Harwood "Eagle" Loomis++
Death Wobble: One of the most widely discussed and misunderstood concepts of owning and driving a Jeep (any model), or any off-road vehicle with a solid front axle. Death wobble is frightening and potentially dangerous, often rearing its ugly head with no warning and for no apparent reason. What is it, and how can it be eliminated?
Before discussing what death wobble is, we must first understand what it is not. What it is not is shimmy or tramp. Shimmy and tramp are controllable; you can drive the vehicle and remain in control. Death wobble is not controllable. Automotive tires and wheels revolve (rotate) at comparatively high speeds. Seemingly small deviations from perfect balance can result in misbehaviors such as wheel shimmy or tramp. These phenomena are found in all cars and trucks, but they are not necessarily “death wobble.”
If a front wheel has too much weight in one spot on the outer or inner rim of the wheel, as speed increases that heavy spot will tend to be thrown more toward the centerline axis of the plane of revolution. This causes the wheel to wobble or shake, especially if there is any free play or looseness in the wheel bearings or the steering components. This kind of imbalance usually starts to show up at speeds between 50 and 55 MPH, and disappears between 60 and 65 miles MPH. It is generally felt as a slight or even moderate shaking of the steering wheel. This kind of wheel shake is called “shimmy.” It is possible to continue driving with shimmy, and shimmy usually disappears if speed is increased to more than 65 MPH.
Now picture the same front wheel but with equal amounts of excess weight at the same point of the wheel circumference, but half to the inside and half to the outside. The weight is symmetrical around the plane of revolution, but around the axle (the axis of rotation) there is one very heavy spot. This is the cause of seeing a car zooming down the highway with one front wheel bouncing up and down off the pavement like a basketball, often with the driver seemingly oblivious to what’s happening. This kind of imbalance causes wheel “tramp,” that severe vertical bouncing of the wheel.
With either shimmy or tramp, it is usually possible to continue driving at legal highway speeds or above. The only damage is accelerated wear of the tire or tires involved, and of front suspension and steering components. Since many cars drive on the highways every day with these problems, they obviously don’t cause death wobble, and they are not to be confused with death wobble.
Death wobble, by informal definition, occurs when BOTH front wheels engage in violent and uncontrollable shaking such that it is impossible to control the vehicle or to proceed. Applying the brakes often only makes the shaking worse, and the only recourse is usually to slow almost to a stop to end the shaking. Although many people write or speak of “a hint of death wobble at 52 MPH,” or “slight death wobble,” or “emerging death wobble,” it’s clear that these people are referring to something other than death wobble. There is no “slight” death wobble. It is not possible to proceed at speed with true death wobble. There is no “emerging” death wobble; either you have it, or you don’t. If you have experienced it, you will definitely know it. It’s when it feels like a demented demon has seized the front suspension of your vehicle and is shaking it violently with the clear intent of flinging it – and you – as far off the road as possible. As one writer expressed it, it’s when you have to slow almost to a stop to control the wheel shaking, then come to a complete stop to control your shaking and to change your under shorts.
What is happening with death wobble is that erratic motion/shimmy/shake/wobble in one front wheel is somehow being transmitted to the other front wheel, and the two wheels interact in a kind of harmonic reinforcement that increases and magnifies the initial wobble or movement until the amplitude reaches a degree that is impossible to control. This harmonic build-up takes place exceedingly rapidly, fast enough that for practical purposes it is virtually instantaneous.
In general, it appears that death wobble may be set off by one of three mechanisms:
Wheel imbalance (shimmy in one front wheel sets off a corresponding shimmy in the opposite front wheel, and the two reinforce each other;
Encountering a severe bump with one front wheel, throwing the axle into death wobble; or Warped brake rotors starting one or both front wheels wobbling.
In each case, it is not the simple fact of one front wheel wobbling or shaking that makes for death wobble. Death wobble occurs when the movement (shake, wobble, shimmy, tramp, or whatever) in one front wheel is transferred through the steering and suspension to the other front wheel, and the other front wheel begins to shake or wobble in a frequency that harmonically reinforces the movement in the first wheel. Thus, each wheel feeds off the other, and the problem escalates so rapidly that to the driver it appears instantaneous.
Unfortunately, there is no one solution to death wobble because there is no one cause. It is obviously inaccurate to say that a lifted suspension “causes” death wobble, both because there are numerous examples of lifted vehicles that do not exhibit death wobble, and because there are 100 percent stock vehicles that do exhibit death wobble. However, there are some common threads to be investigated if you encounter true death wobble with your vehicle.
First, be certain that all front suspension components are free of excess play or slop, and that all bolts and fittings are properly tightened to the recommended torque values. Something as simple (and as easy to overlook) as a loose or worn track bar end or bushing can allow the front axle to dance around under the vehicle if a wheel begins to shake. Each steering knuckle has two ball joints: one upper and one lower. There are tie rod end type fittings where the drag link attaches to the pitman arm (off the steering box); where the drag link attaches to the passenger side steering knuckle; where the end of the tie rod attaches to the drag link; where the opposite end of the tie rod attaches to the driver’s side steering knuckle; and (unless you have a heavy-duty aftermarket track bar) where the upper end of the track bar attaches to the frame bracket. All of these fittings much be checked for excessive wear, and for correct torque.
Second, check the tires both for balance and for out of roundness. It’s a sad fact of life that modern, computerized tire balancing machines don’t assure that tires will be properly balanced. Often, the operators are poorly trained in the correct use of the equipment, and/or they don’t understand the importance of accurate tire balance and they rush through a set of tires with a “that’s close enough” attitude. “That’s close enough” may suffice for small, low profile tires such as those commonly found on many imported cars, but it does not suffice when dealing with the very large, very heavy tires often found on off-road vehicles. If all front suspension and steering components appear to be in good condition and tight, if death wobble occurs while driving straight ahead on a relatively smooth road and kicks in between 50 and 55 MPH, wheel balance is the probable cause. Take the vehicle back to the shop, tell them the tires shake, and make them rebalance the tires. At the same time, ask them to check the tires for out-of-round. One of the 4WD magazines recently (as of September 2002, as this article is being written) wrote a report on the benefits of having tires “trued.” Truing is nothing more than shaving the high spots off the circumference of a tire to make its rotation more perfectly round. Do not discount this possibility. The author has encountered tires from reputable manufacturers which simply could not be made to run smooth.
If death wobble rears its ugly head when the brakes are applied, look at whether the front rotors may be warped. When Chrysler took over the Jeep/Eagle division from AMC, one of the changes they made (affecting all Jeep vehicles) was to change from homogeneous rotors to composite rotors. These composite rotors are notorious for warping, often within less than 5,000 miles of vehicle purchase or rotor installation. The author’s sole experience of death wobble was in a completely stock, almost new 1999 Grand Cherokee WJ—which had warped rotors. Applying the brakes lightly to control speed down a long grade on a 55 MPH road was all that was necessary to induce death wobble.
If all the above considerations can be eliminated, steering alignment (geometry) should then be investigated. There are three settings for front suspension/steering geometry. Camber is measured in degrees deviation from vertical and is factory-set at zero and cannot be adjusted; therefore, it will not be discussed. If camber is other than zero, axle damage is probable.
Toe-in/toe-out is measured in inches, as the difference between the track at the front of the front tires and at the rear of the front tires. The factory service manual (FSM) for the Cherokee XJ specifies a toe-in value of zero, plus-or-minus 1/32 of an inch. The problem is that, especially with large tires, any toe-out at all seems to contribute to death wobble. If there is any slop or wear in steering components, a setting of “zero” on the alignment machine turntable may result in an actual toe-out condition on the road. For all vehicles, toe-in should be adjusted with a spreader bar between the front tires (at the leading edge) and the toe-in set to zero or plus 1/32 of an inch.
Caster is the vertical angle, in degrees, that the imaginary line between the upper and lower ball joints leans back from the vertical. Positive caster is when the upper ball joint is farther back than the lower; negative caster is when the upper ball joint is ahead of the lower. For the XJ Cherokee, the FSM gives contradictory information. In the 1994 FSM as an example, in one place it specifies a caster of +6.5 degrees for XJs with manual transmission, and +8.0 degrees for XJs with automatic transmission. In the table of alignment specifications, however, the specified preferred caster is +6.0 degrees, and the acceptable range is 5 to 9 degrees.
In checking and adjusting caster, it must be kept in mind that the steering knuckles are welded to the axle tubes, and so is the differential. Adjusting the caster angle also adjusts the angle of the front differential and pinion yoke. An excessive angle between the pinion and the front driveshaft can result in damage to the u-joints. The FSM states that in adjusting caster, the pinion angle takes precedence.
With a stock vehicle this is not a problem. Due to the geometry of the front axle and its upper and lower control arms, however, caster does become a problem with lifted vehicles. All XJ lifts are suspension lifts, meaning that the body and uniframe is lifted away from the axles. The front axle remains at the same height relative to the ground but the chassis, and with it the transfer case, are raised. This increases the angle between the pinion shaft and the driveshaft, and at the same time reduces the caster angle. Attempting to adjust the caster angle back to within specified values, however, may result in excessive driveline angles at the pinion yoke. Thus, any attempt to align a lifted Cherokee becomes a balancing act, between achieving sufficient caster to prevent death wobble and tipping the differential so far that the front pinion angle becomes excessive.
Why is caster important? Consider how a tire rides on pavement. If you consider a theoretical tire with an absolutely flat tread and perfectly square shoulders, with the vehicle moving straight ahead the tread is perfectly flat against the pavement. When the vehicle turns, the tire and the tread “contact patch” will rotate around the axis defined by the two ball joints. With zero caster and zero camber, this axis will fall in the center of the contact patch and the tread will remain flat on the pavement.
With positive caster, however, the steering axis intersects the pavement somewhat ahead of the center of the contact patch, and with enough positive caster it falls completely outside of and in front of the contact patch. What happens when you turn the wheel under this geometry is that the contact patch does not remain flat on the pavement; the tire tends to lift up on the inside (or outside) shoulder. When you release the steering wheel, the vehicle weight acting on the tire tends to push the tire back down to the flat contact patch, which is why steering is to an extent self-centering. It is this self-centering effect that helps to combat death wobble. The effect is increased in direct proportion to positive caster, and helps to stop wobble in a wheel before it becomes uncontrollable. As caster is reduced, it becomes easier for a wheel to wobble, and to transmit this wobble to the other wheel, thus triggering an episode of death wobble.
As noted, because caster angle is related to and affects front pinion yoke angle, it is impossible to make any absolute statement regarding what the caster angle “should” be for a lifted vehicle. In general, it is probably safe to say that under no conditions should the caster be allowed to go to less than +5.0 degrees, and preferably not less than +6.0 degrees.
Lastly, to close out this discussion of death wobble it is important to dispel one last myth: that a defective or inadequate steering stabilizer (damper) “causes” death wobble. This statement is a confusion of cause and effect. A steering stabilizer is nothing more or less than a horizontally-mounted shock absorber. It helps to combat death wobble by damping the transmission of wobble and shake between the two front tires, but if one or both tires are not wobbling the damper is inert and performs no function. The steering damper cannot cause death wobble. A weak steering damper may allow death wobble to develop, and it may appear that replacing the damper has “eliminated” death wobble, but in fact all that has happened is that the cause remains but the symptoms have been temporarily masked. There are people driving lifted, highly modified Cherokees with no steering damper. Therefore, even if replacing the damper appears to remove death wobble, it is advisable to continue investigating until the underlying cause can be found and remediated.
© 2002 Harwood W. Loomis (All rights reserved)
Death Wobble: One of the most widely discussed and misunderstood concepts of owning and driving a Jeep (any model), or any off-road vehicle with a solid front axle. Death wobble is frightening and potentially dangerous, often rearing its ugly head with no warning and for no apparent reason. What is it, and how can it be eliminated?
Before discussing what death wobble is, we must first understand what it is not. What it is not is shimmy or tramp. Shimmy and tramp are controllable; you can drive the vehicle and remain in control. Death wobble is not controllable. Automotive tires and wheels revolve (rotate) at comparatively high speeds. Seemingly small deviations from perfect balance can result in misbehaviors such as wheel shimmy or tramp. These phenomena are found in all cars and trucks, but they are not necessarily “death wobble.”
If a front wheel has too much weight in one spot on the outer or inner rim of the wheel, as speed increases that heavy spot will tend to be thrown more toward the centerline axis of the plane of revolution. This causes the wheel to wobble or shake, especially if there is any free play or looseness in the wheel bearings or the steering components. This kind of imbalance usually starts to show up at speeds between 50 and 55 MPH, and disappears between 60 and 65 miles MPH. It is generally felt as a slight or even moderate shaking of the steering wheel. This kind of wheel shake is called “shimmy.” It is possible to continue driving with shimmy, and shimmy usually disappears if speed is increased to more than 65 MPH.
Now picture the same front wheel but with equal amounts of excess weight at the same point of the wheel circumference, but half to the inside and half to the outside. The weight is symmetrical around the plane of revolution, but around the axle (the axis of rotation) there is one very heavy spot. This is the cause of seeing a car zooming down the highway with one front wheel bouncing up and down off the pavement like a basketball, often with the driver seemingly oblivious to what’s happening. This kind of imbalance causes wheel “tramp,” that severe vertical bouncing of the wheel.
With either shimmy or tramp, it is usually possible to continue driving at legal highway speeds or above. The only damage is accelerated wear of the tire or tires involved, and of front suspension and steering components. Since many cars drive on the highways every day with these problems, they obviously don’t cause death wobble, and they are not to be confused with death wobble.
Death wobble, by informal definition, occurs when BOTH front wheels engage in violent and uncontrollable shaking such that it is impossible to control the vehicle or to proceed. Applying the brakes often only makes the shaking worse, and the only recourse is usually to slow almost to a stop to end the shaking. Although many people write or speak of “a hint of death wobble at 52 MPH,” or “slight death wobble,” or “emerging death wobble,” it’s clear that these people are referring to something other than death wobble. There is no “slight” death wobble. It is not possible to proceed at speed with true death wobble. There is no “emerging” death wobble; either you have it, or you don’t. If you have experienced it, you will definitely know it. It’s when it feels like a demented demon has seized the front suspension of your vehicle and is shaking it violently with the clear intent of flinging it – and you – as far off the road as possible. As one writer expressed it, it’s when you have to slow almost to a stop to control the wheel shaking, then come to a complete stop to control your shaking and to change your under shorts.
What is happening with death wobble is that erratic motion/shimmy/shake/wobble in one front wheel is somehow being transmitted to the other front wheel, and the two wheels interact in a kind of harmonic reinforcement that increases and magnifies the initial wobble or movement until the amplitude reaches a degree that is impossible to control. This harmonic build-up takes place exceedingly rapidly, fast enough that for practical purposes it is virtually instantaneous.
In general, it appears that death wobble may be set off by one of three mechanisms:
Wheel imbalance (shimmy in one front wheel sets off a corresponding shimmy in the opposite front wheel, and the two reinforce each other;
Encountering a severe bump with one front wheel, throwing the axle into death wobble; or Warped brake rotors starting one or both front wheels wobbling.
In each case, it is not the simple fact of one front wheel wobbling or shaking that makes for death wobble. Death wobble occurs when the movement (shake, wobble, shimmy, tramp, or whatever) in one front wheel is transferred through the steering and suspension to the other front wheel, and the other front wheel begins to shake or wobble in a frequency that harmonically reinforces the movement in the first wheel. Thus, each wheel feeds off the other, and the problem escalates so rapidly that to the driver it appears instantaneous.
Unfortunately, there is no one solution to death wobble because there is no one cause. It is obviously inaccurate to say that a lifted suspension “causes” death wobble, both because there are numerous examples of lifted vehicles that do not exhibit death wobble, and because there are 100 percent stock vehicles that do exhibit death wobble. However, there are some common threads to be investigated if you encounter true death wobble with your vehicle.
First, be certain that all front suspension components are free of excess play or slop, and that all bolts and fittings are properly tightened to the recommended torque values. Something as simple (and as easy to overlook) as a loose or worn track bar end or bushing can allow the front axle to dance around under the vehicle if a wheel begins to shake. Each steering knuckle has two ball joints: one upper and one lower. There are tie rod end type fittings where the drag link attaches to the pitman arm (off the steering box); where the drag link attaches to the passenger side steering knuckle; where the end of the tie rod attaches to the drag link; where the opposite end of the tie rod attaches to the driver’s side steering knuckle; and (unless you have a heavy-duty aftermarket track bar) where the upper end of the track bar attaches to the frame bracket. All of these fittings much be checked for excessive wear, and for correct torque.
Second, check the tires both for balance and for out of roundness. It’s a sad fact of life that modern, computerized tire balancing machines don’t assure that tires will be properly balanced. Often, the operators are poorly trained in the correct use of the equipment, and/or they don’t understand the importance of accurate tire balance and they rush through a set of tires with a “that’s close enough” attitude. “That’s close enough” may suffice for small, low profile tires such as those commonly found on many imported cars, but it does not suffice when dealing with the very large, very heavy tires often found on off-road vehicles. If all front suspension and steering components appear to be in good condition and tight, if death wobble occurs while driving straight ahead on a relatively smooth road and kicks in between 50 and 55 MPH, wheel balance is the probable cause. Take the vehicle back to the shop, tell them the tires shake, and make them rebalance the tires. At the same time, ask them to check the tires for out-of-round. One of the 4WD magazines recently (as of September 2002, as this article is being written) wrote a report on the benefits of having tires “trued.” Truing is nothing more than shaving the high spots off the circumference of a tire to make its rotation more perfectly round. Do not discount this possibility. The author has encountered tires from reputable manufacturers which simply could not be made to run smooth.
If death wobble rears its ugly head when the brakes are applied, look at whether the front rotors may be warped. When Chrysler took over the Jeep/Eagle division from AMC, one of the changes they made (affecting all Jeep vehicles) was to change from homogeneous rotors to composite rotors. These composite rotors are notorious for warping, often within less than 5,000 miles of vehicle purchase or rotor installation. The author’s sole experience of death wobble was in a completely stock, almost new 1999 Grand Cherokee WJ—which had warped rotors. Applying the brakes lightly to control speed down a long grade on a 55 MPH road was all that was necessary to induce death wobble.
If all the above considerations can be eliminated, steering alignment (geometry) should then be investigated. There are three settings for front suspension/steering geometry. Camber is measured in degrees deviation from vertical and is factory-set at zero and cannot be adjusted; therefore, it will not be discussed. If camber is other than zero, axle damage is probable.
Toe-in/toe-out is measured in inches, as the difference between the track at the front of the front tires and at the rear of the front tires. The factory service manual (FSM) for the Cherokee XJ specifies a toe-in value of zero, plus-or-minus 1/32 of an inch. The problem is that, especially with large tires, any toe-out at all seems to contribute to death wobble. If there is any slop or wear in steering components, a setting of “zero” on the alignment machine turntable may result in an actual toe-out condition on the road. For all vehicles, toe-in should be adjusted with a spreader bar between the front tires (at the leading edge) and the toe-in set to zero or plus 1/32 of an inch.
Caster is the vertical angle, in degrees, that the imaginary line between the upper and lower ball joints leans back from the vertical. Positive caster is when the upper ball joint is farther back than the lower; negative caster is when the upper ball joint is ahead of the lower. For the XJ Cherokee, the FSM gives contradictory information. In the 1994 FSM as an example, in one place it specifies a caster of +6.5 degrees for XJs with manual transmission, and +8.0 degrees for XJs with automatic transmission. In the table of alignment specifications, however, the specified preferred caster is +6.0 degrees, and the acceptable range is 5 to 9 degrees.
In checking and adjusting caster, it must be kept in mind that the steering knuckles are welded to the axle tubes, and so is the differential. Adjusting the caster angle also adjusts the angle of the front differential and pinion yoke. An excessive angle between the pinion and the front driveshaft can result in damage to the u-joints. The FSM states that in adjusting caster, the pinion angle takes precedence.
With a stock vehicle this is not a problem. Due to the geometry of the front axle and its upper and lower control arms, however, caster does become a problem with lifted vehicles. All XJ lifts are suspension lifts, meaning that the body and uniframe is lifted away from the axles. The front axle remains at the same height relative to the ground but the chassis, and with it the transfer case, are raised. This increases the angle between the pinion shaft and the driveshaft, and at the same time reduces the caster angle. Attempting to adjust the caster angle back to within specified values, however, may result in excessive driveline angles at the pinion yoke. Thus, any attempt to align a lifted Cherokee becomes a balancing act, between achieving sufficient caster to prevent death wobble and tipping the differential so far that the front pinion angle becomes excessive.
Why is caster important? Consider how a tire rides on pavement. If you consider a theoretical tire with an absolutely flat tread and perfectly square shoulders, with the vehicle moving straight ahead the tread is perfectly flat against the pavement. When the vehicle turns, the tire and the tread “contact patch” will rotate around the axis defined by the two ball joints. With zero caster and zero camber, this axis will fall in the center of the contact patch and the tread will remain flat on the pavement.
With positive caster, however, the steering axis intersects the pavement somewhat ahead of the center of the contact patch, and with enough positive caster it falls completely outside of and in front of the contact patch. What happens when you turn the wheel under this geometry is that the contact patch does not remain flat on the pavement; the tire tends to lift up on the inside (or outside) shoulder. When you release the steering wheel, the vehicle weight acting on the tire tends to push the tire back down to the flat contact patch, which is why steering is to an extent self-centering. It is this self-centering effect that helps to combat death wobble. The effect is increased in direct proportion to positive caster, and helps to stop wobble in a wheel before it becomes uncontrollable. As caster is reduced, it becomes easier for a wheel to wobble, and to transmit this wobble to the other wheel, thus triggering an episode of death wobble.
As noted, because caster angle is related to and affects front pinion yoke angle, it is impossible to make any absolute statement regarding what the caster angle “should” be for a lifted vehicle. In general, it is probably safe to say that under no conditions should the caster be allowed to go to less than +5.0 degrees, and preferably not less than +6.0 degrees.
Lastly, to close out this discussion of death wobble it is important to dispel one last myth: that a defective or inadequate steering stabilizer (damper) “causes” death wobble. This statement is a confusion of cause and effect. A steering stabilizer is nothing more or less than a horizontally-mounted shock absorber. It helps to combat death wobble by damping the transmission of wobble and shake between the two front tires, but if one or both tires are not wobbling the damper is inert and performs no function. The steering damper cannot cause death wobble. A weak steering damper may allow death wobble to develop, and it may appear that replacing the damper has “eliminated” death wobble, but in fact all that has happened is that the cause remains but the symptoms have been temporarily masked. There are people driving lifted, highly modified Cherokees with no steering damper. Therefore, even if replacing the damper appears to remove death wobble, it is advisable to continue investigating until the underlying cause can be found and remediated.
© 2002 Harwood W. Loomis (All rights reserved)
