Editors Note: Science Behind the Magic is a reoccurring column by Art’s Cyclery web content editor Brett Murphy, who uses his mechanical engineering background to explain the latest industry advances, and breakdown the inner workings of common components. The original article can be found here.
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 ways.
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 9mm 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.
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. Sometimes even while running really low pressure up front, the bike has to be leaned over too far to get the shoulder knobs in contact with the ground. 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.