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Old June 25th 19, 04:39 PM posted to rec.bicycles.tech
Frank Krygowski[_4_]
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Default Steel is Real and Carbon is Lighter

On 6/22/2019 4:24 PM, Frank Krygowski wrote:
On 6/22/2019 3:16 PM, Tom Kunich wrote:
On Friday, June 21, 2019 at 3:55:05 PM UTC-7, jbeattie wrote:

Today's ride:
https://localfreshies.com/wp-content..._2014_V1.jpg¬* Mt.
Bachelor.¬* The winds were howling at the top, and I almost had a
Froome moment coming down -- and I was on modest C35s and not super
deep-dish sails.

-- Jay Beattie.


This was why I wanted to make the point that these cheapo deep aero
clinchers do not run off on me - they are no more sensitive to side
gusts than some of those old Campy Proton wheels which were flat
sections.

I assumption is that it is the spoke tension but it may be the
directional stability of the Victoria Corsa G+ tires. I've used these
clinchers on the Aero frame of the Colnago and the very non-aero frame
of the Pinarello Stelvio and they seem to react that same way.


OK, my guess on crosswind stability and aero wheels:

With an old fashioned, unstreamlined sort of box section rim, the cross
section of the forward part of wheel is a simple bluff body. Air hits it
and goes turbulent no matter what the angle of attack - that is, no
matter if there's zero wind, or if there's a sidewind causing the air to
come at the rim at an angle. So the front and rear portions of the wheel
get roughly the same amount and direction of force.

The more a tire+rim looks like a teardrop or airfoil, the less that is
true. The airfoil shape will certainly give less drag if it's pointed
directly into the relative wind (that is, if there is no sidewind). But
for many values of sidewind, the relative wind is at an angle of attack
that causes a significant sideways "lifting" force on the front part of
the wheel. [Rather, it would be a lifting force if the airfoil were
horizontal, as an airplane wing.] On the bike, this is a lateral force
that tries to steer the front of the wheel away from the wind. The
backside of the wheel sees the same angle of attack, but its airfoil is
oriented backward so it's much less efficient at generating side force.
The sideways force there is much less, so the front and rear side forces
are much less balanced than for a normal wheel.

The more streamlined the wheel+tire, the better this works. So I'm not
surprised aero wheels would be sensitive to side winds.

I don't think loose spokes have anything significant to do with this,
assuming the spokes don't go dead slack. In fact, I don't think tighter
spokes increase the rigidity of the wheel. The stiffness (or modulus of
elasticity) of a spoke with 50 kgf tension is the same as a spoke with
100 kgf tension. They're both within the elastic range of the material,
where strain is proportional to stress, so the same force will cause the
same deflection.

That's what I think. We can discuss.


Yesterday's club ride was at a pretty relaxed pace, but one guy had
brand new and extremely deep section aero rims. Sorry, I didn't catch
the brand.

But as we passed him, I asked "How are those rims in crosswinds?" He
said "Terrible. They're actually scary, really scary."

--
- Frank Krygowski
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