Upside-down suspension forks are not a new concept. The concept dates back over 20 years, with the Marzocchi Shiver SC being an early example of a USD fork design, featuring thin lower legs and thick upper tubes. Despite their presence, they’ve been criticized for being too heavy, too flexy, and too complicated to set up, especially when it comes to brake line routing or mudguard attachment.

So, why has the USD fork design failed to gain mainstream acceptance, even though it has been around for decades? Major suspension manufacturers still favor the traditional RSU fork design for most trail and gravity bikes. Let’s dig deeper.

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The Origins of the Right Side Up Fork

The rise of the standard “right side up” fork can be traced back to the early days of mountain biking in the 1990s, when the cantilever and V-brake systems were dominant. As suspension forks began to replace rigid forks, the need for brake mounting points on the lowers became more important. This led to the design we know today, with lower legs connected by a brace and upper tubes that slide into them.

Over the next 30 years, mountain bikes became more refined, and disc brakes replaced rim brakes, rendering the brake mounts on the lowers obsolete. Still, the “right side up” fork remained dominant because of its established design and performance. Attempts to introduce USD forks were met with limited success. The issue was mainly that the benefits of USD forks weren’t enough to overcome their perceived drawbacks. But with FOX’s new Podium fork, this conventional wisdom is being challenged.

The Pros and Cons of Upside-Down Forks

Let’s break down the key differences between the traditional Right Side Up forks and the newer USD forks.

Weight and Distribution

USD forks are generally heavier than their RSU counterparts due to their construction. However, some believe this weight penalty is worth it for the benefits. Although USD forks add weight, they distribute the mass higher up in the stanchions, reducing the unsprung mass at the bottom. This results in a lighter, more responsive lower leg, which can help the fork better respond to trail feedback.

Torsional Stiffness

One of the main criticisms of USD forks is their reduced torsional stiffness compared to RSU designs. Torsional stiffness is critical when it comes to handling steering forces and the impact of the trail. With the traditional RSU fork, the cross brace between the lowers helps resist these torsional forces. USD forks, however, must rely on the bulkier upper tubes for stiffness, which some argue leads to more flex when the steering forces are applied.

However, this issue is more complex than it appears. Torsional stiffness is lower in USD forks, but the difference is negligible when compared to the overall performance. Some experts argue that modern USD forks can achieve sufficient torsional stiffness, thereby challenging the long-standing belief that USD forks are inherently weak in this area. So, the real question may be: do we need as much torsional stiffness as we’ve always assumed?

Oil Distribution & The Advantage of Gravity

One of the significant advantages of USD forks is their oil distribution. In a USD fork, the lubrication naturally flows from the upper tubes down to the critical seals and wipers, keeping them lubricated throughout the ride. In contrast, RSU forks store their oil in the lower legs, making it harder to maintain consistent lubrication, especially on the upper seals. This design ensures better performance over long descents, as the fork will stay well-lubricated and responsive.

Practical Considerations

One practical challenge with USD forks is routing the brake lines. The brake hose has to be routed upward, past the wheel, and into the upper legs of the fork. Additionally, mudguards on USD forks must be mounted higher up under the crown, as they don’t have the traditional arch. This can be a bit more complex compared to RSU forks, where the mudguard attaches to the fork arch. However, these are minor quirks that users can get accustomed to.

Motocross vs. Mountain Biking

Motocross bikes have long used USD forks, but mountain bikes have been slower to adopt the same design. One reason lies in the historical focus on lightweight designs in mountain bikes. The extra weight of USD forks initially turned off many mountain bike riders, especially given the higher percentage of total bike weight that the fork makes up on a light mountain bike, compared to a much heavier motocross bike.

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Is the Future of Suspension Upside Down?

While USD forks have their drawbacks, mainly in weight and complexity, they offer significant advantages in responsiveness and lubrication. Torsional stiffness, often considered a weakness, might not be as big an issue as it’s been made out to be. FOX’s Podium fork represents a significant push to make USD forks more mainstream, thanks to the company’s market influence.

Ultimately, the choice between USD and RSU forks will likely come down to personal preference and what riders are willing to adapt to. As more manufacturers, like FOX, enter the USD space, the design could become more accessible and accepted within the MTB community.

The ultimate question remains: will USD forks become the new standard, or will they remain a niche product? Only time will tell. But for now, they offer a glimpse into a potential future for mountain bike suspension.

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