Help settle a bet between me and a neighbor.

This is the hypothetical situation:

You are riding your bicycle, on a perfectly flat, perfectly straight road, if the wind blowing perpendicular to your direction of travel. Does the wind help the cyclist by pushing the cyclist along, or does the wind cause resistance for the cyclist?

It's going to slow you down, just a bit. As you ride, you push into the air. More wind, the more air you push into. I don't think you'll notice the effect though.

I'm going to guess that it adds resistance. Either way, if it is strong enough, it will impair your ability to ride in a straight line, so that will slow you down.

What kind of bicycle? If it's got nothing to extract energy from the wind, I think it might slow you down a bit. If you've got something with some sail area, you can get energy from it just like a sailboat:

It will slow you down, compared to zero wind.

Based on observation, it will slow you down some.

There's not any way to just deduce the answer. What actually happens is that you're moving through the air at a skewed angle. Your airspeed is greater than it would be with still air, your "frontal area" is actually the area on a diagonal, the drag coefficient changes, but no telling how. The force from the wind will be at a skewed angle, also.

Get on the leeward side of the other guy..

It slows you down in a couple of ways.. first there's the fluid dynamics of the wind which causes resistance..
But then.. in order to not get blown over you shift to a less efficient riding position so your coefficient drag goes up while you lean into it and this causes your tires to increase the road resistance.

How much of the bet do I get if I tell you the correct answer?

drag is proportional to the speed of the fluid (air) moving over the object (you). With wind at perpendicular angle, you will face a higher wind current than if you were just riding with no wind, in which case the only wind moving past you is in the direction you're facing.

For instance, if you are facing a wind that is as fast as your bike speed, you can add the perpendicular wind vectors together, and the resulting drag would be sqrt(2) the amount you would face if there was no perpendicular wind.

Side loading wind such as you ask about here will slow the cyclist down by adding friction to the direction of travel.

If that wind were water this bet would be settled quick!

Just watch the major cycling races and see what happens with a crosswind.

A significant decrease in speed and a chance to break a race to pieces as unlike a headwing the following cyclists get forced into the wind becaseu the best position is to more to teh side of the rider ahead and roads are onlty so wide.

Hurt. Extra energy is needed to maintain a straight line.

it would only make you go faster if you had a sail set at angle angle, in sailing they call that "tacking" don't they?

Crosswinds, at angles of 90 degree and above, are felt as apparent headwinds. See https://sheldonbrown.com/brandt/wind.html

I take it your neighbor does not ride?

I was thinking the same thing.

Pretty much. With the sail/aerodynamic bike structure at zero angle, the force on it is just pushing sideways. With it held at an angle, there are components pushing to the side and ahead.

Damnit, now I'm going to have to figure out the physics of a sailing bike Maybe some kind of tandem with the stoker sailing instead of pedalling...

Ice and beach sail rigs are all 3 point of contact, trikes..
skate blades or general aviation wheels.

Laying down low so the sail boom doesn't hit you..

It sure slowed me down last night.

Sails do not push a boat. A sail pulls a boat based on the Bernoulli principle (on anything from a broad to close reach). A conventional bicycle and rider make a poor air foil. If anything the wind will cause the cyclist to slow a bit due to lateral resistance.

If the cyclist were able to fashion even a crude foil then some lift would be acquired aiding the cyclist.

Why didn't I think of that. The faster the cyclist moves, the apparent wind will move further forward. This would probably slow the cyclist much more than the lateral resistance that I mentioned.

Yes, cross winds will slow you down. If you are riding with others in a cross wind, rather than drafting directly behind the rider in front, you want to form an "echelon" in which you are behind the rider but off to the left or right of the rider, depending on if the wind is from the left or right.

Actually, cross winds can be more tricky than headwinds, since the wind will move you off course. This is especially true if the wind is gusting. Even more true of you have deep rims on the front.

Back when they started using solid disk wheel, they had them front and rear. They quickly found that disks on the front were a bad idea when the first wind gust blew them across the road.

Now that I see how the conversation is going, I feel I can jump in without persuading the topic. Yes, my neighbor does not ride, and yes he feels that a cross wind is helpful to the cyclist. His argument is that if it is not a head wind, then it is helping the cyclist. Like the wind will wrap around the back of the cyclist and push the cyclist forward somehow.

I think one could argue that if the force of wind 'pushes' the cycles to the left or right, during travel, that this is a form of resistance. The cyclist will 'waste' energy compensating for the wind pushing the off the vector of travel, forcing the cyclist to turn into the wind slightly.

I am not sure the magic angle, but I would even go as far as to say that a tail wind at a steep angle, nearly perpendicular to the cyclist, is still a from of resistance. The cyclist will have resistance greater than the help the wind is doing pushing the bicyclist.

You both better stick with cycling. Tacking is changing directions by pointing the bow through the wind. You're thinking of reaching when the wind is at your side.