Hacker News Reader

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.

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.

> 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.

>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.

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.

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?

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.

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

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.