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. 


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  • Dave on

    Stacey – It could have been a ton of different things, so we can’t narrow it down too much more. This is sure the only time I’ve heard of it with an Al33 or any other rim we’ve ever built with. It could have been just something, wheel or wheel aerodynamics related or not. Certainly something I’ll remember having heard about, but I can’t bring it too much beyond that. Thanks

  • Stacey J on

    They’re the fat ones – 25s balloon out past the rims at least a mm or two on either side (I haven’t measured the actual width, but would guess its around 28mm).

  • Dave on

    Scott – We’ll take that as a good sign

    Colin – There are two things working for you there – the width and the depth of the RCG36. We didn’t get into it here, but depth actually helps to a degree, by smoothing the transition and helping the wheel keep “attached flow” more. Width helps this, too, assuming same tire width between the two wheels.

  • Colin on

    FWIW, Pacenti SL23 wheels on the same bike get blown around way more (i.e. it’s noticeable) than RCG36 wheels on the same bike (where it’s just not noticeable).

    Both bladed spokes. Same tyres. Progress is good, that’s all

  • Scott Booth on

    I love this!! Funny thing about questions… most of what I know about wheels I’ve learned here, and as such there are no burning questions in my brain, at least that I can think of when I try.



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