Are G's a figment of our imagination?

[Brady]

Okay, here is a technical question that I know guys like @Than_Bogan and others will love. I also know this was discussed in the last couple of years, but I couldn't find the thread, and I think this is an interesting topic.

 

I have a neighbor of mine who is a professional drag racer. I won't say his name but his initial are A.O. He also was a great skier in his day (PB 2.5 @ 35 off at 34) He was telling me about all the G's he feels when he races and I told him skiers--especially elite-- feel g's when skiing. He promptly told me I was full of Sh&*. I didn't say skiers have as much g's as dragracers, but nonetheless, g's are experienced. he then said that if you felt even 2 g's, you wouldn't be able to hang onto the handle.

 

So, who is right and who is wrong? I didn't pull up any of my formulas from college, but rather, I searched the internet and came up with this site:

http://measurespeed.com/acceleration-calculator.php

According to the site, if one accelerates from 34mph to 70 mph in the course of 1/2 second, you would experience 3.27 g's. Also, wouldn't your current weight play into this, as a skinnier skier would create more g's than a hefty one?

 

I know that there has to be something going on behind the boat, because the exhileration I feel could only be rivaled going mach 5 in an f-16. Or maybe on a ride along with @SkiJay.

[Gloersen]

http://www.ballofspray.com/forum#/discussion/7033/the-physical-forces-of-slalom-skiing

 

had >2 many "GEEZ!" out of the buoys today; seemed real to me.

[klindy]

Interestingly Wikipedia uses a top fuel dragster as an example and accelerating to 100 mph in .86 seconds is about 5.3g.

 

As far as being able to hang onto the line, you could deal with a pretty high g force if the duration was small. For example I was told years ago that top open men jumpers experience 10-12g's when they hit the ramp. Ive never measured it and it may be a long way off but since the duration is so short it's conceivably possible.

[Waternut]

G's are based on acceleration only. Weight only comes into play as a function of force exerted. I tried to do math last time this came up based on watching videos using distance, time, assumed speeds, etc. and was quickly told I was wrong so I won't be doing that again.

 

Either way, the perception of G forces and the actual peak G forces are wildly different. Sustained G's feel way more dramatic because you have more time to process the sensation and fight it.

[BraceMaker]

The link posted above for instance doesn't ask about weight, since you would divide out weight on both sides.

 

You basically need to mount a skier with an accelerometer at approximately their center of mass and read it, maybe time sync it to video so you know what's actually going on when.

[DefectiveDave]

I'm with @Bracemaker, the easiest way is to just strap an accelerometer to someone. I would guess less than 2 G's but wouldn't be surprised if I saw slightly more.

 

It would be as easy as installing some data logging software onto an android or iphone, putting it into a waterproof case, and strapping it to a competent skier.

[A_B]

I doubt that @OB shows up at the dock in his anti-G suit, but he can probably shed some light on G's...

[Bruce_Butterfield]

By far the highest g load is when you crank a turn and almost come to a stop. My guess is you can get 4-6g, but the duration is really short - on the order of 0.1-0.2sec, so not a big deal. It would be comparable to the g load you get if you jump off a 3 ft high table.

 

Of course if you are smooth, the load is much less.

[Onside135]

The highest G-Load I've experienced skiing is when my lower chest impacted the first wake a few years ago. Not sure the G-load, but enough to break ribs...

[drewski32]

Pretty sure slalom skiers don't accelerate from 34mph to 70. The start speed would be however fast they are at the turn in and the top speed is not 70. I think Horton did some sort of experiment with this one time by measuring the frequency of the whistling sound a ski makes at different speeds, and then recorded it at different times in the course. I'm not sure how accurate the information was, but the results were that a slalom skier doesn't even come close to 70mph.

[BraceMaker]

End course video should be sufficient to extrapolate data.. Is there backwards end course video?

[Brady]

I would think that you would always be at 34mph at the apex of the turn. That is when you are at your slowest speed. I would like to get a radar gun on top skiers as they are crossing the wakes. I would think the speed is much higher than what one would think. Time frame is easy. I measured just under .5 seconds from the apex of the the turn til the first wake.

[A_B]

From Waterski Mag 6/4/03 article with radar gun checking Wade Cox top speed:

 

Wade's Speeds

15 off – 49.8 mph

28 off – 49.3 mph

35 off – 49.7 mph

38 off – 53.5 mph

39½ off – 54.6 mph

41 off – 55.2 mph

[Brady]

@AB. But did you take into account how close Wade's face is to the water when he crosses the wake? I believe it is called the "balls component"

 

So, with the conversion rate, 55.2 mph times the balls component ratio of 1.5, he is traveling roughly 75 mpg!!!

[Ralph Lee]

I ski with an ex air force pilot who flew the F-4 phantom, and F-15 eagle, he never quoted a number, but has told me that we are pulling some good G's in and out of the turn.

[SkiJay]

The G's generated by a skier in mid-corner must closely resemble those of a Moto GP racer because they are supporting roughly the same lean angle. Moto GP bikes generate 1.6 G's, so a water skier likely generates less than 2 G's in mid-corner . . . a number that doesn't come close to capturing how awesome it feels to be cranking a steep turn!

 

http://sphotos-g.ak.fbcdn.net/hphotos-ak-prn2/8653_10151667322800827_736528683_n.jpg

 

 

[gator1]

@SkiJay‌ nailed it on gs in the turn.

 

The force on the line theory is wrong, since the rope is fighting acceleration + water drag.

 

Using @Brady‌ time of half second, and Wade's speeds, and assuming constant acceleration from apex of turn to rooster tail, the acceleration in the pull portion is 1.75 gs.

 

But, you don't accelerate all the way from the apex to rooster. Say it takes Wade .17 sec to complete turn and hook up, leaving only .33 seconds to accelerate. Then he'd be up to 2.6 gs.

 

I'd take 2.5 gs to the bank as a conservative number for a pro.

[Roger]

@SkiJay - Great shot of Lorenzo there on his Yamaha in MotoGP. Can't believe the Honda's are reaching 61 degrees of lean angle (Yamahas are close of course). Great to see Lorenzo and Rossi win races this year after Marcus started off winning every race (first 11). Sorry for the off topic...

[A_B]

Would be really cool to see if Nate accelerates to a faster speed due to his high loading out of the ball and change edge behind the boat technique.

 

@Brady once you and I get up to speed, we need brembo brakes to slow down.

[Roger]

Dave Benzel did some measuring years ago on Andy and Deena (not radar though, I think it was his LISA system so not sure how accurate). Fastest speeds were 57mph for Andy at 39 and 47mph for Deena at 38 as I recall. Always want to laugh when I hear announcers talk about 70mph in slalom.

[MISkier]

I have always told people I thought it was about 55-60 mph, even before seeing this data. I wonder what the speed through the turn is. Maybe 25-28 mph? It seemed to me that I calculated the straight line distance through the course following the zig zag pattern and determined that you had to average just a shade over 40 mph to keep up with the boat traveling at 36 mph. Don't have those calculations handy, though.

[Brady]

@MISkier wouldn't it be impossible to travel slower than the boat speed? That is a constant, and physics would say that you are always traveling at that speed. ?? Maybe not.

[ToddF]

34 mph @ 16.95 assuming 34mph at the buoy and 52mph at wake crossing and acceleration/deceleration are fairly consistent. There are 6 mid wake to buoy crossings and 6 buoy to mid wake crossings in a course one fast to slow, the other slow to fast, where each segment equally has roughly 1.41 seconds. But entrance and exit gates take a little extra time.

So I am guessing: Buoy to mid wake 1.25ish and Mid Wake to buoy 1.4ish

 

To accelerate from 34 to 52 in 1 second is .82Gs, 30 to 54 in 1 second is 1.09G's

So I agree there is no way we are experiencing 3 or 4 g's of acceleration, tremendous loads yes, G's No.

 

On dry land experiencing no G force, probably a lot of us can put 500lb load on a rope for a brief moment. this can be done without any mechanical advantage from a boat. Add in the boat, and the load can increase significantly probably double with bigger skiers/pro's.

 

 

[Than_Bogan]

Ok, what's the deal? A thead on the internet that doesn't contain ANY obviously wrong analysis!!?? :)

 

Meanwhile, let me comment a little on way it may feel like more.

 

Quick nerdy background: As noted, some of the highest g's are at the apex of the turn. Because the forces of gravity and deceleration are essentially perpendicular, the tangent of the angle of the skier's lean (measured from vertical) is a great approximation of the g forces. E.g. 45 degrees is about 1 g and 60 degrees down is about 1.7 g's.

 

But it's shortly after that that we seem to "experience" g's -- that shot out of a cannon feeling. And yet, these g's are typically no more than the turning g's and often less. So where's the disconnect between physics and "experience"?

 

I'm trying to avoid saying "coordinate frame" in my answer, so let me try "what's happening vs. your surroundings." In a turn, or in a spinning carnival ride or NASA-style g simulator, there is a very large acceleration acting on you. But what this acceleration does is PREVENT you from continuing at your current velocity, which is what you'd do if no acceleration acted on you. But you don't experience any acceleration relative to your surroundings: the world looks fairly static except that part where you're being crushed. Another way to think about it is that the direction of your velocity is changing very rapidly, but the magnitude of your velocity is fairly constant (or may be exactly constant in some carnival rides).

 

Contrast that with accelerating in a dragster, or a literal cannon, or going from the end of the turn to the middle of the wakes on your ski. In each of those situations, your surrounding change very rapidly AND you experience a change in the magnitude of your velocity. I claim both of those things contribute greatly to what your senses report. Thus much more "feel" of acceleration.

[MISkier]

@Brady, good question. I guess I never thought that I was actually turning at 34 mph (or 36). It always seemed like the deceleration into to the turn bled off a lot of speed. Maybe that just makes it seem slower than 34 mph.

[Than_Bogan]

@Brady It is not only possible but quite common to go slower than the boat. All that has to happen is that you are losing ground relative to the boat. This will naturally happen right after the apex of a turn as you decrease the rope's angle, but aren't yet going very fast right-to-left.

 

Interestingly, *avoiding* this loss of speed becomes an important shortline technique. One of the more common symptoms you'll see of a skier trying a new line lengths is coming to almost a complete stop right after the one ball and then trying to get going again. A better skier manages to avoid bleeding off quite so much speed.

[DW]

A skier realizes G forces in a few ways. Per SkiJays great Lorenzo pic, center turn G's are there basically being resisted by the soles of your feet. I content that a skier could see more G's that the MotoGP bike due to larger surface area based on the ski footprint compared to the tire(s) footprint although much slower speeds of the skier limit the final G loading. The skier certainly feels the G forces at the exit of the turn as he/she hooks up and undergoes the acceleration phase back across the wake. If you look at the elapsed time across the course and the required distance to travel, simple math would account for an approximate 2 g level (which is not that much actually). If you break it down, your body will get hit with much greater G loads in your hands, elbows and shoulders during the rope hit events. Slack to tight rope short burst G loads are probably well in to the 20-30 G range if not higher.

 

To Than's comment on turn apex G loading being high, if you look at the skier path, the true apex of the turn is very late, look at JTH's fav, pic of Nate, you can see his apex is very late!

 

Motorsport accident study's have revealed G loads on the human body to be much higher than previously thought. It is not that uncommon to hit 100 G's during an accident, I have heard 150+, luckily for a very short time thus life preserved. It is also surprising how low can also be fatal if in the wrong direction.

[Edbrazil]

What can I say about this thread other than "gee".

In the SL course, the direct buoy-to-buoy distance is some 14% longer than the boat path,

but the skier is moving on a somewhat sinusoidal path, so probably going 20% or so longer.

Which makes their average speed thru the course around 43 mph.

In the case of skiing, acceleration and force on the towline don't necessarily relate. When

we were doing Boat Tests for Waterski Mag. some years ago, before the invention of the

Drone, we used a real person for the "distance to 36". Being a big guy and doing a 2-foot

in deepwater on one ski, and measuring with my Dillon Force Gauge, he was running up

to 700 lbs. And pretty much no G's. You can just get behind the boat, rear back and put a

lot of force on the towline, but not go anywhere. Old-old days slalom technique was a

lot like that: sending up a big spray, but not accelerating much cross-wakes.

 

Anyway, there is a part of the sport where there are some serious G-forces. Which is in

the Jumping ramp contact. Than and others can look at the numbers, but for an Open Man

hitting the ramp at 90 feet/second (61 mph, and this is conservative), this speed needs

to be converted into vertical and horizontal velocity. For a 6-foot ramp, this is 90*.271,

or 24 fps vertical, to be achieved in only about .06 of a second, which is about the time

that the jumper has moved up the ramp about 18 inches of vertical.

 

24/.06 = 400 feet/second/second. Or 400/32 = 12+ G's. Would be very interesting

to strap an accelerometer on to a top jumper. Force acting only a very short time, but

if the skier is not in the right position, we see the obvious results.

[ToddF]

Impact and load G's are different than acceleration G's especially when skiing I am pretty sure I can do an otf and have a 20G impact landing without any problem, I can also absorb or take a 20 G slack line hit while only accelerating at a .5 G rate

 

But to keep this on common terms, the rope handle terms should be load amounts and the core/whole body should be the measured as G force acceleration, otherwise we are comparing apples to oranges

[Horton]

Back in the dark ages when I was a jumper (short skis and hand drive) I am sure the acceleration was far greater than I have ever seen on a slalom ski. Not sure how much things are different in the modern era but at one point in my jump career I was trying to turn to the ramp from the pylon and my water speed had to be below 10 mph at apex. 1.5 seconds later (I guess) I was going 55 mph+ (again a guess). Even if it was only a 40 mph delta in 1.5 sec that is a lot.

 

Surprise surprise I jumped farther by turning farther back on the boat and with a lot more water speed at the turn.

 

[Edbrazil]

 

Interesting Horton note. Sometimes, the harder you try, the worse you do.

Just note that Freddie went 170 on a single cut a couple of years back. If you

do it just right, it will work out just right.

 

Historical/Hysterical note: 170 in Open Men was a foot above the World Record

from 1975--1979 until John Mondor eclipsed Wayne Grimditch's 169 at the Masters.

 

Ps. I figure that today's big-big skis add some 15--20 feet for elite Men.

[DW]

Its not the acceleration that is the problem, its that sudden stop or deceleration that is the painful part...

[Horton]

@Edbrazil‌ any idea what the boat speed was for Fred's 170?

[Horton]

@Edbrazil‌ since I quit it is a whole different event.

 

Big skis

Big boats

Constant pull from ZO

Fast second segment

Bigger ramps (in 1993 a 5 ½ ramp was really 5 ½ foot tall)

 

And I had to walk to the dock barefoot in the snow uphill both ways

 

[Edbrazil]

 

Far as I know, Freddie's 170' was done with a legit. pull at 57kph. With fast 2nd segment, of

course. I witnessed that feat. Freddie earned a $1,000. bonus thanks to Vern Oberg.

Who is quite a character, but that is yet another story.

 

Yep, about every jump pull these days would have been a reride in the Olde Days, where there

was no "fast 2nd segment". The idea is so the boat would be catching up with the

airborne skier and not dragging them down, when they passed the level of the pylon.

[gator1]

Is there a Ball of Bone Fragments site where they talk about the importance of keeping your head vertical while driving your motogp around corners at insane bank angles? Or does that just happen instinctively?

[SkiJay]

It may be instinctive for some, @gator1‌, but it's certainly not hardwired in humans. Usually, it's an acquired skill the same way it is for water skiers. Back in the 50's and 60's, there were lots of bike racers who leaned their heads into corners perpendicular to their spines, but now it's considered a rookie move. Keeping our heads perpendicular to the ground while turning is better for inner ear functions that regulate balance. It's instinctual for birds, cats, and other fast moving animals that have evolved through natural selection though.

 

http://1.bp.blogspot.com/_TohuLUB1zNk/S6-GlhsZ5KI/AAAAAAAADJ0/NJgfzox5Obk/s1600/CHEETAH%26gazelle.jpg

[jimbrake]

@Ed Brazil - wasn't Grimditch's world record 180 until Mondor broke it in '79 with a 187? I can still remember the picture of Mondor on the cover of The Water Skier (the small format magazine that it used to be).

[DW]

@gator1: it is an instinctive reaction and it also part of what keeps your internal gyro properly oriented. If you can't, it becomes very difficult to correctly control your vehicle, happened to me early in a race season in a formula car, not enough neck strength, long hot race on a very high g technical track, head started to flop off to the wrong side as I couldn't keep it oriented correctly. Was a bitch to drive for the last couple of laps & luckily did not crash or lose any spots. One reason you see the neck straps on ovals.

[Porkfight]

I don't know how accurate they are, but there are free smart phone apps that can measure G force. A decent skier with a waterproof phone case might be able to provide some data. I am not really that skier.

[Edbrazil]

 

@jimbrake My error. Wayne's 169 was at the 1972 Masters. He did hold the record from

that time until John Mondor's 187' broke it in 1979. In the meantime, Wayne went 180' at

the 1975 Masters. Who would ever have thought that you could go 170' without a double-

cut? Obviously, Freddie did. At that event, he didn't even take any regular jumps, but did

all single-cuts, working up to the bonus money 170'.

 

What a great idea to wear one of those smartphones that measure G's. Maybe in a backpack

or smaller pack, or hooked to the armsling, etc.

[Drago]

@Horton‌ consistent speed with ZO ?

[Horton]

@Drago‌ back in the Hand Drive days every driver was so different than the next. now the computer is consistent

[A_B]

Consistent end time.

[Drago]

@Horton‌ totally agree on that. I thought you were speaking of speed down the lake