“Gravel” does not refer just to rocky, stony, pebbly roads—it’s pretty much all the roads, paths, and unpaved surfaces that lie between paved tarmac and technical singletrack.
Gravel speed is significantly slower than road speed. This might seem obvious, but if you’re accustomed to finishing a century in 6 to 7 hours, and you’re 8 hours into a gravel ride and still have a ways to go, it can be a rude awakening.
Gravel remains largely undefined, which is exactly the point. It’s supposed to be an adventure, so going on a “gravel ride” or signing up for a “gravel race” can mean many different things to different people. One person might imagine quiet, rugged, relatively smooth, if crunchy, roads. Another considers any unpaved surface fair game.
If you’re not prepared for someone else’s definition of gravel, it can turn out a lot like the encounter Chris Hadgis had on a particularly puckering descent in the most challenging, steep, cratered-out, boulder-pocked stretch of unPAved.
Hadgis knew she had brought a butter knife to a sword fight with her tube-filled tires pumped to about 60 psi and unforgiving, 11-28t gearing. But she hadn’t quite anticipated what the course description meant when it referred to “raw” roads. As she wrote in her race report for Pretty. Damned. Fast., a digital content provider dedicated to women’s cycling: “I hear someone say, ‘Oh my God, it’s a rock garden.’ This is no garden. ‘Garden’ is much too gentle a word. This is an endless minefield of sharp, pointy, edgy, uneven rocks, boulders, steep climbs, and steep descents into puddles. Or as I dub it: the ‘gnarly-rocky-pitchy-mud puddle section.’ I let go of the brakes, say a prayer, and fly. Exhilarated, a rush of adrenaline and a thousand thoughts flood my brain: ‘This is it; I’m going to die!’ ‘Well, I’ll die doing what I love!’ ‘I don’t love this, I hate this!’”
I’ve found this oscillation of completely opposite emotions to be natural in gravel riding. But the ultimate goal here is obviously not to hate gravel. (And for the record, Hadgis didn’t end up hating it either. After navigating the dark, scary part of the forest, she found the light, concluding: “I feel an overwhelming sense of calm and gratitude simply to be riding here surrounded by this huge, lush forest with diverse, tall trees, every shade of green, and views of a flowing river. I feel alone—like I’m not a part of an organized ride at all, but rather just a passerby, a witness to the beauty of the Susquehanna woods.” Now that’s love.)
A big part of loving gravel is knowing gravel. That starts with knowing what to expect.
The best interpretative guide to date was created by former pro road racer and all-around badass bike racer Neil Shirley, who went all-in on gravel and had tons of people asking his advice on what tires and equipment to run for various events. Shirley realized that gravel rides are not just literally all over the map, but figuratively, too, in the types of terrain and conditions. He started writing a document he called the Industry Standard Guide to Gravel (ISGG) for Our Outdoor Office, a blog at The Lyman Agency where he worked at the time. He began with tongue planted firmly in his cheek, laughing at the idea that anyone could “standardize” gravel. But then he realized his categorization was actually pretty useful.
Of course this is just a starting point. If a course has a few super rough, unmaintained roads but the rest is pavement, you may not want 2.1-inch mountain bike tires for the whole day. If you’re brand new to riding unpaved surfaces on a drop bar bike, everything may feel a category tougher.
Smooth, well-maintained dirt roads will have either very small gravel chunks or none at all, and are very much road bike friendly. These roads are in better condition than many paved roads in the United States and are hard packed, offering little more difficulty than riding on tarmac. There are a number of roads like this in Colorado that have a magnesium chloride treatment to keep the dust down and harden the top layer of dirt. Roads like these are often found in a number of gran fondos to add a little more adventure to the event.
Tire size: 25–28mm*
Event examples: Haute Route Rockies (Colorado), Battenkill (New York), Hell of Hunterdon (New Jersey), Jeremiah Bishop’s Alpine Loop Gran Fondo[OH1] (Virginia).
*As gravel tires get better and smoother-rolling, even on pavement, you can bump up to 30mm here and travel just as fast, in many cases.
Expect potholes, washboard ruts, and probably loose, blown-out corners. There is likely to be gravel outside of the main tire tracks that could cause an extra challenge if you were to come off your line. Choosing to use a road bike is just fine, but more skill is needed when cornering at speed. Going up in tire size, compared with an everyday road setup, is recommended to lower the risk of pinch flatting and benefit from the pneumatic suspension of a higher-volume tire.
Tire size: 28–32mm
Event examples: Gravel Worlds (Nebraska), Dirty Devil (California), Boulder Roubaix (Colorado), Belgian Waffle Ride (California)
These infrequently maintained roads require a higher level of skill if you're going to tackle them on a road bike. Exposed rocks, tire-eating rain ruts, sand bogs, and any number of other unexpected challenges could arise around the next corner. Unless a Category 3 gravel section has been added to the ride as a short connecter between smoother terrain, a gravel bike with 33–38mm tires that offer side knobs is the recommended equipment to achieve both speed and safety.
Tire size: 33–38mm
Event examples: Rebecca’s Private Idaho, Mid South 100 (Oklahoma), Crusher In the Tushar (Utah), Rock Cobbler (California), Coast to Coast Gravel Grinder (Michigan), unPAved 120 (Pennsylvania), Rasputitsa (Vermont, though be prepared for snow).
Unmaintained forestry roads fall into Category 4 due to deep ruts, rock gardens, and potential landslides left for the next gravel rider to stumble upon. Going with high-volume tires will offer pneumatic suspension, reduce the risk of a pinch flat, and provide greater traction in the corners and on steep, loose climbs.
Tire size: 38–42+mm, or Road Plus (650b wheels with 42+mm tires)
Event examples: Dirty Kanza 200 (Kansas), Grinduro (California), Lost and Found (California), any UltraCross event like Iron Cross (Pennsylvania) or Southern Cross (Georgia).
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