What a Jerk!

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I love Big Thunder Mountain Railroad with the intensity of a thousand suns. I think it is a perfect attraction. On my last trip to Walt Disney World, I rode Big Thunder Mountain Railroad and then used a FastPass for Seven Dwarfs Mine Train. I was surprised by how smooth Seven Dwarfs was compared to Big Thunder Mountain and that got me thinking.

Here at TouringPlans, we like numbers and we use them for good. Actually, we love numbers. We love data and we do beautiful things with them. We collect data on a lot of different things. We track schools in session and weather to estimate crowd levels. We track posted and actual wait times to help you minimize your wait in line and visit more attracions. And now we have a new source of data that will tell use how bumpy rides are.

A quick view of any theme park message board will reveal hundreds of posts about which rides are too bumpy for some guests. For example, some guests report that they cannot ride the Matterhorn at Disneyland because it is too bumpy, or they compare how smooth Seven Drawfs Mine Train is relative to Space Mountain.

Since we love data and theme parks, we thought we would collect some data on how bumpy some of these rides are.

It turned out that “bumpy” isn’t a term that physicists use. But guess what, there is a term that describes how much you get jerked around.

Introducing jerk

Jerk is an admittedly loaded term. It could refer to that person that tries to slip under the ropes from the standby to the FastPass line at Big Thunder Mountain. That person is a jerk, but we are talking about the jerk you feel when riding an attraction. You know that saying “getting jerked around? That kind of jerk.

What is jerk?

Roughly speaking, jerk is how much you get whipped around on a ride. The term jerk isn’t a word we just innovented, it is an actual term used in physics. Don’t believe us? Check out this scientific paper written by @Eager:2016fw.

Let’s talk about motion for a bit, shall we? We experience a wide variety of motion every day, even if we don’t realize it. When we are on a plane or in a car, we are moving but we don’t feel it. But we do feel it when the plane is taking off or we accelerate from a stoplight. We feel a change in motion. If you are slowing down for a stoplight and have to swerve to avoid a cat that just darted in front of you, that is jerk. It is a change in the change of motion.

To summarize:

  • You don’t feel constant motion
  • You do feel a change in motion
  • A change in the change of motion is called jerk

Measuring jerk

We have a handy dandy device called an accelerometer that measures motion for use. Our accelerometer is small enough to fit in our sock when we take it on attractions.

Visualizing jerk

Now that we are all familiar with jerk, let’s look at how much jerk we experience on a ride.

Jerk plots

We are going to create a visual depiction of the data we recorded. The horizontal axis is the time in seconds. Big Thunder Mountain Railroad is about 3 minutes long or 180 seconds. The vertical axis is the amount of jerk experienced on the ride. If the line is between 0 and 1, there is not much jerk. If the line is goes beyond 2, then there is a good amount of jerk. Let’s look at this for Big Thunder Mountain Railroad.

I think this is beautiful, and not just because I made it.

Let’s break this plot down into parts.

From 0-20 seconds there is a little bit of jerk, but nothing bad. This is where the ride vehicle is leaving the station but before the first lift hill.

From 20 to 45 seconds the ride vehicle is on the first lift hill. Not a lot of jerk here.

From 45 to 65 seconds the vehicle is going through the first set of twists and turns and we experience a fair amount of jerk. This makes sense!

From 65 to 100 seconds the ride vehicle is one the second lift hill and the jerk is low.

From 100 to 125 seconds the ride vehicle is going through the second set of twists and turns and we experience some significant jerk.

From 125 to 150 seconds the ride vehicle is on the third and final lift hill.

From 150 to 180 seconds we go through the third and final set of twists and turns and then slow down before returning to the station.

So what?

Great, you can record data and make beautiful plots with it. How does that help me?

With our new power we can finally answer that age-old question, is it smoother to ride in the front of the ride vehicle or in the back?

The pictures that I showed you above were all from row 1 of the front car of Big Thunder Mountain Railroad. How does it compare to the back row of the back car (row 15)?

I’m glad you asked. Take a look. The front row is the top picture, the back row is the picture on the bottom.

It appears the amount of jerk in the rear car is lower than the front car. Huh? What? The rear car gives a smoother ride than the front car. See how much narrower the width of the lines is?

TouringPlans, answering the hard questions you never knew you wanted to ask.

In the future, we will be going to all the parks with our accelerometer to put numbers to how much jerk we experience on these rides. We hope you will be able to use these data to plan which attractions you feel comfortable on. Stay tuned.

Comments? Thoughts? Let us know what you think about these types of experiments. 

22 thoughts on “What a Jerk!

  • September 12, 2019 at 11:25 am
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    Not to be a jerk [*doubles over laughing*], but if we’re going to do this, let’s do this!

    What are the units of jerk on your plots? Are they m/s^3, ft/s^3, or something else?

    Was this measured using a triaxial accelerometer? If so, are you just summing the magnitude of readings from all three axes? If not, what axis are you measuring?

    Assuming that you’re using a triax, this approach could also be meaningfully used on simulators; even though they don’t “go” anywhere, there are still measurable accelerations applied. It would be fascinating to see an overlay showing the levels of intensity between Soarin, FoP, and Star Tours (for example).

    Reply
    • September 13, 2019 at 8:37 pm
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      Hi Andy,

      Check out David and Rob’s responses or the Eager paper for the full derivation.

      I did use a 3-axis accelerometer and created the magnitude of the 3D vector as here, then took the appropriate derivatives.

      You are absolutely correct on all of the applications for this and I’ve already taken it on many of rides you mention.

      Thanks,

      Chris

      Reply
  • September 12, 2019 at 12:19 pm
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    Great article – I was just thinking of doing a “understanding physics using theme park attractions” class for my kids/students, and this fit in perfectly. Does “jerk” correspond to G force (i.e. M/s^2) or such? Cause that would be perfect – although 6G is probably a bit too high to be an accurate measurement (although, maybe if it’s just for an “instant”….)
    https://rollercoaster.fandom.com/wiki/Highest_G-Force_on_a_Roller_Coaster

    Reply
    • September 12, 2019 at 4:08 pm
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      “Jerk” is the third derivative of position. Motion (speed in a direction) is the first, acceleration (change in speed in a direction) is the second, and jerk (change in acceleration) is the third. So… motion would be in m/s, acceleration in (m/s)/s, and jerk in ((m/s)/s)/s or m/s^3.

      G forces are acceleration; if you go from 1G to 3G over the period of a second (+20m/s^3), it will be smoother (less jerk) than if you go from 1G to 3G in a tenth of a second (+200m/s^3).

      (I think I have this right. It’s been *mumble* years since college…)

      Reply
  • September 12, 2019 at 12:23 pm
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    (OK, answering my own question above – a jerk is different than a g-force, although somewhat related – the paper written by @Eager:2016fw that’s mentioned in the article goes into it)

    Reply
  • September 12, 2019 at 12:31 pm
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    Why didn’t you compare this to Seven Dwarves?

    Reply
  • September 12, 2019 at 12:39 pm
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    You, sir, are not a jerk by any stretch of the imagination. However, we are on to your very thinly veiled plot to ride as many roller coaster type rides in the name of scientific research!

    Well played.

    Reply
  • September 12, 2019 at 2:00 pm
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    In addition to the track roughness, the age of the train wheels has a significant impact on “jerk.” These wheels are metal, coated with a layer of nylon(?) on the running surfaces. This nylon layer wears down with time. With new wheels, the trains hugs the rails tightly and feels super-smooth (assuming the track is smooth). But as these wheels age, the nylon erodes away and the train fits more loosely on the track, and may shuffle a bit side-to-side and up-and-down (still within manufacturers tolerances), creating a more rough ride experience. This sensation is more pronounced on faster coasters such as Rock’N’Roller Coaster or Disneyland Paris’ Space Mountain. Regulars immediately notice when a coaster train changes from worn wheels to new wheels.

    Reply
  • September 12, 2019 at 2:05 pm
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    I think this would be a great resource to have for people with some sensitivities to “wild” rides. I have issues with my neck from a car accident and knowing that sitting in the back is smoother than the front, I would take this into consideration on some of the multi-car rides. Having this information on all Disney rides allows us to have informed fun!

    Reply
    • September 13, 2019 at 8:38 pm
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      Thanks for the feedback, we’ll take this into consideration.

      Reply
  • September 12, 2019 at 4:13 pm
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    It took me a second to realize the second and third graphs had a +2 offset applied. You may want to see how the data looks if you take the absolute value of the data, or calculate the RMS over some period of time. Likely, though, positive and negative matters — like on the backward section of Expedition Everest, when it feels like your stomach is trying to take flight. I imagine people vary in their tolerance towards that.

    Reply
  • September 12, 2019 at 5:02 pm
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    Great idea! Though it would need to be updated over time. Rock N Roller Coaster used to have almost no jerk, but as it’s aged, there is a lot more. This would definitely help people to decide what rides they could ride and which they couldn’t.

    Reply
  • September 12, 2019 at 6:25 pm
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    You should compare each side of space mountain. I think one side has more jerk than the other

    Reply
  • September 12, 2019 at 6:36 pm
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    This is great and would have been so helpful to us this week if a complete list had been available. I’m here with my cousin who had unplanned shoulder surgery 7 weeks ago. Just yesterday saw was texting me from DHS asking how rough certain rides were. The information you’re working on providing would have been invaluable!!

    I know many will benefit from this. Thank you for working to add this information to your site!!

    Reply
  • September 13, 2019 at 7:08 am
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    Even though I’m not a scientist, I find this kind of thing interesting. However, I’d be curious to see what your findings are if you were to wear the accelerometer in/on a hat or headband. My least favorite part about BTM is the sudden stop just before entering the station. I feel that more than any other part of the ride like a whiplash.

    All of the warnings advise to avoid rides if you have neck or back issues. Can’t think of any that mention feet or ankle issues (based on sock accelerometers).

    Reply
  • September 13, 2019 at 9:53 am
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    I’m in interesting in a comparison between the jerking at Spade Mountain at WDW and Disneyland. It feels a lot bumpier and jerkier at WDW to me!

    Reply
  • September 13, 2019 at 11:46 am
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    Any chance that the touring plans app could be used to get this information from riders? All phones have accelerometers these days. The UI might be tricky to get right but averaging over many riders could show the average acceleration experienced and also the average change in acceleration (jerk).

    Reply
  • September 13, 2019 at 3:56 pm
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    We’re heading to WDW in Nov.
    Any chance you can incorporate a ‘jerk’ rating on you app for the rides – listing front, middle & rear seat measurements if applicable?
    I know that’s asking a lot, but just saying the last car on TMRR is the least jerky ride will make a big difference to the hubby – his one complaint was it was too rough, but he likes to sit in the first car. . . Will insist we sit in rear seat! ( we just booked our Fast pass+ rides yesterday )
    Have used your book & now phone app for last four trips & wouldn’t think of not using it!!! I swear by it and wish I lived near WDW, I’d help do research for seniors!!!!
    Luv you!!!

    Reply
    • September 13, 2019 at 8:44 pm
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      Great question. It wouldn’t be easy to incorporate that feature into the app. If you want to record your own data there are a number of apps available for iPhone and Android that can access your phone’s accelerometer. I tried out a few different methods to record these data, including my phone, and found that there was a bit more uncertainty around the readings than compared to the accerometer I used.

      Reply
      • September 16, 2019 at 11:06 am
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        Cool study! I’ve been thinking about this for years, but instead every time I go to the parks, I just have fun 🙂

        If collecting crowd-sourced data from phone accelerometers, it would be very helpful to have info on how tightly coupled the phone is to the vehicle. A phone in a cushioned shirt pocket, where it can float will be different from a phone in a back pocket or clipped to a hip. Even so, you might gather useful info. Thanks for all you do at Touring Plans!

        Reply
  • September 14, 2019 at 1:05 pm
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    I found the comparison very interesting since I’ve heard that riding in the rear is a “wilder” ride and one should sit in the front of coasters if they aren’t huge coaster fans and don’t want to get whipped around. This seems to disprove that theory.

    I’m looking forward to more “jerk” comparisons.

    Reply

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