In this article, we’ll be discussing Front Forks for Bicycles by SRAM, US publication 20220355894. The publication date is Nov. 10th, 2022 and the filing date is May 10th, 2021. This patent is not granted.
The USPTO recently changed its search system, so I’m not entirely sure how to link the document directly (Google Patents takes a few weeks to update). Go here and type in that number if you want to read the document itself.
Brief Summary (tl;dr)
SRAM are working on a new linkage fork for, what appears to be, a gravel bike. This new fork utilizes an upper fork and a lower fork. The upper and lower forks are connected via two pairs of links and bearing sets. The lower fork is a single piece, connected with a bridge. The steering tube holds a spring made of plastic or rubber, and the bridge applies pressure to the spring to provide a small amount of damping as it goes through its travel.
Background
I have some pretty strong feelings about front bike linkage systems. You’re going to have a hard time convincing me that the absolute, most correct, bestest answer for a full suspension system is a linkage system at the rear and a telescopic system at the front. Not a chance.
The idea of a linkage fork like this one is to provide some level of support and reduce high frequency, low amplitude vibrations for causing fatigue in a rider. This has been attempted to be addressed by multiple manufacturers (Isospeed, Futureshock, Diverge STR, eeSilk, Buttercups, Lauf and on and on), so we can assume this is a real issue that needs a real solution.
That being said, I can tell you with 100% certainty that multiple companies are working on long travel linkage forks. Recently, we’ve seen the Trust system (now a Specialized product), the insane Structure Cycle Works system (shoutout Mr. Rasmussen), the Motion Ride system, and many others that are trying to normalize this idea. The problem is being addressed, and it’ll be interesting to see who comes up with the best idea.
I understand most of those are mountain bikes, and this SRAM concept is for a gravel system, but SRAM’s is still a linkage system rather than a linear/tubular system like the headset suspension (i.e. Future Shock).
Intended Novelty
Again, this isn’t granted, but the intended novelty is the use of a bridge to connect the two lowers, a spring extending out of the steerer, and the bridge engaging with the spring. This will make more sense as we go.
Why
Ultimately, it appears as though SRAM want to create a more aerodynamic and aesthetically pleasing suspension fork. A telescoping fork must include two cylinders (stanchions and lowers), which must maintain a coaxial connection. As a result, the fork sits pretty wide compared to the front wheel.
Additionally, SRAM appear to be trying to improve upon the current flexing and linkage forks on the market, because they say the lower legs aren’t structurally connected:
…flexing members or linkage type of forks… have a separate lower member on each side of the front wheel, and therefore are not structurally connected… [therefore] allow for excessive independent movement of the lower members relative to each other. In use, when side loading due cornering or bumps occurs, the lower members move relative to each other leading to the wheel position and/or suspension trajectory moving out of center plane of the bicycle, which results in imprecise steering behavior.
What
This document is broken into two parts; the first explains the linkages, and the second explains the spring. So, let’s take a look at the linkages. FIGs. 2 and 3 show this new fork in more detail. We’ve got two primary components, the upper 200 and the lower 202. The upper and lower are connected via two pairs of links. The two pairs of links are mounted to the upper and lower via a pair of bearings.
One of the issues SRAM mentioned earlier is the fact that current front linkage/flexible forks are not connected to one another. FIG. 3 shows bridge 236. This bridge fixes the left and right legs to one another, and ‘…the lower fork 202 may be considered a one- or single-piece fork member.’ This should lend to improved stiffness, though there could be a weight penalty.
FIG. 4 shows an exploded view of the fork. We can see that the single-piece lower is connected to the upper using the two pairs of links, a bunch of bearings, and a few bolts.
FIG. 5 shows a front view of this forker. SRAM say this design is more aerodynamic compared to traditional telescoping forks, as well as being more A E S T H E T I C versus a telescoping fork.
…[the] profile that is more stream lined and aerodynamic than known telescoping tube forks… [and is] more aesthetically appealing because it remains aligned with the rest of the frame 102 (FIG. 1) behind the front fork 108 when viewed from the front. In known telescoping tube forks, the lower tubes extend vertically from the ends of the axle 500 and do not curve inward…[and] remain relatively wide compared to the front wheel, which results in increased drag and is less aesthetically appealing.
FIGs 6A-D show this fork going through its travel. The left images show the profile, and the right images show a cross-section of one of the legs. There’s not much else to say here – it go up and down.
FIG. 7 shows the front axle path of this new fork. We can see that the path is not linear, but curved. It appears to be somewhat rearward, so I know the internet will get all boned up about that.
Alright, now we’re getting to the spring part of this idea. FIG. 8 shows another exploded view, similar to what we’ve already seen, but with the spring 800 and associated components. The spring assembly sits inside the steerer, and abuts against the bridge as the fork goes through it’s travel. The material of the spring will be ‘…at least one of urethane or a thermoplastic’, so it’ll be some kind of hard plastic/rubber material.
FIGs. 9 and 10 show the spring in a bit more detail. We can see the spring 800 is jammed inside the bottom of the steerer tube and is fixed in place with some hardware (at the top). Then, the spring is fixed to the bridge with another screw 814. It’s as simple as it sounds: the fork goes through it’s travel, compresses the plastic, and you get a nice, soft ride – or so I’m told.
There’s another variation that uses a shear spring, but I won’t go into that one.
Here’s a line I’ll leave you to ponder on:
The front fork 108 can be configured to… [@SRAM, there’s a language error here] achieve any length of travel path and/or vertical travel distance. For example, the travel path and/or the vertical travel distance can be modified by changing the length of the links… the attachment points of the links… the stiffness of the springs, the attachment positions of the springs, etc. These parameters can be changed to optimize the travel path and/or vertical travel distance for improved bump absorption, improved behavior braking, and/or reduced rolling resistance. In some examples, the front fork can have a travel path length of 5 mm-200 mm. Thus, in some examples, the front fork 108 can be configured for use on bicycles that only desire small or medium amounts of travel (e.g., road bikes), and can also be configured for use on bicycles that desire larger amounts of travel (e.g., mountain bikes).
Conclusion
Ultimately, in my view, the primary battle linkage forks will continue to fight one of aesthetics. I firmly believe front linkage is the correct answer, which will lend itself to significant advancements in bike performance as designers get more axle path freedom. They’ll allow a new level of tuning and innovation, which is exactly what we’ve seen at the back of the bike. That being said, they’re ugly af, and I can’t come up with an answer for that. This SRAM option might look nice in real life. At this point in time, I’m inclined to say the Trust system hits the eye the best, but it’s still questionable in appearance.
For my racecar folks, back in the 70’s, Plymouth threw a gigantic ugly-ass wing and a nose-cone on a Road Runner, creating the Superbird. The appearance was very questionable and extreme at the time, resulting very low sales volume. Richard Petty went on to win, again and again, even causing NASCAR to change engine rules for ‘aero cars’. That car is now known as one of the best ever to hit the track, changing NASCAR forever, and being one of the most desirable American collector cars. Winning doesn’t care about appearance, never has and never will.
So, how do you legitimize an ugly product that may offer genuine performance characteristics? You race the damn thing. I’m calling this now; I think we’ll see a Trust fork on a Specialized race bike (probably Enduro, hopefully DH) in an attempt to sell the idea to the public. The Trust crew tried to sell it, but I can’t recall it being raced (please comment if they have). If Specialized want to sell it, they need to make it cool, and winning is pretty cool (pee your pants cool). That goes for every manufacturer. You want to sell something ugly af? Win with it.
Keep preaching the gospel. We’ll get there some day!
Another excellent article, a good find and a really well written explanation, many thanks.
I totally agree with you that linkage forks are the way to go. Telescopic forks have a lot of inherent flaws that aren’t easily addressed and it makes sense to go this way.
Unfortunately linkage forks for both bikes and motorbikes seem to follow a common path of performing very well, having some very impressed users become firm fans, but being a commercial flop. It would be great if this pattern could be broken.
To me, this particular patent seems like it might struggle to be considered novel enough to be granted. I’m fairly sure that Moulton will have done something very similar during his developments and it isn’t that different to their standard front fork on the AM bikes apart from having a second pair of links rather than a top guide.
For road bikes; the elastomeric (particularly the combination of shear and compression) spring is a great solution with natural damping, low weight and zero maintenance.
Obviously any success in road racing depends on the UCI not banning it which is by no means certain.
I wonder if we will see it rolled out fairly soon for the cobbled races like Paris Roubaix?
I appreciate the nice words George. I completely agree they always seem to flop, even with the higher levels of design freedoms. Patent law is interesting, it doesn’t take much to convince the USPTO your idea is novel, though they then need to defend it if anyone doesn’t like it. And that would be great to see on Roubaix as an excellent proof of concept.
I have 3 of my 5 MTBs with linkage forks. Anti bob and no dive under braking can also be built into a linkage design.
Do your forks have questionable appearance? I feel as if someone can make one look nice and run races, it’ll be the new standard.
Good looks is in the eye of the person using it. While wild looking, I absolutely drool over Motion Ride fork, absolutely love the novel of the Rocksled design and definetly would proudly have an Adroit Linkage front suspension. Would take any of those instead of a fox 36 any day of the week.
The trust, while very good looking, have proprietary damper and spring. Serviciability of a wild piece is something that I consider quite a lot.
I do like the Motion ride, as well.
I see designs like this and think of bikepacking and touring .
Being able to mount bags or a rack to be isolated from suspension movement is huge.
Giant is doing the rear rack mounted to the front triangle on there new adventure ebike.
With a fork like this I can see a lot of bikepackers adopting such a design .
Hello – stumbled on the site by accident and love what I see. This article got a reaction – big fan of linkage forks. – got a trust shout on my emtb that transformed it from a dive bomber on the tech steep descents and given I am not a jumper and more a rock garden rider it’s fantastic (I cut my teeth of leading links on motorcycle combo and a BMW telelever years ago).
This looks like a springer fork (ala Harley’s vintage) with a twist where the action has moved through 90 degrees
It’s interesting but no doubt will still have the aesthetic police in a froth for sure but if they have a decent air shock that’s easy to access/adjust I’d look at it seriously
Anything with a rear centric axle path up front where I ride will always have my attention off road.
David,
Always nice to hear support for linkage forks! I was head of product during the development of the most successful mountain bike linkage fork. One of the most important elements of the success of the product was to reduce the focus on a rearward axle path and increase focus on dynamic geometry.
Motorcycles – especially those intended for paved or dirt roads (i.e. excluding motocross and trials) – are subject to less weight shift and pitch than mountain bikes, making motorcycles less sensitive to changes in front-centre length and steering trail. Linkage forks reduce friction – especially static friction – so effectively that it’s not necessary to fully optimize the compliance ratio (the alignment between axle path and impact force), freeing the designer to optimize dynamic geometry and handling behaviours.
In other words, don’t judge a (mountain bike) linkage fork by its axle path! Motorcycles and road bikes can be judged somewhat more by their axle paths.