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Comment by ben7799 | Hacker News Reader

Not sure why this is coming up again a year later.

They put it out as a marketing thing. It had a bunch of impractical caveats.

They haven't announced they solved any of the problems with it in the year since they put it out.

The friction thing & drivetrain is mostly a non-issue. If you're riding fast enough to care the % drag from the chain/drivetrain gets smaller and smaller the faster you go. It's pretty much a non-factor compared to air resistance.

You get way more benefit from doing some extra stretching and yoga so you can lower your handlebars and still ride hard than you do changing to ceramic bearings or fiddling with expensive lubricants or chains/belts/shaft drives.

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Agreed, rolling resistance at 30mph (about the fastest you'll ever ride a bike, even down a steep hill) is about 50w of energy, whereas wind resistance at that speed is close to 1000w of energy.

Compare to 10mph (average speed for most casuals) where your total wind + drivetrain is about 50w of energy. Wind resistance goes up by the cube, drivetrain is mostly linear.

> Agreed, rolling resistance at 30mph (about the fastest you'll ever ride a bike, even down a steep hill)

What do you consider a steep hill? There are hills here that I can routinely get up to 35 mph with a little effort. Many cyclists have gone downhill at speeds exceeding 50 mph [1].

[1] https://www.youtube.com/watch?v=sc2nqMXlZWg

Most people who aren't hardcore cyclists with really nice bikes are going to chicken out around 30 MPH.

Or they should. 30 MPH is fast on a bike. If anything goes wrong, it's going to hurt.

Then i've been lucky, since i did that and much more in the past. Nowadays i'm doing 22 to 23 up to 28 peak on flat ground with no headwind. On a decades old 3-speed, with coaster/back pedal brake, mud guards and basket on the (rear)carrier. (27x1 1/4 pumped to 7.5 to 8 bars)

The only thing of going wrong i can think of is insects in the eye, and generally unpleasant airstream in the eyes. Use sports glasses. Problem solved. Anything else is situtational awareness, maybe a helmet, and good maintenance.

We used to routinely do 55mph going down mountain passes in Washington State on $30 goodwill bicycles but I think that is well outside the scope of "about the fastest".

Conceptually you seem correct but your numbers are a bit off.

Down a steep hill on my road cycle I can easily get to 45-50mph.

Rolling resistance doesn't change that much with speed[1].

Rolling resistance energy balance at 18mph is on the order of 10-15W[2]

Drag increases with the square of velocity (D=0.5 * <drag coefficient> * <representative cross sectional area> * <air density> * <velocity>^2).

The drag energy balance (plus some rolling resistance) at 10, 20, and 30mph is on the order of 10, 100, and 300W, respectively[3].

[1]https://www.schwalbe.com/en/rollwiderstand.html (see section "what exactly is rolling resistance")

[2]https://www.bicyclerollingresistance.com/road-bike-reviews

[3]https://www.exploratorium.edu/cycling/aerodynamics1.html (see the calculator)

These #s are way off.

30mph does not require anywhere near 1000w, it's more like 350-400w on flat ground depending on the riders position and size. I'm a former amateur racer. I've done 30mph steady state for a mile on flat ground without a draft before. That's a 2 minute mile and there is no way on earth I could ever do 2 minutes @ 1000w. 1000w is more like a 5s sprint for an amateur, you would put that power out in a burst to accelerate and probably end up doing about 34-35mph on flat assuming no draft. Pros are probably more like 1500w. 30mph is nothing. I probably exceed 30mph every time I get on my bicycle if I'm riding for exercise as opposed to commuting. Almost no race goes on where speeds don't hit 40mph, and 50-60mph is not rare.

Rolling resistance is not drivetrain resistance. Most of rolling resistance is tire resistance. It doesn't scale up at the same rate as aerodynamic drag.

Chain resistance is much smaller than tire resistance. It does go up with power but not really with speed directly because you shift.

400W is too low :) but 750W Bafang motor with non speed locked controller will get you to 50km/s without pedaling (not that I would recommend going that fast).

50 km/hr

50 km/s is over mach 145 :)

Don't use the rated wattage of your motor for this kind of comparison. Some electric motors can run briefly at 10x or more their rated current, and sustained if they have very good cooling.

Likewise, poor cooling can lead to a 10x decrease in motor performance.

Cooling is usually the biggest limiter of power, but max rpm (before the thing blows apart) combined with iron saturation (effectively meaning the magnetic fields inside can't get any stronger, even with more current) can also be a limit.

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