Hi, we are back with our second installment on the subject of Gravel Cycling to talk about all things rubber – the tires we use and how they might differ from what you might be used to in the paved realm. Three tire characteristics will govern the quality of your gravel experience, all of which we need to get right: Tire width, tire tread and air pressure.
If you are riding a standard road bike you are probably on tires ranging from 23mm to 28mm in width. If you have ever had to navigate gravel on 23mm tires you understood the importance of width as those skinny hoops tried to keep you upright ploughing a wobbly groove in the road. Floating on the top beats carving grooves through the gravel. While opinions on tire width vary, most gravel riders would agree 32mm is the minimum width that will keep you on top of the gravel. Go for tires that will keep you comfortable and safe, keeping in mind the maximum width tire you can put on your bike is limited by the clearances in the frame and fork.
There are two main things we need to understand about air pressure, the first is the relationship between air pressure and rolling resistance. If you are used to running something like 25mm tires on the road you have probably been pumping them up to 100 pounds or more of pressure. This is in line with what used to be the prevailing wisdom that higher air pressure equated to lower rolling resistance and hence faster speed. More recent data from both lab and road testing have proven this old wisdom mostly wrong. We will spare you all the details and condense the findings to this: High pressure causes tires to bounce over bumps, converting hard earned horizontal motion into wasted vertical motion and transmitting road harshness into the rider's body leading to fatigue. Low pressure allows the tire to deform on the bumps, absorbing the shock conserving energy and your body. High pressure is good on perfectly smooth roads, but an impediment everywhere else. Our local paved roads tend to fall into that everywhere else category, and out in the gravel world things get even rougher.
How much pressure you need is governed by the relationship between pressure and volume. As tires get larger they hold a larger volume of air and require less air pressure to safely support the weight of bike and rider. In 23mm tires, you need 100psi or more to prevent pinch flats resulting from the rubber from getting pinched between road and rim on bumps.
In a wider tire, say 40mm, 60psi might provide the protection, and have the same feel of firmness as the 100psi in the 23mm tire. If you put this information together with what we said about rolling resistance above, you will see that we don’t need to ride on hard firm tires; we want our tires to be soft to absorb the bumps. The good news here is that with the added volume provided by wide tires, there is plenty of buffer between rim and road for you to ride at surprisingly low pressures. When width and pressure are right, we can be Fat and Fast. The actual pressure you ride at will depend on several factors like rider weight, the relative gnarliness of the roads, and how much road vibration you want to tolerate. One guideline that might be helpful: if you feel like you’re running a jack-hammer while cruising the gravel, you have too much air in your tires. After mounting new tires, I find it helpful to take a gauge and pump out on the road and experiment.
We mentioned pinch flats above, and one of the challenges common to both mountain biking and gravel biking is finding the point where pressure is low enough to smooth out the bumps, but high enough to avoid the pinch flat. Tubeless tires, now standard in mountain biking, takes the pinch flat out the equation - there is no tube to pinch, and hence no flat. The tubeless set-up requires tires and rims specifically designated as "tubeless" and a liquid sealant that rides inside the mounted tire. The sealant is formulated to ooze out through any puncture holes in the tire and congeal to stop the leakage of air. It's not uncommon to ride an entire season with punctures in a tire without ever finding out about it until you take the tire off and examine it.
Finally, the topic of tread. Simply put, you probably need less tread for gravel than you think you do. Traction, or grip, is dependent on the amount of rubber that actually meets the riding surface. The deep tread knobs typical on mountain bike tires can find traction in sand, loose dirt or muddy conditions because the tread sinks in and gets surrounded by the surface material. Gravel behaves differently - the same deep tread typically provides no advantage in gravel. The most contact on gravel is provided by smooth, supple tires that can deform around the individual gravel chunks.
Some riders will choose to ride tires with deeper tread patterns for added traction based on their local gravel characteristics, which may include sandy or wet road conditions. In some places, the Flint Hills of Kansas for example, the roads are covered with very hard sharp flint chips which can be deadly to thin-skinned tires.
A final word: everyone with experience in the gravel has opinions on what works best in choosing tires, tread and pressure, and we are no different. Talk to other riders, do your research, but most of all, get out there on the road and find out what works for you. When you come back wearing a smile, you'll know you're getting it right.
Come back next week, for a discussion on the bikes we love – the All-Road Bikes.
At FBC the owner and staff have a passion for cycling – we all ride to help you ride better. Come and see us in Freeport at 120 South Chicago in Freeport to find out how we can help you get full enjoyment out of your cycling experience.