Words by Dan B. at Wheelbased
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.
Brief Summary (tl;dr)
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 has a non-constant radius of curvature, unlike the linear system where the radius of curvature is zero and unlike a circular motion where the curvature is constant. Yeti claim this may improve anti-squat and anti-rise numbers. More importantly, 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.
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:
In other words, the seven 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, or else this article will be 5 times longer.
The number of IVC’s of a suspension system can be calculated using the equation:
is the number of total number of IC’s and 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. These IC’s are what controls the feel of our bikes.
The intended novelty of this part of the invention is the same as the first part. The bottom pivot has a non-linear migration path, unlike the current Switch Infinity system. The complete Yeti system is a 6-bar system, but the Chebyshev 4-bar system
of this invention contains non-opposing links driving a pivot in a non-linear 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. 2.16, Yeti show a rear view of the seat tube with both the current Switch Infinity system and the proposed suspension system. They claim that…6104 is horizontal distance of the linkage interface between swingarm body 1002-E and link body 1005-E. 5105 is horizontal distance of the linkage interface between swingarm body 5002-E and link body 5005-E. It is clear 6104>5105. The wider interface as disclosed herein allows for a stiffer interface between swingarm body 1002-E and link body 1005-E which translates to a stiffer interface between swingarm body 1002-E and the suspended body 2. 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. Note in figure 2.16 how much wider and shorter this new system is compared to the current Switch Infinity system.
Yeti are introducing another design for the same idea of having a non-linear migration path of the bottom pivot. Again, this is a 6-bar system with a pivot that is not concentric with the shock link pivot due to the solid swingarm. 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. Yeti aren’t 100% clear as to what advantages this design has over the first example.
Yeti state that this design is similar to the design from the first article, but not exactly the same. Yeti say:…embodiment 2 [example 2] exhibits similar anti-squat, anti-rise, and leverage rate properties, although not identical. The higher anti-squat percentage is 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. Thus, suspension performance may be improved through the interrelationship between the 15 IVC migration paths.
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.
Figure 2.1 shows the proposed suspension system in a fully extended position. The swingarm (component 1002) is attached at PIVC 1045 and PIVC 1046. The upper pivot of the swing arm pivot is linked at PIVC 1046 and 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 1045 is the bottom pivot that is connected to the swingarm and will be performing the up-down movement.
Figure 2.11 and 2.12 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 1042. Again, the swingarm is pivoting about PIVC 1045 and 1046. 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 1045). Yeti have a few different configurations of this design that we’ll talk about in this section.
Figure 2.8 shows a simple point schematic of the bottom pivot system from Figures 2.11 and 2.12. Again, note the E, I, and C as extended, inflection, and compressed states. As the bike goes into its travel, the swingarm PIVC 1045 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 1040 and 1041 (attached to the front triangle). All other points in the system move during wheel travel.
There is a concentric rotation of the shock link body 1006 around PIVC 1042. Consequently, because the rear swing arm has zero pivots, this rotation is not concentric around the pivot link body 1005, which is constrained by links 1003 and 1004. This entire proposed configuration is based on a system called a Chebyshev 4-bar system
. The Chebyshev 4-bar system constrains a point (1045) with two non-opposing links (1003 and 1004) in such a way that the point (PIVC 1045) moves in a nearly straight line, using the two links (1003 and 1004) rotating around two different pivots (1040 and 1041). In the case of this invention, links 1003 and 1004 move point 1045 nearly linearly, but not quite. Figure 2.26 shows an example migration path of PIVC 1045 through the bike’s travel. As you can see, the migration path is not quite perfectly linear.
Yeti describe the migration path of PIVC 1045:The radius of curvature is not constant and varies throughout the entire migration path. This is not true with linear motion [old system] where the curvature is 0, or with circular motion where the curvature is constant [other manufacturer system].
Well, that’s the geometry. Now let’s talk about how they’re implementing the concept on the bikes.
Aside from the typical shock linkage, there is a small device behind the seat tube, just above the bottom bracket. Figure 2.15 shows an isometric view of one of the proposed systems installed on a bike compared to the current Switch Infinity system. Note how much smaller, wider, and simpler this new system is.
Figure 2.14 shows an exploded view of the same system. As you can see, the configuration appears to be smaller and possibly simpler/lighter than the current Switch system. There are two links, 1003 and 1004, attached to the front triangle. These links are then attached to link 1005. This is the Chebyshev 4-bar system.
Yeti don’t state any more examples of this system, such as flexible brackets or anything of the sort. It appears as though this is a simpler system compared to the Watts system.
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.