You’ve done everything right—worked with a saddle fitter, kept your horse in a consistent work program, and been diligent about their care. Yet, something still feels slightly… off. Maybe your saddle consistently slips to one side, no matter how carefully you place it. Perhaps your horse feels resistant in one direction, or you find yourself constantly shifting your own weight to feel centered.

If this sounds familiar, you’re not alone. It’s a common sign of a challenge that affects nearly every horse and rider pair: asymmetry.

While often subtle, these imbalances can create significant communication breakdowns. A frequently proposed solution is a shim—a small insert designed to fill a gap and rebalance the saddle. But this is where a simple fix becomes surprisingly complex. The market is flooded with options made from felt, memory foam, gel, and other specialty materials.

Most riders are simply told to “add a shim,” but the crucial differences between them are rarely explained. The truth is, the material you place under your saddle is just as important as the saddle itself. It’s not just about filling a space; it’s about how that material behaves under the immense pressure, heat, and movement of a horse in motion. Let’s explore the science behind these materials to understand what’s really happening on your horse’s back.

Why Shims? Understanding the Goal Before the Material

Before comparing materials, we need to be clear about what a shim is supposed to do. Horses, like humans, are naturally asymmetrical. One shoulder may be larger, one side of their back might be less muscled, or they might have a slight dip behind the withers. For a deep dive into this common issue, it’s helpful to understand exactly what equine asymmetry is.

A correctly chosen shim aims to:

1. Create a Level Foundation: It provides support where muscle is lacking, allowing the saddle to sit squarely on the back.
2. Distribute Pressure Evenly: It prevents the saddle from concentrating pressure on one area, which can cause bridging or hot spots.
3. Support Muscle Development: By creating a balanced surface, it can encourage a horse to use its body more correctly, allowing atrophied muscles to rebuild.

The key word here is support. A shim isn’t just a plug; it’s a functional tool that needs to perform under load. And that performance comes down entirely to its material composition.

The Material Breakdown: Not All Shims Are Created Equal

Choosing a shim based on how soft it feels in your hand is one of the most common mistakes riders make. How a material behaves under the dynamic forces of riding is a completely different story.

1. Traditional Felt Shims

Felt, typically made from pressed wool, is one of the oldest materials used for shimming. It’s natural, breathable, and initially conforms well to the horse’s shape.

• The Problem: Research and field tests consistently show that under the pressure and moisture generated during riding, felt fibers compress and harden. This process, known as “felting,” turns a once-soft shim into a rigid, unforgiving lump.
• The Science: Studies on pressure distribution reveal that compressed felt does not disperse pressure effectively. Instead, it can create sharp pressure points along its edges. It loses its corrective properties quickly, often within a few dozen rides, becoming part of the problem rather than the solution.
• The Analogy: Think of a thick wool sock inside a hiking boot. At first, it’s cushiony. But after a long, sweaty hike, it becomes matted and hard and can cause blisters. That’s what’s happening to a felt shim under your saddle.

2. Standard Foam Shims (Open-Cell & Memory Foam)

This is the most common category, including everything from basic open-cell foam to popular memory foam products. They feel wonderfully soft and squishy to the touch, which often gives riders a false sense of security.

• The Problem: These foams are defined by their high “memory”—they hold an impression. While that sounds promising, it means the foam compresses under weight and is slow to rebound. This phenomenon is called “bottoming out.” The foam flattens completely under the saddle’s pressure points, offering no real support or shock absorption where it’s needed most.
• The Science: Pressure mapping studies show that while memory foam might feel soft, it fails to distribute load. The pressure from the saddle tree simply travels straight through the compressed foam to the horse’s back. These materials are also notorious for trapping heat and moisture, which can lead to skin sensitivity and muscle soreness.
• The Analogy: It’s like a cheap mattress topper. It feels luxurious for a moment, but when you lie down, you sink right through it and feel the hard mattress underneath. It provides the illusion of comfort without any real ergonomic support.

3. Specialty Shims (Closed-Cell Foam & Composite Materials)

This category includes advanced materials specifically engineered for load-bearing and shock absorption. These are often closed-cell foams or proprietary composites that don’t rely on softness for performance.

• The Advantage: Unlike memory foam, these materials have high rebound and minimal compression. They are designed to actively resist pressure, distributing the load over a wider surface area. They function like the high-tech cushioning in a professional athlete’s running shoe—firm, supportive, and built for performance.
• The Science: These engineered materials maintain their structural integrity and thickness even under significant weight. They provide consistent support throughout a ride, absorbing concussion and distributing pressure evenly without trapping heat. This creates a stable and comfortable foundation for the saddle. These same principles are central to innovations in saddle panel design, where the goal is to create a seamless connection that supports the horse’s biomechanics.
• The Analogy: This is the performance athletic gear for your horse. It’s not about being pillowy; it’s about providing dynamic, resilient support that works with the body, not against it.

The image above clearly illustrates the difference. The left side shows the concentrated hot spots (in red) created by a material that fails to distribute pressure. The right shows the even, balanced contact (in green and blue) achieved by a material doing its job correctly.

Putting It All Together: From Theory to Practice

Understanding the science behind shim materials empowers you to look beyond the surface. A shim that feels soft in your hand may become hard and unforgiving on your horse’s back, while one that feels firmer might be providing the exact shock absorption and pressure distribution your horse needs.

This becomes especially critical when dealing with unique conformations, such as those seen in many baroque breeds or sport horses. Riders often struggle with saddle fit for short-backed horses, for example, where even minor asymmetries can have a major impact due to the limited space for the saddle. In these cases, a high-performance shim is not a luxury; it’s a necessity for achieving proper balance and freedom of movement.

The goal is to create a harmonious system of rider, saddle, and horse where communication can flow freely. The right shim material acts as a silent translator, ensuring the saddle’s message is delivered clearly and comfortably to the horse’s back.

Frequently Asked Questions (FAQ)

1. How do I know if my horse needs shims?
   Look for tell-tale signs: your saddle consistently slipping to one side, uneven sweat or dirt marks after a ride, or your horse showing resistance to turning or bending in one direction. The best way to know for sure is to have an evaluation from a qualified saddle fitter who can assess your horse’s musculature both statically and in motion.

2. Can shims fix a saddle that fits poorly?
   No, and this is a critical point. Shims are for fine-tuning and accommodating temporary or conformational asymmetries. They cannot fix a saddle with a tree that is the wrong size or shape for your horse. Using shims to mask a fundamentally poor fit will almost always create more severe pressure points.

3. How often should I check my shimming setup?
   A horse’s back is constantly changing with age, fitness, and training. You should reassess your shimming needs with your saddle fitter at least every 6-12 months, or sooner if you notice any changes in your horse’s behavior or how your saddle sits.

4. Can I use more than one shim?
   In some cases, yes, but this should only be done under the guidance of an expert. Stacking shims incorrectly can create ridges and uneven pressure. Professional systems often use tapered shims that can be layered to create a smooth, gradual adjustment.

The Foundation of a Better Ride

Choosing a shim is far more than just filling a gap. It’s a decision about material science that directly impacts your horse’s comfort, performance, and long-term soundness. By moving beyond the simple metric of “softness” and embracing an understanding of pressure distribution, compression, and rebound, you can make a more informed choice.

Your horse’s back is the canvas for your communication. Ensuring the tools you place upon it are designed for true ergonomic support is the first step toward a more balanced, harmonious, and successful partnership.

If you’re ready to learn more about how the saddle itself contributes to this equation, a great next step is to explore the importance of saddle panel design.