What's Going On With Commencal's Prototype DH Race Bike?

Yesterday, Commencal pulled back the curtain, or should I say removed the blanket, on their new DH race bike, a prototype in the purest sense of the word.

This prototype looks to be allowing Commencal to test out their ideas for the future of their DH bikes at the pointiest end of the sport. If I had Amaury Pierron and Angel Suarez, to name just two of the racers aboard it, available to give my ideas as much of a testing as possible, you wouldn't have to ask me twice.

While there is a lot going on, and we'll get to some of those interesting changes, there are a lot of similarities with the current production bike. Commencal says that they've used lots of the same tubing to make this bike, which makes sense. Tubing moulds can be expensive and with current long lead times for everything down to washers and bolts it's good to use what you already have. And they do have quite the portfolio of bikes to pick and choose frame parts from, which is likely why some of the forged parts actually look quite finished. Maybe that or someone was burning the midnight oil when doing the 3D for this bike.



What's New?
Where we do start to see the changes is in the big CNC parts that go into making the suspension system. And that suspension system is quite the talking point of the new bike.

The production Supreme uses a high single pivot design, with two links used to actuate the shock. The shock is attached to the downtube at about 45°. The idler pivot is connected to the swingarm, meaning that its position, and influence in the suspension, is going to change as the bike goes through its travel.

This new bike, however, moves two steps away from a single pivot design and is a six-bar suspension system, closest to a Stephenson I linkage system. Some other notable six-bar designs include the Atherton bikes and the much mentioned Felt Equilink design, whose name is being thrown around in the comments and forums, as this new Commencal sits closest to this layout.


Easily recognisable in the system once we overlay the kinematic are the seat stay (dark green), chainstay (yellow) and rocker (light green). Where things get interesting though is in the addition of the link between the chainstay and mainframe (blue) and the dog bone link (purple) between this aforementioned chainstay link and the rocker.

One other point to note is that it looks like the shock is connected to the mainframe. There has been some speculation that it might be floating, and connected to that lower chainstay link. But that link rotates clockwise at quite a rate of knots, so connecting the shock to it would mean that the lower shock mount would too migrate around quite quickly, requiring more compression of the shock from the rocker link to account for this lower shock mount movement. It does however have a flip chip on that lower shock mount to move the mount forwards and backwards, adjusting the progression of the system.


As the rear wheel sees an impact, the whole seatstay part is going to begin moving up. This is going to push on the rocker and start it rotating while simultaneously pulling on the chainstay. This chainstay pull is going to start to rotate that small chainstay link. This then means that the purple link, connected to both the rocker and chainstay link, is going to be pulled up by the rocker but also pushed up by the chainstay link.

This purple link is, like all the other links really, a key link in the system. Remove it and the rear wheel would move around freely without ever rotating the rocker or compressing the shock, all the while still technically connected to the mainframe of the bike.

It's also an adjustable link. When we see flip chips in a bike to adjust the geometry and suspension, all we're doing is changing the geometry of the link that the flip chip sits in. For the purple link, that geometry change is the change of its length. And with it the whole bike changes its geometry, and so too the suspension. Extending the purple link pushes the rear wheel downwards, steepening the head angle, raising the BB, lengthening the reach and shortening the chainstay. Shortening the purple link has the opposite effect.

That purple link has two mount points too on the rocker. Seeing as the link is floating, and not fixed to the mainframe, then changing its mount point on the rocker is going to adjust how much rotation and at what point for the rocker and chainstay link as the rear wheel goes through its travel.



This new Commencal also changes the idler position too. A large plate, bolted to the mainframe, now holds the idler, meaning that its position doesn't change through travel like it did with the old bike. That means the interesting span of the chain to take into account when analysing the anti-squat is just from the idler to rear wheel. The span between the bottom bracket and idler has no effect, given that the two points are connected to the same frame member. The idler position is also much lower than the production bike, closer to the chainring.

This idler mounting change likely came about as a result of the other, more important, drivers of the frame. The seat stay has moved up quite a bit, and there are no other frame members in the vicinity that are favourable to connect the idler to. Idler stability is a big point - lots of bikes with single shear bolts on the idlers tend to suffer from problems. Lots of the designs with double shear, capturing the idler from both sides, fare a lot better under the high chain loads, especially if you're one of the racers sprinting with gritted teeth mid race run.


Why?
Well, this is the trickiest but also the most entertaining part. Speculating, or playing Sherlock Holmes, as to why Commencal has gone down this route is always good fun.

Firstly, they wanted to create an instant centre that they could manipulate. On a single pivot bike the instant centre is the main pivot, so it's a physical fixed point in space. On the production bikes the idler is fixed to the swingarm giving some freedom in designing the anti-squat characteristics of the bike. But things like the anti-rise were simply a by-product of the main pivot placement. That gives the production bike very high anti-rise figures and there isn't much Commencal can do about it.

Making the instant centre a virtual point gives them much more freedom in deciding its location in space and so, the anti-squat and anti-rise characteristics of the bike. That freedom, along with the construction of the bike, may have led them down the path to have the idler mounted on the mainframe.

The design still retains a high pivot, with it being way up there for the majority of travel. It does migrate downwards throughout travel, almost following the line of the shock, before dropping really low at the very end of travel. That does give the axle path more of a vertical trend, if you will. While it might never travel forwards of its starting point, it does begin to travel back towards it from about half way on.

Perhaps this was intentional, as one notable point of the truly high pivot bikes, like the production Supreme, is that completely rearward axle path. While it does mean that the axle path allows the wheel to move with a little more ease with the vectors of the oncoming impacts at the rear wheel, in the rebound stroke it actually moves pretty damn quickly back at them at a very aggressive angle. This might go towards explaining why the true high pivot bikes need a lot of rebound damping, especially high-speed, to control the fast returning wheel smacking into the oncoming impacts. It's often magnified too, with these bikes requiring a lot of spring to handle the bikes active tendencies, exacerbating the amount of spring force pushing back when deep in the travel.


While I can try and draw out the kinematic from a side on photo, the sensitivity of kinematics to small changes means that the exact pinpoint numbers on the suspension are very hard to nail down, especially with this short link design. But the general theme is there, and that's something we can report on. As this new bike goes through its travel, the instant centre drops down, slowly at first but then speeds up towards the end of travel. This slow change to quick end stroke drop off is a common trait of short link bikes, as the short links rotate around quite an angle. And with the six-bar design, that lower chainstay link really begins to accelerate as the bike nears the end of its travel. At the end of travel the instant centre might actually be getting pretty close to the ground.

It's also possible to see that this design yields a progressive leverage ratio. The movement of the wheel relative to the shock is quite a lot at the beginning of the travel, meaning a lot of wheel movement for a small shock movement. The two begin to get closer to 1:1 as the bike goes deeper into the travel.

Now why Commencal chose to use six-links to make their instant centre a virtual point instead of using four, much like the GT Fury and DH/enduro bike that we've spotted recently, is a little harder to speculate on. With the added freedom that the increased number of pivots and links bring, they also up the complexity, weight and potential for problems. One question that pops up is that with designs that are sensitive to small changes in pivot placement, how much can Commencal guarantee that what they're riding is matching up with what they designed? Aluminum construction can yield bigger tolerances in angles, lengths and pivot placement. But perhaps they're using some validation process, like 3D scanning, to understand exactly the bike that is being ridden.

Perhaps that added freedom is exactly what they are after and gives them the tools to explore all the ideas that they have for DH racing, and perhaps for their next generations of all their bikes. There is also a fair bit of adjustability in this bike, so Commencal has even more tools to test out a lot of different bike setups in one bike and with very short spaces in between runs to make the changes. With the added freedom of all the instant centres that a six-bar design gives that gives Commencal a lot to play with and learn from, perhaps feeding that into their later production bikes with suspension systems that are a touch simpler.


There's also noticeable differences between Amaury's and Angel's bikes, with the rockers being very different and Angle only seeming to have one mount point for the dog bone link, whereas Amaury has two. The idler position is also very different in relation to the seat stay tube, Angel's is very close indicating that it's higher and further back. While Amaury's is lower down and further forward. That freedom in design of the suspension system that the six-bars are offering is perhaps giving Commencal the tools to fine tune and adjust to suit the rider's preferences and needs.

The beauty of this bike is that Commencal can do exactly what they want to, test ideas out at the highest level. As some of you have questioned, isn't there a UCI rule against this? Well, prompted by that very question a while back, our own James Smurthwaite researched to see if he could find this rule that means a brand has to eventually be selling what they are racing on. No such rule could be found and it seems to have been uttered by a brand owner once and the rumour started spreading.

But this is all just speculation from a bunch of photos. We'll likely see a lot more of the bike from more angles in the coming days now that the veil has been lifted, along with seeing if the fairly radical changes that Commencal have made add up to a winning combination. DH race season is on and I am as excited as a kid at Christmas.

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Reviews and Tech Spotted Behind The Numbers DH Bikes Commencal Commencal Supreme Dan Roberts