Disc Thickness Variation and Lateral Runout Discussion

[Music] Many times vehicles are brought into a repair facility with a complaint of pedal pulsation. It's always diagnosed as warped rotors. Rotors don't warp. Typically what causes pedal pulsation is disc thickness variation. Today we're going to talk about what the causes are and the corrections of that. As we stated earlier, pedal pulsation is not caused by warped rotors. Typically what causes pedal pulsation is a thickness variation in the rotor. But what causes that thickness variation is excessive lateral runout. Lateral runout is actually the rotor wobbling on its axis back and forth. Manufacturers have really tightened that up. It used to be ten thousandths or more. Now it's one (or two) thousandths or less. Virtually, what happens with excessive runout, every time the rotor wobbles back and forth, it will touch that brake pad. Depending on the properties of the brake pad, if it's a semi-metallic, which will wear the rotor... or a ceramic brake pad, which will leave friction material on the face of the rotor. Ultimately, what happens is the wearing or the transfer causes thickness variation. Thickness variation is a build up of the surface, and it's uneven build up. When you step on the brake, what will happen is the pads will oscillate back and forth, displacing fluid and causing pedal pulsation. Here's a prime example of a vehicle that came into the shop. The consumer is complaining about pedal pulsation. As we do an inspection, we can look at the face of the rotor and you can see some discolored marks within the face of the rotor. This application has ceramic brake pads. Virtually, what has happened is the ceramic materials have transferred unevenly onto the face of the rotor... thus causing thickness variation causing a pedal pulsation. So what we'll do is let's check the lateral runout of this and see where we're at. Typically, this vehicle has two thousandths or less lateral runout. We've mounted the dial indicator with the conical washers and we'll check this runout. This vehicle is experiencing six thousandths lateral runout. The specification on this is two thousandths. That is enough to cause that thickness variation in the rotor. Let's talk about lateral runout. When a rotor is manufactured, it's held to very tight tolerances. Anywhere between one to two thousandths lateral runout on a machined rotor. The biggest drawback is today 85-90% of our rotors are hub-less rotors. What does that mean? This rotor has to slide onto a piece on the vehicle, which is a hub bearing. And that may have been on the vehicle for hundreds of thousands of miles. Typically what we see is rust buildup underneath that rotor. A piece of rust can cause as much as 17 to 20 thousandths lateral runout. You have to be sure when installing that rotor to take the time and clean the hub and make sure there's no rust. So we have a few options here to alleviate lateral runout and thickness variation. Number one is we can turn them on a brake lathe. Many shops have a bench-type brake lathe. You have to make sure that that brake lathe is up to specifications. Make sure you hold the tight tolerances. The other option would be an on-the-car brake lathe, which is a very expensive investment but a very good tool to alleviate thickness variation. Or the third option, obviously, is replace that rotor. Ok, our option was to replace the rotor. But a word of warning: whether it's a new rotor install or a rotor that has been re-surfaced, it's mandatory that you check lateral runout. Let's check the runout on this rotor and see where we're at. We're right at one-thousandths lateral runout. Spec for this vehicle was two-thousandths runout. We're well within specification. Just remember, excessive lateral runout ends up with thickness variation. As little as a half-a-thousandths thickness variation on a rotor will shake the steering wheel out of your hand. [Music]

Channel: Raybestos Brakes

Views: 15,613

Rating: 4.9414635 out of 5

Keywords: lateral runout, disc thickness variation, rotor runout, brake rotor runout, rotor runout definition, dial indicator, rotor runout gauge, thickness variation, warped rotor, brake pulsation, brake pulsation in pedal, brake pulsation when braking, pulsating brakes, pulsating brakes when stopping, what causes pulsating brakes

Id: B3JUPt4upxk

Length: 4min 25sec (265 seconds)

Published: Thu Sep 06 2018