2018-2019 Audi S5 Front vs Rear Brakes: What Wears First and What to Replace

By Core Brake Parts Tech Team — Circleville, OH warehouse

On a 2018-2019 Audi S5, the front brakes do most of the stopping and normally wear out first, but do not assume the rears will last twice as long. The front axle carries large 350 mm by 34 mm rotors, while the rear uses smaller 330 mm by 22 mm rotors with the electromechanical parking brake built into the rear calipers. Measure all four corners every time the wheels come off, and replace pads and rotors as axle sets.

How the S5 Splits Braking Between Front and Rear

Every car shifts weight forward when it brakes, so the front tires can put more braking force to the road. Audi sizes the S5 hardware to match. The front rotors are the big, thick parts on this car, and the rear rotors are noticeably smaller and thinner. That difference is the first clue to how the wear splits.

The front rotor has only 2 mm between new and minimum thickness. That is a small window, and it is why many S5 owners find the front rotors need replacing along with the front pads rather than resurfacing. Machining a front rotor on this car usually takes it straight to, or past, the discard limit.

The rear brakes do less of the stopping, but on a modern all-wheel-drive Audi the stability and traction systems apply individual brakes to manage wheel slip, and the rear pads carry less friction material than the fronts to begin with. The practical result: rear pads on these cars can wear closer to the front rate than many owners expect. There is no fixed ratio you can rely on, so measure instead of guessing.

Symptoms: Which End Is Worn?

You can often tell which axle needs attention before you pull a wheel.

• Pad wear warning in the cluster. The S5 uses an electronic wear sensor. Once it trips, the sensor has usually been ground through and should be replaced along with the pads. Check which axle the sensor is on for your car before assuming which end triggered it.
• Vibration in the steering wheel under braking. Points to the front rotors, usually from uneven pad deposits or thickness variation after hard use.
• Vibration felt in the seat or through the body, not the wheel. More often a rear rotor issue.
• Grinding or scraping from one corner. Metal-to-metal contact. Stop driving on it; the rotor on that corner is now being cut by the pad backing plate.
• Parking brake fault warning. The parking brake is part of the rear caliper, so a warning after rear brake work usually means the system was not put into service mode or closed correctly afterward.
• Heavy dust on the front wheels only. Normal. Front pads do more work and shed more dust.

How to Check Wear on Both Axles

Pull the wheels and look through the caliper at the pad material on both the outer and inner pads. Inner pads can wear differently from outer pads on either axle, so check both. Then measure each rotor with a micrometer at several points around the braking surface, not at the rusted outer lip.

• Front rotor at or under 32 mm: replace it.
• Deep grooves, blue heat spots or cracking on either axle: replace the rotor regardless of thickness.
• A raised ridge at the outer edge you can catch with a fingernail: the rotor has worn a step, and new pads will not seat properly against it.

If one side of an axle is worn far more than the other, look for a sticking caliper piston or a pad that is hanging up in its bracket. Fix the cause, or the new parts will wear the same way.

The Rear Job Is the Harder One

On most cars the rear brakes are the easy half. On the S5, the rear is the job that catches people out, because an electric parking brake motor is mounted on each rear caliper. Inside the caliper is a spindle and pressure nut that the motor drives to hold the piston out when the parking brake is applied.

ZF, which makes TRW electric parking brake systems, spells out the correct approach. Before the car goes on the lift, use a diagnostic tool to release the rear calipers and put the system in pad change mode. That retracts the pressure nut. The piston itself still has to be pushed back by hand, and it is a straight push, not a wind-back like older mechanical rear calipers. After reassembly and torquing, use the tool again to close the brakes so the system resets.

Skipping service mode and forcing the piston back can damage the actuator, which costs far more than the pads. If you do not have a scan tool that can command the parking brake on an Audi, this is the half of the job to hand to a shop.

The front brakes have no parking brake hardware, so a front pad and rotor job is a conventional one. Push the pistons back evenly, clean the caliper and pad contact surfaces, fit new clips, and torque the caliper and wheel bolts to the values in the factory service manual.

What to Replace, and What to Replace With It

Always replace pads as a full axle set, both sides at once, and replace rotors in pairs. Mixing a new rotor with an old one on the same axle can cause a pull.

• Front job: front pads, front rotors (usually, given the 2 mm wear window), a new wear sensor if yours has tripped, and new pad clips. Browse brake pads and brake rotors by fitment.
• Rear job: rear pads, and rear rotors if they are under spec or ridged. For the rear axle we carry the Hawk HPS 5.0 rear brake pads for the 2018-2019 Audi S5, a performance street compound with a 0.652 inch pad thickness.
• Hardware: ZF recommends fitting new spring clips with new pads and keeping grease off the pad contact areas. A brake hardware kit covers the clips.
• Brake fluid: brake fluid absorbs moisture over time. If a flush is due on the maintenance schedule, a brake job is a sensible time to do it, using the fluid type Audi specifies on the reservoir cap or in the owner's manual.

A note on pad choice: performance street pads like the Hawk HPS 5.0 are made for iron rotors. Hawk states that its compounds are not compatible with carbon ceramic rotors, so confirm your S5 has iron rotors before buying. After installing any new pads, follow the maker's bed-in procedure so the pad lays an even transfer layer onto the rotor.

How Hard Is Each Job?

For a home mechanic with a torque wrench, the fronts are a reasonable Saturday job. The rears are just as manageable once you have the right scan tool; without one, they are the half to send out.

Frequently Asked Questions

Do the front or rear brakes wear out first on a 2018-2019 Audi S5?

The fronts usually wear first because they do most of the braking. The rears can wear faster than owners expect on this car, so measure both axles whenever the wheels are off.

Can I resurface the front rotors on my S5?

Usually not. The front rotors are 34 mm new with a 32 mm minimum, so there is very little material to remove. Most need replacement rather than machining.

Do I need a scan tool to change the rear brakes?

In practice, yes. The rear calipers carry the electric parking brake, and the correct procedure is to put the system in pad change mode before work and close it afterward with a diagnostic tool.

Should I replace all four corners at once?

Only if all four are worn. Replace what measures out of spec, always as a matched pair on one axle. Many owners end up doing the fronts on their own and the rears on a separate visit.

Why is my parking brake warning on after a rear brake job?

Most often the system was not closed or reset with a scan tool after the pads were installed. Have it checked before driving, since the parking brake is part of the rear caliper.

Sources

1. Alcon Brake Kits - B9 S4/S5/SQ5 front rotors - front rotor 350 mm x 34 mm, 32 mm minimum thickness, 2018+ S5 fitment
2. Alcon Brake Kits - B9 S4/S5/SQ5 rear rotors - rear rotor 330 mm x 22 mm, 2018+ S5 fitment
3. Hawk Performance - HB866B.652 - rear HPS 5.0 pad, 0.652 in thickness, not compatible with carbon ceramic rotors
4. ZF Aftermarket - How to change brake pads with EPB - pad change mode, straight push-back of the piston, new spring clips, no grease, closing the system after