How to ride an electric unicycle - understanding the dynamics

[mrelwood]

On 1/15/2022 at 2:55 AM, Julian Lo said:

https://youtu.be/77WFrQYfytg

1. The faster you go, the harder you hit the ground as the footage showed.

This is probably not exactly what you meant, but I do have to be a bit anal about the physics of your claim.

 Smart physicists long before us already discovered that gravity doesn’t care about speed. If you drop a ball from head height, it takes the exact same time to fall no matter if you’re stationary or standing on the roof of a bullet train at 300km/h. The impact to the ground is the same as well.

Now, does this mean that a crash at 70km/h doesn’t hurt any more than falling down at 0km/h? Absolutely not, because there are many other forces playing their part as well. But still, the ground doesn’t hit you any faster or harder.

On 1/15/2022 at 2:55 AM, Julian Lo said:

1. No matter how long you’ve been riding, you will crash when you are stepping out of your comfort zone. 

That is indeed a large probability. And that’s why we gear up!

[Paul A]

Bullet Fired vs Bullet Dropped - Mythbusters for the Impatient

1,151,845 views

Apr 22, 2016

 

A lesson in classical mechanics: Will a bullet fired horizontally take longer to fall to the ground than a bullet simply dropped from the same height?

 

*Adam Savage was riding an EUC in this video from 2016. 

 

 

 

[Chriull]

1 hour ago, mrelwood said:

Now, does this mean that a crash at 70km/h doesn’t hurt any more than falling down at 0km/h? Absolutely not, because there are many other forces playing their part as well. But still, the ground doesn’t hit you any faster or harder.

Imho one hits the ground harder driving faster, as one normally touches first with the feet/knees and by friction there is a rotational acceleration.

Once one has enough velocity to reach sliding friction quite instantly more speed could maybe not really matter. So falling on a fixed, nicely sliding knee protector could be better as touching with the shoes first.

[.....]

Im not sure hitting with feet first, actually causes an increase in impact (g's) when hitting the ground. Unless you are magically pushing off of something above you and using muscle to force yourself into the ground, its still the same. If you hit with feet first and then your face, you have actually split the impact but overall it remains the same as any other object dropped from the same height. I GUESS if you stick your feet into clay and become a plank, you COULD increase the speed your face hits the ground, as you are now using your feet as a lever. If you can change forward momentum into vertical momentum, then yes, faster is worse.

However, I would guess that most of the time, we dont become levers and we merely skip/slide/tumble loosely. If you add up ALL the skips and tumbles and energy that is transferred into forward release of energy, then add this into the 'impact' equation: of course faster = more energy. However, if we are merely talking about how hard we hit the ground at its 'peak' impact, we may find that faster is indeed less 'peak' impact as it is shared across a longer timeframe. Jumping directly down off a roof plays hell on ankles. Launching from the roof and rolling it out, made my late night teen escapes a breeze. From the euc crashes I've seen at speed, it looks like feet are merely a formality and sometimes hit last....

Edited January 17, 2022 by ShanesPlanet

[Eucner]

16 hours ago, Chriull said:

Imho one hits the ground harder driving faster, as one normally touches first with the feet/knees and by friction there is a rotational acceleration.

Due to rotational movement the head hits harder and feet less hard. On the average the vertical speed of free object is the same with or without rotation.

16 hours ago, Chriull said:

Once one has enough velocity to reach sliding friction quite instantly

This is inaccurate. Sliding friction creates a force in opposite direction to movement. This slows down the speed. More speed means longer slides. There is always some slide if there was some speed.

16 hours ago, Chriull said:

more speed could maybe not really matter.

Correct, if you are just sliding, but not if you hit something.

16 hours ago, Chriull said:

So falling on a fixed, nicely sliding knee protector could be better as touching with the shoes first.

I agree. It is better to take the biggest forces to knees than to the head. Natural reactions works here against us. Falling should be practiced.

[mrelwood]

On 1/18/2022 at 1:00 AM, Eucner said:

This is inaccurate. Sliding friction creates a force in opposite direction to movement. This slows down the speed. More speed means longer slides. There is always some slide if there was some speed.

Friction force is not linear per speed, and at lower speeds like running speed the static/initial friction (the friction to overcome in order to begin sliding) plays a major role, as it can twist your limbs and dislocate your joints. Once you go fast enough, the static friction is surpassed so quickly that it doesn’t make much difference to the fall.

Then there are the differences of both frictions on pavement and on gravel/dirt. On the former the static friction is much larger than the sliding friction, whereas on the latter the difference is small.

 Falling down is always a complex event, and the simple calculations we do here apply only rarely. But on all except one high speed crash I’ve seen on video, the rider falls from a deep crouching position and wears a helmet so they haven’t hurt their head. Therefore calculating only the forces applied to the head aren’t very useful, when the rider can be hospitalized due to trauma everywhere else in the body.

[Eucner]

13 minutes ago, mrelwood said:

Friction force is not linear per speed, and at lower speeds like running speed the static/initial friction (the friction to overcome in order to begin sliding) plays a major role, as it can twist your limbs and dislocate your joints. Once you go fast enough, the static friction is surpassed so quickly that it doesn’t make much difference to the fall.

Dry friction can be subdivided into static friction between non-moving surfaces and kinetic friction between moving surfaces. Friction is nearly independent of speed. In our crash cases there is no melting metal surfaces against each other or anything like that, so friction can be considered a constant enough. When crash starts from speed, it will start with kinetic friction and only when movement stops it goes to static friction. It is opposite to what happens when you start a movement.

The reason for phenomenon you described is the other party of friction force equation. The collision contact force is not a constant. It varies over time. At the beginning of a contact it rises to the peak value very rapidly, and then comes down in lower pace until settles to a constant static force. All this happens in a fraction of second.

48 minutes ago, mrelwood said:

Therefore calculating only the forces applied to the head aren’t very useful, when the rider can be hospitalized due to trauma everywhere else in the body.

I'm with you here. All contact forces need to be considered.

[mrelwood]

 

6 hours ago, Eucner said:

The reason for phenomenon you described is the other party of friction force equation. The collision contact force is not a constant. It varies over time. At the beginning of a contact it rises to the peak value very rapidly, and then comes down in lower pace until settles to a constant static force. All this happens in a fraction of second.

Nope, you are leaving out quite a lot. A panicking rider who’s unsure whether they should take the first hit on their Flexmeter wrist guards, Leatt knee guards, or cheap market elbow guards, after which they try to remember whether they should try to roll or not, but try to do so anyway, quickly stopping it after remembering that they have a relatively new MacBook Air in the backpack, at which point they feel their butt dragging on the pavement so they try to turn on their front side only to stop and hesitate midway because they realize that they just ate a huge all-you-can-eat meal only covered by a T-shirt… At which point they finally stop.

 Please mark the spots where the friction was constant. 

Edited January 19, 2022 by mrelwood

[Tawpie]

41 minutes ago, mrelwood said:

 

Nope, you are leaving out quite a lot. A panicking rider who’s unsure whether they should take the first hit on their Flexmeter wrist guards, Leatt knee guards, or cheap market elbow guards, after which they try to remember whether they should try to roll or not, but try to do so anyway, quickly stopping it after remembering that they have a relatively new MacBook Air in the backpack, at which point they feel their butt dragging on the pavement so they try to turn on their front side only to stop and hesitate midway because they realize that they just ate a huge all-you-can-eat meal only covered by a T-shirt… At which point they finally stop.

 Please mark the spots where the friction was constant. 

NOPE, WRONG. What really happens is you're riding along minding your own business, and magically you're laying on the ground wondering what happened and whether or not your wheel survived. Zero actual time passes between those events, and there was no friction, constant or otherwise… due to the teleportation. In my crashes, the only exception to this was a sneaking suspicion that "that rock there might be a little bit bigger than I prefer". But nothing happens between that and a close up view of the riding surface. It truly is magic (one of many magical parts of riding). 

Edited January 19, 2022 by Tawpie

[Eucner]

42 minutes ago, mrelwood said:

 Please mark the spots where the friction was constant. 

It was constant between each material pair. I think @Tawpie had more realistic scenario of what actually happens in drivers mind.

[Paul A]

Friction could be constant...

 

[mrelwood]

2 hours ago, Tawpie said:

NOPE, WRONG. What really happens is you're riding along minding your own business, and magically you're laying on the ground wondering what happened

Hmm. My crashes have been fundamentally different. Time shows down a bit, and on the other time I remember thinking that “Oh crap, here we go, let me try to soften… No, wait! I vividly remember a biker I used to know telling about his crash when he broke both his arms. Yeah, I don’t want to do that. Arms in!” *thwump*

Maybe you’re just getting a bit… umm… old? You see, I’m still an energetic 25 year young… (inside).

 

1 hour ago, Eucner said:

It was constant between each material pair. I think @Tawpie had more realistic scenario of what actually happens in drivers mind.

Sounds like you haven’t crashed yet. Let’s hope it will be more like mine than Tawpie’s. 

[Tawpie]

4 minutes ago, mrelwood said:

My crashes have been fundamentally different. Time shows down a bit

Yeah! There are some people that actually know what's happening because time slows down. Can you teach that to me? Youtube? I'm like a bird: CAT GOT ME->I'm dead now, lights out.

Edited January 19, 2022 by Tawpie

[RockyTop]

3 minutes ago, mrelwood said:

Hmm. My crashes have been fundamentally different. Time shows down a bit, and on the other time I remember thinking that “Oh crap, here we go, let me try to soften… No, wait! I vividly remember a biker I used to know telling about his crash when he broke both his arms. Yeah, I don’t want to do that. Arms in!” *thwump*

Same! … I REALLY like flying through the air!! I take time to enjoy the pleasure before having to pay up for my mistake. 

[RockyTop]

5 minutes ago, Tawpie said:

Yeah! There are some people that actually know what's happening because time slows down. Can you teach that to me? Youtube? I'm like a bird: CAT GOT ME->I'm dead now, lights out.

You know how some men/ people don’t show emotion? … During an emergency they evaluate and react instead. No time for emotions. We will deal with them later. I think it is tied in with that system. Just a thought. 

[mrelwood]

 

14 minutes ago, Tawpie said:

Can you teach that to me? Youtube?

I can maybe make a YT video out of it myself some day, since I haven’t seen one yet.

 The basic problem though is that we would probably have to make you younger somehow. You’re already EUC’ing, so I really don’t know what else is there. Maybe start playing the guitar and join a rock band? Nah, too many dangers, they tend to die at 27.

 Sorry, I really don’t have anything atm!

[Paul A]

Average reaction time is around 150 to 300 milliseconds.

Watching videos of cutouts at high speed doesn't appear to show riders being able to do much before they impact.

[mrelwood]

9 minutes ago, RockyTop said:

You know how some men/ people don’t show emotion? … During an emergency they evaluate and react instead. No time for emotions. We will deal with them later. I think it is tied in with that system. Just a thought. 

Umm… You are probably new here. There was just somebody here with the same name.

Anyway, let me introduce myself. I’m mrelwood, and I can get a BIT emotional pretty darn OFTEN! I think the only time that I DON’T show ANY emotion is when I’m biting my tongue very hard trying not to insult some jerk who’s wrong in the internet. 

So yeah, I don’t think it’s that. 

[.....]

The beginning of most crashes are the most AWESOME time I ever have in my life. Watching time come to a near halt as the body dumps adrenaline and the mind taps into its reserves, is a drug best served quickly. As @RockyTop mentioned, its really just nearer the end of the entire ordeal, that I really dont care much for.

[mrelwood]

2 minutes ago, Paul A said:

Average reaction time is around 150 to 300 milliseconds.

Watching videos of cutouts at high speed doesn't appear to show riders being able to do much before they impact.

If the cutout happens out of the blue, then absolutely. My crashes have always been very non-blue, and the reaction time passed well before the actual flight. When I realized that I’m riding into mud in a curve, wobbles get slowly worse and worse, or losing grip on gravel.

[Paul A]

Ok, have never experienced the Matrix style, time slowing down thing, out of the blue or otherwise.

Worst crash was at walking pace speed. Couldn't roll, landed on one wrist guard, injured shoulder quite a bit.

Higher speed crashes were actually better, just rolled or slid a bit.

Weird outcomes.

 

[Eucner]

8 minutes ago, Paul A said:

Average reaction time is around 150 to 300 milliseconds.

Only when you are prepared. If not, then it is about 1 sec. Falling down takes about 0.4 - 0.8 s depending on height and style.

[RockyTop]

15 minutes ago, mrelwood said:

Umm… You are probably new here. There was just somebody here with the same name.

Anyway, let me introduce myself. I’m mrelwood, and I can get a BIT emotional pretty darn OFTEN! I think the only time that I DON’T show ANY emotion is when I’m biting my tongue very hard trying not to insult some jerk who’s wrong in the internet. 

So yeah, I don’t think it’s that. 

OK!!! … So not that.

…ADHD??…  I just found out that I have that. I told my doctor that coffee calms me down and helps me sleep at night. He said that I likely have ADHD or something, I wasn’t paying attention.

[.....]

18 minutes ago, Eucner said:

Only when you are prepared. If not, then it is about 1 sec. Falling down takes about 0.4 - 0.8 s depending on height and style.

If the average reaction time is 1 second, we are already all dead. The math of how far cars travel per 1 second at high speeds, is a LOT more than the time it takes me to react. When we are driving and riding, we should ALWAYS be diligent and prepared. Perhaps you are thinking of the reaction time of a typical teeneager on a cellphone, or an euc rider whos also piloting an r/c car or camera or drone? Fwiw, I try to ALWAYS leave 2 sec follow with cars. I am ocd and count it out all the time while driving. Sucks tho, you leave 2 seconds and some asshat takes the spot. You slow down to alot 2 more seconds and happens again. I find it best to just not be in a hurry anymore.

If i walk up behind you and clap my hands by your head, and it takes you 1 second to react.... Well, Ima gna assume you're on drugs, asleep, or lost in deep thoughts. Go ahead, yell at someone walking by and see if it takes a full second for them to react. Keep in mind, they weren't prepared for it either.

https://humanbenchmark.com/tests/reactiontime/

spoiler... avg adult is .277ms.   IF a fall takes .4-.8ms.... simple math right there. However, I have no idea where the .4-.8ms fall time comes from either. Some falls seem to come from nowhere and others seem to blatatly give you a 'heads-up"

@Paul A You are missing out on one of my favorite moments in life. Even as a child, it was usually worth the crash, to fly like a bird or tumble like a rock and capture time like it was your bi*ch. I havent found a drug (yet) that mangles the clock quite as vividly. Yumm! If only they didnt end in pain, it would be perfect!

Edited January 19, 2022 by ShanesPlanet

[Paul A]

How to measure your reaction time