Gravel Myths (2): Smaller Knobs Roll Faster?
In part 1 of our series of ‘Gravel Myths,’ we looked at the idea that you don’t want to run ‘too much tire’ for a given course. We found that, when in doubt, it makes sense to choose wider tires: They roll as fast on smooth surfaces, and much faster when the going gets rough.
‘Too much tire’ can also refer to tread pattern. Most cyclists believe that slick, or almost slick, tires are fastest. What about knobs for rough, loose, muddy and snowy conditions? It seems to make sense that tires with small knobs roll faster than tires with big knobs. The thinking goes that, unless you encounter apocalyptic mud or deep snow that require huge knobs, a diamond ‘file’ pattern or small knobs are good choices. Are they really?
Let’s look at why knobs make tires slow: Knobs flex and squirm as the tire rolls. Flexing rubber takes energy, and that energy has to come from somewhere: The bike slows down. To make a fast knobby tire, you want the knobs to flex and squirm as little as possible. And, paradoxically, that requires large knobs, which are stiffer and thus flex less than small knobs.
To visualize this, look at car tires. Above are the tires used in Formula 1 racing. On the left are three slicks with different rubber hardness (depending on whether a driver wants to optimize wear resistance or cornering grip). On the right are two tires for intermediate and wet conditions.
We’ll focus on the wet-weather tire on the far right. It has tread blocks that are separated by channels to evacuate water. Basically, the wet-weather Formula 1 tire—and the tires on street cars—are knobbies.
The knobs are large, so they don’t squirm or flex excessively. But Formula 1 race cars have about 1000 horsepower, so there’s going to be a little squirm even with these large knobs. That’s why racers prefer slick tires when conditions are dry. Yet it’s pretty amazing that, in rainy conditions, Formula 1 race cars can put down 1000 horsepower through knobby tires.
The strongest pro sprinters put out about 2 horsepower. That’s amazingly strong for a human body. Yet it’s only 0.2% of a Formula 1 race car. With that ‘little’ power compared to a race car, the knobs on bicycle tires can be much smaller than those on a Formula 1 tire. But there are limits: If you make the knobs too small, they flex and squirm. Larger knobs are stiffer and thus faster.
Once you realize this, it seems weird that many tire makers recommend tires with small knobs for dry conditions, when you want a fast-rolling tire above all. It doesn’t make sense, and when we tested these tires on real roads, they were in fact surprisingly slow.
Making the knobs larger would make these tires faster, but that alone isn’t enough. You also want to distribute the knobs so that you always have the same number of knobs supporting the weight of bike and rider. (This also makes for predictable cornering, because you always have the same amount of rubber on the road surface.)
At Rene Herse, we designed our knobbies like wet-weather Formula 1 tires: We took a slick and removed as much material as possible, while still preserving the performance of the slick tire. The objective is different—evacuate water for Formula 1; dig into loose surfaces for bicycles—but the idea is the same. And with cyclists’ smaller power output, we could carve away the surface of our slick tire until only knobs remained.
The big question was: Once we remove so much material that our knobs are small enough to dig into mud and snow, will they still be large enough not to squirm? Our calculations suggested that this was possible. But calculations don’t always tell the whole story: The proof is on the road.
You can imagine our excitement when our 700×42 mm Hurricane Ridge knobbies (left) rolled as fast as our smooth 700×44 mm Snoqualmie Pass tires (right) in our real-road tests. Our knobbies may look burly, but they roll faster than all other knobbies we’ve tested. And they also perform better on loose terrain and in mud, because there’s more ‘negative’ space, which means the tread doesn’t clog up with mud as easily as tires with many small knobs and little space in between.
Rather than thinking of our dual-purpose knobbies as the opposite of slicks, it helps to visualize them as closely related to slicks, with just enough material cut away to improve traction in mud and snow, but not so much that the basic characteristics of the slicks are lost. In other words, our dual-purpose knobbies are the bicycle equivalent of Formula 1 wet-weather tires. The photo above shows our 700×42 mm Hurricane Ridge knobbies and illustrates the concept: A knobby that corners and rolls like a slick—until you need the knobs, at which point it becomes an aggressive knobby.
Back to the original question: Slick tires have a tread that doesn’t squirm beyond a little flex as the tire rolls. The opposite in performance terms are small knobs that squirm and waste a lot of energy. It’s possible to create knobbies that behave like the slicks they are carved from (figuratively speaking). Their relatively large knobs are the secret behind their excellent performance.
Further reading:
- Gravel Myths (1): Too much tire?
- Gravel Myths (3): Wide Tires Need Wide Rims?
- Gravel Myths (4): 700C Wheels Roll Faster?
- Gravel Myths (5): Side Knobs for Cornering Traction?
- Results of Bicycle Quarterly’s tire tests
- Our book The All-Road Bike Revolution explores how bicycles really work.
Photo credits: Pirelli & C. S.p.A (Photos 2, 3); Ansel Dickey (Photo 4); WTB (Photo 5).
