Part 2 - Driveshaft Geometry

Proper driveshaft geometry is critical to getting the most from your driveshafts. Improper geometry will cause vibration, excessive wear, and premature failure. Even the best driveshaft in the world will suck if it is not installed with proper geometry. We also need to know about proper geometry in order to select the best type of shaft for our application.

Before we discuss geometry, a couple more definitions of some terms are in order. For the purposes of this article:

Terms

Angle:

An angle is the measurement of angularity, in degrees, between any two planes. Where those 2 planes meet, they form an angle. In other words, where two lines intersect, providing they are not parallel, there is an angle. In the case of driveshaft tech, where something rotates through that angle, we call it an "operating angle"

Slope

Slope. A slope is a special kind of angle. It is the angle formed between the horizontal surface of the earth (one plane), and the object in question (e.g. driveshaft) (second plane).

When talking about driveshafts, we say that a slope is down if, when viewed from the side of the vehicle, it is higher at the transfer case and lower at the axle (i.e. it descends from the center to the rear of the vehicle for a rear driveshaft or descends from the center to the front of the vehicle for a front driveshaft). The slope is up if, when viewed from the side of the vehicle, it is lower at the transfer case and higher at the axle (i.e. it rises from the center to the rear of the vehicle for a rear driveshaft or rises from the center to the front of the vehicle for a front driveshaft).

Phase:

We say that 2 u-joints are in-phase if they are fitted to yokes that are fixed on the same shaft, such that the 2 opposing bearing caps in the u-joint that are held captive by the yoke on the shaft (the inboard yoke's u-joint caps) are both in the same orientation.

A pic is worth a thousand words. Look at the pic to the left. In the top shaft, the 2 joints indicated by the green arrows (ignore the fact that the right-most is part of a double cardan CV assembly) are in-phase because in both joints, the bearing caps held captive in the shaft are both vertical, while the bearing caps that are free (not held captive in the shaft) are both horizontal. If you then look at the bottom shaft, you will notice that this is not the case with the joints indicated by the blue arrows. The left has captive bearing caps vertically oriented, while the right most joint has it's captive bearing caps oriented horizontally. The joints are therefore 90° out of phase.

Note that this means that they would be in phase if you rotated one or the other 90° (by cutting and re-welding the shaft). Note also that 90° is the most they can ever be out of phase, for as you pass through 90° difference between the two joints orientation, you begin to come back into phase.

U-joint life span.

One geometry factor that is common to all shafts, regardless of type, is the decrease in u-joint life span that is experienced with an increase in the operating angle of that u-joint.

No matter what the shaft style, the greater the angle a u-joint operates at, the shorter it's life span will be. The graph at the left comes directly from Spicer, and shows the range from 100% expected life span at 0° up to just over 15% expected life span at 20°.

As we learned in part 1, there are 2 types of driveshaft that interest us, each with their own separate geometry requirements.

Single-Cardan-style universal joint driveshaft geometry

The single-cardan style driveshaft, also called a "standard" driveshaft, consists of a tubular shaft with 2 tube yokes, one at each end, that each utilize a single cardan u-joint. Recall how, when we have a single u-joint operating at an angle (as will certainly be the case in any 4x4 because the transfer case output will be above the pinion) it causes the driveshaft to speed up twice and slow down twice each revolution. Uncorrected, this change in angular velocity will cause annoying vibration, wear out u-joints, and cause undue stress and strain on the driveshaft itself, transfer case output, and axle pinion.

The solution is simple and elegant. If we ensure that the u-joints at each end of the shaft are both "in phase" and operating through exactly the same angle, the pinion end of the driveshaft will speed-up and slow down opposite to the transfer case end, and therefore the different angular velocities cancel one another out, the pinion is driven at a steady rate, and vibration is minimal (if I did a decent job of describing why the elliptical paths happen in the first place - you should be able to prove this to yourself). This works fairly well up to angles approaching the maximum operating angles of the u-joints in question. As the angles grow, so do the magnitude of the accelerations and decelerations, and the less effective the matched angle are at eliminating vibration.

In other words, eventually, you may have a driveshaft operating at such an angle that, even though the input (transfer case) and output (pinion) operating angles match exactly, the shaft will still vibrate. At this point, it's time for a double cardan CV driveshaft!

Note that in the standard single-cardan shaft "match the angles" geometry the angles do not have to be the same "sign". THIS IS A COMMON MISCONCEPTION. Certainly, the most common method of achieving proper single-cardan shaft geometry is to set the transfer case output and pinion shaft centerlines parallel, thus achieving equal angles between each end of the driveshaft (pic at left).

Normally, this is done by rotating the axle housing (with shims in a leaf-spring suspension, or with relative lengths of upper and lower control arms with a link suspension). This is because the transfer case output is usually considered pretty fixed - the only way to adjust it is to either lower the transfer case (an all around bad idea and bad deal - I speak from experience) or to tilt the engine up (raise the engine mounts) again - not a good idea).

However - this is not the only acceptable method of achieving the proper matched-angle geometry. The angle between the driveshaft and pinion can be opposite to the angle between the transfer case and driveshaft - as long as they are equal. Note that they must still be in phase. This unusual configuration is called "Broken back" or "W" geometry (see pic at left ), and is common on agricultural equipment, marine drives, some tractor-trailers that use stub-shafts between front and rear of a tandem assembly, and most often on power-takeoffs like hydraulic pumps and PTO shafts.

Most of the equipment that uses driven shafts in a broken-back configuration though, are fairly low RPM (but not all). The reason is, due to the nature (geometry) of the configuration (again, have a look at the picture above) with this setup, there's a lot more inherent strain on the slip member as it rotates. because of the opposite angles, the shaft "wobbles" the slip member back and forth as it rotates - like a skipping rope being swung. At high rpm, with anything but the tightest slip-joint assembly, this would cause a horrible vibration - that's why Spicer light duty driveshafts do not normally come factory in this arrangement. Note however, that some Land Rovers do have stock driveshafts in the broken back configuration, so it can work! I don't have any experience with these Rover's, but I imagine that the angles in the stock configuration are pretty darned small. As with the more standard single-cardan setup - start increasing the operating angles of the u-joints and the elliptical paths get more and more elliptical, the angular velocities (amount the shaft speeds up and slows down each revolution) get greater, and when you spin that shaft at 1000rpm, the more likely it is that the shaft will be noisy, harsh, and vibrate - EVEN IF the angles are matched. Again...time for the double cardan CV shaft!

Double cardan (near) constant velocity driveshaft (commonly known as a CV driveshaft) geometry.

This is because of the clever design of the CV joint, or head assembly. If you refer back to the picture of the CV shaft components in pat one, you will see that the CV head assembly (double cardan joint) contains two cardan style universal joints and a centering yoke assembly. This clever arrangement serves to neutralize the effects of the increasing and decreasing angular velocities, right at the head assembly. This relieves us of having to arrange the pinion yoke operating angle to be equal to the transfer case operating angle. Instead, the pinion is arranged so that the operating angle between it and the driveshaft is zero degrees (0°).

Measuring and calculating universal joint and driveshaft operating angles

So, we know what the geometry is supposed to be - how do we find out what ours actually is, and what do we do about it? This section discusses measuring your geometry, calculating the results, interpreting the results, and making necessary adjustments.

The first 2 steps, measuring slopes of components, and calculating operating angles are the same, regardless of whether you have a standard single-cardan shaft or a double-cardan CV shaft. The third step, interpreting the results, will differ, depending on the style of driveshaft.

Before attempting to measure angles ensure that tire air pressure is correct, that the vehicle is at the correct trim (chassis) height (i.e. suspension loaded, operating weight in/on the vehicle, etc.) and the ground surface is perfectly level. The driveshaft also needs to be installed and torqued to spec.

Step 1 - Find the slopes of the components involved.

Recall that the slope is the angle formed between the component in question, and the horizontal. To find these slopes, we measure them with a protractor as follows:

Transfer case

Measure the slope of the transfer case output yoke by placing the protractor or angle finder (inclinometer) on the bottom of the bearing cup, (see pic at left). Position the angle finder on the clean flat surface of the bearing cup, level the bubble (if equipped) and note the reading.

If your transfer case has an output flange, the best way to measure the slope is to temporarily remove the shaft from the flange,place the angle finder against the flat machined surface of the flange, then add or subtract 90° from the reading taken. Remember to re-attach the driveshaft so that you can correctly measure its slope.

Remember that the slope is "down" if it is higher at the t case than at the axle.

Driveshaft

Measure the drive shaft slope, as shown, by placing the angle finder directly against the tube. level the bubble (if equipped) and note the reading.

Remember that the slope is "down" if it is higher at the t case end than at the axle end.

Axle Pinion

Measure the slope of the pinion yoke by placing the protractor or angle finder (inclinometer) on the bottom of the bearing cup, (see pic at left). Position the angle finder on the clean flat surface of the bearing cup, level the bubble (if equipped) and note the reading. Alternatively, the angle finder can be placed against a flat machined surface that lies 90° to the pinion shaft centerline, the reading taken, and then 90° added to or subtracted from the result.

Remember that the slope is "down" if it is higher towards the center of the vehicle, and lower at the end of the vehicle.

Step 2 - Find each operating angle between each pair of slopes as follows:

If both slopes are in the same direction (up or down), subtract the lesser number from the greater to obtain the operating angle.

If both slopes are in different directions (one up and one down), add the lesser number to the greater to obtain the operating angle.

In the special case of calculating the operating angle at the pinion in a CV shaft application, assign the pinion slope a (+) sign if it is DOWN and a (-) sign if it is UP, then always subtract the pinion slope from the driveshaft slope, regardless of their relative sizes, and make note of the resulting sign (+ or -), as this will help in deciding any possible corrective action.

Step 3 - Interpret the results

Assuming a fairly standard 4x4 setup with no pillow blocks or intermediate shafts, once you have measured all the slopes of the components, and calculated all the operating angles, you should have a piece of paper that has 3 slopes and 2 operating angles written on it. It helps to have a sketch too, like those shown below on the left.

Standard (single cardan) Shaft:

The first operating angle must be less than the maximum operating angle of the series of u-joint used, and preferably less than half the maximum (remember - you need to allow for increase in operating angles while off road due to suspension movement, and also how the higher the operating angles, the more likely vibration, even if geometry is correct). If it is not, you have only 2 choices: lower the suspension or drop the drivetrain (lower t-case); or switch to a CV style shaft.

The second operating angle should be within 0.5° (1/2)° of the first AT CRUISE THROTTLE. This is a critical point. virtually every axle (to a greater or lesser degree, depending on power and suspension) will experience some "axle wrap" or pinion rotation ( pinion rotates up in rear axle, and down in front axle) depending on acceleration and to some extent braking torque. This will of course alter the geometry of the pinion, and therefore the whole driveshaft! Since the driveshaft will presumably spend most of it's time (and therefore the effects of it's vibrations will be most annoying and damaging) in a cruise throttle condition, it is standard practice to set driveshaft geometry for this state (If you have a highly specialized vehicle, like a drag car, this may not apply - and you will want to discuss your needs with an expert like Jess at High Angle Driveline). Generally, for most truck rear axles, at cruise, the pinion will rotate up 1-2° from its static position. As such, it is common practice to shim the axle or adjust the links, rotating the pinion and changing the pinion slope at rests, such that the pinion slope is 2° lower than that required to achieve equal operating angles at rest. Read that bit again, carefully! It's a bit of a juggle, because as you adjust the pinion slope itself, so you also actually alter the driveshafts slope, and therefore the transfer case operating angle as well. Once you get close though, you will easily end up at the correct balance point. the point I'm making is, don't just make a whopping 20° change to the pinion angle, then weld those spring perches on and call it done. That big of a change will have effected things, so you'd have to measure and re-calc all the slopes and operating angles again, as you hone in on the final setting.

Example:

Looking at the sample worksheet to the left, and assuming it is for a rear driveshaft

We can see that the front operating angle is 12°. Assuming we are using a 1350 series u-joint, and the vehicle is not a frequent, long-range freeway cruiser, nor does it have a super-flexy suspension with monstrous travel, we decide that this is satisfactory. The axle joint operating angle is 11°. Because it is a rear driveshaft, the rear pinion will rotate up, let's say 2° under cruise throttle. Since our measurements and calculations were done at static, this means that in reality, under cruise throttle, the pinion slope would change from 6° down to 8° down (even though the pinion wraps "up" - remember we describe slopes as up or down depending on orientation between t-case and axle). This would make the axle joint operating angle actually 17° - 8° = 9° at cruise. Since we need it to equal 12° at cruise, we need to rotate/shim the pinion at rest down 3°. This will result in a static pinion slope of 3°. the would net a cruise throttle pinion slope of 3° + 2° = 5°. That would make our axle joint operating angle now 17° - 5° = 12° - A perfect match for the front!

Of course - we would need to make this adjustment (rotate pinion down 3° at rest) and then re-measure and re-calc everything to get it perfect, as the change may effect driveshaft slope and therefore t-case joint angle too. However, this is unlikely with the magnitude of the changes in this example.

Double cardan CV shaft

The first operating angle, the CV joint op angle, must be less than the maximum operating angle of CV joint used. Remember - you need to allow for increase in operating angles while off road due to suspension movement. How much you must allow will depend entirely on your suspension design and the terrain driven. If it is not, you have only 2 choices: lower the suspension or drop the drivetrain (lower t-case); or switch to a higher angle capable CV joint shaft.

The second operating angle, the pinion op angle, should be within 0.5° (1/2)° of zero (0°) AT CRUISE THROTTLE. This is a critical point. virtually every axle (to a greater or lesser degree, depending on power and suspension) will experience some "axle wrap" or pinion rotation ( pinion rotates up in rear axle, and down in front axle) depending on acceleration and to some extent braking torque. This will of course alter the geometry of the pinion, and therefore the whole driveshaft! Since the driveshaft will presumably spend most of it's time (and therefore the effects of it's vibrations will be most annoying and damaging) in a cruise throttle condition, it is standard practice to set driveshaft geometry for this state Generally, for most cars and trucks rear axles, at cruise, the pinion will rotate up 1-2° from its static position. As such, it is common practice to shim the axle or adjust the links, rotating the pinion and changing the pinion slope at rests, such that the pinion slope is 2° lower than that required to achieve an operating angles of zero at rest. Read that bit again, carefully! It's a bit of a juggle, because as you adjust the pinion slope itself, so you also actually alter the driveshafts slope, which directly affects what your pinion angle must be, in a feedback type loop. Once you get close though, you will easily end up at the correct balance point. The point I'm making is, don't just make a whopping 20° change to the pinion angle, then weld those spring perches on and call it done. That big of a change will have effected things, so you'd have to measure and re-calc all the slopes and op angles again, as you hone in on the final setting.

Example:

Looking at the sample worksheet to the left, and assuming it is for a rear driveshaft

We can see that the front CV operating angle is 22°. Assuming we are using a High Angle 1350 series CV shaft, we are comfortably within the proper operating spectrum here. The axle pinion joint operating angle is -1°. Because it is a rear driveshaft, the rear pinion will rotate up, let's say 2° under cruise throttle. Since our measurements and calculations were done at static, this means that in reality, under cruise throttle, the pinion slope would change from 28° down to 30° down (even though the pinion wraps "up" - remember we describe slopes as up or down depending on orientation between t-case and axle). This would make the axle joint operating angle actually 27° - 30° = -3° at cruise. Since we need it to equal 0° at cruise, we need to rotate/shim the pinion at rest down 3°. This will result in a static pinion slope of 25°. This would net a cruise throttle pinion slope of 25° + 2° = 27*. That would make our cruise throttle axle joint operating angle now 27° - 27° = 0° - A perfect setup for a CV shaft!

Note that, in the case of a CV shaft, since we always calculate the angle the same way driveshaft slope minus pinion slope, the sign of the result tells allows us to write an equation, the result of which tells us whether we need to rotate the pinion at rest down or up.

The equation is RR=DS-PS-WA. Where RR = rotation required (of the pinion at rest), DS - driveshaft slope, PS = pinion slope, and WA is the estimated pinion wrap angle. If the result is (-), we must rotate the pinion down at rest that many degrees, if the result is (+) we must rotate the pinion up that many degrees.

Of course - we would need to make this adjustment (rotate pinion down 3° at rest) and then re-measure and re-calc everything to get to perfect, as the change may effect driveshaft slope. However, this is unlikely with the magnitude of the changes in this example.

How to chose a Driveshaft for your Rig.

This is actually a fairly easy one to answer.

First - buy the absolute best you can possibly afford. Why? It is almost impossible to overstate the massive annoyance of a bad driveshaft. Driveshaft vibration is horrendously annoying - street queen or trail-only rig. Believe me, I know. My current buggy, the Wolf, is always trailered, and spends 90% of it's time below 10mph - occasionally it may get to 40mph. I had NO IDEA how bad my cobbled-together shaft was until I replaced it with a High Angle driveshaft. Even at slow speeds, a crappy driveshaft will EASILY suck the fun out of driving your rig. I figured - it's trail buggy - it's loose and noisy anyway - who cares. Well, I learned....it is Sooooooooo much nicer with a decent balanced shaft, operating within it's angle capabilities - and of course - the peace of mind for the components that actually put the power to the axles is priceless.

Do you need a a CV shaft, or will a standard single-cardan style do? Well, in my opinion, unless your rig is only an inch or 2 over stock suspension height, with minimal wheel travel / flex, the answer is definitely YES!

The CV driveshaft offers several HUGE advantages to the off-road vehicle.

First, the pinion and pinion yoke can be rotated up out of harms way, where it will be less susceptible to damage from rocks and other obstacles.

Secondly, the only limit to our operating angle at the CV joint (and thus how much suspension height we can run) is the limit of the double-cardan joint itself. A stock Spicer double cardan CV joint can run successfully at about 22°, and a High Angle Driveline double-cardan CV joint can be run successfully at as much as 32° !!! This allows for successful driveshaft installations in vehicles with much more suspension height than a simple single-cardan shaft can accommodate. Even if you matched the angles on a single cardan shaft exactly, you could never run it safely and vibration free at 30°!

Thirdly, the double-cardan CV joint assembly is simply better and more efficient at reducing or eliminating driveshaft vibration. Even at smaller angles, and even with correct matched-angle geometry, the single-cardan shaft is still susceptible to vibration. The CV shaft will always run smoother, quieter, and with less stress on the u-joints, and transfer case and pinion shafts, bearings, and seals.

The only advantage the single-cardan shaft has is that it is cheaper to manufacture / buy, and you don't have to buy a centering yoke or a third u-joint when rebuilding it. As always - the best costs a little more!