The RST 387L is a budget-friendly suspension fork found on many entry-level mountain bikes and hybrids. Over time, the elastomer spring inside can wear out, leading to a harsh or non-functional suspension. Replacing or upgrading to a compatible elastomer spring restores performance. This guide covers everything you need to know about the RST 387L adjustable suspension elastomer spring, including how it works, adjustment tips, installation guidance, and what to look for when buying a replacement.
Understanding the RST 387L Suspension System
The RST 387L uses a simple elastomer (rubber-like polymer) spring instead of a metal coil or air spring. The elastomer compresses when the fork absorbs bumps, providing a basic level of suspension. The term "adjustable" refers to the ability to change preload, which adjusts how much force is needed to compress the fork. Some versions also offer a lockout feature. The elastomer spring is housed inside one or both fork legs, depending on design.
How the Elastomer Spring Works
Elastomer springs are made of a durable, rubbery material that compresses under load and returns to shape. They are lightweight and require no oil or air, making maintenance simple. However, elastomers can degrade over time, especially with UV exposure or temperature extremes. A worn elastomer may feel spongy or stiff, reducing suspension effectiveness.
Adjusting the RST 387L Fork
Adjusting the preload on an RST 387L allows you to fine-tune the suspension to your weight and riding style. Most models have a knob or hex key adjustment at the top of the fork leg.
Preload Adjustment
- Locate the adjustment knob on top of the left or right fork leg. Some models use a 4mm or 5mm hex key.
- Turning clockwise increases preload, making the fork stiffer (less sag). Useful for heavier riders or aggressive riding.
- Turning counterclockwise decreases preload, making the fork softer (more sag). Better for lighter riders or smoother trails.
- Start with the knob in the middle of its range and adjust based on feel. Too much preload can make the fork harsh; too little can cause bottoming out.
Lockout (If Equipped)
Some RST 387L forks include a lockout feature that prevents the fork from compressing. This is helpful on paved climbs or smooth sections. Use the lever on the top of the fork to engage or disengage lockout.
When to Replace the Elastomer Spring
Signs that your elastomer needs replacement:
- The fork feels completely rigid or doesn't move at all.
- The fork compresses easily but doesn't return (possible broken elastomer).
- Visible cracks or deformation in the elastomer (visible after disassembly).
- Unusual noises like squeaking or creaking.
- Inconsistent travel – the fork feels different on each compression.
Buying a Replacement RST 387L Elastomer Spring
When shopping for a replacement, pay attention to the following factors:
Compatibility
The RST 387L uses specific elastomer dimensions. Most aftermarket elastomers are labeled as compatible with RST 387L or similar models (like 391, 281). The spring rate (hardness) is often color-coded:
- Soft (yellow or light color): for lighter riders (<150 lbs)
- Medium (blue or green): for average riders (150-200 lbs)
- Firm (red or black): for heavier riders (>200 lbs)
Adjustability Kits
Some aftermarket manufacturers sell adjustable elastomer systems that allow you to change the spring rate by swapping inserts or turning a preload adjuster. These can extend the fork's range and lifespan.
OEM vs Aftermarket
Original RST elastomers are available from bike shops or online. Aftermarket options are often cheaper and may offer better durability or more spring rate choices. Check compatibility before purchasing.
How to Install the Elastomer Spring
Replacing the elastomer on an RST 387L is straightforward but requires basic tools.
Tools Needed
- Hex wrenches (typically 4mm, 5mm, or 6mm)
- Flathead screwdriver
- Grease (silicone or fork-specific)
- Clean rag
Step-by-Step
- Remove the front wheel and brake caliper (if necessary) to access the fork.
- Loosen the bolts on the top crown (the part holding the fork legs to the frame). On some models, you may need to remove the top cap.
- For the leg containing the elastomer (usually the left leg), remove the top cap or adjustment knob.
- Carefully pull out the old elastomer. If stuck, gently tap the leg or use a screwdriver to pry it out.
- Clean the inside of the fork leg with a rag. Apply a light coat of silicone grease to the new elastomer for smooth action.
- Insert the new elastomer, followed by any spacers or preload parts in the correct order.
- Reinstall the top cap and tighten according to manufacturer torque specs (usually 5-8 Nm).
- Reattach the crown bolts, wheel, and brake caliper.
- Test the fork by compressing it several times. Adjust preload if needed.
Performance Tips
- Pair the elastomer with the correct preload. If the fork feels too soft even with minimal preload, consider a firmer elastomer.
- For aggressive riding, an aftermarket adjustable system can provide more tunability than stock.
- Regularly clean the fork stanchions and seals to prevent dirt from wearing the elastomer prematurely.
- Store the bike out of direct sunlight when possible to slow elastomer degradation.
Recommended Brands and Options
Several brands offer replacement elastomers for the RST 387L:
- RST Original Parts: Available through dealers, these are direct-fit but can be pricey.
- SR Suntour (compatible): Some models share similar dimensions; check specs.
- Enduro: Makes adjustable elastomer systems (like the "Enduro Max" cartridges) that fit many RST forks.
- Kinekt / Cane Creek: Offer high-quality aftermarket elastomers in multiple densities.
Always verify dimensions (length, diameter) and shape (flat or conical) before buying. User forums and bike shop specialists can help identify compatible options for your specific year and model.
Final Recommendation
For most riders, replacing a worn elastomer with a medium-density aftermarket unit from a reputable brand is the best balance of cost and performance. If you want finer control, an adjustable system allows you to dial in stiffness for different terrains. Stick with a known compatible product to avoid fitment issues. Regular maintenance and proper adjustment will keep your RST 387L fork working well for years.