Rear Upper Shock Absorber Bush Failure on Fiat 500 & Abarth 312 Explained
If you’re chasing a rear-end knock on a Fiat 500, 500C or Abarth 312 or Abarth 312C, there’s a very high chance the issue is coming from the rear upper shock absorber bushes. These small rubber mounts sit at the top of the rear dampers and play a critical role in keeping the suspension controlled, quiet and properly located.
On paper, they’re simple components. In reality, they’re one of the most common failure points on this platform.
What the rear upper shock bush actually does
The rear upper shock bush is responsible for:
- Isolating vibration from the damper into the body
- Keeping the shock absorber correctly located
- Controlling movement under compression and rebound
- Preventing metal-to-metal contact at the mounting point
On the Fiat 500 / Abarth 312 platform, these bushes (including OEM references such as 51857841, 51864833, 81857840, 51860150, 51857842, 50709731, 1540225, 1578532, 1672372, 9S51-18125-AB / AC / AA) are relatively small and made from soft rubber compounds designed primarily for comfort.
That’s where the problems begin.
Why rear shock bushes fail on the Fiat 500 / Abarth 312
1. Compression set (primary failure mode)
The most common issue is compression set, where the rubber permanently deforms under load.
Once this happens:
- The bush loses its original shape and height
- The shock absorber eyelet becomes loosely located
- Small movements develop at the mount under load
This is what produces the familiar rear knocking noise over bumps.
2. Rubber fatigue and ageing
Over time, rubber naturally degrades due to:
- Heat cycles from suspension movement
- Exposure to moisture, road salt and contaminants
- General ageing and hardening
As the rubber hardens, it loses its ability to absorb vibration, cracks begin to form, and the structural integrity is reduced. This is what turns a flexible and comfort-driven bushing into an unconstrained failing bushing.
The Fiat 500 platform may be small, but it is exposed to significant suspension loads due to its short wheelbase, potholes and speed bumps in urban driving and aggressive compression and rebound cycles from the state of the roads in the UK and Europe – Combined with ‘enthusiastic’ driving found in Abarth models, these repeated shock loads cause internal tearing of the rubber, leading to splitting around the inner sleeve, separation from the outer housing, & knocking from the rear of the vehicle.
3. Progressive failure that’s easy to miss
One of the biggest issues is that failure is gradual:
- First signs: occasional light knock
- Progression: more frequent rear-end thudding
- Final stage: constant knocking and instability over bumps
By the time the noise is obvious, the bush is often already significantly compromised.
Symptoms of worn rear upper shock bushes
Common driver complaints include:
- Rear-end knocking over bumps
- Dull thud from the rear axle area
- Reduced rear-end stability
- Harshness over uneven roads
- A feeling of looseness at the back of the vehicle
These symptoms are often misdiagnosed as shock absorber failure, when in reality the damper is usually fine — it’s the mounting bush that has collapsed.
Why replacement rubber often fails again
Standard replacement rubber bushes typically replicate OEM design characteristics:
- Soft compliance for comfort
- Limited resistance to compression set
- Similar material degradation over time
As a result, repeated failure is common on high-mileage or frequently driven vehicles.
How Polybush improves durability and control
Upgrading to polyurethane significantly changes how the suspension behaves under load.
The 24Y Rear Shock Absorber Upper Bush is designed to address the core weaknesses of the OEM design by:
- Maintaining shape under sustained load
- Resisting compression set
- Reducing unwanted shock movement at the mounting point
- Improving consistency of rear suspension behaviour
Rather than degrading over time like rubber, polyurethane retains its structural stability, helping to maintain predictable handling and eliminate knocking caused by excessive bush movement.
Conclusion
Rear upper shock bushes on the Fiat 500 and Abarth 312 platform are a known weak point due to their size, material composition and constant exposure to shock loading. Failure typically begins with compression set and progresses into tearing and loss of location, resulting in rear-end knocking and reduced suspension control.
Addressing this component properly is key to restoring rear-end stability and eliminating persistent suspension noise.
For a long-term solution, BUY NOW!
