Matching Frequency to Component RPM
Matching Frequency to Component RPM
At this point in the diagnosis, the vibration has gone through the following analysis:
* The vibration has been duplicated.
* The vibration has been designated as abnormal.
* The vibration has been related either to engine speed or to vehicle speed.
* The vibration has been assigned a frequency from the EVA or identified based on its feel or its sound.
Automotive vibrations are usually related to the rotating speed of a component. Calculate the speed of these components using either an engine speed method or a vehicle speed method. Use the engine RPM readings taken during the road test in order to diagnose the vibrations that are sensitive to engine speed.
If the vibration is sensitive to vehicle speed, determine the rotational speed of the tires. As long as you operate the vehicle at a constant speed, the tires will operate at a constant speed. This speed is measured in rotations, or cycles per second. The reading is then compared to the frequency of the vibration, which is also measured in cycles per second.
Calculating Tire Rotation
Component Rotational Speed Worksheet:
Determine the rotational speed of the tires in revolutions per second, or Hertz (Hz), based on the vehicle speed at which the vibration occurs. In order to determine the rotational speed, use the following procedure:
1. Determine the speed at which the vibration occurs.
2. Determine the number of 8 km/h (5 mph) increments:
* Divide the complaint speed by 8 when using kilometers.
* Divide the complaint speed by 5 when using miles.
3. Determine the vehicle tire size.
4. Locate on the table below, the Hertz value at 8 km/h (5 mph) for that tire size.
5. Multiply the Hertz value by the number of 8 km/h (5 mph) increments.
6. The result is the rotational speed of the tires in Hertz at the complaint speed. If this figure matches the vibration frequency, a first-order vibration is present in the tire and wheel assembly.
For example, if the complaint speed is 60 mph, the number of increments is 12. According to the following table, a tire with size of P235/75R15 has a value of 1.00 Hz. You can calculate the rotational speed at 60 mph:
12 increments x 1 Hz = 12 Hz
Sometimes, the tire/wheel vibration may be of a higher-order. In order to compute possible higher-order vibrations, multiply the rotational speed of the tires, in Hertz, at the complaint speed by the order number. If any of these matches the vibration frequency, a vibration of that particular order is present in one of the tire/wheel assemblies.
Steering and Suspension Assembly Vibrations
Steering and suspension assembly vibrations are the first level of testing for low-frequency vibrations that are sensitive to vehicle speed. The symptoms of a steering/suspension first-order vibration are a shimmy or a shake. This is usually felt in the steering wheel or in the seat. Inspect the following components for wear or damage:
* The steering tie rod ends
* The suspension ball joints
Calculating Propeller Shaft Rotation
The propeller shaft drives the tires through the rear axle. Knowing the rotational speed of the tires enables you to easily calculate the speed of the propeller shaft. Therefore, you can determine the rotational speed of the propeller shaft by multiplying the rotational speed of the tires by the ratio of the rear axle.
1. Obtain the tire speed by performing the tire speed calculation, described earlier.
2. Determine the axle ratio using one of the following methods:
* Cross reference the axle information from the service information part label to the parts book.
* Cross reference the axle identification code that is stamped on the axle to the parts book.
* Perform the following calculation:
1. Make a mark on the tires and the propeller shaft.
2. Rotate the tires for one complete revolution, counting the number of propeller shaft rotations.
For example, the propeller shaft rotates approximately 31/2 times for every tire revolution. The axle ratio is 3.5:1
3. In order to find the propeller shaft rotation speed, multiply the tire speed by the rear axle ratio.
In the earlier example, at a 60 mph complaint speed:
12 Hz x 3.5 = 42 Hz
Match the frequency of the vibration from the EVA with the rotational speed of a component (either a tire or a wheel). If none of the frequencies match, do one of the following actions:
* Recheck the data.
* Attempt to match the figures again, allowing for 1-5 mph of speedometer error.
When you have matched the speed of the component with the frequency of the vibration, refer to the service information that outlines the repair procedures for that particular component.