1987-1991 BMW 325i Front vs Rear Brakes: What Wears First and What to Replace

By Core Brake Parts Tech Team — Circleville, OH warehouse

On a 1987-1991 BMW 325i (E30), the front brakes wear out first, usually by a wide margin. The front axle runs a larger, vented 260 mm rotor and does most of the stopping, while the rear runs a thinner solid 258 mm rotor that mostly steadies the car. Expect to do front pads more often than rears, and check both axles every time a wheel comes off.

How the E30 325i brake system is laid out

Every E30 325i in these years has four-wheel disc brakes, with discs at both ends. (Early four-cylinder 318 models used a different rear setup, so parts are not interchangeable with them.) The two axles are built very differently, and that difference is the whole story of why they wear at different rates.

A vented front rotor more than twice as thick as the rear tells you where BMW expected the heat to go. The front rotor has cooling passages between two friction faces because it absorbs most of the energy in every stop. The rear rotor is a single solid plate, because the rear axle loses load under braking and cannot use as much brake force without locking up.

Why the fronts wear first

When you brake, the weight of the car pitches forward. The front tires press harder into the road and can put down more braking force before they skid, and the rear tires get lighter. The hydraulic system is proportioned to match: most of the clamping work goes to the front calipers so the rear wheels do not lock and swing the car around. More work at the front means more heat and more pad material worn away on every stop.

On a typical street-driven E30 that means front pads come due well before the rears. How much sooner depends on how you drive. Stop-and-go city traffic, spirited back-road driving and track days all widen the gap. Long highway runs narrow it. There is no single mileage that fits every car, so measure rather than guess.

When the rear pads wear faster than the fronts, something is wrong. The usual suspects on a car this age are a sticking rear caliper slide or piston, a parking brake that drags because the shoes or cable are out of adjustment, or a front caliper that is not applying fully. Any of those deserve a diagnosis before you throw pads at the car.

Symptoms: which end is telling you it is done

Front brake symptoms

• Brake lining warning light on the dash. The E30 uses an electrical wear sensor clipped into a pad on the front axle. When the pad wears down to it, the circuit trips the light.
• Steering wheel shake under braking. Uneven rotor thickness or pad deposits on the front rotors usually shows up through the steering wheel, because the fronts are the steered wheels.
• Squeal or grinding from the front, and a deep lip on the outer edge of the rotor face.
• Pulling to one side. Often a sticking front caliper or a collapsed flexible hose.

Rear brake symptoms

• The same warning light. The rear axle has its own wear sensor, so a lit lamp does not tell you which end tripped it. Look at both.
• Vibration felt in the seat or the whole car, rather than the steering wheel, often points to the rear rotors.
• Rust ridges and scoring on the rear rotor faces. Lightly worked rear rotors often corrode before they wear out, especially on cars that sit.
• Weak or uneven parking brake. The parking brake on this car works inside the rear rotor hat, so rear rotor and parking brake condition are linked.

How to check each axle

1. Pull the wheel and look through the caliper at both pads. The inner pad often wears faster than the outer one if the caliper slides are dry.
2. Measure rotor thickness with a micrometer in several spots around the disc. Compare it to the minimum thickness cast or stamped on the rotor. The rear discs start at only 10 mm, so there is very little material to machine off before they are scrap.
3. Run a fingernail across the rotor face. Deep grooves, a heavy lip or blue heat spots mean replacement, not resurfacing.
4. Check the wear sensor wire on each axle. If the light has come on, the sensor has worn through and must be replaced.
5. Look at the flexible brake hoses. On a car this old the rubber can crack outside or swell inside and hold pressure on a caliper.

What to replace, and what to replace with it

Front brake job. Front pads, and front rotors if they are near minimum or damaged. The vented rotors are 22 mm when new and will often outlast a set of pads, but new rotors with every pad change give the smoothest, quietest result. Add a new front wear sensor. For street use pick from our replacement brake pads or low-dust ceramic brake pads. For dedicated track work we carry the Hawk DTC-70 front race pad set for the 1987-1991 325i, a race compound that belongs on a track car, not a daily driver.

Rear brake job. Rear pads and, because the solid rear rotor is so thin to begin with, usually rear rotors at the same time. Add the rear wear sensor, and while the rotors are off inspect the parking brake shoes and hardware inside the rotor hat. Browse replacement brake rotors and brake hardware kits.

Worth doing on any 30-plus-year-old E30:

• Flush the brake fluid with the grade specified on the reservoir cap or in the owner's manual. Old fluid absorbs moisture and boils sooner.
• Replace cracked or swollen rubber brake hoses. They fail with age, not mileage.
• Rebuild or replace a sticking caliper. See our brake calipers.
• Always replace pads and rotors in axle pairs, left and right together.

Keep the balance in mind. A much grippier front compound shifts bias forward; a much grippier rear compound can make the car unstable under hard braking. Keep compounds in the same family front and rear unless you know why you are changing it.

How hard is the job

Both axles are a reasonable home job for someone with jack stands, a torque wrench and basic hand tools. The fronts are the easier of the two. The rears take a bit longer because the parking brake shoes live inside the rotor hat, and a rusted rotor may need the parking brake backed off before it will come off. Torque the caliper bolts and wheel bolts to the values in the factory service manual, bed in the new pads before hard use, and pump the pedal firm before the car moves.

Frequently Asked Questions

Does a 1987-1991 BMW 325i have rear disc brakes or drums?

Rear discs. The 325i uses solid 258 mm by 10 mm rear rotors, with the parking brake working inside the rotor hat.

How much faster do the front brakes wear on an E30 325i?

There is no fixed ratio, because it depends on how the car is driven. Front pads almost always wear out first, and hard stop-and-go or track driving widens the gap. Measure both axles instead of assuming.

My brake warning light is on. Is it the front or rear pads?

It could be either. The E30 has a wear sensor on the front axle and another on the rear axle, and both feed the same warning lamp. Inspect both ends and replace the sensor that has worn through.

Can I reuse the brake pad wear sensor?

Not once it has triggered. The sensor works by being worn through, so after the light comes on it has to be replaced. Many owners simply replace the sensor with every pad change.

Should I replace the rear rotors every time I do rear pads?

Often, yes. The solid rear rotor starts at just 10 mm and has very little material to spare. If it is near its stamped minimum, rusted or scored, replace it rather than machining it.

Are race pads a good idea on a street 325i?

Not for daily driving. Race compounds are made for high track temperatures and can be noisy, dusty and hard on rotors on the street. Choose a street or street-performance pad unless the car sees regular track time.

Sources

1. AutohausAZ - Zimmermann front rotor 34111160915 - E30 front rotor is vented, 260 x 22 mm
2. AutohausAZ - ATE front rotor 34111160915 - 260 x 22 mm front rotor fits the 1987-1991 E30 325i
3. Zimmermann - Sport brake disc, BMW 3 (E30) rear - rear disc 258 mm, 10 mm thick, 9 mm minimum
4. BimmerWorld - OEM rear brake rotor, E30 - rear rotor fitment across 325i models, early 318 excluded
5. BimmerWorld - E30 front brake pad wear sensor - one sensor per axle, replace once the lining light is on
6. Ireland Engineering - E30 brake pad wear sensor - E30 has wear sensors front and rear; change them with the pads