Normally, this spot is designated for a how-to feature, but we are going to break tradition for this Tech Tuesday and cover some new ground with a think piece on how rim width affects tubeless tire performance. If you don’t care about tubeless tires and future rim design, then this one may not be for you. If you are running tubeless, or have an interest, then I invite you to consider that significantly wider rim designs are the future of tubeless in all disciplines of the sport.
We mounted a set of monster-sized Schwalbe Muddy Mary tires to 21-millimeter (ID) DT Swiss EX 5.1D rims and burped them right away at 35 PSI - a pressure that makes a 2.4 inch tire almost rock hard. Big tires on narrow rims - bad. The same tires rocked on 25-millimeter (ID) AM wheels. Big tires on wide rims - good.
Burping Air Burping air happens when lateral forces on the wheel distort the tire and momentarily unseat the tire’s bead. The tire loses about half its pressure and then the bead magically reseats, leaving the hapless rider with a tire that is neither full nor flat – and absolutely useless for riding purposes.
Having been a first adopter of tubeless, I have experienced the litany of complaints about the subject that readers have posted on Pinkbike, and can say with confidence that two percent are valid technical issues and the lion’s share are generated by well meaning blokes attempting to air up a tire designed by someone who doesn’t give a damn about tubeless, on a wheel that was manufactured by his evil twin. Burping air, however is a complaint that is aired by the most respected riders and mechanics in the sport.
This article is not about wrestling with the impossible. It’s about proper tubeless rim and tire combinations, and a suggestion that we dump the narrow rim standard that cross-country riders adopted in the 1980s. The skinny rim was probably a mistake – especially for tubeless users.
Watch Mavic's Short Video That Explains the UST Tubeless Tire and Rim Interface First, A Word About UST Universal System for Tubeless is a patented interface between the tire and rim, as well as a set of standards that determine utilitarian functions. A UST tire has a square bead that interlocks to a matching groove in the rim flange and its inside has a layer of sealing rubber to allow it to be aired up and run dry. Among other items, the standard requires that the tire be mounted and aired up by hand or with minimal tools. Watch the Mavic video shown above to see how UST works. The bottom line is that UST is tough to beat for reliability and ease of use. It’s the best tubeless system in the bike industry by a long margin. If you ride XC or trail and run UST wheels and UST tires up to 2.3 inches, you probably won’t experience many of the problems outlined in this test. Presently, you can’t beat the UST system for hassle-free tubeless, but the best can always be improved upon.
Mavic's Crossmax SX rim measures 21 millimeters inside to inside and is a UST certified tubeless design. The WTB Weirwolf tire is a 'Tubeless Ready' design the requires sealant. When we mounted it to the wider rim, it changed the profile of the 2.3-inch tire and improved its cornering performance.
Why Rims Need to Be Wider There are a number of valid reasons that make wider rims a good idea. This is my short list: • Weight savings is number one. Consider the tire and wheel as an integrated unit. The tire weighs much more than the rim, so a wider rim, though slightly heavier, adds volume to the tire without adding additional rubber and tire casing. This could be especially beneficial to 29ers because the big wheel already spins way too much rubber.
• Stability is another. Wider rims add a significant amount of lateral support to a tire – especially large-volume tires like the 2.35 and 2.4-inch rubber that has become regular fare for XC/Trail and AM riders. A tire casing could be made lighter weight and boosting the tire’s lateral stability should eliminate burping.
• A wider rim puts more of the tread in contact with the ground in a straight line and while cornering because the tread deflects less dramatically under lateral loads and remains more parallel to the rim.
• Finally, advances in materials like carbon fiber and aluminum alloys have made it possible to manufacture wider rim profiles without significant weight penalties.
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Bicycles make their own gravity" a cycling luminary once told me. Because it must be leaned to turn, the loads almost always go through the contact patch (A) and up center-line of the tire and wheel. In normal riding mode, both wide and narrow rims can equally handle the stress.
What We Know From Experience Experience has shown that switching from skinny, 22-millimeter (OD) rims to 27 -millimeter (OD) width versions dramatically increases the lateral stability of a wheel and tire in corners and in technical terrain. Riders who could burp 22 millimeter tubeless wheels simply by pounding out a climb out of the saddle can hammer away on 27 millimeter rims and never lose pressure. That is a delta of only three millimeters. Once the realm of downhill and freeride, 28 - 30+ millimeter width tubeless rims are now appearing in trail-riding weights. Lightweight high-volume tires like the 2.4-inch Schwalbe Rocket Ron might feel wiggly on narrow rim, but they shred on a wider profile wheel like the Mavic CrossMax SX. I found I can run one size smaller tire casing on a wider rim and it feels like a completely different tire. The 2.3-inch WTB Weirwolf comes to mind. So that got me thinking.
Why Narrow Rims and High-Volume Tires Are Bad for Tubeless A tire may be made from a flexible material, but once it is pressurized, it acts much like a solid object to a surprising degree. Sure, it can flex and rebound when deflected, but the casing is quite stiff and when you put a lateral load on the wheel, one side of the tire tries to lever the opposite side off the rim. Clamp the tire in a vice and pull the wheel towards you and discover for yourself.
Physics dictates that the same sized tire imparts more leverage on a narrower rim than on a wider one. This probably accounts for the feeling of instability when aggressively riding large-casing tires on narrow rims. The difference of three to five millimeters in rim width only amounts to about an 18-percent change in the moment arm (leverage) of a 2.4-inch tire acting between the rim flanges. The abrupt feeling of a tubeless tire folding underneath you and burping air is a far more dramatic event than an 18-percent difference in lateral stability. There had to be more to the puzzle.
The imaginary tires and rims are flexed laterally at 15 degrees to illustrate that the contact patch (A) has shifted to the right. The wider rim's contact patch is still comfortably within the confines of the rim flanges (B), while the contact patch of the narrow rim has shifted to the danger zone over the right flange. The Green lines depict the moment arms of the tire and its fulcrum point at the top of the rim flange exerting an upward force (C) on the opposite bead (D). The tire probably would hold pressure at this point.
Turns out there may be another culprit that may be to blame for the thorn in the side of tubeless. All pneumatic tires flex from side to side under lateral load shifts – it’s in their nature. One reason that wider rims give tires more lateral support is that a wider rim ensures that the tire’s contact point hovers between the rim flanges. It is possible for a tire’s contact point to migrate outside the rim flanges and in such case, the line of force between the tire and rim would go over-center. In the same way that a quick-release lever generates an exponential amount of clamping force when it goes over-center as it is closed, once the tire’s contact patch moves outside of the width of the rim flanges, the tire’s ability to support the rider is significantly reduced, while the leverage it imparts to lift the opposite side bead from the rim would substantially increase. This effect would explain the feeling of a tubeless tire burping – the tire suddenly collapsing under the weight of the rider or the G-force of a turn and then losing pressure. Of course, a narrow tire could not flex far enough to allow the contact patch to get near the danger zone, but a big, flexible 2.35-inch tire on a 22-millimeter (OD) cross-country rim certainly could.
At 20 degrees of imaginary lateral distortion, the wide-rim tire's contact point is just nearing the danger zone over the rim flange (B), but it can still support a load. The narrow rim's tire has flexed to the point where the contact patch is over center (B) and the bending forces are creating tension across the tire casing which pulls on the opposite bead, drawing it up and away from the rim flange (D). The seal has been broken and air may be already leaking.
Why Wider Rims Are the Future of Tubeless So, by now it seems clear that, if a slightly wider rim is good, perhaps a much wider rim could offer even more benefits. In the illustrations that I drew up, I exaggerated the widths of the rims to give a clearer view of the concept – that the first benefit of wider rims for tubeless is to reduce the moment arm between the tire and the bead interface. Secondly and perhaps more important, the wider rim keeps the tire’s contact patch between the confines of the rim flanges under high deflections. Considering that a wider rim is proven to reduce the effects of lateral forces on a tire, it is unlikely that the tire on the wider rim would distort equally as I depicted in the comparison to the narrow-profile rim.
Still at 20 degrees, when we add more down force on the tires, the one on the wider rim can still transfer the extra load to the wheel because its contact point is hovering over the rim. Only a small amount of additional down-force collapses the tire on the narrow rim, pulling the opposing bead away from the rim flange and spraying bystanders with a noxious combination of crusty old Stan's sealant and sweat infused floor pump air. With the pressure reduced, the tension is relaxed from the tire casing and the bead returns to the sealed position against the rim.
Another potential that a wider rim may bring is a suppler ride over small bumps, and at higher pressures than expected. Tests with wider rims and similar road tires by Mavic and HED shows that the subtle change in the tire’s profile made a dramatic improvement in its performance. The switch from the classic “lightbulb” clincher shape created by confining the tire within a narrow rim, to one resembling a continuous arch made a dramatic reduction in rolling resistance over rough pavement at any tire pressure value. Cornering performance was also significantly better. Similar gains are.probable with mountain bike tires as the tire casing scribes a more relaxed arch over a wider rim, and can flex in a more uniform pattern when the tire is deflected.
The 2.35-inch Schwalbe Hans Dampf is perfectly matched with the DT Swiss AM 10 rim. It mounts up with a hand pump and rides like it's on rails. This matchup is standard fare on the 2012 Scott Genius LT.
TubelessTips For Today's Riders What all this information suggests is that riders who run tubeless and plan to use UST or tubeless ready tires larger than 2.125 inches should consider pairing them with tubeless compatible rims with an inner flange-to-flange measurement between 21 to 25 millimeters. A number of lightweight AM wheels in that category exist from Easton, Mavic, DT Swiss and Wilderness Trailbikes. After testing numerous combinations, we know for a fact that the wide format pays massive dividends in the handling department, and the tubeless fuss factor is reduced to almost zero. As far as getting rid of the burp, we recently ran 2.4-inch Schwalbe Muddy Marys on a set of old-school narrow DT Swiss XC wheels and they burped flat at 35 psi. We shredded for a few months on the same tires, mounted to wide-format DT Swiss All Mountain wheels without a single fault. The weight penalty of the wider rm seems insignificant when balanced against its performance gains.
This is an original Keith Bontrager re-rolled Mavic road racing clincher rim mounted to a Fisher (the real one) 1.9-inch Fattrax skin-wall tire - a popular XC racing tire in the mid 1980s. This is the rim that spawned the skinny, 22-millimeter (OD) mountain bike standard. Keith cut some of the circumference from a 700C 36-hole rim and rolled it smaller to make a 32-hole 26er rim. The Mavic rim is pressed together over an internal sleeve.
So, Where Do We Go From Here? The old school 22-millimeter (OD) rim was a product of Keith Bontrager cutting and then re-rolling Mavic road clincher rims to fit 26 inch MTB tires. His creations were so lightweight that it was an overnight game-changer. Every brand that made or wanted to make a mountain bike wheel copied Bontrager’s re-rolled 700C hoops. Nobody really thought it through. Then, however, a 2.125-inch tire was considered huge and tubeless was only a dream. Tire technology has advanced so far, it’s a wonder why rim makers have only recently responded – and who beyond UST has considered tires and wheels to be an integrated system?
For the near future, I say the wider the better. Considering that carbon fiber rims are well proven and silly light, the possibility of an XC race-competitive rim at 23 millimeters between flanges is easily attainable, and rims in the upper 30s – maybe mid 40s – for AM and DH might be the game changers of 2013. If there’s anyone out there making custom rims, I’m thinking that 28 to 30 millimeters is the perfect width for trail riding. Make mine tubeless, please.
Final Notes It's worth repeating that this Tech Tuesday is based upon my observations and theories in an effort to get an industry wide dialogue started to explore the possibility that wider rims are a better format for tubeless and to investigate the performance potential for significantly wider rims for all modes of MTB riding. I'd love to hear any opinions or theories that relate to the subject. So please weigh in.
- RC
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