Yeah, I'm spamming. Sorry.

[jedusor]

LOOK AT THIS THING.

There are not words to express the intensity of my desire to own that contraption. (And don't let my "motorcycles" tag scare you off--it's really more of a segway.)

Comments

[mamagotcha.livejournal.com]

Whoa. Now THAT is a nifty idea! It totally looks like some photoshopped thing. Where did you trip over this?

[jedusor.livejournal.com]

I saw it in an icon and demanded details. Look, the inventor is entering MIT this year. (http://web.mit.edu/newsoffice/2008/gulak-tt0514.html) He's in Liz's class, and she's doing engineering too, so maybe they'll have classes together!

[mamagotcha.livejournal.com]

Oh my... maybe he'll need a test rider?

[gerald_duck]

So… how does it do an emergency stop?

I note they say it only does 25mph currently, so it may not be quite such an issue, but it would be a serious problem if they let it do any significant speed.

XKCD386

[mrmorse.livejournal.com]

I'm trying to respond to this without overreacting, but apparently I can't. Sorry.

So I ride a unicycle everywhere, all the time. (And I have such lust for the Uno, I can't tell you.) I get stupid comments about it all the time. But the one that gets to me is "but how do you stop?"

The answer is, you stop. There's no trick to it. Stopping is so automatic that there's no "how" to it.

I expect the Uno works more or less the same way.

(So, hi, jedusor. I've accidentally invaded your LJ and started insulting your friends. Pleased to meet you.)

[gerald_duck]

A vehicle that's moving forwards has kinetic energy (proportional to the square of its speed). If it wants to come to a standstill, it has to lose that kinetic energy. Neglecting air resistance, the kinetic energy has to be lost at the wheel-pavement interface.

For each individual wheel, decelerating causes the vehicle to experience a forward turning moment relative to that wheel's axle. In vehicles with multiple ranks of wheel, that transfers weight away from the rear wheels towards the front; with just one rank that can't happen: the tendency is for the rider to flip over the wheel and smack face-first into the dirt.

To avoid this, the rider leans backwards when stopping. By transferring their centre of gravity so it's behind the wheel rather than over it, they create a backward moment about the wheel: where this would normally mean they fall off backwards, the trick is to balance the forward and backward moments exactly so the rider remains on the vehicle as it stops.

The faster the deceleration, the further back the rider must lean to compensate. Once the rider is horizontally behind the vehicle's wheel (even assuming they were a good enough rider to achieve that), they've reached the limit. It's impossible to decelerate faster than that without coming off the vehicle forwards.

The faster a vehicle is going, the faster it might need to decelerate in an emergency.

Unicycles travel quite slowly, which is why — even though their emergency braking capability is actually considerably poorer than an equivalent bicycle — this doesn't matter much. However, that Uno goes at 25mph, which means it has four times the kinetic energy of a pretty fast 12½mph unicyclist, twenty-five times the kinetic energy of a much more typical 5mph unicycle.

Googling, I can find no record of anyone ever having taken any vehicle with a single rank of wheels to a higher speed than 25mph. If one ever got to 50mph, that would be fully a hundred times harder to stop in an emergency than the typical unicycle!

[mrmorse.livejournal.com]

I have considerable practical experience with the mechanics of unicycling. I am quite familiar with the need to lean back while slowing down (including being horizontal behind the wheel. Ouch). It is true that the harder you want to decelerate, the farther you have to lean. But it is also true that the balancing force from leaning back depends on the tangent of your angle from vertical, which means that as you approach horizontal, your force approaches infinity. Leaning back far enough to avoid flipping over should never be a problem.

Granted, my experience is in the 6-10 mph range, and things might be different at 25 mph, but the two things that play a bigger factor in how fast you can decelerate are the power provided to the wheel and the friction force of the contact between the wheel and the pavement.

On a unicycle the power is provided by your legs. On the Uno, it's provided by the motor. None of the reports I've seen have mentioned power or acceleration, but I assume it's something the designers are paying attention to.

Friction is another issue. There is a static friction force between the wheel and the pavement which is equal to the decelerating force. If you exceed the grip of the tire on the road, it will start sliding. And if your wheel starts sliding, you are fucked. Again, personal experience.

[1firefly.livejournal.com]

O goody it's a geek fight!!!

[gerald_duck]

Hmm. Fair point about the tangent. I've also just done some force diagrams (for the first time in a couple of decades) and decided the downforce on the wheel's contact patch with the pavement will also remain good.

On the other hand, the degree of poise and balance required to hold a unicycle or similar thing steady at such a great angle will still surely be infeasible? Especially when the vehicle hits even the tiniest bump in the road while braking?

[mrmorse.livejournal.com]

Okay, so in practice a unicycle rider (or unic) is always accelerating, decelerating, turning, compensating for uneven ground, and what have you. But let's assume a unic moving at constant speed in a straight line over perfectly smooth flat ground for the sake of physics. The rider is positioned so her center of mass is directly over the contact point of the wheel with the ground. Gravity exerts a force straight down on her center of mass. The ground exerts a force in the direction from the contact point of the wheel up to her center of mass. Since we're starting in equilibrium, this is exactly equal and opposite to the gravitational force.

Now she unexpectedly hits a rock. The wheel suddenly slows down, but her center of mass keeps moving forward at the same speed. Now gravity is still pushing straight down, but the ground is pushing up at an angle and she begins to fall forward.

She quickly responds by pedaling harder, causing the wheel to accelerate and increasing the magnitude of the upward force vector. The vertical component of the two vectors equal each other and she does not fall down and instead rights herself. But the forward component of the ground vector has accelerated her, meaning she is now traveling faster than before.

To slow down, she basically reverses the process in a more controlled manner, leaning back as she pedals more slowly, keeping the forces and torques in balance.

That is, assuming she can apply the required force to the wheel fast enough. There are lots of reasons why she might not be able to, including her speed, reaction time, position of the pedals, and if she is in fact not traveling in a straight line but is instead playing hockey and has just fired off a slapshot on goal, meaning her body is twisted and trying to move in one direction, while her wheel is trying to move in another direction and angular momentum is trying to conserve itself.

Then she falls down. This happens to me every couple of miles or so. Usually I land on my feet while my uni clatters to the ground behind me. Very rarely I land on something other than my feet, which tends to be very painful.

That's all if the bump is unexpected. If you know it's coming it's easy to get the speeds and forces set up ahead of time. I routinely ride up over two inch curbs and down two or three steps in a row. I knew a guy who rode down over drops of two feet or more, equal to the diameter of his wheel, until he blew out the frame of his uni.

Then he built a stronger frame.

The Uno, however, works the same way as a Segway. You control it by leaning, so if you hit a bump, it should take care of all the speed changes for you. And if you're riding along and someone steps out in front of you, you'll lean hard out of the way, and it will accelerate or corner hard to prevent you from falling off, and you'll dodge the pedestrian and everyone will keep going where they were headed.

I think it is fair to ask how hard it can accelerate, but if they intend it for street riding I hope the answer is pretty hard.

[mrmorse.livejournal.com]

Actually, the other reason I hope it can accelerate hard is that I totally want to be able to make it dance. Throwing my weight around as it pivots, bobs, and weaves beneath me would be totally awesome.

Re: XKCD386

[jedusor.livejournal.com]

Hi! My friends are pretty good at firing back, as I see you've noticed. Nice to meet you, too. :)

Hey, I see you're a Bostonite. I'm going to be joining you later this month, sort of (I'll be in Worcester, but I hope to be able to get to Boston on a semi-regular basis to hang with my peeps).

[deyo.livejournal.com]

It's kinda the opposite of this.

[thatwesguy.livejournal.com]

Holy cripes. I WANT ONE! This might even trump my desire for a Tesla.

...well, probably not but at lest I might actually be able to afford this thing.

[rubrick.livejournal.com]

Holy shit! It's a motorized, headless tauntaun!