Aftermath of driveshaft mishap: Lessons Learned
11-20-2004, 07:24 PM
#1
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Aftermath of driveshaft mishap: Lessons Learned
The driveshaft has been replaced and my truck is back up and running. It seems to be as good as new. What I want to do is share with the group what I found in my research on driveshafts while I was trying to decide how to repair my truck.
I'll break the post down into six sections:
- What Happened
- What I Found
- The Phenomenon
- The Particulars
- Lessons Learned
- Epilogue
What Happened
For those of you who don’t know what happened, here is the story. I installed a cam into my 2003 Chevy Avalanche, which until that point had the following modifications:
Radix with stock 5.3L pulley (3.4”)
McGaughys 2” drop spindles
Eibach 3” drop springs
Hotchkis anti-sway bars and trailing arms
ASA TRS II 20” x 9.5” wheels
Kumho Ecsta STX 305/50-20 tires
Once the cam was installed I planned a trip to the dyno and the track because I had a dyno and track run with all the modifications above but without the cam. I wanted to see what the cam alone added to my numbers. So I was going to the Dyno Day in Santee, CA and immediately thereafter going to Fontana for the last drag race of the season. Or so I thought.
I hooked up my HP Tuners and climbed into the passenger side of my truck to log the first dyno run. The operator asked me if I wanted to go to 5500 RPM’s and I said, “No, six thousand is fine.” As far as I knew, it was. Well, he ran through 1st and 2nd gears, then into 3rd, and as you can all see from this dyno and this picture, 6000 RPM’s was not fine. At around 5900 RPM’s there was a horrible crunch from underneath my truck, followed shortly thereafter by some thunks and big pieces of metal flying out from underneath my truck.
What I Found
It took me a while to figure out what had happened. It was only because of my desire to get the best driveshaft possible into my vehicle that I stumbled upon the information that described what exactly happened on the dyno. The answer? My driveshaft had reached what is called ‘critical speed’ in driveshaft-speak and came apart, from the middle outward. Anyone familiar with the phenomenon, and the specifications of my vehicle, could have predicted beforehand that the driveshaft was going to come apart, and could have even given the exact RPM range when it was going to happen.
Now before any of you get worried that this might happen to you, keep breathing. It probably won’t, and I’ll tell you why. Most of you don’t have 72” aluminum driveshafts. This is the key parameter that my truck possesses which pretty much ensured the thing was going to come apart. This (plus some other measurements) is what determines the ‘critical speed’ of a driveshaft.
Actually, all of the components of the equation are as follows:
(1) Driveshaft length
(2) Driveshaft diameter
(3) Driveshaft material
(4) Driveshaft material thickness
There are other, more oblique parameters such as ‘modulus of elasticity’ and ‘density’, but those can pretty much be derived from the other four parameters above.
The Phenomenon
So what is ‘critical speed’ and what are its characteristics? Well, in my quest for information I found the following info:
Here is another, more in-depth writing on the phenomenon:
This info is what led me to the determination that my driveshaft had reached critical speed: It had broken in the middle and come apart there, just before the section connected to the front sheared off at the front U-joint.
The Particulars
So, once all the parameters of a particular driveshaft are known, it is a simple matter to plug them into the equation to get the outputs, which are: (1) critical speed, (2) tube weight, and (3) torsional yield. Using the calculator at this site, and plugging in the parameters from my Avalanche of 72” driveshaft length, 5” diameter, and 6061 aluminum, we find a critical speed of 6205 RPM’s.
To be clear, this calculator is taking info from strict 6061 spec aluminum, from which I am sure the material from the stock OEM driveshaft from GM deviates. For example, if the modulus of elasticity of the material is changed from 10 to 9 (you can do this manually within the calculator), the critical speed changes from 6205 to 5886 RPM’s. In my case, the material could be sufficiently different such that the modulus of elasticity was different, or the material density was different, with the result being a lower critical speed. Or, my stock OEM driveshaft could have been out of balance to sufficiently lower the critical speed to ~5900 RPM’s. Regardless of the cause, it came apart, and it could have been predicted to do so.
So should you worry? I doubt it. I would guess that most of you don’t drive Suburbans or Avalanches, which I believe are the only trucks The General makes that have one-piece aluminum driveshafts that long. Plugging lower numbers for driveshaft length into the calculator, you can see that even 2” or 4” makes a difference in critical speed. Take a foot or 12” off (going down to 60”), you end up with a critical speed of 8935 RPM’s, which of course is not a problem. For those with 4WD, transfer cases use shorter shafts, and for those with long crew cabs, 1500HD’s, or 2500HD’s, the driveshafts are two piece steel, not one piece aluminum. So you’re probably out of the woods.
Lessons Learned
The #1 lesson here is: Know your critical speed! If you are ever in doubt, dyno your vehicle in 2nd gear and not 3rd. Reason being, if you are in 2nd gear and the motor is running 6000 RPM’s, the driveshaft is not – it is running at a lower RPM, due to the transmission’s gearing.
Here are the gear ratios for the 4L60E/65E and the 4L80E:
4L60E/65E
1st – 3.06:1
2nd – 1.63:1
3rd – 1:1
OD – .78:1
4L80E
1st – 2.48:1
2nd – 1.48:1
3rd – 1:1
OD – .75:1
So, if you’re going 6000 RPM’s in 3rd, your driveshaft is going 6000 RPM’s. But if you’re going 6000 RPM’s in 2nd, your driveshaft is going (6000 RPM’s/1.63 =) 3680 RPM’s with a 4L60 and (6000 RPM’s/1.48=) 4054 RPM’s. No problems approaching critical speed in 2nd for either transmission.
Epilogue
As for the AV? Well, after talking to lots of different places, I determined that no one was going to be able to make me a driveshaft better (or even as good) as The General, at least a 1-piece aluminum one, so I got an OEM replacement. Steel is out for something 72” long, unless I go 2-piece, and I decided not to do that right now. So what I will do is lower my rev-limiter to something under 5800 RPM’s, at least in 3rd....
Hope some of you find this information useful.
I'll break the post down into six sections:
- What Happened
- What I Found
- The Phenomenon
- The Particulars
- Lessons Learned
- Epilogue
What Happened
For those of you who don’t know what happened, here is the story. I installed a cam into my 2003 Chevy Avalanche, which until that point had the following modifications:
Radix with stock 5.3L pulley (3.4”)
McGaughys 2” drop spindles
Eibach 3” drop springs
Hotchkis anti-sway bars and trailing arms
ASA TRS II 20” x 9.5” wheels
Kumho Ecsta STX 305/50-20 tires
Once the cam was installed I planned a trip to the dyno and the track because I had a dyno and track run with all the modifications above but without the cam. I wanted to see what the cam alone added to my numbers. So I was going to the Dyno Day in Santee, CA and immediately thereafter going to Fontana for the last drag race of the season. Or so I thought.
I hooked up my HP Tuners and climbed into the passenger side of my truck to log the first dyno run. The operator asked me if I wanted to go to 5500 RPM’s and I said, “No, six thousand is fine.” As far as I knew, it was. Well, he ran through 1st and 2nd gears, then into 3rd, and as you can all see from this dyno and this picture, 6000 RPM’s was not fine. At around 5900 RPM’s there was a horrible crunch from underneath my truck, followed shortly thereafter by some thunks and big pieces of metal flying out from underneath my truck.
What I Found
It took me a while to figure out what had happened. It was only because of my desire to get the best driveshaft possible into my vehicle that I stumbled upon the information that described what exactly happened on the dyno. The answer? My driveshaft had reached what is called ‘critical speed’ in driveshaft-speak and came apart, from the middle outward. Anyone familiar with the phenomenon, and the specifications of my vehicle, could have predicted beforehand that the driveshaft was going to come apart, and could have even given the exact RPM range when it was going to happen.
Now before any of you get worried that this might happen to you, keep breathing. It probably won’t, and I’ll tell you why. Most of you don’t have 72” aluminum driveshafts. This is the key parameter that my truck possesses which pretty much ensured the thing was going to come apart. This (plus some other measurements) is what determines the ‘critical speed’ of a driveshaft.
Actually, all of the components of the equation are as follows:
(1) Driveshaft length
(2) Driveshaft diameter
(3) Driveshaft material
(4) Driveshaft material thickness
There are other, more oblique parameters such as ‘modulus of elasticity’ and ‘density’, but those can pretty much be derived from the other four parameters above.
The Phenomenon
So what is ‘critical speed’ and what are its characteristics? Well, in my quest for information I found the following info:
“Every driveshaft has a critical speed. When a driveshaft is accelerated, it resonates at a certain frequency. The resonation causes the shaft to vibrate. When the vibration is accompanied by rotation, the center-most point of the shaft distorts. The centripetal force from the rotation of the shaft accentuates the shaft distortion until it eventually snaps.”
“For a driveshaft, critical speed is usually defined as the rpm that equals the frequency of the first natural lateral resonance. Dana/Spicer will only rate shafts to 3/4 of this rpm. The first natural lateral resonance is the motion that you see in a vibrating guitar or piano string, and is actually the frequency a driveshaft would ring (vibrate) if struck with a mallet and measured with an accelerometer and an oscilloscope. It is also easy to illustrate with a phone cord – wiggle it back and forth, and the frequency that makes it look like a uniform, but wide, "U" is the first lateral harmonic. As you increase the frequency, eventually it will look like an "S". This is the second natural lateral harmonic frequency. It depends on the dimensions - diameter and length, as well as the specific modulus of the material. When a driveshaft spins close to an rpm equal to the first natural lateral harmonic frequency, it starts to displace the center in the "U" shape because the rotation is exciting the lateral harmonic just like if it was hit with a mallet or plucked like a string. As the center is displaced, centrifugal force also plays a role in displacing it farther.”
“When shafts break due to critical speed problems, they always break right in the center, because it is actually a bending type break from bending the center in and out over and over. When running close to "critical speed" a driveshaft actually starts to whip it's center round and round, just like you can illustrate with a telephone cord. Picture what happens to a driveshaft at 7300 revolutions per minute. It is doing just what a phone cord does when you twirl it around. That is what shakes the car. If the shaft also has a balance problem it feels a lot worse (actually is a bigger problem). The specific modulus is the ratio of stiffness to density, and is the reason CF tubing works so well for driveshaft applications. Actually, all the metals except Beryllium have pretty close to the same specific modulus. CF tubing has extremely variable specific modulus because it is very easy to manipulate the stiffness of the tubing, by changing fiber type or winding angle, but the density remains pretty constant.”
“When shafts break due to critical speed problems, they always break right in the center, because it is actually a bending type break from bending the center in and out over and over. When running close to "critical speed" a driveshaft actually starts to whip it's center round and round, just like you can illustrate with a telephone cord. Picture what happens to a driveshaft at 7300 revolutions per minute. It is doing just what a phone cord does when you twirl it around. That is what shakes the car. If the shaft also has a balance problem it feels a lot worse (actually is a bigger problem). The specific modulus is the ratio of stiffness to density, and is the reason CF tubing works so well for driveshaft applications. Actually, all the metals except Beryllium have pretty close to the same specific modulus. CF tubing has extremely variable specific modulus because it is very easy to manipulate the stiffness of the tubing, by changing fiber type or winding angle, but the density remains pretty constant.”
The Particulars
So, once all the parameters of a particular driveshaft are known, it is a simple matter to plug them into the equation to get the outputs, which are: (1) critical speed, (2) tube weight, and (3) torsional yield. Using the calculator at this site, and plugging in the parameters from my Avalanche of 72” driveshaft length, 5” diameter, and 6061 aluminum, we find a critical speed of 6205 RPM’s.
To be clear, this calculator is taking info from strict 6061 spec aluminum, from which I am sure the material from the stock OEM driveshaft from GM deviates. For example, if the modulus of elasticity of the material is changed from 10 to 9 (you can do this manually within the calculator), the critical speed changes from 6205 to 5886 RPM’s. In my case, the material could be sufficiently different such that the modulus of elasticity was different, or the material density was different, with the result being a lower critical speed. Or, my stock OEM driveshaft could have been out of balance to sufficiently lower the critical speed to ~5900 RPM’s. Regardless of the cause, it came apart, and it could have been predicted to do so.
So should you worry? I doubt it. I would guess that most of you don’t drive Suburbans or Avalanches, which I believe are the only trucks The General makes that have one-piece aluminum driveshafts that long. Plugging lower numbers for driveshaft length into the calculator, you can see that even 2” or 4” makes a difference in critical speed. Take a foot or 12” off (going down to 60”), you end up with a critical speed of 8935 RPM’s, which of course is not a problem. For those with 4WD, transfer cases use shorter shafts, and for those with long crew cabs, 1500HD’s, or 2500HD’s, the driveshafts are two piece steel, not one piece aluminum. So you’re probably out of the woods.
Lessons Learned
The #1 lesson here is: Know your critical speed! If you are ever in doubt, dyno your vehicle in 2nd gear and not 3rd. Reason being, if you are in 2nd gear and the motor is running 6000 RPM’s, the driveshaft is not – it is running at a lower RPM, due to the transmission’s gearing.
Here are the gear ratios for the 4L60E/65E and the 4L80E:
4L60E/65E
1st – 3.06:1
2nd – 1.63:1
3rd – 1:1
OD – .78:1
4L80E
1st – 2.48:1
2nd – 1.48:1
3rd – 1:1
OD – .75:1
So, if you’re going 6000 RPM’s in 3rd, your driveshaft is going 6000 RPM’s. But if you’re going 6000 RPM’s in 2nd, your driveshaft is going (6000 RPM’s/1.63 =) 3680 RPM’s with a 4L60 and (6000 RPM’s/1.48=) 4054 RPM’s. No problems approaching critical speed in 2nd for either transmission.
Epilogue
As for the AV? Well, after talking to lots of different places, I determined that no one was going to be able to make me a driveshaft better (or even as good) as The General, at least a 1-piece aluminum one, so I got an OEM replacement. Steel is out for something 72” long, unless I go 2-piece, and I decided not to do that right now. So what I will do is lower my rev-limiter to something under 5800 RPM’s, at least in 3rd....
Hope some of you find this information useful.
11-20-2004, 07:34 PM
#3
Just got back from MagnaFlow
Nice to see you again Richard, and a pleasure to meet you Brandin!
Well I must say I am happy with the results of the new exhaust on the CC! It was a little loud at first, but I am used to it now
After a dyno pull and results of 390.57 rwhp and 443 rwtq I am pretty happy! That is approx a 10-13 rwhp gain accross the power band, Plus it sounds AWSOME! The truck seems to pull alot better on the top end, and rips outta the hole. If anyone is thinking about exhaust, www.magnaflow.com is the only way to go!
Well I must say I am happy with the results of the new exhaust on the CC! It was a little loud at first, but I am used to it now
After a dyno pull and results of 390.57 rwhp and 443 rwtq I am pretty happy! That is approx a 10-13 rwhp gain accross the power band, Plus it sounds AWSOME! The truck seems to pull alot better on the top end, and rips outta the hole. If anyone is thinking about exhaust, www.magnaflow.com is the only way to go!
11-20-2004, 07:35 PM
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Thanks fro the write up mate!
But one question, maybe I am still to hung over to get it but shouldn't the RPM but multiplied by the gear ratio? Not divided by?
But one question, maybe I am still to hung over to get it but shouldn't the RPM but multiplied by the gear ratio? Not divided by?
11-20-2004, 07:42 PM
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Originally Posted by F8L Z71
But one question, maybe I am still to hung over to get it but shouldn't the RPM but multiplied by the gear ratio? Not divided by?
11-20-2004, 07:49 PM
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Originally Posted by Naked AV
That would mean that at 6000 RPM's in 1st, my driveshaft is doing (3.06 * 6000) = 18,360 RPM's...do you think that is the case?
Well in my line of thinking. The motor is spinning at X RPM, the gearing through the tranny multiplies that RPM, then it is multiplied again at the rear end with the rear gear ratio.
That's why first gear always has better acceleration than 2nd and 3rd. The gear multiplication is higher. If it is divded by then wouldn't acceleration be worse in each gear after 1st???
Man, I don't feel good and you're messing with my head. LMAO
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11-20-2004, 07:55 PM
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Originally Posted by F8L Z71
But one question, maybe I am still to hung over to get it but shouldn't the RPM but multiplied by the gear ratio? Not divided by?
That was a pretty damn good write up. You recieve an A.
Where are the cliff notes?
11-20-2004, 07:56 PM
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See I'm all bassakwards.. I am still thinking the driveshaft is turning 3:1 with the motor. You know more than I do though so I'm not saying your wrong. I just wanna learn exactly what it's doing. Especially since this is car basics at it's simplist. LOL