Thing That Go Boom..

On Tyler’s truck (with a 3.08 axle ratio), a tire with a 3rd order harmonic vibration can disguise itself as a 1st order driveshaft complaint. This 3rd order vibration can coalesce with the 1st order driveline vibration to produce a phasing vibration (a vibration that seems to increase and decrease in amplitude at a steady speed). The repair is to make the 1st order vibration as low as possible to eradicate the phasing boom.

“I never got that much into vibration diagnosis,” Tyler mentioned. “I use a sirometer that I use for small engine repair.”

A sirometer is nothing more than a coil of wire inside of a two-piece plastic housing. Dialing the top half of the sirometer lengthens and shortens the wire. The vibrations make the wire vibrate when the length is set just right for a particular frequency. Then, the user reads a numeric scale on the housing of the sirometer to see where a little pointer stopped at to get the frequency.

Everything has a preset frequency that it emits when it vibrates. The frequency measured is a fingerprint of the component that is causing the problem.

“Imagine a tire with a single bump on it,” I explained, “It would generate a frequency of once per revolution. If that tire had another bump, then it would be twice per revolution and then a third bump would be three times per revolution. That’s your First order, Second order and Third order vibration.”

“That sounds easy enough,” Tyler said.

“There can be a lot of math involved in vibration analysis,” I said.

“Bob, I never liked math. This is a hands-on business,” Tyler said, “I don’t like math.”

“Geez, you sound like some students I know, Tyler. It is a hands-on business but you have to think with your brain what your hands are going to do.”

I showed him on paper. Tyler gave me the tire size of a vehicle in the shop: 255/70R16. We used are tire size chart to find the diameter: about 30-inches. The chart also identified the tire circumference of 94.42-inches (although π x diameter works).

“Then divide the 63,360 (inches in a mile) by 94.4”. The revolutions per mile is 692,” I said.

“Okay—now what?”

 “Now we can determine the frequency, or revolutions per second, by dividing by 60 for an answer of 11.5 Hz. When using this formula, it will provide a Hz reading for the vibrating tire at around 60 mph.”

“I still don’t get it,” Tyler said.

“The tire will produce a first order vibration with a 11 Hz to 12 Hz reading if it were the source of the highway speed vibration. If there are two bumps on the tire, then it will produce a second order vibration reading between 22 Hz and 24 Hz,” I explained.

We used another truck in Tyler’s shop. The rear differential ratio on the truck was 1:3.73 and the tires were 255/70R16. The customer said his truck had a vibration around 40 mph and 1,500 rpm that was hardly noticeable, but was there. My vibration analyzer picked up a vibration of 36 Hz measured at the center console.

“Let me guess—more math,” Tyler commented.

“We could do that, but let’s try some special software that does the calculations for us.” After plugging the rear differential specs into the software, and the engine rpm at which it occurred, we get an answer of first order driveshaft vibration. A first order driveshaft vibration will cause one disturbance for each revolution of the driveshaft. 

“Anything that’s out-of-balance will cause only a first order vibration, never any higher order vibrations,” I said.

First order driveshaft vibration is usually caused by a component rotating at the same speed as the driveshaft that’s out-of-round or out-of-balance. 

“Driveshaft balance issues can generally be felt at vehicle speeds over 30 mph,” I said.

Tyler realized that a ‘hands-on’ business such as the repair business involves plenty of flexing of the gray mass between your ears.