Gravel Cyclist's Tire Dilemma: Knobbies or Slicks? 4/8
Read this, if you are a critical thinking gravel or allroad cyclist
Cornering on Dry Tarmac and Dry Gravel
I found slicks to have more predictable cornering grip not only on tarmac but also on gravel.
When cornering on tarmac, knobbies vibrated nervously (the bouncing mentioned in part 3 of this article), in particular if their knobs were big and hard (Schwalbe Smart Sam). Small soft knobs (Schwalbe Racing Ralph) made cornering unpredictable in another way. They squirmed and produced a sudden wobbly pull when I leaned my bike more aggressively into corners. What I found most annoying was that I never knew exactly at which moment the pull will happen in the next corner. These tires didn't communicate with me as a rider at all.
After a couple thousand km on the soft knobbies I inspected them. I was shocked by what I saw: cornering forces torn out the lateral knobs for up to 1/3 of their width on the front tire, and for up to 1/2 of their width on the rear tire!
On gravel, the problematic cornering features of knobbies didn't disappear. Bouncing and squirming were joined by sliding. Contrary to popular belief, even big knobs failed to dig into gravel. They slid in gravel corners instead, producing the same bouncing effect as they did on tarmac. Similarly, the small soft knobs retained their squirm also on gravel. The only positive thing I noticed about cornering with knobbies on gravel was purely psychological. Their problematic cornering features were hard to distinguish from vibrations and sliding caused by gravel. This might be a reason why many cyclists don't recognise problematic cornering features of knobbies when cornering on gravel: they think all vibrations and wobbliness come from gravel.
On the other hand, cornering predictability and stability of slicks on gravel was confidence-inspiring on the condition that these were low inflated. Inevitable sliding was predictable, never bouncing or wobbly. As slicks lack knobs, they were capable of evenly conforming themselves to irregularities of gravel without disturbances caused by knobs.
To sum up, my finding that slicks outperform knobbies in cornering on dry gravel, is the polar opposite of popular belief that cornering on gravel is best with knobbed tires.
Turns of Uniküla road ready for gravel sliding (Surprise the Edelweiss M-route). On this winding gravel descent, slicks easily outperformed knobbies in terms of comfort and speed.
Cornering on Wet Tarmac and Wet Gravel
On wet tarmac, cornering grip of both knobbies and slicks is diminished in comparison to their cornering grip on dry tarmac. The diminishment is more noticeable in slicks, since their grip in dry is excellent. In knobbies, their cornering grip is already bad on dry tarmac, so rain doesn't make it considerably worse. On wet coloured surfaces like zebra crossing, cornering grip of both slicks and knobbies is very bad.
On wet gravel, cornering grip depends on how much water is there in the gravel.
When amount of water is minimal (damp gravel), cornering is even better than on dry gravel. Water mollecules bind the smallest particles of gravel (which actually are sand) together, thus reducing the sliding of cornering tire. This is true both for slicks and knobbies. For knobbies, a positive thing happens. They start digging in the riding surface which improves their grip. At the same time, the digging into softer riding surface takes some discomfort due to their bouncing away. Thus, on damp gravel, difference in cornering between slicks and knobbies is slightly smaller than on dry gravel. That is to say, on damp gravel, slicks still perform better than knobbies, but not far better as it was the case on dry gravel.
On moderately wet gravel the performance of slicks is slightly better than that of knobbies. Slicks start loosing a bit of their grip, yet in totally predictable way. Knobbies, on the other hand, increase their grip by digging into wet surface, and their bouncing is further reduced. In this case, they perform approximately the same well as slicks. But this is only true under two conditions. Knobbs have to be, firstly, big and firm (e.g. Schwalbe Smart Sam), and, secondly, effectively mud-shedding. Not exactly what a typical, small-knobbed, gravel tire is. Small grooves in between small knobbs fill with wet gravel (gravel mud), and that results in tires' loosing of their digging effect. Consequently, small-knobbed tires perform worse than slicks which have no grooves.
In very wet gravel (gravel mud), I noticed no significant difference in cornering when riding on slicks or knobbies. Namely, in such conditions the corner entering speed with both tire types was so low that traction was not an issue. I never bombed down winding descents covered in gravel mud. But I noticed an adventage of knobbies when I hit big puddles. Knobbies enabled me to corner/climb out from the big muddy puddles, or ride on the tiny, less muddy inclined verge of ruts. Slicks didn't allow me that. On slicks I simply had to follow the rut instead. This often led me directly into puddles. Yet at the same time, this didn't make me and my bike more dirty. After my ride on slicks, my clothes were alwas noticeably less dirty than of those who rode on their knobby MTB tires. Knobbs gather dirt in between them, making wheels considerably heavier, and also throw dirt into your face. Slicks don't.
To sum up, slicks outperform knobbies in cornering on gravel which is up to moderately wet. This, too, came to me as a surprise contradicting popular belief that knobbies, not slicks, are the best for riding wet gravel. The popular belief may only be true for cornering in gravel mud at higher speeds which I didn't dare to try.
Cornering with slicks Vittoria Voyager Hyper 40 mm had to be done very carefully in this watery and slippery gravel mud. Slicks' cornering features highly depend on the amount of water in gravel.
THE END OF PART 4/8
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.