"A bicycle cannot stop any faster than it's wheels can hold friction."
Nothing can stop faster than "it's wheels can hold friction". But on a bicycle when you brake hard then momentum is transferred on to the front wheel, which is typically ahead of where the bulk of the weight (the rider) is situated.
"You can stop any type of board-device and stay on, as long as the friction between the board and the rider is greater than the wheels and the road."
On a skateboard, the front wheel is MUCH closer to the center and the centre of gravity is MUCH higher. You'll be flung off if you stop as fast as a bicycle with properly maintained brakes can safely.
It's really not that difficult - you shift your weight all the way back, flex the rear knee into a very low stance with the front leg 'pushing back' against the deceleration. With proper technique you can brake almost as well as a bike.
Simply incorrect that you can get bike like braking safely, even using the described technique.
Edit: I really want to prove this, but the only way I can think of to do it is you buy one of these for $1200, I buy a bike for $1200 and we spend $1200 in airfares to meet somewhere and try it out. Perhaps someone with physics qualifications can weigh in?
There is simply more inertial mass on a Moto, which allows you do use the front brake to its full potentail without pitching a faceplant. That and the suspension absorbs some energy. Also, the rider position is usually COG to the rear.
On a bike, your body position on drop bars puts you more COG forward, steeper head angle, more levered over the front friction patch (tire/road). The bike is light, suspension if any not as robust, its prette easy to stop a 20lb bike vs a 250-500lb moto, and for you to fly off. You are like a pole vaulter at 60 deg vs 45deg on the stick (not precise, just directional)
Normal skate is same thing, its light, close to ground, The board has a low COG. The rider a much higher one. The board has no suspension. So, it can stop on a dime if it meets and object that makes one wheel "not round". You are like a pole vaulter at 90 degrees.
It it possible to ride like a ninja, super low and crouched to the back and de-weight at precisely the right time to go over XYZ obstacle. Maybe. But the issue really is one of probabilities. At what stage do you not foresee the obstacle? And heaven forbid an hazard that is only slightly more complex, say, 10-20 meters of sand or grit on the tarmac, that you cant just skip over. That is a very normal type of defect to urban road surfaces. Ther is a reason they have "street sweepers" and alternate side parking and all of that.
I think the problem here is that people are conflating an enforced stop - hitting a rock - with a normal braking stop, and even then with an emergency stop.
If the issue is - is an electric skateboard more dangerous than a bike? well, then of course it is. You're less visible, less stable, and have less braking ability and much smaller wheels.
But that's not to say you can't safely brake a skateboard without throwing yourself off the front. Like any device - motorised or not - it's up to the operator to control their speed for the conditions.
I totally agree. If you run into something on either vehicle it's going to hurt. If you aren't wearing a helmet, you could really get hurt. It is pretty apparent with the behavior of typical commuter boarders vs. bicyclists that they know which is safer. Most cyclists on streets actually don't wear helmets. Most boarders do.
With regards to breaking when you don't have a rock, curb, door in your face...You can program anti-lock braking into an electronically controlled breaking system. If you have enough understanding of the friction of the rider to the board and the boarder is experienced enough to know how position themselves during braking, that system should win as it can avoid skidding. Regarding experience level: on a bike it takes some experience not to just use the front brake only and drop your head on the pavement.
Until we see some stats about which group actually sustains more serious injuries, commuter bicyclists or commuter boosted boarders, the jury is still out.
This is a fair point. The only note here is that the "enforced stop" as you put it is a function of the wheel diamter. If you think about making a wheel "non-round", its a function of the surface hardness of the wheel (ability to deofrm) and the relative size. so a 1/2 inch rock is just so much more likely to hurt a 2inch or whatever wheel than a 17/21/26 or whatever (would be like hitting a 6-8inch piece of concrete).
The frequency of this hazaard is more the problem. I skated in college to work all the time, but had the benefit of clean/smooth/concrete. It was a great mode of transit, for that. Who knows maybe someday the will have such lanes for these things?
& Obviously, the enforced stop scenario sucks on bike. Think, NYC taxi-cab doors, and etc. So thats fair.
Nothing can stop faster than "it's wheels can hold friction". But on a bicycle when you brake hard then momentum is transferred on to the front wheel, which is typically ahead of where the bulk of the weight (the rider) is situated.
"You can stop any type of board-device and stay on, as long as the friction between the board and the rider is greater than the wheels and the road."
On a skateboard, the front wheel is MUCH closer to the center and the centre of gravity is MUCH higher. You'll be flung off if you stop as fast as a bicycle with properly maintained brakes can safely.