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Pinion angle - interesting theory - discuss

Discussion in 'The Hokey Ass Message Board' started by Weasel, Aug 23, 2013.

  1. Weasel
    Joined: Dec 30, 2007
    Posts: 6,698

    Weasel
    Member

    I came across this interesting piece when researching how to set up an O/T - or so I am told - rear end. I would be interested to hear comments from professionals and trained suspension/driveline engineers. Please discuss....

    First and foremost a universal joint is NOT a constant velocity joint.(this is where the concern lies). this means if its driving a shaft at any angle other than zero, although the half of the joint that is connected to the transmission is rotating at a constant velocity the other half of that same joint is NOT rotating at a constant velocity it is actually constantly accelerating and decelerating on each revolution as the joint is working itself around the angles .at one point it has further to travel because of the angle and so speeds up and at another point it has less distance to travel and so slows down(relative to the trans speed).this means the prop shaft is rotating in a 'jerky 'movement constantly slowing down and speeding up even though the trans is at constant speed .now down at the diff end this 'jerky ' rotation will be transferred to the diff u joint which will be transferred to the wheels so what we would have is the trans trying to drive the wheels at a smooth constant speed but the u joint is trying to slow down and speed up the wheels. this means they are in conflict together causing stress and vibration. the more the angle the worse this 'jerky movement becomes. now to get over this if the diff pinion is set parallel with the trans the half of the u joint at the diff end connected to the diff pinion is also rotating with this 'jerky' movement, slowing down and speeding up each revolution but its equal and opposite to the trans u joint so they cancel each other out and the result is the diff pinion and trans are rotating together at constant speed however the prop shaft is still slowing down and speeding up. twice per rev in fact .if the diff pinion is not set parallel with the trans then this canceling effect does not take place and so the diff will not be rotating at a constant speed in relation to the trans. it is for this reason front wheel drive cars use constant velocity (CV)joints on their driveshafts not u-joints because if you imagine a front wheel on full steering lock the angle of the wheel compared to the drive shaft would be so great this slowing down and speeding up of the wheel joint would be so severe while the drive shaft is trying to go at a constant speed it would shake the whole thing apart instantly. saying all this because the angles we're dealing with are relatively small compared to the front wheel drive scenario if the diff pinion isn’t parallel with the trans then maybe its not noticeable from a driving point of view but this jerky movement is still going on and can cause premature wearing of the joints but as many rods aren’t driven as much as a daily probably not noticeable.
     
  2. Dude, that was givin' me a headache.

    Let's try this:

    First and foremost a universal joint is NOT a constant velocity joint. (This is where the concern lies)
    <O:p</O:p
    This means if its driving a shaft at any angle other than zero, although the half of the joint that is connected to the transmission is rotating at a constant velocity the other half of that same joint is NOT rotating at a constant velocity it is actually constantly accelerating and decelerating on each revolution as the joint is working itself around the angles .
    <O:p</O:p
    At one point it has further to travel because of the angle and so speeds up and at another point it has less distance to travel and so slows down(relative to the trans speed).
    <O:p</O:p
    This means the prop shaft is rotating in a 'jerky 'movement constantly slowing down and speeding up even though the trans is at constant speed .
    <O:p</O:p
    Now down at the diff end this 'jerky ' rotation will be transferred to the diff u joint which will be transferred to the wheels so what we would have is the trans trying to drive the wheels at a smooth constant speed but the u joint is trying to slow down and speed up the wheels.
    <O:p</O:p
    This means they are in conflict together causing stress and vibration. The more the angle the worse this 'jerky movement becomes.<O:p</O:p
    Now to get over this if the diff pinion is set parallel with the trans the half of the u joint at the diff end connected to the diff pinion is also rotating with this 'jerky' movement, slowing down and speeding up each revolution but its equal and opposite to the trans u joint so they cancel each other out and the result is the diff pinion and trans are rotating together at constant speed however the prop shaft is still slowing down and speeding up, twice per rev in fact.
    <O:p</O:p
    If the diff pinion is not set parallel with the trans then this canceling effect does not take place and so the diff will not be rotating at a constant speed in relation to the trans.
    <O:p</O:p
    It is for this reason front wheel drive cars use constant velocity (CV)joints on their driveshaft's not u-joints because if you imagine a front wheel on full steering lock the angle of the wheel compared to the drive shaft would be so great this slowing down and speeding up of the wheel joint would be so severe while the drive shaft is trying to go at a constant speed it would shake the whole thing apart instantly.
    <O:p</O:p
    Saying all this because the angles we're dealing with are relatively small compared to the front wheel drive scenario if the diff pinion isn’t parallel with the trans then maybe it’s not noticeable from a driving point of view but this jerky movement is still going on and can cause premature wearing of the joints but as many rods aren’t driven as much as a daily probably not noticeable.<O:p</O:p
     
  3. txturbo
    Joined: Oct 23, 2009
    Posts: 1,771

    txturbo
    Member

    Exactly why the angle needs to be just right. If not that jerking will eventually cause a rear end failure.


    Posted using the Full Custom H.A.M.B. App!
     
  4. bluthndr
    Joined: Oct 4, 2004
    Posts: 254

    bluthndr
    Member

    This is true. You can feel it in any solid axle 4x4 with the front wheels engaged when turning on pavement - it "hops". Ideally you want the trans yoke and pinion parallel. The further away from this ideal, the more driveline vibration you will get. From building and driving lots of lifted trucks, I can tell you that the vibration is usually relatively minor, and some vehicles are more tolerant of it than others. Since most of our cars here are scratch built it's good to get it as close as possible to parallel, but I wouldn't loose too much sleep over it either.

    For what it's worth, other than extreme use (think drag car with slicks or sled pulling) I have never seen even extreme cases cause imminent part "failure" per se. A u-joint that might have gone 60K may only last 10-15K if really misaligned.
     
    Last edited: Aug 23, 2013

  5. So, in a radically Z'd, channeled, cut up, exaggerated stance hot rod, how do you manage the u joint angles while keeping the trans and rear end parallel?

    [​IMG]
     
  6. jmh
    Joined: Jun 30, 2008
    Posts: 438

    jmh
    Member

    Watching.. John
     
  7. On my channeled '40 Coupe, I can change the ride height at the rear by changing the bolt position on the bottom of the coil-overs. Unfortunately, with a 3 degree engine incline and a fully lowered rear end, the u-joint angle at front and back is about 5 degrees. This is pushing the practical limit.

    Doubly unfortunately, on my drive out to Bonneville from Maryland last year, the driveshaft had a severe vibration that destroyed the front u-joint by the time I arrived in Laramie, WY. It turned out that the new driveshaft had been built with the two u-joints out of phase by 95 degrees! If the 5 degrees had been only about 2 degrees, I probably would have hardly felt the problem.

    What I'm trying to illustrate is that the speeding up and slowing down of the main body of the driveshaft is not cancelled out if the u-joints are out of phase, even if the tranny and pinion shafts are parallel. They are all part of a system which requires a fair attention to precision to work smoothly.

    The drive back to Maryland was a LOT smoother.......

    I lucked out. I got a new driveshaft fabricated in Las Vegas at Dan's Driveline and delivered to Bonneville and all was KOOL!
     
  8. Regardless in the difference in height between the transmission and rear end you can still attain a parallel situation as long as you are using angles that are close to the same with one being positive and the other being negative relative to the center line and not exceeding the maximum allowable working angle of the joint . Also it should be noted that even though the ideal setup would be zero angle the u joint bearings will experience brinelling and premature wear. A one degree angle will prevent this.

    [​IMG]
     
  9. greybeard360
    Joined: Feb 28, 2008
    Posts: 2,074

    greybeard360
    Member

    According to just about ANYONE that is in the business ... you do NOT have everything in a dead straight line. There needs to be a minimum of a 1 degree "working angle" between the joints. The illustration above in the second "parrallel" image is where that working angle comes from, a slight difference in the angle of the driveshaft .....

    I wish I could find it, but Dana has a real nice video showing how all this works and a good visual (and sound) of the u-joints speeding up and slowing down as described in the first post. Hard to visualize until you see and hear it.
     
  10. Dan Timberlake
    Joined: Apr 28, 2010
    Posts: 1,530

    Dan Timberlake
    Member

    in post 8 Is all the misalignment in one plane as shown in these pictures ?
    In the bottom situation, is the angle between drive shaft and trans the same as the angle between drive shaft and diff?
    If so, I believe the bottom picture is equivalent to 2nd one down as far as u-joint kinematics ( both non-constant-velocity, and "secondary couple" trying to straighten out and align all 3 shafts) is concerned, because the shaft speed up/slow down happens twice per revolution.
     
  11. mgtstumpy
    Joined: Jul 20, 2006
    Posts: 9,214

    mgtstumpy
    Member

    Another perspective on working angles
     

    Attached Files:

  12. Rickybop
    Joined: May 23, 2008
    Posts: 9,606

    Rickybop
    ALLIANCE MEMBER

    All that just to say that the trans and pinion need to be parallel.

    Not in-line and on the same plane like this: ---- ----

    But on slightly different planes, yet parallel like this: ------ _____
     
  13. landseaandair
    Joined: Feb 23, 2009
    Posts: 4,485

    landseaandair
    Member
    from phoenix

    Although I agree with the importance of having proper U joint angles on both ends I can't really wrap my head around U joints causing the shaft to increase and decrease speed through a revolution as stated in the original post, at least until I see scientific proof. Perhaps the author misunderstands the principles involved and the real issue is related to angular speed or the whip of the heavy U joint body in one direction, pausing, whipping the other way and pausing again? In a Constant velocity joint the only thing bouncing around would be the rollers.

    Just my .02
     
  14. Rickybop
    Joined: May 23, 2008
    Posts: 9,606

    Rickybop
    ALLIANCE MEMBER

    The driveshaft itself doesn't increase and decrease in speed...only the speed that the "working angle" of the u-joint changes. The driveshaft is spinning at Xrpm...but the working angle or inclination of the u-joint is constantly changing...back and forth, back and forth. Due to the difference in angle between the centerline of the driveshaft and the centerline of the pinion, the speed that the working angle changes is constantly...um...changing. It's very similar to why a piston speeds up and slows down throughout it's cycle...slower near tdc and bdc and faster in between...because the relationship (the angle) between the crank journal location (and angle of motion) and the centerline of the cylinder is constantly changing. To move the piston along the bore, the crank journal needs to move toward and away from the bore...but it does so at a slower speed when the connecting rod is at less of an angle to the bore.

    Imagine a tire slowly turning at a constant speed. Pick a single spot on the sidewall...say, the "F" in Firestone. When the "F" is near 3:00 and 9:00, it's moving up and down fairly quickly. But nearer to 12:00 and 6:00, it's moving up and down slower...even though the tire's rotational speed remains constant. Same with a crank journal, and the piston connected to it. And basically the same for the working angle of a u-joint.
     
    Last edited: Aug 23, 2013
  15. 19Fordy
    Joined: May 17, 2003
    Posts: 8,027

    19Fordy
    Member

    Burnt Out Old Mechanic, Thanks for retyping that "essay" into a readable format. I got a headache trying to read that first OP.
     
  16. This is correct, and here's why:

    The movement of the 'U' joint allows the rollers to rotate a bit, thus spreading the lubricant around, and moving the stress points as well.

    If the 'U' joint does not move ('80's Chevy van, front joint) the 'U' joint will fail quickly due to the rollers brinnelling into the cups.

    1º should be considered a minimum, more is better, up to a point (5º?).

    Those of you having trouble visualising the acceleration difference - take out 2 extensions and your 'U' joint from the 3/8" socket set. Rotate them at a fairly extreme angle. Should be obvious.

    Cosmo
     
  17. Truckedup
    Joined: Jul 25, 2006
    Posts: 4,661

    Truckedup
    Member

    Maybe I'm missing it...but.. I always heard you need two U joints so the jerky movement will cancel out. Like a Cardan that has two joints in the same carrier.Or the very common straight shaft with a u joint one each end, or the two u joints on Corvette half shafts.
     
  18. Thus my statement above in post 8....Also it should be noted that even though the ideal setup would be zero angle the u joint bearings will experience brinelling and premature wear. A one degree angle will prevent this.
     
  19. phatim
    Joined: Dec 13, 2012
    Posts: 15

    phatim
    Member

    im setting up my rear end as we speak ( 31 chevy 1/2 ton p/u) I have the rear leaf springs hung and have new spring pads to weld onto the axle. I have the frame level. The rear end is centered. Now I have to determine the pinion angle , then tack weld the spring pads. What would be the ideal angle ? 3 degrees? Sorry if this is a newb question.. im a newb... ha.... Thanks in advance..
     
  20. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,719

    1971BB427
    Member
    from Oregon

    If all this stuff is true, then how did the old enclosed drivelines ever work so well? They had a joint that moved up front with the rearend travel, but always straight into the differential in the rear. From the original post and what others have indicated, this single angle would create a vibration that would not be cancelled by a 2nd u joint.
    I agree that joints need to have a slight angle to work, and self lubricate, and also that the angles can't be too severe, or in the wrong direction, or you'll get vibration. But I'm not buying everything in the original post. A driveline doesn't speed up and slow down. A U joint constantly changes angle as the driveline spins, if it's got any angle to it at all, but that doesn't change driveline speed.
     
  21. I think the enclosed drivelines had something more like a CV joint than a U joint up front(?)
     
  22. Hnstray
    Joined: Aug 23, 2009
    Posts: 12,355

    Hnstray
    ALLIANCE MEMBER
    from Quincy, IL


    None of the common makes did. Chevy, Buick, Ford all had torque tube driveline with a single ordinary u-joint at the transmission end.
     
  23. Hnstray
    Joined: Aug 23, 2009
    Posts: 12,355

    Hnstray
    ALLIANCE MEMBER
    from Quincy, IL



    I would have to disagree with this conclusion. How does the u-joint change "angle" when the driveshaft to transmission angle is constant (excluding normal movement from bumps in the road) ?

    And if the driveshaft doesn't change speed (Velocity) with a common cross type u-joint, why are the other style called "constant VELOCITY Joints"?

    And why would they be used if not to produce smoother, vibration free power transmission ?

    No, I think the original 'theory' is correct. That being, a unique characteristic of cross type u-joint is, when operating at an angle, it does accelerate and decelerate the driveshaft as it turns through it's angle, even though the trans and differential shafts remain at constant speed.
     
  24. blowby
    Joined: Dec 27, 2012
    Posts: 8,661

    blowby
    Member
    from Nicasio Ca

    So if you had a drive shaft where the joint eyes were 90 degrees out of phase from each other front to back, would it self cancel in the bottom image scenario?

    [​IMG]
     
  25. If you were to watch a "U" joint move through an angle (the operating angle) from an end view . You would see that the joint in the driven shaft has to move through an ellipse. Because the joint has to move through each of the quadrants of this elliptical path in a fixed amount of time, the velocity or surface speed of the driven shaft increases & decreases 2 times per. revolution. With a conventional two joint drive shaft, if your second "U" joint has an equal or intersecting angle , The second "U" joint will be decelerating at the same time and at very near the same rate that the first "U" joint is accelerating, resulting in a smooth power flow through to your pinion.

    I would guess that with a torque tube setup with only one joint up front at the transmission the acceleration and deceleration of the joint and subsequent vibration are being absorbed by the torque shaft itself due to it's much smaller diameter and its abuility to wrap up and unwrap under varying loads.

    [​IMG]
     
  26. pitman
    Joined: May 14, 2006
    Posts: 5,148

    pitman

    Phatim,
    3 degrees would be ideal!, in my read as a machine designer, moose has posted what describes the u-joint motion correctly. The pair of shafts, needs to be parallel, and such that an equal and opposite 3 degrees results in the pair of u-joint coupling angles.
     
  27. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,264

    gimpyshotrods
    ALLIANCE MEMBER

    Yes. The last setup is not "wrong". It is a perfectly valid setup. Old Rovers are setup like that.
     
  28. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 6,250

    Ebbsspeed
    ALLIANCE MEMBER

    Absolutely correct! I believe it is called a "W" arrangement, read that somewhere but lost it long ago.


    Gimpy, the answer is No. Like you said, the last illustration is perfectly valid, but the u-joint yokes still need to be aligned front to rear. 90 degrees out of phase front to back would exaggerate the acceleration/deceleration induced by the angle of the u-joint.
    HOTRODDERS.COM definitely has it wrong in saying that the last illustration is a No-No.
     
  29. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,264

    gimpyshotrods
    ALLIANCE MEMBER

    It is in-place, in full-time 4-wheel-drive Land Rovers, across the globe.
     
  30. How the drive shaft in a torque tube works with only one joint is the surface speed is much slower than a shaft of say 3-4 inches of cross section. The surface of a drive shaft does speed up and slow down and the bigger cross section the worse thing are. Its the driving yokes the need to be at 90* JW
     

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