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Comment by bdamm | Hacker News Reader

There’s been some pack of the napkin research on this, and the conclusion is that a cyclist stopping at every intersection and then speeding up to 20mph vs a cyclist that doesn’t stop and just cruises at 20mph burn 500W vs 100W. Unfortunately I can’t find a link right now but based on my own experience I’d say that is spot on. Stopping and resuming at every stop sign or stop light is far more taxing, so much so that many people wouldn’t even be able to do it and maintain a better than walking reasonable pace through a city.

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I made an estimate and while it was higher than I expected, the increase in power is not anywhere near what you mentioned.

I'm using the value of Cd * A from here (0.6 m^3): https://www.sheldonbrown.com/rinard/aero/formulas.html

That page also indicates that the vast majority of the energy expenditures on a bike are due to drag.

The fraction of energy needed to get up to speed divided by the energy expended due to drag works out to: (mbike + mhuman) / (Lblock * rho * Cd * A) where mbike is the mass of the bike, mhuman is the mass of the human, Lblock is the length between stops, rho is the mass density of the air, Cd is the drag coefficient, and A is the projected area. I can post the derivation if anyone is interested.

Assuming an 8 kg bike, 90 kg human, 200 m between stops, and an air density of 1.2 kg/m^3, stopping adds 68% more energy.

This is dependent on the weight. If I crunch the numbers for my own weight, stopping adds about 56% more energy.

So, this is higher than I expected, but I don't think it's so much higher that it justifies running red lights.

I don't think any amount of effort justifies running red lights.

I think this calculation is basically right, but might be a bit worse for the average commuting cyclist. Many commuting bikes are heavy and badly maintained, so rolling resistance is likely to be a larger factor. Also at the low (max ~25km/h) speed of many commuting cyclist rolling resistance is almost as significant as air resistance. [1] indicates that up to around 17km/h it is as big a factor as drag, but that is on a well maintained bike.

[1] https://www.gribble.org/cycling/power_v_speed.html

(20 mph is really fast for a bike, eh?)

Anyway, 400W to avoid getting creamed by a car seems cheap to me.

Eat a Clif bar; stop at red lights.

Goddamnit.

The worry is that you won't be able to output 400W. My approach is to reduce speed earlier and hope the green comes before I'm forced to stop.

That's a good idea. What I want is some gadget that caches my momentum (in a little compressed air cylinder or something) for the stop.

"Caching the momentum" is essentially what "regenerative breaking" does.

An engineering student designed something like that, using a flywheel: https://www.treehugger.com/bikes/engineering-student-builds-...

But if you want something commercially available, you have to get an e-bike with that feature, and those models seem quite pricey.

I have an ebike w/ regenerative breaking (a Bionix conversion kit, $1500 w/ labor) but I can't say it seems to do much. But then again, with no other non-regen bike to compare to I can't be sure. It doesn't recover anything like the charge it spends going up a hill (with assisted pedaling) when you're coming back down, but that's to be expected, eh? Thermodynamics and whatnot.

The flywheel thing is exactly it, except I think it would be safer and lighter to use compressed air, also stores longer than a flywheel.

With something like that you'd eliminate most of the work penalty for brief stops.

Compressing air isn't a very efficient operation, pumps usually have leak, spring compression, friction, and heat losses.

20 mph sounds way over the top for average city cycling. Anecdotally most people running stoplights seem to not exceed 12 mph, as I frequently overtake them shortly after getting my green light.

I was thinking that too, but it turns out that the percentage increase in energy is independent of the speed in a fairly simple model: https://news.ycombinator.com/item?id=22257938

I think the reasonable alternative then is to walk, not to dangerously run through intersections when it's not your turn.

Edit: I will admit that I rarely come to a complete stop at a stop sign. It definitely is a considerable inconvenience to get back up to speed, but a lot of safety precautions are inconvenient.

The problem with walking is the same issue. Unless the lights are timed for a walking pace you will almost always have to wait 3-8 minutes at a light for your turn. Walking in a place with a lot of crosswalks is excruciatingly slow. Especially since a lot of crosswalk signals will only add you in on the next cycle of green rather than let you cross on a green right away like a car or a bicycle.

> pack of the napkin

That's back of the napkin. Bone apple tea!

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