Balancing driveshaft

[beastie_3]

I understand the importance and need for balancing the driveshaft, but I have read that when you take your old driveshaft off, you should mark it so it goes back on in the same spot. Why is that? Its not like you can balance the driveshaft while it is ON the vehicle.

[Psycho71]

Never worried much about how they go on the vehicle. Never noticed a difference either. But I do mark the yoke(s) when swapping u-joints. get one of them backwards, and it's a trip to the drive-shaft shop for a re-balance.

[Da67goatman]

I have mine marked because it has a conversion joint at the axle and one axis is heavier due to bigger caps.

[VileZambonie]

Phasing the driveshaft is good practice to prevent unwanted vibrations.

Witness marking the companion flange to the driveshaft is something you should always do as a technician. Let's just say for example you've owned your truck for 10 years and you're used to it and all it's little vibrations at certain speeds. Now you bring it into the shop and the shaft is installed 180° opposite of how it was. You drive your truck and notice a change in feel/sound
- Customer returns with the infamous "Ever since you worked on my car...."

[beastie_3]

Phasing? Is that what it's called?

[Grim 82]

I have heard it several times, and have always made the marks to return everything just the way that it was. I absolutely agree with the logic behind it, but beastie brought up an interesting point that got me thinking and wondering...

Phasing the balanced driveshaft to the front shaft/slip yoke (and calling it one driveshaft assembly) is obviously very important, given that the two pieces that have significant spinning mass that can vary in machining are matched together with a u-joint. But, as far as being 180 degrees off at the pinion, wouldn't any vibration still occur, just half of a revolution later?

I only ask for the sake of learning something new. I would compare it to taking a tire that is balanced on a wheel, and mounting it on the vehicle. If it's independently balanced, it doesn't care what rotation it's in when you bolt it on. In other words, if you remove a wheel, rotate it 180 degrees, and bolt it back on upside down, you don't get a vibration, because the wheel and tire combo are balanced to themselves. Wouldn't a driveshaft obey the same rules?

Again, it's quite possible I'm missing something, and I only ask for the purpose of learning something new.

[bake74]

     I have built many a jeep, and with very short, very angled driveshafts, it is important to have it balanced/phased properly.  If not you won't get far and then you wont have a yolk/u-joint any more.  It only takes once and you don't do it again.

[Grim 82]

I agree and follow the practice, and don't dispute the part failures from ignoring the practice, I'm just trying to wrap my mind around the why of it.

[chevyh20]

how do you proceed if you switch out the rear end with this topic?

[beastie_3]

and how do you know the previous owner never screwed with it? I still dont see how its important because you cant have it all balanced together in the first place.

[Irish_Alley]

I'm with beastly. I've taken a front drive shaft for a th350 cut it down and made it fit to a 465. Have been running it without any problems yet lmao. Now a rear drive shaft yes I would balance don't really think which way the yoke goes on matters. But vile you have me doubting my self

[VileZambonie]

Phasing of the driveshaft is when the two pieces of the shaft are matched and balanced. Again in the vehicle you don't want to cause a change in vibration after you've removed it. Harmonics are generated and transmitted into the passenger compartment. It may become worse depending on wether or not it is a hunting or non hunting ratio. If it's your own vehicle and you don't care neither will anyone else. It's just good practive to prevent the "Ever since you worked on my car, there's a vibration at 60mph" Well it may have been there at 65 and was never noticed until it changed.

[bake74]

     Longer driveshafts are more forgiving, the shorter the driveshaft the more you have to worry about balancing.  Our trucks have pretty long driveshafts, on a lifted jeep, every time you replace a driveshaft you get it balanced out of the jeep, kinda like a tire then you run it for awhile, and if the vibration is there then you move the weights around a bit till you get rid of most or al the vibration.  If you ever get to drive a short driveshaft vehicle, then rotate the driveshaft 180 degrees and test drive it again, you will then know why it is important.