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.
What happened between you guys?
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.
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?
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...
♂️
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.
When it is refered rst, intend... it's strange to say the least
True
I ran a RS Dual crown Sid on my rocky mountain hardtail back in the day in highschool
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.
If feasible, might introduce another standard (insert recoil and shriek in horror here) but it would be an interesting experiment at least.
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.
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.
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.
www.pinkbike.com/photo/20434047
But diameter is far from the only thing that keep the shiver SC from being stiff.
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!
+ 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!
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
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)
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.
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.
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).
But why is inverted better? I haven't really hear anything that is substantially better about them.
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.
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.
www.downhillnews.com/storage/damper-tpc-plus-diagram.jpg?__SQUARESPACE_CACHEVERSION=1373819084853
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.
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.
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.
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