5 ms·

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 t

by mgdlbp 4y ago

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 can be prevented by gradually letting off the brake before reapplying it fully upon stopping, but few drivers and rapid transit systems seem to be aware.

WalterBright 4y ago
> 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 comment on it, they always deny doing that :-/ If I'm on my game, I can shift smoother than an automatic. The bonus is the clutch will last a very long time.
- 2143 4y ago
  "They way somebody treats their car is the way they treats themselves" — Frank Martin (paraphrased for gender neutrality). And you can tell how somebody treats their car by examining how long the clutch lasts, if they drive a manual.
  - InCityDreams 4y ago
- yellowapple 4y ago
  > What the heck are drivers ed instructors doing? Dinging me over their interpretations of ambiguous driving laws, if my experience is any indication.
- irjustin 4y ago
  > 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.
  - jonsen 4y ago
    Yes it's their job. It's a kind of passive safety.
- seper8 4y ago
  I can shift smoother than an automatic Not a double clutch one, e.g. DSG
lisper 4y ago
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.
- mgdlbp 4y ago
  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_causes_the_tiny_jerk_when_a_car_comes_to_a/ https://www.reddit.com/r/askscience/comments/20mljk/what_cau...
  - lisper 4y ago
    > The occupants visibly move backwards relative to the car body The only video linked to in that thread has been removed so I wasn't able to see this, but I will point out that the timing is critical to interpreting this observation. During deceleration, the occupants will move forward relative to the car body so that the car can apply a decelerating force to their bodies (via seat belts or friction against the seats and floor of the car). At some point after the car has stopped, they will necessarily move back to their neutral position. This will have nothing to do with the transient motion of the car when acceleration drops to zero. I will also note that it is not that difficult to come to a complete stop with no perceivable transient if you ease off the brakes as the car comes to a halt. It's actually a useful skill to cultivate IMHO.
userbinator 4y ago
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.
- ssl232 4y ago
  > Also, when fully stopped, only minimal pressure should be necessary to keep the car still. Why? To keep the brake fluid lines from bursting or something?
  - jstanley 4y ago
    More pressure won't do any harm, it's just not necessary if you only want to keep the car still (on a flat road, in neutral, with nobody trying to tow you, etc.).
    - userbinator 4y ago
      It's more tiring, however, which can be an important factor if you're taking a long trip.
- lqet 4y ago
  > Everyone whose car I've ridden in knows to do that I also do that, and don't know anyone who doesn't. I vaguely remember that I learned it in driving school, most likely because my driving instructor didn't want to be jerked around on the passenger seat for an hour every week. Train/tram drivers here also usually do that in stations, except when they try to make up for delays, or when they have wrongly estimated the breaking distance.
  - iggldiggl 4y ago
    Re train/tram drivers: Especially with more modern(ish) rolling stock that can also depend on how well the manufacturer has set up the braking system. For one instance multiple units (especially electrically powered ones) commonly have computerised braking controls, often transition from dynamic to friction braking shortly before coming to a standstill, and might possibly have some sort of automatically applied parking brake. If the manufacturer didn't properly adjust this whole system, the friction brake as it takes over for the last few kph might be applied with too much of a "bite" and therefore cause a jerky stop which even a skilled driver might not be able to fully prevent.
tetris11 4y ago
Given that train drivers have cameras pointing at passengers, I'd argue that they are indeed aware of the effect, but don't always roll the stop due to delays on the line and/or personal reasons.
Gordonjcp 4y ago
> This effect can be prevented by gradually letting off the brake before reapplying it fully upon stopping, but few drivers and rapid transit systems seem to be aware. Is this why it seems to be mostly Americans that are into the idea of self-driving cars, because the standard of driving is so low?
jaclaz 4y ago
>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 https://en.wikipedia.org/wiki/Euler_spiral
yojo 4y ago
This was particularly noticeable to me riding San Francisco light rail to work. When trains run underground they start and stop under computer control. Nice, smooth acceleration and deceleration. On the surface (or when the computer control system was borked) the starts and stops were a lot less pleasant.
phkahler 4y ago
This is fun to try in cars. The drivetrain can have a bit of twist under deceleration, and you can feel it spring back after the wheels stop. For best comfort you need to gradually reduce the braking force not just for human comfort but also to relieve that twist.
tzs 4y ago
OT: that reminds me of an interesting physics problem. If you have a ball sitting on the floor in the aisle of a stationary bus and the bus starts accelerating forward the ball rolls toward the back of the bus. If you have a bus moving at a constant velocity and it start decelerating the ball rolls toward the front of the bus. Suppose you also have a helium balloon floating in the bus. Does it also move toward the back of the bus when the bus accelerates and toward the front when the bus decelerates? Or does it stay where it is? Or does it move toward the front when the bus accelerates and toward the back when the bus decelerates?
- joexner 4y ago
  The balloon would probably move toward the front when accelerating and toward the back when the bus slows, if it moved at all. It helps (me) to imagine an air bubble in a sealed, nearly-full fish tank on that same accelerating bus. The heavier water gets "flung" harder away from the direction of acceleration, and the bubble gets pushed out of the way in the opposite direction. Same principle.
  - tzs 4y ago
    Yup. The movement is quite noticeable. Here's someone trying it [1]. [1] https://www.youtube.com/watch?v=XXpURFYgR2E https://www.youtube.com/watch?v=XXpURFYgR2E

by mgdlbp 4y ago

WalterBright 4y ago

> 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 comment on it, they always deny doing that :-/ If I'm on my game, I can shift smoother than an automatic. The bonus is the clutch will last a very long time.

2143 4y ago
"They way somebody treats their car is the way they treats themselves" — Frank Martin (paraphrased for gender neutrality). And you can tell how somebody treats their car by examining how long the clutch lasts, if they drive a manual.
- InCityDreams 4y ago
yellowapple 4y ago
> What the heck are drivers ed instructors doing? Dinging me over their interpretations of ambiguous driving laws, if my experience is any indication.
irjustin 4y ago
> 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.
- jonsen 4y ago
  Yes it's their job. It's a kind of passive safety.
seper8 4y ago
I can shift smoother than an automatic Not a double clutch one, e.g. DSG

lisper 4y ago

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.

mgdlbp 4y ago
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_causes_the_tiny_jerk_when_a_car_comes_to_a/ https://www.reddit.com/r/askscience/comments/20mljk/what_cau...
- lisper 4y ago
  > The occupants visibly move backwards relative to the car body The only video linked to in that thread has been removed so I wasn't able to see this, but I will point out that the timing is critical to interpreting this observation. During deceleration, the occupants will move forward relative to the car body so that the car can apply a decelerating force to their bodies (via seat belts or friction against the seats and floor of the car). At some point after the car has stopped, they will necessarily move back to their neutral position. This will have nothing to do with the transient motion of the car when acceleration drops to zero. I will also note that it is not that difficult to come to a complete stop with no perceivable transient if you ease off the brakes as the car comes to a halt. It's actually a useful skill to cultivate IMHO.

userbinator 4y ago

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.

ssl232 4y ago
> Also, when fully stopped, only minimal pressure should be necessary to keep the car still. Why? To keep the brake fluid lines from bursting or something?
- jstanley 4y ago
  More pressure won't do any harm, it's just not necessary if you only want to keep the car still (on a flat road, in neutral, with nobody trying to tow you, etc.).
  - userbinator 4y ago
    It's more tiring, however, which can be an important factor if you're taking a long trip.
lqet 4y ago
> Everyone whose car I've ridden in knows to do that I also do that, and don't know anyone who doesn't. I vaguely remember that I learned it in driving school, most likely because my driving instructor didn't want to be jerked around on the passenger seat for an hour every week. Train/tram drivers here also usually do that in stations, except when they try to make up for delays, or when they have wrongly estimated the breaking distance.
- iggldiggl 4y ago
  Re train/tram drivers: Especially with more modern(ish) rolling stock that can also depend on how well the manufacturer has set up the braking system. For one instance multiple units (especially electrically powered ones) commonly have computerised braking controls, often transition from dynamic to friction braking shortly before coming to a standstill, and might possibly have some sort of automatically applied parking brake. If the manufacturer didn't properly adjust this whole system, the friction brake as it takes over for the last few kph might be applied with too much of a "bite" and therefore cause a jerky stop which even a skilled driver might not be able to fully prevent.

tetris11 4y ago

Given that train drivers have cameras pointing at passengers, I'd argue that they are indeed aware of the effect, but don't always roll the stop due to delays on the line and/or personal reasons.

Gordonjcp 4y ago

> This effect can be prevented by gradually letting off the brake before reapplying it fully upon stopping, but few drivers and rapid transit systems seem to be aware. Is this why it seems to be mostly Americans that are into the idea of self-driving cars, because the standard of driving is so low?

jaclaz 4y ago

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

yojo 4y ago

This was particularly noticeable to me riding San Francisco light rail to work. When trains run underground they start and stop under computer control. Nice, smooth acceleration and deceleration. On the surface (or when the computer control system was borked) the starts and stops were a lot less pleasant.

phkahler 4y ago

This is fun to try in cars. The drivetrain can have a bit of twist under deceleration, and you can feel it spring back after the wheels stop. For best comfort you need to gradually reduce the braking force not just for human comfort but also to relieve that twist.

tzs 4y ago

OT: that reminds me of an interesting physics problem. If you have a ball sitting on the floor in the aisle of a stationary bus and the bus starts accelerating forward the ball rolls toward the back of the bus. If you have a bus moving at a constant velocity and it start decelerating the ball rolls toward the front of the bus. Suppose you also have a helium balloon floating in the bus. Does it also move toward the back of the bus when the bus accelerates and toward the front when the bus decelerates? Or does it stay where it is? Or does it move toward the front when the bus accelerates and toward the back when the bus decelerates?

joexner 4y ago
The balloon would probably move toward the front when accelerating and toward the back when the bus slows, if it moved at all. It helps (me) to imagine an air bubble in a sealed, nearly-full fish tank on that same accelerating bus. The heavier water gets "flung" harder away from the direction of acceleration, and the bubble gets pushed out of the way in the opposite direction. Same principle.
- tzs 4y ago
  Yup. The movement is quite noticeable. Here's someone trying it [1]. [1] https://www.youtube.com/watch?v=XXpURFYgR2E https://www.youtube.com/watch?v=XXpURFYgR2E