Tire Pressure Testing for Unbound Gravel
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Unbound Gravel is often called the Super Bowl of gravel. It is at least the most watched gravel race in the US, if not the world. If you spend a couple days in Emporia, KS that first weekend after Memorial Day you will talk tires and tire pressure until you are blue in the face. We thought, we might as well go do some tire pressure testing in the Flint Hills to help you optimize your set up for Unbound or any other gravel race for that matter.
We brought Ben Delaney to do plenty of laps of Divide Road along with myself to add some extra data points. Thanks to Ben for joining us and you can check out his video on his YouTube Channel, The Ride.
Choosing a Tire for Unbound Gravel
Before we get to optimizing the tire pressure for Unbound Gravel, we need to select a tire. If you are starting down this post, then you likely familiar with the intense tire debate at a race like Unbound. When it comes to selecting a tire for a gravel race, I think it comes down to three main topics and the order in which you would prioritize these changes depending on the course. For this year's edition of Unbound Gravel, I put them in this order:
1. Flat Protection
The slowest tire possible is the one you are on the side of the road trying to plug or put a tube in rather than riding. The 2025 edition of Unbound went north which has a reputation for being absolutely relentless sharp flint rock punctures. Having done this event a few times now and countless discussions with everybody from the pointy end of the race all the way to the breakfast club hopefuls, not puncturing is a big deal.
I look at puncture protection for this race as a go or no go type of requirement. The tire either has enough protection for the course, or it doesn't. This can be done in a number of ways from, rubber compound, to size, to puncture protection belts, or all the above. The first step here is to make sure you have a tire that is going to be able to reasonably complete the race.
2. Speed and Rolling Resistance
There are plenty of tires that go way too far on the puncture protection and it makes the tire really slow. You can pretty easily be looking at a difference of 25-30w between some of the fastest tires vs some of the slowest. This means this is no small consideration.
If you think you shouldn't be worried about speed on the course, you should be. Fourteen hours is a pretty common finishing goal as it will give you a 15-minute buffer to beat the sun. For somebody like me personally, that means I need to average about 200w for 14 hours to hit that mark. If I were to give up 25w in tire choice, that means I will finish about 1.2mph slower or 1 hour 17 minutes and 33 seconds slower. Obviously speed is important for the front group trying to win the race, but as somebody limping to the line after a really long day in the saddle, if you realize you could be done but you chose not to pay attention to the details so you still have another hour and 15 minutes to ride, it will feel really important riding through the dark.
3. Tire Clearance On Your Frame
This might seem like an easy selection, but like most things involved in optimizing equipment, there are many different pieces to keep in mind.
If the course is dry, the selection becomes a little bit simpler. From what we have seen on rough terrain, the answer is run the biggest model of your tire that you can fit. On my current Lauf Seigla, that means I can fit up to 57mm and leaves a ton of options.
The introduction of the infamous peanut butter mud of the flint hills, this decision gets much more difficult. You then are worrying about running a smaller tire to be able to clear mud, but not too small that you increase your puncture risk and rolling resistance. Selecting this tire size is really a judgement call that can get better with more experience riding events with mud and listening to the locals on how the roads typically respond to rain.
With all of this being said, I brought a few selections of tires with me to Emporia this year and had a pretty clear hierarchy of which ones I wanted to run in which order. My first option was the trendy 2.2" Continental Race King. Behind that I had Pirelli Cinturato H's in 45mm, and for the more apocalyptic conditions, I even brought a Pirelli Cinturato M in a 40mm variant.
As the course ended up drying out before race day, I ran the Continental Race Kings, but if it was expected to have quite a bit of mud, I was comfortable running the 45mm Pirelli's and if mud clearance was going to be the only thing that mattered, a set up with with 17mm of clearance felt like the least bad way to get through the day. Luckily, it was dry and I was able to ride my first choice in tire.
Selecting a Segment to Test Tire Pressure
There are a bunch of ways to select a segement to optimize your setup for, but I like to look at it as one of three options. I'll outline each one and why you might want to choose each of the options.
1. Where the Race Will Be Won
This is looking for the place that the winning move is most likely to happen. This can be because you want to attack here, somebody else likely will and you need to follow, or the sprint finish. 99% of people doing a race like Unbound don't need to worry about this. There are likely 20 people in the race with a real shot at winning the event, and its only those that should consider this option. It is also risky because the race might go somewhere else and you wasted all this optimization for something that never materialized.
2. The majority of the course
For a race like Unbound this is a really popular option. It can be tricky to use this in an event like BWR that is notoriously rough in a few sections but has a lot of road as well. For Unbound you are likely looking at fairly rough to relatively smooth gravel with some really chunky sections, so you optimizing for that category 3 gravel is likely a great option.
3. The roughest section of the course
If comfort is your priority or the roughest sections will be really bad and might end your race day if not addressed properly, then optimizing for these roughest sections is likely ideal.
For our testing these year, we chose two segments with the goal of highlighting the differences. We went to Divide Road at mile 40 which ended up being the roughest section of the course on race day. Then we also did a pavement loop nearby to show the polar opposites of how the testing could go.
Tire Pressure Testing Protocols
For our pressure testing we used the SILCA Tire Pressure Calculator and selected Cat 1 Gravel (23psi), Cobblestone (20psi) , Cat 4 (17psi), and what we referred to as "Stupid Low (14psi)" as the pressures we were testing. With Ben Delaney as my testing partner this gave us two sets of data to make sure we were seeing similar results as well.
We have done an in depth Chung Method interview with Dr. Robert Chung himself if you want to take a deep dive, but this Chung Method or Virtual Elevation method is what we used.
In a very short explanation of how this works is that speed is relative to the cube of power where rolling resistance is linear. This massive difference helps tell the difference between what is being impacted by rolling resistance and what is an aero change.
There are a bunch of different protocols but our favorite four are the ones below:
1. Out and Back - We used this on Divide Road. You want a course that has undulating terrain to help highlight the differences in elevation. You need to find a place where you can turnaround without the use of any braking.
2. Loop - This is what we used on the road in Eskridge. A small loop with a slight rise allows multiple spots to match the elevation.
3. Up a Hill - We have used this plenty of times in the past but riding up and down the same hill at a slow, medium, and hard intensity, helps line up what is an aerodynamic and rolling resistance change. We didn't use this on the day.
4. Rolldown - This is the easiest to use when you don't have a power meter. You simply roll down a hill multiple times. The power is a constant 0 so if you are short the power meter it can be a great option.
The big keys to this out and back course is to turnaround at the exact same point each time. We helped achieve this by placing a cone in the middle of the road and did a u-turn at the top of a roller each time to make sure we could turn as close to that as possible.
The next thing to keep in mind is that with a surface as variable as a gravel road, try to hold the same line as close as possible. It is never going to be perfect, but this will help give you the best average surface of the area you are riding.
Equipment Limiations
There are a few pieces of equipment required for the Chung Method to work well.
1. Power Meter - A dual sided power meter is better than single sided. Most people have power meters these days and the power ends up allowing you to determine the input on the bike. Without this you really are limited to the roll down protocol.
2. Speed Sensor - GPS Speed might be great for your general riding, but it is often times not accurate enough for this kind of testing and you will likely need to lower your expectations of a result if you try to use GPS speed for the test. You will likely not be able to get exact crr numbers but could still get good relative data such as this is faster or slower than that. You may also spend a couple hours testing only to get home and find out the data wasn't good enough and you wasted a couple hours riding back and forth, so be warned and get a speed sensor.
3. Altimiter - Most computers these days have a fully serviceable altimeter in the computer, but I have also had ones in the past that couldn't be relied on at all. if that is you, then it might make sense to borrow a computer or find an altimeter that could be added to the file as well.
5. Wind Sensor - Accurate wind measurements can be helpful, but the most important thing is to try to test on as windless of a day as possible or at the very least, a non-variable wind day. A steady 3mph wind will end up getting good results, but gusts of 20mph are going to make the results look off.
6. Aerosensor - The last piece of equipment that can be used is an Aerosensor. There are a few commercially available options on the market right now, but as of right now they are almost all pretty difficult to use and need lots of attention to detail to be useful. They certainly aren't needed, but could become helpful in the future.
Unbound Tire Pressure Testing Results
The testing we did was on Wednesday morning of race week and that is important because this road was virtually unridable about 24 hours before. I had my own ideas about what would be fastest and what I was thinking we would show is that we would continue to see lower pressure go faster until we went to the "stupid low" (14 psi) that would ultimately be too slow. This would give us an idea of how much bigger of a deal it is to be running too high of pressure vs too low. My thought was that the "stupid low" would be just a little slower, while the Cat 1 gravel would a pretty big penalty.
The thing I didn't expect was how soft and sand-like the surface would be on the day. It was pretty clear early we might see something unexpected when I stepped out of the car with mtb shoes on and immediately couldn't clip in because my cleats were so clogged from stepping in the middle of the road.
We saw about a 12w decrease in rolling resistance going from Cat 1 to Cobblestone pressures. Then another 10w from Cobblestone to Cat 4 for a total of 22w saved in tire pressure. This is about where I expected the optimal pressure to be.
The real surprise here was that the "Stupid Low" or 14psi on my set up was another 2w faster on this surface. This implies that the fastest pressure for that area on that day was likely between 14-17psi because this was the flattest portion of the curve, and as the pressure went up by 3psi increments, it was slightly more than a linear penalty.
Now that we saw the lower the pressure, the faster the set up was the question is why? This sort of breaks the rules of what we typically see in our calculator and why would that be. The answer was fairly simple. The surface was so soft that the surface itself was deforming under the tire. Just like trying to ride on sand, even the worst performance possible from a tire is still more efficient than the surface deformation. It a true sandpit, there really isn't a lower limit of pressure and on this particular surface we just saw the optimal pressure being extremely low.
What Pressure You Should Run at Unbound!
So after spending all this time finding that the optimal pressure on this surface was 14psi from what I tested, is that what I should run? In short, No. pinch flat risk, rim damage, and the fact that this section was only a couple miles of the 200, it doesn't make sense to optimize for that.
We looked at the road testing that I left out here (because it was mostly just a confirmation of expected behavior) we were already below optimal pressures. The majority of the course was somewhere between the road and soft gravel roads so I opted to run 18psi front and 19psi rear. This ended up being great and likely an optimal pressure for somebody trying to shorten a long day on the bike.
For everybody else I would say "it depends" on the course conditions on the day but for Unbound's north course the Category 3 or Category 4 gravel selection on our Tire Pressure Calculator will get you to a great option for race day, but if you can get out on course to confirm the days prior, that would be ideal.
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Travis Verhoff
Travis is a Product Manager at SILCA focusing on bringing products to the market that improve every aspect of your riding experience. He's also a huge fan of all things Unbound and dogs.
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