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

Getting A Grip On Things

Hey All,

Something really impressed at work these past few days, and again it has to do with the same GT3 RS that we took a look at in my first post. I talked briefly about how this certain costumer opted for an after market flywheel and a multi-plate clutch on his car. These two words, flywheel and clutch, which are so mundane within the automotive vernacular literally made me change the way I look at race car engineering. Yet, the two things I'm about to discuss are relatively simple in design, however, make huge leaps and bounds to increase the performance of the vehicle. So let's again dive into things.


The flywheel is typically a steel or chromoly disc with a ring of holes at the center which allow it to be bolted to the crankshaft and an outer gear profile that allows the starter to crank the engine over (not always for some race cars). Steel and chromoly are strong materials and are great for chassis, roll cages, anti-swaybars, and wheel hubs but are quite dense. Freeing up weight at the flywheel allows for a more instantaneous throttle response and quicker acceleration as rotating mass is reduced. The guys at AASCO have taken this truth to the extreme by creating an aluminum based flywheel. Now this may sound weird seeing that aluminum is soft and quick to deform under heat, which is what friction creates. The flyweel takes a good amount of bashing, especially from bad drivers, as it comes in direct contact with the friction discs of the clutch. Having a purely aluminum flywheel would be extremely detrimental as it would mostly likely contort, expand, or shear the surface of it. So as seen in the picture above, the center portion of this specific flywheel has a concentric black ring. This ring is not some anodized aluminum piece, but rather a thin chromoly disc which is attached to the aluminum flywheel using flat socket head cap screws. This allows for a widely lighter flywheel yet still allows for normative grip when compared to typical clutch discs and conventional chromoly flywheels. Another cool thing about this specific piece is that if the chromoly wears down, you simply unfasten the flat head screws and install another chromoly ring.







Next is the Tilton multi-place clutch. The theory behind multiplate clutches is the more discs, the more faces of contact which allows for a higher torque capacity before the clutch slips. Traditional clutches rely on the face which comes into contact with the flywheel. That's why most single disc after market clutches which say they have higher torque capacities have a very stiff clutch-pedal feel. They simply increase the torque capacity by using a pressure plate with more static tension which applies more pressure on the disc. Multi-plates don't have to use this stiff pressure plate method to gain higher torque capacity numbers because your using both sides of several friction discs. Just think of it as sliding two sheets of sand paper on each other versus trying to slide stacks of sand paper upon one another. I have a feeling that the stack is going to be harder...


Lastly, probably the coolest thing that caught my eye about the Tilton setup is the use of a hydraulic release bearing. See typically, the clutch master cylinder moves fluid when the driver depresses the clutch pedal. Since, the fluid has nowhere to go but to the slave cylinder (kinda derogatory calling one cylinder the master and the other the slave, IMO), the fluid from the master cylinder extends a rod which protrudes out of the slave cylinder. This rod on the slave cylinder actuates a hinged fork (kinda like a see-saw) which pushes a ball-bearing. The ball-bearing moves with the motion of the slave cylinder rod and is pushed against the pressure plate, engaging or disengaging the clutch. This specific system eliminates the fork and slave cylinder as the clutch pedal pushes the nearly incompressible fluid into a sleeved cylinder. The pressure causes a volume change and so the height of the hydraulic release bearing rises in the same fashion as a brake piston when the you slam on the brakes. This motion is directly applied to the pressure plate which again engages or disengages the clutch. So essentially the hydraulic release bearing is: 1) the slave cylinder, 2) hinged fork, and 3) release bearing all in one which you have to admit is pretty bad ass. As you can see in the picture above, steel braided line is used to transfer clutch fluid into the hydraulic release bearing. The other line is used as a line for bleeding (removing fluid or air pockets in the clutch system). The bleed line is attached to an external bracket on the side of the transmission case. This bleed line is not necessary but prevents "sailor-mouth" syndrome in mechanics who won't have to remove the transmission every time the clutch system has to be bled.



Well I know there has been a decent pause between this post and the one prior, but I'll try to be a bit more consistent. Till I find something else interesting to write about, I'm over and out.

-Josh