In this article, we’ll be discussing a Bicycle Suspension Components by SRAM, US publication 20210394860. The publication date is Dec. 23rd, 2021 and the filing date is June 17th, 2020. This patent is not granted, yet.
12/30/21 edit: These are the Buttercups from the new Flight Attendant system. I missed that press release a few months ago. There isn’t much out there on these things so hopefully you have a better picture of how they work. Still can’t win them all…
Brief Summary (tl;dr)
SRAM are developing a new little shock mount for their forks to help with high frequency, low amplitude vibrations. It’s called an ‘isolator’, and it’ll probably have some silly marketing term when it comes out. I still haven’t gotten over Super Deluxe Select Plus. This little isolator sits at the bottom of each fork shaft and decouples a direct connection of the shaft from the fork lower. The end of the shaft sits between two rubber disks, which are housed in a cup/cap system. The cup/cap is then bolted to the fork lower. As a result, the fork can absorb really small vibrations more effectively because the static friction of the seals won’t need to be overcome during those small vibrations.
Today, we’re talking about stiction, high-frequency vibrations, and small bump compliance; all the things we’re reading about in articles, blogs, and forums that suspension manufacturers are tackling at the moment.
Stiction is effectively the force needed to get a fork/shock to start moving, from rest, and affects small bump compliance, especially high frequency, low amplitude vibrations. In short, the fork needs to overcome a small amount of static friction to begin traveling or changing direction. This isn’t unique to mountain biking, as literally any suspension component can have this issue, though I believe it’s more pronounced in air suspension due in part to larger diameter tubes. I wish I could show you a graph, but I can’t seem to find anything on the internet, even though I know I’ve seen it somewhere. If you have a graph with the little fucked up part at the beginning, I’d be happy to throw it in this article.
As bike development has progressed, this stiction issue has improved dramatically. For example, my wife has a 2020 Lyrik and I have a 2021 Zeb and I can, very clearly, tell a difference between the two forks. My Zeb is noticeably easier to get moving from rest, when the bike is standing still. Though, if I’m honest, I don’t notice when I’m riding – or I don’t think I notice. Engineers are seeing this stiction data in testing. But, it looks like SRAM want to further improve small bump compliance to keep them hands and arms from burning out on chatter.
This isn’t granted, but it appears the current intended novelty, for now, is the use of an isolator that couples the shaft to the fork lower, where the isolator includes an elastomeric material to absorb vibrations. I’m sure this will change at a later date. We’ll get more into this isolator in a minute.
SRAM are doing this to circumvent stiction, not avoid it. Pistons and seals cause friction, which must be overcome to begin the travel process. In particular, high frequency (above 5htz), low amplitude (magnitude of the frequency) vibrations may not be absorbed by the fork. As a result, the forces are more pronounced in the frame and then the rider.
…every time the direction of movement changes (e.g., expansion to compression), this static friction needs to be overcome. As such, there is a slight delay while the force builds up before the upper and lower leg portions begin to move. This results in a stick slip feel that can be felt by the rider at the handlebars. Further, high frequency (e.g., frequencies above 5 hertz (Hz)), lower amplitude vibrations, such as those caused by a washboard terrain, are typically not absorbed by the damper and spring.
There’s a little tidbit for you other manufacturers. This line suggests SRAM’s threshold for high frequency is 5 Hz, meaning these frequencies may be a definitive frequency for them. Ultimately, this design is to:
…improve shock absorption and absorb high frequency vibrations…
FIGs. 2 and 4 shows a picture of what the fork may look like. Ignore the straight steerer; I think that’s just for simplicities sake. The isolators are 370 and 380.
FIG. 3 shows the insidy parts of the fork. We’re focusing on the little circles 390 and 395 at the bottom of the fork. I’m unsure if any other part of this fork is new, so I’m not going to focus on anything else. If you spot something, shoot a comment at the bottom. Those little circles are showing the isolators, which include the rubber cushioning members. The cushioning members include a pair of ‘elastomeric members’, meaning they’re just rubber. But, SRAM also say they can be springs, though they’ll probably be some kind of rubber based on the images and claims. They say the cushioning members:
…enable relative movement of the upper and lower leg portions without having to overcome the friction in the seals of the damper and spring components.
So, these little isolators effectively allow the fork to move over high frequency, low amplitude bumps without having to overcome the friction of the seals, as is typical in a fork. They’re doing this by:
…enable[ing] the lower leg portions (which are attached to the wheel) to flutter or vibrate independent of the upper leg portions, thereby reducing vibrations that are felt by the rider.
Typically, the shaft couples directly to the bottom of the fork lowers (shown below). As the bike goes over terrain, the fork lowers compress relative to the fork uppers, and you have travel. I won’t go much deeper into the composition of a fork, but all you need to know is (with this new design) the shafts 324 and 336 couple to the isolator, rather than the bottom of the fork lower. The isolator then couples to the bottom of the fork lower. As a result, there isn’t a direct connection between the shafts and the fork lower. That’s the entire idea, in a nutshell.
FIG. 5 shows an exploded view of the isolator. Here’s how this thing works. The shaft will thread onto the translating coupler 508. Notice the plate 532 on the translating coupler; that thing is sandwiched between two rubber cushioning members. Then, the cap 500 threads into the cup 512, which houses the translating coupler and two rubber cushioning members. Lastly, the cup is then bolted to the fork lowers.
FIG. 6a shows an example of one of the cushioning members. There isn’t much to talk about here other than its rubber and has an optimized shape to allow it to accept the plate and fit inside the cap/cup.
There’s another little special seasoning they put in this design. It’s probably not advisable for the rubber and translating coupler to rotate, so they’ve come up with a solution. The pin 514 threads into the cup and travels through holes in both pieces of rubber and the translating coupler. This prevents the system from rotating inside the cap/cup. S M A R T
FIGs. 7 shows a close-up of the isolator on the damper-side. The translating coupler with the plate sits between the two cushioning members. Everything is housed in the cap/cup and is bolted to the lower. The pin then keeps everything in line. The air-side will look exactly the same.
FIG. 9 shows an alternative design, where the cushioning members are on either side of the lower. This one is probably a little more of pain in the ass to assembly.
This is a really cool idea – yet another one from the big-brains at SRAM. In the end, this is a shock bushing, which are used in plenty of other industries. As far as the design goes; it looks solid. And I know you’re thinking it, but the weight penalty wouldn’t bother me. The weight weenies won’t like it, but it is what it is. Everything is a compromise. That being said, this is something I’d need to try out before I’d be able to develop an opinion. I just don’t have enough issues with my Zeb where this is something I feel like I’d absolutely need, but I don’t know what I need.
The small bump and friction improvement on the Zeb is noticeable compared to my Lyrik. Was the Lyrik bad enough to justify a Zeb? No, they’re both great. But I got one anyway because needed to have the hottest bike at the park, and they know that. I don’t ride throwback whips.
Could uneven degradation of the elastomers from side to side lead to higher stiction from the slight extra tilting of the hub/axle as one coupler moves more than the other?
Check out the Flight Attendant maintenance on the SRAM website. They show these in great detail and they are calling them Buttercups.