Wheel Covers used in Time Trials

Discussion in 'Cycling Equipment' started by Brian, May 10, 2003.

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  1. Brian

    Brian Guest

    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
     
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  2. Jay

    Jay Guest

    I would not recommend them for reasons.

    It's difficult to get a smooth transition from the cover to the rim. Because they are designed to
    work with spoked wheels, they are not symmetric on both sides of the wheel. Each side follows the
    dish of the wheel, therefore you end up with more curvature on one side of the wheel than the other.
    This creates a side load on the wheel just like that of an airplane wing, resulting in drag.

    When I removed my covers, my time went up.

    Jay

    "Brian" <[email protected]> wrote in message news:[email protected]...
    > 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
     
  3. "Jay" <Jay.Guthrie [email protected]> wrote in message
    news:<[email protected]>...
    > I would not recommend them for reasons.
    >
    > It's difficult to get a smooth transition from the cover to the rim. Because they are designed
    > to work with spoked wheels, they are not symmetric on both sides of the wheel. Each side follows
    > the dish of the wheel, therefore you end up with more curvature on one side of the wheel than
    > the other. This creates a side load on the wheel just like that of an airplane wing, resulting
    > in drag.
    >
    > When I removed my covers, my time went up.
    >
    > Jay
    >
    ...if your time went UP when you removed them, perhaps you should put them back! (unless you are in
    a slow bicycle race, of course) ;-)
     
  4. Shock

    Shock Guest

    [email protected] (Brian) wrote in news:[email protected]:

    > 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~~
     
  5. Shock <[email protected]> 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 <[email protected]> Distortion Field!
     
  6. Dave Lehnen

    Dave Lehnen Guest

    David Damerell wrote:
    > Shock <[email protected]> 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.

    An airfoil, aligned with the direction of travel, can generate forward thrust whenever
    arctangent(Drag/Lift) < angle of attack. Angle of attack depends on the vector difference of the
    true wind velocity and the bike velocity. For example, a bike going 25 mph with a pure side wind of
    7 mph has an angle of attack of 15.64 degrees. If the wheel has a Lift/Drag ratio of better than
    3.57 at this angle of attack, it will generate net thrust rather than net drag. Wheels don't make
    very good airfoils, but L/D of 3.6 is very low. For comparison, an old thick NACA 0025 symmetrical
    airfoil has an L/D of about 12 at this angle of attack. Rotation of the wheels complicates things,
    especially for spoked wheels, compared to a simple fixed airfoil, but it still doesn't take much of
    an airfoil to generate some thrust in the right side wind conditions.

    Dave Lehnen
     
  7. Mark Hickey

    Mark Hickey Guest

    David Damerell <[email protected]> wrote:

    >Shock <[email protected]> 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.

    Actually, as unlikely as it may sound, wind tunnel testing has shown that it is indeed possible to
    get a "sail effect" from some disc wheels in certain conditions.

    Mark Hickey Habanero Cycles http://www.habcycles.com Home of the $695 ti frame
     
  8. Jeff Wills

    Jeff Wills Guest

    David Damerell <[email protected]> wrote in message
    news:<gHm*[email protected]>...
    > Shock <[email protected]> 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
     
  9. A Muzi

    A Muzi Guest

    > Shock <[email protected]> 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" <[email protected]> wrote in message
    news:gHm*[email protected]...
    > 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
     
  10. On Tue, 22 Jul 2003, Dave Lehnen wrote:

    > David Damerell wrote:
    > > Shock <[email protected]> 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
     
  11. Jeff Wills <[email protected]> wrote:
    >David Damerell <[email protected]> wrote in message
    >news:<gHm*[email protected]>...
    >>Shock <[email protected]> wrote:
    >>>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?

    A sail does not present a symmetrical profile to a crosswind!
    --
    David Damerell <[email protected]> Distortion Field!
     
  12. A Muzi <[email protected]> wrote: [Disc wheels in crosswinds]
    >"David Damerell" <[email protected]> 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.

    But in all but the highest winds the apparent wind direction is from the front and/or side, never
    from the rear, because one can always ride faster than the wind.

    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.
    --
    David Damerell <[email protected]> Distortion Field!
     
  13. Dave Lehnen

    Dave Lehnen Guest

    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
     
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