In the recent article on the QR PR3 I read the following;

Fork: (...) One thing about all the PR series bikes is that the surface area of the fork is behind the rake line, and any surface area that’s attached the fork or steer column that sits behind the rake line offsets the steering torque the rider experiences in a sidewind.

I recall the from another article on the LG Gennix,

The Gennix, above, attempts to fight this force by building into its fork surface area behind the steering axis.

So... rake line, steering axis. It's not clear... To explain my problem I cite the picture below (you can find the original at the article on the LG Gennix)


In contrast to the original, I've highlighted an area (by a yellow hatch).

• The yellow line is the actual steering axis, going straight through the middle of the headset bearings.
• Hatched is the area made up by the yellow line (steering axis) and the point at which the fork contacts the tire (at the wheel hub). That's the area which is bothering me.

Why is this bothering me? Because I think this triangular area is not contributing in the described offset of steering torque. Whereas the purpleish area behind the steering axis obviously is.

As far as I am able to test this on my ordinary road bike, gently pushing any area of the fork (even towards the 'back' side of the fork) is provoking a motion that would turn the front wheel in the direction of the wind direction. This is in line with what I would suspect, as the fork has a net moment about the steering axis.
In the same way, all of the yellow hatched area should also have a net moment about the steering axis (alas less, because the perpendicular distance to the steering axis is smaller.)

Of course this reasoning seems valid as you are close to the headset and far from the tire contact patch with the ground.
Reason being that, close to the ground the 'rotation' (it's not a pure rotation I believe) is more about the contact patch than it is about the steering axis.

It's just that this becomes a real thing when you start looking at other forks. For example QR's newest PR6.


Here the yellowish line is, again, drawn straight through the headset bearings. One can easily see that almost the entire fork area sits in front of that.* And thus would either fall into my yellow purple hatched area or the green area.

So... rounding up the unclarity. My questions;

Why would the yellow-purple hatched area contribute to crosswind stability? (if so) Can this be explained moment-wise?
And, how would you explain the beneficial effect of the fork on the QR PR6 for example? (if possible at all)


* by the way: even if you would draw a straight line from the lowest headset bearing to the wheel hub, most of the fork area would still be in front of that. So either way -yellow hatch or not- in the case of the PR6 I'm having a hard time seeing this as beneficial for crosswind stability. I would go as far as thinking that it could instead provoke crosswind instability?)