THIS is why you use a CV after susp. lift - (long) - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 02-24-2002, 12:08 PM
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THIS is why you use a CV after susp. lift - (long)

I recently went out and bought myself a manual transmissions and drivetrains book, and I must say it was a VERY well spent $80. I just got through reading a chapter on CV joints and U-joints, and I think I now almost fully understand what the true purpose of a CV joint is. I know that some of you big-time techies already know everything I'm about to say, but I KNOW there are some others out there (like me previously) that this will be completely new to. The following little clip from my book explains exactly why you don't use U-joints on lifted Jeeps.


If a U-joint's input shaft speed is constant, the speed of the output shaft accelerates and decelerates to complete a single revolution at the same time as the input shaft. In other words, the output shaft falls behind, then catches up with the input shaft during this revolution. This acceleration and deceleration of the U-joint is known as speed variation.

These changes in speed are more easily visible after examining the action of the U-joint. A U-joint serves as a coupling between two shafts that are not in direct alignment. It would be logical to assume that the entire unit simply rotates. This is only true of the joint's input yoke.

The output yoke's rotational path looks like an ellipse because it can be viewed at an angle instead of straight on. The face of a clock can be used to illustrate the elliptical action of the output yoke (attached pic). The blue clock is the path of the output yoke, while the normal clock represents the path of the input yoke. As you can see, although both yokes cover the same amount of distance in the same amount of time, the output yoke must travel at different rates of speed to accomplish this. The greater the angle between the two yokes, the greater the speed variation.

The resulting acceleration and deceleration produces fluctuating torque and torsional vibrations characteristic of all U-joints. (COMMENT: That is why you get vibrations when you lift your Jeep. You have increased the angle so greatly between the input and output yokes that the output yoke is having to greatly accelerate and then greatly decelerate to keep up.)

The torsional vibrations set up by the changes in velocity are transferred down the drive shaft to the next U-joint (the one mated with the axle pinion). At this joint, similar acceleration and deceleration occurs. Because these speed changes take place at equal and reverse angles to the first joint, they cancel each other out whenever both occur at the same angle.


Well, that's a U-joint in a nutshell. Now you know why people use CV joints on lifted Jeeps. CV stands for constant velocity and because of the way it is designed, the input and output always spin at the exact same speed. U-joints are obvisouly NOT constant velocity, because the output must speed up and slow down to keep up with the input yoke. If they are spinning at the same speed all the time like in a CV joint, then no (or very few) torsional vibrations occur.

I love this kinda stuff so I hope you found it interesting also. If you don't have books on this kind of stuff, I suggest you get some. Hours of reading.

93 YJ/4.0L/33x12.50s/D30 front/D35 rear/NP231/AX-15/3" Rough Country suspension/1.25" shackles
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File Type: gif 9-697562-YokeDiagram.GIF (6.0 KB, 1 views)
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post #2 of (permalink) Old 02-24-2002, 01:20 PM
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Re: THIS is why you use a CV after susp. lift - (long)

Very good post. It is for the same acceleration/deceleration that high rpm motors are smoother with longer connecting rods. The rod is still moving the same stroke (up/down) and lateral (max 9 o'clock and 3 o'clock on the crankshaft) but the rate of acceleration is greatly reduced with a longer connecting rod because the angle has been reduced.

Mike H.
1983 CJ-7 Laredo, 1991 Cherokee Ltd.
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