Disc Thickness Variation and Lateral Runout Discussion
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- เผยแพร่เมื่อ 3 ก.พ. 2025
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Many times, vehicles are brought into a shop with a complaint of pedal pulsation. Oftentimes, the cause of the pedal pulsation is diagnosed as a warped rotor. This diagnosis is wrong. In this video, we discuss the real cause of pulsating brakes.
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What is Disc Thickness Variation?
The cause of pedal pulsation is disc thickness variation. Disc thickness variation happens when a rotor varies in thickness on its surface.
What causes a thickness variation to occur? The answer is excessive lateral runout. Lateral runout is the amount of side to side movement of the rotor as it rotates.
Today's vehicles have extremely tight lateral runout specifications. Most vehicles have a specification of two-thousandths of an inch or less. If the runout is excessive, the rotor will begin to make contact with the brake pads as it wobbles back and forth. This contact between the rotor and brake pads won't be constant. It will happen periodically as the rotor wobbles back and forth.
If you use a semi-metallic pad, the pads will begin to grind away on the spots of the rotor that it contacts. This happens because a semi-metallic pad has an abrasive effect on rotors. If you use a ceramic pad, the pads will leave a thin layer of friction material on the spots of the rotor that it contacts. This material transfer happens because ceramic pads have an adhesive effect on rotors.
Either way, this contact with the rotor is what causes a thickness variation. As the portion of the rotor with the thickness variation passes past the brake pads, the caliper piston has to extend or compress. This causes the pressure of the brake fluid to rise or fall during braking. This change of pressure is what leads to pedal pulsation.
What Causes Lateral Runout?
Lateral runout can be caused by uneven torque or a brake stud. However, the most common cause of lateral runout is rust buildup on the hub assembly.
Hubs are often in use on a vehicle for hundreds of thousands of miles. As you can imagine, a significant amount of rust will build up on that hub over that period of time. It is imperative that you take the time and clean the hub properly every time during a brake job. This will help ensure that you don't receive a comeback.
Steps to Checking Lateral Runout:
1.) Clean the hub
2.) Install the rotor and conical washers
3.) Torque lug nuts down to specifications: Specifications can be found in a specification guide or the vehicle's service manual.
4.) Install dial indicator
5.) Check lateral runout by turning the rotor: Specifications can be found in a specification guide or the vehicle's service manual.
Correcting Lateral Runout
There are a few options you can choose from to correct lateral runout.
The first option is to machine a rotor with a bench lathe or an on-the-care lathe. The other option is to replace the rotor with a new rotor.
No matter what option you choose, make sure that you re-measure the lateral runout and take the vehicle for a road test to ensure the pedal pulsation has stopped. - ยานยนต์และพาหนะ
Excellently and clearly explained. Great work. Great video
Had this problem when replacing rotors and hubs on my car. Problem was the new hubs that I discovered after rotating rotors 90° and side to side. I was able to cure with hub correction plates from Raybestos. Thanks!
This is the perfect example of what technicians should be. Today, most to all repairs facilities including new car dealership service. Do nothing close to this. They will replace at your expense.
Back when labor was cheap and parts were expensive, this is how technicians were. Today, parts are cheap and labor is expensive. Why spend 2 hours at $150+/Hr measuring rotor run out and cutting rotors when they can simply be replaced (with a warranty) for $40-100/ea?
Magnifique, quel beau travail, merci !!!
Old school, right info.
Don't believe the new school garages. All what they do nowadays it is to change, oil, filters, tyres and brakes. And very often they don't do this properly.
Great explanation, thanks!
Thanks - great vid :)
Excellent video sir!! Mr. Spock could not have described the associated logic any better. BTW, I did check your ears just in case! lol!
Really good video :)
Rotors don’t warp period! People think you’re nuts when you tell them this.
Great video. So I'm guessing the hub surface or brake hardware was corrected to explain why a new rotor (after run-out inspection) was sufficient to send a customer down the road with.
When your mechanic and doctor are the same guy.
and talks like Jesse Ventura
yeah and i am sure that every shop out there does it just like in the video
Ride your brakes down a long hill, until they really heat up, then come to a complete stop and keep the brakes depressed. That'll transfer some of material which will cause unevenness...
I just take some emery cloth to the rotors to remove the excessive material transferred from the pads. Then new pads are installed and there are zero issues with pulsation.
This sounds crazy dam. Can't be this easy
I heard this is a process McLaren recommended for their cars except you need to very fast if it’s for a McLaren.
Nailed it ta
I wonder if there was another way of saying lateral runout so your average person with no mechanical knowledge could understand what the mechanic was trying to say...I dont know sometging simple ...perhaps Warped!? since they arent flat anymore and if you put them on a turn table , youd watch the needle arm go up and down ...like a warped LP...or are you going to say the LP is Laterly Runed out?.
"Rotors don't warp"
Subsequently measures rotor warp after stating manufacturers have reduced lateral runout to
Seems like the the difference between warp" and "thickness variation" is purely semantic. If the rotors weren't "warped", why don't you just grind out the "thickness variations" and make the rotors uniformly even. In any event, what's the difference between warp and thickness variation if the rotors have to be replaced.
@@ItsVideos The term "warp" has been accepted in common parlance among auto mechanics to mean thickness variations on the surface. Get a grip on how words obtain different meanings according to common usage over time. There's no reason why the term "warped" can't be applied to differing contours of the surface of the rotor in addition to a full bending of the rotors especially when there is no meaningful difference between the two. If there were some meaningful difference the guy in the vid should have turned the rotors, but he replaced them. So who cares about some meaningless distinction made in the video.
@@ItsVideos He suggested it as one option to alleviate 3 types of warped rotors, but then in this instance he replaced them. So he did unnecessarily more expensive work than was actually needed. Some honest mechanic you got there.
@Fish hey where'd you go? Not confident enough in your comments to let them stay public? Not surprising.
I have been asking this question over and over again myself for a long time now also why is there a need to check the rotor if it is new out of the box good question Paul
@@joehyundaitech7961 Because most rotors are Chinese junk with sloppy tolerances. Quality parts stopped being produced in the 90s.
Lol
New Raybestos dics with 0.006" lateral runout - are you serious!!!??? Repeatedly! It's not like it was a one-off bad disc. Crap quality control. They're wobbly dragging on my pads like a drunk warped disc. New calipers dragging - pistons not retracting properly. Severely disappointed with this company and Rockauto selling their crap as top of the line products.