Shudder in 3rd - 3 speed top-loader

[Joe_68_F100]

I just recently got my truck running during the course of a frame-off rebuild. It is still on jack stands, but to make sure I had the shifter adjusted right, I fired it up and ran the gears a little in the garage.

There is a noticeable shudder in third, especially at lower engine RPM's. It killed the engine the first time I put it in third since I didn't bother gasing it because the truck was on stands. I didn't notice this in any of the other gears. I had the tranny over-hauled when it was out of the truck and I replaced all of the u-joints. Any ideas? I didn't have to open the throttle to get it to idle in first, second, or reverse.

Thanks!

[BobbyFord]

Wait until the truck is on the ground and driving and I'll bet there is no shudder.

[Joe_68_F100]

I tried it again and I think it shudders in all of the gears, it is just the most noticeable in 3rd. First and second start to shudder when I pick up speed.

Could I have the drive shafts out of balance? It has stock drive shafts, the front one is from a '69 and the rear is from a '68. Do they have to be balanced as a set? I wouldn't think so. Are the weights supposed to go towards the rear, or does it matter?

Thanks

[BobbyFord]

Two-piece driveshafts need to be a matched set, in phase and balanced.

[Joe_68_F100]

Darn...

How do I tell if they are "in phase?" (and don't tell me "when it doesn't shake like crazy")

[BobbyFord]

Darn...

How do I tell if they are "in phase?" (and don't tell me "when it doesn't shake like crazy")

"In phase" means, as it was factory balanced, as it came from the factory, with all of the u-joints lined up. Since yours is combined from two different vehicles, I recommend that you take it to a driveshaft shop and have it balanced.

[sargentrs]

I agree with Bobby. Mixing and matching driveshafts is tricky and a rebalance is good idea. Shouldn't be that expensive and would eliminate a variable.

[Racer Z]

It's probably running at too low an RPM and just stalling. Wait until the wheels are on the ground and you can drive it.

Drive shafts seldom loose their balance. If you get to the point of needing the drive shaft re-balanced, there are shops that specialize in drive shaft work that can fix you up. The drive shaft will spin faster as you drive faster. At slow speeds you probably won't feel anything. The faster you drive, the worse the vibration. Don't be fooled by an out of balance tire. ... Or a bad shock or ....

[70_F100]

A matched set is not necessary.

The factory did not balance them as a set. Think about it. You could buy either shaft separately as a replacement part.

As long as they are balance correctly independently, the only thing you need to be sure of is that they are in phase.

[Racer Z]

Explain PHASE to me please.

"In phase" means, as it was factory balanced, as it came from the factory, with all of the u-joints lined up."

I don't see where having the knobs of the u-joints lined up will make a difference. Each shaft half is balanced independently of each other.

[forrestbump]

Explain PHASE to me please.

http://www.4crawler.com/4x4/CheapTricks ... ml#Phasing

[BobbyFord]

Explain PHASE to me please.

"In phase" means, as it was factory balanced, as it came from the factory, with all of the u-joints lined up."

I don't see where having the knobs of the u-joints lined up will make a difference. Each shaft half is balanced independently of each other.

Every two piece driveshaft I've ever worked on has had the u-joints lined up.

[Joe_68_F100]

I will check to see if I lined the U-joints up. If I did, it was purely by accident, because I just put it together without worrying about anything else.

I was thinking like some of these other guys, that each shaft should have been balanced at the factory independently and it doesn't matter how they get bolted together.

Thanks!

[70_F100]

Having the U-joints aligned puts the driveline in phase.

Without getting into a lot of physics, if the u-joints aren't aligned, it causes them to sort of work against one another.

Look at any driveshaft, and you'll find the welded yokes are aligned with one another.

The slip yoke, for all intents and purposes, becomes the second yoke on the front driveshaft, so it has to be aligned with the yoke on the front of the shaft to work properly.

Balance is a totally different subject.

[Racer Z]

The phasing article is directed at one-piece drive shafts. I can see how the u-joint phase on the double-ended section could be an issue with "vibration". That section is welded and won't change.

The forward section of our trucks has a joint nearest the transmission, the other end is fixed in the center support. Not only do not see how aligning the forward joint to the other two joints will do any thing, but to reinforce my thinking,
"Several months ago I serviced my two piece drive shaft. It never occurred to me to check the phase. I just now went out and looked at it. The forward joint is out of phase by nearly 45 degrees. 35 or 40 is closer to where it is. I have never experienced any vibration or "shudder". This includes a 6,000 mile trip across the USA with my girlfriend. Little things like that I might be willing to overlook, but she is, err... sensitive... to those kinds of things."

That's my two cents worth on "phasing" a two piece drive shaft.