Mountain bike suspension forks have traditionally been designed with slender upper tubes and bulkier lower legs. To this day, upside-down (USD) forks remain a niche product. But could the new FOX Podium turn our understanding of “right side up” on its head? Our deep dive with developers in technical interviews reveals whether the USD design has untapped potential.

Is it all about to change? Technically speaking, upside-down (USD) suspension forks for mountain bikes aren’t exactly a new concept. Over 20 years ago, the Marzocchi Shiver SC was released as an early USD fork, featuring thin lower legs and thick upper tubes. And even today, there are single crown USD forks on the market, such as those from Intend, EXT, Cane Creek, PUSH, and Bright. Never heard of them? No surprise! While USD forks do keep finding their way onto modern mountain bikes, the design has yet to establish itself in the mainstream. Too heavy, too flexy, too expensive, too complicated to route brake lines or mount a mudguard – and let’s not forget those vulnerable stanchions down there, so close to stray rocks! These are the most common concerns. To this day, the niche USD fork concept is met with skepticism in the MTB scene and remains mainly a curiosity for custom builds. When it comes to stock setups on trail and gravity bikes, major suspension manufacturers still stick to the conventional concept. That’s why we still refer to them as Right Side Up forks – the “correct” way up.

But now, for the first time, a major player in the suspension market is launching a USD fork designed for aggressive downhill riding: FOX. This move could finally catapult USD forks out of the nerdy basement workshop and straight onto the big stage of the MTB world. The new Podium fork enters the highly competitive single crown segment for heavy trail use with serious travel, a massive 20 mm thru-axle, and a system weight of a little less than 2.8 kg – a market currently dominated by the Right Side Up design. What smaller brands haven’t managed so far, FOX might just pull off thanks to their sheer market power: turning upside down into the new “right way up.” But is that really what we want? What advantages – and drawbacks – would we be inviting into our garages if this decades-old standard of lowers and uppers were to be turned on its head? We dug deep into the technical specifics of both concepts and even took a look at the world of motocross. Is a radical suspension revolution on the horizon – and if so, why only now?

To find out, we picked the brains of some of the brightest minds and suspension gurus in the MTB scene: Jordi Cortes, Global Race Department Manager and suspension expert at FOX; Cornelius Kapfinger, founder of Intend and USD pioneer, known for fine-tuning countless custom bikes; Cesar Rojo, founder of Spanish MTB brand UNNO; Ruben Torenbeek, creator and head of RAAW Mountain Bikes; and Kalle Nicolai, founder and CTO of German machining legends NICOLAI. Each of them has their own take on the upside-down world of mountain bike forks. Ready to dive in? Let’s go!

Of suspension forks for rim brakes

How did we even end up with the now-standard Right Side Up suspension fork on mountain bikes? In the beginning, there was… the cantilever brake. When the first suspension forks began to replace rigid forks on early mountain bikes in the 1990s, rim brakes were the state of the art. The soon-to-follow V-brake also required mounting points near the rim’s braking surface to ensure consistent alignment of the brake arms with the wheel. This led to the development of today’s typical “lowers” – the lower fork legs – which are connected by a bridge and feature dedicated mounts for rim brakes.

Over the following thirty years, mountain bikes became more capable, their technology more refined, and disc brakes eventually became the new standard. The mounting holes for rim brakes were no longer needed, but by then, the distinctive look of Right Side Up forks was firmly established. “Right side up” was born – this is simply how an MTB fork is supposed to look! Boosted by increasingly powerful models, there was little reason to fundamentally turn the concept of the suspension fork… on its head. Early attempts to develop USD forks revealed weaknesses, further validating the RSU design. But times change – and with them, perspectives and priorities in the MTB scene. Were Right Side Up forks really the ultimate solution from a technical point of view? The launch of FOX’s new fork is not the first thing to cast doubt on the old standard.

Pros and Cons of Upside Down Suspension Forks

Looks aside – which design actually makes more sense technically, and why? After three decades of mountain bike suspension development, the conversation around USD and RSU forks is full of buzzwords, half-truths, and hard facts — all of which need to be clearly separated and properly understood.

Slender at the top, thick at the bottom – or the other way around

The basics: a conventional Right Side Up fork has two lower legs, or “lowers”, which are connected by a brace and form the lower part of the fork assembly. Attached to these are the axle, disc brake mount, a clamp for the brake line, and – a somewhat contentious topic – usually a mudguard. The upper part of the fork consists of the thinner stanchions, or “uppers”, which slide into the lowers. These are fixed in position with the crown and steerer tube, forming a rigid assembly. At the transition between lowers and stanchions, dust wipers keep dirt out and the lubricating oil inside the lowers. Each side has two or three bushings guiding the stanchions within the lowers. The internals of both legs house the damping and spring systems.

USD forks flip this principle upside down. The bulkier upper tubes are positioned at the top, firmly joined to the fork crown and steerer tube. The thinner lowers carry the axle, the brake caliper, and – a unique requirement for USD forks – a guide that neatly routes the brake line upward past the wheel. Since there’s no additional cross brace between the fork legs, any mudguard must be mounted high up, just under the crown. As with RSU forks, bushings allow the lowers to slide within the uppers. A key difference is that the lubrication oil is located in the upper tubes. So far, so clear – but what are the concrete advantages and disadvantages?

USD forks tend to be heavier

Low weight remains a priority for many riders when it comes to mountain bikes and their components. But on the scales, upside-down forks initially come off worse – due to their construction, they’re generally a bit heavier than comparable Right Side Up models. Even Jordi Cortes from FOX acknowledges this – hinting that the FOX Podium won’t be the perfect fork for everyone. That said, we’ve long known from riding a good eMTB that even bikes tipping the scales at over 20 kg can still deliver a proper party on the trail. A glance at the current DH World Cup shows that some riders are even taping additional weights to their bikes. Still, every extra pound on the fork doesn’t just affect the overall weight, but also alters the weight distribution on the bike – and that has to be well-balanced in the end.

It’s always been different on motorcycles – but why?

By the time the first suspension forks appeared on mountain bikes, motocross bikes had long since adopted disc brakes as standard and almost exclusively used upside-down fork designs – typically with dual crown setups. And yet, the fundamental demands on suspension components are quite similar in both disciplines. So why wasn’t the MX scene used as a technical blueprint, leading to earlier adoption of USD forks on MTBs?

Part of the answer lies in the roots of the MTB movement: early mountain bikes had to first distance themselves from their road bike ancestors, slowly shedding their obsession with ultra-lightweight builds and skinny designs – a legacy that still lingers today. The fact that USD forks are inherently a bit heavier than their RSU counterparts can therefore act as a deterrent. Additionally, the fork’s share of the bike’s total weight is significantly greater. On a 15 kg mountain bike, a 2 kg single crown fork makes up a hefty 13% of the total weight. For comparison’s sake, an 8 kg fork on a 100 kg MX bike accounts for just 8%. Weight savings are therefore much more critical in MTB forks than in MX.

There’s another key difference in riding technique: while mountain bikers steer through corners standing up with significant weight over the front wheel, motocross riders usually sit down and load the rear wheel as they open the throttle. Whether it’s because of this or simply the added stiffness provided by the dual crown design – torsional stiffness in USD forks is hardly a point of contention in motocross, while it remains a hotly debated topic in the mountain bike world.

“Upside down or right side up – the basic function is the same.” -Jordi Cortes, FOX

Lower Unsprung Mass

Despite their inherently higher weight, USD forks partially compensate by reducing unsprung mass – a direct benefit of their construction. Unlike RSU forks, where much of the weight sits in the lower legs, USD forks concentrate their mass higher up in the stanchions. This is advantageous because extra weight in the uppers doesn’t move with every bump in the trail. The relatively lighter lowers on a USD fork move more freely and can respond more sensitively to trail input. Cesar Rojo from UNNO even considers this the most significant advantage of the USD design.

“The biggest and most obvious advantage of USD forks is their sensitive response.” – Cesar Rojo, UNNO

Oil Follows Gravity

… and that means downward. In USD forks, the lubricant naturally flows from the uppers down to the critical dust wipers and seals. This results in continuous re-lubrication, offering another theoretical performance benefit in terms of responsiveness. By contrast, RSU forks store their oil in the lowers, which makes it more difficult for lubrication to reach the upper seals and wipers consistently.

No Binding with Proper Axle Installation

When the front wheel is removed from a USD fork, the two lowers can compress and rotate independently. That’s why it’s crucial to install the axle precisely and securely, ensuring both legs are perfectly aligned and able to compress without binding. With a bit of experience, this alignment process becomes second nature with USD forks. In contrast, with traditional RSU forks, the rider has no control over this alignment. If the factory-molded unit consisting of the lowers and arch isn’t manufactured with an exact axle-to-axle spacing, the stanchions will inevitably be forced apart or squeezed together when the axle is tightened. This creates internal stress and friction, which can significantly compromise sensitivity and suspension performance.

“If the bore spacing isn’t precisely 150 mm — say it’s 150.1 mm due to built-in stress — the fork binds. Game over. With USD forks, that simply isn’t a concern.” – Cornelius Kapfinger, Intend

Stiffness: Frontal vs. Torsional

When it comes to frontal stiffness, USD forks have a structural advantage over RSU designs. The solid assembly of steerer tube, crown, and thick upper tubes creates a stiffer unit around the head tube compared to conventional forks. This means that USD forks are technically better at handling impacts coming from the direction of travel. But – and this leads us to one of the most persistent and damaging criticisms USD forks continue to face – what about torsional stiffness? This kind of stiffness is crucial when steering forces and ground impacts oppose each other. The dirty go-to test for the parking lot: clamp the front wheel between your knees, grab the handlebars, and forcefully twist them left and right. Yep – there’s noticeable movement, even if the stem isn’t slipping!

“USD forks undoubtedly have more torsional flex.” – Jordi Cortes, FOX

In RSU suspension forks, the arch between the lowers plays a key role in resisting torsional forces. Due to the lack of stabilizing cross brace, USD forks must find their torsional stiffness elsewhere. This often means accepting added weight as a trade-off — or settling for reduced stiffness. But is that really an issue?

Here, opinions diverge sharply among our interviewed suspension experts. For Jordi Cortes of FOX, it’s clear: torsional stiffness is lower in upside-down forks compared to Right Side Up models. Ruben Torenbeek (RAAW) and Cesar Rojo (UNNO) actually see this as a genuine weakness of the USD design, echoing a long-standing criticism that quietly persists throughout the MTB scene. Cornelius Kapfinger of Intend, however, is convinced that modern USD forks can absolutely achieve sufficient torsional stiffness – and he challenges us with a pointed question: how objective can our judgment on the trail really be, when we’ve been conditioned since the early days of USD forks to assume they inherently lack torsional stiffness? Touché!

“A disadvantage in torsional stiffness simply doesn’t exist and isn’t an issue in practice – otherwise we wouldn’t still be around.” – Cornelius Kapfinger, Intend

Unlike the unrealistic parking lot test with knees clamped around the front wheel, out on the trail, the tire’s tread is the only thing anchoring the wheel to the ground. In a moment of strong torsional load, the wheel will simply follow the rider’s steering input long before the fork’s torsional stiffness is ever fully tested. Any remaining difference in ride feel could just as easily be interpreted as a welcome degree of compliance – much like with Zipp’s 3ZERO MOTO wheels or other components that intentionally dial back stiffness. It’s a trend we’ve seen emerge in recent years, moving away from the old “stiffer = better” mindset toward one that values appropriate stiffness instead of maximum rigidity.

Cable Routing, Mudguards, and Fenders

USD forks require a dedicated brake line guide to route the hose cleanly past the wheel and up to the upper legs. A mudguard must be mounted directly beneath the steerer tube. Since its distance to the tire changes dynamically during compression, it needs to be significantly larger than those on RSU forks, which can snugly wrap around the fork arch near the tire. As a result, mudguards on USD forks often resemble the long-beaked fenders found on MX bikes. There’s also a common concern about whether the delicate coatings on the lower legs of USD forks – which sit close to the ground – need extra protection from debris. Cornelius Kapfinger says: no need! Still, Intend offer stanchion guards for their forks – likely because customer demand persists. FOX equip the new Podium with carbon guards as standard to protect the precious Kashima coating. These aren’t so much disadvantages as they are quirks in handling and aesthetics – things you can simply get used to, if you’re willing. For Ruben Torenbeek of RAAW, these usability and visual aspects play a key role in determining how widely USD forks will be accepted.

“The importance of practical details, like easy brake line and fender mounting, shouldn’t be underestimated.” – Ruben Torenbeek, RAAW

The People Will Decide

So how does this showdown end? Are upside-down forks the future of trail riding, or just another trend that conventional forks will shrug off with ease? Or will both types of suspension forks share the trails of tomorrow in equal measure – some going one way, others the other?

We can find part of the answer by looking at the tech. USD forks offer higher fore-aft stiffness, better self-lubrication, and the potential for more sensitive small-bump compliance due to their lower unsprung mass. Their supposed weakness – torsional stiffness – might not be the major drawback it’s often made out to be. In fact, could it actually be an overlooked strength? Maybe it’s time to rethink just how much torsional stiffness a mountain bike fork truly needs. On the flip side, USD forks tend to be heavier and more complex — which also makes them more expensive to manufacture.They also require users to get familiar with a few new procedures. But for those willing to adapt, things like axle installation, brake line routing, mudguard and fender setups become second nature quickly.

“With a Right Side Up design, it’s much easier to build a fork with low lateral and torsional flex. A good USD fork on a budget bike will be tough to pull off.” – Kalle Nicolai, NICOLAI

Still, technical pros and cons are only part of the equation when it comes to deciding whether USD forks like the new FOX Podium will challenge our three-decade-old perception of what a “right side up” fork should be.The MTB scene has developed a strong visual identity over the years. With that comes a clear expectation of how a mountain bike – including its suspension fork – is supposed to look. With its market influence, FOX has the ability to steer trends and spark enthusiasm for a product that has so far lived in the shadows. Can the distinctive look of USD forks challenge the aesthetics of traditional MTB forks? Do you want to find them sexy? In the end, it will be up to the people to decide.

Our conclusion about Upside-down suspension forks

Upside-down suspension forks have the potential to challenge the dominance of the Right Side Up design. Their drawbacks in terms of weight and complexity are real, but their handling and aesthetics are something riders can get used to. The concept’s key strengths lie in its potential for sensitive responsiveness and consistent lubrication. And the oft-cited lack of torsional stiffness? Upon closer inspection, that criticism may well be unjustified.

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Words: Moritz Geisreiter Photos: Peter Walker, Dave Trumpore, Rupert Fowler, Trevor Worsey