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Bike handling: steering torque & crosswind stability
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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?)
Last edited by: InspectahChilla: Apr 28, 18 10:55
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Re: Bike handling: steering torque & crosswind stability [InspectahChilla] [ In reply to ]
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I'm not sure I understand your question because its unclear to me what you are illustrating with the yellow line. All forces between the fork and the wheel must be exerted at the wheel axle/quick release. This is the only place the interfaces are attached there is no other area where they can exert force on each other. As a result if you want to understand torque you need to draw a line from the axle to the headset bearings. This is then the axis around which torque will occur.

The area you have highlighted in yellow is behind the relevant axis and will therefore increase cross wind stability. The PR6 which has dropouts at the extreme front of the fork which as Dan notes places the fork behind the relevant axis and aids cross wind stability.

This relationship is described by the front center (FC) on the first of images. The relationship between a bikes reach and its FC then has a big impact on the responsiveness/handling a bike because it determines the relationship between rider steering input and the movement of the wheel
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Re: Bike handling: steering torque & crosswind stability [scott8888] [ In reply to ]
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scott8888 wrote:
I'm not sure I understand your question because its unclear to me what you are illustrating with the yellow line.


He's drawing the axis of rotation there. Which is pertinent to the discussion. The wheel contact point is off-axis. But all torque calculations are presumably done using the axis of rotation as the...axis of rotation.
Last edited by: trail: Apr 28, 18 9:17
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Re: Bike handling: steering torque & crosswind stability [scott8888] [ In reply to ]
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First off, thanks for taking time to consider my thoughts.

I follow you in what you say first, but here I'm not sure anymore:
scott8888 wrote:

(...)
As a result if you want to understand torque you need to draw a line from the axle to the headset bearings. This is then the axis around which torque will occur.
(...)


Well, I think this indeed touches the clue. I supposed the headset can only take moment about the steering axis because the fork rests in the headset bearings. (This of course aside from the weight bearing moment in the yz-plane, or saggital plane if you will.)

This steering axis is not equal to the line from the axle to the bearings, hence my problem in understanding how force/moment is exerted.

Trough the construction of the fork, I don't think it is possible to think of moment solely about the axis from the wheel axle to the lowest headset bearing.
Last edited by: InspectahChilla: Apr 28, 18 10:50
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Re: Bike handling: steering torque & crosswind stability [InspectahChilla] [ In reply to ]
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I think you may mean "moment" where you're using "momentum."
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Re: Bike handling: steering torque & crosswind stability [trail] [ In reply to ]
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Thanks ;) that's been edited.
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Re: Bike handling: steering torque & crosswind stability [InspectahChilla] [ In reply to ]
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I have attached a diagram I hope better explains the situation. On the diagram you have a standard fork shown in red, the LG fork shown in green and PR-6 form in blue. All the forks have the same rake and FC and therefore the handling comes down to the fork shape.

The critical thing to remember is that the PR6 is a super bike meaning you end up with a whole lot of bike in front of the steering axis. This is equivalent to the orange area on Dan's diagram. The way the PR6 tries to counter this is by not centering the dropout but ensuring most of the fork is behind the fork rake line. I agree its still not behind the steering axis but I still think it does the same job because its behind the fork rake line. At a minimum centering the dropouts on a similar massive fork while maintaining the rake would only put more surface area where you don't want it.

The LG takes the same principle but because its a standard bike the whole lot is shifted back hence the fork is behind the steering axis. This should provide a much more stability that than the PR-6 but is only possible because its not a superbike.
Last edited by: scott8888: Apr 28, 18 11:56
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Re: Bike handling: steering torque & crosswind stability [scott8888] [ In reply to ]
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Think you forgot to attach the diagram? :)
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Re: Bike handling: steering torque & crosswind stability [InspectahChilla] [ In reply to ]
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Should work better now.
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