Aero Helmets: Cheap Speed
Posted in Tech Talk, Watches & Race Equipment by Jordan RappGoing faster on a bicycle can be achieved in four different ways. The first, and simplest, is to apply more power to the pedals. This requires a lot of hard work, training, and dedication. So let’s ignore that. The other three are all basically “free” speed. They are, in order of importance, aerodynamic drag, rolling resistance, and weight.
Weight does play a minor role in determining rolling resistance, i.e. a lighter bike and rider will have lower rolling resistance than a heavier bike and rider but the difference is so small as to be essentially negligible. Rolling resistance, which is governed by your tires and tubes, is a really interesting topic that we’ll cover in the future. For now, our focus is on aerodynamic drag and how to minimize it, while also minimizing your expenses
Ironically, weight is probably the most emphasized of the three methods of “free speed,” but is actually the least important, at least among most bicycles. Dropping from a 40lb. hyrid to a 20lb. mountain bike will make a huge difference, but between say a 12lb. ultralight or an 18lb. average tribike, there is not much difference. Also, by weight, I am talking about strictly bike weight, not the weight of the rider. It is even less important for triathlon than for cycling, where it is also overrated, since there are no summit finishes and even the hilliest of triathlons is quite flat compared to a hilly cycling race.
The hour record for the bicycle, which is up over 50mph, was set on a recumbent bike that probably weighs about as much as a downhill mountain bike, maybe more. The advantage of the recumbent - superior aerodynamics. Everyone wants the lightest this and that, but on most triathlon courses, the average speed of most finishers is in the range where aerodynamics matters much more than weight.
So what causes drag? To keep it extremely simple and general, drag is caused when something pushes air out of the way so that it can be where the air was. The shape of this object is important, since certain shapes push air of the way more effciently that others. And the size of this object is important, since a bigger object needs to push more air out of the way than a small object in order to move through it. Things like texture also have an effect on drag, but we’ll leave that out of this discussion since it is, generally speaking, a much smaller factor than the other two.
So how does this relate to you? And, more specifically, how does it relate to helmets, our topic of discussion for today? Simple. Racing with an aerohelmet can make a big difference in how much drag you generate on your bike. Aerohelmets work because of two things, their smooth covering (none of those massive vents of your regular road helmet) and their long profiles. How much an aerohelmet benefits you will be determined, to some extent, by your position on the bike.
If you are sitting up into the wind on regular road bars, then a piar of aerobars probably should be your first investment. But if you can ride comfortably on the aerobars, an aerohelmet is the cheapest way to gain some free speed. When you consider that you need to race in a CPSC-certified helmet in the US, meaning that your aerohelmet must be as good at protecting your brain as your regular helmet, you can actually gain speed totally almost for free, since you could get away with only owning one helmet.
Whenever I recommend an aerohelmet to someone, I hear a number of different arguments about why someone doesn’t want one: “They are don’t make much of a difference.” This is occassionally followed with “for people riding less than 30mph.”
Before addressing the “not much of a difference” issue, I’d like to cover the 30mph issue, which seems to come up often when discussing aerodynamics. Without going into too much detail, let’s first off say that there is nothing about 30mph that is special. Nothing happens to air at 30mph that makes it much different than air at 25, 20, or even 15mph.
It is true that if you are lucky enough to ride at 25 or 30mph, you will save more power than someone at 15 or 20mph. BUT, because the person at 15 or 20mph is going proportionally slower, they will be saving a higher total amount of time. Basically, someone going faster will save a higher percentage of time, but the person going slower will save more time in total.
And this true for everyone at virtually every speed achieveable by bicycle. In order for drag not to make a difference, you’d be going so slowly that you’d miss the cutoff to finish the race. So things that help time trialists in the Tour de France will help you, even if they ride more than twice as fast.
As far as not making a difference in general, let’s look at the average road helmet, which now seems to be marketed by the total number of “vents” on the helmet. All these vents which are designed to keep you just as cool as if you were not wearing any helmet at all. But all those vents come at a price. Those vents don’t just allow air to pass over your head. There is also a lot of air that comes and slams into the surface of those vents. And that slows you down.
The difference, according to a recent issue of Bicycling which contained an issue with some engineers at MIT who do research in the windtunnel there, between a regular helmet and an aerohelmet is rougly 400%. A regular helmet generates four times the drag of an aerohelmet. They didn’t discuss the brands used, so it’s not like that number is hard and fast for all helmets, but it gives you an idea as to the difference between helmets.
Of course, you may say, but 4 times very little is still very little. And that is true. But as more and more athletes head to the windtunnel, we have even more information on what sort of difference equipment makes. One easy to remember comparison is that an aerohelmet saves about as much drag as a disc wheel in the back as compared with a regular high-spoke count training wheel.
Now you might say “but everyone’s position is so different.” And that is definitely true, but everyone’s bike is also different, and their legs are different shapes, all of which affects the airflow over a disc, yet a disc still is superior because a solid shape is much smoother through any air than a whole mass of spokes.
The easiest way to approach is to look at an aerohelmet and a regular helmet, and compare the amount of area where air contacts a flat shape, like hitting the front of a bus. On most aerohelmets, there are only a few tiny vents, and a nice smooth profile. Even if you are sitting up into the wind, you will be more aerodynamic with an aerohelmet, just because the shape is more efficient.
“They are hot.” There is some truth to this. In order to give up speed, you are giving up some ventilation. But even in very hot weather, cycling does an amazing job of cooling the body. Even at relatively low speeds, an amazing amount of air passes over your body and does a great job of cooling you. Given how many people I see wearing dark clothing and dark helmets at races, I also think there are plenty of other places to make yourself cooler than needed a massively ventilated helmet.
In addition, there are number of different helmets to choose from. The Rudy Project Syton, which the Timex Team uses, is a great helmet since it leaves the ears exposed. Does this sacrifice some pure aerodynamic speed for comfort? Probably. But it also means it is a great choice for people that don’t want to put a really full helmet on their dome. It also comes in all-white, which means it is as cool as any helmet can be in sunny weather.
As much as I hate to rely on anecdotal-type stories as support, I don’t know of any athlete for whom the reduction in ventilation correpsonded to overheating so much that they were slower. It does make sense to train with the helmet, so you can get a feeling for it. But by the time it is universally “too hot” for an aerohelmet, most of us won’t be riding our bikes anyway.
You may think you sweat more in the helmet as well, but you can easily check this with a scale weighing yourself before and after rides in similar conditions with different helmets. I think that a lot of the perception of higher sweat rate comes from the sweat dripping off your head rather than being blown out the back off your ventilated helmet.
Lastly, “they look stupid.” I don’t really know what to say to this one, except to say that nothing looks cooler than going fast. :)
So hopefully that gives you a reason to rethink your helmet choice on race day. An aerohelmet can be a huge asset to athletes at all speeds. And given the relatively low cost (most aerohelmets cost no more than a top quality “regular” helmet), they represent the best dollar-for-dollar investment in speed after a pair of aerobars, which are practically ubiquitous in triathlon these days.
