Comment by geor9e | Hacker News Reader

Comment by geor9e | Hacker News Reader

The website seems to end with a 2015 electric unicycle but not continue on with covering that form factor. Maybe because most people don't refer to those as monowheels anymore.

http://www.douglas-self.com/MUSEUM/TRANSPORT/motorwhl/motorw...

Also airwheel is kind of a random brand to showcase - Trevor Blackwell made an earlier one https://www.trevorblackwell.com/eunicycle and Solowheel got the majority of the earlier patents (and royalties to this day).

That form factor has come a long way since 2015 though.

Check out this MotoGP style race, they're hitting 50+mph: https://youtu.be/fbX6qaWINBk?si=l2G0JnBxxCMP8a7e

I see them every day whizzing across San Francisco, especially on Van Ness passing all the cars.

I know there's at least two shops in SF selling them, one in Potrero and one in Hayes Valley - https://alienrides.com/collections/electric-unicycles - https://lastmilepev.com/collections/electric-unicycle-euc

Unicycle seems to be the winner. The "wrap around the body" form factor seems doomed from a physics point of view. That big of a diameter has way too much leverage, and the person sitting inside has too little. So you end up doing a flip instead of stopping. As such, their required stopping distances are very long compared to unicycles which can stop as well as a bicycle, if the rider is skilled.

In theory, the braking performance could be the same as the bicycle. In practice, the rider needs to guess the traction very precisely and to move their weight backwards to an angle just a smidge higher than arctan(static friction coefficient). Do you start to see a problem here?

While the bicycle rider only needs to move their weight so that the combined center of mass is projected between the contact patches of the wheels during the deceleration, the EUC rider absolutely needs this to be as close to the contact patch as possible. Shifting the weight too far back means an almost guaranteed wipeout, and you don’t know if the road is squeaky clean or has some trace amounts of sand that would reduce traction, so any sane rider would err on the side of caution and under-brake to avoid falling on their back.

Did you ever ride on one? They brake using the motor so it doesn't wipe you out. At worst you stand on the ground, but mostly you just start riding backwards.

I own one and I can assure you it’s very easy to fall on your back if you overestimate the friction when braking aggressively. Not even close to real life bike braking with disc brakes if you know how to transfer your weight properly (granted, a newbie would totally fly over the handlebars if they tried to brake to the limit without shifting their weight behind the seat..)

Odd - I ride EUC and bike mainly, and bike is easier to skid on accident. Opposite experience as you. It makes sense too, since on an EUC 100% of the normal force is utilized for friction force. On a bicycle you can only utilize 100% if you're extremely good at applying the front and back brake at the perfect ratio to each other.

>arctan(static friction coefficient) >needs this to be as close to the contact patch as possible

That's not true, or perhaps you mean something other than how those words are directly interpreted? Static friction force isn't an arctan, it's just the normal force (which is you+wheel weight) x the friction coefficient. That's the force that's slowing your motion. And being close or far from the contact patch doesn't affect that at all. Elaborate what you're thinking?

Arctan(friction coefficient) gets you the "friction angle", which is a meaningful angle to know in a lot of physical systems. For example, it tells you how steep of an angled surface your object can rest on before it starts sliding downhill.

In this case, it sounds like the friction angle is important because it's the angle at which the reaction force is being applied to the unicycle by the road. (The normal force and friction force components make a right triangle with the resultant force, and their ratio - the friction coefficient - is by definition the tangent of that angle). I know nothing specific about unicycles, but it sounds like they're saying the rider needs to lean back so that the "shaft" between the seat and the axle is oriented along that line of force or else the system will become unstable.

I don't think (electric included) unicycles will ever have the same stopping performance of conventional vehicles - the center of gravity is just much further forward than in any two-wheeled vehicle. The requirements imposed by the balancing mechanism limits the possible braking force to how far behind the wheel a person can get.

They can still stop plenty fast enough for most purposes, though, and it's a very natural action compared to almost any other vehicle.

On wheel size - yeah, its the case that the smaller the wheel, the more responsive a unicycle becomes to torques applied by a person's body. It becomes a tradeoff between the resilience of a large wheel to bumps and terrain, and the responsiveness afforded by a tiny wheel (which takes less torque to apply a given forward/backwards force)

I disagree. Having a single wheel has the benefit of maximizing traction. With multiple wheel vehicles, if you apply the brakes to some wheels and not others, the tires lose traction. This is because those tires don't have enough normal force (weight) pushing them into the ground. I know my car only applies brakes to the front wheels. Any all-wheel-braking vehicle like a unicycle can beat my car to a stop, for that reason alone. It takes a sporty rider to be good at hard braking on EUC but I've seen them stop extremely fast. Especially some of the riders who race around manhattan. They have a handle on their EUC, and throw their entire weight behind the wheel in a crouch, like a cowboy bucking a bull. Their center of gravity gets near the ground. The motors on their wheels are extremely powerful, 10kW or more in either direction. It takes a skilled rider though. The main danger of super hard braking in the EUC community is the notorious "death wobble" that amateur riders sometimes get, where it starts oscillating right and left the harder you brake. Good riders grip things stiff enough to not have that issue though.

Having fewer wheels does not mean you have more traction - contrary to popular belief, a racecar can trivially out-brake a MotoGP bike despite weighing an order of magnitude more. The contact patch size matters more than the number of contact patches. This is ignoring the issue of perfectly balancing a unicycle to stop effectively, where just the time it takes to transition from leaning forward to backwards is a significant cost in time and distance for braking compared to any vehicle that has inherent lateral stability (such as a car or a motorcycle).

> I know my car only applies brakes to the front wheels.

I doubt this. What kind of car do you have? Almost all modern cars apply brakes to all 4 wheels. In most cars which have most of the weight in the front the brake bias is approximately 80% front and 20% rear. For mid-engine cars this usually shifts to 60% front and 40% rear. Some cars might be somewhere in between depending on their weight distribution. The limiting factor in braking is tires, and the way to improve that is by increasing the contact patch size - which is done by increasing tire widths, diameters, and the number of tires.

>a racecar can trivially out-brake a MotoGP

Racecars are essentially wings due to the aerodynamic plate on their underbelly, sucking themselves hard downwards into the ground, that's why they can brake better than vehicles that don't do that. And contact patch has to be the most misunderstood and overanalyzed concept in racing. It's baked into the coefficient of friction already. The force it exerts is still directly proportional to the normal force on each wheel.

>What kind of car do you have? Almost all modern cars apply brakes to all 4 wheels.

That's good if it's true. Maybe they will make 18-wheeler trucks start having brakes on all 18 wheels so they can get similar stopping distances as those cars. Point is there are plenty of vehicles on the road where only 2 or the 4 tires go into a skid when they lose traction. Parent comment was absolutist about how unicycles will "never" match any other vehicle in braking. I figured I'd defend unicycles. Don't want to jump off the rails too much here.

I know my car has rear drum brakes actuated by the hand brake. It can lock up the rear tires for a drift. And front disc brakes actuated by the foot pedal. Perhaps it does use all 4 when I press the foot pedal - my mistake if so.

I think it's funny you're objectively wrong but continue to insist you're right. Anyway, I'm not gonna waste any more time correcting someone that's allergic to learning.

Ah, knocking all the chess pieces off the board and strutting away. It's funner to actually think about the interesting problem tho. I just got back from a weekend EUC race at Alameda naval tarmac, I love pushing what the things are capable of.

You joined the comment tree of "I don't think (electric included) unicycles will ever have the same stopping performance of conventional vehicles" and have taken the position of defending that absolutist statement.

I get it, not everyone is familiar with EUC racing. To help, here is a photo of the average hard brake from 40+mph on a modern EUC (probably a ET Max or Lynx or similar high kilowatt wheel), since you might not be familiar with what we are discussing -

https://i.imgur.com/pDzPtvB.png

Notice how far you lean to decelerate from speed. Note the handle you can yank as you buck your bodyweight back. That all forces the controller to compensate with more kilowatts of stopping power, instantly righting the gyro sensor. The more you try to tip it backwards, the harder it brakes to force itself back upright.

I don't have a mythbusters style video to make it crystal clear that you're wrong, but sounds like its a good idea, by how folks are reacting to the news. I'll suggest it to my youtuber friends.

For now, try to imagine a vehicle that invalidates your position. We only need one example after all, it being an absolutist statement. Think of edge cases. Something that skids around a lot. Fixie bike in NYC? The kind you see messengers whizzing around on all the time, that can only skid their rear wheel, and have no front brake, by design. Still defending the claim?

And to your point - an F1 car with thousands of pounds of aerodynamic downforce will stop shorter than an EUC, absolutely. We agree there.

> I'll suggest it to my youtuber friends.

Ah, now I see the problem. You get your information from clickbait YouTube videos instead of actually trying to learn things.

I'd point out more ways you're wrong, but you don't listen to what everybody else has already said so it's pointless.

>With multiple wheel vehicles, if you apply the brakes to some wheels and not others, the tires lose traction.

Which vehicles don't brake on all wheels? I can only think of cruiser bicycles.

I think GP mistook a short summary for FWD that it “drives” on front wheels, as in it delivers engine output to front wheels.

fyi to all, brake hydraulic lines are still distributed to all four wheels in low-end cars, but “drum” brakes are often used in the rear, which stretch out to slow inner wall of a cylinder rather than squeezing together to hug on surfaces of a disc.

Are there any modern (post-1990) cars with drum brakes? Discs are simpler and cheaper and work better and are much easier to service.

Discs are also exposed to wind and sand, drums aren't. Front discs and rear drums is just fine for commuter cars, ID.4/Q4 E-tron uses that if an example is needed.

Thanks for the example. They’re not regenerative brakes, just rear drums?

I don't see the point of continuing this comment tree.

Other guy lost interest, but a wheel being able to regen brake just means theres an ungeared electric motor on it. It's basically free for the manufacturer to add the feature, so they all do it. Mechanical brakes are seperate and still required for harder braking.

My 2009 has rear drum brakes, actuated by the handbrake. The front are disc.

It sound like folks are adamant that the brake pedal actuates all 4 of my brakes. I didn't know that. Sounds plausible, I never really thought too much about it. I'll take their word for it.

Thank you - you're right I did assume that. Plus the fact that my handbrake only actuates my rear drum brakes (and sends me into a fishtail drift). I didn't realize my foot pedal was also actuating that, but if so sounds like a smart design.

> I know my car only applies brakes to the front wheels.

If that's true, your car needs urgent brake maintenance. Most cars' brakes are forward biased, and all cars apply brakes to all wheels.

Braking performance is limited by:

- angle of CG to front wheel, tan(vert angle) == max stopping acceleration in g (assuming front/all wheel braking, rear wheel only is different)

- the coefficient of friction (generally in the rangeish of 1)

- shear strength of the rubber in the tires (wider/more is better if you're at other limits)

- other braking system issues -- heat dissipation, crap brake pads

The thing about single wheels is that the CG -> contact patch vector basically defines all of the instantaneous dynamics of the system. If you want to go from turning to braking, you have to change position, and there are limits about how fast that can happen that don't apply in a multiwheel system.

Emergency braking feels more like shoving the wheel forward than shifting oneself behind the wheel. You ignore balance for a while, get yourself into a sitting position fast, the system can right itself (or not) after you've bled enough speed.

> The website seems to end with a 2015

With the style of the website, I'm shocked it has anything after 2000

ok, that race video was as fun to watch as it was unexpected

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