Why Upgrade the E39 M5 Differential, Struts, and Bushings?

The BMW E39 M5 is already a legendary sedan, praised for its balance of luxury and raw performance. However, even the best factory cars leave room for improvement, especially after miles of hard driving. Upgrading the differential, struts, and bushings are three of the most impactful modifications you can make to sharpen handling, improve traction, and restore the car's youthful composure. These upgrades work together: a limited-slip differential puts power down more effectively, performance struts control chassis motion, and stiffer bushings eliminate slop in the suspension linkages. The result is a more predictable, responsive, and confidence-inspiring machine whether you’re on a back road or a track day.

Upgrading the Differential

The stock differential on the E39 M5 is an open differential (or optional limited-slip in some markets), which sends torque to the wheel with the least resistance. Under hard cornering or on slippery surfaces, this can lead to inside wheel spin and lost momentum. Upgrading to a true mechanical limited-slip differential (LSD) transforms how the car delivers power.

Benefits of a Limited-Slip Differential

  • Improved traction: An LSD transfers torque to the wheel with grip, reducing wheel spin on wet roads or during aggressive exits.
  • Better cornering performance: The car maintains a more neutral balance, reducing understeer and allowing earlier throttle application.
  • Enhanced acceleration: With both rear wheels pulling effectively, 0-60 times and corner exit speeds improve.

Choosing the Right LSD for Your E39 M5

Popular options include Wavetrac and OS Giken units, which offer different clutch-pack or helical gear designs. Helical diffs (like the Torsen type) are smoother for street use and require less maintenance, while clutch-type diffs provide more aggressive lock for track driving. Consider your driving style: if you daily drive the car, a helical LSD is often preferred for its seamless engagement. If you track the car regularly, a clutch-type unit with adjustable breakaway torque can be tuned to your needs. Always use the correct BMW rear diff fluid (75W-90 or 75W-140 with limited-slip additive) to ensure longevity and proper performance.

Installation Tips for the Differential

  • Fluid choice matters: After installing an LSD, you need specialized gear oil. Many owners choose Red Line 75W-90 NS or Motul Gear 300 to protect the clutch plates.
  • Check backlash and pattern: If you’re replacing the entire differential or ring and pinion, proper gear mesh is critical. Have a shop with a differential press handle the setup if you lack experience.
  • Torque spec note: The differential mounting bolts and output shaft bolts must be torqued to factory specification (check Bentley manual). Loose bolts will cause noise and driveline vibration.
  • Consider a diff cooler: If you plan on hard track use, an aftermarket differential cooler can prevent overheating and oil degradation. Turner Motorsport offers kits for the E39.

Upgrading the Struts

The E39 M5 came from the factory with Sachs dampers that balanced comfort and control. After 20+ years, these struts are often worn, leading to floaty handling, excessive body roll, and poor damping response. Upgrading to a modern performance strut restores (and exceeds) the original ride quality while giving you adjustability for different road conditions.

Strut Options: Coilovers vs. Traditional Performance Shocks

Two main upgrade paths exist: replacing the factory struts with sport-oriented replacements (like Koni Yellow or Bilstein B6/B8) or installing a full coilover system. Each has its strengths.

  • Coilover systems (e.g., KW Variant 3, Ohlins Road & Track, BC Racing) offer ride height adjustment, adjustable damping, and often a stiffer spring rate. This is ideal for track-focused builds or drivers who want to lower the center of gravity for sharper turn-in. Coilovers also allow corner-weighting for perfectly balanced handling.
  • Performance shocks with factory springs or aftermarket lowering springs (like Bilstein B8 with H&R springs) provide a more budget-friendly route that still improves body control and reduces dive under braking. The ride quality is firmer but still daily-drivable, and installation is simpler because you reuse the factory spring perches and top mounts.

What to Look for in Strut Upgrade

  • Adjustability: Even a non-adjustable performance strut like the Bilstein B8 is an upgrade over worn OEM units. Adjustable dampers (Koni Sport with external adjusters, or KW Variant 3 with independent rebound/compression) let you fine-tune the balance between comfort and handling.
  • Spring rate compatibility: Coilover kits come with matched springs. If you mix and match shocks with springs, ensure the strut is valved for the spring rate you choose. Too soft a damper with stiff springs causes bouncing; too stiff a damper with soft springs yields a harsh, jittery ride.
  • Top mount condition: While upgrading struts, inspect the top mounts and bump stops. Worn rubber mounts will negate the handling benefits. Upgrade to powerflex or monoball top mounts for maximum precision if using coilovers.

Installation Considerations

  • Alignment reset: Any suspension height change requires a full alignment. After installation, take the car to a BMW specialist or alignment shop that knows the E39 specs. Camber plates (if added) allow fine-tuning for track use.
  • Proper spring seat orientation: When replacing struts, the spring must sit correctly in the lower and upper perches. A spring compressor is essential for safety.
  • Torque strut bolts at ride height: Always tighten control arm and strut bolts with the suspension loaded (car on the ground) to prevent bushing bind.

For a deep dive on strut options and experiences from E39 M5 owners, check out the M5Board forums where thousands of upgrade threads exist.

Upgrading the Bushings

Bushings connect suspension arms to the chassis and subframe. Over time, rubber bushings harden, crack, and develop slop, turning your precise handling into a vague, wandering experience. Replacing them with polyurethane or spherical bearings transforms the car's responsiveness.

Key Bushing Locations on the E39 M5

  • Thrust arm bushings (TAU bushes): These are the single biggest improvement you can make. The front thrust arms (lower control arms) have large rubber bushings that deflect under braking and cornering. Replacing them with Powerflex polyurethane or Moog problem solver bushes eliminates brake shimmy and sharpens steering.
  • Rear subframe bushings: The E39 rear subframe mounts to the body with rubber that allows movement. Upgraded bushings (like Powerflex or Turner solid) lock the subframe in place, reducing wheel hop and improving rear traction during hard launches.
  • Rear trailing arm bushings (RTABs): These control the rear wheel's toe and camber under load. Worn RTABs cause loose rear-end behavior. Poly bushings here give immediate, predictable rear grip.
  • Sway bar end links and bushings: Replacing stock sway bar bushings with poly versions reduces body roll transition delay and sharpens turn-in response.

Bushing Materials: Polyurethane vs. Rubber vs. Spherical Bearings

  • Polyurethane (Powerflex, SuperPro) offers a great balance of stiffness, longevity, and NVH. It is the most popular choice for street-driven E39 M5s. Expect slightly more vibration and road noise, but it is manageable.
  • Hardened rubber (Meyle HD, Lemförder) is a close-to-OEM option that improves life span without harshness. Best for daily drivers who want to eliminate play but keep the ride civil.
  • Spherical bearings (monoballs) are the stiffest option, used by track cars. They transmit every road imperfection and require periodic lubrication. Not recommended for street cars due to noise and wear.

Installation Tips for Bushing Upgrades

  • Use a bushing press: A hydraulic press makes bushing removal and installation clean and safe. If you don't have one, many shops will press bushings for a small fee. Some bushings (like Powerflex) can be installed using a bolt and washer method, but a press is faster.
  • Lubricate properly: Polyurethane bushings require assembly lubricant (usually supplied) to prevent squeaking. Apply it generously to the inner sleeve and bushing bore. Spherical bearings need anti-seize or light grease.
  • Inspect surrounding hardware: While replacing bushings, check ball joints, tie rods, and control arms for excessive play. It's smart to replace all wear items at once to avoid repeat alignments.
  • Torque bolts at ride height: Just like with struts, tighten all chassis bolts with the suspension loaded. Poly bushings do not have the same compliance as rubber, so preloading them can cause premature wear or bind.

For a comprehensive guide on bushing replacement, including torque specs and part numbers, consult the RealOEM parts catalog to cross-reference factory and aftermarket parts.

Synchronizing the Upgrades for Maximum Performance

Upgrading these components individually yields benefit, but installing them as a set transforms the E39 M5 into a truly modern sports sedan. The combination of a limited-slip differential, responsive struts, and solid bushings eliminates the car's main weaknesses: wheel spin under power, soft damping, and vague steering. When planning your upgrade sequence, start with the bushings and struts — they restore the chassis' stiffness and control — then tackle the differential. This order allows you to feel the effect of each change. Many owners report that after these three upgrades, the E39 M5 feels like a new car, with turn-in precision that rivals much newer machines.

Maintenance Considerations After Upgrades

  • Regular inspections: Poly bushings last longer than rubber but still need checking for wear. Inspect annually for cracks or excessive looseness.
  • Fluid changes: LSD gear oil should be changed every 30,000 miles or after every track season. Use the correct friction modifier to prevent chatter.
  • Damper service: Coilovers and adjustable struts can be rebuilt. Schedule rebuilds every 40,000–60,000 miles or when damping degrades. Bilstein and Koni offer rebuild services.
  • Alignment recheck: After any suspension work, and at least once a year, have the alignment verified. The E39 benefits from slight negative camber in the front and a touch of toe-in at the rear for stability.

Final Thoughts

The E39 M5 remains one of BMW's greatest achievements, but age and mileage take their toll on its suspension and drivetrain components. By upgrading the differential to a modern LSD, installing performance struts (coilovers or sport shocks), and replacing critical bushings with polyurethane or hardened rubber, you can recapture — and exceed — the car's original driving dynamics. These three modifications are the foundation of a sorted E39 M5 that is a joy to drive on any road. Invest the time in proper installation, use quality parts, and your M5 will reward you with years of razor-sharp handling and reliable performance.