Body weight -- does it affect glide?

The recent discussion of BMI made me wonder -- what are the
advantages of being heavier? One that occurred to me
is that a heavier skier may get better glide
than a lighter skier. I mentioned this to a friend and he
scoffed, saying that if a skier is on a ski that is
properly flexed for his/her weight and the ski is
well-waxed then body weight has no effect on glide.
I'm not convinced because he has a quiver of skis from
which he selects the proper one for the day and waxes
obsessively, yet I outglide him despite having
only one ski and waxing only occasionally; the only
thing that I can think of as a cause for this is that
I am a lot heavier than he is.

-Everett

Ah- gravity enhanced!

I ride bikes quite often. There is no "naturally occuring" benefit of
weight in riding down a hill. Heavy and light riders seem to roll down
slopes at a similar speed for a similar distance. So my gut (pardon the
pun) says that gravity isn't "doing you a favor" at least on a bike.

Skis are more difficult to analyze. You are gliding on a thin film of water
molecules under your ski. More pressure from more weight might enhance this
film effect. However, there have to be a bunch of "environmental"
variables - the flex of the ski, the track temperature, snow density,
waxing, etc. I don't know if you can solve it by analysis of a situation,
but I think in theory the likely answer is that the laws of physics aren't
working for you.

"Everett" wrote in message
om...
The recent discussion of BMI made me wonder -- what are the
advantages of being heavier? One that occurred to me
is that a heavier skier may get better glide
than a lighter skier. I mentioned this to a friend and he
scoffed, saying that if a skier is on a ski that is
properly flexed for his/her weight and the ski is
well-waxed then body weight has no effect on glide.
I'm not convinced because he has a quiver of skis from
which he selects the proper one for the day and waxes
obsessively, yet I outglide him despite having
only one ski and waxing only occasionally; the only
thing that I can think of as a cause for this is that
I am a lot heavier than he is.

-Everett

I believe that body mass is beneficial on a downhill when gravity is the
significant driving force. Since gravity force is proportionate to mass,
and air resistance is to a great extent propotionate to frontal area (given
the same shape factor, etc) a heavier bike rider/skier will probably have a
more favorable ratio of downhill gravity force to retarding forces from air
resistance.

Ski retardation forces are a little more complex than bikes. With bike
descents, air resistance is more significant that rolling resistance.
However, ski descents are affected by air resistance, friction between skis
and snow, and any displacement of snow (tip plowing etc).

If ski physics are similar to boats, a long boat will be faster than a short
boat of the same displacement (not counting hydroplanning behavior). I
wonder if a longer ski has less plowing resistance than a shorter ski all
thing being equal.

Edgar

"Pieter Litchfield" wrote in message
news
Ah- gravity enhanced!

I ride bikes quite often. There is no "naturally occuring" benefit of
weight in riding down a hill. Heavy and light riders seem to roll down
slopes at a similar speed for a similar distance. So my gut (pardon the
pun) says that gravity isn't "doing you a favor" at least on a bike.

Skis are more difficult to analyze. You are gliding on a thin film of
water
molecules under your ski. More pressure from more weight might enhance
this
film effect. However, there have to be a bunch of "environmental"
variables - the flex of the ski, the track temperature, snow density,
waxing, etc. I don't know if you can solve it by analysis of a situation,
but I think in theory the likely answer is that the laws of physics aren't
working for you.

"Everett" wrote in message
om...
The recent discussion of BMI made me wonder -- what are the
advantages of being heavier? One that occurred to me
is that a heavier skier may get better glide
than a lighter skier. I mentioned this to a friend and he
scoffed, saying that if a skier is on a ski that is
properly flexed for his/her weight and the ski is
well-waxed then body weight has no effect on glide.
I'm not convinced because he has a quiver of skis from
which he selects the proper one for the day and waxes
obsessively, yet I outglide him despite having
only one ski and waxing only occasionally; the only
thing that I can think of as a cause for this is that
I am a lot heavier than he is.

-Everett

The physics is simple. The gravitational force is proportional to
mass. If the frictional force is also proportional to mass, then the
dynamics will be identical for large and small skiers.

However, there are lots of situations in which friction is not
proportional to mass. For example, wind resistance is all about drag
coefficients, which depends on cross-sectional area, not mass. Note
that area goes like r^2, where as volume/mass goes like r^3, so the
frictional force increases like m^(2/3). Because the frictional force
increases less than proportionately to mass, this means big objects
fall faster than small ones. Do a google search on "Stoke's settling"
to see some math that describes this.

In skiing, there is also no reason to believe that friction should be
proportional to mass. See above for air resistance (important at high
speeds). Also, the width of the ski is the same for all skiers, so
the loose snow that needs to be displaced/packed in front of the ski
is independent of mass (important out of track). This will
slow down lighter skiers more than heavier skiers (a=F/m). As for the
contact of ski base with packed snow underneath, I'd guess that that
might be reasonably described by the sort of friction coefficient you
see in 1st year physics (i.e., friction proportional to normal force,
which is proportional to mass), so the effect might indeed be the same for
skiers regardless of weight.

So, my thoughts: yes, it's complicated, but in many situations it
seems very likely that heavier skiers will glide better than light
ones.

Now, there are some exceptions. There are times when the track hasn't
set up and the snow is soft. In this case, heavy skiers may break
through, while lighter skiers glide over the top. This is most
definitely in the favor of the lighter skiers. At 135 lb, these are
the sort of conditions I dream about!

Cheers,
Brian

In article , Everett wrote:
The recent discussion of BMI made me wonder -- what are the
advantages of being heavier? One that occurred to me

"Edgar Hee" wrote in message
...
I believe that body mass is beneficial on a downhill when gravity is the
significant driving force. Since gravity force is proportionate to mass,
and air resistance is to a great extent propotionate to frontal area
(given
the same shape factor, etc) a heavier bike rider/skier will probably have
a
more favorable ratio of downhill gravity force to retarding forces from
air
resistance.

Ski retardation forces are a little more complex than bikes. With bike
descents, air resistance is more significant that rolling resistance.
However, ski descents are affected by air resistance, friction between
skis
and snow, and any displacement of snow (tip plowing etc).

I tend to agree with what Edgar's written. Concerning the original
poster...

It could be that even though your skiing buddy has more skis and seems to
spend more time prepping them, yours simply glide better than any of his,
for any one of may reasons (fit to your weight and bottom condition being
the two most common).

It could be that even though he looks like he's in a good body position,
yours might be better. Two ways this might happen:

He might be a little knock kneed in the tuck position, thus weighting the
inside edges a little and increasing friction. This is pretty common I
think. Think "bowlegs" when going downhill to keep those bottoms flat on
the snow.

He might be plowing his tips a little because his weight is too far onto the
balls of his feet when gliding, especially in the tuck position. Think
"weight heel" to counteract this. Especially true with striding skis, in
my experience.

Grissy.

And of course there's gliding UP whatever amount of grade which brings in the
power/weight ratio... : )

--

Jeff Potter
****
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On Thu, 29 Jan 2004 wrote:

As for the
contact of ski base with packed snow underneath, I'd guess that that
might be reasonably described by the sort of friction coefficient you
see in 1st year physics (i.e., friction proportional to normal force,
which is proportional to mass), so the effect might indeed be the same for
skiers regardless of weight.

To complicate this a bit, it has been argued that since the ski is not
a rigid body the flex of the ski comes into play. A ski flexed properly
to you body weight will in theory have the force distributed well across
the length of the ski, thus avoiding any high-pressure points. This
seems to work in the wet-glide scenario.

In dry-glide (extreme cold) things seem to change. I once talked to
Anssi Juutilainen, former world champion in ski-orienteering, about
this. He said the Finnish team's practice at that time was to go with
stiffer skis in extreme cold in order to minimize contact area, since
all friction at these temps is bad friction. (Speaking strictly of
skating skis here.)


Now, there are some exceptions. There are times when the track hasn't
set up and the snow is soft. In this case, heavy skiers may break
through, while lighter skiers glide over the top. This is most
definitely in the favor of the lighter skiers. At 135 lb, these are
the sort of conditions I dream about!

Well... as I'm sure you're aware, ski-o encounters soft-track
conditions far more often than cross-country. And since you've already
demonstrated you have a talent for ski-orienteering...

-Mitch

I weigh 195 lbs and I would challenge any of my spindly comrades
skiing at Tahoe to beat me down a hill, all other things being equal.
I think there's some law of physics involved here. I'm sure one of
the many propellerheads who frequent this newsgroup will help me out
on this one. I failed physics in high school.

bt

(Everett) wrote in message . com...
The recent discussion of BMI made me wonder -- what are the
advantages of being heavier? One that occurred to me
is that a heavier skier may get better glide
than a lighter skier.

Do you mean gliding uphill or on the flat or downhill? Due to gravity,
you might have an advantage over your friend downhill. On the flats or
uphill, no way.

JT

Isn't there a story, supposedly true, about a smallish Columbian
climber from the big European bicycle tours back in the 70/s or 80's,
who could dlimb with the best. After awhile it was noticed that
at the top of all the major climbs, he'd be handed a new waterbottle,
but he never seemed to drink from it, and generally it was flung away
at the bottom of the descent. When one was recovered, it was found
to be full of (solid, not liquid) lead, perfectly legal then but
maybe not now. Maybe JFT or another of the bike racers on this ng
can tell us whether that actually happened. I wasn't a propeller-head
back in high school, my beer drinking/pool shooting friends assure me,
but maybe I got to be one later. Putting on that cap, I'm certain
the gravitation arguments for weight being an advantage downhill are
correct, but surely you lose more on the uphills than you'd ever
gain going down. In any case,, drafting in bike racing nullifies the
advantage there in the majority of situations. On the level on skis,
a bigger skier should glide farther in a straight test, but that's
no advantage in racing. He or she needs to expend more energy in
order to pick up an extra 1km/hr say on the flats, compared to
a smaller skier (leaving aside any question of a difference in friction),
just because kinetic energy is proportional to mass at a given velocity.

Best, Peter