Choosing the right rims, tires, and tire pressure affects your comfort and the capabilities of your bicycle.
Bicycling is a pastime and a sport. That makes it personal. We all have favorite places to ride. We all have preferred riding styles – ranging from fast on a day ride on perfectly paved roads to more slowly while bikepacking for a week or more on a route that includes unpaved back roads. Once off the paved road, no two tours or gravel rides will be the same. “Rail trails” and many Midwest gravel roads are nearly as smooth as paved country roads. More remote forest roads change from easy to impassible with every rainstorm.
We love the adventure and challenges of gravel cycling! The previous article, "Why Your New Bike – or NEXT Bike – Should Be a “Gravel” Bike!" covered the MANY reasons why cyclists are rediscovering the joy of unpaved roads. But we don’t want to flounder because we were unprepared. In the words of Selene Yeager, author of Gravel, The Ultimate Guide, “Gravel remains largely undefined, which is exactly the point. It’s supposed to be an adventure.”
Selene gets to the main point by stating: “TIRES! Forget about the bike! Those metal triangles with handlebars are just a vehicle for your tires! Okay, that’s hyperbolic, but not as much as you might think. Gravel bikes, by nature, are designed with primarily one thing in mind: being able to fit big, ground-gripping tires in the frame while still delivering a fun, functional ride. It’s impossible to overstate the difference the right tires and tire pressure can make on your overall gravel riding experience.”
The best bike and rider are rendered helpless with the wrong tires. How do you sort out the many tire and wheel options available? Scan the specs of gravel bikes, touring bikes, and MTBs and you will be bombarded with a confusing array of inches and millimeters, plus designations with “C” and “B” added to rims and tires.
First, let’s look at the rims found on gravel bikes. Gravel bikes fit between road bikes and mountain bikes, with some overlap on both ends of the range. In his book, Gravel Cycling, Nick Legan tries to sort out the options. He writes, “When describing a rim, road bike companies will describe it in metric terms, while mountain bikers stick to imperial measurements. This is maddening, especially when they are describing the exact same dimension.”
For example: 700c = 29 inches
This is by far the most common size on road bikes, cyclocross bikes and gravel bikes. Using the same diameter rim, mountain bike companies also use this size, calling it a “29er”, the approximate overall diameter with a 2-inch tire.
All bike frames are narrower at the apex of the rear triangle and fork crown. Therefore, many road bikes are limited to 32mm tires – which is also the limit imposed by ICI, the governing body for cyclocross racing. In contrast, many gravel bikes can fit tires up to 45mm with 700c wheels.
To accommodate wider tires, you need to use smaller diameter wheels. 650b = 27.5 inches
This size is the middle ground between 29” and the traditional 26” wheels, which have become largely obsolete on mountain bikes. Gravel riders tackling single track trails have adopted this the rim size to allow for tires 47 to 50 millimeters, or wider. Round-the-world bike tourists opt for 650b wheels for the larger tire cross-section, which provides more weight capacity.
With the slightly smaller diameter 650b rims, you gain space for larger tires on the same frame. The bike that can fit 700c x 40mm tires may have clearance for 650b x 50mm tires, or larger. Note also that many riders mistakenly assume bigger is better. They assume that 700c wheels are ALWAYS the fastest wheels on and off road (or 29ers, in the case of mountain bikes.) However, the rolling resistance is factored by both tire diameter and width. You will find that 700c rims and 38mm tires have basically the same diameter as 650b rims and 50mm tires. Since the gear ratio of your bicycle factors in the tire diameter, these two wheel sets share the same gearing, as well identical bottom bracket height for handling and clearance, along with identical toe clearance when steering.
Which tire and wheel combination is BEST? How do you choose?
The answers depend on many variables, but the two most important factors are the total weight of the rider and bike, then the road/trail conditions.
First, let’s discuss rider plus bicycle weight. The heavier the cyclist, bike, and gear the higher the tire pressure that is required. Higher pressure provides a rock hard tire that rolls with less effort but is also tiresome as it transmits more road shock and vibration. Lower pressure offers a softer ride on rocks and gravel. But lower pressures also increase the likelihood of “pinch flats” when the inner tube is cut by the rim on rough trails. Add 20 or 30 pounds of bikepacking gear and you will need higher tire pressures.
For example, a 110 pound woman may need only 50 PSI while a man with gear totaling 200 pounds may require 75 psi while riding the same rims and tires on the same gravel course. That is 50% higher air pressure for the same tire on the same road.
At this point we need to take an important sidetrack to discuss TUBELESS tires, especially for gravel and MTB tires. Selene Yeager doesn’t hesitate when she says, “There’s one thing all the experts agree on 100 percent: Go tubeless. No matter what size and tread you choose, make sure they are tubeless. The ability to run lower pressures will dramatically improve your ride experience.”
Modern tubeless tires really have NO downside*. The ability to ride with lower pressure will result in a more comfortable ride. By removing the inner tube, you also benefit from a significant reduction in wheel weight. More importantly, this means a reduction in rotational mass, which means you need to expend less energy to rotate a lighter set of wheels. Third, the latex tire sealant instantly plugs pin holes from nails, glass, and other roadside debris. Who doesn’t LOVE fewer flat tires!?
By design, tubeless tires also ELIMINATE the chance of inner tube pinch flats Selene Yeager relates her personal experience, “My first foray into DK200 (the 2013 Dirty Kanza 200-mile gravel grinder), I was still running tubes, as was most of the field since tubeless technology hadn’t really taken off yet. Despite running 60 PSI and babying every rocky section and cattle grate, I still pinch flatted a little more than halfway through. The second time, I ran tubeless with a cushy 30 PSI and rolled start to finish, barreling through rock beds and stream crossings, with no tire trouble.”
Today, most quality bikes are equipped with rims that are TUBELESS COMPATIBLE. Most bikes are sold with inner tubes, but the tires and rims are easily converted to tubeless with the addition of sealing rim tape, a tubeless valve, and tire sealant.
Back to our original example but substituting tubeless tires, a 110 pound woman may now need only 23 PSI while a man with gear totaling 200 pounds may require 36 psi while riding the same rims on the same gravel course. This is still the same 50% air pressure differential for the same tire on the same route, but the comfort level, speed and efficiency are increased exponentially.
Now let’s mix tire sizes: the 110 pound woman may run 23 PSI in her 38mm 700c tubeless tires. But the 200 pound man can safely run only 25 PSI in his 47mm 650b tires. The larger tire has more air VOLUME to support the load and requires proportionally less air pressure. It is quite common for gravel bike models to offer interchangeable 700c and 650b wheels. Or in the case of mountain bikes, interchangeable 29er and 27.5-inch wheels.
You might be a lightweight, aggressive gravel racer or a bikepacker that is loaded for a cross-country expedition – or BOTH at different times of the year. There is a wheel and tire combination that will match your riding style and goals. Attending a distant “gravel grinder”? You can travel to the event with BOTH sets of wheels and decide when you get there whether 700c or 650b tires are the best combination, based on the route condition and weather!
This brings us to the second consideration when choosing tires: road or trail conditions. While pavement is fairly predictable, unpaved roads vary tremendously in trail quality, elevation, and more. Some rides include rough single track while others have a mix of hard-packed gravel and pavement. The same route can vary tremendously in different seasons and weather. The Mid-South Gravel in Oklahoma is notorious for deteriorating into a mud bath when it rains. The Unbound Gravel is famous for chewing up tires in the Flint Hills of Kansas. Every event is labeled a “gravel grinder” but there is no standard definition of “gravel”!
With that background, Neil Shirley decided to create some categories. Neil is a former professional road racer on the National and International circuits. More recently, he has become a gravel cycling convert and is a past champion of the Gravel Worlds in Nebraska. Using his experience, he created the “Industry Standard Guide to Gravel” (ISGG), and it has been adopted by event promoters and cyclists alike. For a complete description of the various categories, see “Making Gravel Standard” at GranFondoGuide.com. Here is a summary of the categories with the size tires that Neil recommends for each. Ken Avery of Vittoria Tire has added the tread recommendations:
Category 1 Gravel: Smooth, well-maintained dirt/gravel roads. The crushed gravel secondary roads in the Midwest are often hard packed and suitable for road bikes with tires as narrow as 25mm (1-inch) – when dry. The original 10-speed bikes had 1.25” tires (32mm) and logged millions of miles on these roads before “mountain bikes” or “gravel bikes” were conceived or named. Modern gravel tires with a slick center tread and light texture on the shoulders up to 35mm are perfect. See the Vittoria Terreno Zero below:
Category 2 Gravel: These lesser-traveled roads have packed tire tracks, but loose dirt and gravel between. Expect lots of potholes, washboard, and loose dirt on curves – all requiring more tread and more finesse. Tires up to 38mm provide protection against pinch flats and cushion in rough sections. The centerline needs a light texture, with more pronounced tread on the shoulders. Avery says, “I use a fish scale design in the center. When you’re rolling, the scales are flat and smooth, but when you hit the brakes, those edges stand up to give you traction. See the Vittoria Terreno Dry below:
Category 3 Gravel: These “Jeep” roads may be mapped but are seldom maintained. You will encounter deep, eroded ruts and exposed rocks, stretches of sand washed or blown over the trail. If it has been wet or thawing, expect puddles, runoff, creeks, and plenty of mud. Your tires must be wider for traction and protection from the obstacles. Plan on 38mm to 40mm widths, or wider. For muddy roads, Avery has these recommendations: “A good mud tire has a mixed tread, with the knobs tighter in the center so it still rolls faster, but with more spaced out toward the outside.” See the Vittoria Terreno Mix below as an example of a tire that can handle all conditions:
Category 4 Gravel: These hardly qualify as roads. We are describing forest trails, ATV trails and mountain bike single tracks. Your bike must roll over washouts, ruts, tree roots, rockslides, flowing water, snow, and slush – all while climbing or descending sharp hills. These primitive trails are also a key element in most ultra-endurance events. You need wider tires for traction and cushion. Look for 700c tires up to 45mm - if they will fit your frame. If not, or for self-supported ultra-endurance events, go with 650b wheels with 47mm to 55mm tires.
Typically, mountain bike tires start at about 55mm (2.1”) and get wider. The fast-rolling 27.5 x 2.1-inch Vittoria Mezcal is a favorite of endurance racers. This tire size is at the narrow end of the tire spectrum for between mountain bikes and the upper, widest tires that will fit some (not all) gravel bikes.
Sharing this 2.1” or 55mm tire presents a new choice in bicycle models for racing and bikepacking. For hard-core singletrack, riders can opt for a traditional hardtail or full-suspension mountain bike. But for gravel racing on muddy routes or “mountain bike” racing on forest roads a drop handlebar bike with wide 650b (27.5”) wheels is lighter and faster.
These wide tire bikes with drop bars have been dubbed “Monster Cross” for blurring the line between mountain bikes and gravel bikes and have become favorites for extended bikepacking and endurance events like the Tour Divide. Many GDMBR riders choose lighter gravel bikes without suspension forks and rely on the fat tubeless tires for both ride comfort and load carrying. You may think that the wider 650b tires are slower than 700c tires. In fact, the wider and fatter 650b/27.5” wheels and tires are usually lighter than the larger diameter 700c wheels. That means less “unsprung” weight and lower rotational mass which contributes to quicker handling and better acceleration. Here is an overview of the bikes and equipment selected by participants in the 2021 Tour Divide/Great Divide Mountain Bike Race.
Note that there are MANY exceptions that experienced riders add to these categories:
1. If you are brand new to riding unpaved surfaces on a drop bar gravel bike, consider moving up to the next larger tire size for increased traction and stability.
2. Use local information to choose the best tire size. Have the roads recently been graded? Have recent rains exposed sharp ledge rock and roots? When in doubt, err towards a larger rather than smaller tire.
3. High pressure tires actually INCREASE rolling resistance on rough roads. The hard bumps throw off your momentum and cause fatigue. Larger, softer tires soak up the bumps – and smooth is FAST! For this reason, many experienced racers (and long-distance bicycle tourists) choose the next larger tire size over the recommendations above. This is especially true for longer distance events and extended bikepacking where comfort and flat protection are worth the slightly increased rolling resistance.
I recently completed a fast, 120-mile S24O (Sub 24-hour Overnight) bikepacking ride with over 50-miles of crushed rock roads. While my 700c x 32mm tires were perfectly capable in terms of traction and load carrying, I longed for wider 650b tires for a smooth and supple ride!
4. It is better to use a narrower tire in the mud. You want to slice through the top layer into the firm base. A fatter tire will squirm and throw you. Even worse, reduced tire clearance will quickly jam the frame with mud, stopping you completely if not wrecking your drivetrain. Some riders use a narrow tire on the rear wheel, and one size wider on the front as a compromise.
5. There is no law that requires you to run the same tread on the front and rear tires. After all, your bike is “one wheel drive” and you may like a stiffer, heavier tread on the rear wheel, paired with a smoother tread for the front tire.
6. Narrow tires have less air volume, locking you into a narrow range of pressures. Wider tires have a greater range of volume, permitting you to “air down” on soft or rocky sections – then add air for hard packed or paved sections. Narrow, hard tires may lose traction and spin out on hills, forcing you to dismount and push. More air pressure is needed to support the weight of rider and gear for bikepacking. Again, if in doubt, choose the next larger size tire.
7. Finally, each event or tour may encounter many road surfaces and combinations of conditions – from smooth pavement to rocks to sand and everything in between. In the end, the best you can do is play the percentages. Determine what are you expecting to see the MOST of – then gear up accordingly. Don’t make your decision based on the worst short sections of gnarly trail. This means that you may be totally unprepared for some stretches. The goal is simply to be equipped for 98% of what you expect to encounter. On a multiday tour, you can’t change tires unless you find a bike shop. For the unrideable hike-a-bike sections, well, “Embrace the Suck!”.
One last point: You should also consider the rim WIDTH when choosing tires, that is, the inner tire bead seating surface. If you install narrower or wider tires on the same rim, the result will be different outer dimensions and side profile. Similarly, diameter and cross-section changes when you install the same tire on a narrow versus wider rim. Consult the specs for your preferred tire to see the recommend rim width.
Of course, the discussion of tires would not be complete without discussing flats – which can still occur with tubeless tires. Here are some Pro Tips:
· Tubeless tire sealant is a thick latex liquid that dries out over time. You need to check the sealant every few months (and before traveling to every gravel grinder), then refresh if necessary. Adding and checking your sealant requires a valve core tool.
· Tire sealant won’t clog a big puncture. For big holes, carry a tubeless plug kit. The plug is soft, sticky rubber that reduces the leak and allows the liquid sealant to fill the remaining gap.
· If all else fails, install a tube. Carry one of the tubes you saved when converting your tires to tubeless. It’s also a good idea to carry a patch for a slashed tire sidewall, although a dollar bill works remarkably well! To make roadside repairs, you will also need to carry two or three tire levers.
· Don’t forget a tire pump. Some riders swear by CO2 cartridges, but you still need a pump. Might as well add a few inner tube patches, just in case.
· Some bicycle tourists choose to rely on tubes. Fixing flats may be a pain, but even with tubeless tires a long-distance tourist must carry at least one inner tube and a patch kit anyway. Tourists also typically opt for heavier tires with Kevlar inner layers to repel most punctures.
*There is one minor downside to tubeless tires. Some cyclists consider gravel bikes to be the ultimate commuting vehicle. The added frame clearance allows both wider tires and mud guards. For warm climates, tubeless tires are perfect. However, if you are a winter commuter, you will need special cold-weather sealant that won’t freeze. The good news is you can run the “anti-freeze” sealant year round. But you should be checking and refreshing your tubeless sealant at least every six months anyway.