bicycle physics question

Andy Gee said:

Okay, this is weird. I was doing my usual after-work timed 5-miles
down the Hudson River Greenway. It's going south. There was a 25
MPH west wind off the river. There's a heliport at 34th street --
sometimes the choppers land or take off at an angle where a cyclist
can really feel it. Anyway, this one chopper had over-corrected and
was recovering just as I was passing by and I got hit with the
combined west wind and his rotor wash, and the entire bike "slipped"
sideways. I was lucky I didn't get blown into the highway. I never
stopped moving forward, the wind just pushed the bike sideways
without me tipping over.

Quoted message said:

I figured the rotational inertia (angular momentum?) of the wheels
kept the bike upright while the force of the wind was being applied
laterally, and it overcame the static friction of the very small
area of contact between the smoothish tires and the smooth road
surface. Afterward, I tried to "tip" my bike while moving but
_without_ turning the handlebars, and I couldn't do it, not that I
tried too hard.

Inertia of its wheels plays no role in keeping a bicycle upright,
except when riding no-hands. In that event, wheel inertia (gyroscopic
forces) are used to steer the bicycle... and that does not work with
even a moderate crosswind.

Quoted message said:

Does this make physics sense?

No.

Quoted message said:

What should I have done if I was really in danger of entering the
highway? Fortunately, there was a wide dirt buffer. Should I have
steered into the "slip" direction, or away from it, or just jumped
off?

Make all the usual crosswind corrections that only work up to a
certain lean angle and turbulence. At times, stopping is the only
recourse.

Jobst Brandt

You blew with the wind because you steered that way to keep the
bottom of the bike under the top of the bike.

Otherwise you would have fallen over.

--
Ron Hardin
[email hidden]

On the internet, nobody knows you're a jerk.

Ron Hardin said:

You blew with the wind because you steered that way to keep the
bottom of the bike under the top of the bike.

Otherwise you would have fallen over.

The way I read it he didn't steer the bike that way, it slid that way.
Which seemed pretty unlikely if he was on dry pavement, IMHO, but that's
how I read it.

I think he was very lucky.

Rich

Rich said:

Ron Hardin said:

You blew with the wind because you steered that way to keep the
bottom of the bike under the top of the bike.

Otherwise you would have fallen over.

The way I read it he didn't steer the bike that way, it slid that way.
Which seemed pretty unlikely if he was on dry pavement, IMHO, but that's
how I read it.

I think he was very lucky.

Rich

He steered that way whether he noticed it or not.

--
Ron Hardin
[email hidden]

On the internet, nobody knows you're a jerk.

Ron Hardin said:

Rich said:

Ron Hardin said:

You blew with the wind because you steered that way to keep the
bottom of the bike under the top of the bike.

Otherwise you would have fallen over.

The way I read it he didn't steer the bike that way, it slid that way.
Which seemed pretty unlikely if he was on dry pavement, IMHO, but that's
how I read it.

I think he was very lucky.

Rich

He steered that way whether he noticed it or not.

Are you saying that Andy corrected for the extra force perhaps without
realizing it? That sounds like the only way he might have avoided being
tipped over by it.

catzz66 said:

Ron Hardin said:

Rich said:

Ron Hardin wrote:

>You blew with the wind because you steered that way to keep the
>bottom of the bike under the top of the bike.
>
>Otherwise you would have fallen over.

The way I read it he didn't steer the bike that way, it slid that way.
Which seemed pretty unlikely if he was on dry pavement, IMHO, but that's
how I read it.

I think he was very lucky.

He steered that way whether he noticed it or not.

Are you saying that Andy corrected for the extra force perhaps without
realizing it? That sounds like the only way he might have avoided being
tipped over by it.

I suspect he is saying that - and I would second it. I've ridden in
VERY windy conditions along the ocean in Florida, and often the wind
would be blowing VERY hard one direction and shift 180 degrees almost
immediately due to turbulence caused by a wall of condos.

I've had rides where I'm leaning heavily into the wind to compensate
for the force, and been blown several feet one way or the other by a
gust, a wind direction changes or a car passing (and blocking the
wind). There was never any "sliding" though - it's just a matter of
the change in lateral pressure causing me to lean more or less into
the wind, and the correction the bike HAD to make to stay under me
(fortunately).

Spooky, but not "sliding" by any definition.

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $795 ti frame

Ron Hardin <[email hidden]> wrote in
news:[email hidden]:

Quoted message said:

Rich said:

Ron Hardin said:

You blew with the wind because you steered that way to keep the
bottom of the bike under the top of the bike.

Otherwise you would have fallen over.

The way I read it he didn't steer the bike that way, it slid that
way. Which seemed pretty unlikely if he was on dry pavement, IMHO,
but that's how I read it.

I think he was very lucky.

Rich

He steered that way whether he noticed it or not.

I think I would know if I was steering or not! I was riding fast with
enough corrective lean _into_ the wind to keep moving straight, then the
rotor wash hit me. If I were steering I would have steered _into_ the
wind and the bike would have travelled diagonally. At all times in this
process both wheels were parallel to the center lane marker.

--ag

Quoted message said:

"Mark Hickey" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

catzz66 said:

Ron Hardin said:

Rich wrote:
>Ron Hardin wrote:
>
>>You blew with the wind because you steered that way to keep the
>>bottom of the bike under the top of the bike.
>>
>>Otherwise you would have fallen over.
>
>The way I read it he didn't steer the bike that way, it slid that way.
>Which seemed pretty unlikely if he was on dry pavement, IMHO, but that's
>how I read it.
>
>I think he was very lucky.

He steered that way whether he noticed it or not.

Are you saying that Andy corrected for the extra force perhaps without
realizing it? That sounds like the only way he might have avoided being
tipped over by it.

I suspect he is saying that - and I would second it. I've ridden in
VERY windy conditions along the ocean in Florida, and often the wind
would be blowing VERY hard one direction and shift 180 degrees almost
immediately due to turbulence caused by a wall of condos.

I'm with Mark. You can get very odd wind shifts around skyscrapers in
Chicago on windy days. It does feel like the bike slides, but I think it's
autonomic compensation -- from years of practice keeping yourself upright
while walking / cycling / etc. your body knows how to compensate and does so
before you have time to think about it.

"andy gee" wrote: (clip) If I were steering I would have steered _into_
the wind and the bike would have travelled diagonally. At all times in this
process both wheels were parallel to the center lane marker.
^^^^^^^^^^^^^^^^^^^
Balancing a bike is ALWAYS done by steering. As Ron Hardin said, you
steered the bike AWAY from the wind. This is not sonething special that
happens in a cross wind. It is going on all the time you ride. Tiny
balance errors are corrected by introducing a turn.

This is also how you turn corners. You steer AWAY from the corner, and the
bottom of the bike puts you into a lean which is proper for the turn, and
you then ride a curved path with that lean. To complete the turn, you steer
INTO the turn, putting the wheels back under you, standing the bike up.

If you froze the headset bearings in a straight ahead position, and somehow
managed to ride the bike, a cross wind would not slide you sideways--it
would blow you over.

Andy Gee said:

Okay, this is wierd. I was doing my usual after-work timed 5-miles down
the Hudson River Greenway. It's going south. There was a 25 MPH west wind
off the river. There's a heliport at 34th street -- sometimes the
choppers land or take off at an angle where a cyclist can really feel it.
Anyway, this one chopper had over-corrected and was recovering just as I
was passing by and I got hit with the combined west wind and his rotor
wash, and the entire bike "slipped" sideways. I was lucky I didn't get
blown into the highway. I never stopped moving forward, the wind just
pushed the bike sideways without me tipping over.

I am just speculating here, but maybe the rotor wash was at a much
higher velocity close to the ground and so it exerted more pressure
below your center of gravity than above resulting in the tires slipping
sideways rather than you getting blown over. In my minds eye I am
picturing a column of air shooting down towards the ground from the
rotor and then spreading out laterally when it hits the ground,
creating a very high velocity zone close to the ground. Who knows if
that is the way a column of air hitting a flat surface really behaves,
but it makes intuitive sense to me.

Another Andy

In article <[email hidden]>,

Andy Gee said:

Anyway, this one chopper had over-corrected and was recovering just as I
was passing by and I got hit with the combined west wind and his rotor
wash, and the entire bike "slipped" sideways. I was lucky I didn't get
blown into the highway. I never stopped moving forward, the wind just
pushed the bike sideways without me tipping over.

Nah, the wind didn't have anything to do with it.

You just unwittingly rode through a Hilbert space :-)

cheers,
Tom

--
-- Nothing is safe from me.
Above address is just a spam midden.
I'm really at: tkeats [curlicue] vcn [point] bc [point] ca

Tip vortices probably added to the effect of the downwash. There's
plenty of research on these if you want to look it up.

"Andy" <[email hidden]> wrote in
news:[email hidden]:

Quoted message said:

I am just speculating here, but maybe the rotor wash was at a much
higher velocity close to the ground and so it exerted more pressure
below your center of gravity than above resulting in the tires slipping

I was thinking that might be the answer... but I'm still trying to
calculate in general terms exactly what force would be required to overcome
the "gyroscope effect" of the wheels. But yeah, a directed blast would do
it.

--ag

Andy Gee said:

Okay, this is wierd. I was doing my usual after-work timed 5-miles down
the Hudson River Greenway. It's going south. There was a 25 MPH west wind
off the river. There's a heliport at 34th street -- sometimes the
choppers land or take off at an angle where a cyclist can really feel it.

You may find this weird as well, but your post made me think of fire
engines. I ride past a fire station regularly, and a few times the
engines have emerged as I went by or passed me down the road.
I proffer the advice that anytime a fire engine, police car, or
ambulance gets anywhere near you with its lights and/or siren going, you
should do the same thing you should do when there's a helicopter taking
off or landing nearby: get completely off the road ASAP and stay there
until the vehicle or aircraft is gone.
Things like that distract drivers, and may motivate them to pull off
the road (as they should for emergency vehicles). They won't be looking
for cyclists.

Bill

__o | Look to the past and remember no empire rises
_`\(,_ | that sooner or later won't fall.
(_)/ (_) | --Al Stewart

Andy Gee said:

I was thinking that might be the answer... but I'm still trying to
calculate in general terms exactly what force would be required to overcome
the "gyroscope effect" of the wheels. But yeah, a directed blast would do
it.

You were there and are the only one who knew what you felt. I am not
making light of your observations, but sometimes I have done some things
on my bike that made feel it was just the "E.T." effect that kept me
upright.