Re: Wheel Covers used in Time Trials

[Shock]

(Brian) wrote in
m:

> Has anyone had experience with wheel covers (lens shaped plastic
> covers for the rear wheel)for use in time trials? Are they effective?
> Who sells them? Any help is appreciated. Thanks.
>
> Brian

I have used the UNI wheel covers on both the front and rear of my old Shogun
Kaze (was state of the art back when "funny bikes" were cool).
Noticed a considerable drop in times. Had little problem with crosswinds
unless the front cover was installed by itself. Must admit that it DID get to
be rather "touchy" above about 90kph, though still not unrideable.

On my funny bike, wheel covers were very beneficial and not as dangerous as I
was told they'd be in a crosswind. Would certainly use one on the rear but
would not use one on the front on race day unless the wind was calm or head-
on. Also, I'd suggest not using a front wheel cover for the FIRST time on
race day. Maybe not at all on a 700C front wheel (remember, I'm only dealing
with a 24-inch front wheel so any crosswind won't affect me as much).

While it's true that the wheel covers will follow the dish of the spokes on
each side and thus present an assymetrical profile to the air, the smooth,
continuous surface provided by them will certainly produce substantially less
drag than all those spokes whipping up the air. There has even been some wind
tunnel data to suggest that disc wheels and wheel covers may even provide
some small amount of propulsion in certain crosswind situations....something
that cannot be said for non-disc rims under any conditions.

That's MY 10-cents' worth anyway.

~~Shock~~

[David Damerell]

Shock > wrote:
>drag than all those spokes whipping up the air. There has even been some wind
>tunnel data to suggest that disc wheels and wheel covers may even provide
>some small amount of propulsion in certain crosswind situations...

Which is evidently nonsensical - the wheel is symmetrical fore-aft.
--
David Damerell > Distortion Field!

[Jeff Wills]

David Damerell > wrote in message >...
> Shock > wrote:
> >drag than all those spokes whipping up the air. There has even been some wind
> >tunnel data to suggest that disc wheels and wheel covers may even provide
> >some small amount of propulsion in certain crosswind situations...
>
> Which is evidently nonsensical - the wheel is symmetrical fore-aft.

No, he's correct. It's similar to a sailboat getting propulsion from
a sidewind or even quartering headwind. Haven't you ever seen a
sailboat tack into the wind?

Jeff

[A Muzi]

> Shock > wrote:
> >drag than all those spokes whipping up the air. There has even been some
wind
> >tunnel data to suggest that disc wheels and wheel covers may even provide
> >some small amount of propulsion in certain crosswind situations...



"David Damerell" > wrote in message
...
> Which is evidently nonsensical - the wheel is symmetrical fore-aft.

Couldn't it if the wind was from one side and behind ? Unlikely but not
nonsensical.

--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

[Doug Milliken]

On Tue, 22 Jul 2003, Dave Lehnen wrote:

> David Damerell wrote:
> > Shock > wrote:
> >
> >>drag than all those spokes whipping up the air. There has even been some wind
> >>tunnel data to suggest that disc wheels and wheel covers may even provide
> >>some small amount of propulsion in certain crosswind situations...
> >
> > Which is evidently nonsensical - the wheel is symmetrical fore-aft.
>
> It may be counter-intuitive but it's not nonsensical. Disc wheels,
> and some other wheels such as the tri-spoke wheels with airfoil
> spokes and deep section rims, can generate some forward thrust in a
> crosswind. This has been measured in wind tunnels and does not
> contradict theory. The thrust is not nearly enough to overcome drag
> of the rider and the rest of the bike, but it is enough to make a
> difference. FWIW, vertical-axis wind turbines typically use
> symmetrical airfoils, with the airfoil's plane of symmetry tangent
> to its circle of rotation. They won't self-start, but generate power
> after being started by external power.

<Snip of the rest of a nice post>

For a practical example of a symmetric airfoil (essentially no camber)
making thrust, see one of my hpv's here:

http://www.interq.or.jp/cool/yutakais/gallery/aero4/main.htm

As with the windmills you note above, it has to be pedaled up to some speed
first, then the wind takes over. With a steady crosswind this bike sails
quite nicely. I've done many miles without pedaling, on the flat, at 15-20
mph. Of course with that much side area, gusty winds or high speed traffic
(opposing trucks are the worst) can be scary--small boat warnings apply(!)

Wind tunnel tests also confirm these results--the x-force (force along
the centerline of the bike) becomes thrust at some yaw angle (angle of
the relative wind).

-- Doug Milliken
www.millikenresearch.com

[Dave Lehnen]

David Damerell wrote:
<snip>
>
> I am not convinced by the sail analogy; a sail produces thrust
> perpendicular to the plane of the sail (yes, yes, it's curved)
> and matters can be arranged such that vector points fowards; but
> perpendicular to the plane of a disc wheel will always be
> directly sideways.

While this seems intuitively reasonable, it's not true. You may need
to look at airfoil test results to convince yourself of this. What
is true is that the resultant force (vector sum of lift and drag)
can never be at an angle of 90 degrees or less to the apparent wind,
otherwise perpetual motion would be possible; sailboats and wind
turbines would work in a dead calm.

Dave Lehnen