SCOTT_A

iTrader: (1)

Hey Swerve, What is your driveshaft made out of? Do you know the diameter of the shaft?
Your driveshaft should be a similar length to mine.

Swerve7rpm2000

iTrader: (9)

3 1/2" steel ill get the length for ya, spicer ends and yoke

Swerve7rpm2000

iTrader: (9)

This Critical Speed stuff has me worried, why wouldnt a driveshaft shop tell you this when you order a aftermarket driveshaft of these lenghts? They think were only gonna turn 5700 rpms???? 57 3/4" from center of u-joint cap to u-joint cap and heres a pick of the yoke....

SCOTT_A

iTrader: (1)

OK, we basically have the exact same shaft... 3.5 inch steel ~ 56 inches between ujoint centers. Go to the link below, there is a chart of critical speeds. I have been reading into this like crazy as it seems to be the only possible theory that explains what I'm experiencing. Good practice is to not exceed 75% of the critical speed. Critical speed for our shafts is 6300 RPM. 75% of that is only 4700 RPM. I know I'm exceeding that and my confidence is growing that this caused my tailshaft bushing to wear 0.025 inches out of round as the driveshaft whipped around like a jump rope.

http://www.pstds.com/driveshaft_crit...peed_chart.htm

SCOTT_A

iTrader: (1)

I don't know why a shop would sell you a shaft for a performance application and not have a conversation about critical speed. The shop that built mine is local to me and typically builds shafts for dump trucks and certainly is not a performance shop... I don't blame them a bit. To be fair, I havn't proved this is the problem.

Swerve7rpm2000

iTrader: (9)

Great info though! But to be honest I had the stock drive shaft cut down and slip yoke welded on and never had a vibration and I took it well above the critical mark... Since I went to the 3 1/2 I have had nothing but problems...even sent it back to be rechecked and they said it was fine, drove it down the road again and had no problems other than a minute vibration on deceleration, haven't had a chance to keep testing cause the while rearend is out now... I'll have more info myself this weekend and keep me updated... Looks like we would be better off with an aluminum 4" drive shaft or a relatively expensive carbon fiber to be up to technical par lol

SCOTT_A

iTrader: (1)

I think we are both fighting the same battle. I have a hunch that your stock driveshaft had a thinner wall thickness compared with the 3.5 inch tube. If the wall thickness is thinner, the shaft will weigh a lot less and therefor its critical speed will be much higher.
When I was in highschool, I worked at a Dodge dealer and if you remember the 5.9 RT Dakota's... We sold an extended cab 5.9 RT. It had a single shaft steel driveshaft and it kept coming back for service for bad vibration at high speed and smoked tailshaft bearings. The owner fought to lemon law it, but Chrysler kept trying to fix it. The first fix was an aluminum shaft, that failed and then a carbon fiber shaft was tried. The truck was super smooth after that.
I think the aluminum shaft didn't help the RT because it was extra long with the extended cab. I think it would solve our vibration, based on the critical speed charts.

Gadgetized

iTrader: (9)

When I had my shaft made I had it high speed balanced. Denny's asked me how fast I was going to drive it, and I told them 150. They balanced it up to 150 even though it will more than likely never be going that fast. I like a little overkill. One thing that the guy as Denny's did mention is you can not turn the yoke 180 degrees. Basically when they balance a shaft it is adding weight to the lighter side. It is near impossible to weld a shaft perfectly centered where it wouldn't need weight. If the yoke was turned 180 (like if you replaced a joint and didn't reference the yoke in the right position) the balance would be 2X off what it was before balancing. I don't think you guys removed your yoke, but that is something.g to think about.

As far as the whip effect, you could check this. I would put the truck in the air, and secure a dial indicator to the shaft and check for out of round. I would check this to see how bad it is. Another thing I would do is remove the tires, and put the truck on a lift. I would try this first with the tires off, and see if you could see anything moving or get that unwanted vibration. This could be dangerous obviously, and isn't the best method because your suspension is hanging. Another thing I am wondering if everything comes out straight is checking the pinion angle. I'm not sure of the way to do this, maybe use a jack under the front of the rear and check the speed then. Hope this helps.

SCOTT_A

iTrader: (1)

Thanks for all the ideas Gadgetized, I talked to a guy at Denny's yesterday and he explained this whipping or critical speed theory to me. He said my steel 3.5 inch 57 inch long shaft can't be safely spun up to the speed I am feeling my vibration. He also told me if it was pinion angle problems, I would start to feel a vibration at slower speeds, and I am completely smooth until ~75 and it gets scary bad above 100. No matter how straight and true the shaft runs, if it hits its critical speed, it will whip. Since Denny's balanced your shaft up to 150 mph, I assume its made of Aluminum. If its steel, well I will be very confused.
A year ago I had my steel shaft balanced at a local shop and they don't do high speed balancing. They told me if its balanced at slow speed, it will be balanced at high speed as well. Thats true, as long as the shaft is short or light enough to not hit its critical speed. I believe if they spun my shaft up to ~100, it might have jumped out of the balancing rig.

Gadgetized

iTrader: (9)

My shaft is a 3" steel. It might be made of stronger steel or something. It is what they call their nitrous ready version and is good over 2000hp. It is a pretty heavy duty shaft.

I honestly think it is a pinion problem. If you have the tail bushing becoming oval shaped, it tells me there is stress on the top and bottom (if it were oval on the side I would assume the engine/tranny/rear end isn't lined up properly) causing it to wear. When you are at higher speeds, you are on the throttle more, and it is pushing the front of the rear up causing it to change. When you are slower, it is down where it is supposed to be. I would be interested to see a pic of the tail bushing. Im wondering what would happen if you pointed the front of the rear down a couple degrees.

This is another reason why I am trying to talk myself into a Go Pro, I would love to see what it is doing.