In the first installment of this feature, which appreared in the February 2000 issue, we discussed some of the fundamentals of vibrations. We discovered that each part of the car has a specific frequency at which it will respond to vibrations. This is known as natural frequency. When a vibration occurs in any area of the vehicle, the other systems/components will respond to vibration only if their natural frequency is equal to the frequency of the vibration present.
We also stated that when the frequency of the vibration equaled the natural frequency of the system or components in question, they would start to respond to the vibration by vibrating themselves. This is the point at which the customer feels the vibration is at its worst and it generally causes complaints. The highest amplitude (severity), as far as the customer is concerned, is reached at this point, commonly known as resonance.
This fundamental is easy to demonstrate. Say the car you’re working on has a torque converter that is improperly balanced. At idle it feels fine. But as you raise the engine speed slightly, the vibration starts to appear. As the rpm continues to rise, the vibration continues to increase in severity until a specific rpm is reached. At that point, even though the engine rpm is driven higher, the severity of the vibration starts to decrease. Is the vibration really getting better at the higher rpm value? No! The faster the imbalanced component spins (in this case, the converter) the more vibration amplitude is produced. But since we are above or below the other components’ (chassis, body, cross members, etc.) natural frequency, the vibration does not appear to be present.
Many times you will deal with a vibration that appears to come and go, repeating itself, even though you hold the speed/throttle steady. This type of concern is known as beating or phasing. It is caused by two separate vibrations with almost identical frequencies. This causes the vibration forces to add to each other’s amplitude at times, but subtract for each other’s amplitude at other times, causing the vibration to appear as though it is coming and going. Neither vibration is generally bad enough by itself to cause the customer to complain. Therefore, eliminating one of the vibrations generally will take care of the problem.
Vibrations are classified according to their order. The order of a vibration refers to how often the vibration is present in on rotation of the component. See Figure 1. Each vibration is described as first order, second order, third order, etc.
As shown in Figure 1, a tire afflicted with one rough spot would have the spot strike the ground only one time in a rotation; two bad spots in the tire would force the tire to move up two times in one rotation. As you can see, we can make as many bad spots in the tire as we want, and the vibration order simply corresponds to the number of disturbances in one rotation of the component. Therefore, the order of the vibration can be any conceivable number.
You may be wondering why we are concerned with the order of the vibration. The reason is quite simple. As the order increases, the frequency of the vibration will also increase by a multiple of that order. In simple English, if the tire in Figure 1 is rotating at 10 times per second (10 cps), a first order tire concern would register as a 10 Hz vibration. If the same tire had a second order problem, the reading would be 20 Hz (10 tire revolutions per second multiplied by two equals 20 Hz as a second order concern).
A converter or engine balance concern would be a first order vibration. If the engine vibration occurred at 600 rpm we would simply convert the value to Hz by dividing 600 rpm by 60 seconds for a value of 10 Hz. In our example, a crankshaft running at 600 rpm is actually rotating 10 times for each second the engine is running. Since a first order problem occurs once per rotation, our engine/converter vibration concern would register as a 10 Hz vibration at 600 rpm.
Note: Static imbalance or runout concerns are always first order problems. Remember you always have one spot that is heavier (which is where the compensation weight will be placed) than any other when the component has an imbalance problem, no matter what type of component you are dealing with.
To find orders for components such as wheels/tires and propshafts, we will need to know a few things about the vehicle and the vibration that is plaguing it, such as:
• What is the road speed at which the vibration occurs?
• What is the vehicle’s axle ratio?
• What size tires are being used?
If we know the above information, we can determine the exact cause of our vibration, which will then allow us to determine what it will take to fix our concern.
Note: Over 90% of vibration concerns are not caused by faulty components! Therefore, replacing components without knowing what type of failure is present and/or which component is causing the concern is an expensive and time-consuming method of diagnosis.
Charts are available to determine how fast tires rotate at specific speeds. Since this varies with the tire’s loaded rolling circumference, it is important to know this information if we want our calculations to come out perfect. If we don’t mind our calculations being off slightly, we can use a rule of thumb of one tire rotation for every 5 mph in road speed. Most common tires rotate anywhere from 0.9 to 1.2 times for every 5 mph of road speed. Remember, the larger the tire, the fewer times it rotates per 5 mph increments.
Next, we need to know the vehicle’s axle ratio. This can generally be determined by locating the vehicle’s option sticker and interpreting the option code for the axle ratio (some, but not all, vehicles have an axle ratio tag). This also can be determined by rotating the pinion, and noting the number of pinion rotations needed to achieve one rotation of the vehicle’s tires.
After we have acquired this information, we can determine which area of the powertrain is producing the vibration as well as what type of concern is present by using some simple calculations. Let’s see if we can determine what the frequencies would result from a subject car:
Example Car:
Vibration occurs at 50 mph
Axle ratio is 3.73-1
Tire size P195/70R-14 (This tire rotates at 1.17 times per each 5 mph road speed.)
Tire/Wheel/Axle/Brake Drum Speed
50 mph (Vibration Speed) ÷ 5 mph = 10
10 x 1.17 (tire rotations) = 11.70 Hz (This figure represents the frequency that will be registered for a first order tire/wheel/axle/rotor/brake drum concern. This represents a balance or runout concern.)
To determine second order:
Frequency of first order concern times two.
11.70 x 2 = 23.4 Hz
Propshaft Speed
(First order wheel/tire frequency calculations multiplied by the axle ratio.)
11.70 Hz x 3.73 = 43.64 Hz (This figure represents the frequency that will be registered for a first order propshaft, which is either a prop runout or balance concern.)
To determine second order prop:
(First order propshaft calculated frequency multiplied by two.)
43.64 x 2= 87.28 Hz (This figure represents the frequency that will be registered for a second order propshaft, which is either a prop U-joint phasing or U-joint angle concern.)
Notice that each type of failure concern created a different frequency (the fingerprint of the guilty party). As you can see, we can determine what frequency we will see for all types of problems. There is no guess work and there is no changing of parts until we find the one that is causing the problem. Just some simple calculations and a test drive is all it takes!
Repairing Common Vibrations
Now we will discuss how to repair some of the more common vibrations.
In order to diagnose a vibration we must first have the facts from the customer regarding the vibration such as:
• When did the vibration first occur? Could it be related to work previously performed on the vehicle? Has this vehicle been in an accident prior to the concern?
• Is the concern road speed related? If so, at what speed and in which gears? Could it just be wind speed related?
• Does a specific accessory have to be operating for the concern to be present?
• Is the customer concerned with a noise or a vibration or a combination of the two?
• Have others attempted to repair this concern? If so, what has been replaced while trying to repair the concern?
Most vibration concerns have simple origins. Therefore, your strategy should always start with thorough questioning of the customer and a test drive with the customer so he may show you exactly what he wants repaired. I cannot count the number of times I have seen shops struggle repairing what they interpreted to be the customer’s problem only to find out later that the customer was really after another noise or vibration the tech had not even noticed or tried to address.
Your strategy should always concentrate on investigating the simple causes before the complex ones. An engine vibration concern is far more likely to be caused by a loose accessory bracket than by an out-of-balance crankshaft. Your time will be spent much more productively if you keep a few of these simple guidelines in mind.
As we previously explained, we need to develop a strategy to help us isolate the concern quickly and accurately. One such strategy is shown in Figure 2.
We’ve discussed how vibration frequency can help us pinpoint the concern, now it’s time to put what we have learned to work.
Subject Vehicle
• The vibration is observed at 60 mph.
• The vehicle’s axle ratio is 3.08-1; tire size: P235/75R-15 (this tire rotates one rotation per every 5 mph of road speed).
• Predominate frequency measured at complaint speed: 75 Hz (measured with a reed tach or an electronic vibration analyzer).
• The concern is road speed sensitive, not torque sensitive or wind speed sensitive.
• Vibration can be heard and felt in the seat.
Calculations
60 mph (complaint speed) ÷ 5 mph = 12 (5 mph increments)
1st order tire: 12 x 1 = 12 Hz
2nd order tire: 12 x 2 = 24 Hz
1st order propshaft: 12 (1st order tire calculation) x 3.08 (axle ratio) = 36.96
2nd order propshaft: 37 (1st order propshaft calculation) x 2 = 74 Hz
As you can see, our complaint frequency equals one of the calculated frequencies from above. In our example, it appears we have a second order propshaft concern.
First order prop concerns are due to balance or runout problems, while second order indicates a "U-joint phase or a working angle" concern. In this instance we would have been wasting our time if we were to concentrate our efforts on the wheel/tires or if we were to rebalance or replace the propshaft in an attempt to repair the concern.
Some simple calculations and some basic knowledge regarding vibration diagnosis has decreased our diagnostic time and the cost of the repair.
Angles
Universal joints require a minimum working angle in order to properly lubricate the needle bearings of the joint. The U-joint has a maximum working angle which is determined by the rotational speed of the prop shaft. In other words, we need a little working angle for the U-joint or it will bernell and prematurely fail. We cannot handle too much angle or a vibration and joint failure will occur. As the shaft turns, the U-joints speed up and slow down the propshaft twice per rotation (a second order vibration). The steeper the working angle, the greater the speed changes in the shaft will be, thus, the greater the vibration.
That leads us to the question, "What is the maximum working angle for the shaft?" This could range from as high as 11-1/2° for a shaft rotating at 1,500 rpm to as low as 3-1/2° for a shaft rotating at 5,000 rpm. Generally, for the applications we deal with we would like to see a minimum of 1/2° to a maximum of 5° for the typical passenger car or light truck.
To check U-joint working angles, we need an inclinometer. This is a relatively inexpensive tool (generally around $50-$80) and you will find it will pay for itself the first time you use it.
To check working angle, simply rotate the shaft in question until a U-joint is pointing straight down. Attach the inclinometer and move the gauge until the bubble is centered. Note the reading on the tool. Remove the tool, rotate the shaft 90°, reattach the tool to the U-joint cap and center the bubble. Note the reading now on the gauge. To determine the angle, simply subtract the larger of the two readings from the smaller one. See Figure 3.
The above procedure is followed for all joints on all shafts. As each working angle is determined, keep this information in mind:
• Joints on opposite ends of a shaft should be within a 1/2° working angle of each other;
• On shafts with only one U-joint, the angle of that joint should be 1/4 to 1/2°;
• We have been measuring the longitudinal angles only. The other angle the shaft may have is called its lateral angle. This angle represents the amount of sideways offset of the propshaft in the vehicle. Vans are a good example of this. To make room for the driver’s feet, the powertrain is offset to the passenger side of the vehicle. It is possible to have both angles within specs but yet have a vibration from an excessive combination of the two angles known as the "equivalent angle." (Note: Lateral angle is a visual measurement, as typical shop tools are unable to calculate this angle.);
• Changing the angle at one end of the shaft also will affect the angle at the other end of the shaft, so double check all U-joint working angles;
• When dealing with a "launch shudder" concern, keep in mind that the static angles may appear okay. But remember, the nose of the rear axle rotates upward toward the vehicle floor pan when load and engine torque are applied. This must be taken into consideration when working on launch complaints; and
• U-joint angles are checked with the vehicle level and loaded with a weight comparable to the weight it normally carries.
U-joint angle concerns are addressed differently based on the type of rear suspension on the vehicle:
• Coil spring rear suspension - The rear working angle is adjusted on these vehicles typically by replacing the upper control arms for the rear suspension. Most manufacturers make different length upper control arms to allow you to position the nose of the rear axle where you need it.
• Leaf spring rear suspension - The rear working angle is adjusted on these vehicles by placing a tapered shim between the axle spring pad and the leaf spring. Which side of the rear axle you insert the tapered shim under is determined by which direction you want to move the angle, as well as which side of the rear axle the spring pad is mounted to. Tapered shims are available in different offset values from both the OEM as well as the aftermarket. (Note: If you are using a large offset/degree shim, be aware the center bolts’ heads will need to be extended or the axle may shift position causing a thrust angle/tire wear concern);
• Front joint angles are also adjustable. To raise the angle simply install a shim between the trans/transfer case mount and the cross member. Cross member shims are available from both OEM and aftermarket sources. To lower the angle it may be necessary to drop the position of the cross member lower in the vehicle. Shims are not available for this purpose so you will need to fabricate spacers to do the job.
Phase
The other concern we said our example vehicle may be suffering from is incorrect U-joint "Phase." Many prop shafts are equipped with removable slip yokes. This is done to allow us to change the phase of the U-joints. U-joint phase is used for one of two purposes:
1. Counteract the speed increase at one joint of the shaft by creating an opposite phase speed decrease at the opposite end of the shaft. Thus the speed increase at one end is canceled by the speed decrease at the other end, giving an even output speed from the shaft.
2. In some instances the factory will intentionally "mis-phase" the joint to create a vibration to cancel out a vibration from some other cause.
Note: The joint at the slip yoke end is aligned with the joint on the fixed end of the shaft itself. See Figure 4.
Note: Be sure to mark the shaft-to-joint position and the shaft to its mounting flange position before removing the shaft from the vehicle. This is true even on vehicles that do not use conventional slip joints, such as all-wheel drive applications. Failure to follow this rule may result in a vibration after other work was performed on this vehicle.
Phasing of the U-joints consists of aligning the removable slip yoke joints with the joints on the other end of the shaft. This means the slip yoke joint is parallel with shaft joint at the other end. See Figures 4 and 5.
Runout
If our calculations had indicated a first order propshaft concern we would have needed to check the propshaft runout and the balance of the shaft. Runout measurements are accomplished by using a dial indicator in at least three spots along the shaft (3" from each end and in the middle), see Figure 6 above. Always check your runout before you attempt to balance. Balance is used to compensate for slight runout variations in the shaft. If runout is too high, you will be unable to balance the shaft and it must be replaced.
The amount of runout acceptable is determined by the length of the shaft and whether it is a single-, two- or three-piece shaft. Generally, the longer the shaft and the more pieces, the lower the acceptable runout. Check with the manufacturer as shaft runout tolerances can range from as high as .060" to as low as .015" depending on the shaft.
If runout is excessive, first mark the shaft position, mark to point of highest runout, unbolt the U-joint and rotate the shaft 180°. Reinstall the U-joint cap bolts and remeasure the runout. If runout is now acceptable, you’re done. If not, note whether the runout followed the shaft movement or stayed with the flange. This will tell you which component is at fault.
Balancing
Prop shaft balance can be accomplished by sending the shaft out to be balanced; by manually balancing it or by strobe balancing it.
I much prefer on-vehicle balancing for a couple of reasons. First, it compensates for any imbalance in the components attached to the shaft (such as flanges, which is quite common). And second, you make the money, rather than someone else.
To manually balance the shaft proceed as follows:
• Check the shaft runout and mark the spot on the shaft with the highest runout.
• Place two hose clamps with their heads aligned 180° from the point you marked indicating the highest runout. Tighten the clamps in this position. (The heads of each clamp weigh about 1/4 ounce. You are putting the heads opposite the max runout area in an attempt to compensate for the imbalance caused by the runout.)
• On jack stands, run the road speed of the vehicle up to the complaint speed and note the amplitude of the vibration. Stop the vehicle, loosen the clamp and separate the clamp heads slightly in opposite directions. Retighten the clamps. (Note: Separating the heads will act to reduce the total weight available.)
• Repeat the above step if the vibration amplitude is decreasing. When the point of lowest amplitude is achieved, the correct amount of weight and its position have been determined.
• If the amplitude gets higher as you separate the clamp heads, you need to add more weight, so add another clamp. Move the third clamp head while leaving the first two clamp heads aligned opposite the maximum runout position. Continue moving the third clamp until an acceptable amplitude can be achieved. If the amplitude increases as the head of the third clamp is moved from its original spot, more weight is required, which means the shaft should be replaced.
• After determining the correct amount of weight and its position, tighten the hose clamps into position and paint them to match the drive shaft.
Note: Using hose clamps to balance prop shafts is deemed an acceptable service practice by all the major vehicle manufacturers. Tires and brake drums should be removed prior to attempting to balance the prop shaft.
On-vehicle strobe balancing requires a strobe balancer or an electronic vibration analyzer to accomplish the task. See Figure 7.
The procedure is almost identical to manually balancing a shaft, with one exception: The shaft must be marked in at least four spots using separate identifiable characters or letters. This will allow you to pick out the heavy/light spots on the shaft. The heads of the clamps are then placed 180° from the heavy spot.
The strobe light will then indicate if the weight is too light, too heavy, needs to be shifted, or when the shaft is in balance.
Now, this is easier said than done. It takes some practice and some experience. Knowing if the amount of weight is wrong or if the weight needs to be shifted is dependent on where the clamp heads appear as the strobe fires. In addition, most strobe balancers fire on the heavy spot, meaning the weight is placed opposite where you’ve seen the strobe continually flash. EVA flashes on the light spot, meaning you place the weight where you’ve seen the strobe continually flash.
Just like manual balancing, the weight is added, subtracted or shifted until the strobe light indicates that the shaft is in balance. A properly balanced shaft is indicated when the strobe randomly fires, which will appear as random numbers/letters being illuminated as the prop turns or by very low strobe amplitude, which is indicated on the display of most strobe balancers.
In the next segment we will look at wheel/tire and engine/accessory service.