Gravel From The Ground Up: Tires & Pressure

In a couple days, I'll return to race the Peloton Gravel MOB in Ojai, CA. This was, in 2017, my first ever foray into gravel racing - and really bike-racing (without a swim before and run after) more generally. I'd not yet officially retired from triathlon, though I was pretty sure I was going to. I had spent the month of October - my first month at Zwift - doing a lot of running. My most ever - with two 100+ mile weeks, and I was pretty sure I was going to become a trail runner. But I was intrigued by gravel riding; in particular, at that time, because I thought gravel-tri was the one thing that might keep me as a triathlete.

I asked Diamondback to send me their most basic gravel frame - the $1200 (roughly) Apex1-equipped aluminum Haanjo Comp. It was a great bike that I very much enjoyed riding, though it was a bit on the heavy side and the geometry wasn't an ideal match for me. But I logged a bunch of miles - and two hard races - on it before I decided gravel riding was something I wanted to pursue more seriously. My gravel season culminated with the truly epic (even as much as that word is overused) Dirty Kanza - 206 miles through the flint hills of Kansas. At just under 11-1/2hrs, it was not only my longest bike ride, but my longest race of any kind, eclipsing my 10:09 finish at Norseman 2017.

With Gravel MOB, Rock Cobbler, Belgian Waffle, and Kanza - and the miles and miles of preparation I did for them - under my belt and more miles in 2018 on my bike that any other year in my entire career as a triathlete, I come into the 2018 edition of Gravel MOB as a very different racer. While I'm still more than a bit rough - especially on technical descents, I have a solid base and a real understanding of the equipment side of gravel riding that is vastly different than where I was a year ago.

It's hard to remember exactly what I thought then, but what I know now is that gravel riding is almost entirely about two things: tire choice and tire pressure. Front-end geometry is an important third factor - and we'll discuss that over the course of what will be an ongoing series (I foolishly thought at first I could fit this all into one - maybe two - articles) about gravel riding. I'll cover equipment from the ground up - tires all the way to helmets - as well as a lot of what I learned during my big training blocks to start the year. But more than anything, I want to drive home that gravel riding is not about the bike. It's about the tires. To quote the under-appreciated classic "Days of Thunder" - "tars wins races."

Tire Selection: all-ROAD vs ALL-road

Gravel, as a term, is an excessively broad catch-all. All-road is maybe a better term, though that is perhaps also deceiving since there are plenty of gravel races that involve long stretches on things that are most definitely not roads and very often venture into the realm of mountain-bike single-track. I extensively tested two sets of tires that I think neatly summarize the two approaches to gravel riding. The ALL-road Specialized Trigger Pro (38mm). And the all-ROAD Compass Snoqualmie Pass (44mm). I found both of these tires to be excellent, and while I rode them on many of the same stretches, I found them to be particularly suited to very different types of terrain. They are both competent on pretty much anything, but which tire is the right choice - and which is less than ideal - is what I'd like to cover in this first article. I will say that I think anyone serious about gravel riding should have both tires in their quiver. To offer a summary in advance, the Compass Snoqualmie is the tire that I wished I had chosen (though I didn't have a bike that would accommodate them at the time) for the Belgian Waffle Ride. The Specialized Trigger Pro is the tire I'm glad I chose for Dirty Kanza, where it has a multitude of wins on both the men's and women's side.

Tire choice and utility isn't just a matter of the tires themselves; it's also at least as much about tire pressure. When I first came to gravel riding, I ran the Trigger Pro at 50-55psi, which seemed insanely low at the time. For Dirty Kanza, I ran them at 42psi. And, critically, I found there to be a noticeable difference between the feel and performance of the tire off road between 38, 42, and 46psi respectively. While at first that seemed to be splitting hairs, I realized that difference was - in terms of percentage - roughly equivalent to the difference between 110 and 100psi for a 23mm road tire, something I'd definitely notice. 50psi - what I initially thought of as low - felt almost unrideable once I got used to the grip of the tire at 42psi. At 42psi, long stretches on the road were still manageable, though the Trigger is certainly not designed for pavement. As compared with the Compass Snoqualmie, it costs about 25w at 30kph/20mph based on some back of the envelope math.

For the Snoqualmie, Compass provided me with the "Extralight" version, which I think is fine for mostly-road application, but which I would not advise for anything with sharp rocks. I definitely pushed the envelope on what this tire was designed for, taking it on some trails that I previously rode most happily on 2.1in (53mm) aggressive MTB tires. And it performed admirably. I did suffer a few small nicks and punctures - in particular, the area where the tread meets the sidewall seems prone to damage when used in MTB-esque terrain with pointy rocks such as we have in Southern California, but the tire always sealed up. I used a mix of Orange Seal and Stans, and I will say that - at least in the Extralight version, these tires do live up to their reputation for requiring regular top-ups of sealant. I never suffered excessive bleed-through on the sidewalls, but I did get some. However, the tire is so universally good - it's been tested by Slowtwitch forum regular Tom Anhalt to roll as fast as a Continental GP4000S - both on and off road, it's worth a bit of hassle. On dirt fire-roads the tire has admirable grip and is just screaming fast. Plus, at 35psi, it just rolls over everything, on road and off. Yerba Buena is one of the nicest climbs in the Santa Monica Mountains, but I find it generally unrideable because the pavement is so bad. But with the Snoqualmie, I rode it regularly and loved it, especially because that same bad pavement keeps cars and motorcycles off of it. If you can steer clear of pointy and sharp objects, it's pretty close to the perfect tire. And, with a generous amount of good sealant, it handles the occasional puncture without issue; it seals reliably and quickly. The tire is made for Compass by Panaracer, so if you're a fan of their casings, you'll probably like this tire. And, if not, you might not.

Tire Pressure

Dialing in pressure for both tires was made much easier thanks to the Quarq TyreWiz. I like the TyreWiz for two big reasons. The first is that it's really easy to do field testing of different pressures. I did bleed-down testing - starting with obviously too high pressure and testing subsequently lower pressures until I was happy and then going even lower until I was unhappy again. As a testing and training tool, I think superb utility. In this case, you can simply pair it to Quarq's iOS or Android app and check your pressure for subsequent runs over a given stretch of trail.

But it's also useful as a tool for racing. For this, you really need a Garmin device that can run a ConnectIQ app. The ConnectIQ app displays pressure from two separate TyreWiz modules, though you'll have to figure out which field (left/right) corresponds to which TyreWiz since there's no way to forcibly designate one sensor as front and one as rear. But the ANT+ setup is seamless and easy once you have the ConnectIQ app installed; it just finds the TyreWiz sensors - probably makes sense to do it somewhere where there aren't a bunch of them around, as is the case when pairing any sensor. But once you're set up, you get 1hz updates on your tire pressure, which is far more useful than it might seem.

Tubeless sealant is miraculous stuff. And it keeps getting better. But it costs air pressure to seal a tire. How much? That can be a big question. If you puncture during a race, how much pressure have you lost? Can you ride it or do you need to stop and air up? And especially at a race like Kanza, where the terrain varies so much, it's also nice to know whether or not you have punctured in the first place, since the shift from washboard to smooth road to rough trail happens over and over and over, and it can - and does - play with your mind, especially as the miles grind on you. Being able to monitor my tire pressure in real time during Kanza was huge peace of mind. On the neat-but-not-essential side, it was also interesting to see how temperature affected pressure, with my tire pressure peaking at about 45.5psi at the height of the day's heat and sun exposure and then dropping as the temperature dropped - and air bled out over the course of the very, very long day - to about 38psi by the finish. Quarq blog post with data is here.

Closing Thoughts

I won't cover 650B tires in this post, because for most races they are a worse choice than 700C. But they do offer some interesting utility on bikes that accommodate them, and I'll cover why you might want a pair of 650B wheels in your quiver in the larger piece on overall equipment choice. Likewise, I'll save the impact of tire size on steering geometry for the next post, which is all about bike geometry and gravel since, after tire choice/pressure, that's the most important aspect of bike "tech" when it comes to gravel.

Riding the Compass Snoqualmie, I was shocked at the fundamental impact of contact patch. These are totally slick tires. The tread is no different, really, than what you'd find on any standard road race tire. The tires are just massive. And that massiveness - and the accompanying ability to run extremely low pressures - just gives you a ton of grip on most terrain. Loose sand is pretty sketchy, but it's always sketchy. Overall, I was astounded at how well a tire that rolled fast on the roads performed off-road over very technical terrain. I took a fully-rigid bike with drop-bars on trails that I would previously have only considered riding on a full-suspension MTB. And I never felt that my tires were holding me back. The Compass tires made me a more focused rider - I did, of course, have to be more judicious in choosing my lines, but they also didn't keep from being able to just enjoy myself ride.

The Trigger Pros made me appreciate the overwhelming value of durability and of the difference that a bit of extra grip - especially at the sidewall/tread junction - can make in looser terrain. But what really makes me love these tires is just how robust they are. I've abused these tires and put them, in the case of Kanza, through one of the harshest tests a tire can go through. And they've performed with aplomb. If you want a single choice for "gravel" riding, I cannot imagine a better tire.

But the nice thing about gravel is that it welcomes variety. Of definition. Of bike. And, of course, of tire. While I'd say an air compressor is a fairly necessary investment, and changing tubeless tires full of sealant is never the neatest of jobs, the variety and enjoyment by a choice of tire is hard to beat. I loved both of these tires, but for very different reasons. And as gravel continues to grow, I expect there to be even more great choices.

In the next piece, I'll continue working up from ground level and focus on geometry - particularly front-end geometry (front-center and trail) - and gearing.

[Disclaimer: Specialized and Compass both provided tires free of charge but without obligation or any other compensation. I am an ambassador for Zipp, SRAM, and Quarq and was provided the Quarq TyreWiz free of charge but without obligation or any other compensation. As a professional triathlete, I was sponsored by both Specialized and SRAM, but I have no existing monetary relationships with either company.]