Prototype Saracen Carbon Myst - Val di Sole World Cup DH
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The common belief is that it makes much more sense to pursue weight loss and strength gains by going with a carbon fiber front triangle than it does to use the same material at the back of the bike. Even today, a handful of companies offer extremely high-end carbon bikes that feature aluminum swing arms, or at least chain stays, citing the minimal grams that can be shaved versus the presumed high relative costs involved. That didn't stop Saracen from doing things backwards, though, with the company debuting a carbon swing arm for their 203mm travel Myst DH bike in 2012 that they claimed to be a whopping 400 grams lighter than the aluminum version. That old alloy swing arm was either a boat anchor, which is entirely possible, or the carbon version is extremely light. It's likely that the weight loss came from a combination of both.
If Saracen was able to drop nearly a pound from just the Myst's swing arm a few years back, how much have they been able to lose by going to the full carbon fiber front triangle that the team will be aboard at this weekend's World Cup DH race in Val di Sole, Italy? My guess is that it's a hell of a lot more than a pound, especially considering that the original aluminum Myst wasn't a lightweight when compared to the latest carbon downhill race bikes on the circuit these days. Saracen is a relatively small outfit and probably wouldn't design a brand new carbon frame unless they could say it's significantly lighter and stronger than the aluminum model.
The production Myst's aluminum linkage.
The prototype bike's carbon fiber linkage.
While Saracen dabbled with carbon by using the material for the Myst's swing arm back in 2012, they've done a cannonball into the pool in 2015 by going as far as employing carbon for the bike's linkage. Both the swing link and the driving link are made from the stuff, although it's hard to tell if there have been any changes when it comes pivot locations that might alter the bike's travel, leverage rate, or geometry from what the team had been using for the aluminum bike.
As you'd expect, the new carbon frame is much sleeker than its aluminum predecessor, with Saracen doing away with the old bike's forward shock mount that connected the top and down tubes. The shock now simply bolts onto an extension formed out of the down tube. It also looks like Saracen have stuck with the oversized head tube that we spotted earlier this year on Matt Simmonds' aluminum race bike. The design allows riders to tinker with both head angle and top tube lengths by fitting different offset headset cups, which in itself isn't a new idea, but the keyed interface on the Myst does ensure perfect alignment and zero guessing.
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Photos by Dave Trumpore
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Props on the threaded BB.
I am far from an expert considering the production techniques, i'm more of a total newb (as far as a mechanical engineer interested in bike design can be a newb in this regard), but still. There have to be some differences, otherwise people wouldn't invest 50+k USD/EUR/etc. in steel tooling for carbon frames. They'd just make a cheap mold for polistyrene frame cores and lay the frames up like that, it's all hand work either way.
If you've ever done fibreglass work, carbon fiber isn't terribly different at the one-off scale. I think the reason most test mules are aluminum though is that welding up some tubes is less time intensive than hand molding a carbon frame, & you can actually modify the aluminum model, like shortening the top tube, where that's a whole new mule for a carbon process.
@hankbrowne: Glad to be of help
Wasn't aware of that procedure. Interesting. Thanks
As for using styrofoam cores for mass production, you could make (an aluminium) mold, inject/expand polystyrene quickly and easily in it and use those for the mass produced layup. And then disolve the cores via the above written process. As i said, it's hand work in both cases, but the outer steel old process in the autoclave MUST be yielding better results, otherwise people wouldn't spend a lot more money and time for making these complex outer molds.
Also, why exactly was my previous comment downvoted? o.O
Sure, the outside will be all wonky, but it's function over form. And you usually have some clearance around the pivots and the tires anyway, so a slightly thicker frame in those areas is not much of a problem.
In the end of it, i'd do it that way really for a one off or maybe if a special geometry was needed to be tested and just to prove a concept. Mass production should be done in a more controlled way.
If i'm imagining things right, in the case of bladders the cloth is laid up onto the bladders (or into the mold), which is closed, the bladers are further inflated (well, the heating in the autoclave should be enough to increase the internal pressure) and the mold is autoclaved. That means the outside dimensions don't change, but the inside dimensions of the tubes enlarge. That should have a straightening effect on any wrinkles in the layup.
The foam core and vacuuming over it should give the opposite effect, bunching up any wrinkles even more, since you're squeezing the layers down to a smaller diameter.
Process control therefore should be better in a proper outer mold with an autoclave compared to the foam core manufacturing process.
As for stifness, there are ALWAYS variations in tube dimensions and thicknesses, even when working with aluminium, which will give you some differences between mass produced frames (same frames mind you), which i suppose should be in line with the stiffness and strength variations of a one-off foam core produced carbon frame. That and difference bearing clearances in full suspension bikes, when looking at the rear end stifnesses, etc.
I think we (I at least) should stop complicating the parts with little to no impact to the final product
EDIT: seriously, WTF, why is my first comment in this thread downvoted?!?!
With a single pivot frame that's not a faux-bar (like a Kona) they might be seeing some big stiffness benefits by going carbon in the rear as well.
When you have all these values, you can calculate how unwilling the suspension system is to movement. At a certain acceleration (of the system, when it starts compressing for instance), you'll get X amount of needed force at the rear axle. Compared to the F = m*a equation, where m would be the complete system weight, X will be less than F.
Hope i made it at least somewhat clear. If not, ask away!
*pneu
Oh... wrong language.
1+ pounds is pretty standard (450g+)
Hmm... the Yeti ASR-5 used a carbon swing arm as well, which resulted in one of the lightest frames in its class for years... so much for that theory.