Gravel Cyclist's Tire Dilemma: Knobbies or Slicks? 3/8

Read this, if you are a critical thinking gravel or allroad cyclist

Speed / Rolling Resistance

I noticed that slicks rolled significantly faster on tarmac, and slightly to noticeably faster on gravel (depending on the size of gravel). In other words, due to their absence of knobbs, slicks have lower rolling resistance than knobbies, irrelevant of the road surface we ride them on.

For example, when we ride knobbies on chunky gravel road, the rolling resistance due to knobbs is the same as when riding them on super smooth tarmac.

This might sound counter-intuitive to those who think that on chunky gravel, knobbs don't slow them down much. In reality, knobbs slow you down on chunky gravel exactly as much as on tarmac. What happens on chunky gravel is that in addition to the knobbs-caused rolling resistance, RR1, which is the same on both tarmac and chunky gravel, we deal with the chunky gravel-caused rolling resistance, RR2. As chunky gravel is significantly bigger than knobbs, also RR2 is significantly higher than RR1. With slicks, on the other hand, there is no RR1, while RR2 is exactly the same as with knobbies because chunky gravel is chunky gravel no matter what tire we ride it on. Thus, even on big-sized gravel, the overall rolling resistance is slightly, though insignificantly, lower with slicks.

To sum up, the knobbs-caused rolling resistance stays the same all the time, because no matter what surface we ride knobby tires on, the knobbs will always be there. It's simple like that.

Mosaic road surface on Gravelest's Mother and Daughter L-route: tractor and car ruts, compressed and hidden loose sand, lime (whitish on the right), loose (some crumbled) bricks. This road definitely speaks many languages at the same time, and they have to be recognised and translated patiently by a cyclist, if one is not to fall down. Do you think knobbies would do their job better than slicks on this particular road?

Sound (Humming, Buzzing)

Humming is a sound caused by knobs as they deform under cyclist's weight which pressess them against riding surface. The humming is the sound of tread's rolling resistance which slows us cyclists down. That's why I don't like to hear strong buzz from under my tires.

I noticed that knobbies hum both on tarmac and gravel. On tarmac, the hum is amplified by resonance, and thus much more noticeable. On gravel, the resonance usually doesn't occur due to greater irregularities of gravel road surface, and the rest of the hum is drowned in the crushing sound of tires' sliding over different layers of gravel. How much the hum is drowned, dependens on gravel's roughness.

Not hearing the drowned and unamplified hum of knobbs is yet another reason why some erroneously think that knobbs don't cause rolling resistance on rough gravel.

Slicks's humming, on the other hand, is barely audible on tarmac. On average-sized gravel, slicks are only insignificantly more silent than knobbies. The crushing sound of slicks' sliding over different layers of gravel is dominant, just as with knobbies.

On a gravel road that even, smooth and dry, knobbies hum as strongly as on tarmac. With slicks (Vittoria Voyager Hyper 40 mm), the ride through this sector of Gravelest's Great Larch L-route was silent.

Comfort

I found knobbies to be less comfortable than slicks both on tarmac and gravel.

The bigger the knobs and the smoother the tarmac, the more easy was for me to feel that knobbs made tires slightly bounce as they rolled forward. In other words, knobbies' main forward-directed movement is disturbed by small vertical movements, bouncing. (Hey, when cycling, I want to move along riding surface, not up from and down to it!) Consequently, these small declines in the moving direction make my bike vibrating and require my body's energy to neutralize them. Thus, cycling on knobbies over longer period of time inevitably results in lessened comfort and increased fatigue.

On gravel, the knobbs-caused bouncing, B1, is joined by bouncing due to greater surface irregularities, B2. The rougher is the gravel, the bigger are both B2 and the margin between B1 and B2. When riding slicks, on the other hand, there only occurs bouncing due to surface irregularities, B2. Consequently, slicks are more comfortable even on gravel.

It's only on very rough gravel or rocks where the constant knobbs-caused B1 becomes insignificant in regard to very big B2. Theoretically, that's the point where tread type makes no difference in comfort. Yet knobbs make the tire tread thicker, diminishing the tire's suppleness and riding comfort even on chunky gravel.

THE END OF PART 3/8

Credibility of the Article

The author of the article is Matej Goršič [Matey Gorshich], 41, male, PhD, who has been an active sportive cyclist for 28 years. In his youth, he trained, and competed in, road cycling and athletics. Once finally out of the competitive waters, he started consciously re-defining his understanding and practice of cycling. Today, he feels most at home in a creative mixture of non-competitive, electronic-devices-free, self-exploring, adventurous allroad cycling. During last 3 years, he spent over 25.000 km mostly on tiny gravel roads of South Estonia.

The tires tested for the article were bought by its author at normal market prices from different cycling shops in Europe. There have been no sponsors or donations for writing the article nor for mentioning brands and items in the article and showing them in its photos.

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