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Joined: Jul 2002
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From: Mannheim, Germany
New vibration thread That thread was too long like 5 pages ago. I saw this post and I think it sums it up very well.
This should be a sticky! Maybe I'll put together an organized grand summary of the other thread on my website if I get bored this week. A lot of good info but it's so long and crazy. This is the kind of info that message boards are all about! 
Originally Posted by jyeager
Which inigma? Everyone who experiences this could have a different cause. Just to summarize my thinking on the possible causes:
1) anything that rotates can be out of balance. Some are unlikely culprits (like accessory pulleys, engine rotating assembly) but the likely ones are:
a) flywheel, clutch, PP (MT only)
b) flexplate (auto only)
c) driveshaft
d) axles
e) wheels/tires
2) something bent/damaged
a) axles
b) rims
c) driveshaft
d) u joints
e) damaged tranny mount
3) something worn
a) pilot bearing
b) slip yoke bushing (in tranny tailhousing)
c) tires cupped (most likely culprit in general!!!)
d) u joints
e) axle bearings
f) pinion bearing
g) ball joints, tie rod ends, idler arm
h) rubber suspension bushings
4) something out of adjustment
a) pinion angle (due to lowering, or changing tranny mount, or changing torque arm
b) loose suspension parts.
1) anything that rotates can be out of balance. Some are unlikely culprits (like accessory pulleys, engine rotating assembly) but the likely ones are:
a) flywheel, clutch, PP (MT only)
b) flexplate (auto only)
c) driveshaft
d) axles
e) wheels/tires
2) something bent/damaged
a) axles
b) rims
c) driveshaft
d) u joints
e) damaged tranny mount
3) something worn
a) pilot bearing
b) slip yoke bushing (in tranny tailhousing)
c) tires cupped (most likely culprit in general!!!)
d) u joints
e) axle bearings
f) pinion bearing
g) ball joints, tie rod ends, idler arm
h) rubber suspension bushings
4) something out of adjustment
a) pinion angle (due to lowering, or changing tranny mount, or changing torque arm
b) loose suspension parts.
That's a good list. Driveshaft critical speed should be added to it though, with a lot of people swapping to deep gears that can spin it too fast. My point is, you can have a driveshaft that is perfectly balanced and with no bends or damage at all, but it will still shake itself to pieces if you spin it too fast. This is because it has a natural frequency based on it's mass and stiffeness. Larger diameter and/or lighter driveshafts can solve this.
wheels can also not be aligned and may require hub centric rings. this is especially true to people running wheels designed for other cars (i.e. HP Racing wheels with 5x120 bolt patterns)
Cal,
I saw your post in the other thread and I did some math. I don't think we're getting that close the the critical speed.
Someone want to double check my math?
70mph = 4,435,200 in/hr
25.66 in diameter tire = 80.57 in circumfrence
4,435,200/80.57 = 55046 axle revolutions per hr
55046 rev/hr X .02 hr/min= 917 axle RPM
917 axle RPM X 4.11 ratio = 3771 DS RPM
3.5" X 44" alum 6061 DS Critical Speed = 10,609 RPM
I think f body DS are around 40" and the shorter the DS the higher the critical speed.
Did I miss anything?
I saw your post in the other thread and I did some math. I don't think we're getting that close the the critical speed.
Someone want to double check my math?
70mph = 4,435,200 in/hr
25.66 in diameter tire = 80.57 in circumfrence
4,435,200/80.57 = 55046 axle revolutions per hr
55046 rev/hr X .02 hr/min= 917 axle RPM
917 axle RPM X 4.11 ratio = 3771 DS RPM
3.5" X 44" alum 6061 DS Critical Speed = 10,609 RPM
I think f body DS are around 40" and the shorter the DS the higher the critical speed.
Did I miss anything?
All I know is LS1 drive shafts start shaking at about 110 mph with 4.11 gears, but you can't really feel it until you get to about 125 mph. At 145 mph, the shake is really bad, with the driveshaft spinning at about 8000 rpm. LT1 steel drive shafts are much worse. Your calcs look good, but I don't believe a stock LS1 driveshaft is good to 10,000 rpm. Some aftermarket ones are, though.
I think it was LG Motorsports that sold an aluminum DS good to 10,000 rpm, but they stopped offering them. IMO carbon fiber is the way to go; not only do they have a higher critical speed, but they "broom" when they break, so they will never go through the floor pan. Expensive as hell though.
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TECH Enthusiast
Joined: Jul 2002
Posts: 653
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From: Mannheim, Germany
Originally Posted by BlackHawk T/A
Out of that whole list i'm really surprised I didn't see warped rotors on there...i'm pretty sure that's part of my problem...
Someone explain to me how with the 10bolt there were never any issues. Installed the 9" and a PST 3.5" DS and all of a sudden its there??? How did the rotors warp? How did the aligment on the wheels go out of whack?
