Taking the Inside Line: A Technical Conversation With Chuck Texeira

Taking the Inside Line: A Technical Conversation With Chuck Texeira

Our new Control SL is the lightest, fastest and most cutting edge mountain bike wheel we’ve ever made. In addition to feathery weight and awesome ride characteristics, it is also incredibly strong. We put an insane amount of work into each and every Roval product, from the first design briefs, through an exhaustive prototype and test cycle, sometimes including many pre-production revisions before we are ready to even begin the initial manufacturing phases. In the case of the new Control SL wheels we burned through 16 different prototype rim designs before achieving the results we were seeking. To glean a little more insight into the work that goes on during the development stage we are going to take a deeper dive into what goes into our wheels in the next couple blog posts, and explain our goals and process in more detail than the sales brochure allows.

Chuck Texeira played a huge role in the development of the new Control SL, and his wisdom and voice are our guides here. In this post, he’s going to talk us through the rim design, how it is different, what makes it awesome, and why this matters.

“Everything starts with a product charter; it’s gotta be this wide, gotta be lighter, and we don’t want to give up any strength. [With the Control SL] We wanted less weight, more width, and more strength. That’s tough, usually you have to give up any one of those to make the other two happen.

So I set off in two different paths. Started with some basic FEA, trying to get the best cross section – the shape, geometry of the rim was super important. We went through a ton of those to get the starting shape.

In the other direction I am always asking questions - why why why? The big question I had was when we test the rims in the lab they are always super strong, so, why do rims still break out in the field? That led me down the rabbit hole of maybe people are getting flats, and the next nanosecond is when you are blowing through your rim. This led to a design objective; prevent flats, and then your rim will be strong enough in the real world. If you can keep air in the tire, your strength demands on the rim go way down.

So I made 16 different prototypes focused on the geometry of the rim edge shape, that interface between the rim edge and the bead of the tire. With rims and rocks you have a hammer (rim) and an anvil (rock), and something in between. Eventually you are pinching something in between them. How do we go about protecting that soft thing? How do we protect it? What shapes work? What shapes hurt?

I tested each shape three times with 20psi, then three times with 25psi. To do this, I needed special tires - mtb casings but no knobbies, because knobs interfere with the effect of the test guillotine on the rim.  The tire knobs have more rubber than the space between them, and this introduces variables. So, we started with clean slicks, a few hundred of them specially molded for this experiment. Every time you run the test you fail the tire. So, it was an exhaustive, expensive, tedious test. Three tests, per pressure, per rim profile.

We learned what shapes worked well, and what shapes don’t, so in spite of the tedium it allowed us to figure out a sound geometry. From there we were able to apply what we had learned to the final shape of the rim. Shapes that we intuitively thought would perform well, did not. We intentionally tested profiles that we knew would be bad, just to be sure. We set out to understand the effect of rim width in terms of flat protection, handing & rolling resistance. We learned from the test that within a 5mm or so range, width didn’t necessarily improve or hurt flat resistance.”

That rigorous design and testing cycle led directly to the final shape of the Control SL rim. The 4mm wide flat surface at the top of the rim wall, with its carefully designed edge radiuses, requires 22 percent more force to cause a pinch flat than its predecessor. It may not look like a revolution, but that’s a pretty huge leap forward in reliability and durability, and in terms of getting from point A to point B as fast as possible.

“Typically when we do our field testing with our riders, and we tested this more than any wheel we’ve done in the past,” Chuck offers anecdotally, “it’s really common that you are gonna find people who pinch flat. With this latest design, the evidence went beyond the lab out into the real world – nobody was pinching these. Out of all the riders and miles tested, we might have had one pinch related flat throughout that.”

In addition to the flat-reducing shape of the rim bead, the Control SL rim is asymmetrical. Chuck explains: “The whole strategy with asymmetric rims is to seek a better balance with spoke tension. Wheels are often dished in order to fit cassettes or disc brakes, with the spokes on one side of the rim taking a deeper angle to the hub flange than the other, flatter side. That creates an imbalance in tension. Imbalance is never good. Ideally, you want tensions to be even, that’s the holy grail. You really want equal amounts of tangentially paired spokes on either side of the rim. So, asymmetry allows for similar dish and greater bracing angles, which creates greater stability and more even spoke tension. The kicker here is that we were able to use the same length spokes entirely across both front and rear wheels. Asymmetrical was the best way to do that.”

Other less obvious, but crucially important, design considerations were brought into play as well. There are recesses in the tire bed of the rim so that rim plugs can be used, saving weight but still allowing for a trouble-free tubeless setup. Chuck explains: “The asymmetrical rim shape pushes the spoke holes to one side so I had to make the spoke bed non-uniform in order to get the plug to interface right. This allows the plugs to sit flush, and that allows tires to bead up easier. Plugs are lighter, significantly lighter than a rim strip, and about 10 grams or so lighter than the lightest tapes.”

Those are the visible touches that differentiate the Control SL rim from the rest. Beyond what we can see with our naked eyes, however, there has been extensive manipulation of the carbon fiber layup to achieve the desired strength, weight and ride quality goals. “The rims are significantly thicker where the spokes go, and significantly thinner where the spokes are not. Little squares of pre-preg are laid by hand into the rim - until you cut one open you’d never know it’s there. Thicker material stops spokes pulling through, but that is the only place that thickness is needed. You reinforce where you need it. One of the great things about working with carbon fiber is that you can do this. You are building these wheels one ply at a time, and you can manipulate thicknesses to get the characteristics you want. Meanwhile, we iterated through eight different layups to get the ride and stiffness characteristics we wanted.”

It’s not just a lighter rim. It’s not just a stronger rim. It’s not just a less likely to pinch flat rim. It’s not just a more comfortable rim. It’s all of that. Pretty damn impressive. Take a breather, and then we’ll dig into some more of what went into making the new Control SL. We’ve got some hubs to talk about!

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