SJG Forester – tracking down noisy wheel bearings

Our Forester has developed a noise that is similar to a turbo-prop aircraft. This is not the first time it did, as the front bearing have been recently replaced.

Unfortunately this time the noise was not as directional as with the front wheel bearings. The whole cabin was filled with roar above 70km/h, and sometimes it appeared that the noise would come from the front.

Whit the reports of faulty new OEM bearings on various internet forums I started doubting the new front wheel bearings.

To make the matter worse, I cannot just load parts cannon and fire new wheel bearings as local Subaru parts people (Winger) were asking $400+GST (~$300USD), which was double of what they are sold from subaruparts.com. I was looking at almost $1000 in parts, thus I needed to be sure that the rears were faulty. BTW it is not unusual for Winger to ask 100-300% more than one would pay through amayama.com or subaruparts.com. It is a very normal occurrence and I despise them for that. I digress, as it is very common for NZ retailers to rip off fellow kiwis.

Since I do not have an easy access to a hoist at the moment, and I am unwilling to have a car doing ~50km/h while lifted on axle stands, I had to come up with some other method determining where the noise is coming from.

I came up with this:

piezo transducer

The above is a piezo element from a random buzzer connected to a simple inverted op-amp and powered by a random LiFePO4 battery (can be couple of AAs, or 9V battery). Regular headphones were the output.

I needed to attach the piezo securely, yet temporarily to a knuckle being listened to, so I glued the piezo to a magnet from an old HDD.

piezo on a magnet
back side of piezo

When tested in rainy conditions the piezo got shorted by water, so I have insulated it with hot melt glue.

Insulated piezo

Here it is attached to a knuckle:

The circuit is fairly simple:

It might pay to replace R1 with a potentiometer as 100k has resulted in too much gain.

The test confirmed that left rear was the source of the noise, and right rear was failing. It also provided the piece of the mind as the fronts were silent.

The vehicle needs new rear bearings, but we are definitely not buying them from Winger.

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