Rick's Free Auto Repair Advice

Car shakes when braking — Most common cause

Learn why your car shakes when braking

If your car shakes when braking, the cause is usually due to disc thickness variation. Most people blame brake pedal pulsation or shaking while breaking on warped rotors, but rotors really don’t warp. That’s a myth. In fact, there isn’t a street vehicle in the world that can generate enough heat to warp a cast iron rotor. The brake pads would have burned up long before temperatures reached cast iron melting point.

What causes the shake when braking?

In order for brake parts to apply equal force to the brake rotor, the rotor must sit perfectly parallel with the wheel hub and steering knuckle. If it’s off by as little as .002″, the rotor disc will rotate in a wobble and that can cause uneven wear on opposite sides of the rotor or brake pad deposits on opposite sides.

It’s this disc thickness variation that causes the “car shakes when braking” syndrome. What causes the brake vibration is really disc thickness variation (see this post on disc thickness variation) which is caused by lateral run-out.

It all starts with lateral runout — What causes lateral runout

Sloppy brake installation is the root cause. Not cleaning corrosion off the wheel hub is the #1 cause of lateral runout. All you need is .002″ of corrosion buildup on the hub to prevent the rotor from sitting perfectly parallel with the hub.

Not using a torque wrench to tighten lug nuts is the #2 cause of lateral runout. Uneven lug nut torque causes the rotor to be uneven in contact with the hub.

Lateral run-out causes the rotor to wobble during braking and that causes uneven wear and brake friction buildup and THAT’s what causes pedal pulsation. The rotor isn’t actually warped. Read on for more information about how to prevent warped rotors and brake pulsation.

Brake job Mistake #1 Buying Cheap Parts

I can talk all I want about the difference between a name-brand top-of-the-line rotor and an economy rotor, but I’ll let the photos do the talking. Look at the photos shown hereThey show two brand new rotors for the same vehicle. One is a “white box” or store brand economy rotor and the other is a brand name top-of-the-line rotor. Notice the difference in weight. Then notice the difference in the thickness of the rotor surfaces. What you can’t see from these shots are the differences in the cooling vanes. The cheap rotor has fewer cooling vanes. And cheap rotors usually don’t match the OEM design vanes. Rotor cooling is essential and some OEM rotors have curved vanes to get maximum cooling. Those curved vane rotors are much more expensive to duplicate, so knock-off companies just cast straight vanes. But you can’t just rely solely on a brand name because most companies offer two quality levels; a “service” grade for penny-pinching customers, and a “professional” grade that’s the company’s top-of-the-line product.

Brake job Mistake #2 Not cleaning new rotors properly

Let’s assume you buy the best brake rotor. You take it out of the box, spray aerosol brake cleaner on it to clean the brake rotors before installing to remove the anti-corrosive “oil” coating. Then you slap in on the wheel hub. STOP! You’ve just made two mistakes! Aerosol brake cleaner is great at removing the anti-corrosive coating, but it DOES NOT remove the manufacturing machining residue. No matter how much spray you use, you’re still leaving machining particles on the face of the rotor. If you install them without further washing, the metallic particles will embed into the new pads and cause noise problems. That’s why ALL rotor manufacturers REQUIRE cleaning with hot water and SOAP!

I know, you’ve never heard of that or done that in any brake job over the last 40 years. Well, get over it. Times have changed and this is now the “best practices” way to clean new brake rotors. Even professional technicians are having to learn how to do it right. So quiturbitchin and start doing it NOW. Then clean the hub.

Brake job Mistake #3 Not cleaning the hub

rust on wheel hub

Corrosion on the wheel hub causes lateral runout

Next, you have to clean the wheel hub mating surface. The wheel hub accumulates rust and that rust can introduce lateral run out. And I’m not just talking about a quick wipe with a rag. If you leave rust on the hub or you’re reusing an old rotor with rust inside the rotor hat, that extra thickness will cause run-out. During each revolution, one face of the rotor will hit the inboard pad and the opposite face will hit the outboard pad. The pad’s friction material will build up on each of those faces and you’ll wind up with rotor thickness variation. And THAT’s one major cause of pedal pulsation. So what to do about it?

Brake manufacturers specify a maximum of .002” of runout measured at the middle of the rotor. That means you must remove all the rust from the wheel hub. 3M has come out with a system that chucks into your drill. See it here.  Just slide the unit over each stud and pull the trigger. The abrasive pad will remove rust without removing metal from the wheel hub.

Brake job Mistake #4 Improper Lug Nut Torque

Now let’s talk about lug nut torque. If you’re tightening lug nuts without a torque wrench, you’re begging for trouble. I know, you never had to do that in the old days. Well, it’s not the ’60’s anymore. You can introduce lateral run out imply by torquing lug nuts by hand without a torque wrench. All the nuts have to be torqued evenly. If you don’t, you’ll “cock” the rotor and introduce lateral run out.

Of course, all of this assumes that the wheel hub is true. If it’s not, all your work is in vain. Your new brake job will develop pedal pulsation in about 3,000 miles, even with good pads and quality rotors.

Finally, you have to ensure that the caliper slide pins, pad hardware, and caliper abutments are clean and coated with high-temperature synthetic brake grease. This is no small matter because is the caliper can’t “float” and the pads can’t retract, you’ll wind up with rotor overheating and pedal pulsation. Anti-seize is NOT the proper grease. Buy a tube of synthetic brake grease. If you find any corrosion on the caliper slide pins, REPLACE THEM.

Finally, perform the proper pad break-in procedure. Perform 30 stops, each from 30MPH, allowing 30-seconds of cooling time between each stop. That’ll heat up the pads and cure them, transfer a film of friction material evenly over the two rotor faces, and set you up for a perfect brake job. Avoid hard panic stops for about a week, because that can overheat the pad and cause glazing.

By cleaning the wheel hub, using good quality parts and using a torque wrench on the lug nuts you can say goodbye to the common car shakes when braking symptom.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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