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This is going to be a long posting, but bear with me, because it covers a topic that is surprisingly complex: the SSR driveshaft. It’s going to be worth your while to bear with me because the end result is a dramatically better driveshaft for your SSR, if you happen to be one of the SSR owners who is experiencing notable driveline noise.

Driveshafts are viewed by most people, even car enthusiasts as “simple”. Unfortunately, they are NOT in truth simple. Simplifying a very technical subject a lot, I can summarize by saying that there are several things that make them complicated:

1. The connection between transmission, driveshaft, and rear axle is NEVER a pure straight line. Because the axle has to move up and down over bumps, we have to have universal joints (“u-joints”) at each end of the driveshaft. The smoothness and durability of these u-joints are extremely sensitive to the relative angle between the transmission, the driveshaft, and the rear axle pinion gear. In fact, NEITHER of the two vertical angles involved, and neither of the two potential horizontal angles involved, should ideally be more than a very few degrees, and the DIFFERENCE between the angles must be even tighter. Furthermore, the angles must NOT be zero!

2. The longer a driveshaft, the more prone it becomes to destructive deflection under high rpm. This is why commercial trucks with their long wheelbases generally have two piece driveshafts (with a supported bearing connecting them), unless they operate at VERY low driveshaft rpm

3. The longer a driveshaft is, the lighter it must be built, in order to prevent self-destruction at high rpm

4. For every driveshaft, there is a “critical speed”, where resonance of the shaft will cause large magnitude vibration. A driveshaft that reaches critical speed will self-destruct, and will unfortunately often severely damage the vehicle and can severely injure or even kill the occupants. Any vehicle manufacturer, and every aftermarket driveshaft supplier, must ensure that the driveshaft used will never hit its critical speed under any reasonably forseeable conditions. To get more specific, the critical speed rpm (i.e. the point of self-destruction) occurs at lower and lower shaft rpm as you increase either the length of the driveshaft or its weight. So, if you MUST have a long driveshaft, it needs to be as light as possible. In addition, the greater the shaft diameter for a given length and weight of shaft, the higher the critical speed. Bottom line: To keep the critical speed at a very high rpm that the shaft is never likely to see, you want short length, low normal operating rpm, light weight, and large diameter.

5. A driveshaft can be quite heavy (a steel one is commonly 20 to 35 pounds or more), AND it rotates at high speed. This means it hurts fuel mileage, dampens acceleration and deceleration (because of inertia effects), and can make a LOT of vibration and noise if not PERFECTLY constructed and balanced.

The GM engineers on the SSR faced some real obstacles when they designed the SSR driveshaft. They started with an SUV chassis that necessarily had a long driveshaft because the SUV was an extended version SUV (bad). They shortened the chassis and driveshaft a few inches for use in the SSR (good), but that changed the relative angles discussed in 1. above (bad). It also still left a driveshaft so long (55.75” u-joint to u-joint), that it is at the extreme ends of most “critical speed” tables for performance driveshafts! The SSR has large diameter tires (bad because it necessitates a higher rear axle ratio than normal and therefore a higher driveshaft rpm, all other things being equal). The SSR is also very heavy for a performance vehicle (which is how GM has marketed it), and therefore requires a higher rear axle ratio to get decent acceleration (again, bad because it necessitates a higher driveshaft rpm, all other things being equal).

The GM engineers addressed the obstacles in a predictable manner. They specified an aluminum driveshaft (for lightness). They limited the rear axle to 3.73, which is really too numerically low for any performance vehicle with tires this large and carrying this much vehicle weight. They went with as large a diameter as practical (4.5”), and went to as light a metal gauge as they could. All this made the long driveshaft light, in an effort to ensure good fuel mileage and to keep the driveshaft critical speed in a range unlikely to be encountered by any SSR under any reasonably forseeable conditions.

The results of the GM approach have unfortunately not been perfect. That thin wall gauge, large diameter, aluminum driveshaft makes an absolutely terrific “musical instrument” that amplifies any noise present in either the rear axle or transmission, and also adds it own “music” – the result of its own high speed rotation and imperfect balance (There is a limit to how much a mass production manufacturer can spend on balancing driveshafts!). The inherent problems associated with the large diameter thin wall shaft are in fact accentuated by the need for GM to swage (reduce by mechanical deformation) each end of the shaft to get it back down to a 3 7/8” diameter compatible with GM’s chosen u-joint castings. This swaging process is rife with opportunity for introducing imperfect symmetry. This driveshaft noise issue is a GM-acknowledged problem on not only SSRs, but also on other GM SUV models that use the similar extended SUV chassis (there is a paper tail of technical service bulletins).

In an effort to address customer complaints, GM has tried multiple SSR (and Trailblazer) driveshaft versions, including one that includes a large dampening donut that is a combination of heavy steel and rubber. BUT the factory solution necessarily mounts the heavy steel and rubber dampener IN FRONT OF the front u-joint, where it is entirely supported by the whipping u-joint at one end and the transmission tailshaft at the other. It doesn’t take an engineering degree to figure out that this arrangement will NOT work long term, as the rubber in the dampener will be “worked” until it starts to break down and work its way out of the dampener body, and / or the heavy dampener, accelerated minutely each revolution by the whipping and heavy driveshaft and u-joint assembly, will eventually destroy the tailshaft seal in the transmission, precipitating heavy vibration and loss of transmission fluid. (Please GM reps monitoring this website, don’t bother to deny this: BOTH the rubber in the dampener AND the tailshaft seal on my SSR have suffered exactly the failure described above, and GM has had to replace the tailshaft seal AND yoke as a result. Check your warranty records).

The noise problem appears to vary by individual SSR, with some exhibiting noticeable and offensive driveshaft noise, and others being relative benign. This is symptomatic of a design that is accidentally “near the edge” – sometimes being a problem and sometimes not, depending on individual tolerance stackups during manufacture, and maybe even on driving style.

GM has not apparently noticed, but I have, that the driveshaft noise issue seems to be pretty much exclusive to SSRs equipped with the automatic transmission. I have not yet seen a single credible report of driveshaft noise issues on a 6-speed SSR, even though I actually asked for 6-speed owners with a noise problem to respond to a posting I did on this board some time ago. This is significant. There are two big differences between the automatic SSRs and the manual SSRs:

(a) The automatics have a Torsen (gear mechanism) differential, whereas the manuals use a conventional friction disk differential

(b) The automatics use a rear axle from American Axle, whereas the manuals use a much stronger rear axle from Eaton

My theory is that the axle assembly in the automatic is simply a lot noisier, and probably because the Torsen is a gear mechanism versus the friction disks in the Eaton differential. (Yes, I know that a GM rep has stated that the Torsen is not noisy, but Chevy mechanics at two different dealerships respectfully disagree.) I think the factory SSR driveshaft is amplifying this noise plus adding more noise of its own, because it’s just way too “ringy” when excited by either mechanical vibration or noise, and because it is “built to a price point”. You can experience the “ringiness” of the factory driveshaft yourself by flicking it with just your finger or knuckle the next time your SSR is on a hoist. It will ASTOUND you with how loudly and persistently it rings after you flick it with just this “finger strength” excitation. Imagine what it does when excited at a few thousand cycles per minute by either the rear end or the transmission. This is why so many SSR owners are complaining about driveshaft “whine” or “ringing”, and it is also PROBABLY the cause of the dreaded “ting” as well.

continued . . .
 

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Discussion Starter #2
So what is the solution? Well, Dennis Reinhart of Reinhart Automotive in Orange Park, Florida has actually proposed a solution: a better, hi tech driveshaft. He’s serious. Serious enough that he sent me one via UPS OVERNIGHT late last week and asked me to test and evaluate it asap.

Dennis had had a bad experience with a souped up Chevrolet Avalanche driveshaft actually failing on a dyno a while back, and it scared the heck out of him. He replaced it with a custom designed hi tech driveshaft from a reputable supplier.

When he contacted me recently about a problem he was having tuning a supercharged SSR, He mentioned the Avalanche problem, and I mentioned the SSR driveshaft problems. Dennis jumped on this, did some thinking and talking with his supplier, and called me back telling me that he believes he has a solution, and inviting me to test it and give my honest opinion. One of his customers, our very own Fanatic “Donnyson”, subsequently called me as Dennis mentioned the test to him. Donnyson enthusiastically endorsed Dennis as a stand up guy.

The driveshaft that Dennis sent me differs from my current factory one in a number of very significant ways, some of them visually obvious, and some more subtle. When I compared Dennis’s driveshaft to the facotry one currently on my SSR (my 4th factory driveshaft in just 20,000 miles on the odometer), I could readily see the differences.

The length of both driveshafts is the same u-joint to u-joint, BUT the factory one includes the heavy steel and rubber dampener, while Dennis’s does not need this undesirable crutch.

The diameter of the factory driveshaft is 4.5” while Dennis’s is 3.5”.

The factory one requires the swage to get back down to the correct diameter for the u-joint castings, while Dennis’s is the correct diameter to begin with.

The finish is no comparison. Just look at the attached photos below. Judge for yourself.

The u-joints on Dennis’s driveshaft are visibly higher quality. Again, print out the photos and examine them. Dennis’s driveshaft uses Spicer u-joints.

The weight of the factory shaft is a very light 16.6 pounds. The weight of Dennis’s shaft is even outstandingly better: 14.2 pounds, or 15% lighter. In a rotating part with significant moment of inertia effects, that’s a “home run” magnitude of difference.

The “finger flick test” was incredible. When the factory driveshaft is struck, it rings loudly and the ringing persists for several seconds. When Dennis’s driveshaft is struck, it sounds “dead” and what little sound there is decays to nothing very rapidly.

The material of construction is clearly different. The factory appears to be simple thin gauge aluminum. Dennis has not shared the material specs of his shaft with me, but it is pretty apparent from the combination of appearance, weight, and response to excitation that it is a metal matrix composite (MMC) aluminum shaft. Assuming I am correct, this material is about 40% stronger than typical 6061 aluminum, is far more rigid, weighs less, and as seen above, is incredibly quieter than the straight aluminum. MMC material is a hi-tech “solution” of silicon carbide moelcules within aluminum (up to 20% silicon carbide). It can be made by a number of processes, including liquid casting into billets, which are then drawn into tubing.

The splines which engage the transmission tailshaft are MUCH more accurately machined than the ones on the factory shaft. The fit is very dramatically better.

Once installed, the radial play in the front end of the driveshaft is MUCH smaller on Dennis’s driveshaft than on the factory one. No comparison. The radial “looseness” of the factory driveshaft has been noticed and negatively commented on by three different skilled mechanics.

Take a look at the attached photos. Note especially the one of the factory shaft that shows the steel and rubber dampener. Look at the rubber on the righthand side of the photo. Randy pointed out to me as soon as he had it off the SSR that it is already breaking down and coming OUT of the dampener assembly. This particular shaft has less than 1600 miles on it. Looking at the history of the two dampener-equipped factory driveshafts I have had, I have to say that I now regard them as having a high probability of causing damage to the transmission tailshaft seal and tailshaft splines, and I personally would not accept another one as a GM warranty repair. I had heard that GM was NOT using this dampener fix any more, and I now understand why I think. My dealer probably received it not realizing that it had already fallen out of favor at GM.


To ensure an absolutely proper and “typical” professional installation (although there is not that much to installing a driveshaft), I asked my friend Randy Peurifoy, a very talented and experienced mechanic with over 35 years experience, to install the shaft and road test it while I took notes in the passenger seat. Then, I drove the SSR myself with Randy in the passenger seat. Then, to ensure testing solo as well as with two aboard (in case it affected the results), I road tested again alone. Each of us noted and remarked on differences in noise volume, apparent directionality, rpm dependence, speed dependence, etc.

The results are clear, dramatic, and persuasive. The driveline is WAY quieter. Furthermore, the remaining, much diminished noise that is still present is very clearly coming from the rear of the vehicle. That was very clear to both Randy and me, as the driveshaft ringing that has been there with the factory shaft with every factory shaft that I have had is just plain gone now, and so the source of any remaining noise is readily determinable. I’m not talking about a ”subtle” reduction in driveshaft noise here guys. I’m talking BIG, like virtually total.

Furthermore, the removal of the distracting and masking driveshaft ringing allowed Randy to do some much better diagnostics on the remaining noise. He quickly determined that the remaining noise occurs mostly when the load on the driveline is very light – at that point where the vehicle is just cruising at very light throttle setting. Decelerating causes the noise to cease entirely, and accelerating diminishes it a lot while not entirely eliminating it. An experienced mechanic would first guess ring to pinion fitting, but since my 4.56 ring and pinion combo has been checked and optimized by a very skilled mechanic and performance shop owner (Reese Cox at MTI Racing in Atlanta) who once installed EIGHTY ring and pinion sets within 3 months while on the Mobil One race team, I suspect this is the Torsen. And, I suspect my 4.56 ratio, 22% higher than the factory stock 3.73, is aggravating it.

Would installation of this shaft solve the “ting” problem? I honestly cannot say for a very simple reason: My last three factory shafts have NOT had the “ting”. (If you don’t know what the “ting” is, do a search on this board using “ting”!). So, someone who HAS the ting currently, and replaces his or her shaft with Dennis’s, will have to answer that question.

Why does Dennis’s driveshaft work better than the factory one, despite its smaller diameter? Probably because of more sophisticated design, superior material, closer tolerance control, and better manufacturing. In essence, it’s simply a better shaft because whoever designed it was evidently very meticulous, and did not need to meet a ridiculously low mass-production price point.

So, what do I honestly think of Dennis’s driveshaft? Dennis, you have a winner, and you’d better brace for an onslaught of SSR owners calling you to get one. I hope you have a supplier with LOTS of production capacity, because he is going to need it.

I haven’t asked Dennis about the cost, but don’t expect this high a quality shaft to be “inexpensive”. And, don’t be angry that GM didn’t provide it as stock equipment on the SSR. Driveshafts that cost hundreds of dollars apiece are never going to be standard on any mass production vehicle (Although carbon fiber driveshafts ARE standard on certain Nissan and Infinity models). Yes, we can say all day that GM “should” have done better than they did, but I suspect that until the closure of the Lansing plant was announced, GM engineers were probably working at finding a solution less costly than an MMC shaft and weren’t yet ready to swallow the cost of a really good shaft made of superior material and built and balanced to tight standards. Dennis has skipped the futile penny-pinching, and gone straight to a GREAT solution.

I really like this driveshaft. I’ll report later on extended long term performance, but for now, the report is a solid “A Plus”. If you have a noisy factory driveshaft and have been unable to get it resolved by GM, here’s a solution that at least so far looks darn good to me.

Jim G
 

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Discussion Starter #3
In the photos:

1. The factory shaft with dampener

2. Close-up of the dampener end of the factory shaft, showing the already degrading rubber in the dampener (right hand side of the photo)

3. Dennis's driveshaft (the part with the funny appearance near the end is where I removed a shipping sticker)

4. One end of Dennis's driveshaft

5. The other end of Dennis's driveshaft

Jim G
 

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This is a GREAT drive shaft

As I've said before, I came the SSR from a Mercury Marauder, and this is the drive shaft of choice by those owners upgrading their factory shafts. We never had the problems with noise, tinks, pops, that the SSR had since the Marauder came with a Police Interceptor shaft rated to 130 MPH. Since some of us wanted to exceed that limit (the EEC limited speed because if the shaft), on went this shaft which is rated up 1o 160 MPH on the Marauder.

It's a work of art as well.

Dennis will certainliy offer these as a group buy to save you some bucks.

here's a few shots of my MM shaft when I was getting exhaust work done:

edit: this shaft had 35,000 miles on it when these shots were taken

edit 2: that last shot was my beauty (snif) at the Indianapolis Speedway in 2004
 

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Thanks Jim

One of my items on my list of Dealer Warranty fixes is the drive shaft noise. Mine has the dreaded "Tink"

I am very interested in the cost of this new shaft as well as the possibility that it will address the Tink issue. (My 04 SSR Automatic is completely stock) I will forward this thread to Dale King (GM Master mechanic) and see if he is willing to work with me on the tink issue and get his input on the new shaft.

Keep us posted on the availability.

Thank you for the thorough report
 

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Donnysan said:
As I've said before, I came the SSR from a Mercury Marauder, and this is the drive shaft of choice by those owners upgrading their factory shafts. We never had the problems with noise, tinks, pops, that the SSR had since the Marauder came with a Police Interceptor shaft rated to 130 MPH. Since some of us wanted to exceed that limit (the EEC limited speed because if the shaft), on went this shaft which is rated up 1o 160 MPH on the Marauder.

It's a work of art as well.

Dennis will certainly offer these as a group buy to save you some bucks.

here's a few shots of my MM shaft when I was getting exhaust work done:

edit: this shaft had 35,000 miles on it when these shots were taken

edit 2: that last shot was my beauty (snif) at the Indianapolis Speedway in 2004

I am not a vendor here yet, but I am working on it, I would gladly do a group buy on the MMDS but lets wait till I am approved, in the mean time here is a article, a friend of mine wrote about CDLV He got me started in the high performance business, he is the best in the business and soon will be offering GM tuning, just like he has been selling for years for all Ford Lincoln and Mercury vehicles, this will be a major advantage for all GM owners.

What is Critical Drive line Vibration

Every rotating object has a “critical” speed or resonant speed, which is a function of its design, mass and stiffness. This is when the driveshaft is whipping in the middle, rather than spinning on a true centerline. For a driveshaft, this is also called “first bending mode”, indicating the shaft actually bows out into a boomerang shape (on a micro-scale). This first mode bending speed is usually referred to in a driveshaft frequency.

What it does –
The energy stored and released through the deflection of the driveshaft through the resonance creates lateral and vertical accelerations of >10g at the problem frequency, which results in broken transmission extension housings, cases and causes moderate to severe vibration at highway speeds (> 70 mph), particularly with axle ratios numerically higher than 3.27:1. This energy release, when compounded by excessive driveshaft imbalance (some is good, too much or too little is not), companion flange run out/imbalance and excessive driveline angles provides the driver with excessive vibration and boom and tortures the driver and driveline components in general.

Because of this, most vehicles have a speed limiter to prevent from entering this mode and causing damage to the driveline.

Some detail –
As mentioned above, the driveshaft rotates at a certain speed based on rear axle ratio; tire size and road speed, but is independent of engine speed (unless you have a vehicle such as a Porsche 944 or C5 Corvette which utilize torque tubes and transaxles, in which case the driveshaft turns at engine speed).

The factors governing driveshaft critical speed include its material properties (i.e., Bulk Modulus of Elasticity which is roughly analogous to material stiffness), diameter, and length and to a lesser degree, wall thickness.

The only factor you can really modify to affect critical speed is material choice. Length is package-dictated, and diameter is usually constrained by driveline tunnel space as well. The answer then becomes a bit simpler – replace your steel shaft with an aluminum or MMC (metal matrix composite) shaft. Both offer reduced weight, which is key in this frequency range. MMC offers the additional bonus of additional damping and stiffness over a typical aluminum alloy.

As mentioned above, at the frequencies in question, a change in rotational mass has a greater impact on resonant frequency than a change in stiffness does, partly since it is easier to reduce mass than increase stiffness (adding stiffness almost invariably means adding mass -- a vicious circle), but particularly since resonant frequency is equal to the sqrt (k/m), where m is mass and k is stiffness. Here m is a stronger function being the in the denominator of a square root. So you can see that as “m” gets smaller, the resonant frequency “f” gets much bigger.

The use of an aluminum shaft provides a dual purpose – increasing critical speed out of the operating range AND directly reduces the rotational forces since those rotational forces are governed by:

F = mr w**2
Where w is rotation speed, m is the mass and r is the radius at which it is spinning.

This means that a 50% reduction in rotational mass results in 50% less rotational force. So, when a driveshaft rotates out of true, due to run out of the shaft itself or due to trans output shaft or axle companion flange run out, the reduced mass * the radius of gyration (i.e., run out) product is smaller than for the same conditions with a steel shaft.

This becomes important not only at critical speed, but at more normal operational speeds where the effects of run out and mass imbalance are more evident than those of resonance:

For a typical Fox or SN95 Mustang, driveline critical speed is around 95-100 Hz. Using stock tires we have the following:

225-60R15, 225-55R16, 245-45R17 all rotate at 812-820 revs/mile at 60 mph.

This give is 13.5 Hz wheel frequency at 60 mph, and assuming a 3.27 axle, we then have:

812/60*3.27 or 44.25 Hz , driveline frequency.

So, 100/44.25*60 yields a driveline critical VEHICLE speed of 135 mph. A good rule of thumb states that the objectionable driveline forces will start becoming significant at 70% of resonant frequency, so for the case of the 3.27 axle, the boom and vibration may be felt beginning at 95 mph.

Typically, 3.27 axles don’t provide the driver with much to complain about; it is 3.73 and above which create the concerns. Using a 3.73, we find that

13.53*3.73 gives 50.5 Hz wheel frequency at 60 mph (substantially higher than the 3.27)

And the critical VEHICLE speed then becomes

100/50.5*60 or 119 mph.

Taking 70% of 119 mph equals 83 mph, certainly a speed at which some Mustang drivers experience occasionally.

For a 4.10 axle, the “70% speed” is 76 mph!

Compounding this problem are factors like transmission output shaft run out, imbalances and run outs from components such as the reverse sun gear, driveshaft, companion flange and pinion pitch line run out (a torque induced run out created when the pinion tries to crawl up the face of the ring gear involutes).

Combine these factors and the already marginal NVH resulting from proximity to 1st bending (critical speed) and the NVH becomes absolutely agricultural.

The aluminum shaft minimizes the contribution from companion flange run out and the driveshaft’s own run out, directly due the lower mass. The pinion is free to pitch +/- 20 degrees and adding in any run outs of the companion flange or driveshaft at the pinion end results in the driveshaft mass having a large eccentric path to wobble about. It is this path times the mass of the driveshaft, which gives the characteristic boom and vibration at highway speeds.

Thus, as Newton predicted, as mass decreases so will the forces. That is why an aluminum shaft is your friend when coupled to 3.73s.

One side note: that great big mass on your pinion nose, fondly named by driveline engineers after the appendage on a male moose, is tuned to 45 Hz, the frequency at which the 2nd order forces created by u-joints as they rotate, force the pinion to bounce or pitch up and down and shake you by the seat of your pants and create an uncomfortable boom in the car. Once again run outs and imbalances will modulate this 2nd order driveline phenomenon to make it worse, so the moral is, LEAVE THE MOOSEB-, uh, DAMPER ON the pinion nose!

Another item: you CAN expect more axle noise when using an aluminum shaft however, which does not necessarily mean the pinion depth or side shims are incorrect, or that the gear cutting process is flawed. It just means that the aluminum shaft is more willing to “bend” circumferentially, torsionally and in a double hump (2nd bending) much more easily than a steel shaft.

Recall my prior statements at the very beginning about aluminum stiffness vs. steel? Picture a piece of sheet metal ducting. Bend it and it makes a WA-WA sound. That is pretty much what a driveshaft does, but at a much higher frequency – higher than even the dreaded “critical speed” of 100 Hz.

Axle noise will occur from about 350 Hz all the way through 500 Hz, sometimes even higher than that. The energy comes from the teeth meshing at the pinion/ring gear interface. This energy is transmitted to the driveshaft (and suspension components) and makes them deflect in the same sense as a piece of sheet metal goes WA-WA. Aluminum is less stiff than steel and takes less energy to deflect it, so it is far more inclined to make your axle go WOOOOO as you drive down the road at 45-70 mph.

Assuming again a 3.73 axle ratio, which has 11 teeth on the pinion and 41 on the ring gear, the axle noise frequency is calculated as (at 45-70 mph):

815/60*3.73*11 or 557 Hz at 60 mph.

This means the WOOO you hear at 45 mph is about 418 Hz and the WEEEEEE you hear at 70 mph is way up there at 650 Hz. You can’t SEE the driveshaft is bending and breathing and twisting, but it is telling you that precisely that is occurring.

So, now armed with this information, you now understand the basics of your vehicle’s driveline.
 

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Wowzers!!

Beautiful driveline parts make me drool. I am now sitting at my computer in a pool of my own spit!

Blast
 

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I can almost follow this.. :)
Dennis Reinhart said:
F = mr w**2
Where w is rotation speed, m is the mass and r is the radius at which it is spinning.

This means that a 50% reduction in rotational mass results in 50% less rotational force. So, when a driveshaft rotates out of true, due to run out of the shaft itself or due to trans output shaft or axle companion flange run out, the reduced mass * the radius of gyration (i.e., run out) product is smaller than for the same conditions with a steel shaft.
Now he has to go into the numbers aspects... I am not an engineer, and i am sure some of you all follow this, but I have not even had my coffee yet. :confused

Dennis Reinhart said:
This becomes important not only at critical speed, but at more normal operational speeds where the effects of run out and mass imbalance are more evident than those of resonance:

For a typical Fox or SN95 Mustang, driveline critical speed is around 95-100 Hz. Using stock tires we have the following:

225-60R15, 225-55R16, 245-45R17 all rotate at 812-820 revs/mile at 60 mph.

This give is 13.5 Hz wheel frequency at 60 mph, and assuming a 3.27 axle, we then have:

812/60*3.27 or 44.25 Hz , driveline frequency.

So, 100/44.25*60 yields a driveline critical VEHICLE speed of 135 mph. A good rule of thumb states that the objectionable driveline forces will start becoming significant at 70% of resonant frequency, so for the case of the 3.27 axle, the boom and vibration may be felt beginning at 95 mph.

Typically, 3.27 axles don’t provide the driver with much to complain about; it is 3.73 and above which create the concerns. Using a 3.73, we find that

13.53*3.73 gives 50.5 Hz wheel frequency at 60 mph (substantially higher than the 3.27)

And the critical VEHICLE speed then becomes

100/50.5*60 or 119 mph.

Taking 70% of 119 mph equals 83 mph, certainly a speed at which some Mustang drivers experience occasionally.

For a 4.10 axle, the “70% speed” is 76 mph!
Someone get me some aspirin, my head is spinning out of the proper and aligned resonant frequency. Watch it as it goes spiing off into space. :willy:

Dennis Reinhart said:
Compounding this problem are factors like transmission output shaft run out, imbalances and run outs from components such as the reverse sun gear, driveshaft, companion flange and pinion pitch line run out (a torque induced run out created when the pinion tries to crawl up the face of the ring gear involutes).

Combine these factors and the already marginal NVH resulting from proximity to 1st bending (critical speed) and the NVH becomes absolutely agricultural.

The aluminum shaft minimizes the contribution from companion flange run out and the driveshaft’s own run out, directly due the lower mass. The pinion is free to pitch +/- 20 degrees and adding in any run outs of the companion flange or driveshaft at the pinion end results in the driveshaft mass having a large eccentric path to wobble about. It is this path times the mass of the driveshaft, which gives the characteristic boom and vibration at highway speeds.

Thus, as Newton predicted, as mass decreases so will the forces. That is why an aluminum shaft is your friend when coupled to 3.73s.
Ahh that Newton is a clever fellow, and I love his cookies. :yesnod

Dennis Reinhart said:
One side note: that great big mass on your pinion nose, fondly named by driveline engineers after the appendage on a male moose,
Where does animal husbandry come into play?
Dennis Reinhart said:
is tuned to 45 Hz, the frequency at which the 2nd order forces created by u-joints as they rotate, force the pinion to bounce or pitch up and down and shake you by the seat of your pants and create an uncomfortable boom in the car. Once again run outs and imbalances will modulate this 2nd order driveline phenomenon to make it worse, so the moral is, LEAVE THE MOOSEB-, uh, DAMPER ON the pinion nose!

Another item: you CAN expect more axle noise when using an aluminum shaft however, which does not necessarily mean the pinion depth or side shims are incorrect, or that the gear cutting process is flawed. It just means that the aluminum shaft is more willing to “bend” circumferentially, torsionally and in a double hump (2nd bending) much more easily than a steel shaft.

Recall my prior statements at the very beginning about aluminum stiffness vs. steel? Picture a piece of sheet metal ducting. Bend it and it makes a WA-WA sound. That is pretty much what a driveshaft does, but at a much higher frequency – higher than even the dreaded “critical speed” of 100 Hz.

Axle noise will occur from about 350 Hz all the way through 500 Hz, sometimes even higher than that. The energy comes from the teeth meshing at the pinion/ring gear interface. This energy is transmitted to the driveshaft (and suspension components) and makes them deflect in the same sense as a piece of sheet metal goes WA-WA. Aluminum is less stiff than steel and takes less energy to deflect it, so it is far more inclined to make your axle go WOOOOO as you drive down the road at 45-70 mph.

Assuming again a 3.73 axle ratio, which has 11 teeth on the pinion and 41 on the ring gear, the axle noise frequency is calculated as (at 45-70 mph):

815/60*3.73*11 or 557 Hz at 60 mph.
And now we come to the part where WooHoo got her handle

Dennis Reinhart said:
This means the WOOO you hear at 45 mph is about 418 Hz and the WEEEEEE you hear at 70 mph is way up there at 650 Hz. You can’t SEE the driveshaft is bending and breathing and twisting, but it is telling you that precisely that is occurring.
I guess she should change it to WooHee

Dennis Reinhart said:
So, now armed with this information, you now understand the basics of your vehicle’s driveline.
All in all, this is some pretty heady stuff, and I am just a (smartazz)driver. But if you can improve the dynamics of my vehicle, making it safer, and less likely to incur damage while someone is driving it, then I am all for it. Please keep us posted.
 

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I'd want to see the price, and read Jim G's longer-term test report, but I'm a potential buyer. It would be worth the drive down bumpy old I-35 to Austin to meet Jim and have his "guy" do the install.
 

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JAustin: So far I have about 110 miles on it.

I actually have TWO different guys who can do the install for anyone interested.

Maybe a "driveshaft party" at my house if enough people are interested? It takes just a short time to do the swap, and because my driveway has a concave drain shaped into it, Randy was able to do it withOUT using the ramps even!

Hey Dennis, have you become a site sponsor yet? Can I buy a number of them to resell to Texas SSR owners within driving range who can order them through me?

Jim G
 

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SSR03: Slow down! I am actually working again by the looks of things! It took the company 4 months to decide to expand its Austin practice, but it finally did so and brought me in yesterday! It LOOKS like they have work for a while.

So, I now have to "go to the office" (almost forgot how to do that!). I experienced the Austin commuting traffic again this morning - took me 45 minutes to go 19.8 miles!!

Weekends though are still available for the really important things in life, like SSRs!

Jim G
 

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Jim G - congrats on the job.. Also my compliments on the writeups in this thread. I'm totally on board with the info presented.
What piqued my interest is the lower mass and smaller diameter - lowering the MMI of the driveshaft.
I'm very interested in some longer term use reports and while I don't have the tink or expect to have it this looks like a well thought out and worthwhile mod. Especially with the 4.56 gears (someday I hope).
 

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Who needs soap operas? I'm staying tuned to find out if this is a permanent fix, if Jim G. can stand the pressures of the office, and who will install these new "thingies" and oh yeah will I have to change my name to woohee?? Tune in tomorrow for the answers to all these and possibly more life changing questions.:jester
 

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TUNE IN TOMORROW AND FIND OUT,... Does Jim keep the "Shaft", Does Jim sell the "Shaft" or does Jim get the "Shaft"..


Same Bat Time,... same Bat Channel! :lol
 
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