Why roller coaster loops aren’t circular anymore • Hacker News Board

Re: Why roller coaster loops aren’t circular anymore

#61

Post by mgdlbp » Sat, Jul 09, 2022, 9:02 AM UTC

userbinator wrote: post #37

mgdlbp wrote: post #22

Earlier quoted context omitted.

Also seen in the planning of curves in roads (where jerk corresponds to the rate at which a steering wheel must be turned) and railways. And this is also why the passengers jerk of a vehicle jerk backwards after it comes to a complete stop. Their muscles statically counter the relative forwards acceleration of their torsos during braking and require time to react to the acceleration suddenly going away. This effect c…

This effect can be prevented by gradually letting off the brake before reapplying it fully upon stopping, but few drivers and rapid transit systems are aware. This is surprising to read. Everyone whose car I've ridden in knows to do that, and it's only in extremely urgent and unexpected stops where it's neglected. Also, when fully stopped, only minimal pressure should be necessary to keep the car still.

There's definitely confirmation bias causing me to not notice stops where there isn't a jerk. And perhaps by the Baader-Meinhof effect you'll begin to notice them more frequently, too. But stops without any perceptible jerk are rare enough that when they happen I get an odd feeling of floating forwards like when the train beside your own at a station departs—perhaps it's because my subconscious, still anticipating the jerk, believes that I'm still in motion.

In some cars there's hysteresis in the brake pedal (perhaps caused by the booster or self-energizing system) that makes it hard to smoothly release the brake even when the driver tries to. But metros seem to increase their braking force—visible in standing passengers leaning progressively more—as speed decreases. Is there some physical cause to that?

> only minimal pressure should be necessary

I gathered that it was best practice to fully brake when stopped in case someone hits you, especially at a light where you might be rear-ended and roll into the intersection.

Re: Why roller coaster loops aren’t circular anymore

#62

Post by jzl » Sat, Jul 09, 2022, 9:05 AM UTC

jorgesborges wrote: post #3

> More people paid to watch others ride these early coasters rather than ride themselves I had no idea roller coasters have been around this long. The photos are laugh-out-loud terrifying. I was shocked that anyone would pay to ride them until I read the quote above. Now it makes sense. I’d pay to watch that too!

One of the most mind-blowing images on Wikipedia is this one of the first ever Ferris Wheel from the Chicago World's Fair in 1893:

https://upload.wikimedia.org/wikipedia/commons/6/68/Chicago-...

Note how large the cars relative to the tiny people standing in them. This thing was unimaginably massive. It's easy to think of 1893 as being before the modern technological era, but we were more modern than most people like to think.

Re: Why roller coaster loops aren’t circular anymore

#63

Post by rkagerer » Sat, Jul 09, 2022, 9:09 AM UTC

yccs27 wrote: post #55

cperciva wrote: post #33

Earlier quoted context omitted.

And 'jerk' has to be kept below a certain threshold for humans to find movement comfortable. It's not strictly a matter of threshold -- people might tolerate a higher jerk if it's for a much shorter duration, for example. In practice it doesn't much matter which metric you minimize; you'll end up with similar results. The simplest option is to minimize the mean absolute jerk, which has the side benefit of utterly con…

Note that 'mean absolute jerk' can also be described as 'total variation of acceleration over time'. That implies it does not depend on how fast the acceleration changes, only about the difference between minimum and maximum (as long as the acceleration increases/decreases monotonically to/from the maximum). This may or may not be what you want.

I see what you did there

Re: Why roller coaster loops aren’t circular anymore

#64

Post by mgdlbp » Sat, Jul 09, 2022, 9:13 AM UTC

lisper wrote: post #26

mgdlbp wrote: post #22

Earlier quoted context omitted.

Also seen in the planning of curves in roads (where jerk corresponds to the rate at which a steering wheel must be turned) and railways. And this is also why the passengers jerk of a vehicle jerk backwards after it comes to a complete stop. Their muscles statically counter the relative forwards acceleration of their torsos during braking and require time to react to the acceleration suddenly going away. This effect c…

It's a little more complicated than that in a passenger car. The deceleration compresses the front springs. When the car comes to a stop, the springs decompress and the front of the car pops up and the body of the car moves slightly backwards even though the wheels are now stationary.

Interestingly, in this /r/askscience thread:[1]

1) lots of comments are attributing the jerk felt by occupants to the vehicle suspension. But that isn't the case! The occupants visibly move backwards relative to the car body, but the backwards motion of the car itself should rather have the opposite effect.

2) a commenter contradicts what several commenters here have noticed:

> I wager this has to do with the driver; most drivers I've noticed don't ease up off the breaks when slowing down, and so the 'slowing force' feels like it ramps up along the deceleration profile, up until the point when the car comes to a complete start and there's a 'jerk'.

[1] https://www.reddit.com/r/askscience/comments/20mljk/what_cau...

Re: Why roller coaster loops aren’t circular anymore

#65

Post by jaclaz » Sat, Jul 09, 2022, 9:25 AM UTC

mgdlbp wrote: post #22

tobtoh wrote: post #18

If I remember my uni engineering/calculus maths class correctly, the third derivative of position is used in planning these sort of curves. The first derivative of postion (with respect to time) is velocity. The second derivative is acceleration (ie rate of change of velocity). And the third derivative is jerk (rate of change of acceleration). And 'jerk' has to be kept below a certain threshold for humans to find mov…

Also seen in the planning of curves in roads (where jerk corresponds to the rate at which a steering wheel must be turned) and railways. And this is also why the passengers jerk of a vehicle jerk backwards after it comes to a complete stop. Their muscles statically counter the relative forwards acceleration of their torsos during braking and require time to react to the acceleration suddenly going away. This effect c…

>Also seen in the planning of curves in roads (where jerk corresponds to the rate at which a steering wheel must be turned) and railways.

Just for the record, the transition curve is usually (not always but very often ) a clothoid (or Euler's spiral or Cornu spiral)

https://en.wikipedia.org/wiki/Euler_spiral

Re: Why roller coaster loops aren’t circular anymore

#66

Post by [deleted] » Sat, Jul 09, 2022, 9:27 AM UTC

ben_w wrote: post #34

keithnz wrote: post #20

Earlier quoted context omitted.

I have a t-shirt which has "don't be a" and the equation for the third derivative

Reminds me of someone I knew at university whose T-shirt was the definite integral from 10 to 13 of 2x dx followed by a question mark.

[deleted]

Re: Why roller coaster loops aren’t circular anymore

#67

Post by jonsen » Sat, Jul 09, 2022, 9:39 AM UTC

mgdlbp wrote: post #61

userbinator wrote: post #37

Earlier quoted context omitted.

This effect can be prevented by gradually letting off the brake before reapplying it fully upon stopping, but few drivers and rapid transit systems are aware. This is surprising to read. Everyone whose car I've ridden in knows to do that, and it's only in extremely urgent and unexpected stops where it's neglected. Also, when fully stopped, only minimal pressure should be necessary to keep the car still.

There's definitely confirmation bias causing me to not notice stops where there isn't a jerk. And perhaps by the Baader-Meinhof effect you'll begin to notice them more frequently, too. But stops without any perceptible jerk are rare enough that when they happen I get an odd feeling of floating forwards like when the train beside your own at a station departs—perhaps it's because my subconscious, still anticipating th…

If rear-ended with your brakes engaged more of the impact energy will go into crumbling your car than if it's allowed to transfer into forward speed. If you see it coming, let go of the brake, and then step on it after impact. That's the advise I got.

Re: Why roller coaster loops aren’t circular anymore

#68

Post by seper8 » Sat, Jul 09, 2022, 9:41 AM UTC

WalterBright wrote: post #24

mgdlbp wrote: post #22

Earlier quoted context omitted.

Also seen in the planning of curves in roads (where jerk corresponds to the rate at which a steering wheel must be turned) and railways. And this is also why the passengers jerk of a vehicle jerk backwards after it comes to a complete stop. Their muscles statically counter the relative forwards acceleration of their torsos during braking and require time to react to the acceleration suddenly going away. This effect c…

> but few drivers or rapid transit systems seem to be aware I find that amazing. What the heck are drivers ed instructors doing? It's not just hard on the passengers, it's hard on the machinery. It's the same with the clutch. I've driven with enough people who fancy themselves as great shifters, but they jerk the hell out of the clutch every time, never attempting to match the shaft speed with the engine speed. If I…

I can shift smoother than an automatic

Not a double clutch one, e.g. DSG

Re: Why roller coaster loops aren’t circular anymore

#69

Post by thaumasiotes » Sat, Jul 09, 2022, 9:58 AM UTC

atoav wrote: post #46

tobtoh wrote: post #18

If I remember my uni engineering/calculus maths class correctly, the third derivative of position is used in planning these sort of curves. The first derivative of postion (with respect to time) is velocity. The second derivative is acceleration (ie rate of change of velocity). And the third derivative is jerk (rate of change of acceleration). And 'jerk' has to be kept below a certain threshold for humans to find mov…

A very similar thing is done in the creation of reflective surfaces on car bodies (typically in CAD software). They call these constraints by G and a number. G1 would be a positional constraint: the two surfaces meet each other at the same point G2 tangential: same as G1, but the surfaces are tangential G3: same as G2, but the curvature (radius^-1) of the surfaces is the same at the point where the two meet. This ess…

> G2 tangential: same as G1, but the surfaces are tangential

This makes me think "tangential to what?".

Do you mean that, along the seam between G1 and G2, the tangent plane to G1 at a given point is equal to the tangent plane to G2 at the same point?

Re: Why roller coaster loops aren’t circular anymore

#70

Post by jonsen » Sat, Jul 09, 2022, 10:00 AM UTC

irjustin wrote: post #51

WalterBright wrote: post #24

Earlier quoted context omitted.

> but few drivers or rapid transit systems seem to be aware I find that amazing. What the heck are drivers ed instructors doing? It's not just hard on the passengers, it's hard on the machinery. It's the same with the clutch. I've driven with enough people who fancy themselves as great shifters, but they jerk the hell out of the clutch every time, never attempting to match the shaft speed with the engine speed. If I…

> What the heck are drivers ed instructors doing? I mean to be completely fair - this is not their job. Driving instructors are there to keep everyone on the road safe - first.

Yes it's their job. It's a kind of passive safety.