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Things most people don’t know about bikes

August 5, 2022

A friend of mine long ago told me that I forget that most people don’t know what I know, and don’t figure things out as quickly.  And even so, it took time for me to figure things out, I know of things that were right in front of my face for decades, and I did not notice them.  So, after over 40,000 commuting and errand miles on cargo bikes since 2006, and as someone who has more than one copy of Bicycling Science, as well as my own personal copy of Food, Energy, and Society, here’s some stuff about bikes that I’ve learned and other people appear to be less clear on.

Stopping power, turning ability, bike geometry

The physics behind the numbers below is discussed, in detail, in Bicycling Science.

A useful thing to know is that rubber on road has a sticking grip force that is about the same as the force against the road, for dry pavement.  That means that a vehicle that is low enough to the ground that it will not flip will have a maximum deceleration before it skids of 1g (g = earth’s gravitational pull), or 32 feet per second-squared (9.8 meters per second-squared, or 22mph per second).  That is, if you are in a car traveling 22mph, the quickest you can possibly stop is one second (your speed decreases by 22mph per second of 1g braking), and in that one second you will travel 16 feet (the formula for distance traveled is x = v0t + 0.5at2), t is 1, a v0 is 32 ft/s, a is -32ft/s2, so 16).

On a “normal” bicycle, two things limit this.  Because the rider is most of the mass and is positioned relatively high, before the front tire exceeds its grip on the pavement, the rider will instead rotate over the front of the bike onto the road.  This occurs at about 1/2 g. A corollary of this is that it’s not a great idea for a cyclist to tailgate a car; if the car stops hard, the cyclist is physically unable to stop as quickly without being flung onto the pavement or the back of the car.  The exceptions to this are bikes where the center of mass is further back; a tandem with two riders, or a box bike, or a long-tail cargo bike that is also well-loaded to the rear.  On the other hand, a penny-farthing or high-wheeler, where the rider is positioned almost on top of the front axle, has very limited ability to brake without flipping.

The other problem is that even on differently designed bicycles where the rider will not flip, because a bicycle rider uses their front wheel for steering, when it goes into a skid it becomes very likely that the bike (and its rider) will fall down.  This is not guaranteed, and with years of experience I have survived such skids once or twice, but the first time I had a front wheel skid I hit the ground so fast I was down and in pain before I realized what had happened; it’s much faster and more violent than a rear wheel skid.

A further problem is that stopping quickly requires a fair amount of arm force to keep you on the bike, and in the worst case you’ll just keep moving while the bike stops underneath you.

Rear-wheel braking is slower yet; because stopping shifts your center of force forward, it reduces the force on the rear wheel, which if it is the braking wheel, will have less road grip.  On a normal bicycle this limits rear wheel braking deceleration to about 0.25g, whether you do it with a caliper brake, coaster brake, or by jamming your legs on a fixie.

In theory (not to be confused with practice), a skilled rider with enough spare room on the road could turn their (normal) bicycle with a full g of centripetal acceleration in the same forward displacement needed to stop it (with 1/2 g of forward deceleration).  (Math: for centripetal acceleration, the radius of the circle is v2/acentripetal; for forward deceleration, the distance to zero velocity is 0.5v2/abraking; however because a normal bicycle can turn twice as hard as it can brake, the circle radius and stopping distance are equal.)  HOWEVER, in practice this would be stupidly risky, because it does not reduce your kinetic energy and if you fail (failure is always an option) the resulting crash will be far more dangerous.  This daring maneuver also requires much more clear road space than simply braking.  And, on a tandem or cargo bike of any sort, the longer bike’s improved braking ability beats its unimproved ability to turn.

Perception and reaction time

Bikes lack stopping power, but for most people on bicycles (upright bicycles, rider not wearing headphones, not seriously impaired hearing) a person on a bike is far more able to perceive what is going on around them.  They are (usually) seated higher, don’t have an additional layer of glass in front of their eyes, or supports for that glass obstructing their view, or bulky hood hiding who knows what, and don’t have the noisy engine or layers of acoustic insulation obscuring sounds around them.  And, because the front of a bicycle is much shorter than almost any car’s hood (excepting front-box cargo bikes) their riders are able to position themselves far forward and look around corners.

Reaction times for bicycle riders seem better (from videos of my own reactions) than the norm assumed for drivers.  I think this is mostly a result of better human factors in the brakes; to stop a car, a driver must lift their food from one pedal, move it over, and depress a different pedal, whereas a cyclist with hand brakes can maintain their fingers over brake levers in traffic, and activate the brakes in a single motion.  I’ve measured real-world on-bicycle reaction times as low as 0.6 second, and some perhaps as low as 0.5 second (which through the camera lens, looks superhuman).  0.9 second, which is about the estimated driver reaction time, is what I get when I am distracted — it looks fine on the video, but at the time it felt like I had made an enormous mistake.  The failure modes for panic stops in cars and bikes also differ; on a bike, there’s a risk of a header from braking too hard, on a car the risk is that your foot will miss the proper pedal and you will accelerate instead.

There’s an additional problem related to cognitive load; if you’re actually evaluating everything within your stopping distance that could go wrong, as your speed increases, that distance increases, and it increases at a greater rate than the speed increase.  A driver traveling 30 miles per hour has 2.5x their stopping distance at 15mph; to understand what’s in front of them, they need to know 2.5x as much “stuff” about their surroundings, and they need to update that knowledge at twice the rate.  The same thing applies to someone on a bicycle, but common case there is 20mph or below, not even 25mph, so this is less of a problem (people biking should be really careful at “high” speeds like 30mph, because we have so little experience at those speeds; my time over the last 16 years traveling 12-20mph is measured in months, my time above 25mph is measured in minutes.)

The combination of better reaction time but lower stopping deceleration means that up to about 20mph, hand-braked bicycles and cars have about the same stopping distance, with bikes slightly ahead at 17mph and slower.  However, because people on bikes have much better perception and less cognitive load, they’re more aware of what’s around them and can make more sense of it.  A corollary of this is that to a driver, a cyclists’ choices may appear “random”, but this is because the cyclist is (often) acting on information that the driver lacks.  Just for example, if I hear a car approaching an intersection from the left, I may stop without ever looking in that direction, even if the right is obviously clear.

There are other not-obvious-to-drivers effects at work.  When stopped at traffic lights, because they take up so little space, cyclists usually are stopped at the front, and over time, can collect a lot of information about signal timings, local road conditions, and local traffic patterns.  This can mean things like “the walk signal comes 3 seconds before the green” or “the light is long to allow pedestrians to cross, the side traffic usually clears after 5 seconds and then it is safe to run the light”.

 Two wheels versus three wheels

People who have problems with balance or coordination can’t necessarily use a bicycle, and can instead use a tricycle.  For lower speeds, the tricycle is more stable because the center of weight sits well within its wheels.  At high speeds, however, tricycles become riskier to turn because most tricycles cannot tilt, and because they cannot tilt, they risk flipping.  This is not universally true; there are tilting tricycles, very-low-to-the-ground recumbent tricycles, and experienced riders can throw their weight around on the tricycle to counteract this effect, but these are not common case.

So, basically, three wheels is more stable at low speeds and easy turns, less stable at higher speeds and with rapid turns.

“Motor” efficiency

Bicycles are efficient for carrying one or two passengers because bicycles have small weight and move relatively slowly (both compared to cars), but viewed as a motor, we humans are only about 25% efficient; 75% of the food that we eat for physical energy, we turn into heat, and the food that we eat took energy to produce. Because humans are such inefficient motors, and modern batteries, motors, and their controls, are quite efficient, it is entirely possible for an e-bike to be the more efficient choice, depending on details:

  • Humans have varying diet; the more carnivorous someone is, the greater the energy cost of their diet (generally, there are further details, but meat tends energetically expensive).  But, contra that, what matters is the marginal calories, not the average calories.  When you exercise more, you may find yourself craving carbs more than usual, not lobster.
  • An e-bike, being easier to ride, will tend to generate more travel and thus consume more energy.  However, if that extra travel would have occurred in a car, then it is still a win.  An e-bike makes it easier to ride more quickly (up to 20 mph in the US), which is somewhat less efficient than riding at typical commuting speeds (12-15mph seems typical without e-assist).  However, if the alternative to a rapid trip by e-bike is a trip in a car, then again, it is still a net win.  Notice how in both cases, the unfavorable comparison is to a bicycle trip that might be purely hypothetical, whereas, if the actual other trip is in a car, the e-bike is far and away the energy-saving choice.

Another under-appreciated corollary of the wastefulness of the human engine is that we get hot and need the airflow that a bicycle provides.  Climbing hills is famously hot because our energy (and heat) output go up, while the speed of the cooling wind goes down.  Stationary bicycles tend to require fans.  And pedal-powered electricity generation gadgets are usually a bad idea; yes we get the exercise, but we also get very hot, will get extra-sweaty, and may require a fan for ventilation, and the fan is not energy-free.

A happy corollary of our wasteful human engines is that in cool weather it’s not that hard to stay plenty warm.  Our extremities still need protection from the wind (so, toes, fingers, ears) but everything else tends warm, after we have physically “warmed up” to the exertion.

Ventilation

This is a little odd, but one thing people miss is that when you bike there is a lot of airflow.  Yesterday I biked in 95F-ish temperatures, about 40-50% humidity, and as long as I was rolling, actual physical exertion in that heat was still comfortable.  Rolling along at 12mph, or 18 feet per second, I sweep through about 10 square feet of air (crudely, 2 feet by 5 feet), for 180 cubic feet per second of ventilation, or 10,500 cubic feet per minute.  That’s about 5 20-inch box fans on high, all aimed at me.  At the same time, in the winter, because of this airflow, one of the most important ways to stay warm is to block the wind.

Double-counting bicycle time.

Because humans are not the finest motors, and because some (tasty!) food has a high energy cost, in some cases the end-to-end miles-per-gallon of a bicycle can be as bad as some fossil-fueled automobiles.  However, up to at least 100 miles per week, we get to double-count time/distance on a bicycle as exercise; time spent on the bicycle is time not spent at the gym, and calories burned on the bicycle are calories not burned at the gym.  So for example, five days each week, I get about 30 minutes of exercise before and after work, and then in zero time spent and zero energy consumed, arrive at work.  (To be fair, that is a lot of exercise, but it’s not wasted; there were measurable changes at my annual physical, worse from the low-commute Covid year, and then recovery to the better place in the next year.)  Energy expended at the gym also incurs a ventilation cost; to cool you down, gyms tend to be air conditioned and often include fans, where each fan consumes 50-100 watts.

And, the same as when you drive, you can also listen to podcasts or books on tape while you bike to work.

Typical speeds

People sometimes make wild estimates of bicycle speeds.  Cyclists who can cruise at 25mph are not common; well-trained cyclists can, but most people are not well-trained cyclists.  Even cruising at 20mph is not that common; I could, as a teenager, but it took a lot of practice, and I can’t do that now.  Sprints are faster, but similarly limited.  E-bikes come with various limits; the US federal law specifies two speed levels for assist, 20mph (still a bicycle) and 28mph (a “type 3” e-bike).  California state law uses these same rules, I think that this is the general plan for new e-bike legislation, especially the 20mph part.  But, for federal law purposes, if the assist can propel you faster than 28mph, then it is not a bicycle, at all.

Cargo capacity

Most people have a poor intuition for how much you can or cannot carry on a bicycle or a trailer.  Far and away the most important factor is the interaction of loads and hills; the greater the load, the smaller the hill that you will be able to manage.  Very low gears make hill-climbing with larger loads possible, but every rider has a minimum speed at which it becomes very hard for them to balance a bicycle, and there are physical limits on drivetrains.  On a cargo tricycle, however, balance is not a problem, only the torque limits of the drivetrain.

A second problem is managing the ability to brake; default bicycle brakes are sized for a default bicycle load.  Larger disk brakes and drum brakes help with this; managing downhill speed also helps with this.

On flat ground, the main limit is the rider’s ability to handle the load at very low speeds.  As the load gets heavier, more time is spent at low speeds, and the harder the load is to rebalance.  In practice I can pretty easily start and balance a load that weighs about as much as I do (over 200lbs), but I have also seen a video of someone riding a cargo bike loaded with 500lbs of bananas, and they needed help to get started.

Using a trailer avoids the balance problems, though the hill problems remain, and braking can be more difficult depending on how weight is distributed on the bicycle and trailer.  

Weird rules

Your intuition about what is or is not a “bicycle” may not agree with the law, which in turn depends on where you are.

The rules about what is a legal bicycle are a little odd, sometimes devolved to the states, and tend to differ in important ways from Europe.  So, at the US federal level, a “bicycle” is defined by its power and number of wheels; if it has no assist and 1, 2, 3, or 4 wheels, it is a “bicycle”.  If it has up to 750 watts of inhuman power assist, its assist is limited to 28mph, and 3 or fewer wheels, then it is also a “bicycle”.  There appears to be no US federal limit on “bicycle” width or weight.  But, if it has an e-assist propelling it faster than 28mph, then that is not a bike, that is not an e-bike, legally, that is some other device.

However, at the state level, there are a variety of rules and regulations, with a variety of e-assist power and speed limits.  California has rules that conform to the federal standards, with additional use requirements on so-called “type 3” e-bikes that have assist past 20mph.  As of this writing Massachusetts treats them as mopeds, though that may change within a month. New York State has a 36-inch width limit on cargo bicycles, with a bill proposed to increase that to either 55 or 48 inches, but that bill also includes a lower speed limit (12mph) and insurance requirement for e-cargo bikes.

Rules in the EU are more detailed and still evolving. Older rules limited assist for cargo bikes to 250 watts, which is completely inadequate for actual cargo in hilly places.  Four-wheeled e-bikes and e-cargo-bikes are legal.  The new rules allow more assist, but also have detailed remarks about brakes and bicycle durability.  As of that cited document, EU rules were silent on width, but Germany’s DIN has proposed regulations — 1m width and 250kg for 2-wheels, 2m width and 300kg for 3 or 4 wheels.

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