Rock Solid ABS

Hey readers and those who are paid monetarily to read this,

My next post all began from an installation error at work. Now don't be quick to judge the mechanics at work because they are truly amazing at what they do; they are the most OCD people I know when it comes to torque specs and alignments, and they were fully unaware of how "redunkulous" Porsche engineers can be. This all started when a regular costumer with a GT3 needed some serious work done. He went off course at a race track and hopped around 90mph in the dirt sideways. If you think the dirt is forgiving due to the lack of lateral grip your entirely wrong because this car got hurt in the same way the last time you went snowboarding and landed your pale ass over a solid patch of ice. The rear wheels bearings felt more like the fan speed dials on the center console of a cheap car, suspension linkages were bent and battered, and one of the wheels was even bent! So after all these things were replaced, repaired, and installed, the car's ABS light came up. This may have been due to the fact that a rat lived inside this $110,000+ car munching away at wires, but was actually due to the wheel bearings.

After a call from a local dealership who inspected the car due to these warning lights, we learned that the wheel bearings have a particular orientation when they get pressed into the uprights. Now, believe me there is barely a way to have known that these bearings needed a certain direction to get pressed in. The inner and outer races were more similar than the Olsen twins. There aren't any visible arrows or imprinted faces of Heidi Klum telling you this side is "IN" and the other side is "AUS." Rather a very, very close look and perhaps a translator would have been helpful. The local dealership even unknowingly installed a couple wheel bearings in the wrong direction until they received confirmation from Porsche that these things are critical in their orientation in the upright.

Now, even though the bearing can be spun counter-clock or clock wise, why should this set off an ABS light? When you examine the GT3's uprights and look on the back of them (the side not exposed to the guy who is going to steal this car), you'll notice that there are no splines or gear profiles. Unlike the GT3, your average car will have a gear/spline profile behind or somewhere on the upright which spins concentrically with the wheel which ensures proper operation of your anti-lock brake system (ABS; look right 'cause that's probably what you have). An inductive sensor picks up these patterned skips of metal and air as your wheels rotate. If your wheel locks up, the sensor will output a non-pulsating signal because its not picking up any wheel rotation. Your car's ABS system will recognize this and reduce the brake pressure to that wheel or the group of wheels that are locked-up. Porsche decided to bust out their .44 magnum and become Clint Eastwood-badass by not using this method for their ABS sensors. Rather their wheels bearings contained an eccentric magnet at one end.

Magnetized wheel bearings? I know it sounds weird, but just give me a minute and it will all make sense. Porsche in trying to maximize performance perhaps by reducing unsprung weight uses a discreet magnetic sensor which does not need to read off a spline/gear profile. By having a magnet on the wheel bearing, the magnetic sensor can pick up the magnetic field of the magnet on the wheel bearing and create a sinusoidal output to the ABS computer. So if a change in magnetism is not registered by the magnetic sensor, guess what, the wheel or wheels are locked up and the ABS system will reduce pressure to the needed calipers. See why it is so critical to have the bearings pressed in the right direction? Press them in backwards and you have a magnetic sensor that doesn't register anything but air as the magnet on the wheel bearing is too far by a mere 50mm to have it's magnetic field get picked up by the sensor. So why don't most car manufacturers use this method? For one, it is more expensive then your typical inductive sensor layout. Two, it's a GT3. Your paying north of $110,000 for one of the best engineered vehicles in the world and believe me it shows in numerous aspects (just refer to the past posts).

A simple and short post I know, but when I found about these magnetized wheel bearings I felt bad for the technicians BUT again (and as always) appreciated the intellectual pragmatism of Porsche. Sad (I know) that I find these small things exciting, but hey if you're reading this, so do you.

-Josh