4 Bar Link Suspension Mountain Bike Parts: A Complete Guide

The 4 bar link suspension system is one of the most common rear suspension designs on modern mountain bikes. It uses four pivot points and three linkages to control wheel movement and absorb impacts. Understanding the individual parts can help you maintain, upgrade, or choose the right bike. This guide breaks down each component and its role.

Key Components of a 4 Bar Link Suspension

A 4 bar link suspension consists of a front triangle, rear triangle, and several connecting links. The four pivot points are typically located at the bottom bracket, seat tube, chainstay, and top tube. The main parts include:

Main Frame (Front Triangle)

The front triangle holds the bottom bracket, head tube, and shock mount. It connects to the rear via pivots. On many 4 bar designs, the shock is mounted between the down tube and the swingarm or between the seat tube and the rocker link.

Rear Triangle

This includes the swingarm (or seatstay/chainstay assembly). The rear axle attaches here. In a 4 bar link, the rear triangle may be one piece or split into a seatstay and chainstay, connected by a pivot. The rear triangle often incorporates the lower link and upper link attachment points.

Links

The defining feature: two connecting links (upper and lower) that control the rear wheel's path. The lower link usually runs from the bottom bracket area to the swingarm. The upper link connects the swingarm to the front triangle near the seat tube. These links determine the suspension's leverage curve and anti-squat characteristics.

Pivots and Bearings

Each pivot point uses a bearing or bushing. Common types:

• Cartridge bearings: Sealed, low friction, common on high-end bikes.
• Max-type bearings: Larger surface area, durable.
• Bushings: Simpler, less expensive, but higher friction. Pivot hardware includes bolts, washers, and spacers. Proper torque and maintenance prevent play and creaking.

Shock Absorber (Rear Shock)

The shock dampens and absorbs impacts. Mounts vary: some designs use a trunnion mount (horizontally mounted shock) or a standard eyelet mount. The shock's stroke and tune must match the frame's leverage curve. Common brands: Fox, RockShox, DVO, Ohlins. Air shocks are adjustable for rider weight; coil shocks offer consistent feel.

Hardware and Linkage Plates

Some bikes use separate linkage plates that connect the shock to the links. These can affect the progression of the suspension. Replacement linkage plates may change the suspension feel.

How Each Part Affects Performance

The geometry and stiffness of each component influence ride quality, climbing efficiency, and descending capability.

Pivot Location and Anti-Squat

Anti-squat is the suspension’s tendency to resist compression under pedaling. It is determined by pivot locations and link lengths. A well-designed 4 bar link provides efficient pedaling without bobbing. Higher anti-squat (above 100%) helps on steep climbs, but may feel harsh. Lower anti-squat reduces pedal feedback but can bob more.

Leverage Curve

The leverage ratio changes as the suspension compresses. This curve is shaped by the link lengths and pivot positions. A progressive curve (stiffer toward end of travel) prevents bottom-out and suits plush initial stroke. A linear curve offers consistent feel. Parts like the shock tune and air volume spacers also affect progressiveness.

Rear Triangle Stiffness

Stiffness comes from frame material (carbon, aluminum), tube shapes, and pivot joinery. Stiffer rear triangles improve tracking and power transfer. However, overly stiff designs can feel harsh. Carbon frames often have tuned compliance. Pivot hardware must be tight enough to avoid flex but not so tight that bearings bind.

Common Upgrades and Replacement Parts

Many riders upgrade suspension components to improve performance or replace worn items.

Shock Upgrade

Swapping the rear shock can drastically change ride feel. Consider:

• Air to coil: Coil shocks offer more consistent damping for heavy or aggressive riders.
• DebonAir or MegNeg air cans: Increase air volume for more progressiveness.
• Tune: Lighter or heavier damping for different terrains. Make sure the shock's length and stroke match your frame. Check eyelet diameter and hardware compatibility.

Linkage Kits

Aftermarket linkage kits alter the leverage curve. For example, a “flip chip” in the linkage changes geometry slightly. Some companies sell complete link replacement sets to adjust progression or anti-squat. These require careful installation and alignment.

Bearings and Bushings

Replace worn pivot bearings (usually standard sizes like 6902, 6802) with sealed cartridge bearings. Some frames use bushings that can be replaced with Igus or similar low-friction bushings. Perform regular cleaning and greasing.

Hardware

Bolts can strip or corrode. Titanium bolts are lighter but require anti-seize. Stainless steel lasts longer. Use a torque wrench to avoid overtightening.

Maintenance Tips

• Check pivot bolt torque after every few rides.
• Clean and lubricate bearings annually or after wet rides.
• Inspect shock seals for dirt. Rebuild shocks every 100-200 hours.
• Listen for creaks: often from pivot bolts or shock mount hardware.
• Replace bearings when they feel gritty or have play.

Choosing Replacement Parts

When buying parts, prioritize compatibility:

• Shock: Match eye-to-eye length, stroke, and mounting hardware.
• Bearings: Confirm inner diameter, outer diameter, and width. Often printed on the bearing seal.
• Linkage bolts: Thread pitch, length, and head type (countersunk or flange).

If upgrading the shock, consult your frame manufacturer’s specs. Some bikes require specific shock tunes for proper damping.

Final Recommendations

For most riders, the biggest performance upgrade is a well-maintained or tuned shock. If your bike feels overly harsh or bob-heavy, consider a shock rebuild or valve kit. Pivot bearing replacements are inexpensive and can restore original feel. Avoid unnecessary upgrades like linkage kits unless you understand your frame's geometry and have specific goals (e.g., more progression for aggressive riding). Always source parts from reputable brands to ensure durability. If you’re unsure about compatibility, consult a local bike shop or the frame manufacturer’s support documentation.

Remember: the 4 bar link is a proven design, and its parts are robust. Regular care will keep your suspension performing optimally for years.