If your steering wheel vibrates or your brake pedal pulsates, this story is for you.
So you do your own brake jobs and you’re proud of it. You slap on a new rotor (leaving the rust on the hub) and then the pads and caliper. Then you mount the wheel and crank down the lug nuts so it takes a 1” drive impact wrench and 2,000ft/lbs of torque to get them off. And you wonder why you develop pedal pulsation in about 3,000 miles? Well, I’ve got news for you, you caused the pulsation yourself.
People will tell you the pulsation is caused by warped rotors. They’re wrong. The rotors aren’t warped. In fact, rotors rarely warp (if ever). What you’ve done by not cleaning the hub and over-torquing the lug nuts is create lateral runout. In plain English, the rotor is not sitting completely parallel with the hub. Either rust has caused the rotor to “cock” slightly, or the uneven or overtorqued lug nuts have caused distortion in the rotor “hat.” If you leave the rotors in that condition, you cause disc thickness variation, and THAT’s what’s causing your pedal pulsation. If you slap on new pads and rotors without addressing the root cause, it’ll just happen all over again in another 3,000 miles. But there are certain things you can do during the brake job that’ll improve your chances of getting a “no pulsation” brake job, and I’ll walk you through them.
Cleanliness and lug nut torque are far more critical than you think.
Most DIY home mechanics don’t pay any attention to the condition of the hub. You may think I’m nagging you, but if you leave as little as .003” of rust on the hub face, it’s enough to cock the rotor “off parallel,” creating lateral runout.
![steering wheel vibrates, brake pedal vibrates]()
Rust on hub causes brake pedal pulsation
As the rotor spins out of parallel, the high spot on one side of the rotor slaps the inboard pad and 180° later the high spot on the opposite side hits the outboard pad. Depending on the type of pad material you use, the pad will either deposit friction material onto the rotor, making it thicker in one area on each side, or the pad material will wear down the high spot in one area on each side of the rotor.
The same result occurs if you don’t use a torque wrench to tighten the lug nuts. Think of this as a sandwich. You’ve got a hub, a rotor and the wheel. The rotor is sandwiched between the hub and the wheel by the clamping force of the lug nuts. If you don’t use a torque wrench, you’ll have uneven clamping force between lug nuts. A lug nut that’s over tightened will clamp the wheel and rotor closer to the hub. While a looser lug nut will provide less clamping power. In other words, if you don’t tighten the nuts evenly, one side of the rotor is tighter against the rusty hub than the other side. The only way to tighten evenly is with a torque wrench.
EXAMPLE:
Let’s say you didn’t clean the hub before installing the rotor and the rotor cocks .003” at the outside edge of the hub. Assume you’re driving on 205/55R16 tires. That means the rotor high spots slap the pads 836 times each mile. In 3,000 miles, the rotor has hit the pads 2.5 million times. Can you see how that many slaps could wear thin spots or deposit extra pad material to form thick spots?
The type of friction material you use determines whether the rotor’s high spot get thicker or thinner
Pads generate friction when pushed against the rotor. But there are two kinds of friction—abrasive and adherent. Abrasive friction is present in a pad where the friction material is harder than the rotor. So the rotor wears as well as the pad. This is the kind of wear action you’d find in a low metallic pad. If you have lateral runout with this type of pad, they’ll wear a thin spot each time they slap the inboard and outboard pads. Now here’s where the pulsation comes in; As the thinner portion of the rotor reaches the pad area, the depression causes the caliper piston to move out of the bore. That causes a resulting drop in brake fluid pressure and pedal drop. Then, as the rotor continues to rotate, the thicker portion of the rotor comes into place, pushing the piston back into the bore. The increases fluid pressure and pushes the brake fluid backwards, lifting the pedal. This is how disc thickness variation causes pedal pulsation. You’ll find low metallic pads on European, Asian, and some high performance American cars
Adherent friction, on the other hand, is an action where the pads transfer a thin layer of friction material into the pores of the rotor, creating a film of friction material over the entire face of the rotor. Stopping action in this type of system is the result of film-on-film shear. As heat builds, the film on the rotor disintegrates and new friction material re-deposits on the rotor face. To understand the adherent friction, think of winter snow tires. Winter tires achieve greater traction in fresh snow because the large voids between the tread blocks compress snow and push it against snow on the street. In this case, the increased traction is created by snow-on-snow contact (called snow shear). That type of traction is far greater than the traction that can be created by rubber on snow. Don’t believe it? Take two snowballs and slide them against each other. That’s snow-shear. Next, slide a chunk of dry rubber against snow. The rubber produces far less friction.
The point here is that for adherent braking to work, it must deposit friction material onto the rotor. If the rotor isn’t parallel to the hub, the pads wipe too much friction material onto the high spots of the rotor, causing increased thickness. The result is increased fluid pressure and pedal pushback, followed by a return to normal rotor thickness and then piston outward movement, a drop in fluid pressure, and a lowered brake pedal.
In both cases, the end result is disc thickness variation CAUSED by lateral runout (rotor not parallel to the hub. Here’s an example:
It’s usually not rotor warp
Most people think rotors warp due to high heat—like an LP record left in the sun. That kind of warp is almost unheard of. The amount of heat required to cast a rotor is 3-5 times higher than you could possibly create from even the most aggressive downhill braking. Based on the way rotors are designed and built, the metal will crack long before it can warp. Still don’t believe me? Well, think about what would happen to a floating caliper IF the rotor was indeed warped.
As the high wave spot approaches the inboard pad it’ll push against the caliper piston. If we assume the caliper slides are adequately lubed, the caliper body will respond by sliding away from the rotor. The caliper will pull the outboard pad along with it. There will be no increase or drop in pressure because the rotor thickness is still uniform. The floating caliper will slide back and forth on the slide pins, riding the “waves” of a warped rotor instead of causing pedal pulsation. No thickness variation equals no pedal pulsation.
Of course if the slides aren’t properly lubed, a caliper can’t float along a wavy rotor. In that case, a ‘warped” rotor could cause. And, if the lateral runout exceeds .010” the caliper can’t move (float) fast enough at high speeds and “riding the waves” will create a pulsation. At low speeds, that kind of runout will feel like a surging movement, almost like the wheels are out of round. And, of course, a warped rotor would definitely cause pedal pulsation in a vehicle with fixed (not floating calipers).
The rotor manufacturers tell us that in almost all cases where rotors that are returned under warranty because they “warped,” the rotors actually have disc thickness variation (DTV).
It’s the DTV, stupid. Now let’s prevent it.
The best way to eliminate disc thickness variation is to clean the hub and make sure it doesn’t have more than .006 lateral runout. Your job is to clean the rust off the hub
![steering wheel vibrates, brake pedal vibrates, pedal pulsation]()
Clean hub with 3M hub cleaning pads or wire wheel
![steering wheel vibration, brake pedal pulsation]()
3M 07547 Hub Cleaning Kit
without removing metal. 3M makes some hub cleaning products. Or you can use drill and wire wheel, sandpaper, or scrub pad. Once you’ve cleaned the hub, apply a very thin film of synthetic high temperature brake grease to reduce future rust buildup. Do NOT use anti-seize compound—it isn’t designed to handle the clamping loads. And clean the inside of the rotor “hat” area if you plan to re-use the old rotor.
![brake pedal pulsation, steering wheel vibration]()
Clean the inside of the rotor hat
Apply the same high temperature grease to the pad slide areas to prevent end-tab rust on the pad backing plate. And apply high temp grease to the caliper slide pins. If you find any corrosion on the slide pins, replace them. They run about $10 for a set of two.
Torque wrench on lug nuts
Proper lug nut torque is CRITICAL to maintaining true rotors. In a recent GM seminar, an engineer disclosed that if you under-torque a single lug nut, you can cause up to .003” of lateral runout.That means you MUST use a torque wrench. I’m not a big fan of those cheapies you can buy at Harbor Freight. But, if you buy one of those, at least you have a chance that each of the lug nuts is tightened to the same torque (or close to it). The correct way is to tighten the lug nuts to 50% of the spec, and in a criss-cross pattern. Then return and tighten to the full spec.
So here’s the best way to prevent lateral runout and disc thickness variation
1) Clean the hub with the 3M products above or a wire brush. Then clean the rust off the back side of the wheel.
2) To prevent rust from forming again, apply a thin coat of anti-seize on the face of the hub. Do not EVER lubricate the studs!!! This is EXTREMELY important. If the lug nut is hard to install on the stud, get a new lug nut or replace the stud—it means the threads are distorted. Do NOT coat the studs with anti-seize.
3) Install the wheel and torque the lug nuts in a star shaped pattern to one-half the specified torque. This is known as pre-stretch. Then reset your torque wrench to the specified torque and tighten them all to full torque. It’s a two-step process.
I know. I know—nobody does it that way. Then again, everybody complains about warped rotors, too.
Got a car question? Click here and send it to me. I’ll do my best to get you an answer.
For more information on this repair or any others for your vehicle, buy an online subscription to either Alldatadiy.com or eautorepair.net. Click on this link to compare the two services: Compare Alldata and Eautorepair.
If you just need information for a single repair and want to save money,eautorepair offers a lower price 1-week subscription for only $11.99. Or, if you’ll be working on this vehicle in the future, you can buy a 1-year subscription (Alldatadiy.com for $26.99, or eautorepair.net $29.99)
© 2012 Rick Muscoplat
![Alldatadiy.com]( "Alldata, shop manual, wiring diagrams, tsbs")
Alldatadiy.com
![eautorepair.net]( "online manual, shop manual, repair information")
eautorepair.net
Alldata and Eautorepair are both professional grade shop manuals. You’ll find more in-depth information and more complete wiring diagrams, along with descriptions of how the systems work. And, you’ll get all that at a price you can afford. Check them out!
Click HERE to read AUTO REPAIR ARTICLES sorted by topic
Click HERE to read some cool DIAGNOSTIC TIPS
Click HERE to read ACURA articles
Click HERE to read AUDI articles
Click HERE to read BMW articles
Click HERE to read CHRYSLER articles
Click HERE to read DODGE articles
Click HERE to read FORD LINCOLN MERCURY articles
Click HERE to read GM articles
Click HERE to read HONDA articles
Click HERE to read HYUNDAI articles
Click HERE to read INFINITI articles
Click HERE to read JEEP articles
Click HERE to read KIA articles
Click HERE to read MITSUBISHI articles
Click HERE to read NISSAN articles
Click HERE to read SUBURU articles
Click HERE to read TOYOTA articles
Click HERE to read VOLKSWAGEN articles
![]()
The post Brakes vibrate or pulsate appeared first on Rick's Free Auto Repair Advice.
