WHY is the front Driveshaft out of phase?

[MDarius]

I had my front U-joints replaced and driveshaft balanced and the driveshaft shop balanced it in phase. I called C-Dan to confirm (because I was away from Ih8mud and my FSM) and he assured me it needs to be out of phase. I agree with C-Dan in all things, including politics, beverage choice, favorite color, and anything else you've got. I questioned him once and my birfields started clicking and I got 3 new rust spots within a month. I'll never do it again!

The question is, WHY is it out of phase? I called the shop back and they insist that it makes no sense to have it out of phase. I can't quite wrap my head around it, so I'm hoping someone can explain WHY (besides the fact that the FSM shows them that way) they should be out of phase, and what will I damage if I install it in parallel (in phase)?

I'm printing the page from my FSM tonight and will take it back in to hav eit rebalanced, but I'm leaving first thing in the morning for Southern Utah and the activities my 80 was designed for, so I don't really have time to do it right now. I'd like to be armed with more knowledge than just, ''Because C-Dan said so" when I go back in to make my case.

 

[sleeoffroad]

I
The question is, WHY is it out of phase? I called the shop back and they insist that it makes no sense to have it out of phase. I can't quite wrap my head around it, so I'm hoping someone can explain WHY (besides the fact that the FSM shows them that way) they should be out of phase, and what will I damage if I install it in parallel (in phase)?

The only reason I can think off is because of the broken back orientation of the pinion and transfer case flange. The flanges are oriented like this \------/ from the factory at stock ride height. I have no idea of this plays a role, but that is the only thing that is different from the 80 to most other 4x4 vehicles. Also might be something to do with the radius arm setup and how it travels and rotates that axle when going up and down. All this is pure speculation.

 

[-Spike-]

How about 'Because Mr. T says so'.

-Spike (What Christo said is what I believe)

 

[80t0ylc]

What's wrong with telling the shop: "I'm paying for this. Do it the way I requested it. If it's wrong, then I'll pay to make it right."? I know they're supposed to be the experts, but I would consider another shop if they have a problem with it. You need Robbie (Powderpig) to chime in.

 

[scottm]

A U-joint doesn't transmit rotation smoothly, it's output shaft accelerates and decelerates twice per rotation. This is more pronounced at higher joint angles. A second U-joint 90* out-of-phase will decelerate and accelerate at exactly the opposite times to cancel out the first U-joint, and the final output will be smooth rotation. Both U-joints have to be at the same joint angle. On our rear driveshafts, the rear axle moves pretty much straight up and down, and the front and rear joints are at pretty much the same joint angles. They are in the same plane, you could lay them on a table and they would lay flat like that. Looking at the front driveshaft, you can see the front U-joint kind of angles to the side, and the rear angles another way. They aren't on the same plane, you couldn't lay them on a table like that. Looking from the front of the truck, the rear driveshaft, including input and output shafts in the axle and t-case, would form a straight line. Looking at the front driveshaft, you'd see a couple angles, about 90* off from each other. The front U-joints in that configuration are pretty close to 90* out-of-phase when they're installed to match each other. The rear U-joints are 90* out of phase when they're installed to mirror each other.

The original 80 FSMs had the front driveshafts drawn with the joints mirrored like the rear driveshaft. Were the trucks sold that way originally? I've wondered if they got vibrations in the front and realized the angles of the joints were not in plane, and they rotated a joint to fix that.

Sorry for the long-windedness, I guess I could've just posted this:
Universal joint - Wikipedia, the free encyclopedia

ps: this phase thing only matters at moderate to high speeds. The angles get funky with suspension travel on the trails, but if you're doing it at speed you have bigger worries. If you lift your truck you only need to get the angles right for it sitting on level ground.

 

[MDarius]

Scottm,
Uhhh...yeah (EDIT: In reference to the Wikipeida entry, not your post). Your explanation was perfect and way easier to understand than the formulas on Wikipedia. 15 years ago when I was taking physics I probably would have understood all that. Now, I understand your longwinded explanation. Thank you. Now I feel like I can speak intelligently about this to the driveshaft guy. Thanks!

I am constantly amazed at the knowledgeable people on this site. I guess smart people drive Land Cruisers...and some of us dumb ones.