing arms are not in the same plane anymore; if the trailing arms and bushings were absoultely rigid the car could not roll. Since the roll resistance contribution of the trailing arms and bushings is not significant, they have to flex to accomodate this relative motion.

How much flex? For given cornering conditions on a given car, it will follow these variables:
- roll resistance of the rest of the suspension;
- stiffness of the trailing arms
- stiffness of the bushings.

With the stock suspension on a 105 car, the soft rubber bushings combined with the little torsional rigidity of the arms, they will flex easily; obviously they have been designed to do so.

With urethane bushings on the same car, some of the flex will transfer from the bushings to the arms; except perhaps under racing conditions, it would be unlikely that this change would cause a failure of the arms as they flex so easily.

Any upgrades in roll resistance would reduce the need for the arms and bushings to flex, hence reduce any problem.

Where there is a problem, it`s on a car with rigid tubular trailing arms such as a Giulietta. Any increase in torsional resistance of the bushings and trailing arms results in increased stress at the trailing arms attachment points. On Giuliettas, these attachment points on the rear axle are flimsy and have been known to rip apart under racing conditions resulting in massive accidents.

I have seen broken trailing arms not on Alfas but on transit buses, where they are massive 3 in tubes with forged ends. And yes, the broken ones did result from the experimental use of poly bushings.

So poly bushings are OK for some applications, including 105 trailing arms, but this cannot be made into a general rule for any application.
 

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Where there is a problem, it`s on a car with rigid tubular trailing arms such as a Giulietta. Any increase in torsional resistance of the bushings and trailing arms results in increased stress at the trailing arms attachment points. On Guiliettas, these attachment points on the rear axle are flimsy and have been known to rip apart under racing conditions resulting in massive accidents.
Alfa increased the size of the bushings on the 101 Giulia cars versus the earlier Giuliettas. It is my belief that this was done to increase compliance and reduce stress on the trialing arm attachment points.
 

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risks of poly bushings

Alfa increased the size of the bushings on the 101 Giulia cars versus the earlier Giuliettas. It is my belief that this was done to increase compliance and reduce stress on the trialing arm attachment points.
105 axles also have much sturdier attachment brackets than Giuliettas. On a 101 - 1300 Spider I owned, these brackets had been bent sideways then sort of straightened; the car had no track history and was dead stock. Clearly, there was a weakness in the Giulietta rear suspension design. Which is why I consider poly bushings to be potentially dangerous when used on a pre-105 Alfa.

Another place where I would think twice before using stiffer than stock bushings is in the 105 front suspension: the upper arm caster bushing also needs some degree of compliance.
 

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... i have toyed with the idea of a bearing on the rear trailing arms,,mostly the front bushing area of the trailing arm..i have extra pair of arms..night look into this feb.march..
 

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Another place where I would think twice before using stiffer than stock bushings is in the 105 front suspension: the upper arm caster bushing also needs some degree of compliance.
[/QUOTE]


Please elaborate. :) So far the received wisdom re: poly bushings is that the upper caster bushing was not only safe but substantially contributed to improvements in overall handling.

(Sorry. If this elicits a lot of response, I'll move it to a new thread.)
 

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... i have toyed with the idea of a bearing on the rear trailing arms,,mostly the front bushing area of the trailing arm..i have extra pair of arms..night look into this feb.march..
Yeah, don't try it. A mechanic around here told me a story of how in his foolish youth he once put poly at the rear and used a solid aluminum bushing at the front of the trailing arms. Ripped the mount off in a hard corner.
 

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on my last post,, i said i toyed about thinking about useing a' bearing' in place of the front trailing arm bushing..not a soild alum bushing..a spec. bearing.. sorry about the misinderstanding..
 

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180OUT said:
Please elaborate. :) So far the received wisdom re: poly bushings is that the upper caster bushing was not only safe but substantially contributed to improvements in overall handling.

(Sorry. If this elicits a lot of response, I'll move it to a new thread.)
This is where the confusion regarding poly bushes lies.

As discussed poly bushes are not the best when needed to be squished (unless details have changed recently) and thus if you need to alter your caster angle you will be squishing your upper inner poly bush and thus putting load on the mounting points and the suspension arm. Poly bushes are best when everything is nice and parallel all the time and they then simply have to rotate.

I'd rather put in a spherical bearing myself.
Pete
 

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Poly bushings are not all solid pieces anymore, and depending on where they are designed to go, their stiffness varies... They've come a long way in the last few years, and there aren't any reports that I have heard, about poly bushings from known vendors, such as Centerline and Performatek, causing problems. I certainly haven't had any issues, and one of my Spiders has poly everywhere, including the motor mounts. As bushings need to be replaced in the other ones, they will get poly as well.

Best regards,
 

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Danyl
Why do you feel the need to try & teach us fracture mechanics & metallurgy?
Dunno,... Perhaps he figured we did a lot of partying in college?... (Didn't you? :D)

Best regards,
 

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Danyl
Why do you feel the need to try & teach us fracture mechanics & metallurgy?

I think it's mean to make a mechanic work with a fracture. I'm sure it really hurts.

When I a kid I rolled in some grass and got metallurgy all over. It was real itchy and I had to put lotion on it for a week!

:)
 

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elaborata

Quote from 180OUT :

Please elaborate. :) So far the received wisdom re: poly bushings is that the upper caster bushing was not only safe but substantially contributed to improvements in overall handling.

(Sorry. If this elicits a lot of response, I'll move it to a new thread.)


When the suspension moves up/down, the upper arm/balljoint pivots around its attachment to the body; at the same time the caster tube pivots around the caster ball joint. These two trajectories meet in a single or neutral position; the caster bushing has to link these two components together in all positions. To do so it has to stretch and twist more and more as we get away from the neutral position; the inner bushing will twist too in a team effort. The amount of stretch required is determined by geometry, not by stiffness. If both the caster and inner balljoint bushings are made stiffer, the stress in the bushings and adjacent components increase. If overdone, at some point and over time, there would be a risk of breaking either the fork of the tube or the loop that holds the caster bushing. Either of these breaking at speed would likely cause a loss of control.

The stress level with overstiff bushings would actually be less on a race car with a stiff, lowered suspension than on a softly sprung road car as supension travel would be less on the former. Worst would be a road car with a stock suspension occasionally used on the track.

I'm not saying poly caster bushings can't work; urethane can be formulated in many duro hardnesses. I'm just saying we should not believe blindly in the "harder is better" philosophy.

The other thing we don't know is the hardness of available poly bushings compared to a) a new rubber bushing; b) a dead, 30 years old one. I have a durometer, if anybody has the samples...
 
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