Driveline Geometry 101
Driveshaft Angle Explained
This video will demonstrate the proper angles for a drive shaft in a stock Jeep or Truck and how the rules change when you add a suspension lift.
The video below demonstrates what is happening to the shaft at different angles, how improper angles can cause speed oscillations, and it focuses more on 2WD vehicles and applications that are not lifted Jeeps and trucks. Plus it's just down right fun to watch. As the title screen suggests, in many ways it is a better driveshaft video!
The article below offers an in depth explanation of universal joint operation at angle. Specifically we are focusing on drive shaft angle on lifted trucks and Jeeps. Read if you want to fully understand why drive line geometry is important and how it affects the type of shaft required. If you are looking for guidance on measuring angles, read our guide to measuring drive shaft angles.
O.K. so now you've done it. You put that lift in your vehicle or changed the engine, transmission or transfer case or differential maybe all of the above and now your go anywhere four wheel drive baby rides like a out of balance washing machine. Or now that you've got all that raw power & torque you keep breaking your drive shaft. What do you do? The lift kit manufacturer may tell you one thing and the local drive line shop or mechanic will tell you another. You certainly haven't put this much time, effort and money into creating the ultimate 4X4 to live in fear of the possible catastrophic consequences which can come about (usually at the worst possible moment) from neglecting drive line considerations.
So what do you do? Who do you believe? Larger joints? Possibly a C.V.? Quite frankly, only YOU can answer these questions. As with many problems in life the solution can usually be found by arming yourself with information. Unfortunately for you there is a lot of misinformation out there especially regarding proper u-joint angles. I hope to clear up a lot of this here. Please bear in mind that we are not working with an exact science. Some of the time things that in theory should work, do not, and other times people seem to be happy with a drive-line that by all standards should cause a horrible vibration or short life. Although your chances for success are greater if you do your homework and design around established principles.
My opinions and recommendations are based on numerous sources of information and many years of personal experience. By no means do I know all there is to know about drive lines (or any thing else). The intent here is to give broad general information, realizing that for the most part we are dealing with highly modified vehicles, requiring other than factory approved solutions.
In addition to a straight and properly balanced driveline, proper geometry is the most important design factor to consider when smoothness of operation, ultimate strength, and long life are desired.
If you are like me, rather than relying on just what someone may tell you. You tend to believe something more readily if you have a basic understanding of the principles involved. It is very important that you understand the concept of non-uniform velocity of your drive line caused the u-joints working through an angle:
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.
Now I hope you noticed I stated "very near" when describing this cancellation of non-uniform velocities. this is because the rates of acceleration and deceleration, minimum and maximum velocity, are NOT reciprocal numbers. Min./Max. velocities are a function of the cosine of the operating angle. If for example, to make the numbers easy, the cosine of the angle were .90 and the velocity on the driving shaft were 100 F.P.S. the min. velocity would be 90 F.P.S. and the max velocity would be 111 F.P.S. It is for this reason that on your drive shaft there is an upper limit to how steep you can run a drive shaft. Even with equal or intersecting angles.
So, How steep can you run a drive shaft? Again, this is entirely up to you. However, most manufacturers, such as Dana/Spicer, recommend a maximum of 7 degrees. I personally believe they are conservative (they have to be). I also think they base their recommendations on the math for the largest semi-truck sized driveline and call it good for every thing else (which it would be). Doing the math for an automotive sized driveline, using a 4" swing diameter and assuming the transfer case out-put and pinion shafts are parallel, the actual cosine for an angle of 15 degrees. I calculate the result of the net difference in linear distance traveled through the arc of each of the u-joints' path, to be roughly .0014" per occurrence . I believe there are enough clearances in the universal joint , the slip yoke & spline stub along with a torsional modulus of elasticity in the tubing and other components to accommodate this. Beyond this point the the power train components must themselves flex and distort to allow for this extra motion. This repetitive and continuous flexing will fatigue these components and cause premature failure.
There are other factors to consider though. Beginning with what you are willing to live with. Bear in mind that with a driveline pushed to this 15 degree limit you may notice a slight (slight can be a matter of definition) vibration on smooth highway at about 45-50 M.P.H. when you flutter the gas just right. Most people can live with this. When in doubt or if you are near this upper limit, I recommend that you install a double cardan (CV) type drive shaft.
The geometry you need to maintain with a double cardan drive shaft is different from that of a conventional 2 joint driveline. In many cases, the cost differential between the two types of shafts is minimal and the performance/life gain will pay for itself in the long run.
Another factor seldom considered is the vibrations which will be caused by the forces required for acceleration & deceleration of the mass of your driveline. A driveline which is too heavy and/or having radius which is too large along with running through a steep angle can accentuate a problem here.
Also, you need to know your u-joint life expectancy. Basically a u-joint is rated for specific, continuous operating load @ 3000 R.P.M. for 5000 hrs. with a 3 degree joint angle, and assuming proper periodic maintenance. If you double the angle you halve the life, halve the load you double the life, and vice versa. Because your driveline seldom sees a constant load, u-joint life becomes a difficult number to crunch. While 5000 hrs may not seem like much it's roughly equal to driving 8 hours a day, 5 days a week for 2 & 1/2 years. So 20% of life expectancy may not be such a bad number after all.
Most drive shafts will, depending on components used, incur a binding interference at about 25-30 degrees. You DO NOT, I repeat DO NOT want to run a drive shaft at any where near this angle. You need to allow for axle droop, frame flexing and differential roll. All of which can momentarily alter the operating angle of the u-joint to the point that it will cause what I refer to as an IMMEDIATE & CATASTROPHIC FAILURE. Ultimately you need to be certain that your driveline will rotate freely under full axle droop.
In leaf spring vehicles, it is also very important that you consider the upward pinion movement, caused by spring wrap, on the differential under high torque situations. You can usually get a pretty good idea of how much the differential will roll up, know as axle wrap, by watching this short video.
Many people mistakenly believe that a double cardan (cv) type drive shaft will allow for greater operating angles than a conventional 2 joint or single cardan drive shaft. This is not true. Some types of CVs will actually incur a binding interference at less of an angle than a standard two joint drive line, depending on the individual components used. Additionally, the CV itself is longer than more conventional components and will create a greater operating angle on the driveline, due to a shorter run in the rise/run equation. This is especially true on very short shafts.
The real benefit to a CV (double cardan) drive shaft is smoother operation at higher operating angles and longer life. The CV assembly works by intersecting the joint angles at the center pivot point and delivering a smooth rotational power flow or surface velocity through the drive line. What about the 3rd joint in the double cardan shaft? With this type of driveline it is important to rotate the differential upward so that you have minimal joint operating angle at the differential end of the driveshaft. Any substantial joint angle would cause the pinion to try to speed up & slow down two times per revolution. Causing what is known as a torsional vibration. Torsional vibrations will also be created in a 2 joint driveline that has unequal angles at each of the u-joints or too much angle for each of the u-joints to fully cancel each other out. Rotating the differential upward will lessen the total operating angle at each end of the driveshaft. Now at the transfer case end of the driveshaft you have two joints equally dividing the total angle . This will double the life of the joints at this end, additionally you will be back up to full rated life for the joint at the differential end. I also believe a CV is stronger than a conventional driveline when turning through the same angle. This would be the result of transmitting the torque in a plane more perpendicular to centerline of the driveshaft.
One word of caution though, rotating the differential changes the location of the differential fill plug. Make sure the pinion tail bearing still receives adequate oil. An over-fill may be required, overfilling the differential may cause a problem with foaming of the differential fluid. Adding about a cup of a Dextron type II automatic transmission fluid to your gear oil will lower the surface tension of the oil and should help control the foaming. Additionally while this rotation of the differential is easily done with the rear, front ends create a different problem. Because, unless you are willing to cut the differential housing away from the tubes and reweld, anything you do to correct for driveline angles up front will adversely effect the steering geometry of your vehicle. Most people do just what is done by vehicle manufacturers every day. That is, make some kind of a compromise, get things as right as possible for the high speed rear shaft and live with less than ideal performance from the generally lower speed less used front shaft.
If you understand and apply the concepts that I've attempted to convey here and do your homework, you should be able to figure out the right type of driveshaft for your application and how to properly adjust your angles. Understand that installing a lift or lowering your suspension does more that just make your Jeep or truck sit higher or lower, it changes the angles on your driveline. Understanding these changes will help you to make the right adjustments and/or to get the right driveshaft the first time around. After all, it really is a lot less expensive to do the job right the first time.