A pulsating brake pedal is irritating and can be dangerous when emergency braking. You are correct to question why your rotors keep warping, I’m a mechanic, and very shortly, we’ll have this figured out.
Brake rotors commonly warp when they overheat. When replacement rotors keep warping, there may be an underlying issue, such as:
- Inferior quality rotors
- Wrong type rotors fitted
- Brake caliper issue
- Excessive brake use
In this post, you’ll learn about the most common reasons brake rotors keep warping and what you can do to prevent it.
- Why rotors warp
- Worn rotors
- Inferior quality rotors
- Wrong type rotors fitted
- Brake caliper issue
- Excessive brake use
- Why warped rotors are dangerous
- Rotor buying tips
- Fitting rotors
- Sum up
Why do Brake Rotors Warp?
As said previously, brake rotors typically warp because they overheat, and so we are really asking why our brakes are overheating.
Some reasons are obvious, like worn-out rotors, and we’ll cover that below, but since your replacement rotors keep warping, I suspect you may have an underlying issue, and we’ll cover that below too. But first, we’ll need to eliminate a few of the more common reasons rotors warp.
The most common reason brake rotors overheat is wear. As rotors wear, they become thin, and the thinner the metal, the faster they heat up and the more likely they are to warp.
When a mechanic inspects brakes, they check pad material dept and rotor wear. All manufacturers have a min rotor thickness and runout spec. If the rotors are within this min spec, the rotors may be turned. Turning, also known as machining the rotors, removes a thin layer of metal from both sides of the rotor, renewing the pad contact surface and also making the rotor true.
Tools – In the workshop, to check rotors, we typically use a dial gauge to check runout and a micrometer to check disc thickness in five locations.
Both runout and thickness must be within min spec to machine the rotors.
Diagnosis – When inspecting vehicles, the first job I do is a test drive. This gives me a ton of information before I ever drive into the workshop. If I have a pulsating brake pedal, I know I have a warped rotor, and while the rotors may be within spec when measured, experience has shown me that the best solution for a warped rotor is replacement.
Inferior Quality Rotors
Rotors are under a ton of stress and must be up to the job; some after-market rotors simply aren’t up to the job. I’ve fitted rotors for over twenty-five years. While OEM (Original Equipment Manufacturer) and spurious replacement parts may look alike, the materials and manufacturing processes make a huge difference to their performance and durability.
In my experience, many replacement rotors just aren’t up to the job. It’s okay to use inexpensive spurious parts in noncritical low-stress parts like window motors, door locks, etc.
Brakes, steering, and suspension need top-quality OEM components or at least a top-rated parts maker.
Inexpensive rotors will work for low mileage low stress braking, but a heavily used vehicle will warp replacement rotors in no time at all.
Wrong Type Rotors Fitted
As an apprentice, my job included running to the local parts store with a shopping list of parts which would inevitably include some wrong parts. My solution, bring all the old parts with me and cross reference.
Happy to say wrong parts have largely been eliminated. Your auto part store access a central computerized system that automatically lists parts for your vehicle by chassis number.
Ordering parts this way is almost foolproof; note I say almost.
Rotors come in two main flavors – solid and vented. The performance between these types of rotors is day and night. While it isn’t likely you have the wrong rotors fitted, it is possible and worth checking.
Solid rotors are, as their name suggests, the friction surface is solid. This type of rotor is fitted to smaller, lighter vehicles that aren’t likely to be traveling at high speeds or towing a trailer, etc.
The advantage of the solid rotor is its cost; it’s inexpensive when compared to the vented rotor.
The disadvantage of the solid rotor is it’s inefficient at shedding heat and, if stressed by excessive braking, can cause overheating and rotor warping.
Vented brakes are just like their name suggests; there is a void between the friction surfaces of the rotor. Vented rotors are fitted to luxury, commercial, and performance vehicles where the vehicle’s weight alone requires better brake performance.
The advantages of vented rotors – the void between the contact surfaces allows airflow and removes friction heat efficiently.
The disadvantage of the vented rotors is the cost; they cost more than solid rotors as they vented are more complex to manufacture.
Diagnose – Check your rotors when cold; most vehicles won’t require wheel removal. The rotor edge will sport the aforementioned void between either friction surface.
Brake Caliper Issue
Rotors, calipers, and pads all work hard, and the caliper, in particular, can cause a few common issues. Rotor warping is, as you know, caused by excessive heat; a dragging brake is a likely culprit. And a dragging brake is commonly caused by a sticking caliper; the caliper brake remains partially on, and that, as you can imagine, causes the rotor to heat excessively.
The obvious question is, what causes the caliper to stick? There are two common issues, they are:
1 Seized caliper piston – The caliper employs a hydraulically controlled piston that forces the pads against the rotor.
In older brake systems, moisture inside the brake lines can cause piston corrosion, which prevents the piston from retracting, meaning the brake remains partially on.
Seized caliper slide pins – A seized caliper slide pin/bolt only applies to floating-type calipers, which, to be fair, are most calipers fitted to regular-type vehicles.
I covered what a floating caliper is previously, and you can check that out here – “Are calipers suppose to move?” but since most calipers are floating, this will likely apply to you.
Diagnosis – Common symptoms of a dragging wheel include excessive brake dust on one wheel, heat from one wheel, particularly, and vehicle pulling to one side. To diagnose a suspected dragging break, – jack the affected wheel up and spin it; it should spin freely; if it won’t budge or only moves a little, you found a dragging brake, and you’ll need to remove the wheel for further diagnosis.
With the wheel removed, try the following:
- Open the brake fluid reservoir cap one turn
- Remove the caliper retaining bolts
- Remove the caliper
- Attempt to retract the piston with channel locks
- If it fails to retract, suspect a frozen piston
- If it retracts without issue, check the slide pins
- If the slide pins are sticking or seized, repair or replace
- Close reservoir cap again when complete
I wrote a complete post about diagnosing and fixing a dragging brake “What causes a brake to drag?”
Excessive Brake use
Driving style and use will dictate how quickly our brakes wear or overheat. One customer may get fifty thousand miles from a set of rotors, and another customer might not make twenty thousand.
It depends on many factors; the three that contribute the most to rotor wear and overheating are; the type of driving your vehicle does, the weight of your vehicle, and driver type.
Symptoms associated with excessive brake use include – Heavy brake dust on wheels; Glowing rotors; Smoke from rotors; Grinding noise under braking; Brake fade when brakes are hot. All of these symptoms contribute to warped rotors.
Type of Driving
Vehicles generally either live and work in a city or an urban environment; a mix of both.
City living and working vehicles tend to be hard on brake rotors and pads, no surprise since they do a ton of stop-start type driving.
Urban living and working tend to be the easiest on the brakes, but as you’ll learn later, this type of light brake use can cause other issues.
The commuter vehicle, urban living but works in the city, wear brakes at a rate somewhere between the previous types. Vehicles that lean more on highway driving than city driving are generally easier on brakes.
Weight of Vehicle
The weight of your vehicle obviously plays a large part in how hard your brakes are working and wearing. It makes sense, a pickup truck, for example, will wear brakes faster than a family sedan, and more mass takes greater force to control.
If your vehicle ordinarily carries a ton of kit in the trunk, or maybe you have a hatchback and regularly carry lots of heavy gear, you can expect heavier brake temperatures and wear.
Similarly, if your pickup spends most of its life fully loaded or hauling a trailer, then you can expect heavier than normal rotor wear, and as we know, the thinner the rotors get, the more likely they are to overheat and warp.
It might be worth accessing if the vehicle is up to the workload. Modern SUV Crossover type vehicles look impressive and are great vehicles, but many just aren’t suitable for hauling a trailer or lugging heavy loads regularly.
Type of Driver
We had customers from both sides of the spectrum at our shop, and different driving styles caused their own particular issues.
After working on a ton of customers’ vehicles, you develop a sense of the type of drivers they are; typically, drivers fit into one of three groups:
- The high speed, heavy load late breaker type – This type of driver likely haul a heavy trailer or just enjoys driving a little faster than most. Typically this driver brakes hard and late, which generally shows in the maintenance records of the vehicle. Rotors might not last this driver more than twenty thousand miles.
- Average speed, average brake use type – This type of driver makes up the majority, and rotors might last thirty to forty thousand miles.
- The low-speed light breaker type – The low-speed driver can expect to get fifth thousand miles plus from their rotors. But because this type of driver is so easy on the brakes, it causes rust to build up on the rotors, and the rotors often require replacement not because of wear but because of corrosion.
Why Warped Rotors Are Dangerous
You already know what a warped brake rotor feels like; pulsing pedals, brake grab, and excessive brake noise are common, but so too is a long brake pedal, which can be dangerous.
The pads are designed to ride close to the rotor, but a warped rotor pushes the pads further away, meaning it takes greater brake pedal force and travel to make pad rotor contact. And greater travel means it takes longer, and time is crucial when emergency braking.
Yep, a warped rotor is dangerous, and not just because they take longer to work but also because a warped rotor is far more likely to fracture and break up, causing either a lack of brakes or wheel lock up, both highly dangerous.
Rotor Buying Tips
As said earlier, I recommend buying OEM parts when working on critical systems like Brakes, steering, and suspension, or at least a top-tier spare parts manufacturer. Go to the parts store armed with your chassis number and explain your issue, most good parts stores are happy to help, and they’ll recommend the best rotors they have, manufacturers like Delphi, ACDelco, and Bosch.
If you need superior brake performance, you’ll need to shop in the performance brakes aisle, EBC, Brembo, Willwood, etc. I wrote a post about Wilwood brakes previously, and you can check that post out here “Are Wilwood brakes worth it?”
Fitting rotors is a job the home mechanic can easily do, but you’ll need a few tools to nail this successfully.
Tools, supplies – You’ll need the following:
- Axle stand
- Torque wrench
- Socket set and breaker bar
- Selection of wrenches
- Torx & Hex bit selection
- Flat screwdriver
- Channel locks
- Wire brush
- Copper grease
- Anti squeal paste
- Silicone grease
- Brake cleaner
You’ll find all these tools here on the “Brake repair tools page”
Parts – You’ll need the following:
- Two rotors – we fit brake components in pairs for equal brake performance on the same axle
- Set brake pads – we always fit new brake pads with new rotors
The rotor removal process looks like this:
- Remove wheel
- Remove the rotor to hub locating fastener (typically a countersunk Torx)
- Remove the caliper fasteners and secure the caliper with a bungee cable (do not allow the caliper to hang from flexi line)
- Remove pads
- Remove two brake carrier fasteners using the breaker bar and socket and set the carrier aside
- Remove the rotor (if it’s stuck, hit it with a hammer)
The fitting process looks like this:
- Clean the new rotor thoroughly using brake cleaner. New rotors come with protective grease, which, if not removed, will contaminate your new pads
- Wire brush the hub interface
- Fit the new rotor and fastener
- Wire brush the brake carrier (clean outdoors is best)
- Fit the carrier and tighten the fasteners to spec (typically about 195 Nm (144 ft-lbs)). Many manufacturers recommend these bolts are replaced once removed.
- Open brake fluid reservoir cap one turn
- Use channel locks or wind back tool to retract piston
- Apply anti-squeal paste to pads before fitting (check the pads are location specific)
- Check and lube slide pins
- Fit the caliper, taking care not to twist the brake flexi hose.
- Fit the caliper fasteners and tighten to spec (typically about 30 Nm (22 ft-lbs)). Many manufacturers recommend these bolts are replaced once removed.
- Repeat the process on the other side
- Important Pump brakes several times before driving the vehicle
- Check brake fluid reservoir level before closing the cap
- Bed in brakes
Bed in brake procedure looks like this:
- Pump brakes several times before starting the vehicle
- The brake pedal should feel normal
- Drive the vehicle below 30 mph and apply brakes; repeat five times
- Go ahead and drive at normal speeds and test the brakes five times
No need to brake hard or excessively; this only serves to damage the new components.
Check your fluid level again, and you are done; nice work!
I covered the whole process step by step with pictures previously, and you can check that out here – “Are rotors hard to change?“
Rotors warp when they are exposed to repeated cycles of overheating. Rotors commonly overheat and warp as they wear out and become thin. Other possible reasons are poor quality rotors, excessive brake use, and a binding brake caliper.
A thorough diagnosis is needed; inspect the brakes; check the quality of rotors fitted; access the driving style and type of use.
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