We know, there’s no such thing as a stupid question. But there are some questions you might not want to ask your local shop or riding buddies. AASQ is our weekly series where we get to the bottom of your questions – serious or otherwise. This one is about upside down suspension forks, also known as inverted forks. Hit the link at the bottom of the post to submit your own question!

Upside down forks, or inverted suspension forks are the norm in motor sport. We see them used in Moto GP, Motocross, on touring bikes, and pretty much every type of motorcycle in between. Why then, is this design so rarely seen in mountain biking?

A custom RockShox RS-1 inverted fork for XC MTB spotted at Nove Mesto World Cup in 2015

If the upside down fork design works so well for motorcycles, what is stopping the major mountain bike suspension brands adopting the same design? We have seen a few of the major brands release inverted forks over the years. RockShox came out with the RS-1 in 2014, an upside down XC fork that is no longer in production. Why did the design never take off?

prototype RST inverted xc trail suspension fork
A prototype inverted fork for XC from RST

We sent this question along to many major suspension manufacturers, none of whom returned comment on the matter. No surprise really that the only brands willing to provide comment were those who currently offer upside down forks; Intend and Wren Sports. Here are their thoughts.

AASQ Inverted Forks

Upside down forks are very popular in motocross and on motorbikes across the industry, even in Moto GP. Why then, does the mountain bike industry use upright forks?

Intend: The reason in my eyes is the current public perception of upside down forks, which goes back to the first Marzocchi Shiver fork from back in the day. This fork was the first USD fork brought to a wide range of costumers.

The upside down design was new – right side up forks weren’t possible in the early days because of V-brakes. Human kind is always skeptical of new things, this is part of our nature.

Intend Edge New Age MTB fork, 180mm upside-down air XC-Enduro mountain bike fork, Revel Rascal by Flowrider Racing
Intend’s New Age 180mm travel upside down fork for enduro

With an inverted fork, if you put the front wheel between your legs and turn the handlebar with full power, you will always see movement. Because of this observation, riders are quick to judge that the fork is not rideable. But, the truth is, you will never get a load this high through the system while actually riding.

The observations from this simple off-the-bike test, combined with the general public perception that USD fork are bad, is the main reason the design has been unsuccessful historically.

Rockshox RS-1 inverted suspension fork review and actual weights

The Manitou Dorado, the RockShox RS-1 and the latest entrant, the Magura Boltron – all of them were made with the knowledge that inverted forks have advantages over the right side up forks. Companies would not have tried it otherwise.

Wren Sports: That is a good question. My guess is that it was and is cheaper to build the upright style fork. It continues because many of the companies are heavily invested in tooling to make this type of fork. As an aside, we feel the inverted style fork is the “right side up” fork.

magura boltron upside down e-bike suspension fork could be reworked for enduro
The Magura Boltron upside down fork for e-bikes

How does riding an upside down fork compare to riding an upright fork?

Intend: Upside down forks benefit from better lubrication of the bushings. They can provide a lot of stiffness back and forth and they have less friction at the bushings. This is because the bushings are moving down and this minimizes the leverage on the bushings when fully compressed.

IntendEdgeForkUpsidedownforkanodisedbluedeviateguideeurobike2018
The Intend Edge Upside Down fork on a Deviate Guide at Eurobike 2018

So, the ride feel is more stable with reduced vibration because the wheelbase is not changing that much. The friction of the system is lower especially during heavy hits and you do not need to turn your bike upside down to lubricate the system.

Wren Sports: You will feel greater stiffness and stability when riding an inverted fork. This is especially true for hard chargers and for fat tire bikes where greater forces on the fork come into play. There are a couple of reasons for this:

wren inverted mtb suspension fork

  1. The inverted design moves the longer, stronger tubes to the top connected to the crown making for a very strong and stiff chassis. Upright forks position the stanchions at the top, which are the shorter, lighter tubes. The top of the fork is key to absorbing the forces entering from the axle. A stronger chassis is a big advantage.
  2. The inverted design moves the shorter, lighter tubes to the bottom, reducing the moving weight of the fork. This allows our fork to move faster and easier to changing trail conditions. This keeps the tire planted. Also, having the shorter tubes on the bottom reduces the leverage factor. Typically, a fork can flex where the two tubes join.

Wren-inverted-suspension-forks-upside down

 

Our shorter stanchions reduce the forces acting on this junction point and allows the stronger, stiffer chassis to easily absorb them. The longer, lower tubes on an upright fork can amplify these forces asking the weaker chassis to absorb them. Just put an inverted style fork next to an upright fork and you can immediately see these advantages.

Do inverted forks have a shorter service interval than regular upright forks? Seems to me the seals would be more exposed as they are closer to the trail.

Intend: No. My experience is that the overall service intervals are equal. The main problem is the loss of oil from the inside to the outside and the amount of dirt coming in to the system.

Intend Edge New Age MTB fork, 180mm upside-down air XC-Enduro mountain bike fork, Revel Rascal by Flowrider Racing

Using the same seals, I had dark oil with uspide down and right side up forks after several month of hard riding. The seals of the inverted fork are lower, but you also have no dirt coming from the tire to your upper legs. So, in total there is no difference.

Wren Sports: We recommend a complete clean and lubes every 100 hours of typical riding. More often if you ride in dirty, muddy, wet conditions. We use a maintenance-free all-weather sealed damper cartridge, so no damper maintenance is necessary.

wren mtb suspension all-weather damper upside down fork inverted
Wren’s all-weather sealed damper cartridge for their upside down forks

The wipers would seem to be more exposed because they sit lower on the fork. However, our wipers face down allowing for improved cleaning of the stanchions with each stroke with the help of gravity. The wipers/seals on upright forks face up allowing dirt and moisture to collect and sit there. Eventually, each compression stroke can then suck dirt and moisture into the fork.

Would inverted fork technology work better for long travel enduro/DH bikes than short-travel XC bikes?

Intend: Upside down fork technology especially has benefits for dual-crown forks, because you can extend the distance of the bushings up into the full length of an upper tube.

intend infinity dual crown dh fork downhill mountain biking inverted upside down design
Intend’s Infinity dual-crown DH inverted fork

With a normal lower casting you are limited with 200 mm travel and the length of the casting (can’t be made that long). So, bushing distance with inverted dual-crown forks is approximately twice as much, leading to a reduction in friction of up to 50%.

Wren Sports: We feel the added strength and stability of an inverted style fork is an advantage for all types of riding. The disadvantage of inverted style forks historically has been torsional movement. Wren use a key system to prevent this movement from affecting the performance of the fork.

Internals of Wren’s upside down fork

How does the damping work on an upside down fork?

Intend: The damping function has no difference to other forks.

Wren Sports: The damping function basically works the same across all suspension forks. The design of the damper differs between manufacturers. The damper controls the movement of the fork by controlling rebound speed. Without damping, you would essentially be riding a pogo stick.

Thank you to Cornelius Kapfinger of Intend Bike Components and Russ Johnson of Wren Sports for contributing to this week’s Ask A Stupid Question!

Got a question of your own? Click here to use the AASQ form to submit questions on any cycling-related topic of your choice, and we’ll get the experts to answer them for you!

8 COMMENTS

  1. USD forks do come with the advantages Intend and Wren mentioned- they’re more rigid fore and aft, and the bushings stay lubed because they’re lower in the fork. But they come with other big disadvantages- almost every USD fork is both heavier and a lot torsionally flexier than a traditional fork. Heavier and flexier- that’s a poor combination. The fact that they’re also usually more expensive adds insult to injury.

  2. I’ll say the same things that I say every time an inverted MTB fork comes out:
    1) The Cannondale Lefty is the only USD fork that is equal or superior to standard forks in terms of torsional rigidity
    2) Nearly all inverted MTB forks use special techniques to regain part of the rigidity that they lose by using a braceless design: two crowns on a non-DH fork (Cannondale, Maverick); bigger thru-axle (Maverick- 24mm thru axle, Rock Shox RS-1, faux 27mm); crowns with collets instead of bolts, press-fits, or bonding (DVO Emerald- prototype only); one piece carbon crown-uppers (Rock Shox RS-1); key-in-lock axle-dropout interface (Manitou USD forks); integrated axle-lower legs (Cannondale); square legs instead of round ones (Cannondale); inner and outer legs that lock into each other with special channels in the fashion of dropper posts (can’t remember who); leg guards doubling as a very long brace (DVO Emerald)
    3) Most of these stiffening techniques could be also be used, and indeed a couple occasionally have been used, on conventional forks to make them even stiffer
    4) In spite of all the efforts to make up for the loss of stiffness that comes with eliminating the brake brace, most of the inverted forks (to my knowledge, correct me if I am wrong) are heavier and torsionally flexier than other forks in the same class.
    5) Most of the companies that make USD forks stopped making them and/ or went out of business.
    The last time I wrote this, someone accused me of being a shill for Cannondale. Not so- I don’t have a Cannondale, too many dumb standards, too high prices.

    • That was a great summation you put together there! I think the one with the channels allowing the inner/outer legs to interlock that you couldn’t remember the name of is the Xfusion Revel, which seems to be in a perpetual state of semi availability. I’ve always been a little skeptical that sliding keyways would hold up in a fork, as the number of cycles and loading is a totally different ballpark to a dropper post, where keyways admittedly seem to work fine. Having said that, I keep hoping that someone can crack the code and make a USD fork that doesn’t have the typical Achilles heal issues.

      • A good way to do that might be to make a giant hex thru-axle, and maybe combine it with oval legs. But the latter might be hard on bushings.

  3. I think the one you are trying to remember with the keyways interlocking the lowers and uppers is the Xfusion Revel, which seems to be in a perpetual state of semi-availability. I am skeptical about keyways holding up in a fork as the number of cycles and loads are so much higher than a dropper post, but I will continue to hope that someone cracks the USD fork code and finds a way to make them competitive on weight and torsional stiffness.

  4. Another thing, in case anyone is still reading: it is often claimed that USD forks have lower unsprung weight than conventional forks. I doubt that the two fork types have significantly diffferent unsprung weights. USD forks use smaller diameter, thicker walled lower legs made from relatively dense aluminum whereas conventional forks use larger diameter, thinner walled lower legs made of lighter magnesium, which should actually result in conventional forks having lower unsprung weight. Of course, then you have to throw in the weight of a brace on a conventional fork, so unsprung weight probably ends up being very comparable on the two types of forks. I’d like to see the lower legs of each type of fork on a scale before deciding that USD forks really have lower unsprung weight.
    Also, how much of an impact does lower unsprung weight of fork legs really have when we’re running heavy 2.5″ tires, 30mm rims and so on? Proportionally, even like 100g of difference in unsprung weight probably doesn’t make much of a difference in bump sensitivity.

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