Proffitts 4-Link - OOPS (1 Viewer)

[Mikesta]

Wow, thanks for taking the time to write this up. Good stuff. Its all tacked in and will remain that way for a while. I Know I'm probably doing this all backasswards but my goal is to get it done and get it done right. I hope to have the engine tranny/transfer mounted within 3 weeks.... then the front axle cut and turn, then the 4 link alligned. I'm going to definitely read this a few more times as I have tons to learn and it takes a while for this knowledge to sink in.

Maybe this is a moot point because you had the frame set up at ride height and the links positioned relative to that when you were designing but I don't see that here. I just see a frame and an axle being put together. In particular, I don't see the powertrain anywhere. Although the heims are "adjustable", you don't want to run them out a lot, so the lengths of the links aren't going to be a major variable.

Have you addressed the pinion angle?

Once you finalize the frame end mounting points on the links, the pinion angle won't be highly adjustable.

One of the interesting properties of a triangluated 4 link suspension comes from the fact that from the side-view, it's either a parallelogram or some sort of trapezoid variation. If the uppers and lowers are perfectly parallel and the frame mounts are parallel to the axle mounts, then the pinion will point the same angle anywhere in the suspension's travel. If the frame end mounting points are all on the same axis, then the links will look like a triangle from the side and the pinion will point toward center point at all times (or whatever fixed angle above or below center it started at). If the frame end mounting points are MORE separated than the axle end, the pinion will swing AWAY from parallel in the same direction as the travel. On uptravel, the pinion angle will go up, and on droop, the pinion angle will go down.

Your frame end mounting points are very close together and the movement of the pinion angle will therefore approximate the behavior of the triangular side view configuration mentioned above. The pinion will basically point at a nearly fixed angle relative to the axis on which the frame ends of the links rotate. It'll basically always point to a spot near that crossmember that the links are attaching around.
Since the t-case output and the origin of the drive shaft will be forward of this crossmember, this means that the pinion angle is likely to be pointing above the drive shaft, and this will be accentuated with droop.

According to what I've read, with leaf springs you want the pinion angle to be a couple of degrees below the driveshaft angle so that with loading, it twists TOWARD perfect alignment. With a linked suspension though, you have any angle you want because you've eliminated the flexible leaf. The issues all come down to U joint or CV capabilities and strength, driveline strain and vibration, and ultimately, keeping the angles within the range that the joints are designed to operate in.

I *REALLY* recommend that you don't finalize your link positions until you're looking at the output from your transfer case at least mocked up in it's intended position. That way you won't get any nasty surprises about the pinion angle.

 

[BellyDoc]

Thanks for not flaming. It's painful to try to describe this stuff clearly. I'm thinking in pictures and writing in words.

 

[etrnlbzz]

Who the hell could flame that??? Most readers are still REREADING the second paragraph.
Thanks DOC, I've followed your build completly and I'm in the middle of my own. This has brought up some new thoughts to my design. I looked at Proffits 4 link but thought I could save a few bucks by building my own.....I'm glad I did. I'm not talking about saving a few bucks, it's the fact that no 4 link is the same!

anyway, thanks again, and looking forward to the current build!

 

[mrqtipp]

where are the updates

 

[spotcruiser]

where are the updates

No updates. Typical mikesta make-believe project.

 

[Mikesta]

Still in a box

Thanks Todd, yes its still in a box. I got busy lately with other projects. Hoping to pull out the boxes before the new year!