Lately, there’s been a lot of discussion about the pros and cons of carbon fiber versus aluminum when it comes to rims. Here’s what we think.
Plenty of carbon rims on the market are mostly fashion accessories, and that’s cool, if you’re into that sort of thing. But we’re not. In fact, we make some damn fine aluminum rims that cost a lot less than carbon. Those aluminum rims helped make tubeless what it is today, winning championships around the world in the process. A great deal of thought and testing has gone into our aluminum rims, and still does. We’re talking static and dynamic testing, high-speed imaging, material analysis of alloys, constant improvements to tooling and manufacturing processes, calculations, formulas--scary stuff you dealt with in high school and hoped you’d never see again. With five patents just on rim shape and tubeless design, we do all this to make sure our aluminum rims actually solve problems and make rides better.
And that’s how we approached carbon.
What problem is a carbon rim trying to solve? What is this thing going to do that our aluminum rims can’t? As a company, if you don’t ask yourself that question--if you let the marketing department drive development--then you’re just selling people overpriced matte black billboards for your stickers.
For us, the answer was “ride quality.” When you see the word “carbon” in the bike industry, it’s usually shorthand for “carbon fiber reinforced polymer composites” (CFRPs). We can make a CFRP rim that rides better and has better fatigue life than our aluminum rims (and we don’t say that lightly, because we’d put our aluminum rims up against most brands’ carbon models). As a design material, CFRPs are anisotropic, meaning the material has different characteristics depending on which direction you’re considering. The infamous marketing phrase “laterally stiff and vertically compliant” really does describe what carbon can do, but that’s only the beginning. Carbon can actually be tuned to yield distinct behaviors under tension, torsion, bending, and during impacts. The ability to tune a carbon rim to respond in different ways depending on the force being applied to it is the real advantage.
Tuning carbon to do what you want it to do is no small feat, though, when you consider the vast array of layup options. The shape of the rim and the lay-up also need to be designed in parallel so each is optimized to work with the other. There are a whole lot of ways to get this equation wrong. Having more tuning options is wonderful, but it doesn’t automatically make for a great riding rim unless you choose the right options. In fact, it can do the opposite.
Remember when crazy stiff wheels were all the rage? “Our carbon rims are X% stiffer than our aluminum,” and all that? It started on the road and went to mountain bikes. Most carbon rims were really stiff, and rode like crap. And still do. From our earliest designs, though, we never wanted our carbon rims to ride stiff. Sure, they had to resist lateral loads and twisting forces, but everything we knew about tubeless had taught us higher-pressure tires actually rolled slower, not faster. Stan believed the same principles could be applied to makings rims faster.
And Stan was right. The more we tested, the clearer it became that a wheel engineered to roll with the punches instead of constantly ricocheting off the terrain rolling underneath it does indeed go faster. Even with high-volume tires, and even with suspension, wheels have a key role to play in absorbing impacts. We liked the way our radially compliant test wheels felt so much, we created one of those acronyms: RiACT for “Radial Impact Absorbing Carbon Technology.” It’s like there’s some horrible law that you have to use acronyms in the bike business, but in this case, it actually described exactly what we had going on.
By the time our Valor and Bravo carbon wheels were introduced, we knew we were on to something with this whole “radial compliance” thing. We aren’t one of these gigantic companies, but it sure felt like some other brands were soon using more and more of our terminology. Rim designs didn’t necessarily change, but catalog-speak started including more emphasis on “absorbing.”
The engineering behind all this is more than just words, though, which brings us back to those subjects from school that made a lot of us run screaming.
What it Really Means: Physics and Stuff
OK, so we all know how to develop the stiffness matrix for a complex half-disc shape constructed of an anisotropic laminate, right?
Wait, no. I think that's actually a recipe for making quiche using the Large Hadron Collider. The point is, a few of us here spend a lot time running calculations on computers, measuring everything, and then we make stuff and test the hell out of it. Good times on the bike take a lot of hours on the computer and on our testing machines.
Even without all the scary-ass equations, though, understanding our RiACT system basically comes down to the material, construction, and shape of the rim. These three factors all work together to achieve a single goal: balance. Combined with the expertise of the wheel builder, these elements let us create rims that do more than just absorb impacts and vibrations. Combined with a quality wheel build, RiACT rims actually deform elastically to distribute forces more efficiently, protecting the rim and tire from extreme, localized stresses. Like a higher volume, lower pressure tubeless tire, a rim that absorbs impacts instead of resisting them also just plain rolls faster. RiACT rims ride better because they do a better job of balancing forces.
It starts with material. While most brands work with the same grades of carbon fiber, the bulk of the mass in a rim is resin, a specially formulated thermoset epoxy. Resin is basically the filler around the layers of carbon fiber, but it can have a tremendous impact on a rim’s strength and ride quality. Getting the level of radial compliance we wanted meant using an upgraded resin. The resin compound in our RiACT rims includes nano elastomers. This special ingredient in the resin increases the radial compliance of our rims, while creating an even stronger bond between the carbon layers and allowing our rims to avoid brittle fractures. This special toughened resin plays an important role in the strength and radial compliance of our RiACT carbon rims.
The second component to our RiACT system is the unique arrangement of the carbon fiber layers in the rim. Each of our rim models is designed for a slightly different type of riding, so each uses a different arrangement or lay up schedule. The unique arrangement of layers is what allows a carbon rim to resist lateral flex while controlling impact distribution throughout the rim and transferring the load to the spokes. “Lateral stiffness and vertical compliance.” That kind of “cake and eat it too”--stiffness where you want it and not where you don’t--just isn’t possible with aluminum, and it’s the foundation for RiACT.
Rim shape is the final, and most critical, element of the RiACT design. The low-profile sidewall of our rims is specifically engineered to optimize tire profile and control impacts. Considered holistically in conjunction with layup and resin system, our patented rim shapes produce a rim that absorbs, distributes, and dissipates impacts. We’ve all come a long way since rims had to have flat sidewalls for rim brakes. Through persistent calculations, modeling, and testing, we’ve identified key design features and developed complementary curvatures, from spoke bed to bead wall and in between, that enable the distinct ride characteristics offered in RiACT designs.
There are no other rims on the market today capable of balancing forces the way our RiACT designed rims can. From the race-ready 1300g Podium SRD wheelset, to our new Crest CB7 and Arch CB7 carbon rims and wheelsets, Stan’s carbon rims combine lateral stiffness with radial compliance for a better overall ride. The materials, construction, and shape of our RiACT rims all work in harmony to make our RiACT rims and wheels last longer and roll faster.