Are gravel bikes slower than road bikes?Jan Heine
The euphoria about gravel bikes is hitting a snag: Many riders feel that their gravel bikes are slower than their road bikes. For example, James Huang, the technical expert from CyclingTips.com, posted:
“I’ve been spending too much time on gravel and mountain bikes lately. Good to be reminded what real speed actually feels like.”
Global Cycling Networks even did some – admittedly unscientific – testing:
They ran a set of racing tires and a set of gravel tires. They did three tests: on a roller, on the road and on gravel. The result: The racing tires were faster on all surfaces.
Is it true? Are gravel bikes slower than racing bikes? The answer is: ‘It depends.’ There are a few things that can make a gravel tire slower:
Aerodynamics of the bike: Wider tires catch a tiny bit more wind, but a 10 mm increase in frontal area doesn’t make a huge difference. Otherwise, steel frames with skinny tubes would be faster than carbon bikes with large-diameter down tubes.
Differences in airflow over tire and rim can be measured in the wind tunnel, but the effect is very small compared to the overall wind resistance of bike and rider.
(Bicycle Quarterly’s wind tunnel tests found no significant difference in the wind resistance with a 25 and a 31 mm tire.)
Aerodynamics of the rider: The rider is responsible for most of the wind resistance. Many gravel bikes have a more upright riding position, which is less aero. There is no reason why a gravel bike couldn’t use the same riding position as a racing bike (above). After all, gravel roads are roads, and there is no need to change the position just because the ground is a bit rougher.
(The more upright position may be more comfortable, especially for long rides, but this applies to road bikes just as much as it does to gravel bikes.)
Rolling resistance: Most gravel tires today are built like mountain bike tires, with stiff casings that absorb more energy and transmit more shocks than supple racing tires. GCN’s testing (above) shows what happens when you compare apples to oranges: The supple racing tire (GP 5000) requires far less power than the stiff gravel tire (Terra Speed) – not just on pavement, but also on gravel.
The higher rolling resistance is due to the tire’s casing, not its width. We know that with the same casing and tread rubber, wide Rene Herse tires roll as fast as their narrow cousins (at 22 mph/35 km/h). The (very small) increase in wind resistance of the wider tires is compensated by a (very small) decrease in rolling resistance.
(GCN’s table also shows that high pressures roll faster – that is only true on rollers, where the convex roller digs deeper into a softer tire.)
Weight: Wider tires are heavier than narrow ones, but not by much. A 650B x 48 mm Rene Herse Switchback Hill Extralight tips the scales at just 413 g. Still, a set of Switchback Hills will weigh about 450 g more than our featherweight 700C x 26 Cayuse Pass (183 g each) – but that difference is little more than half a water bottle.
The Rene Herse Cayuse Pass is one of the lightest tires you can buy. If you are running a more standard racing tire, like the Continental GP 5000 (221 g), the difference to the 48 mm-wide tires shrinks to just 384 g. And you get some of that weight back, because the smaller 650B rims and shorter spokes weigh a bit less.
(If you are the type who notices whether your water bottle is half-full or totally empty, then the weight difference may matter to you. For the rest of us, bikes like the sub-17-pound Open U.P.P.E.R. above are plenty light.)
Q factor: Many ‘gravel’ cranks are very wide (above). Road cranks rarely have Q factors of more than 150 mm. For many cyclists, a wide Q factor makes the cranks harder to spin. However, the wide Q isn’t necessary: Many modern gravel and all-road bikes are designed to accept standard road cranks.
(Due to carbon’s lower density, you need wider chainstays on a carbon bike. However, the dropped chainstays pioneered by Open have solved the problem of fitting wide tires between narrow cranks.)
Cornering feel: The rotational inertia of wider (and usually heavier) tires makes many gravel bikes feel sluggish compared to a good racing bike, especially when climbing or sprinting out of the saddle. The solution is simple: Use smaller wheels and lightweight tires, so the rotational inertia is the same as on a good racing bike.
(Some riders prefer the more stable feel of bigger wheels and tires. For them, ultra-wide 700C tires are a great choice.)
So we’ve seen that gravel bikes don’t need to be slower than racing bikes, if they are built to the same specs. Why does the industry insist that racing bikes are faster?
I suspect the reason is simple: The industry wants to sell more bikes. The thinking seems to go like this: Now that many riders have bought a gravel bike, let’s convince them that they need a new road bike. Of course, most already have a road bike, but that one has outdated rim brakes. It’ll be easy to convince them that they need a disc road bike with tubeless tires.
(You don’t want to get dropped on your Sunday morning ride!)
Perhaps that will work for some riders. The rest of us know that swapping the heavy, stiff ‘gravel’ tires for a set of supple, fast-rolling tires will make your gravel bike as fast as a road bike. That is why we coined the term all-road bikes and don’t call them ‘gravel’ bikes.
(As speeds increase, wind resistance matters more, so it’s possible that at time trial speeds, narrow tires are a little faster. On the other hand, if you ride slower than 22 mph, you’re probably faster on wider tires.)
Many non-cyclists are excited about the idea of the all-road bike. They want a bike – one bike – that combines speed with comfort and the ability to go off the beaten path. They view these new bikes with a level of excitement that I haven’t seen since the mountain bike boom.
I’m surprised that the bike industry is turning its back on these would-be cyclists. Instead, they keep trying the old strategy of selling bikes to those who already have plenty. Rather than trying to fill the garages of those who already ride, we should bring potential cyclists into the sport. Let’s put them on all-road bikes, where they’ll have more fun than on (slow) mountain or (uncomfortable) road bikes. They’ll be more likely to stay engaged and become life-long cyclists. And that is good for everybody – even for the bike industry!
Update 11/17/2020: We’ve just published our new book ‘The All-Road Bike Revolution’ with all the research that has changed cycling in recent years. Find out why wide tires can be fast, how to find a frame that optimizes your power output, and how to get a bike that handles like an extension of your body. More information about the book is here.