A5INKY
Vendor , w/Business number
Just had a second car in the shop in only a few weeks with the same issue - brake pulsation. Both cars had front brake jobs done by mechanics not unknown to this forum using quality parts from trusted vendors within less than 20K miles and were pulsating badly again already. For one of those customers it had been a chronic problem over many brake jobs. In both of these recent examples the root cause was the same and had nothing to do with the brake parts.
I'm calling this instructional thread a "102" course because Wingnut has a nice 101 level how-to posted in the TDI 101 Stickies HERE. This thread is to merely expand on those basics. If you are new to brake service, Wingnut's thread is a good place to start as this thread will assume one has a good grasp on the material shown there. Also assumed will be that you are using quality parts, OEM or better. Poor part quality alone can lead to pulsation, but even the best parts can fail if what I am showing here is ignored in the service work - as was the case with these two cars.
Wingnut mentioned cleaning up the hub face before mounting a new rotor but did not explain why or what could happen if you don't. This one was not too bad:
I've seen much worse. Nevertheless, you want that surface as flat and clean as the day it was manufactured before mounting the new (or old) rotor. If you are re-using a good but used rotor, both sides of the rotor hat should get the same treatment. My die grinder mounted scotch brite pad works great and fast for this without risking good metal removal.
So what has all this to do with pulsation or warp you ask? In both of these cases, even with new rotors and proper attention to detail, the rotors were not going to spin "true". "Lateral Runout" is the proper term for side to side wobble of a brake disc as it spins. Every car has a maximum spec for it for a reason. Some are as little as less than a thousandth of an inch, though zero is always what is strived for. In the case of the subject MKIV Jetta, the spec is a maximum of 0.004" which is pretty liberal IMO.
Remember how Wingnut said that calipers and pads must move freely side to side? Well, if they are doing so properly, excess lateral runout will go unnoticed by the driver for a relatively long time as the caliper quietly shakes back and forth to the wobble of the rotor. Pulsation only starts to rear its ugly head after this has been allowed to go on long enough for the rotor to wear unevenly in it's thickness, known in the industry as rotor "thickness variation". When rotor thickness variation occurs, the caliper's piston is hammering back and forth in the caliper body violently and transmitting that pulsation to the master cylinder to be felt in the brake pedal, as well as through the suspension and steering systems to be felt in the steering wheel. Left unchecked long enough it can cause the whole car to shake and be dangerous to vehicle control. Thickness variation is all too often labeled "warpage" due to heat, materials, pad selection, driving habits, etc. Parts get improperly indicted, driver is blamed for something out of their control, another brake job gets sold. After all, by now the previous brake job warranty is up anyway, right?
OK, so we now know that thickness variation due to excessive lateral runout is the leading cause of pulsation. How do we prevent it? We prevent it by ensuring the rotors are spinning as true to the axis of the axle as possible. At the very least it needs to be within VAG's maximum lateral runout specification.
So, we have already cleaned the surface of the hub face and rotor. Next we need to bolt the two together in a way that will mimic the mounted and properly torqued wheel. I do this with a stack of hardened washers and the lug bolts:
Next step requires an accurate lateral runout measurement of the rotor face as it spins on the hub. I have a special brake rotor service dial indicator tool from Snap-On that makes easy work of this measurement. However, any dial indicator can work, just may need to get creative with how to mount it.
At this point both of the example cars measured lateral runout was well in excess of the 0.004" maximum spec on just one side of the car with new rotors properly mounted. What could cause that? The hub! Both of these cars had bent hubs and one of them had a bad bearing on the same side too. The worse of the two had ~0.005" of play in the bearing with an additional 0.009" bend to the hub face. That car scared me so bad on the initial test drive I cut it short.
Both owners had spent hundreds on multiple brake jobs while believing they had either gotten defective parts or were just simply too hard of drivers. Not so!
There is one more detail a good 102 level brake instructional should include - brake fluid maintenance. Brake fluid is hygroscopic, or water loving. It will absorb moisture from the air in the master cylinder reservoir. This moisture ends up in low parts of your brake system and causes corrosion and eventual failure of those brake parts. This is why service manuals typically recommend a brake fluid flush every two years. Keep that fluid refreshed that often and your chances of needing caliper or master cylinder replacement drops significantly. In fact, failures of these parts is pretty rare when proper preventative fluid maintenance is performed over the life of a car.
I'm calling this instructional thread a "102" course because Wingnut has a nice 101 level how-to posted in the TDI 101 Stickies HERE. This thread is to merely expand on those basics. If you are new to brake service, Wingnut's thread is a good place to start as this thread will assume one has a good grasp on the material shown there. Also assumed will be that you are using quality parts, OEM or better. Poor part quality alone can lead to pulsation, but even the best parts can fail if what I am showing here is ignored in the service work - as was the case with these two cars.
Wingnut mentioned cleaning up the hub face before mounting a new rotor but did not explain why or what could happen if you don't. This one was not too bad:
I've seen much worse. Nevertheless, you want that surface as flat and clean as the day it was manufactured before mounting the new (or old) rotor. If you are re-using a good but used rotor, both sides of the rotor hat should get the same treatment. My die grinder mounted scotch brite pad works great and fast for this without risking good metal removal.
So what has all this to do with pulsation or warp you ask? In both of these cases, even with new rotors and proper attention to detail, the rotors were not going to spin "true". "Lateral Runout" is the proper term for side to side wobble of a brake disc as it spins. Every car has a maximum spec for it for a reason. Some are as little as less than a thousandth of an inch, though zero is always what is strived for. In the case of the subject MKIV Jetta, the spec is a maximum of 0.004" which is pretty liberal IMO.
Remember how Wingnut said that calipers and pads must move freely side to side? Well, if they are doing so properly, excess lateral runout will go unnoticed by the driver for a relatively long time as the caliper quietly shakes back and forth to the wobble of the rotor. Pulsation only starts to rear its ugly head after this has been allowed to go on long enough for the rotor to wear unevenly in it's thickness, known in the industry as rotor "thickness variation". When rotor thickness variation occurs, the caliper's piston is hammering back and forth in the caliper body violently and transmitting that pulsation to the master cylinder to be felt in the brake pedal, as well as through the suspension and steering systems to be felt in the steering wheel. Left unchecked long enough it can cause the whole car to shake and be dangerous to vehicle control. Thickness variation is all too often labeled "warpage" due to heat, materials, pad selection, driving habits, etc. Parts get improperly indicted, driver is blamed for something out of their control, another brake job gets sold. After all, by now the previous brake job warranty is up anyway, right?
OK, so we now know that thickness variation due to excessive lateral runout is the leading cause of pulsation. How do we prevent it? We prevent it by ensuring the rotors are spinning as true to the axis of the axle as possible. At the very least it needs to be within VAG's maximum lateral runout specification.
So, we have already cleaned the surface of the hub face and rotor. Next we need to bolt the two together in a way that will mimic the mounted and properly torqued wheel. I do this with a stack of hardened washers and the lug bolts:
Next step requires an accurate lateral runout measurement of the rotor face as it spins on the hub. I have a special brake rotor service dial indicator tool from Snap-On that makes easy work of this measurement. However, any dial indicator can work, just may need to get creative with how to mount it.
At this point both of the example cars measured lateral runout was well in excess of the 0.004" maximum spec on just one side of the car with new rotors properly mounted. What could cause that? The hub! Both of these cars had bent hubs and one of them had a bad bearing on the same side too. The worse of the two had ~0.005" of play in the bearing with an additional 0.009" bend to the hub face. That car scared me so bad on the initial test drive I cut it short.
Both owners had spent hundreds on multiple brake jobs while believing they had either gotten defective parts or were just simply too hard of drivers. Not so!
There is one more detail a good 102 level brake instructional should include - brake fluid maintenance. Brake fluid is hygroscopic, or water loving. It will absorb moisture from the air in the master cylinder reservoir. This moisture ends up in low parts of your brake system and causes corrosion and eventual failure of those brake parts. This is why service manuals typically recommend a brake fluid flush every two years. Keep that fluid refreshed that often and your chances of needing caliper or master cylinder replacement drops significantly. In fact, failures of these parts is pretty rare when proper preventative fluid maintenance is performed over the life of a car.
Last edited: