Multi-Body Vehicle Suspension Linkage by Yeti: Part 1
In this article, we’ll be discussing a Multi-Body Vehicle Suspension Linkage by Yeti, US publication 20200247500. The publication date is August 6th 2020 and the filing date was Jan 31, 2020. This patent is related to numerous provisional applications.
This document will be broken down into 4 different articles. There are 4 different distinct ideas that cannot possibly be put into one coherent article. The first through third articles are 3 different concepts for the new suspension system, while the fourth will incorporate a motor. Yes, it seems like Yeti are getting into the e-bike game.
Brief Summary (tl;dr)
This patent is a doozy. Yeti have created a new 6-bar suspension system that utilizes a similar Switch Infinity-type concept, where the bottom pivot moves upward to an inflection point and back down at the end of the travel. Unlike the linear movement of the Switch Infinity, the bottom pivot of this system moves in a linear path in the middle of the migration path and has a small amount of curvature at each end of the migration path, at both full extension and full compression. Yeti claim this may improve anti-squat and anti-rise numbers. Additionally, Yeti claim the 6-bar system will allow the designers more freedom in separating kinematic dials such as anti-squat, anti-rise, and leverage ratio. Yeti are implementing this design with both traditional pivots and flex stays with solid pivots.
If you’re done reading here, check out part 2.
If you’re super done, check out part 3.
We choose our suspension systems based on our riding style, risk tolerance, or local terrain. People wouldn’t typically buy a Commencal Clash if they lived in South Florida and others probably wouldn’t buy a Specialized Epic if they were park rats. But there is a large market for a bike that can do it all. Yeti’s Switch Infinity claims to be a do-all type suspension system, providing good pedaling efficiency with good downhill characteristics. Having lived in Golden, Colorado the last few years, I can say that these bikes are wildly popular. And with Richie Rude and Casper Wooley putting down excellent race results, maybe the hype is real.
Compromise is a central concept that is considered in the design world. In any product, especially high-performance products, one dial will turn another dial. In race cars, it is particularly difficult to find downforce without adding drag, though not impossible. In mountain biking, it is difficult to make a suspension system for an ideal pedaling bike with ideal descending characteristics. Pedaling and descending are, and will always be, mutually exclusive.
There are multiple rear suspension systems in the mountain biking industry, such as Horst-link, single pivot, VPP, and Yeti’s Switch Infinity to name a few. All these suspension systems provide unique kinematic characteristics, allowing us, as consumers, to argue about which design is the best. Each system turns every single kinematic dial in such a way to produce a bike that will perform how the manufacturer wants the bike to perform.
For a little technical background, we’ll define some important vehicle dynamic terms. Instant velocity centers (IVC), physical instant velocity centers (PIVC), and dynamic instant velocity centers (DIVC) are used in all suspension design. An IVC is a point of a moving body that does not have a velocity at a point in time. Additionally, Yeti defines PIVC’s as:
“PIVCs are defined at the pivotal axes or virtual pivotal axes of jointed linkage body members. There are four PIVCs in a 4-bar linkage while there are seven PIVCs in a 6-bar linkage”
In other words, the 7 PIVCs are physical pivots and do not need to be derived. You can view them on the bike as it sits. DIVCs are migrations of IVCs, so they’re moving IVCs. In this article, we’re only going to talk specifics about the PIVCs.
Except for this little part. The number of IVC’s of a suspension system can be calculated using the equation:
where N is the number of total number of IC’s and n is the number of links (bars). In the case of a 4 bar system, there are 6 total IC’s. In the case of a 6 bar system, there are a whopping 15 IC’s.
Additionally, Yeti are introducing flexible braces. Trek are using flexible braces with their Procaliber and Transition are doing it with their Spur. The idea isn’t new. Formula 1 has used flexible suspension pieces for some time now. The flexible nature of carbon provides certain kinematic characteristics and variances that wouldn’t be possible with traditional pivot-driven linkage. But Yeti are using them in a very specific application.
For the sake of time and text savings, here’s a quick overview of the current Switch Infinity system. Yeti say this design ‘gets exactly what we want…’ in pedaling, descending, and leverage ratio characteristics, whatever that means.
In this publication, Yeti are introducing a fresh suspension design using similar characteristics to their current system. I can only dream that it’ll be called Switch Infinity 2: Electric Buggaloo. They’re sticking with their up-down style bottom pivot point, but achieving it in a completely different way, using a 6 bar linkage system with a smaller 4 bar linkage system. Specialized are already using a pseudo 6-bar in their Demo and Enduro, and Atherton are doing a 6-bar, but none are doing it like Yeti are.
The intended novelty of this invention is a non-linear migration path of the bottom pivot. This is achieved with a 6 bar system, with a small integrated 4 bar system. With the current Switch Pivot system, the migration path of the bottom pivot is linear. With this new system, the bottom pivot path is linear, then curved at the ends of the migration path.
From a practicality perspective, Yeti claim multiple advantages of this design. Yeti state “The smaller envelope of the linkage design as disclosed herein can have several advantages structurally: For example, there is more clearance between the rear tire and the suspended body allowing for a shorter distance from the driving cog axis to the driven wheel axis. This can be a performance benefit allowing for quicker turning. The added tire clearance provides more room for dirt and mud that can build up when riding. This added clearance also allows room for a larger “bridge” tying together the drive and non-drive sides of swingarm body which aids in torsional stiffness. The added clearance in front of the driving cog axis provides more room to fit a water bottle and other accessories within the frame of [front triangle]”.
From a kinematic perspective, Yeti further claim that “The smaller envelope of the linkage design as disclosed herein can have several advantages kinematically because there is more freedom to locate PIVC 45 and therefore a greater ability to tune parameters such as antisquat, anti-rise, and leverage rate which translates to greater performance. Also, PIVC migration paths are able to have an extremely large minimum radius of curvature, or unique curvature profiles with inflection points within this small linkage envelope. This is not possible with traditional links and allows for increased tunability of suspension behavior”. So, Yeti believe this is a more tunable, and refinable suspension system for the future.
From a compliance perspective, in Fig. 1.17, Yeti show a rear view of the seat tube with both the current Switch Infinity system and the proposed suspension system. They claim that “5104 is horizontal distance of the linkage interface between swingarm body 2-E and link body 5-E. 5105 is horizontal distance of the linkage interface between swingarm body 5002-E and link body 5005-E. It is clear that 5104>5105. The wider interface as disclosed herein allows for a stiffer interface between swingarm body 2-E and link body 5-E which translates to a stiffer interface between swingarm body 2-E and the suspended body 1. This allows greater performance by improving the handling accuracy of the vehicle”. In short, the wider profile of the proposed system would provide more stiffness, leading to better handling.
Yeti have designed a 6 bar suspension system, with a smaller 4-bar system, with a pivot that is not concentric with a second pivot due to the solid swing arm. The result should be similar to the current Switch Infinity system with an inflection point and a reversal of the bottom pivot. The important distinction is that the linear migration path of the bottom pivot in the current Switch Infinity system is not the same as this invention.
The proposed method creates a linear migration path of the bottom pivot in the middle of the migration path and has a small amount of curvature at both ends of the migration path, at the beginning and end of travel. Yeti state that this development creates
“…higher antisquat percentage…for pedaling efficiency in the beginning of the travel while the lower anti-squat percentage minimizes the anti-squat force where bump absorption takes precedence”
Yeti are also showing an example scenario using flex links. For the cynics, Yeti are claiming that the use of the flex links may provide up to an infinite life-time.
Flexural components 4 and 3 may be made from titanium or composite such as carbon fiber, which has a high or infinite fatigue life ideal for the cyclic bending loads that will occur with these components.
We’ll start with some brief background. There are 3 different states of a Yeti bike; an extended state (E), an inflection state (I), and a compressed state (C). As with the Switch Infinity system, Yeti like to use an inflection state, where the bottom pivot of their suspension system changes direction. The extended state is a static, no-load state. The compressed state is fully hucked to flat state.
Next, let’s talk about important components. The table below shows links and pivots in the system that contribute to the new suspension system. PIVCs 43, 44, and 45 are the extra important components that we’ll talk about throughout this article.
|Swingarm 2 (not shown)||Rear triangle – connected to PIVC 45 and 46|
|Link 3||Bottom link to the bottom pivot attached to front triangle|
|Link 4||Top link to the bottom pivot attached to front triangle|
|Link 5||Middle link to the bottom pivot attached to swingarm|
|Link 6||Link body attached to swingarm and front triangle|
|PIVC 40||Front triangle and link 3 pivot|
|PIVC 41||Front triangle and link 4 pivot|
|PIVC 42||Front triangle and link 6 pivot|
|PIVC 43||Link 3 and link 5 pivot|
|PIVC 44||Link 4 and link 5 pivot|
|PIVC 45||Pivot at Center of link 5|
|PIVC 46||Swingarm and link 6 pivot|
|PIVC 47||Link 6 and shock pivot|
Figure 1.1 shows the proposed suspension system in a fully extended position. The swingarm (component 2) is attached at PIVC 45 and PIVC 46. The upper pivot of the swing arm pivot is linked at PIVC 46, pivots around PIVC 42. The swingarm is also attached to a bottom pivot system. This bottom pivot system is the novelty of this invention. It’s important to remember that PIVC 45 is the bottom pivot that is connected to the swingarm and will be performing the up-down movement.
Figure 1.11A and 1.12 shows two detailed views of the seat tube with the suspension in an extended state and a compressed state, without the swingarm. As the bike goes through its travel, the shock is compressed around PIVC 42. Again, the swingarm is pivoting about PIVC 45 and 46. Here’s where it gets tricky. There is binding in the system due to the swingarm being a solid piece and the lack of concentricity with the 2 pivots. This is solved (and was solved in the Switch Infinity) by using a movable bottom pivot (PIVC 45). Yeti have a few different configurations of this design that we’ll talk about in this section.
Figure 1.8 shows a simple point schematic of the bottom pivot system from figure 1.11A and 1.12. Again, note the E, I, and C as extended, inflection, and compressed states. As the bike goes into its travel, the swingarm PIVC 45 travels upward to an inflection point then downward during a fully compressed state. Note the two lines marked with a ‘2’ are the swingarm. The only pivot points that are not moving are PIVCs 40 and 41 (attached to the front triangle). All other points in the system move during wheel travel.
So, are you as confused as I am? I’ll try to break this down a little more. There is a concentric rotation of the shock link body 6 around PIVC 42. Consequently, because the rear swing arm has zero pivots, this rotation is not concentric around the pivot link body 5, which is constrained by links 3 and 4. This entire proposed configuration is based on a system called a Watts 4 bar system. The Watts 4 bar system constrains a center moving point in such a way that the center moving point (PIVC 45) moves in a nearly straight line, using two links (3 and 4) rotating around two different pivots (40 and 41). In the case of this invention, links 3 and 4 move point 45 nearly linear, but not quite. Figure 1.35 shows an example (possibly exaggerated) migration path of PIVC 45. As you can see, the ends of the migration path have a small amount of curvature. This curvature is the novelty of this migration path.
In another figure example of the bottom pivot system, Figure 1.31 shows PIVC 43, 44, and 45 in their extended, inflection, and compressed states. Additionally, this shows the migration path (noted with -M) of each PIVC. As you can see, the pivots are shifting up and down as the bike compresses, but not in a linear migration.
Well, that’s the geometry. Now let’s talk about how they’re implementing the concept in the real world.
The first configuration of this system is fairly simple. Aside from the typical shock linkage, there is a small device behind the seat tube, just above the bottom bracket. Figure 1.13 shows an isometric view of one of the proposed systems installed on a bike. Figure 1.14A shows an exploded view of the same system. The configuration appears to be smaller and possibly simpler/lighter than the current Switch system. There are two links, link 3 and 4, attached to the front triangle. These links are then attached to link 5. This is the Watts 4 bar system. Link 5 is then connected to the swingarm.
Another, wilder configuration for this device is one single piece that only uses two bolts attached to the front triangle. This is where the flex system comes in.
Remember figure 1.8? Well, this is the exact same configuration using flex links. PIVC 45 moves in the same migration path as the prior example, but with flexible links. Yeti say “This assembly achieves the kinematics as if bodies [3, 4, and 5] [from the Fig 1.8] were separated with pivotal joints, but with a lighter, simpler assembly with fewer parts”. Therefore, Yeti have developed another option for a lighter system, though I would be hesitant to say that this would be used on something like this on a rough enduro rig. I would assume this assembly is for their reduced travel, lightweight XC bikes. But who knows…
Another possible configuration is shown in figure 1.60. This is a combination of the first and second configurations, using an assembly of parts and flex links.
Separating the combined body into several assembled components has several advantages. First, the parts may be composed of varying materials with varying material properties, each of which may be chosen to optimize the properties such as weight, cost, stiffness, manufacturability, fatigue life, etc. For example, central body 5 may be made from aluminum, which is lightweight and easily machinable. This component does not see cyclic flexural loads as with flexural components 4 and 3.
Yeti also state that the materials and thicknesses of the flex links may vary for differing properties.
The materials of flexural bodies 3 and 4 may also differ from one another allowing for different flex properties and therefore different forces that flex the system. This may aid in the tuning the spring force the combined body has on the suspension system as discussed above. The form or thickness of flexural bodies 3 and 4 may also differ to provide varying spring forces of the system. This offers more tunability then if only one flexural body was in the system.
I’m going to take a wild guess and say this is probably the configuration that would be mass-produced. All of the parts are more serviceable and replaceable. Additionally, manufacturing of these would be easier as it appears that the spacers, bearings, and screws are standard parts. More importantly, if a flex bracket breaks, it would be very easily replaced.
Words by Mike Kazimer at Pinkbike:
When will we see this system released on a new bike? Well, according to Yeti, not any time soon – “We are constantly developing and exploring new ideas. However, not all R&D projects make it to production. We have several test mules of various suspension designs that we’ve been on for years. At the present, we are planned out through 2023 and this patent isn’t in our production line.” All the same, it’s interesting to get a glimpse at what’s being developed, and to examine the different elements that make up a new suspension system.
In the end, Yeti are really moving this Switch Infinity idea forward. I’m both extremely impressed with the idea and this patent document. It gets very deep into the background of the concept and is fairly clear as to what the design is. The combination of a 6 bar system, with a tiny 4 bar system, with flex links is really wild and I’d love to give this bike a go. The only thing I’m not happy about in this document, which is typical with most patent documents, is the advantages aren’t clearly stated. As a bike enthusiast, I’m not a fan of that. As a patent engineer, I understand the lack of explicitly stated advantages, as it’s not required nor advisable to put those in a patent document. Either way, this idea is excellent and I hope you enjoyed reading about it!
For part 2, I’ll be talking about the same idea but implemented in a slightly different fashion with a different 4 bar system.
If you think you see something wrong in this article, please email me. We can’t have misinformation out there. Even if you’re not 100% sure you’re right, I’d still like to hear from you.