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Scary Chair Lift Experience

 Poster: A snowHead
Poster: A snowHead
rolling eyes Yesterday I was at our local hill Bogus Basin with my nephew. We were cruising up the quad lift when it came to a sudden stop (normal). We rocked a bit on the chair from the stop (normal).

The lift started to move for a fraction of a second l, then stopped again. What was strange, though, is the chairs started violently bouncing up and down I'd estimate 10-15 feet at a time. My nephew is only 8 so I held onto him pretty tight, but even so he got tossed around a little bit.

I've been snowboarding for over a decade on upwards of a dozen mountains and never experienced anything like it. Much different than usual stop/go chair rocking.

Lasted round 20 seconds, then we started cruising again.

Any lift operators or other folks have an explanation?
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 Obviously A snowHead isn't a real person
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@kazbob48, welcome to Snowheads snowHead

I've no explanation but just to say my wife would have had a dicky fit if this had happened to her and our kids.
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 Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
Lol thanks Layne. As far as I know nobody fell or was injured, but still pretty spooky stuff.
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could be due to a small amount of resonance (google it) when combined with the wind.

Another similar ish real life example (although on a larger scale) is when bridges oscillate a lot in the wind.
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Thanks Alex. I'll look into it as a cause, but I'm doubtful. We had beautiful sunny skies yesterday with only a slight breeze. With the exception of the chair bouncing, the day and weather we're awesome.
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i have been on a chair lift that did that once, it is indeed rather scary, and I too never found out quite why it did it!
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Simple harmonic motion....
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@kazbob48, it doesn't have to be a heavy wind to cause it, just matching frequencies.
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i.e. if a strong wind can cause an entire bridge to move then it could be a light wind causing a small chair to oscillate massively.
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@alex 99 It seems possible, but if so why wouldn't this phenomenon occur more frequently?

Like I said, upwards of a decade on a variety of different mountains and never experienced it before (amd hopefully won't have to again).
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I think your situation was an extreme example from the sounds of it, so its not very likely to occur a lot.
I've seen many chairs oscillate on occasions (i.e. bounce up and down), not to the extent you saw probably, but that's because they design them so that 99.9% of the time they don't oscillate like you saw. Same goes for my bridge example.....it CAN happen to a bridge, but not very often.

p.s. I'm not a liftie (or physics expert), I'm just making an intelligent guess based on my own experiences on lifts, and what you have said. But happy to be corrected if there are any lifties on here who can offer any insight.
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I've seen/experienced that quite a lot, generally as a result of very sudden stops, and more like +/- 5 feet up and down.
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Also @kazbob48, as @clarky999 says, actually its not that uncommon.
Previous thread below.

http://snowheads.com/ski-forum/viewtopic.php?p=2180063
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@clarky999 I have experienced plenty of those small +/- 5 foot bounces. This was something else entirely. Saw this on youtube that is pretty close to what hapenned yesterday. I'd estimate the bounces to be closer to 15 feet.


http://youtube.com/v/vgozRGDNlVc
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I've had this happen on a gondola before, it's alot more pronounced the further you are from a pylon (obviously).

I spend alot of time in small planes for work, so am used to getting chucked about. When it happened in the gondola I barely noticed yet, whereas everyone else (my girlfriend, her family and a ski instructor) all got instant nausea. I won't lie I found it a bit amusing.
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 Poster: A snowHead
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@huntleth makes a good point about how far away from a pylon you are.
It could be that on this occasion @kazbob48 it was unlucky that you were pretty much exactly in between two pylons and so the movements were much more extreme than you may have experienced before.
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Agreed the position from the tower can impact the severity of the movement especially for normal stop/go lift operation.

However, looking at the chairs in front of us as it was hapenning, it appeared that the chairs were being impacted relatively equally regardless of position from the pylon.

I know that it impacted all the chairs because the empty chairs across the way were also wildly bouncing (Which was interesting to watch as I considered our chair was doing the same thing).
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The cable is not straight between pylons - there is some sag due to the weight of the cable, chairs and occupants. There is some slack in the system. Normally, this evens itself out between pylons.

If the lift stops suddenly then all the accumulated 'sags' will run up the mountain so that the chairs between the top pylons drop significantly. The weight of all the other chairs will then pull them back and the big 'sag' will run back to the bottom of the mountain. This continues as the 'sag' runs back and forth.

A bit like compression waves running back and forth along a Slinky except the chairs sag instead of the spring compressing.

Same for any system involving springs and masses - or, indeed, a transmission line in electronics.


http://youtube.com/v/y7qS6SyyrFU


Last edited by Well, the person's real but it's just a made up name, see? on Mon 26-01-15 16:25; edited 1 time in total
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Quote:

it appeared that the chairs were being impacted relatively equally regardless of position from the pylon.


I find that hard to believe, and it might just be that because you were scared/worried the human eye wasn't operating well.

How can a chair only a few metres from a pylon move the same amount as one between two pylons. If its a general movement affecting the whole chairlift then there has to be some relationship between individual chair movement and the distance from the pylon?
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A relatively famous vid of a bridge which was badly engineered..


http://youtube.com/v/j-zczJXSxnw
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The Millennium Bridge in London was briefly closed shortly after opening due to an oscillation problem as well. IIRC that was caused by pedestrians walking in-step as they crossed, and is why you see signs on bridges that say "troops break step".
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@Scarlet, the problem on the Millennium Bridge was that once the oscillation starts, you can't but walk in step, which makes things worse. I think the 'break step' sign would work on a more substantial bridge (like the Albert Bridge?), presumably because the oscillations are much smaller amplitude.
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It's due to the chances of stuff like this happening that I get annoyed when people lift the bar unnecessarily early. I now keep my skis on it to stop them.
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@Yellow Pyranha, the break step sign isn't necessary on the Albert Bridge because it doesn't oscillate anything like as much as the Millennium Bridge.
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I witnessed the bouncing chairlift thing about a week ago in Le Tour. It began after the chairlift stopped and then started again and the chairs were only boucing about a metre. It didn't last long and no one screamed.
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@altis, has it right. It can be exacerbated by the lifties either having to (for safety), or choosing to (for kicks), stop and start (or slow and speed-up) the lift in sync with its natural frequency. One of the first things my wife was shown when training as a liftie was how to bounce the chair deliberately, usually done as a game when the liftie's mates were at the "sweet-spot" Skullie . She didn't play the game btw!
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clarky999 wrote:
I've seen/experienced that quite a lot, generally as a result of very sudden stops, and more like +/- 5 feet up and down.


this...I quite like it
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@alex99, perhaps, but The Royal Borough will tell you "Signs on the bridge order troops to break step while marching over to avoid damaging vibration."
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sugarmoma666 wrote:
It's due to the chances of stuff like this happening that I get annoyed when people lift the bar unnecessarily early. I now keep my skis on it to stop them.
+1 me too Toofy Grin
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@kazbob48, you won't be swinging as wildly as empty chairs the motion will be less thanks to you being in it. The swing will definitely be more in the middle and less as you approach the pylons if not you'd be watching the cable jump off them. The most likely cause as mentioned above is just simply the very sharp stop, the cable is elastic, there is likely a uneven distribution of chairs between each pylon and almost definitely an uneven distribution of weight with different loads in each chair. A sharp stop causes everyone to swing forward and all these differences and elasticity cause the resulting bouncing motion.
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 Poster: A snowHead
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Had my own scary chair experience yesterday here in Grimentz, there was a paragliding competition on and one of them flew into the chair in front of us on the lift!
Knocked both skis off a young kid on that lift with his canopy, then fell to the ground, luckily uninjured........
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Had my own scary chair experience yesterday here in Grimentz, there was a paragliding competition on and one of them flew into the chair in front of us on the lift!
Knocked both skis off a young kid on that lift with his canopy, then fell to the ground, luckily uninjured........
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 Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
Levi215 wrote:
A relatively famous vid of a bridge which was badly engineered..


http://youtube.com/v/j-zczJXSxnw


Of course, the famous example of resonance is not actually resonance.
At Tacoma, it was an aeroelastic instability in torsion, to be strict about it.
The bridge did not fail because of increased amplitude of vibration (ie the deck getting progressively higher and lower)
Instead it failed in rotational torsion along its deck axis.

Ruins a good o level Physics paper if you wrote that however. I could just imagine the University of Lond on Examining Board's examiner reading that and not having a clue
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@KenX, Shocked
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From my own experience of liftieing (the shonky old Crystal Chair on Blackcomb) the golden rule we were taught was to always wait at least 20 seconds or so after any stop without a controlled deceleration beforehand. The reason being that if the chairs don't settle properly after a stop and the lift moves again too soon, you can experience an amplified bounce in that compression wave moving up and down the line if the timing happens to be right.

As for the game, @shep, that tended to only happen when we had the one guy going up first thing on the safety inspection to make sure he was awake Twisted Evil
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Quote:

ad my own scary chair experience yesterday here in Grimentz, there was a paragliding competition on and one of them flew into the chair in front of us on the lift!
Knocked both skis off a young kid on that lift with his canopy, then fell to the ground, luckily uninjured........


Fuuuuuuuurrrrk that is WAY more scary than a bit of bouncing! Evil or Very Mad
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