Blown away: wind and wheels and steering

Blown away: wind and wheels and steering

A few years ago (like 6 - we're getting old here) we were I think the first to actually present some steering force data that the A2 wind tunnel had started providing with testing there. Because this was all new and our expertise in how this was all measured and worked was, I will generously say, developing, we chose not to include a hunk that I'll discuss here. 

In general, you have two dynamics at play - how big the push is, and where the push is relative to the steering axis. There is a third dynamic of consistency, which gets very complicated. Let's stick to the first two for now. 

If it looks like these guys are using huge sails, it's because they are. Given what the conditions look like, these are probably 12.5m2 sails. They're about half the size of a tennis court, and have a HUGE amount of power. I used to race in this class and it's exceptionally fun. The illustration is sail area - deeper wheels have more sail area, shallower wheels have less. Spokes contribute to sail area as well. We'll discuss the history and future of spokes in depth later this week, but a few years ago it was common to have these massive spokes (lower end Fulcrum wheels feature prominently in the memory bank) and the surface area on them was a killer. Take a 1'x1' piece of plywood out on a windy day and hold it perpendicular to the wind, and you'll feel it push but you can control it. Take a 4'x8' sheet of plywood out and do the same, and you'll be on the ground with a bloody face in about .02 seconds. That's the effect of sail area. 

Back to the windsurf sails, if you know what you are doing, you can pick one of these sails up and hold it over your head with one hand and walk around with it. You just have to keep the angle of attack ("yaw angle") right, and keep moderate flow going against it, and it develops a small amount of lift and all is well. But if you mismanage the sail's interaction with the wind, you get hammered - in the ground with a bloody face time, again. So the sail area dictates how much leverage or risk you have of being pushed around.

Steering axis is a funny one. Your bike's steering axis is the line drawn from the fork crown (or middle of the head set's bottom race) to where the tire hits the ground. This axis is behind the wheel's axis, by an amount basically equal to your fork's rake. If your fork has a 43mm rake, then the steering axis is about 43mm behind the axle. Where the steering force is in a wheel determines how much you "get steered" by the wind when using various wheels. If the steering force is on the steering axis, you get pushed over but you don't get steered. If the steering force is behind the axle, you get steered into the wind as the force (wind) increases. If the steering force is in front of the axle, you get steered away from the wind as the force increases. This last one is generally the terrifying one that people object to.

The steering axis effect can be managed by design. When designing the Rail 52, I tried to make the section shape as symmetric as possible - so that the shape would be the same with the tire in front on the leading edge of the wheel as it is with the tire in the back at the trailing edge of the wheel. If you read the old post we linked to up top, you see that the Rail 52 scored exceptionally well relative to other rims of similar-ish depths, and I attribute this to that design feature.

The reason we don't see a lot of sharp v-point rims anymore is, among other things, because they're generally terrible with steering control. The wind is easily able to "tack" from one side to the other, reversing the high and low pressure sides and creating havoc. The more rounded shapes that are ubiquitous now do a lot to ease that effect. 

The thing I hinted at that we chose not to discuss in 2014 is tire size's effect on this. The wider the tire relative to the rim, the crappier the steering gets. To me, this is clearly due to the breakdown in aerodynamic flow. You get turbulence, and the wheel is going to steer funny (and also be slower). So the worst cast would be using an old style v-shaped rim (bad shaping) that's very deep (a lot of sail area) with a big big tire (creating turbulent flow). And that defines the axis along which I think of this stuff. 

Anecdotally, I chose not to go sailing yesterday because it was too windy (we're allowed to sail in RI as of now). So I went out for a 50 miler instead (we're allowed to go out for a 50 miler in RI as of now), using RCG36s with Schwalbe One 28s. The tires measured 29mm wide last I checked, which was one day after initial install and inflation. They might have grown a small bit since then. There is a wind meter network that covers every place where I rode yesterday, and it was legitimately over 20 mph form the last 2/3 of the ride. My impression is that people WILDLY overestimate wind speed most of the time. When it blows 20, it is the overwhelming weather trait. 20 is f-ing windy. The wind certainly affected me, but my wheels were not steering me all over the place or anything like it. RCGs don't have very much sail area and have a good design and the tires are a pretty close match for the rim. That axis, again. 

As an aside, yes I am trying road tubeless again. I'm not even 200 miles into it, though, so there's nothing to report other than the tires set up easily and nothing has gone out of whack. It'll take another 1000 miles before any thoughts are even worth discussing, unless something goes wrong. 

This topic lends itself to a more interactive discourse rather than a 12 page boring discourse, so if you have questions, please leave a comment with the question and based on how many there are and how much explanation each needs, it might turn into a separate Q&A post or we'll just respond in the comments. But we'll let comments build up in order to make that decision.

For the first time since I don't know when I actually planned a week's worth of posts. It seems that people enjoy this, traffic has been really really high, unsubscribe rate is near zero, and while we have the bandwidth to do this we will keep doing it. 

Back to blog


Darrell – Yeah, it’s fun and informative but also REALLY expensive and time consuming and at the end of it all it drives traffic but not sales for us. After the latest round we heard a lot of “gee thanks, you helped me make the decision to buy x wheels.” And the conventions have gotten super muddy in the last several years, so now you just wind up with an argument about protocols on your hands. There aren’t that many bike industry people who’ve spent more time at the tunnel than I have, and it’s been transformational to my understanding, but the marginal utility has gotten smaller.

Stacey – That’s unusual for a couple of reasons, the big one being that if it was a headwind there isn’t that much impetus trying to steer the wheel, and in the grand scheme your situation with a 25 mph apparent wind is really normal. The wind tunnel doesn’t even report the steering force until 5* as I recall. But unusual and possible definitely coexist, so it could have been. Is this the older Schwalbe One that was huge (25s were like 28s) or the current-er ones where my 28s actually measure 29?


I sure miss all the wind tunnel tests and aero drag discussions.


Had an interesting experience earlier this week: Riding ~16mph on a gradual downhill directly into a reported 9.4 mph headwind on a set of AL33s with 25mm Schwalbe One Tubeless tires, I had a few moments of major instability (felt almost like a death wobble on a fast descent), which abated as soon as the road turned a couple of degrees. I assumed that this was due to momentary variations in pressure on either side of the leading edge of the front wheel, but would be interested to hear your take on it.

Stacey J

Thanks Harry. It’s always been given to me as I described it. Still behind the axle by about the same as the fork offset, no?

Garret – Those spokes were windmill blades.


Your description of steering axis is incorrect. The steering axis is the line through the center of rotation of the headset bearings. It intersects the ground ahead of the tire contact patch, which is itself directly below the hub axle. The trail is the distance from the point steering axis intersects the ground back to the center of the tire contact patch (ignoring pneumatic trail). The fork offset serves to reduce trail by moving the contact patch forward toward the steering axis/ground intersection point.


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