Nachdem Cy im Video schon etwas langatmig über das Rocket Projekt berichtet hat, schreibt er jetzt noch sehr ausführlich über die Federung des neuen Rocket. Der Test ist mehr was für eine ruhige, besinnliche Stunde. Wer's es eilig hat liest nur bis zum zweiten Strich
Dear Carsten,
this turned into a complete epic of write up, as there's a lot of background I wanted to fill in to give you the whole 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.
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droplink:
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:
1) The suspension performance is almost identical to the Hemlock, with just a little more chain growth and pedal feedback in the small chainring.
2) It's incredibly stiff. When combined with the steel seat tube it's easily a match for the stiffest bikes available now.
3) The frame rate curve is what I wanted. The small link gives a really nice controlled progression.
4) The tyre clearance is huge because there's no need for a brace on the seatstay between the tyre and the seat tube.
5) It has stable braking performance. The forward brake mount and integral seatstay pivot/brake bolt point the forces in a helpful direction.
6) The weight is low down compared to a rocker link design.
7) The seat tube is straight with full saddle drop potential.
8) It looks great. The seatstay lines work nicely with the top tube, and the little droplink itself tucked behind and under the seatstay means it is very neat.
So that's what we achieved, and this is what it looks like. 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.
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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 sentance 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.
Let's talk about the Rocket and it's droplink suspension, eh? Why no chainstay pivot? This is clearly the first thing to get past: Why after running with a chainstay pivot on the Hemlock, did I 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 as a nod to the new 2 and 3 x 10 drivetrains), with only a little more chain growth/pedal kickback.
Frame Rate:
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 it's 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 diliberation, I settled
on a progression rate of nearly 20% for the new bike.
Stiffness:
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 it's 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.
Braking:
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:
1) Maintain the suspension performance parameters of the Hemlock in terms of anti-squat, chain growth and pedal feedback.
2) Stiffness - it had to anchor to the seat tube, have room for the 15mm pivot and be as short as possible.
3) Had to achieve the frame rate I wanted.
4) Big tyre clearance.
5) Stable braking performance.
6) Ideally keep the weight as low as possible.
7) Allow a straight, uninterrupted seat tube.
8) Clean lines. Slightly controversial this one, but Cotic's hardtails are known for their crisp, clean, uncluttered design and we felt the new bike should follow this as far as possible whilst still achieving the technical goals.
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droplink:
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:
1) The suspension performance is almost identical to the Hemlock, with just a little
more chain growth and pedal feedback in the small chainring.
2) It's incredibly stiff. When combined with the steel seat tube it's easily a match
for the stiffest bikes available now.
3) The frame rate curve is what I wanted. The small link gives a really nice controlled progression.
4) The tyre clearance is huge because there's no need for a brace on the seatstay between the tyre and seat tube.
5) It has stable braking performance. The forward brake mount and integral seatstay pivot/brake bolt point the forces in a helpful direction.
6) The weight is low down compared to a rocker link design.
7) The seat tube is straight with full saddle drop potential.
8) Finally, it looked really really neat. The seatstay lines work nicely with the top tube, and the little kicker link tucked behind and under the seatstay meant it is very neat.
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.
The ride:
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. The added bonus over the old bike is that the progressive frame rate holds the rider up much more firmly when riding up steep climbs, the Rocket keeping it's shape much better in this situation where you'd sometimes find yourself sagging backwards on the Hemlock.
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.
Nearly there:
Finally for the conspiracy theorists out there who spotted on the video that my BOS shock was mounted with the main body at the seatstay, there's nothing special about this. It's simply that BOS didn't have the spacer kits to fit our bike when the shock become available so we knocked some up and they fit better with the shock in the bike that way around. Nothing more than that, and I'll probably put it the other way when I get some proper ones from BOS.
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 Rocket 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 the Hemlock went into this bike. I do feel it's still very much a development of the old design, albeit a revolutionary rather than evolutionary development.
Cheers,
Cy
Dürfte ein großartiges Enduro mit Reserve nach oben werden. 32cm Tretlagerhöhe...lecker, lecker, lecker.
