Nre = 100K; Taping Helmet Holes, Dimpled Skin Suits, Fat Gas Turbine Nozzles

[Bretcahill]

About a year ago a cyclist in the NY City area told me some riders were taping the ventilation holes
in their helmets to reduce wind resistance.

I pointed out that a 10" diameter helmet going around 25 mph was in the same flow regime as a golf
ball going about 200 mph -- the Reynolds number (Diameter x velocity x density of fluid)/(viscosity
of fluid) for both situations is about 100,000. Since the dimples of a golf ball dramatically
*reduce* wind resistance I figured that taping holes on helmets was counterproductive. Someone else
actually calculated the increased drag as either 0.1% or a thousandth of a % or something pretty
insignificant. Later I heard about dimpled skin suits which pretty much confirmed the above
reasoning, even if they don't quite work at sea level velo speeds.

He then asked why they don't dimple cars or 747s. I told him to calculate the Nre. If cars only went
2 mph or widebodies only went 0.4 mph, then dimpling might work there too.

Nre = 100,000 is magic in fluid mechanics. It's a big deal in high altitude jet design. If the Nre
of the flow through a gas turbine drops below 100,000 due to thin air or low speed or small nozzles,
power drops off dramatically.

Bret Cahill

 

[Kinkycowboy]

On 16 Sep 2003 23:11:20 GMT, [email protected] (BretCahill) wrote:

>About a year ago a cyclist in the NY City area told me some riders were taping the ventilation
>holes in their helmets to reduce wind resistance.
>
>I pointed out that a 10" diameter helmet going around 25 mph was in the same flow regime as a golf
>ball going about 200 mph -- the Reynolds number (Diameter x velocity x density of fluid)/(viscosity
>of fluid) for both situations is about 100,000. Since the dimples of a golf ball dramatically
>*reduce* wind resistance I figured that taping holes on helmets was counterproductive. Someone else
>actually calculated the increased drag as either 0.1% or a thousandth of a % or something pretty
>insignificant. Later I heard about dimpled skin suits which pretty much confirmed the above
>reasoning, even if they don't quite work at sea level velo speeds.
>
>He then asked why they don't dimple cars or 747s. I told him to calculate the Nre. If cars only
>went 2 mph or widebodies only went 0.4 mph, then dimpling might work there too.
>
>Nre = 100,000 is magic in fluid mechanics. It's a big deal in high altitude jet design. If the Nre
>of the flow through a gas turbine drops below 100,000 due to thin air or low speed or small
>nozzles, power drops off dramatically.
>
>
>Bret Cahill
>
>

Aren't the holes in helmets a bit more complicated than dimples? After all, the air goes right
through them and then, after interfering with your hairstyle, exhausts out of the back. There have
been "aero" helmets with no holes and small (~5mm dia.) dimples in a golf ball type pattern, head
fairings with no holes or dimples and trackies using holey helmets with the holes blanked off but
not flush with the main surface. Which of these is "best"? And what about whatever happens to
arrive in the wake of the helmet, usually the rider's shoulders and upper back? A helmet which
flies really well through the air all by itself might be nice, but it's not serving any useful
purpose for the cyclist!

Kinky Cowboy

*Your milage may vary Batteries not included May contain traces of nuts.

 

[Benjamin Weiner]

BretCahill <[email protected]> wrote:
> About a year ago a cyclist in the NY City area told me some riders were taping the ventilation
> holes in their helmets to reduce wind resistance.

> I pointed out that a 10" diameter helmet going around 25 mph was in the same flow regime as a golf
> ball going about 200 mph -- the Reynolds number (Diameter x velocity x density of
> fluid)/(viscosity of fluid) for both situations is about 100,000. Since the dimples of a golf ball
> dramatically *reduce* wind resistance I figured that taping holes on helmets was
> counterproductive. Someone else actually calculated the increased drag as either 0.1% or a
> thousandth of a % or something pretty insignificant. Later I heard about dimpled skin suits which
> pretty much confirmed the above reasoning, even if they don't quite work at sea level velo speeds.

I would not be quick to argue that two fluid flows around bluff bodies are the same because they
have similar Reynolds number. The shape and dynamics of the body can have large effects.

Dimples on a golf ball reduce overall drag by affecting boundary layer separation, but they also
work by producing extra lift, because the ball has lots of backspin. See e.g.

http://math.ucr.edu/home/baez/physics/General/golf.html

Boundary layer separation over a cyclist is not always intuitive. Dimples might make a difference
but the size and location are critical. Witness various discussions over (a) Armstrong's funny
shaped aero helmet and (b) riding with head down, which may reduce drag even though the aero helmet
point sticks up in the air. Here are some contributions from Kraig Willett:

http://www.bike.com/template.asp?date=7%2F16%2F2002&lsectionnumber=6

http://groups.google.com/groups?q=armstrong+helmet+group:rec.bicycles.tech+author:willett&hl=en&lr=-
&ie=UTF-8&oe=UTF-8&selm=nK9k9.16023%24CJ5.2873850%40news3.news.adelphia.net&rnum=1

Bottom line: I don't know whether taping over the holes in a helmet reduces drag, and it's unlikely
anybody does unless they've tested it in a wind tunnel. Kraig Willett may have tested commercially
available USCF-legal aero helmets in a tunnel, if so the results might be available with a
subscription to his site.

I'm pretty sure that taping over the holes will make the helmet hotter, though. Or one could just
buy a BMX helmet and have done with it.

 

[Chris Zacho "Th]

Gee. Just how technical is this group gonna get?

May you have the wind at your back. And a really low gear for the hills! Chris

Chris'Z Corner "The Website for the Common Bicyclist": http://www.geocities.com/czcorner

 

[David L. Johnso]

On Tue, 16 Sep 2003 23:11:20 +0000, BretCahill wrote:

> About a year ago a cyclist in the NY City area told me some riders were taping the ventilation
> holes in their helmets to reduce wind resistance.

Just a little reality check. Are you sure that guy had the reason right? I have taped over vent
holes on my helmet. In winter. It keeps the wind off my head. Makes a big difference, and it does
get cold in New York in the winter.

Just seeing riders with the vents taped over does not mean it is for aerodynamic reasons.

--

David L. Johnson

__o | The lottery is a tax on those who fail to understand _`\(,_ | mathematics. (_)/ (_) |

 

[Bretcahill]

Benjamin Weiner <[email protected]> in
>Message-id: <[email protected]> writes:

. . .

>I would not be quick to argue that two fluid flows around bluff bodies are the same because they
>have similar Reynolds number.

Not only that, but how do you define characteristic diameter?

>The shape and dynamics of the body can have large effects.

No one will deny that fluid mechanics is messy at best, however, taping holes is a waste of time
unless you are cold.

Bret Cahill

 

[Bretcahill]

[email protected] (Chris Zacho "The Wheelman") in
>Message-id: <[email protected]>
writes:
>
>Gee. Just how technical is this group gonna get?

>May you have the wind at your back.

But supposing you DON'T have the wind at your back?

What then?

Bret Cahill

 

[Bretcahill]

Benjamin Weiner <[email protected]> in
>Message-id: <[email protected]> writes:

. . .

>Kraig Willett may have tested commercially available USCF-legal aero helmets in a tunnel, if so the
>results might be available with a subscription to his site.

The tear drop helmets should work if they are positioned right.

>I'm pretty sure that taping over the holes will make the helmet hotter, though.

That may be the biggest factor. I told my friend about a 30 something year old guy dying in a DC
area race.

Bret Cahill

 

[Bretcahill]

Benjamin Weiner <[email protected]> in
>Message-id: <[email protected]> writes:

. . .

>Witness various discussions over (a) Armstrong's funny shaped aero helmet

Lance Armstrong isn't winning by cheating the wind by a fraction of a percent any more than Warren
Buffet is making money on the markets by selling at the exact right second or Bob Graham is winning
elections on cute sound bites.

Most only see the tip of the iceberg with those guys -- VERY deceptive.

Bret Cahill

"Fun -- the father of invention."

 

[Werehatrack]

On 17 Sep 2003 04:45:08 GMT, [email protected] (BretCahill) may have said:

>Lance Armstrong isn't winning by cheating the wind by a fraction of a percent any more than Warren
>Buffet is making money on the markets by selling at the exact right second or Bob Graham is winning
>elections on cute sound bites.
>
>Most only see the tip of the iceberg with those guys -- VERY deceptive.

True. And sometimes, a winning team will intentionally hang a red herring gadget where everyone can
see it, without comment, and wait to see who grabs on to it as their "magic bullet" and shows up
with one at the next race. This used to be a regular prank in certain car racing circles; I would be
surprised if it were unknown in cycling.

The admonition "use what works *for you*" has multiple levels of applicability.

--
My email address is antispammed; pull WEEDS if replying via e-mail. Yes, I have a killfile. If I
don't respond to something, it's also possible that I'm busy.

 

[Werehatrack]

On Tue, 16 Sep 2003 23:06:07 -0400, "David L. Johnson" <[email protected]> may have said:

>On Tue, 16 Sep 2003 23:11:20 +0000, BretCahill wrote:
>
>> About a year ago a cyclist in the NY City area told me some riders were taping the ventilation
>> holes in their helmets to reduce wind resistance.
>
>Just a little reality check. Are you sure that guy had the reason right? I have taped over vent
>holes on my helmet. In winter. It keeps the wind off my head. Makes a big difference, and it does
>get cold in New York in the winter.
>
>Just seeing riders with the vents taped over does not mean it is for aerodynamic reasons.

Guy #1 tapes because it's cold out. Guy #2, who does not ride in that weather, sees Guy #1 ride by
on a better bike, and at a higher speed, than Guy #2 can do on a good day. Guy #2 thinks "wow,
taping those holes must make a lot of difference", ignoring the fact that riding *all year* will put
you in better shape than being a fair-weather cyclist. Spring warming comes, and Guy #2 tapes his
helmet...and proudly tells all his friends about this killer go-fast trick. Some believe it and
spread the word. Others, say nothing, probably thinking "what an idiot!"...and before long, the
rumor-believers have become much easier to identify.

--
My email address is antispammed; pull WEEDS if replying via e-mail. Yes, I have a killfile. If I
don't respond to something, it's also possible that I'm busy.

 

[Alexey Merz]

Benjamin Weiner <[email protected]> wrote in message news:<[email protected]>... [...]
> Boundary layer separation over a cyclist is not always intuitive. Dimples might make a difference
> but the size and location are critical. Witness various discussions over (a) Armstrong's funny
> shaped aero helmet and (b) riding with head down, which may reduce drag even though the aero
> helmet point sticks up in the air. Here are some contributions from Kraig Willett:
>
> http://www.bike.com/template.asp?date=7%2F16%2F2002&lsectionnumber=6
>
> http://groups.google.com/groups?q=armstrong+helmet+group:rec.bicycles.tech+author:willett&hl=en&l-
> r=&ie=UTF-8&oe=UTF-8&selm=nK9k9.16023%24CJ5.2873850%40news3.news.adelphia.net&rnum=1

...and a contribution from John Cobb: http://www.slowtwitch.com/mainheadings/techctr/helmets.html

 

[Andy Coggan]

"Werehatrack" <[email protected]> wrote in message
news:[email protected]...
> On Tue, 16 Sep 2003 23:06:07 -0400, "David L. Johnson" <[email protected]> may have said:
>
> >On Tue, 16 Sep 2003 23:11:20 +0000, BretCahill wrote:
> >
> >> About a year ago a cyclist in the NY City area told me some riders were taping the ventilation
> >> holes in their helmets to reduce wind
resistance.
> >
> >Just a little reality check. Are you sure that guy had the reason right? I have taped over vent
> >holes on my helmet. In winter. It keeps the wind off my head. Makes a big difference, and it does
> >get cold in New York in the winter.
> >
> >Just seeing riders with the vents taped over does not mean it is for aerodynamic reasons.
>
> Guy #1 tapes because it's cold out. Guy #2, who does not ride in that weather, sees Guy #1 ride by
> on a better bike, and at a higher speed, than Guy #2 can do on a good day. Guy #2 thinks "wow,
> taping those holes must make a lot of difference", ignoring the fact that riding *all year* will
> put you in better shape than being a fair-weather cyclist. Spring warming comes, and Guy #2 tapes
> his helmet...and proudly tells all his friends about this killer go-fast trick. Some believe it
> and spread the word. Others, say nothing, probably thinking "what an idiot!"...and before long,
> the rumor-believers have become much easier to identify.

Or some guys who have better things to do than pontificate on newsgroups actually get into a wind
tunnel and test the effects of taping over the vents of a regular helmet to see if it works or not.
And others, who want to know the real answer, ask guys like this first one.

Andy Coggan