2017 Subaru WRX STI Front vs Rear Brakes: What Wears First and What to Replace

By Core Brake Parts Tech Team — Circleville, OH warehouse

On a 2017 Subaru WRX STI the front brakes wear first, and usually by a wide margin. The fronts carry most of the stopping load, and on this car they also do extra work in corners because the stability system brakes the inside front wheel to help it turn. Plan on front pads more often than rear pads, replace everything in axle sets, and confirm your car has the 4-piston front Brembo system before you order parts.

How the 2017 WRX STI Brake System Is Laid Out

Every WRX STI of this generation uses Subaru's Brembo Performance Brake System: ventilated discs at all four corners, anti-lock brakes and Electronic Brake-force Distribution (EBD). Subaru's press kits for the 2015 and 2016 STI give the layout: front discs of 13.0 inches in diameter clamped by 4-piston calipers, and 12.4 inch rear discs with 2-piston calipers.

For the 2018 model year Subaru announced an upgraded system with stronger monoblock 6-piston front calipers, monoblock 2-piston rears, larger drilled rotors and new pads with more surface area. That change matters when you shop. A 2017 STI sold in the US carries the earlier 4-piston front setup, so parts built for the 6-piston 2018-and-later cars are not a match for it, even when a listing is dated "2017 and newer" to follow an overseas model year. Check the caliper before you order: count the pistons, or compare the rotor diameter to the 13.0 inch front spec.

Why the Front Brakes Wear First on the STI

When you brake, weight moves forward onto the front tires. The front axle then has more grip, so the system is designed to send it the larger share of braking force. That is why the fronts get the bigger discs and the 4-piston calipers, and it is why the front pads are the ones you replace most often.

The STI adds two things that push that further. First, Active Torque Vectoring, part of Subaru's Vehicle Dynamics Control, can apply brake pressure to the inside front wheel to make the car corner more neutrally. Every time it steps in on a spirited drive, the front pads and discs take a little more wear and heat. Second, Super Sport ABS uses lateral g, steering angle, yaw and brake pressure sensors to control braking force at each rear wheel independently in hard cornering. That helps stability, but it does not shift enough work rearward to change which end wears out first.

How you drive decides the gap. A commuter STI that rarely sees a back road will wear its fronts steadily and its rears slowly. A car that sees autocross or track days can use up front pads in a weekend, and the rears will start to show wear too. There is no fixed mileage that fits every car; measure the pads rather than guessing.

Symptoms and How to Tell Which End Needs Work

• High squeal or scraping on every stop: Wagner lists worn-out pads as a cause of high squeal and noise. On an STI, check the fronts first.
• Grinding: the friction material is gone and the backing plate is touching the disc. Stop driving on it; the disc is being damaged with every stop.
• Pedal pulsation or steering-wheel shake: uneven pad wear or disc thickness variation. Shake through the steering wheel points to the front discs; a pulsing pedal with a steady wheel can come from the rears.
• Pull to one side: Wagner notes a severely worn pad can make the car pull. A sticking caliper piston on one side causes the same thing.
• Weak braking with a normal pedal: pads contaminated by oil, grease or brake fluid lose grip. Find the leak before you install new pads.

The quickest check is visual. With the wheels off, look at pad thickness through the caliper window at both ends and compare inner and outer pads. On the 4-piston fronts, uneven wear between the inner and outer pad points to a piston that is not moving freely. Measure disc thickness with a micrometer and compare it to the minimum thickness stamped on the disc or listed in the factory service manual.

What to Replace, and What to Replace With It

Always replace pads as an axle set, both sides at once. Then decide on the discs and the rest:

• Front pads: the regular service item. Choose a compound that suits how you drive: a street pad for commuting, a higher-temperature compound if the car sees track time. Browse brake pads or low-dust ceramic brake pads and confirm the listing covers the 4-piston Brembo 2017 STI.
• Front discs: replace them when they are below minimum thickness, have a lip, show heat cracks, or cause pulsation. Wagner's procedure is to check lateral run-out, parallelism and minimum thickness against the manufacturer's specification and replace the disc if any is out. See brake rotors.
• Rear pads and discs: inspect them at every front job. They often outlast a set of front pads, but do not assume it on a car that is driven hard.
• Hardware: install the new pad hardware supplied with the pads and lubricate the contact points with brake lubricant. A brake hardware kit covers clips and pins if yours are corroded.
• Brake fluid: hard driving heats the fluid. Flush it on the interval in your owner's manual with the fluid type Subaru specifies, and sooner if the car sees track days.
• Lines and hoses: check the rubber hoses for cracks and swelling while the wheels are off. Braided brake lines are a common upgrade on track-driven STIs for a firmer pedal.

How Hard Is the Job?

Front and rear pad changes on the STI are straightforward for a home mechanic with a jack, stands, a torque wrench and basic hand tools. The fixed Brembo calipers make pad swaps easier than on many cars: the pads come out of the caliper body without removing it, and the pistons are pushed back evenly before the new pads go in. Changing the discs means removing the calipers, so torque every caliper bolt and wheel fastener to the values in the factory service manual rather than guessing.

Two steps make the difference between a good job and a noisy one. Clean the hub face so the new disc sits flat; Wagner points out that rust on the hub leads to run-out and pulsation. And wash new discs with soap and hot water to remove oil and fine particles. Then bed the pads in. Wagner's standard break-in is 30 stops from 30 mph with 30 seconds between each stop, which transfers an even layer of pad material onto the disc. Follow the pad maker's own bedding instructions if they differ.

Frequently Asked Questions

Do the front brakes on a 2017 WRX STI wear faster than the rear?

Yes. The front axle takes the larger share of braking force, and the STI's Active Torque Vectoring also brakes the inside front wheel in corners. Expect to replace front pads more often than rear pads.

Will 2018 STI brake parts fit my 2017 STI?

Not directly. Subaru changed to 6-piston monoblock front calipers and larger drilled rotors for 2018. A US 2017 STI uses the 4-piston front Brembo system, so pads and discs must be listed for that setup.

Should I replace front and rear brakes at the same time?

Only if both ends are worn. Pads must be replaced in axle sets, but the fronts and rears wear at different rates. Measure both ends and replace what is actually worn.

Do I need new rotors every time I replace the pads?

No. Reuse discs that are above minimum thickness, run true and have no lip, cracks or heat spots. Replace them if they fail any of those checks or if you have pulsation.

What size are the 2017 WRX STI brake rotors?

Subaru lists 13.0 inch front discs and 12.4 inch rear discs for this generation of STI with the 4-piston and 2-piston Brembo calipers.

Sources

1. Subaru U.S. Media Center - 2015 WRX STI Press Kit - Brembo system, 13.0 in front discs with 4-piston calipers, 12.4 in rear discs with 2-piston calipers, Super Sport ABS, Active Torque Vectoring
2. Subaru U.S. Media Center - 2016 WRX STI Press Kit - same disc sizes and caliper layout carried forward, EBD
3. Subaru U.S. Media Center - 2018 WRX and WRX STI debut - 2018 change to monoblock 6-piston front calipers, larger drilled rotors, new pads
4. Wagner Brake - Signs of Worn Brake Pads - squeal, pulsation, pull and contamination symptoms
5. Wagner Brake - Replacing Brake Pads and Rotors - new hardware, hub cleaning, washing discs, 30-30-30 burnish