A Historic Overview of Cycle Front Suspension

For nearly 80 years, telescopic front “forks” have been the popular choice for two-wheeled cycle front suspensions including motorcycles and mountain bikes. The application of new materials and manufacturing methods has advanced telescopic suspension performance in recent decades, but there are issues inherent in the design of telescopic suspension forks that limit their performance.

Linkage front suspensions have been around since the late 1800s, falling in and out of favor over time as major manufacturers adopted one design or another. Linkage front suspensions continue to be a fascinating option for many engineers because of the potential performance improvement over telescopics.

Leading versus trailing linkage systems

Historically, linkage front suspensions have been divided into two categories: leading and trailing linkages. Each type of linkage is defined by the location of the axle relative to the linkage.

Most linkage front suspensions of the past have been designed to exploit the advantage of countering brake dive. However, steering stability has rarely been a design consideration for cycle front suspension designers. In fact, most linkage front suspension designs of the past have had decreased stability performance compared to their telescopic counterparts.

If engineered properly, trailing linkage front suspension designs have the potential to actually improve stability, preserving handling and improving the ride experience.

A sticky situation

Stiction (static friction) is friction force inherent to telescopic forks that must be overcome to set objects in motion. Reducing stiction has been a focus for much of the telescopic fork development in the last 50 years. Until you overcome stiction, there’s no suspension.

The fore/aft stiffness of a suspension fork is critical to maintaining traction. To increase stiffness, fork manufacturers have increased stanchion diameters. Forks with 40mm stanchions are much stiffer than those with 32mm stanchions, but the increased surface area of the fork’s sliding components — stanchions, seals and bushings — also increases stiction.

The problem here is that even the best telescopic suspension systems rely on oversized chassis for steering precision, utilizing oversized seals and bushings to increase stiction. It's a paradox; make one area better, and you make another worse.

Unlike front suspensions, most rear suspension systems use compact dampers with smaller shafts, shorter strokes, and low friction pivoting linkages. These design elements allow rear suspensions to operate with far less stiction and better performance than telescopic front suspensions.

Linkage front suspensions are different than telescopics in that they separate suspension and structural duties — similar to most rear suspensions. This allows the chassis to be optimized for stiffness and leaves the suspension free to move smoothly with very little stiction and better overall performance.

It really comes down to this: have you ever wondered why your rear suspension feels better than your front? It’s because excess stiction, fixed offsets, and diminishing caster are fundamental issues with even the best telescopic forks. As riders, we deserve better.


The Advantages of Leverage Ratio

If you’ve driven a car, ridden a motorcycle or full suspension mountain bike, then you have firsthand experience with leverage ratios.

Simply put, a leverage ratio refers to the relationship between the incremental distance the rear wheel moves relative to the corresponding distance the shock compresses. Typically leverage ratios allows designers to use shorter stroke and smaller shock absorbers, which present a wide variety of performance benefits.

Depending on the arrangement of the suspension, the ratio of rear wheel movement to shock travel can vary throughout the range of travel, a key benefit. It’s this ability to make incremental adjustments to leverage rates that allows suspension designers to fine-tune suspension performance throughout the total range of travel.

By employing a linkage system, front suspensions can reap the benefits that rear suspension designs have enjoyed for years — supple performance early in the stroke, supported with a progressive mid-stroke and more bottom-out resistance at the end of the travel.


The Predictability of Caster

Modern mountain bikes use slack head angles and shorter fork offsets to increase caster as a way to improve high-speed handling. Longer front centers (the distance from the center of the bottom bracket to the front axle) increase stability. Creating shorter offsets pull the front axle back toward the bottom bracket, which give the rider the sensation of quicker steering. But the result of the steeper head angles ends up decreasing stability, especially at speed.

The human body is wonderfully adaptable and able to counteract many geometry changes while riding. Each time you adjust your position on the bike, you’re subconsciously redistributing your weight to preserve handling and overcoming the natural reduction head angle. And those shifts in geometry are most dramatic at the front of the bike. As such, caster changes drastically as a telescopic suspension fork compresses in its travel.

There is a better way

The limitations of telescopic forks don’t apply to Trust Performance suspension. Our trailing multi-link design uses an axle path that isn’t forced to travel in-line with the steering axis. Instead, it counteracts the natural steering angle change with a corresponding adjustment in offset, which maintains caster throughout the range of travel, giving you predictable handling in a wider range of trail conditions.

To put it simply, handling remains the same through the stroke as the suspension compresses. Your bike always steers the same whether it’s early in the turn, at the apex, through the exit, climbing or descending. Consistent handling is one of the primary attributes of Trust Performance suspension. And, the first time we rode it, we finally understood what we had been missing.

Anti-dive

Anti-dive has been one of the primary motivations for developing linkage front suspension systems, though improved handling and suspension performance are also benefits that linkage suspensions can provide. You may already be familiar with anti-squat in rear suspension design (the ability of a suspension design to resist squatting or bobbing under acceleration). You can think of anti-dive as the applicable reference for front suspension.

When you apply the brakes, your weight shifts forward, compressing the front suspension. Once the suspension dives, there is less travel available to handle impacts and the head angle is greatly reduced. Unlike telescopic forks, linkage suspensions can combat dive through the use of kinematics. The result is a front suspension that retains its available travel during braking to absorb impacts and provide predictability.

Contour travel

When the front wheel contacts a rock or root, telescopic suspension forks compress but also flex backward. This flex creates binding in the fork’s bushings, limiting the suspension’s ability to absorb the impact.

Unlike telescopic forks, Trust Performance front suspension allows the front wheel to move in more than one direction when it encounters impacts. The axle moves away from the obstacle as well as up. This ability to move in two directions at once means our trailing multi-link design is better able to follow the contours of the trail and maintain contact longer, thereby improving traction.


The Culmination is the Trust Effect

Let’s rewind to our early days of product development. We planned to build a whole line of front suspensions in 10mm increments for different wheel sizes, because that’s what everyone else does.

Once we started riding our prototypes, we felt a few things that had us scratching our heads and changed our perceptions. We swapped traditional telescopic forks for our linkage design and were immediately able to push harder through turns and take new lines on our well-worn test tracks without thinking twice.

The improvement wasn’t incremental; it was awesome. We dubbed this sensation “The Trust Effect” and quickly realized it was the combination of three things: the separation of structural support from the suspension, reliable handling that comes from consistent caster, and the use of a leverage rate curve.

This combo created satisfying results — silky-smooth travel that eliminated hand fatigue and provided levels of front-end traction and predictable steering that we had never experienced with telescopic forks. In addition, it provided more support when pedaling than we had ever felt before.

Numbers…

We also found with trailing multi-link design that the numbers aren’t as critical as they once were. After years of riding prototypes, we learned that our contour travel is more versatile than telescopic travel and that one product is suitable for a wider range of axle-to-crown lengths and travels.

Trust Performance front suspension works differently, so changes to steering angles and axle-to-crown heights (called “AC heights”) have less influence on how your bike will handle. For example, the Message can replace 29/27.5+ telescopic suspension forks with 110mm to 150mm of travel, and 27.5 telescopic forks with 130mm to 150mm of travel.

Skeptical? We don’t blame you, we felt the same way. We didn't set out to build a product that replaced a wide range of forks—it’s something we learned from riding it. But one thing is remarkably clear: it’s caused us to question everything we thought we knew about front suspension fit.

Lag performance

The word hysteresis is derived from a Greek word meaning “deficiency” or “lagging behind”. In terms of suspension systems, hysteresis is used to refer to an undamped response that is felt as the suspension compresses and rebounds. It’s the product of pressure fluctuations within the damper. These unwanted pressure changes happen in as little as one tenth of a second, but they happen frequently.

Worst of all, this undamped lag isn’t consistent. As the name implies, hysteresis is a direct result of the suspension lagging behind current inputs. So, while you’re focusing on the obstacle in front of you, your suspension may still be struggling to recover from the last few impacts. As a result, suspension systems with high levels of hysteresis may feel different in identical situations.

Suspensions systems with low levels of hysteresis offer better small-bump sensitivity and more predictable performance. This is particularly noticeable when traction is critical, such as flat corners.

The result is counterintuitive. Low hysteresis suspension systems often feel like they have more damping compared to those with higher levels of hysteresis. This is because the suspension is uninhibited by lag and able to react to impacts faster, increasing traction and improving control.

Here’s why it works

Every frame designer knows the benefit of having a lower center of gravity; it’s a key factor in improving cornering and climbing performance.

Since telescopic forks experience so much variation under pedaling, they have capitalized on axle-to-crown heights to preserve frame geometry (particularly the bottom bracket height). Even with today’s long-travel forks, many riders run minimal sag or fill their fork legs full of volume spacers to keep the front end up and prevent pedal strikes. But running super high axle-to-crown heights on a telescopic fork with raked-out head angles create a very binary left-to-right steering sensation.

Trust's linkage ratio works like a well-designed rear suspension. It absorbs impacts and is capable of maintaining geometry regardless of axle-to-crown height. The Trust Effect lessens the importance of head angle, axle to crown height, and fixed offsets on ride feel and bike geometry. Our design takes over the important duties of pedaling support as well as stability control and bump absorption.

We know this is a very different approach to front suspension, and if you’re willing to think differently, we think you’ll be impressed. As riders, we invite you to try our trailing multi-link design objectively and look forward to hearing about your experience.