Science Behind the Magic | Wide MTB Rims

Technology in the mountain biking world is changing quickly these days. In the blink of eye, frame geometries, suspension designs and now wheel profiles change. New wheels are significantly wider than those of the past. Is this just the latest trend in the industry—an attempt to force the consumer to buy new gear —or is there a science behind it that has led to a revolution in wheel construction and bike handling?

The answer is yes! It’s all about the science! Wide mountain rims change the dynamics of a rolling tire in many different ways.

Ride Quality

Bigger tires offer more air volume to provide a more supple ride. Tires provide another inch or two of suspension on impact depending on pressure. In the same way an air volume reducer in a shock changes ride characteristics, the same is true with a tire. Comparing the inner width of a Mavic EN 821 Rim and an American Classic Wide Lightning shows 9 millimeters of difference in inner diameter between the rim flanges. This difference widens the base of the tire and can increase air volume by almost 10% in the tested Continental Trail King 29er or similar tire. The resulting smoother ride makes it possible to add a click of compression damping to your suspension, thereby providing even more control without sacrificing comfort. That increased compression damping means less brake dive and more support when pumping rollers.

Traction

Wider rims create a wider contact patch between the tire and the ground. With the same tire mounted to a wider rim, the cross sectional view of the tire is less rounded and more square. This results in a larger tread area parallel to the ground. Wider rims also put the cornering knobs in more constant contact with the ground. The change in width ends up pushing the cornering knobs up by stretching out the tire.

The result of pushing the cornering knobs out can have varied effects. It is very noticeable in tires like the Maxxis High Roller 2 or the Geax Goma. In Brad’s product review of the Geax Goma he states, “Even while running really low pressure up front, the bike has to be leaned over too far to get the majority of those big shoulder knobs in contact with the ground. By the time you do, you better have your foot out.” In spite of the pronounced center and cornering knobs, the lack of transition knobs between the two is the major culprit responsible for this handling trait. When leaning a bike over, a lack of knobbies in the transition zone can leave a rider with a feeling of vagueness. This is where a wide rim can play a significant role in changing tire behavior. Creating a squared off profile and raising the cornering knobs puts them in more constant contact at smaller degrees of lean and thereby increasing handling predictability. A wider rim has the potential to really open up some tire combinations that you would have never considered before.

Beyond cornering traction, wide rims provide better climbing. With the larger air volumes, lower pressures—about three psi lower than a narrow rim as tested in-house —will result in a noticeably better hook up while climbing.

Stability

Mountain tires have tall sidewalls, especially wider tires like a 2.35 or 2.4. Wide rims will add a lot of stability to these tires. When laterally loading the tire, there can be a tendency for the normal force from the ground to be directed into the tire off-axis. With narrower rims, there isn’t a lot of guidance offered by the rim and the tire will fold and can even separate from the rim bead. Bead separation can prove especially problematic with tubeless setups, as it results in a sudden loss of air pressure. When this occurs it is referred to as “burping”.

p4pb11074809 copy 2

Lateral Force Tire Deflection – Photo Courtesy of Specialized

When cornering or traversing uneven ground, the lateral forces exerted on the tire will unseat the bead, if only for a moment, allowing air to escape. Hopefully the impact will be quick and the bead will reseat. However, downhill racers tend to run tubes in their tires because of the extreme impacts that can occur during DH sections. If a burp occurs due to an impact in downhill racing, it is unlikely that the bead will reseat, and the tire might loose all pressure in these conditions.

While burping is less problematic in cross-country and all-mountain racing, it is still not desirable, and the wider rim reduces the likelihood of burping occurring. The extra rim width makes it much more difficult to deflect the tire to a point where the normal forces are being applied off-center, creating enough lateral leverage to unseat the bead. Think of this in relation to widening your stance while playing a defensive sports position. If you spread your legs out, it will be more difficult to push your upper body out past a point of no return—widening a rim allows the tire to assume a more stable stance.

p4pb11074809

Specialized Deflection Test Results

The difference in width and how it affects this off-angle deflection is fully illustrated in the tests that Specialized conducted on their Roval Traverse Fattie wheels. Specialized switched a tire between a 22mm internal rim width and a 30mm “wide” wheel. The tire and wheel were then loaded on an angle and the deflection was measured over a range of different loads. The test bench and deflection measurements can be seen below. Results show that at 900 newtons of loading, the narrower, less supportive rim allows for 50 millimeters of tire deflection, while the wider rim supports the tire more, only allowing for around 33 millimeters of deflection. 900 newton’s is around 200 pounds-force of load; a fairly realistic load to exert for a full grown male coming off a small drop on singletrack at an angle off-center from normal tire loading. This is a significant change that will result in some serious on-trail behavioral differences. Burping will also be far less likely to occur with this 34% reduction in deflection.

Rolling Resistance

When mounting our test tire to the narrower Mavic wheels and the Wide Lightening’s we discovered that the circumference is exactly the same. Despite pulling the tire sidewalls further apart, width is gained in the form of a less rounded sidewall appearance. Rollout tests of 29’er versions of both wheels with a Continental Trail King produced a 230.5-centimeter circumference for both.

With the same circumference, there is still room to play with rolling resistance. The added air volume allows for a greater range of pressures. As indicated previously, we found that while holding all other conditions constant, a wide rim can run a tire at three pounds per square inch lower than the same tire on a narrower rim.

Weight

Wide rims have the potential to save a fair amount of rotational weight. Comparing rims with different widths we see the WTB Frequency TCS rims only add 26 grams between their 23 and 25mm models. Although performance characteristics are not directly comparable with added rim width and added tire width, we can loosely compare this 26 gram increase due to rim width to the 202 gram increase in tire weight when moving from a Geax Goma 2.25 to a 2.4. The point being: It will save a lot of weight to add air volume and contact area by increasing the rim width instead of the tire width.

Another weight saving potential comes in the form of sidewall thickness. As discussed in the stability section, wider rims result in less lateral deformation. Tires offer a variety of sidewall thicknesses that add lateral support but also add significant amounts of weight. In the same conditions it is totally possible to run a wider rim, with a thinner, lighter sidewall tire with the same amount of lateral deformation. Rotational weight savings are far more noticeable than savings in non-rotating bodies. A 200-gram weight savings in a frame will not reduce exerted effort like a 200-gram savings in rotational wheel mass will.

In summary: wider rims offer you a softer ride, more cornering traction, better climbing, less burping, weight savings and a potential to run a wider array of tire profiles. I think wide rims will only improve in the near future as manufactures dial in the designs, desired widths and pair up with tire makers to examine sidewall construction requirements for wider rims to save weight. Luckily the transition will be far less controversial than the whole 29’er vs. 26” revolution.

Science Behind the Magic delves into the inner workings of your two-wheeled steed. Web Content Editor, Brett Murphy, uses his mechanical engineering background to explain the latest industry advances and breakdown component design.

2014-09-23T09:10:35+00:00