Thursday, September 15, 2016

Mountain bike wheel upgrade: DT Swiss XM401 rims

Every five years or so there seems to be enough cumulative small improvements to bicycle technology that riding the newest bike models can feel like a whole new exciting sport. Short of replacing your entire bike though, there are some strategic and affordable upgrades you can make to reinvigorate your riding and extend the life of your bike.

On mountain bikes in particular, you’re probably already familiar with the wide range of handling and control improvements achieved by simply changing tires to better match terrain and riding style. Less familiar to many riders though is how profound a difference upgrading your wheels—or at least your rims—can make in how your bike feels and performs. There are some solid technical reasons why wheels make a sensible upgrade.

Let’s use my bike, a 2014 Santa Cruz Tallboy 2 Carbon, as an example. This 29er is at the XC end of the spectrum but is built much more durably than a fragile race machine. The wheels are WTB i9 Frequency rims at 19mm wide, laced with DT Competition butted spokes to DT Swiss 350 Classic hubs. Pretty decent spec that’s representative of many mid-level XC bikes.

The problem with 19mm wide rims is that their narrow platform offers little stability for mountain bike tires. You can see this by grabbing the hub of your wheel and pushing it from side-to-side to flex the tire against the ground. That sideways movement is amplified by your body weight when riding, causing a squirmy feeling in the wheels. Large knobs increase the squirmy feeling because they enable more flex to occur between the rim and the terrain.

Mounting the same knobby tire on a wider rim increases the tire’s lateral stability and reduces the squirmy feeling. It’s the same principle as why standing with your legs wide apart can brace you against a sideways push. But there’s another source of lateral movement that’s less obvious: rim flex that occurs as the rim itself tries to resists forces from pedaling and trail bumps.

In cross-section, a typical bicycle rim is essentially a “D”-shaped tube. Since the spokes are all attached to the rim in one line (unlike on some fatbikes and adventure motorbikes like BMWs, where spokes are laced tothe outside edges of the rim), there is no lateral bracing to help the rim resist twisting forces that can arise, especially when hitting bumps. Torsional/bending strength therefore relies mainly on the rim design itself. Large diameter wheels (29ers) are more susceptible to these forces for a given rim width than 27.5” or 26” wheels. However, increasing the cross-sectional area of the rim—making it a larger “D”—greatly increases the rim’s ability to resist twisting. As a result, upgrading to a wider rim not only improves tire bracing, it provides a more rigid platform for the tire to push against as it engages with the terrain, reducing overall lateral flex in the system.

Sure, interesting theory--but how does it actually feel in seat-of-the-pants riding? Is there enough benefit to justify changing your rims?

To test this, I rebuilt my back wheel using the same hub and spokes, but swapping the WTB 19mm rim for a gorgeous new DT Swiss XM401 23mm wide rim. (As a DT Swiss dealer, I ordered a bunch of these—let me know if you want some.) The XM series is aimed at the more aggressive end of the XC spectrum ("All Mountain" on the DT Swiss scale) and is meant to offer strength and durability over pure light weight, but it isn't as heavy as their Enduro or DH rims.

Here's the new rim, still in its plastic, resting on the wheel. 

As you can see, edge-on the original rim had taken some damage beyond what truing could fully overcome without creating a weak and unreliable wheel.  

Rebuilding was straightforward: I taped the new rim to the wheel and simply transferred each spoke one at a time to the new rim, using the included Squorx aluminum nipples and washers instead of the original brass nipples. 

Almost done!

Careful truing and tensioning to the DT Swiss spec resulted in a beautiful, strong wheel.

Some DT Swiss rim tape and a lighter valve stem completed my tubeless build.

A dose of Stan’s and a good shake resulted in a perfect seal the first time. No air lost overnight.

Even though it’s a wider rim, the XM401 it’s still remarkably lightweight: the complete rear wheel weighs only about 40g more than the original build with the 19mm WTB rim! You may balk at replacing anything on your bike with something even slightly heavier, but consider what you gain: the increased tire bracing, torsional strength, and air volume together achieve a remarkable performance upgrade.

Indeed, a two hour test ride on fast, flowy trails with rock gardens and roots showed a profound improvement in the feel and handling of the bike. The rear wheel simply felt more planted in response to bumps, and more precise in its control. There was none of the squirmy or vague feeling I had previously felt on off-camber corners or in rock gardens, and cornering traction improved as a result. It also felt like the suspension was more in control, not like there were two separate systems of wheel flex and shock conflicting with each other in reaction to rider and trail input. While I can’t quantify these effects, qualitatively it almost felt like I was riding a new bike. All this from just changing a rear rim! 40g is peanuts to trade for this massive improvement.

Update: I've also swapped out my front rim but haven't had a chance to try it other than a few laps around the driveway. Front end feels noticeably more solid, although a more rigorous test is needed to assess how it affects steering and suspension feel. 

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