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Picking tires for gravel racing and riding is complicated. There isn’t one right answer. Sometimes there isn’t any right answer. Tire selection is a game of overlapping variables and endless options. We attempt to match a course, surface type, rider skill, rider size, and rider intention to a tire’s qualities, its strengths and weaknesses. The right tire for you isn’t the right tire for me, and the right tire for this gravel race certainly isn’t the right tire for that one. That makes picking the right tire for a gravel race rather daunting.
So, let’s try to simplify it.
This story started out as a little back-of-the-napkin doodling, sitting in the grass after last weekend’s Wild Horse Gravel event in western Colorado. That doodling led to a realization that’s obvious in hindsight: the relationship between tire size and the amount of dirt or gravel in a given course is roughly (and that’s the key word: roughly) linear. More dirt, more tire.
Let’s start with the doodle, and then we’ll go into a step-by-step process for honing in on a good tire for your event.
Step 1: How much dirt and gravel will you be riding?
Your first consideration is the balance between off-road and on-road sections. You want a tire that can handle the off-road, but still rolls well on-road. Many events will list an approximate split between road/off-road sections, or you can take a close look at any provided course maps and estimate.
Our rules of thumb:
100% pavement -> 25mm tires. This is called a “road race.”
75% pavement -> 28-32mm tires. You want a bit of extra reliability, but not at the expense of speed on asphalt.
50% pavement -> 32-36mm tires.
25% pavement -> 36-40mm tires.
0% pavement -> 40+mm tires.
These figures are purposefully oversimplified, because there are a million and one caveats. But they are are a good place to start.
Let’s dig into other considerations.
Step 2: How comfortable are you on dirt and gravel?
All of the figures above assume average comfort on dirt and gravel. You may have raced a bit of cyclocross, but aren’t a ripper mountain biker, and don’t love it when your tires start sliding sideways.
As a rule, riders with above-average off-road handling skills (don’t confuse these with on-road handling skills, a totally different skillset) can get away with a slightly smaller tire. Subtract 2-3mm or so. This doesn’t mean these riders should always go smaller; it means they can if they want to. On courses with a bunch of pavement, for example, a smaller tire may be faster overall.
Riders with below-average off-road handling skills (this is most pure roadies, or anyone who hasn’t spent much time on loose surfaces) may want to increase tire size by 2-3mm. So if an event has a 50% pavement/dirt split, you’re looking at a 36mm-38mm tire, instead of one in the 32-36mm range.
Step 3: What does the surface look like?
Not all dirt and gravel are created equal. The flint hills of Kansas, where Dirty Kanza is raced, shred tires that would be more than adequate elsewhere. The smooth dirt roads of Colorado can be ridden on road tires, if you’re careful.
Surface changes will have an impact on both tire size selection and tread selection.
This means you need some intel. What do the gravel sections at your event look like? Are they rocky? Sandy? Smooth? The rougher the surface, the more girth you need to add to your tire setup. If sharp rocks are an issue, extra tread will help protect against tire cuts.
The looser the surface, the more tread you need. Packed surfaces are easily rideable on semi-slicks or slicks. Kitty-litter-over-hardpack, as is common in the Western US, is best with a small knob. Mud requires a bigger knob.
Don’t forget that your front tire is the one that does the steering, so if you’re looking for a bit of extra bite, throw a bigger/knobbier tire on the front. The rear doesn’t matter as much.
As a general rule, if a course has extended sections of sand or loose gravel, go up 2-3mm in tire size. If its off-road sections are predominantly packed dirt, go 2-3mm smaller.
A pile of other caveats
All of this makes a couple important assumptions. First, that you’re running tubeless. If you’re running tubes, size up and increase pressure.
Second, this assumes you’re running 700c wheels. 650b setups, usually paired with wider tires of 40-60mm, are fantastic for events in the 50-100% dirt range. I ran a 50mm-wide 650b tire setup at Grinduro last year, for example, because of the long, singletrack downhill section. 650b does tend to be a bit slower when there’s more pavement. Note that 650b and 27.5” are exactly the same thing; mountain bikers just like to use inches.
The third assumption is that you’re trying to go as fast as you can. But a lot of these gravel events are mullet races — business at the front, party at the back. If you’re in the long, flowing locks of the party mullet, go a bit bigger with your tires. You’ll be more comfortable, less likely to flat, and will therefore have more fun.
Tire selection is as much art as science, really. It comes down to personal riding style, one’s own propensity for flatting, the specific surfaces you’re going to encounter, and how fast you really want to go. There are no right answers, and, for the right rider, no wrong answers either. But the rules above are a good place to start. From there, experiment. Find your gravel tire happy place.