After a wild season of gravel-racing bike development, BikeRadar's Liam Cahill and Simon von Bromley sat down with Nathan Haas, a Girona-based gravel racer who loves bike tech.
The conversation spanned all of the hottest tech trends from the gravel-racing world. We also got Nathan’s take on whether the UCI Gravel World Championships are difficult enough and the tech he can’t wait to try.
There was even a surprise in store for the eventual winner of the 2012 Tour of Britain when it transpired that we possess the jersey Haas never received. The Australian rider was awarded the victory after Jonathan Tiernan-Locke was banned following the identification of anomalies in his biological passport data.
But before we attempt to reunite Nathan with his jersey, here's a selection of his thoughts on 2025’s gravel trends, including ever-wider tyres, super-wide rims to match, what really constitutes gravel, and tyre pressure adjustment systems.
Mountain bike tyres for gravel
Liam: We’ve seen one of the most interesting trends in gravel racing being the use of mountain bike tyres. Is this something you’re tempted to do yourself?
Nathan: I think some of the science is flawed on how people test what is faster for a gravel tyre. They're using a roller connected to a strain gauge. They get the roller going to 30kph, or whatever the set speed is, and they measure the rolling resistance.
At that speed, some of the mountain bike tyres now are rolling as fast as a 40 or 45mm tyre on a comparative tread pattern. But the one thing that they don't take into account is the amount of energy it takes to get that tyre to that speed. So now we're actually talking about how many kilojoules of energy it takes to accelerate from 5kph or 10kph up to 30kph.
For me, the biggest limiting factor of these wider tyres is they take a hell of a lot more time to speed up, and people also forget that we're not in this perfect straight plane on a gravel bike all day long. When we’re accelerating, most of the time we're tipping the bike because we're out of the saddle, which is engaging more of the shoulder tread as opposed to that smooth middle that they’re testing.
So you're getting a huge bag of tyre, which is creating a whole lot more friction. It's also a heavier tyre. So there's that secondary part of the tyre, which is like 'how much rotational weight are we now adding to the bike?'. When you get a bigger tyre, it's like an exponential loss.
In Girona, Chad Haga and I have been doing a little bit of testing of our own. We tried to make it statistically as significant as we could with a small sample size, a climb that we do in Girona called the San Miguel challenge.
We test on days where it's less than 5kph wind, so essentially no wind. It's also quite covered, so even if there is a little bit of wind, it virtually doesn't affect the time on the climb.
So he tested his mountain bike tyres, which were the same tyres that Keegan [Swenson] raced at Unbound. He averaged 432 watts and the normalised power was within 2 watts of that. So it was a very smooth controlled effort and at the top his time was 12:25.
Chad went back and we did the test again a few days later, with similar conditions. He did it on a 40mm tyre and he did 434 watts, so 2 watts more, though the normalised was still within the same range.
Chad went 27 seconds faster up the climb. So, to me, sometimes the testing they do in these theoretical environments doesn't apply to real life. One of the challenges on this climb is there are quite a lot of switchbacks and steep kicks.
So it's about how you accelerate back up to speed and the mountain bike tyre holds you back in those situations. I should probably live by the saying of, 'never tell your enemy when they're making a mistake'. But I think I hate bad science more than I want my competitors to have a disadvantage because I think this trend that everyone is going towards mountain tyres is wrong.
However, for the masses of people that just want to enjoy riding their bike, have fewer flat tyres and definitely have more grip going downhill, you could definitely go wide.
Simon: You mentioned it's a climb and I wonder whether you think the mountain bike tyres in that test are disadvantaged by the fact that that test is all uphill, because they're heavier. Is it a weight thing and are there many gravel races that include sustained climbs like that?
Nathan: I would say I agree with everything you’ve said. Regarding the test, we think that's probably the most fair test there is to do for the fact that the biggest limiting factor in a gravel race is the climbing.
It's where I would say 99 per cent of races are won and lost. But I would say more in gravel it’s where they're lost. People tend not to make attacks like they do on the road. Gravel is a little bit of a last-person-standing event.
We don't hit speeds on the flat where sitting on a wheel can become challenging. So as soon as you're on the wheel at any speed that a gravel bike can do, that's not where you're getting dropped.
I’d say there’s only one or two races in the USA that are technically challenging. In Europe, that's almost the complete opposite. You have races like Houffalize, which is literally the old mountain bike World Cup downhill from the cross-country race. The Trakka, here in Girona, is insane. Wide tyres here can be a benefit.
But to come back to the main point, it's where you win and lose the race, so you need to become as efficient as possible on the climb. So if you're going to be losing 25 seconds just based on the tyre choice, I don't care how good you are on a downhill, there's very few people that can close down 25 seconds to a group.
I would prefer to be in the group and then managing myself on the downhill, if I'm choosing my preference, any single day of the week. So no MTB tyres for me.
Super-wide gravel wheels
Liam: Brands have been pushing rim width in recent years, but Zipp went super-wide with the 303 XPLR. What do you make of the trend for wider rims?
Nathan: Bigger has definitely become better, going from a 21mm internal width to a 23mm and into a 25mm internal width. It's been a really positive change.
But where does it become a negative? Where does it tip to the other side and become detrimental? The question can be answered in a few parts.
I think the first way to start is to talk about an ice-cream cone. I think it's a lovely metaphor for describing how tyres sit. So if you have your cone (the rim), and then you have your ice-cream ball on top (the tyre), the ice cream can roll off that cone quite easily.
You try to corner at speed on the road, but you get that sensation like the tyre's going to pop off and it's a very scary thing.
But there is an advantage to it. When you're hitting rocks, roots and everything, you want it to be able to move, and that is an advantage as soon as you get off-road.
My take is that internal rim width needs to be proportionate to the size of tyre that you're using. If you put a 55mm tyre on a 25mm narrow rim, it’s going to feel unstable. Conversely, when a tyre is sitting more inside the rim, your sidewalls are more exposed to getting flats and you're potentially spending less time on top of the tread, making you slower.
It’s all about balance.
Are the UCI gravel worlds really gravel?
Liam: I saw a lot of grumbling from fans after the UCI Gravel World Championships because, in some people’s eyes, the course wasn’t tough enough…
Nathan: I always laugh when people say it wasn’t gravelly enough. It's like, are you seriously saying that a gravel rider would have done better against Mathieu van der Poel if it was more technical? Wake up man, he's a better bike handler than any of us on any day of the week, even if he was holding the handlebars with one arm.
So it's one of the gripes that a lot of people always say, when they didn't win a race. “Oh no, it was too much like a road race, I'm better at gravel”. So what you're saying is it was more about fitness than skills? You can say that a little bit, but even if it was more about skills, Marianne Vos, Mathieu Van der Poel – the two winners – will out-ride anybody in the gravel peloton on any day of the week.
So… It wasn't gravelly. Well, I'd ask the next set of questions: would you have used a road bike? Hell no. Would you have used the mountain bike? Definitely not. So, quite literally, the definition of what this race is, is a gravel race because the first bike that everybody would choose is a gravel bike.
I thought it was one of the coolest bike race courses that I've done. My only issue is that the UCI continues to disrespect the sport of gravel by calling this a UCI gravel worlds with how they rank the riders on the start line. I think someone showed me a statistic of the top 30 riders on the grid. I believe three of those riders were gravel riders.
So I think the UCI needs to wake up and I think they need to also make rules that people have to have done a UCI gravel race to be eligible for the world championships. Wouldn't it be smart of the UCI to force those riders to have to go to some of the races all across the world?
And it will also help the local promoters who are putting on these UCI races, who are paying a lot of money to have the UCI letters on top of their banner. So I think the UCI needs to consider a few more things to make the Gravel World Championships fairer.
It would also make the Gravel World Championships something that's actually representing the sport that it is, because it's absolutely its own sport now.
Our favourite gravel tech of the year
Liam: Nathan, I don’t know if you saw this, but Matt Holmes’ prison phone was my favourite gravel tech of the year. It’s simply ridiculous and brilliant. What’s your favourite tech from this year, maybe something you want to try in 2025?
Nathan: I'm working on a bike I think will make a bit of noise. I'm not gonna give anything away. Not at this point. But I just think that the one piece of tech that's really gonna change gravel going into the future is Gravaa.
This is the biggest jump in bike tech since we found disc brakes or wireless shifting. This is huge. The fact that I could maybe bring my tyres down to 15psi for technical sections and then when I get onto the road, put it up to 50psi is amazing. And then if I start getting a flat, I'm using my own power to pump the tyre up and to keep air in it.
I think [it's] one of the biggest future developments for a racing circumstance anywhere. I don't know how it applies necessarily to a recreational rider, but for racing, Gravaa is going to change the game.
I think Gravaa is going to be a pretty exciting new thing to jump onto the scene and I'm hoping to get my hands on one as soon as they're available to the public.
