# Why do deep section front wheels get pushed around in crosswinds?

If a bike with a deep front wheel or a front diskwheel were to receive crosswinds with a 90 degree yaw angle handling would not be affected since the pressure would be equally distributed among the entire wheel.

So why does the wheel turn at lower yaw angles?

This front "wheel" seems to be designed to handle optimally in crosswinds at normal yaw angles. Maybe one day somebody can make adjustments to the fork to reproduce the same effect.

• A wheel is an airfoil. The "lift" of the airfoil differs depending on whether you're speaking of the leading edge or the trailing edge. – Daniel R Hicks Nov 5 '16 at 18:00
• So I understand there is a higher pressure difference at the front of the wheel than on the back. Ok I think I get it. – AzulShiva Nov 5 '16 at 18:23
• If the whole bike is pushed by the wind, then handling has been affected, whether the front wheel is twisted by the wind or not. – rclocher3 Nov 5 '16 at 21:40
• I guess this is one of those "try it for yourself" questions. Try filling in your front spokes with something like plastic corrugated board, then go for a ride. – Criggie Nov 5 '16 at 23:11
• Your first paragraph contains an error. Wind from the side does not act equally on the front wheel. Imagine a line from the head tube straight to the road contact point (slightly further back than the pictures below.) That is the axls that the wheel will turn on. There will be more "wheel" in front than behind, so a 90 degree side just hits more area in front than behind this axis, turning the wheel away from the wind. This is in addition to the aerofoil and shunting effects of the wind. – Criggie Feb 1 '17 at 23:43

The answer to your title question is "moving air exerts a force on obstacles", and deep section front wheels have more area affected by side winds so there's more force.

In any crosswind there will be some side force that applies a turning torque on the front wheel of almost any bike (viz, zero rake bikes have been made, but they have less trail than is common today). The diagram below shows the normal design, somewhat exxaggerated as was the style in the 1960's:

(via the esteemed Dave Moulton whose post I got the image from contains an interesting discussion about why we have trail)

Note that the centre of drag/centre of the wheel is forward of the pivot axis. Thus, wind force from the side will tend to turn the wheel downwind, in the same direction that the wind is pushing the bike. So the rider will tend to stay upright, unless the turn makes them crash into something.

The bike you have pictured has the opposite design - as the wind pushes the bike and rider away, the drag behind the steering axis will turn the steering into the wind. I predict that in many cases the centre of mass will move downwind while the support moves upwind, resulting in a fall. But note that the front wheel design pictured is at best going to require clean, dry conditions to operate, and those are normally only found on indoor velodromes, making side winds a non-issue.

The OP asks about airfoil effects: More streamlined shapes can generate lateral forces due to airfoil effects, as often helping propel the bike as slowing it down - this is why speed records must be set in zero wind conditions.

Ignoring drag just for this example, the direction of the force from the wind is affected by movement for a streamlined shape, but not for a round shape. The apparent wind direction changes, but the vector components are still just movement + wind. But because drag is nonlinear the total drag force will be greater on the sum (that's why headwinds are so annoying), but the side force will not change by a huge amount.

The key point, though, is that the side force is still present regardless of the shape of the section. You could have a perfectly flat plane and it would still experience the side force.

And what matters to the rider is that the side force exists. Once you have that force, the above discussions of geometry applies. Where the force impacts and how that changes the balance and steering of the bike is what makes the wind hard to deal with. Whether there's a force-aft component to that force doesn't affect balance or handling.

It's definitely possible that turbulence from the fork will mean that there's less lateral force on the wheel behind the fork. But the more streamlined the fork the less true that will be. For the bike you've pictured above there is no effect at all, since the "fork" is merged into the trailing edge of the wheel. Instead that bike will have the opposite effect - there will be more lateral force on the rear of the wheel because there is so much more surface area behind the pivot axis of the wheel (the head tube).

• Daniel R Hicks has written the correct answer as a comment to my question. Your answer is incorrect. – AzulShiva Jan 31 '17 at 19:49
• @AzulShiva you're saying steering geometry has no effect? It's purely the differential airfoil effect from fork shadowing? Please write an answer explaining how it all works, so I can understand. Thanks. – Móż Jan 31 '17 at 20:40
• You have not mentioned airfoil in the answer. For people who do not know how an airfoil works your answer could be misleading. Sorry your answer is not literally "incorrect" but can be misleading and also doesn't explain why deeper section wheels and disk wheels (if used in the front) react stronger to sidewinds. I have since educated myself in the subject so I will try and give a more complete answer. However I am no expert in this field – AzulShiva Feb 1 '17 at 20:45
• @AzulShiva I really want to see your explanation for why airtfoil effects dominate. I think you've misunderstood what "shallow section front wheels are also affected by crosswinds (but less than deep section because they have less facing area)" means, since I think that completely explains why deep section rims are more affected by wind. I have reworded that and edited more explanation into my answer to try to help you. – Móż Feb 2 '17 at 1:38
• I didn't want to sound like a dick, I have critizised because you didn't mention airfoils at all. However the answer is still not complete. I am not sure wether to answer as comment or as a seperate answer since I really am not expert in this field, but I will get back to you as soon as I can – AzulShiva Feb 3 '17 at 9:35

It's not true that a 90-degree crosswind doesn't affect handling. Suppose you're cycling in such a crosswind, coming from the right. That wind is trying to make you fall over to the left. As it pushes you over to the left, your bike steers to the left so you don't actually fall (this is why bikes are actually stable to ride). You can see this by holding your bike by the saddle indoors: as you lean it to either side, it automatically steers to that side. So, the net effect of the crosswind is to make you steer away from the wind, even if the wind is putting an equal force forward and aft of the hub.