Your vehicle’s braking system is one of the most important systems in your vehicle. To put it in more simple terms, if it fails… you crash. The good thing is that in most cases you are given fair warning before the system totally fails. Whether it’s a light on your dash, a pull to one side, a grinding noise, a funny smell or an annoying vibration in your seat or steering wheel there will be a symptom letting you know that something is wrong. In this article I am going to discuss the all too familiar vibration or surging that is felt when the brakes are applied.
If you have ever experienced the shaking or surging that I am talking about then you will agree that it can be very disturbing. In bad cases it makes you feel like the vehicle is going to shake apart as you apply the brakes. So what is going on in your braking system that causes this and what can your service shop do to make it go away?
Before I do this, I need to give you a quick lesson on the parts that make up a disc braking system. All of today’s passenger and light truck braking systems are hydraulically operated. Your brake pedal is connected to what is called the master cylinder (mounted under the hood on the driver’s side directly in front of you) by way of a linkage made up of various rods and/or levers. The master cylinder is supplied with brake fluid from a reservoir typically mounted on top of it. When you press down on the brake pedal the linkage moves a small piston forwards inside the master cylinder which builds up pressure inside the brake lines running from the master cylinder to the braking components near the wheels. Because brake fluid isn’t compressible it only takes a small amount of piston travel to build a large amount of pressure in the system (this isn’t the case if there is air trapped in the fluid… but that’s for another article). In a disc braking system there are three primary braking components near the wheels; the caliper, rotor and brake pads. The rotor is a steel or composite disc that typically resembles a very low top hat (like this -> ___|*****|___). It is mounted on the hub in a vertical fashion and its outer ring serves as the friction surface that is squeezed by the brake pads to stop the vehicle. The brake pads are mounted inside of the caliper which is mounted to the knuckle and fits around the rotor. The caliper is connected to the master cylinder mentioned earlier by brake lines and hoses. Inside the caliper is a piston that is moved by the pressure created by your foot pressing the brake pedal. When the piston move forwards, the brake pads squeeze the rotor and the frictional force stops the vehicle.
Whew! I know… lots of information there. I hope I didn’t lose you. Stay with me, the interesting part is coming up. If you go to you local dealer or service shop and tell them that you feel a vibration when you apply the brakes, nine times out of ten they will tell you that your vehicle has “warped” rotors. I hate to tell you, but they are wrong. The funny thing is that they don’t even know their wrong. You see, in our industry much of what is learned is passed from technician to technician. Many times technicians don’t really know why something happens, even though they know how to fix it. An all to common misconception among technicians in our industry is driving through a puddle of water after heavily braking will warp the rotors due to the cold water hitting the hot rotor surface. Again, this is not the case. Many studies have been done to prove that rotor do not warp even under the most extreme conditions.
If the run out is more than the acceptable range then the high spot of the rotor will rub ever so slightly on the brake pad with every rotation of the tire. If you think about how many times your tire makes a revolution then it won’t take you long to realize that this rubbing of rotor and pad is going to gradually wear down the rotor in a specific spot. This spot becomes a different thickness (thinner) than the rest of the rotor hence the name “Disc Thickness Variation”.
Once the DTV gets bad enough you will begin to feel a vibration during braking. This is because the brake pads on either side of the rotor squeeze against a rotor surface that has thick and thin areas. At high speeds this can cause a very rapid shimmy or pulsation in the steering wheel if the front rotors have DTV, or in your rear seats if the rear rotors have DTV. At slow speeds DTV will make it feel like the brakes grab and release or lurch as the vehicle comes to a stop.
So now you know what causes vibration when braking, so now let’s talk about how to fix it. The method of fixing the problem depends on the condition of the rotors when you bring the vehicle to the dealer or service center. Normal braking wears down rotors over time and once they reach the minimum thickness specified by the manufacturer they must be replaced. Replacing your rotor with new ones will fix your vibration problem in the short run (more on this in a second). If your rotors are thick enough to be resurfaced then a lathe can be used to restore them to an in-plane condition by shaving off a thin layer of material on both sides of the rotor. This too will fix your vibration problem in the short run.
Why do I keep saying “in the short run”? I say this because there is no guarantee that new rotors or the resurfaced ones have an amount of run out lower than that recommended by the manufacturer. If the amount of run out is too high, even though you won’t experience vibration when you leave the dealer or service center the vibration will re-emerge over time when the DTV gets bad enough.
There are ways to measure the amount of run out and make appropriate adjustments as well as a special lathes that can be used to virtually eliminate run out, but that will be covered in a future article.
Thank you for reading. I hope that you enjoyed the article and learned something by reading it.