Aerodynamics has been the talk of the pro peloton, the bike manufacturers, and the local group ride for the last 10-15 years. It is becoming more and more accepted how important aerodynamics is for road racing. The number once force to overcome in a road race is easily aerodynamics. That is because of its relationship to speed. The faster you go, the more important it gets. While this is now fairly widely accepted, the questions of aerodynamics in gravel riding and racing are popping up left and right. This is partly due to the growing popularity, the increasing speeds, and frankly the commercial attention on this side of our sport. This brought us to our desire to find out what makes a fast gravel bike. Surely aero matters for the pros, but what about the majority of riders who are out there just trying to complete a new challenge. That is what grassroots gravel is all about anyways right?
Recently SILCA took one of our pro gravel racers, Dylan Johnson, to the wind tunnel to answer the question of how important is aero in gravel racing. When you are looking at a top 10 at unbound the answer is pretty easy, it makes a massive difference. Looking at a few of the graphs comparing power savings at different speeds we thought it might be important to highlight the story behind the numbers. This post is sure to be one for the marginal gainers and maybe (hopefully) bring a few of the skeptics over to our side. Aerodynamics matter for everybody trying to complete one of these big gravel adventures no matter the speed, and we will walk you through why.
Aerodynamics and Speed
It is now ubiquitously understood that the faster you go, the more aerodynamics matter on a bicycle. The question is, but how much or when do they stop mattering? The relationship between speed and power is relative to the cube of speed. If you have ever ridden a stretch of road at 20mph and it takes you 200w to do that, how much harder is it to go 25mph? 390w! Maybe you haven’t been able to quantify it, but we have all felt that. Over 80% of the forces slowing you down on a bike are caused by aerodynamic drag and the faster you go, the higher that number gets.
As a general rule of thumb to overcome the aero losses if you 2x speed, you 4x the drag, and 8x the power required to ride at that speed. This simplification of the math does leave out the linear relationships of rolling resistance and drivetrain efficiency, but it does give us a good rule of thumb to go off of. If you are going from 20 to 25mph, you simply do 253/203x200=390w.
When most companies are quoting power savings for a bike, helmet, clothing, etc, those are almost always quoted at the 40kph or 50kph numbers. When you do the math and bring them down to your typical gravel riding speeds of maybe 15-20mph, the savings or penalties become really small. That leads a lot of us to say “aero doesn’t matter for gravel.” It is an easy jump to make, but it is absolutely not the entire story.
When most people think about speed on a bicycle, they think about ground speed. How far you are traveling on the ground relative to the time it takes to do so. That is for the most part what is important for bike riding and racing because it will tell you how much time it will take you to get from the start to the finish line. The thing about aerodynamics is that it is all around wind speed.
Windspeed and Groundspeed
What does that mean for the person finishing Unbound 200 in 13 hours (about 15mph)? It means that if you are riding 12mph into a really stiff 18mph headwind, the aero savings you are looking at are now 30mph or about 50kph!
It is easy to look at a graph of the power savings of a particular equipment choice and say, “well I’m not getting in an aero position on the bike for an 11w gain.” For the entire 200 miles of a race like Unbound, that is probably a fair cost/benefit analysis that it is too uncomfortable to get into a really aero position for 11w of benefit. What about when you get that 18mph headwind though, and it all the sudden becomes a 90w gain? Most would agree that it is absolutely worth 90w to get down on your bars and get a little more aero in the headwind to save 90w of effort for the same speed.
The same can be said for running aero bars. Lifetime has banned them for the elite field here in 2023 but us amateurs can still ride them. When you are using them, our testing shows aero bars are worth about 15w at 15mph. That is far and away the biggest gain you are going to get on your bike, but is it really worth all the extra equipment, loss of hand position, etc for 15w? Maybe not. It certainly is worth the 120w at 50kph!
Between mile 50 and 60 at the 2022 edition of Unbound, the course went almost directly into a headwind so that is a perfect place to run our simulation. Estimating the time savings here is what we are really after. Our baseline of putting a respectable age group power of 180w average over this 10 mile stretch would take 55:49. The 120w savings from going to the aerobars with that headwind would save a whopping 14:12!
We are not here to tell you how to ride your bike, simply to explain how you might be able to improve if you want to. If you look at some of our recent test data, it is really easy to look at it and say it doesn’t matter to be aerodynamic in gravel races because you aren’t going fast enough. That simply isn’t the whole picture.
If you are looking for some real world validation for this concept, think back to a ride where you had a massive tailwind. You are flying down the road while putting out seemingly no power, but you are riding at 25 or 30mph. You get out of your drops and take your hands off the bars and sit up. Not much happens. You drop 1 or 2 mph. That is the same thing in reverse. If you have a 20mph tailwind and are traveling at 25mph, the effective air speed you are facing is just 5mph. The aerodynamic drag on the rider is almost nothing that that point. The only forces you need to overcome are the rolling resistance and drivetrain losses on top of the minuscule aero penalty you might be facing.