The Effect of Crosswinds
 

On my commute home from work tonight, I hit a pretty steady 15 mph westerly cross wind for the 15 miles of river trail. The trail meanders some, but never gets more than 30 degrees off of due south. From my sailing days, I know even the steadiest of winds meander some too.

But statistically speaking, the amount of time spent at dead across, slight help, slight hurt, was a pretty even balance.

All in, it felt like much more of an into the wind, rather than with the wind or no wind experience.

I could probably research it on my own, find the science, and get one of my engineer sons to explain it to me, but that would be ignoring the resident wisdom here.

So my question is: does a dead across wind hurt or is it neutral?

Play with this: HED Cycling - Yaw Calculator

As a practical matter, a dead crosswind won't help unless you have a sail mounted on your helmet. Rather, you spend energy trying to maintain a straight course by leaning and turning into the wind, to counteract the wind pushing and turning you away from the wind. Any energy spent maintaining "go straight" is energy subtracted from "go fast".

A 90 degree cross wind becomes a headwind componet as your speed increases. Plain science.

Some bikes and wheels claim a forward lift from side wind, even wind coming more from the front. Probably minuscule compared to the total.
Riding with a group means you loose draft in a side wind. Definitely slows you down.
Solo it seems to wear on me psychologically more, which in turn cuts into my speed. I encounter side wind enough that it affects my equipment choice; no aero-profile wheels, low-trail steering geometry.

May not be "scientific", but I can tell you without doubt that it takes a higher wattage (according to both PT G3 and Vector 2 meters) to maintain a given speed in a cross wind than in a no wind situation. I was curious enough to check this several times on the same piece of road, and the results were quite consistent. What's worse is the effect of a cross wind, unlike a straight head wind, cannot be noticeably mitigated by assuming a more aerodynamic position. Cliff notes version... I hate cross winds!

Actually, this part isn't really true per-se. (Sciencey nitpicking of no real consequence ahead!) ;)

Your self-generated headwind component is your self-generated headwind component, regardless of the crosswind component. The crosswind won't add to your headwind, and your self-generated headwind will be the same even with no wind at all, since it's the "headwind" generated by you pushing yourself through the air. The added burden crosswind provides to your forward motion, will come entirely from the friction of the tires on the road as you counter-steer against the crosswind...

The crosswind and self-generated headwind vectors add together to make it feel more like a headwind, however, so that the faster you go, the more messed-up the wind situation feels to you, as a rider.

Now that I'm thinking about it, I wonder if it would work to mount little spinnaker sails on the spokes of your front wheel, to capture the energy of the crosswind and turn it into forward thrust, just as a sailboat does... or maybe (easier) a transparent sailboard attached to your handlebars/front fork.

Actually it's a bit more complicated than that because wind resistance isn't linear. When you combine the crosswind with the bicycle's forward speed you get an apparent wind speed higher than either. Since wind drag is proportional to the square of wind speed, when you calculate the drag at the higher speed, then divide it back into the two vectors, the forward drag component is higher than it would have been in still air.

So it's not at all an illusion that riding across the wind is harder than riding in still air.

I think that you're all making it too complicated.

A cross wind is trying to blow you sideways. Consequently, you have to steer into the wind in order to go straight. That's power that's not moving you down the road. One could argue that missing the lead echelon in a crosswind on the second day of the Tour cost Quintana the race.

I respectfully disagree with your analysis. Drag exists only in opposition to motion. Since all of the motion is in the forward direction, in the case of a cross wind, only the self generated head wind contributes to drag. As others have explained you still have to expend extra energy fighting a cross wind even though it doesn’t increase drag.

You're trying to balance on the head of a pin. We both agree that the crosswind does increase the total resistance. If we don't, then I await an explanation for the greater effort needed. So if you don't want to call it drag, you need to give it another name, and explanation of the mechanism.

Part of the problem is that you're only looking at the bicycle with respect to the ground. But while the bike is riding ON the ground, it's moving through the air, and it's the effects of moving through that air which is the issue.

If in doubt, I suggest you draw a free body diagram and account for all the forces needed to maintain equilibrium when a bicycle moves at constant speed in a cross wind.

Help! We need a Physicist with expertise on fluid dynamics, STAT! :) I came to doubt myself when "square of the velocity" was introduced, since I knew that the square of a sum is not the same as the sum of squares, so my calculation was thrown into a vector of despair.

If you're a former sailor, then you recall that as your boat speed picks up, your apparant wind moves forward? The wind is on your beam, you improve sail trim, boat speed increases, and you look at your windvane to see the wind is now more towards your bow. Same thing on a bike.

I assume it's the reason why as a cyclist my perception is that I have a headwind more frequently than I have a tailwind.

The result of moving forward in a crosswind is indeed known as apparent wind direction, and it angles further
ahead the faster you ride.

Happiness is riding fast enough to make a 15mph tailwind feel like a 15mph headwind..................

Your perception of headwinds being more common is spot on for a simple reason. You're riding at speeds above the average wind speeds, so you always have a headwind of your own making, Plus or minus the wind speed, if head or tail on, and shift in apparent speed and direction that sailors are already used to thinking about.

Since most of a riders effort is combating wind resistance, the main effect of tailwinds is to allow higher speeds until we're going faster than the wind and have a headwind again.

I am not whom you seek, but the "v" is flow velocity relative to the object and my understanding (perhaps flawed) is that v squared in the equation is scalar. The magnitude of velocity component parallel to the bike's motion.

Your swept area is greater in a crosswind which you've pulled forward with your bike's motion. With a dead-on headwind, you can get aero and reduce your cross section. But with an apparent wind forward of your beam, getting low doesn't do much. On our tandem, it's a real PITA because normally Stoker hides behind Captain and our aero drag is only about 50% greater than that of a single bike. But in a cross wind, we are both exposed and our drag is double that of a single rider. We hates it.

That said, I don't like any wind that's abeam or forward of our beam. Winds only from aft, please. Dead-on headwinds do decrease speed more than the same wind from the beam, however beam winds increase energy use because it's harder to control the bike and thus also increase danger. So it's all bad.

Apparently, sailbicycles have been a thing for a while. I just never knew.

I encounter a lot of side winds on long rides along the ocean sections of those rides. In my experience without a doubt any cross wind that isn't coming to some degree from the rear as opposed to a straight 90 degrees, (strictly purely sidewind) is a functional headwind requiring more power to maintain the same speed as no wind or tailwind.

I just remembered why I refuse to think about any of the physics involved when I ride. I have to do this stuff at work, but when I am riding, and when I visit here, I shut down the engineering segment of my little, pea-sized brain. I am glad that so many are conversant, and knowledgeable, of the physics/mathematics for bicycles, it gives me hope that new, sweet components and frames will be developed. Carry on, please.

Bill

IMO, a cross wind is harder to ride against than a headwind. Headwinds can be overcome by going into the drops. But tucking under a cross wind is much more difficult, if not impossible.

That one is confusing!

Whatever the drag force of a crosswind might be, there is one thing for certain. Winds coming from the direction of the road shoulder are not nearly the issue that we have with winds coming from the direction of the opposite lane. On a cross country motorcycle trip, many years ago, I happened to ride through Oklahoma when I had a very strong crosswind from the opposite lane. That of course tilts the bike toward traffic. I slowed down a bit and once in a while a truck would pass me. It was very hard not to get killed. So great caution under those conditions!

I've only skimmed it, but when I saw they tested yaw angles up to 90 degrees, I lost all interest. Even for slow riders it's extremely rare to see yaw greater than 25-30 degrees (I believe Zipp and Cervelo have published white papers on why this is) so I have to question their understanding of cycling and interpretation of the data.

Perhaps this has already been said in a different way, but a significant crosswind combined with your forward motion creates more turbulence than you would experience from the same forward motion through otherwise still air. And turbulence is bad, of course. This is the reason for so much aerodynamic design - not just getting a low drag profile, but also to minimize turbulence. (Think about those funny time trial helmets). it's also why you can go a little faster with people drafting you than you can solo for the same effort - the people behind you reduce the turbulence.

Citation to the assertion that there is an aerodynamic advantage when people are drafting behind you:
Training and Racing With a Power Meter Journal: Does drafting benefit the leading rider?