Vibrations in a vehicle are more than just annoying; they can cause damage. Solving vibration problems is easier with a logical approach. A few simple tests will help find the source of most problems.
The driver experiences most vibrations through the steering wheel, floor or the seat of their vehicle. These items are not the problem, but they are the transponders; To feel an automotive vibration, we need a source, a transfer path and the transponder, where it is felt. To solve the issue, we must address the root-cause or isolate it from the transponder.
Finding the cause of a vibration involves identifying the things that affect it. For instance, a shake in the steering wheel, only from 45 to 50 MPH is speed-dependent. It could be a bent wheel, an out-of-round tire or even a torque converter out of balance. Anything that has mass and rotates, can cause a vibration.
They design modern vehicles to reduce engine speed. At 45 to 50, MPH the engine may be rotating just above the idle speed. A slight misfire can feel very much like a vibration. With all these possibilities, a huge amount of money can be spent on trial and error.
Vibration terminology
Shimmy, shake, shudder and many other terms describe automotive vibrations. A vibration is a repetitive force that may be felt, heard or both. This article will not get into the technical differences as they are unimportant to the purpose. Vibrations come in two main types:
1. Driven vibration is one that results from something rotating, such as a wheel, engine or drive-shaft. Driven vibrations continue if the frequency is correct and the driven force continues. An example is a steering wheel that wobbles continuously at 55 MPH. Most of the automotive vibration is this type.
2. Non driven vibrations result from a force acting on something else, for a limited time. For instance, a tire hits a hole and produces a quick shake, that goes away. This is often the result of slack in components. Non driven vibration is far less common in automotive situations than the driven vibration.
Vibrations that aren’t vibrations
Three common automotive problems are technically a vibration. From a standpoint of repair, they are different. We call the shake in the vehicle, only on braking, a brake shudder. This is a brake problem, because it only occurs on braking. These Detailed Topics cover this subject.
A torque converter shudder, is a quick vibration, that last only a few seconds. This will normally occur around 40 to 60 MPH, when the torque converter locks up. Like a brake shudder, this feels like a vibration, but is considered a transmission issue. A quick test for a torque converter shudder is to touch the brake pedal when it occurs. If the shake immediately stops, a torque converter shudder is suspected. This Detailed topic gives more information on the torque converter shudder.
An engine misfire may also feel much like a vibration. A misfire is often intermittent and engine temperature may be a factor. It is also often a short-term event, and related to the engine load, more than the speed. These Detailed Topics have more information on diagnosing engine misfires.
Tools will help to find the vibration
This article is for the home enthusiast and does not require extensive tooling. Equipment is available to measure frequency and amplitude of a vibration. Such tooling is handy but not economically feasible for someone trying to solve their own problem.
The engine tachometer gives the revolutions per minute or RPM of the engine. This is the speed that the engine rotates. RPM will change, depending on vehicle speed and the gear-ratio selected by the transmission. Noting the RPM at which a vibration occurs will help to learn if it is engine-speed of vehicle-speed related.
The vehicle speedometer is another very handy tool. Noting the miles per hour or MPH of the vibration will give insight. A comparison of RPM to MPH is another handy method of determining the cause of the vibration.
The vibration transfer path
Engines vibrate as part of their normal operation. We cannot repair this, because it is not broken. Engineers isolate this shake from the passengers, by addressing the transfer path. The engine mounts absorb this normal vibration, so the occupants do not feel it. When the mounts fail, they pass the vibration through. To stop the vibration, the repair person addresses the transfer path, rather than the source.
Other examples of transfer path failure include exhaust hangers and hoses that touch the body. When the exhaust system insulators fail, engine vibration enters the passenger compartment. Finding and repairing the transfer path is the solution. A pry bar is handy to shift the exhaust and see if it changes the vibration.
Power steering hoses and air conditioner lines may also transfer engine vibration. Engineers carefully route and insulate these lines to prevent vibration. An improperly routed hose will often cause such vibration. A vibration, when turning the steering to full lock is often the result. All pressurized hoses need to be secured and kept away from chassis and body components.
Transfer path vibrations can normally be felt, without the vehicle in motion. Engine speed controls this vibration rather than vehicle movement. Next week’s Detailed Topic covers several simple driving tests to isolate engine vibrations from suspension and drive line problems.
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