Throwback Thursday: Fox's Upside Down Prototype Downhill Fork

Jul 29, 2021
by James Smurthwaite  

Along with bar-mounted lockouts and grip shift, inverted forks are one of those ideas that seems to keep coming back around in the mountain bike industry. Perhaps the most famous USD fork of all is the Manitou Dorado, which was updated a couple of weeks ago, but the design is also currently championed by Intend and RST.

While they will normally catch your eye on a mountain bike, the inverted design is the norm in the world of motocross. The touted advantages include less unsprung weight to allow the wheel to track the ground better, as well as far more fore/aft rigidity, however their Achilles' heel in the mountain bike world has always been the torsional stiffness. The car park test of sticking the front wheel between your legs and twisting the bars can be off-putting regardless of its actual validity on the trail.

Regardless of their advantages and drawbacks, almost all suspension manufacturers have given the design a go. From the RockShox RS1 to the Bos's burly Obsys, they have come from all corners for all uses. Today though we're looking at one that never made it to market but definitely caused a stir, an unreleased prototype downhill fork from Fox.


This fork was first spotted on Gee Atherton's Commencal in a practice run at the US Open in 2011 but it was also later tested by Aaron Gwin. The US Open was the first public viewing, but Fox had apparently been working on the design for a while and also had a number of prototypes that never made it to the public eye.

Digging into the details, the fork didn't use the same 40mm diameter stanchion from Fox's regular downhill fork. Instead, it used 36mm stanchions that that slid into massive 48mm upper tubes. A number of axle setups were investigated, presumably to try and increase the lateral stiffness, but in the end a standard 20mm axle setup was what Gee was riding in 2011. The only info we had on the internals was that it used the same FIT RC2 as the Fox 40 of the time. However, from the outside, the spring leg in particular looked to be something different with its preload dial at the bottom of the leg and a 5mm hex bolt at the top.


The fork never made it past the prototype stage and now simply exists as a curio in Fox's in-house museum. Word is that both Gee and Aaron were big fans of how the prototype fork handled fast, rough sections of trail head on - thanks to the increased fore/aft stiffness of the inverted design - but felt that the standard right-side-up arrangement of the current Fox 40 had the inverted fork soundly beaten in the corners. Fox could potentially have fixed this by adding some more material, but the inverted fork was already significantly heavier than its counterpart so the project was shelved.

That's not to say it was a waste of time though - Fox doesn't look at this project as a failure, but rather an exploratory exercise to learn from. You may never be able to use one of these forks, but you may be currently riding suspension technology that owes some of its DNA to the fork pictured above.


145 Comments

  • 151 5
 Ive noticed pinkbike never brings up DVO when talking about suspension..

What happened between you guys?
  • 132 41
 DVO don't cough up money at PinkBike like others do.. Simple
  • 30 1
 That is a good question. I absolutely LOVE my topaz, and a Diamond will be on the front of my bike someday. Not to mention, their customer service is A+.
  • 149 18
 We don't really see that many bikes come through for testing that are spec'd with DVO. Fox and RockShox are much more common, so they end up being the points of reference. There's no drama between us though - they're welcome to send up a fork or shock for review if they'd like.

And @Healelw1, that's not true at all. If a product is interesting and relevant we're open to reviewing it - we don't charge for reviews.
  • 37 57
flag Balgaroth (Jul 29, 2021 at 12:31) (Below Threshold)
 @mikekazimer: I mean sure but lets be honest if you don't splash some cash in the media you will never get talked about by said media. You may be different but my job is to work with people like you and basically not a single French media works otherwise so I would find it very difficult that PB is any different, even more now with the Outside gig.
  • 21 1
 @mikekazimer: When shall we expect a comprehensive fork shoot out in all categories?
  • 93 13
 @Balgaroth, again, that's simply not true. We review plenty of products from companies that aren't advertisers. I can't say it any more clearly: it doesn't cost anything to receive coverage or have a product reviewed on Pinkbike.
  • 12 11
 @mikekazimer: honest question though, would you find time to prioritize a review with a small company that doesn't support your site with advert $? I know the articles are 'sponsored' in some cases, which I assume means paid for vs the articles that are PB exclusives that are more for general content....I get it either way, just curious.
  • 13 3
 @mikekazimer:
I for one, believe MK.

Let’s use a huge media platform as an example - Tye Joe Rogan Experience.

Does a best selling author pay Joe to be on, because that would increase his book sales. Or does Joe pay the author, because that would increase his listens?

Maybe nobody pays anyone, right?
  • 9 0
 @Balgaroth:

Had the same opinion until I became distributor for a bike related product.
The manufacturer did not pay anything to pb for the very positive review they got on this website...

‍♂️
  • 18 21
 @mikekazimer: I would have be very surprised if you asked for a fee to test products, it is never the case here either. But let's be real, you want to do a test field about flat pedal shoes you can probably talk about 5 to 10 products while there is probably 20 brands on that market. If a product is unreal, wether you got advertisement money or not you cannot not talk about them. But for the average products media always favour brands that spend some cash with them than the others. Same goes for random articles like this one, unless you are seriously innovative you won't be talked about unless you spark some "journalistic" interest with fresh cash, more often than not with the strong recommendation of said media sales guy.
As for DVO let's be honest, aside from the USD fork they released at the launch of the company, they offering has been average and not particularly exciting (unlike Intend for instance which gets some attention from PB). Once they stop releasing blinged out Suntours it may change but until then they better spend some ad money if they want attention. And for those who want some DVO, buy some Suntour, same stuff, less green bling, save some money.
  • 24 0
 I would like to see more DVO testing by PB.
  • 2 0
 @mikekazimer:That makes sense, thanks is for the reply.
  • 10 7
 @ukli: if the product you distribute is innovate or fashionable (think EXT, Intend, Forbidden, Acto.5 or any exotic "porn" brands) Pinkbike or any media will happily speak about your products as they are what the audience is craving for. If you are more mid pack, generic, mass products then things change. And I mean when you receive 20pairs of shoes roughly identical it's only fair to select first the brands that help you put food on your table and run the media and if there is room left then include the others. You can't expect things to always go one way in business but saying otherwise isn't giving a realistic, transparent info about how things work.
  • 2 0
 @cassonwd: in Sweden there barely any customer service, my lbs even complain saying: if you can get hold of a service kit I'll do it but if you dont then no.
  • 10 0
 @mikekazimer: the original question wasn’t asking about reviews, but why PB never mentions DVO when talking about suspension. I understand about reviews and that they are a small player, but this was an article about inverted forks and mentioned the current and past manufacturers, but did not mention the DVO Emerald, which I also found an obvious omission. I will say, though, that I have read about upcoming DVO product on PB previously, so I won’t say they are never mentioned.
  • 4 0
 @Zeust: I call them directly and they are very happy to help. If you can’t get support in Sweden then try reaching out to them by phone or email.
  • 5 0
 @hllclmbr: somebody has to get paid……..otherwise the laws of the universe are violated
  • 11 1
 @Balgaroth: Calling DVO "blinged out" Suntours is a petty comment since SR Suntour makes parts for nearly every suspension brand. The ONYX and DIAMOND forks are pretty amazing products and I see nothing in the Suntour line that looks anything like them. The Triair rear shock and the Topaz do share a body but thats about it. The valves, damper, and bladder are all DVO. As far as attention in PB, most companies would love to recieve the reviews that companies who pay the outrageous ad rates do. Its simple, know what side your bread is buttered on!
  • 1 0
 @mikekazimer: there's nothing wrong stating that was forgotten.

When it is refered rst, intend... it's strange to say the least
  • 1 0
 Probably offended them
  • 7 4
 @mikekazimer: It does cost them something, even if that is cost just sending over a test item. You guys have even mentioned that you wanted to included a certain bike or even a specific trim but didn't because it didn't get sent. The more samples a company can afford to spread around, the more coverage they effectively "buy". Not saying this isn't the norm, but it's not a zero sum game, there is a value proposition.
  • 4 0
 @gonpalco: also left out Maverick, but he might not be old enough…. Wink
  • 2 0
 Where's the BMX coverage? - just watched the finals. GB done good
  • 6 0
 @mikekazimer: when was the last time a Santa Cruz product or a SRAM product received criticism? And I don’t mean ‘with this new version they’ve fixed an issue we didn’t talk about in our review of the old one but everyone now knows exists’. Payment happens in many different ways.
  • 4 0
 DVO Rock....Best customer service I've ever used...
  • 2 0
 I also highly doubt if a new brand or one that doesn’t do advertising reaches out to you to send product they are getting as quick of a response or a response at all. Large bike companies likely have a direct line to media outlets and if the media outlet ghosts them then the company is less likely to advertise. It’s not a quid pro quo but being an advertiser is beneficial for having your product showcased.
  • 2 0
 @jfkusa: You would be wrong on this. If the product is highly innovative and will bring excitment to the PB nerds that we are they will definitely make room for them. Free content that will bring a lot of engagement from the community is always welcomed since this is what keeps PB relevant and a good place to spend advertisement money. Now if you are small/starting and offering dull products, chances are you will not be talked about but is this a loss for the community ?
  • 46 1
 "Perhaps the most famous USD fork of all is the Manitou Dorado" .... Pinkbike, come on, the most famous was Marzocchi Shiver Wink
  • 4 0
 I wanted some shivers so badly back in the day
  • 2 0
 Mini shivers were so cute back then
  • 1 0
 this
  • 2 0
 @blacktea: my kind of adult entertainment
  • 1 0
 Anyone remember the Maveric DUC32?
  • 1 0
 @GoWithTheFlo: 6" monster

I ran a RS Dual crown Sid on my rocky mountain hardtail back in the day in highschool
  • 16 0
 I've got a DVO emerald review for ya... "They're f*cking tits bitches."
  • 2 0
 Same, love mine. But I will admit that I am not a pro level rider, and I am not putting anywhere near the same forces on it that they would. So while a lack of torsional stiffness thing may not be something I would ever notice, it may be an issue for higher level riders.
  • 8 0
 "The touted advantages include"

Why do you guys always leave out the ground clearance issue? A good part of the reason long travel moto forks aren't right side up is because there would be so much leg below the dropouts that it would literally get in the way on things.

The unsprung weight thing is pretty much nothing, too. Sure RSU lowers seem bigger on the outside, but they are state of the art castings with amazing weight-to-stiffness, while USD lowers are both longer and have more and denser material dedicated to adding stiffness being slippery.
  • 1 6
flag thechunderdownunder (Jul 29, 2021 at 21:08) (Below Threshold)
 I think Fox will really look at this comment and that they will probably decide not to test any further lol. What other brilliant tips do you have for them?
  • 10 0
 What about the Foes F-1 inverted fork. Back in the late 90s and early 2000s.
  • 1 2
 How would you ever know about those @weaselssubie Big Grin
  • 8 0
 Avalanche and white bros too
  • 2 0
 Let's not forget the Stratos Superstar 8! Made in Santa Barbara CA. Never had one, but I had a non-inverted Stratos FR4 bumped up to FR5....what a boat anchor!
  • 2 0
 Better yet, what about the Penske Racing Shocks inverted fork that the Yeti team were running way back in the early 90's. Upside down fork with QR axle, talk about flex!!! but the internals were the most advanced for a bike fork at the time, everyone else was doing rubber bumpers or simple valve stacks. Penske was next level!
  • 1 0
 If weren't going that far back, we could just as well include the WP/Rond/Magura Big Ego (and the WP/Rond Mid Ego mid travel fork).
  • 8 2
 Not even a mention of the Emerald...It was the most advanced USD fork on the market years ago and still has features like the CTA and OTT(external negative spring adjustment)which no one else has copied. Manitou was the first to use carbon(for bling) DVO was the first to use carbon to actually solve a problem(flex).
  • 9 4
 no one copied it cause it breaks all the time
  • 3 0
 CTA - yeah, that’s DVO-patented and is neat, but Manitou feels it doesn’t need 1, they release a flexy one only to be able to stop the mud. OTT - Manitou has a different take on this, the IRT (OTT works at the beginning of the travel, IRT - starting from the middle and at the end). Oh, and carbon for the Dorado wasn’t only for bling (it indeed was a huge bling) - served the purpose to shave 250g. Smile
  • 4 0
 I'm not saying this is the 'right' thing to do - has any company figured out the equivalent axle size required to maintain torsional stiffness? I can imagine it is in the range of 30mm. Would add a fair chunk of weight...

If feasible, might introduce another standard (insert recoil and shriek in horror here) but it would be an interesting experiment at least.
  • 5 1
 At least it would ne intentional. Not just making up another standard just because (*cough,cough*- 15x110 axle...)
  • 4 0
 I believe Maverick USD forks used 24 or 25mm axles.
  • 6 0
 A new standard with tangible benefits will never be allowed. Only ones that have a marginal theoretical benefit are allowed. Like a 20.11mm axle.
  • 3 3
 The specialized Enduro came with a 30mm in a regular fork. When the dampener stayed together, that fork was unreal.
  • 2 0
 A keyed axle would solve this issue without needing to go 30mm and specific hub.
  • 2 0
 @bonfire: it had 25 mm axle
  • 4 0
 My dirt bike has a 22mm axle. It weighs 225 pounds. This isn't how you solve the torsional stiffness "problem".
  • 1 0
 @Balgaroth: The Dorado single crown fork had 20mm Hexlock axles for awhile. Not sure how stiffly they rode, though. There's no reason they couldn't do both- say, a 30mm keyed axle.
  • 1 0
 FWIW, the RS1 USD fork had a fake 27mm thru axle- the 27mm part stopped at the dropouts, where it tapered down uselessly to the now standard 15mm.
  • 2 0
 I think the problem isn't axle stiffness, but that it's far too easy for the torsional forces to flex both the uppers and lowers, causing them to 'scissor'. A bigger and bigger axle can make the fork too laterally stiff, taking away cornering traction. They need a solution to the twisting between the uppers and lowers if an UD fork is ever going to be the best option.
  • 1 0
 @Balgaroth: Perhaps, I was using a simple analogy with conventional manufacturing tolerances and techniques. The 'real' challenge is how to generate stiffness without overloading bearings or creating stichon for the sliding surfaces. More precise machining (in general) would also help... but might also make wheel removal not a simple affair!
  • 1 0
 @JeffreyJim: Guessing the components are also steel... approx 3x modulus of elasticity and 3x density by volume. Suspect an all steel mtb fork would not be an attractive option.
  • 1 0
 @ByStickel: Hi there, by increasing the 'axle stiffness' you are preventing the ability for 'scissoring' to occur as the two lowers would not be able to move independently of one another. Not sure how you could 'tune' lateral flex while increasing torsional stiffness at the same time - variable thicknesses throughout the components maybe - not an easy task!
  • 1 0
 The Foes F-1had a 30mm axle with 32mm lowers and they were actually pretty stiff and quite light!
  • 1 0
 @Balgaroth: the Dorado is keyed with hexes on both ends.
  • 2 0
 @JeffreyJim: Exactly! However, motorcycles do have much wider crown spacing which improves torsional and lateral stiffness. There was a great analysis done by Primoz over on the VitalMTB forums. One of the take-homes was that increasing axle diameter beyond 20mm granted only minimal stiffness improvements. On the other hand, increasing spacing helps a lot more because you are increasing the moment of inertia of the entire structure. It'd look silly, but I think an 8 or 9" fork tube spacing on an inverted MTB fork would probably fix all the "problems".
  • 1 0
 @DaveRobinson81: This is not quite true. Aluminum has a higher stiffness-to-weight ratio than steel so most aluminum structures sharing the same ultimate load as a steel equivalent will come in at a higher stiffness.

I've been pondering this issue a lot, along with some other brains over on the Vital forums. I think the best answer we've arrived at so far is that motorcycles place much higher loads on forks than bicycles. These loads are big enough to create deflections that will excite different oscillatory modes (like the "death wobble", or "tank slapper"). Bicycles are not heavy enough, nor do they generate enough tire grip to excite these modes so the main driver for development has been creating enough strength to survive impacts. With that in mind, the RSU fork will always provide the best strength to weight ratio, granted that a bridged lower casting is used.
  • 1 0
 @justinc5716: The stiffness of a material can depend on all parts of each metal forming process - for example, rolled products have a different modulus of elasticity in the 'as rolled' and 'transverse' directions.

Ultimate load and stiffness of a material are pretty independent, same with structures.

The stiffness/mass ratio is almost the same between Al and steel, Al benefits by being able to then use geometry to its advantage so a Al structure can be stiffer than a steel structure of the same weight.

However... it also needs to be bigger by volume, hence my comment above.

So my original comment holds - you can make an Al axle that will be as stiff as your 22mm steel axle, but the volume will need to be bigger. I haven't calculated it, but it's proportional to d^4, hence my guesstimate considering the lower stiffness of aluminum which is about 1/3 of steel.

By 'oscillatory modes' do you mean excitation of natural/resonant frequencies? That is a function of mass and stiffness... obviously we have damping to reduce/negate this by damping the energy.

If a RSU fork is always stiffer why don't motorcycles use it? Increasing the stiffness would increase the resonant frequency, reducing the likelihood of excitation which you mentioned above.

As with everything, compromises need to be made. I am guessing that for a bicycle application reduced weight takes priority over stiffness or sprung/unsprung mass ratios, and damping and effective spring rates are manipulated to mitigate the compromises. Maybe that will change in the future, hence my original comment. Would be good to see 'what it takes' to get the same stiffness in a USD.
  • 1 0
 @justinc5716: Good observation - could start with inverted fat bike forks??
  • 2 0
 @DaveRobinson81: I invite you to join the discussion on the vital forum, as we've covered a lot of this and I think you'd find it interesting and enlightening.

The oscillatory modes of a motorcycle are really strange as they aren't vibrational resonance as we are used to thinking about it. An article I read once described them as "orbits of stability" that arise when dealing with a system that has a high number of degrees of freedom. This gets a bit beyond my mathematical background to explain in detail, I'll admit.

Note I did not say that an RSU fork is stiffer, I said stronger for the weight. In terms of fore-aft stiffness a USD fork of equivalent weight will always outperform a RSU design of equivalent weight. The lack of a bridge however, removes a load path present during torsional loading, meaning that the RSU design will prevail in terms of torsional stiffness.

In regards to ultimate loads though, the RSU design will always come in lighter if you just want to make something that will survive. Chiefly, this approach compromises fore-aft stiffness, and bushing overlap (hence more friction results). On motorcycles it is common for the mass of the entire machine to rest on the front forks during braking. The fore-aft loading can be quite severe therefore as you can imagine. This happens with bicycles too, but the static loads are about 200 lbf instead of 600 lbf (a typical sportbike with human attached). Add in the much higher dynamic braking loads and all of a sudden you are asking the fork to support a substantially higher load in the fore-aft direction. This drives you to design something much stiffer, that will resist binding, and maintain steering geometry at these load levels. Hence the USD design comes in.
  • 6 1
 Just throwing this out there, but a single crown inverted Fox fork with 36 or 38mm stanchions would be awesome. Heck, make it 40mm stanchions.
  • 3 0
 Yeah, X-Fusion makes that already. But idk, every single one advantage of USD forks comes from the length of the stanchions, which is impossible to achieve on a long-travel single crown design. Marzocchi tried, but that thing was heavy and not too well working imho.
  • 2 0
 @speedy-fox2: The problem Marzocchi had with the Shivver SC was the stanchion diameter. It only had 120mm of travel but the stanchions were 30 or 32mm if memory serves. If we bumped up the stanchion size we could easily up the travel to 140 or 150mm I think.
  • 1 0
 @seraph: 30 on the SC and 32 on the DC and maybe bigger after 2004.
But diameter is far from the only thing that keep the shiver SC from being stiff.
  • 2 0
 @faul: 35mm on the Shiver DC.
  • 1 0
 @faul: Exactly, in this sense the only thing keeping the stanchions together is the crown. Imagine the moment it needs to counterbalance!
  • 7 0
 Best looking Fox fork ever...
  • 3 0
 I just got the new Dorado Expert and it is far superior to the Ohlins DH, Boxxer, and Fox 40/49 forksI have used in the past. Torsional stiffness is not an issue for my riding but perhaps it is at the pro level. Would love to see more upside down forks.
  • 1 0
 How did you manage to get a new Dorado so fast???
  • 1 0
 @hitarpotar: I ordered one from Hayes the day they came out. Showed up that week and I've got 50ish park laps on it. Best fork I have ridden yet!
  • 1 0
 @mgs781HD: can you compare it to the previous gen fork, the 36mm one (until 2021)? Smile
  • 1 0
 @hitarpotar: Never road the previous Dorado, so can't compare but the new fork is awesome!
  • 5 0
 Wasn't one of these inside of the Fox truck/trailer that was stolen at a world cup in Quebec, way back when?
  • 9 0
 Came here to say this. Someone out there has one...
  • 6 0
 Yes somebody stole the whole fox trailer at MSA that same year. They found the trailer but it was cleaned out. Some French dude in Quebec probably has that prototype.
  • 5 0
 My Dorado’s were probably the nicest feeling fork I’ve ever had. I never felt they weren’t stiff enough.
  • 3 0
 Yeah, the only people who call the Dorado stupidly flexy are the ones that don’t own one! Big Grin
  • 2 0
 Can you please re-test the Halson Inversion fork?
I had serial #3 that came on my Norco TNT w/ the first year XTR (gift from Norco and was the interbike show sample)
It was really nifty since it could work with cantilever brakes. We all could use more niftyness in our lives!
  • 7 0
 I had a couple of those forks, and remember them well! I can do a full review for you...
+ Rebound control and bump absorption. The spring stack was very long, it compressed only 27% at full compression (some other forks of the era compressed fully 50%- forks like that rebounded back open like pogo sticks). That meant that they had really manageable rebound, and absorbed big hits well. Small bump absorption was also great.
+ Rapid tuning. The elastomer stack was removable without tools. This meant you could easily pull the springs and replace them by hand, for example for changing temperatures.
+ Wide tuning. The elastomers were available in four different color-coded hardnesses, so you could easily experiment with different ride qualities, and even tune the progressiveness of the fork (for example, by putting in several hard elastomers, you could get a very progressive spring rate). For their later PDS fork, Halson made two additional spring densities which could also be used on the original fork, so you eventually ended up with six different hardnesses. Plus, Manitou elastomers from the 3/ 4/ EFC/ Magnum and later Mach 5 forks also worked, so you were never short on tuning options.
+ Ease of maintenance. You only needed a single 4mm Allen to get the fork apart.
+ Well sealed. The rubber boot covered the fork lowers well and kept everything clean.
+ Durable finish. The black hard anodization on the sliding surfaces didn't wear out, unlike the ones on Manitou forks.
+ Fore-aft rigidity. Torsional flex was the real issue back then, but the 37mm upper legs did provide great fore-aft rigidity.
+ Price. This fork cost 100 bucks less than competing forks.
- All of the offset was in the crown. This meant high rotating weight, and that the fork steered noticeably slower than other forks, especially Manitous, which had 100% of their offset in the dropouts.
- High unsprung weight. Halson liked to claim that their fork was inverted and therefore had low unsprung weight, but the opposite was true- the legs and bridge were both heavier than those of, say, a MAG or Quadra. The negative effect of this higher unsprung weight was more than offset by the other positives of the design, mainly the superior spring stack, but still...
- Heavy, ugly, flexy bridge. The clunky cast aluminum bridge weighed more and flexed more than the excellent hollow cast magnesium ones on Rock Shox or the extensively milled out ones on Manitous. Especially with V-Brakes installed, you could see the bridge flex like mad under heavy braking. There's no excuse for a bridge that flexy, especially when you're touting your fork's supposed rigidity as a selling point.
- Heavy, ugly, flexy dropouts. Although the dropouts were pretty thick, they also hung a long way below the fork's lower legs, so they flexed easily.
- Unreplaceable bushings. Unlike on RS and Manitou forks, you couldn't replace the bushings. What were you supposed to do when they wore out, send them back to Halson to have new ones pressed in? Buy a new fork?
- Poor durability. I had two of the original forks, and in one of them, a stanchion came unpressed from the crown. I also broke the internals on their later PDS fork.
The later PDS fork solved a lot of these problems, but also suffered from new ones. But that's another review that at most one person will read!
  • 2 0
 I still have a Halston inverted fork! its in my fork collection now hanging on the wall!
  • 1 0
 Holy cripes and great review! How much travel does the Tange Switchblade have on my Race Lite? For some reason it feels like zip! Bravo mi amigo. @Insectoid:
  • 1 0
 @NWintheUSA: Glad you liked the review! I'm surprised that more than one person read and liked it!
  • 1 0
 When the investment on cast lowers is high and needs "A LOT" of forks to be sell for breakeven, and make money,
Convencional forks can be made lighter/rigid (it has that bridge connection both sides)


So... higher price, higher weight for same rigidity...
If all were about performance, several Enduro rigs would have double crown forks
  • 5 0
 I think I know why they dropped the idea. It covers up the kashima.
  • 3 0
 Just because pb doesn’t review a product doesn’t mean it isn’t legit, don’t be a pb groupie.
  • 2 1
 Well, I've been told many times by all the band wagon jumpers that Mullets are good enough for Motocross Bikes so they are good enough for Mountain Bikes. Then Upside forks should be good enough for them too.
  • 2 2
 If I worked at a PM at Fox, I'd look *really* hard at bringing this back. You could even keep the 40 right where it is, even though this will cannibalize a lot of sales. Some riders will like the feel of an USD fork better than RSU. How much technology have we borrowed from moto? (geometry, damper design, rear end progressiveness, handlebar width, "stem" length, weight distribution, mullet wheel configurations etc etc) How many moto products haven't worked? (its very few; Lenz sport seats?)

By mountain biker's standards, moto forks are also "flexy". But I can corner better on my 225lbs EX300 than I can my 38 pound Enduro with a 38. Why? Because we don't twist the handlebars to turn, there its a much more dynamic movement with forces going to the wheel in very difficult to model (using the parking lot twist).

Plot twist, motos are downsizing axle diameter and engineering flex into swingarms. Why? Because stiffer isn't always better.

I'll take a dorado with the right crown offset over my Fox 38 on my enduro bike. I'm also heavy. If I could put Fox internals in it and slighty bigger upper tubes, more bushing overlap and crowns that are the right offset I'd never ride anything else*

(*in a gravity setting)
  • 4 2
 I could be wrong but when you have an engine to power your Moto the energy loss that flex represents is a lot less of an issue. When you are powering your bike with your meat sticks, we call legs, the energy loss that happens with flex is more meaningful. Although, I have no clue if this has any real basis as I am not an engineer.
  • 3 0
 I agree. I run a dorado on my bike that came with 38’s. It is an absolute dream. It just does everything better and yaw flex is a non issue.
  • 3 0
 @ptrcarson: to climb that is true but when you are talking about inverted forks you are more interested in going down. Fork torsional rigidity won't do anything for or against pedaling efficiency. Frame rigidity or the lack of it can also be an attribute looked for by athletes. Vouilloz was famous for riding noodle wheels and the recent bike check from ALN seem to show the same for her team. And if you have ever rider a Meta v4 or Suprem v4 their back ends are really flexy which makes them great when riding difficult terrain, off camber and such reducing tiredness quite a bit in the end.
  • 2 0
 @ptrcarson: no. if you have an overly flexy frame, crankset, etc this can be true. But we're talking visually flexing every pedal stroke. More to the point, your fork has nothing to do with power transmission.
  • 1 2
 @JeffreyJim: I was aware aware that a fork had nothing to do with power transmission. Was thinking more when climbing if every time you hit a small square edge your fork flexed in towards you rather that just pushing over it would make climbing take more energy. I guess as Balgaroth said this is less about the up than the down.
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 @JeffreyJim: Ah, so I have now read it is only torsionally less stiff not fore and aft so my question no longer is necessary.
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 I would love to see Pinkbike do a review of the Wren sports inverted fork. I ride one on my hardtail and like it alot. Would be curious to see what the reviewers take on it is!
  • 5 2
 But... We would've bought millions of them...
  • 11 0
 Oh, and we could have had an all black version that had "shiver" on the sides.
  • 3 1
 @hubertje-ryu: Shivers for the win!
  • 2 1
 Is there a reason why an asymetrical fork (inverted on one side, regular on the other one) isn't a thing? Would'nt it give half the benefits with less torsion issues?
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 If you can put a bridge on it, but if not, it will be worse than anything else. Intend is making a single and half crown fork, tho.
  • 1 0
 This is an idea waiting on material research and time. Manitou stepped up. Ebikes can hide some weight and they can adopt quirky new steerer interfaces.
  • 1 0
 Fox should just say "It's heavier and not as good as the 40, but dope as hell" and I'd bet they'd still sell a shit ton. I'd buy one...
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 USD forks are a "solution" that doesn't work very well, but that gets trotted out every once in awhile for technical rather than marketing reasons. Every time a USD fork comes out, it reliably generates the same conversation about the pluses and minuses of USD- just look at this thread- and makes people ask if this time they will succeed or even take over, which of course has yet to happen.
Think of the hype that went with the crummy RS1! That fork also had another great non-technical advantage going for it- its upper leg/ crown assembly melded well with carbon frames in both shape and color. In my opinion, those were the only reasons for that fork's existence- generate hype, look sexy.
  • 3 0
 I want one
  • 2 0
 i Love my dorado, and my old school 303WC
  • 2 0
 Imagine a modern 303 with the rails and everything
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 @stormracing: that would be such a dope bike. I would so get one. And will when Yeti is ready.
  • 2 0
 Real shame it never made it to production.
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 Are the old Lefty forks not an invert design? Look terrifying but supposed to be stiff. No personal experience.
  • 2 0
 They are. Super stiff too. IIRC airplane landing systems have the same single strut design. Mechanically speaking that’s as solid if a design as you can get.
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 @GPP2117: yes. Our large uav landing gear struts are single sided and the axle is cantilevered like a lefty. It’s absolutely no issue. I can’t remember precisely what size axle we use but I believe it’s 20-30mm. However the front are also braced laterally and the rear are braced longitudinally as well.

The whole system we use is taken from a larger aircraft for ease of maintenance but the oleo strut is revalved for a lighter maximum landing weight. Whilst the principle is similar, the operation is completely different. The inside of an oleo strut is there to absorb and dissipate one large and slow impact. It’s really close to as simple as a gas and oil shock can be.

The wheels on some of our specialised trailers are effectively a single (unbraced) strut with a cantilevered wheel. It’s really a very common setup.
  • 1 0
 The last lefty I had (100mm with the dual crown design) was so stiff and so smooth. That thing tracked so well.
  • 1 0
 when will hope cnc a fork?
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 I don't see a benefit unless it would be lighter and stiffer.
  • 3 1
 Have you ever rode one?
  • 1 5
flag tacklingdummy (Jul 29, 2021 at 14:26) (Below Threshold)
 @blacktea: No, have you? What is substantially better about it?
  • 4 2
 Have ridden many. Also ride moto. Like everything else we've borrowed from moto, its only a matter of time. "Stiffness" is not everything. As we are seeing today, riders commonly detension wheels, run softer aluminum rims etc to get the bike to be more compliant and actually hook up *better*.

Inverted is the future, but everyone is going to have to reframe what they've been told for eons. "Stiffer is always better" (no, not its not).
  • 3 1
 @JeffreyJim: Yes i bought stiff deep section carbon rims like an idiot and now my forearms pay the price
  • 1 2
 @JeffreyJim: I'm not a fan of detentioning wheels. For the first time in a while, I bought some good aluminum wheels and they were noodle-ly in the really rocky tech. Had stiff carbon rims for several years and prefer them. However, it might be good to have a stiff carbon rear wheel and more compliant aluminum front wheel. Haven't tried that yet.

But why is inverted better? I haven't really hear anything that is substantially better about them.
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 @tacklingdummy: inverted dual crown forks slide better under loads. That's the only thing better they make that is nearly always there, due to a bigger bushing overlap.
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 @tacklingdummy: Upside-down forks are not as stiff as normal ones and forgive you some mistakes in riding. It's a feeling to get used to but it pays off in the long run.
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 @tacklingdummy: USD have less torsional stiffness. But they have greater fore/aft stiffness, greater lateral stiffness, greater bushing overlap, greater lubrication of the bushings and seals to help them slide better increases sensitivity, dirt and crap doesn't pile up on the seals reducing wear, greater ease of servicing as one leg can be removed at a time. Also, the further the stauchions move up into the outer legs the stiffer they get, so the bigger the hit the stiffer they get.
I would much prefer to have the greater fore/aft stiffness to deal with *heavy landings and charging head long into chunk. As well as the greater lateral stiffness to deal with the side loads you can get in hard and sharp berms. I'd also take the sacrifice in lateral stiffness to help grip and composure over off camber nastiness and baby headed rocks, rather than overly stiff setups that glance and ping off every angular strike.
The lower torsional stiffness does become a pain in deep wheel ruts though, seeming to be overly eager to grip the sides and climb out of the rut rather than hold the line through it. But there are exactly zero ruts on the trails I frequent, so I will overlook that.

*One particular model line of right side up fork I, a 96kg rider, have ridden seems to flex out forward on heavy landings and seems to suffer binding.
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 @riderseventy7: The stiction aspect is a valid point. I can see how the oil is better able to lubricate the seals by gravity. However, the stanchions being so exposed to damage by rocks and other stuff kinda bothers me. I'd have to try them to really see if it is worthwhile though.
  • 1 2
 @tacklingdummy: Actually, lubricating seals is an issue when designing USD forks. On a conventional fork, the oil in the lowers will splash and create a tiny film of oil that does the job. On an USD fork the oil in the inside tube has to find a way to go lubricate bushing (and also seals but seals frictions is order of magnitude lower than bushing friction). The oil that is there either stay there if you have tight seals, or will flow thru them if you want low friction seals (at low shaft speeds). So lubrification is harder to design on USD fork... but still, better bushing overlap, so less friction overall.

And you don't want fore/aft stiffness on heavy landings, you want bushing not binding (what USD does better than non USD). Fore/aft flex help a lot reducing vertical accelerations, especially if the rear shock compress first.
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 @faul: The lower bushing is right next to the seals in a USD fork. Both are sitting in a pool of oil. The damper is also inverted in a USD fork, meaning the bath oil sits at the bottom of the upper tubes and the end of the lower inner tube is sealed off. Meaning the seals/lower bushing are running in the bath oil, and the upper bushing is a lot closer to the bath oil.

www.downhillnews.com/storage/damper-tpc-plus-diagram.jpg?__SQUARESPACE_CACHEVERSION=1373819084853
  • 1 0
 @Civicowner: a closed damper in any fork is in whatever orientation you want. An open bath has the compression unit in the bottom.
the "pool of oil" won't stay on his own if the design don't take care of it. The dorado damper has no particular reason of being this way, other than... help lubrification of the leg by closing the tube. It seems that manitou's engineers know what they are doing.
Marzocchi did a hole in their stanchions for the job as they used some open bath cartridges.
The USD forks you can buy are well lubricated because someone thought about it, not because the USD design itself help.
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 @faul: so it's not a fundamental issue with the design then, but rather a simple fix?
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 @Civicowner: Yes, but don't tell it's an advantage of USD fork. It's not. Only real advantage is bushing overlap, everything else can be done with a conventional fork, even bigger (than current conventional forks) frontal stiffness could be obtained.
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 @faul: How is it not an advantage only because the stanchion needs to be sealed? Following that logic no fork orientation has any inherent advantages.

In a right side up, the increased torsional stiffness is not an inherent advantage, only designed into the fork because the fork requires a brace to be added....
The USD fork does not necessarily have increased bushing overlap. because it requires a second pair of bushings to be added....
The lefty does not have reduced friction, because it must be designed in such a way that has low friction...

I'd like to see a right side up fork that has the bushings and seals actually running in bath oil and not relying on splash/foam rings.
  • 1 2
 @Civicowner: It's not an advantage of USD fork if it doesn't do better.
The ability to put a brace is an advantage.
The sealing needed to keep oil adds more friction to the seals. conventional forks can use less efficient seals that are engineered to decrease frictions at all shaft speeds. If you put them upside down, the seals will leak at high-ish speeds. Or if you soak the foams in oil, you'll see an excess of oil on the stanchions during the first few runs. Because they don't have to keep all the oil inside by fighting gravity. Only wipe out the excess of the oil film.
You can have a fork that uses an oil bath for bushing and (most of the time) seals. Open bath cartridges does just that. But open bath cartridges can also uses less seals than sealed cartridges so the lack of frictions doesn't come mainly from the added oil.
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 @faul: Gravity is not really an issue because seals have to hold in the pressures in the lower leg at bottom out, it's not like the mass of 30ml or so of oil is enough to cause really high pressures at the seal compared to the pressure in the lowers from the displacement of the stanchions.

However more important is that the bushings and seals are constantly running in oil, rather than relying on oil being splashed up somehow from the lower lets where it pools. USD Does actually lubricate better, regardless of what seals are needed or not needed. They are still running in oil.


BTW here is a graph of the spring only and then the complete fork. Notice how much extra spring force is added by the pressure in the lower legs... www.mtbr.com/threads/the-spring-effect-of-trapped-air-in-fork-lowers.1190110
compare this to the pressure caused by the mass of 30ml of oil
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 @Civicowner: 99% of the time, your bike isn't riding.
  • 2 0
 @faul: what's this supposed to mean? The seal holds in all the pressure at bottom out, so it holds in the minimal pressure from the mass of the oil no problem. It doesn't slowly leak through, just like it doesn't slowly find its way past foot nuts on conventional forks.

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