There's a lot to get through here as there's a lot of background I wanted to get across in order provide the complete story. However, I'm well aware that this level of detail can put some people off so I'm going to do this a little back to front and put the summary first.
After an awful lot of design concepts, layouts and iterations in my suspension software, the droplink layout began to form into something we could really work with. It achieved all of the goals we set:
So that's what we achieved. How we got there is covered in the essay below. If you're easily bored, just want the bike to work, or haven't got a few mins to spare it's time to pull the rip cord. As you're reading the Geek page of the website, I'm guessing you're wanting more. Here goes....
WARNING! This section will involve words like anti-squat and chain growth
Still with me? Great! Time for some detail then. There's been quite a lot of talk about the new suspension system on the Rocket as we've given it a name; droplink. So let's start with the sentence that would have the marketing men fainting in the aisles; there's nothing revolutionary about droplink. It's a manipulated macpherson strut design to give it the proper title, or linkage driven single pivot if you like. There are some very good reasons we've adopted it, and it's got some great performance characteristics and I'll explain those below, but I'm not claiming it's some ground breaking new design. We've given it a name this time because this suspension will become our new 'platform', and we have plans for several other models based upon it. More of which much, much later. for now, let's talk about the Rocket and its droplink suspension.
Why no chainstay pivot?
This is clearly the first thing to get past: Why after running with a chainstay pivot on the Hemlock (our original FS frame design), did we move away from that for droplink? As I mentioned in the video, after trying out a bunch of different bikes I was more convinced than ever that the balance of anti-squat, chain growth and pedal feedback I designed into the Hemlock was what I liked and preferred. I did want to improve the feel of the bike with a different progression rate on the frame, but the building blocks were right. The key thing to understand about this is that the suspension performance of the Hemlock - in terms of anti-squat, chain growth and pedal kickback - isn't very classicly "4-bar" in terms of feel.
Most chainstay pivot bikes I've tried (Lapierre being the honourable exception due to the really low chainstay pivot) work a lot like a Specialized. Very little chain growth and very little anti-squat. This gives a very neutral suspension feel, but with no chain interaction it's hard to work with it using the pedal input and to my mind always feels mushy and needs a lot of managing by the shock. This is what people mean when they say "4-bar feel". The thing about 4-bar links is that you can do virtually anything you like with them in terms of feel, anti-squat and performance because the instant centre isn't fixed. I used this flexibility in approach to make the Hemlock different. I designed it to be fairly neutral in the middle and big chainrings, but with some 'dig in' i.e. some pedal feedback, a reasonable amount of anti-squat and some chain growth, in the small chainring. This is because I use the little chainring a lot. So does Paul, and Ian does too (but he'll never admit to it because he's a guide). We're the main guys steering the direction of the bike development, and we like trying to ride up steep, technical climbs and seeing how far we can get. It's a hangover from a youth well mis-spent trials riding on street furniture. We're also twiddlers rather than gear mashers, and don't subscribe at all to the 'walking is quicker than the small chainring' school of thought. If we're out riding our bikes, that's what we'd rather be doing. Not pushing or walking. So, getting down off my soapbox now having presented my manifesto, this is what did and still does inform my choices when designing a suspension system.
Moving into the new bike design, I was interested in whether it would be possible to achieve the suspension characteristics I preferred without using the chainstay pivot. There were other factors driving this too (see below), but the key thing was this desire to create some suspension interaction with the small chainring. With a bit of research and design iteration I found that a single pivot could be placed in a sensible position on the seat tube and give me the anti-squat characteristics very similar to the Hemlock (tweaked slightly to give the equivalent anti-squat with a 24t small chainring rather than 22t to match 2 x 10 drivetrains), with only a little more chain growth/pedal kickback.
Frame rate is the amount of leverage ratio change you get intrinsically from the mechanical suspension layout. Nothing to do with shocks or damping, just how the forces into the shock change as the suspension moves through its travel. The Hemlock was designed in 2006. Back then ProPedal hadn't been invented and air shocks were still quite progressive (i.e. got harder the further into their stroke you go), so the bike was designed to have just a small amount of progression on the frame rate so the shock progression wouldn't get amplified and cause any odd effects. This was also informed by my training as a rail vehicle dynamicist. I'd been taught to go neutral on the mechanical set up and tune using the springs and dampers. The Hemlock frame progression was around 3% in 120mm mode and 8% in 150mm.
The rate of progress in shock performance and technology in bicycles over the last 5 years is astonishing. What has happened now is that even air shocks are extremely linear in their spring rates, and even a little 'hammocky' in the mid-stoke, which makes the spring rate curve look like a smile i.e. it's a little softer in the middle of the range than the start and ends of the stroke. This change made the Hemlock incredibly effective at keeping its wheels on the ground, but I did feel that some of the fun from my hardtail was missing; that 'pop', the ability to get the bike off the ground and improvise a little. For the new suspension I wanted a much more progressive frame rate to make the frame push back against the shock a little, not absorb ride inputs quite so much. After much research and deliberation, I settled on a progression rate of nearly 20% for the new bike.
Another thing to consider when you have a bike with a chainstay pivot is how to make the back end stiff. It's not that easy when you have a degree of freedom across the chain drive, and by the end of its life the Hemlock had gained a fair amount of weight around the chainstays to increase the stiffness of the back end. However, the progression of riding and tyre technology meant that even this improvement wasn't enough to make the Hemlock feel as together as I'd like when really pushing on. Part of this was also to do with the long linkage (particularly in 150mm mode) coupled with the 8mm pivot axles. I found the back wheel has a tendency to 'stand up' on me when cornering hard or changing direction quickly. This was definitely something the new bike needed to address.
There's no doubt that a swingarm will be stiffer for a given weight than a chainstay pivot system. Once I'd found I could achieve the suspension performance I wanted from a single swingarm, the stiffness improvement was significant. Combined with the Syntace X-12 thru axle system, the very short link from the seatstay to the seat tube, and both seat tube pivots being 15mm, the droplink back end is incredibly well tied down.
Let's lay the 'isolates braking' myth to bed. A 4-bar linkage in and of itself does not intrinsically neutralise braking force inputs simply by having a pivot on the chainstay. This myth has grown out of the Specialized marketing machine, because IN THE PARTICULAR CONFIGURATION SPECIALIZED USE the braking inputs are managed very effectively. This isn't true of all 4-bars. In fact, it emphatically wasn't true of the Hemlock. The early prototypes suffered horrendously from brake jack, and the key to neutralising this was caliper position rather than anything to do with pivot positions. By moving the caliper as far forward and above the rotor as possible, the brake force on the Hemlock pointed at the instant centre so the braking stabilised. When the caliper was just on the back of the seatstay on the early prototypes the force interacted with the bike in a very odd way. And it's not just the Hemlock. The most significant brake interaction I've ever felt on a bike was on an Ellsworth Moment, which has a chainstay pivot. What I found from my work with the Hemlock and some other designs is that caliper position is key. The droplink suspension has a very forward position for the caliper (in fact I integrated the seatstay pivot axle and the top brake mount into one bolt to maximise this) which gives very neutral and stable braking responses.
The Early Designs:
During the initial layouts I was working with a rocker link design combined with the swingarm/seatstay pivot locations I'd settled on. The key thing was an oversized 15mm rocket pivot to match the main pivot and a much shorter linkage for several reasons. Firstly, it would be lighter and easer to make stiffer than the long Hemlock design, and secondly it would look better! However, after several iterations there were serious problems trying to get decent tyre clearance on the seatstay, my ideal brake mount position was troublesome for the rest of the layout and I was finding it virtually impossible to achieve the frame rate I wanted. So it was time to look at other options. The key specifications were:
And now we're back where we started. After an awful lot of design concepts, layouts and iterations in my suspension software, the droplink layout began to form into something resembling the design we finished with. It achieved all of the goals we set:
The added bonus from driving the shock directly from the seatstay is that the droplink itself is only needed to tie the suspension to the front triangle and tweak the frame rate, not to drive the shock. This makes the loads into the frame from the link very low. Much, much lower than the loads in a rocker link suspension like the Hemlock where the rocker drives the shock. This made the long term durability of the frame is easier to achieve.
Moving to the larger wheelsizes for the 27.5" and 29" wheels projects brought another packaging issue - a curved seat tube. On the Rocket26 the droplink mounted to a bracket in front of the seat tube. The curved seat tube moved this a long way from the pivots on the seatstay, and prototyping on the Rocket29 project showed the longer links allowed too much flex. Going back to shorter links meant repackaging this area, but one thing about the curved seat tube is that we no longer had to worry about the seatpost dropping past the link, so the new Rocket275 uses a new thru axle layout, with the bearings and housing moved to the centre of the seat tube, and the stout 15mm axle we have always used now clamped to the droplinks similar to a pinch bolt style fork. This minimises the size of the parts and packaging and always the short, stiff links that are even more important as wheels get bigger and frame members get longer.
So what does this engineering mumbo jumbo all mean in practice? The result is in the riding, and the progressive frame rate achieved everything that I wanted. It gives a fantastically fun feel to the bike which completely belies it's 150mm of travel. It's interactive, predictable, has that 'pop' to get you up and over things rather than ploughing through and gives much more support in the mid-stroke so the bike feels poised and ready for action.
When climbing, particularly in the small chainring on steep climbs, the suspension has that familiar high traction feel, with the rider being able to feel each edge and bump subtly through the pedals and adjust power to suit.
During the later project moving from Rocket26, we had to consider 1x drivetrains. Luckily there was very little needed doing, as the smaller chainrings that are required for 1x drivetrain use (most people use 28t-32t rings in our experience) mean that the anti-squat characteristics still work well with these drivetrains, and the 42t sprocket increases the torque effects as well which is a good thing when feeling for traction.The stiffness of the back end has to be experienced. It gives so much confidence to the handling of the bike, with the rider really being able to hang low on the pedals and push the bike through turns from the cranks. It also pops through flick-flick direction changes with great alacrity, whilst big stops prove to be no problem at all with the lack of brake induced movement from the back end of the bike.
In other words, I love it and I think you will too.
If you've stuck with me this far, thanks! I hope it was worth it. I think the background on where we came from on this project was important to get across, because although the Rocket26 and droplink suspension share very little with their immediate forebears in terms of looks and mechanical layout, even material use, a lot of what we liked and what we learnt with our earlier bikes went into this one. It has simply been further refined and adapted to the larger wheels as we have moved along, keeping everything we liked about the first Rocket and bringing it to the 27.5" wheel version.