What's Your Urban Speed Limit?
Some hold the theory that instead of braking, the urban cyclist should be quick to accelerate so that they can go around swerving cars and leaping pedestrians. But that isn't always possible. Earlier this week during the holiday shopping rush, I found myself in a situation where I was basically trapped between several moving objects simultaneously and had no choice but to slam the brakes: Two car doors in a row swung open ahead to my right while, at the exact same time, a pedestrian jumped into my line of travel (which was out of the door zone). I could not swerve right because of the car doors, I could not swerve left because of the moving cars in the travel lane, and I could not continue straight because of the pedestrian. Within milliseconds, I had to come to a complete stop.
Over time I have determined that my self-imposed "speed limit" when cycling through areas where such situations are possible needs to be 12mph at most. Any faster than that, and I cannot guarantee that I can come to an immediate stop safely. Do you have an urban speed limit?
Not a number, but I know I slow down somewhat. One of the reasons I looked into riding a longtail, long ago, was that it puts more weight low and to the rear, which gives me somewhat more resistance to flipping. The front disks seem to be capable of plenty=enough braking force; the rear disks are mostly useful when loaded or on ice.
ReplyDeleteI don't aspire to ride faster than 25 anywhere, even when the wind and slope favor it, because crashes of all sorts rapidly become more deadly. The speed also induces a sort of "go-go-go" mental process that is not necessarily wise -- that is, you might not stop/pause when otherwise you ought to, because you would "waste" all that lovely momentum.
The other issue, if your bike is sturdy enough, is that you might not need to get all the way to zero. That car door has a little give to it, and the hinge will crack at a surprisingly low force.
I think that 12 mph sounds about right. That is one of my frustations. People in cars often don't understand that when they swear in front of a bike, it takes the cyclist time to stop too.
ReplyDeleteI feel the same way and ride no more than about 12mph in those situations too....that is why my friends don't like to ride with me!
ReplyDeleteI do go much slower when I'm in a busy area, but have no idea what speed I'm going (no cyclecomputer). Accelerating quickly just isn't an option I'm going to rely on on a 40lb bike.
ReplyDeleteI slow it down even more if for some reason I'm forced to ride in the door zone, or too close to the door zone for comfort. Ditto if I'm "filtering" which I only do if the traffic is well and truly stopped and likely to stay that way (some kind of horrendous traffic jam).
I think that there are a few important factors to consider:
ReplyDeletea)Bicycle set up (bike weight/position + gearing + braking capacity influence your acceleration and deceleration. Also, going quick with 15 holiday shopping bags oddly strapped to your bike will make a difference)
b)Rider experience (can you quick stop? Are you comfortable wailing on a front brake without dire consequence? Are you good at anticipating traffic and processing multi-inputs? Are you okay being in close proximity to vehicles?)
c) Road /traffic conditions (can you take the lane or are you stuck in a sub-par bike lane next to parked traffic? Potholes? Rush hour? Terrain?)
d) The “giveacrap” factor: are you late to the office? Are you enjoying a beautiful day? Hungry, angry, lonely, tired?
e) Weather: wet? Cold?
If all factors are positive for fast movement, I’ll do vehicle speeds, take the lane, and maintain my rights as a vehicle on the roadway.
If conditions are not conducive, I’ll take it easy – 10 – 15 mph, probably – It’s hard to know because I don’t keep a computer on the commuter.
I try to ride outside the bike lane to keep my speed up - say 16mph (I can't physically go much faster then 18mph for long periods of time on a flat). When in the bike lane in door zone, or just in door zone with no bike lane I try to stay closer to 10-12 mph. I go even slower if there are multiple turns or is an area where I know there are cars changing directions.
ReplyDeleteYour column had me looking at numerous youtube videos of doorings or near doorings. A Boston local example.
http://www.youtube.com/watch?v=f1r_-qT-m90
There doesn't seem to be a speed much over 7mph to avoid getting doored if you are in the door zone.
I don't know about anyone else, but if I feel endangered by car doors due to distance, I just pull out more in to the road and make the cars behind me wait, although this is still situation dependent. I don't recall anyone unexpectedly opening the door while I'm riding by.. but I don't do much urban cycling anyway.
ReplyDeleteI agree with you, my recent accident, 20 miles. Today was an adventure that I look unnecessary, if you were 12 miles, everything would be fine.
ReplyDeleteI'm very curious to see the answers to your question. I'm hardly an urban rider, being in the outermost suburbs of a major urban center, and my TOP speed is 13 mph. :) My commute/cruising speed is somewhere between 10 and 11 mph. It could be my loaded-down hybrid is slowing me, but it can't be that much heavier than a standard city bike.
ReplyDeleteI absolutely have a speed limit in busy or dangerous areas.
ReplyDeleteI find the amount of time I spend watching out for potential dangers naturally slows me down.
If I am looking in dozens of cars for people who might enter my path or there are a hoard of kids and I am looking at their little legs trying to determine where they will go next or there is a lot of parking lots and cars are moving in all directions I find that I have to slow down just to look at everything that is happening.
I would guess around 10 miles an hour or so.
Cars can brake faster than bikes. Also, pedestrians respond differently to cyclists than to cars. Peds will often walk directly in front of a cyclist in a way they would never do in front of an automobile. I don't want to claim to rigidly adhere to a speed limit, but consider 12 mph in a dense urban environment to be the optimum maximum speed. That isn't to say that there aren't circumstances where I might go up to 20+ mph on a downhill, but if I do that I'll take the full lane and be sure that I have clear sight lines to protect me from any potential oblivious pedestrians. But the closer I get to the right, the progressively slower I will ride. At 6 feet from parallel parked cars, that speed will be no faster than 12mph. Anything closer than 6 feet I'll try to ride slower than 5 mph.
ReplyDelete20 mph? Wow. I guess I'm a slowpoke. I have no idea how fast I go, but my top speed has to be 15 mph. In congested areas it's more like 10 mph.
ReplyDeleteCan't go over 25 mph here in Burlington also because of the recent speed limit change. And they are cracking down on cyclists as it is if they're caught without lights at night.
Best be safe this holiday season and cycle slower than normal.
I rarely go over 40. But my streets make that safe. Around your parts, I might not want to do even half that speed.
ReplyDeleteI had a computer on my commuter bike briefly, and found my speed tended to VARY the most in town centers. On the more rural parts of my ride, I just cruise at whatever speed my bike lets me go comfortably (usually around 17mph) but in town depending on density I might be doing 12mph when lane splitting with cars and dodging pedestrians, but if there's no room and I have to take the lane I kick my speed up into the low 20s and keep pace with traffic (and one "don't try this at home" evening had me drafting a NASCAR fan for several miles at around 30mph). My current bike and fitness level don't let me hold a 20-something mph speed for very long, but it gets me through some tight spots without drivers trying to squeeze around me.
ReplyDeleteit varies for me ... I will keep my speed slow in places like Mass Ave. or Downtown Crossing. However, there are certain other places where it helps to stay fast and at least try to keep up and be able to merge with traffic to take a lane: the Harvard Square Rotary (esp. when transitioning northbound or going into the underpass by the Science Center), the McGrath Highway when making the left onto Cambridge St. after the Museum of Science, and the Stuart -> Charles -> Park Plaza transition when going from Downtown/Leather District to Back Bay are the ones that most frequently come to mind. I tend to go back and forth about whether to be fast or slow on getting through Kenmore Square. The signage and lane changes there require adaptable tactics.
ReplyDeleteI'm pretty slow anyway, but I have a hard time imagining myself travelling at 20 mph in an urban setting, what with required stops, having to watch out for cars and pedestrians, road hazards, etc. Plus, as far as I can figure out, the stopping distance for a bike is quite a bit greater than for a car (due to higher tire pressure, ultimately). The occasional time when I've had to do an emergency stop from a high speed (for me) when a car turned or pulled out in front of me I've found it very hard to control the bike -- I tend to skid and turn to the right -- and don't stop quickly enough to avoid a collision, though I do reduce the impact quite a bit.
ReplyDeleteDepends on your brakes and the total mass of you and your bicycle. Though I wouldn't think anything over 16mph even with the best brakes and a low mass generating momentum would be advisable. Even locking both front and rear wheels a bicycle can skid quite a few feet before stopping. There just isn't enough rubber connecting with the road surface to stop the built up momentum at speed as quickly as a car can.
ReplyDelete12mph does seem a good average for most people. Not an absolute, since I think a fully loaded cargo bike or an ultra lightweight would perform differently in a short stop panic situation, but a good average.
I don't have an urban versus country speed limit, per se. Rather, I'm constantly varying my speed in response to my comfort level. On one city block, I may hit 20mph, while on another I may stick to 10-12mph. It really depends on the micro-dynamics of a given street, or stretch of street, and I adjust my speed in real time as opposed to having an arbitrary limit. For example, if I know I'm not in a door zone and I'm taking the lane and I have clear sight lines, I will definitely go faster than when I know I'm in the door zone and might not see a person walking into the street. And if I'm on Mass ave in Central Square? Ha! I'm crawling around 6-8mph. Waaaay to many unpredictable variables along that stretch. So, 'urban' to me is too diverse in terms of traffic conditions and risks to impose an arbitrary limit. I need to assess my personal comfort in real time, using gut instincts.
ReplyDeleteThat said, I think the fastest speed I ever hit around the city is around 22mph.
Years ago when I had a computer on my commuter bike, it would consistently report long-term averages of about 10.5mph. This average included 6mph climbs up those notorious Somerville hills, as well as 25mph downhill sprints on said hills. On level ground I would average around 12-13mph.
I try to make this my habit when driving by car, as well. Sometimes I will go the limit, sometimes over, and sometimes under, but I try to adjust according to my gut feeling. So far my gut has spoken wisely: I haven't been involved in an accident in 22 years and hundreds of thousands of miles driven.
Well, I don't have a computer on the bike, so i don't know precise speeds. I do know what 18mph and 21mph feel like in my fixie's cadence, from having ridden alongside friends with computers. If the road's clear and I'm not tired, I'm going about 17-18; as it gets more crowded, I slow down, but to what speeds I don't know. Whatever's comfortable for my reaction time that day, given mood, quality of sleep, etc.
ReplyDeleteI rode motorcycles in traffic for years, though, so am probably trained up to a little more speed than I would have been otherwise. (Think dooring hurts? Try getting T-boned by a Buick when you're moving 60! Instant lesson in defensive driving--and in suspicion of all motorcar operators.)
Flipping over the handlebars can be avoided with good technique. It helps to practice braking really, really hard a few times. When we tested brakes for Bicycle Quarterly, we were surprised how hard you need to pull on the brake to lift the rear wheel, even on a 10% slope.
ReplyDeleteWith experience, you also learn to judge the surface to see how much traction you have. (Concrete roadways have much less than asphalt.)
Finally, you bike makes a huge difference. On an upright bike, you cannot easily brace yourself against the handlebars, and thus will have a hard time stopping as quickly.
So I think your "speed limit" depends on your skills and bike. In a modern car with ABS, all you need to do is press the brake pedal with full force, no skill required.
I'm lucky to have a commute across San Francisco that doesn't include any heavily trafficked areas. My speed tends to vary between as fast as I can pedal my single speed up the torturous hills, and as much speed as I can pick up on the way down. When I do ride in places with more jam to to the traffic I slow down a bit, it's getting doored that really frightens me.
ReplyDeleteCommuting on a 50-lb. (really!) Vélib' bike in Paris, often with weak brakes, I tend to take it easy. My top speed on the flats is around 15 mph, but I'm usually going around 10-12. As Adam says, pedestrians and car passengers opening doors often seriously underestimate how fast a bicycle is going. On several occasions a driver or passenger has looked straight at me, then opened a door. For that reason, if I have to ride in the door zone, I slow down to 7-8 mph. I also slow down if I'm in a separated bike lane with cars on the left and clueless peds on the right.
ReplyDeleteOn my "go faster" bike (I can't claim to go fast), I will ride in the city at 15-17 mph on the flats, but only if the traffic is light. Usually in those cases I'm either in a bus/taxi/bike lane or I'm on my way out of the city.
I ride in urban conditions on my regular commute and tend to cruise at 15-18mph where I have a clear view of the road, the sidewalk, side-streets, parked cars, etc. (essentially any potential hazard), and much slower when I don't have a clear view (passing a stopped bus/truck, for example.)
ReplyDeleteI follow the same general approach I take when driving: Always have multiple ways "out" of any situation. If a car creeps out from a driveway, for example, I slow down and make sure I have at least one good option to avoid them if they decide to pull out. Often this means I have to slow down "just in case," but I think it's worth it.
I have neither a speedometer/ cycle computer nor a set selfs posed speed limit but what feels right, or more correctly feels safe at the time for the existing conditions has nothing to do with the posted speed limit, that is set by traffic engineers who are only begrudgingly acknowledging that non-motorised vehicles even exist.
ReplyDeleteI have been quoting my motorized speed uphill as about 12 mph. I feel like that is a decent commuter speed. I like it slow and I def brake on downhills. the trike needs to maintain a slow speed or I'll pop a wheelie on a turn. With two wheels too I don't think I go that fast. I need a new bike computer to really tell what my speed is but yeah- 12 sounds about right. ( my computer on my Sorte once put me at 16 mph on a downhill and so I gage by that that uphill I am not going faster than that day. Downhill I am probably at 16 on the two wheeler...
ReplyDeleteI almost got doored with the sorte years ago and I swear my speed saved us. I was moving slowly enough that I saw the door open, shouted "watch out" and she shut it all from being at her back bumper to the driver;s door. ( flat lands with slight downhill)
finally I am not nearly skilled enough to react while moving fast.
( and I like what Adam above said...)
I don't have any way to measure my actual speed, but I definitely consciously ride in such a way that I feel like I can stop/swerve/etc if I need to, in any situation. Prevention is the best medicine, and I think riding consciously within your limits to react to a situation is one of the best methods of preventing a collision with something.
ReplyDeleteThis is simply common sense.
ReplyDeleteWhy would anyone ride fast in a urban environment? This is so dangerous!
Recently, while bringing my bike to the shop for a tune-up, the front desk guy had changed. There was this guy, a bike messenger, who was replacing as he had had an accident. Turns out the dude was on the safest bike path ever (the two Wikicommons images):
http://montrealize-montrealize.blogspot.com/2011/09/important-public-consultation-on.html
So, safest ever as anyone can see, yet he managed to break a leg. Courious, I ask how: he was biking during rush hour, i.e. fender to fender, and got sick of the slow pace as lots of people carry children or groceries. So he decided to attempt jumping the line by passing people on the left while other were coming his way... Ahem...
"Well, didn't you see other were coming?"
"Yeah but I am fast you know (I am a messenger, duh), I could have pulled it out if someone didn't jerk out of line".
Anyways, as he was the only moron trying to go fast, skip line, manoeuvering in a full-packed rush hour cycling zone, he ended up breaking a leg as no one made room for his bull$hit.
These kinds of folks are extremely dangerous. If you have long stretches in the countryside or in a suburban environment, fine, do as you please. But in a urban environment?
I strongly believe it is because of these guys that authorities want to put helments on everybody's head. They are the ones making up the majority of accident statistics...
"A driver is unlikely to flip their vehicle over if they brake suddenly at 20mph, but a cyclist is quite likely to either go over the handlebars or be unable to come to a complete stop quickly enough."
ReplyDeleteA bike should be set up so going otb isn't possible yet stop on a dime, bad hands notwithstanding. But it's also about the rider getting better technique.
In the above illustration I'd either limit my speed or take the lane. There you take out some variables and introduce others, like motor pacing.
Out of curiosity what was the traffic speed when the clusteref happened?
Hmmm...something I had never thought about,very good food for thought!
ReplyDeleteThe road's speed limit (I think) doesn't exceed 35MPH where I do 95% of my riding (which means cars rarely exceed,say...50),but now that you mention it,there are places I ride at differing speeds for the conditions,for example,on State Street (the state's border,one side's VA,the other TN),when riding with Nick (my 9 year old) we're often forced onto the sidewalk (where 90% of cyclists ride on that street,it's expected) for his safety,and I tell him "Go slow and walkers have the right-of-way,don't buzz by anybody!",which means 3-5 MPH,but when alone I'm out taking that lane,front wheel on the bumper of a car doing 20-25,while on the few carless paths here and I'm on them,that sometimes has other cyclist as well as peds,I roll around 12-15 MPH,unconsciously for just the reasons you mentioned.
I'm glad you wrote of this,I didn't have enough to occupy my thoughts this close to Christmas,LOL! :p Good,thought provoking read,my friend! :)
Disabled Cyclist
I slow to a crawl around kids, peds and peds with dogs. When I'm in a car...if I see a kid on a sidewalk I do the school zone 20 thing. It's just not worth the hassle of having to stop after hitting someone. :)
ReplyDeletewithout a cyclometer i have no idea of what speed i'm going but i do have a general rule wiich is to basically stay out of trouble and not cause anyone else trouble. if the traffic is heavy and speeds slow i'll get in an auto lane in order to avoid potential dooring and stay with the flow. it really depends on a lot of factors. i keep my eyes open though and try to anticipate as much as possible. can cars really brake faster than bikes? it seems i can stop on a dime when needed and avoided more than a few incidents by grabbing both brake levers ..... but then again, maybe that just means i pedal pathetically slower than i think i do ....
ReplyDeleteIt depends. I normally travel between 15-18 mph on a flat route, sometimes traffic slowing me down to a crawl. If I take the lane, I feel totally comfortable going that speed in general, or even faster, up to ~25 mph downhill. Again, if I take the lane. Any faster than that (and it can happen on steeper hills) and I have to have an open roadway with zero traffic on it, like closer to my neighborhood - not a city street. In Seattle, few downtown streets have actual bike lanes and even if there is a bike lane, one is not required to ride in one. And as for breaking/quick manoevers, experience makes this process easier and more natural, especially as one learns to anticipate potential situations further and further ahead.
ReplyDeleteI have a 'basic' speed limit at all times: "Don't go faster than it's safe!". That is dictated at all times by the situation. Fog, wet roads, busy urban environments, downhills, etc. (You DO know all vehicles brake more efficiently uphill than downhill...:-)
ReplyDeleteProbably the BEST reason to ride FIXED has nothing to do with hipness, efficiency, or anything but SPEED CONTROL. You can instantly get your bike to speed up or slow down without having to reach for and pull on levers, without having to reverse your pedaling, nothing. It's the reason why most couriers in *San Francisco* (of ALL places!) ride fixed. If you've ever watched Track races, you know it's a game of positioning and speed control until you get to the final sprint. Watching expert track riders is an amazing study in all the ways one can control speed.
Final point is YMMV. If you have a heavy Dutchie with steel rims, block-style brake pads hardened with age, you better slow way down. Narrow high pressure tires skid more easily than fatter low pressure ones; Bike setup and cockpit competence formulate speed control.
Val said........
ReplyDelete"You can do 20 and still be fine." But I don't believe that's accurate. "
I agree.......
Only the fool would overreach their ability to ride safely just because the speed limit for cars is faster than they ride.
I think in most cities, there are two distinct speeds possible:
ReplyDelete1. Car speed: If you can keep up with the cars, take the lane, be ready to brake hard, and you will be fine.
2. Low speed: If you go significantly slower than the cars on a given road/street, you'll feel safer if you are out of the way, in a bike lane or at least in the right-most part of the main traffic line. Here, you need to go slow, as you tend to be in the door/ped zone and hard to see for oncoming, turning traffic.
I use both approaches, depending on where I ride.
A couple of years ago, I was heading toward home on a training ride, pedaling at a brisk pace, but I was well within the speed limit because I frequently check my cycling computer. At a stop sign, a guy in a truck pulled up and sneered, "You know, the speed limit is 25 mph here," before he hit the gas and sped off.
ReplyDeleteAs tempted as I was to give him a middle-finger salute, I simply nodded my head and went on my way.
I have indeed skidded after mashing hard on the brakes, but have never lost control or gone over the handlebars while riding on pavement. It would be an interesting experiment to test the relative stopping difference between a bike and a car going at the same speed. It seems as though a bike with good brakes would stop quicker because there's much less mass.
MT cyclist
Bikes stop slower than cars but bike riders have much better situational awareness and much quicker reflexes. Simply because you are already doing things with your body, as opposed to loafing in an easy chair, you have more time to deal with traffic situations.
ReplyDeleteOver the bars excursions are amazingly common offroad. Onroad last time for me would be 46 years ago. Drunk driver exited an enter only alley at speed and created a new center of rotation for me with her fender. Less dramatic circumstances than that it's not likely you're going over the bars. If it ever feels it could happen in normal riding it's time to lower the saddle and move your weight back. Triathletes with the 80 degree seat angle go over the bars with boring regularity, Do the reverse of what they do.
caught myself going 13mph when I was in a bike lane and passed one of those signs telling me what speed I'm going. I was cycling comfortably, upright, and felt confident about my ability to brake. 12mph is very reasonable.
ReplyDeleteLike you I move at a cautious speed in the "door" zone/"pedestrians taking a look to cross the street" or "get in their car" zone. Or I take the lane and move around 20mph. Looking ahead and being aware is my obligation as a moving vehicle.
ReplyDeleteIt depends but I do not bike very fast anymore and have in fact become a total scaredy cat. In urban environments with cars and pedestrians and especially shopping it is wise to slow down. It scares me how people will walk right out in front of me when very clearly I am going steadily. I sometimes have to remark that I can't just stop on a dime! My commute is on a sketchy highway and I ride on the brakes very slowly. I've become tired of it, it is so unsafe. My husband rides very very fast on the highway when cycling alone.
ReplyDeleteIn Vancouver for example, I know the bike routes very well and ride much more quickly than out in the boonies! On parkway bike lanes it can get tricky when there is non stop cyclist traffic and a great number of them do not know what they are doing. Some people ride as if they are in a bike race, others have dogs on the handlebars and weave left and right. Or are texting! It can be spooky.
But for years I lived in a city with little cycling infrastructure, but a huge parkway system with walking/cycle paths and was able to get all over without being on busy roads nor encountering wandering pedestrians.
Within the city I average about 15-16mph, on the open road alone about 18mph, and in a good group about 21-22mph.
ReplyDeleteI think I tend to agree with most ppl on here; in dense, truly urban situations, speeds around 12-15mph seem appropriate in most weather conditions. When all forms of traffic (peds, bikes, autos, etc) clear out, I tend to go as fast as my mood/fitness will allow. (Usually not too fast. I'm typically in a foul mood and poor physical shape.)
ReplyDeleteI tend to play it cautious b/c I worry about reaction time more than braking distance. Maybe I'm wrong, but I feel like most of my bikes stop in a shorter distance than my car does. (Of course, the 90lb roadster with the rod-brakes takes a ridiculously long distance to stop, even at slow speeds. But the bikes with dual-pivot calipers and v-brakes stop on a dime.) With proper braking methods and with brakes in-tune, you're simply not going over-the-bar on paved road, unless you hit a large piece of debris/possum at speed.
I just don't wanna miss something and brake too late; hence, the more cautious speeds.
-rob
Interesting post and discussion. In general, I'm in the camp of determining speed based on what feels safe for the conditions and my bike and ability. For the most part, I stay out of traffic when I am moving much more slowly than the motor vehicles, and am willing to take the lane as needed or when I am traveling at a similar speed to the cars. That said, I was recently commuting through downtown Seattle cruising on a modest downhill grade with very light traffic (mid-morning, Thanksgiving week). I was going 20 or slightly more, so I took the lane. But then one of "Seattle's finest" pulled me over for "impeding traffic." (Her SUV was pretty much the only traffic on the road at the time.) She only issued me a warning, but I have filed a formal complaint as I am concerned of possible anti-bike bias. Seems she didn't agree with my basic right to occupy a lane as a legitimate vehicle on the road. So I guess the hazards of going to fast are many!
ReplyDeleteFriction between the tires and the road creates the force that stops a vehicle. That force is directly proportional to the weight of the vehicle and to the surface area of the tires in contact with the road. As a result, when you increase the weight of the vehicle, you also increase the force stopping the vehicle in direct proportion. So the weight of the vehicle is canceled out by the proportionately greater friction between the tires and the road, and plays no role in stopping distance.
ReplyDeleteWhat does matter is the air pressure in the tires. The vehicle's weight deforms the tires so the surface area of the tire, times the air pressure, equals the vehicle weight. So for a bicycle with tires pumped to 75 psi and a rider + bicycle that weighs 150 lb there would be 2 square inches of tire in contact with the road. The maximum force you can generate is whatever force is required to get 2 square inches of rubber with 75 pounds/square inch weighing down on it skidding on the road.
Cars have lower air pressure, so more of the tire rubber is in contact with the road. That means there is more friction available to stop the car. So for a 3,000 lb car with 30 psi tire pressure you would have 10 square inches of rubber in contact with the road, the force available to stop the car is greater, and the car will stop in a shorter distance than the bike.
The tire shape plays a role, too, of course. Very wide tires like they use on racing cars give more friction and therefore more force available to push the car forward, or stop it. Bike tires tend to be rounder than car tires and so have even less rubber in contact with the road than car tires do, even taking into account air pressure. This increases the stopping distance for a bike even more.
FWIW, any conventional bike with an adequate front brake is capable of throwing a rider over the handlebars, regardless of tire choice, rider weight, and even in the case of a lot of tired brake pads. It is one aspect of riding that people are taught about in bike ed; specifically in the "quick stop" parking lot drill. But I think Velouria was really asking about traffic condition influences on speed rather than the physics of bicycle braking.
ReplyDelete@Jon Webb - I may be wrong, but I think you've got your friction formulas a little mixed up. I believe the force that friction can exert is determined by the weight of the object and the coefficient of friction of the material. The size of the contact patch doesn't matter because it is the same amount of weight spread out across more or less area. Of course, this all assumes perfectly smooth, clean surfaces. A 1" oily spot on the road won't affect a car tire because the tire covers much more than an 1", but for a bike, for a brief moment your entire contact patch is on oil!
ReplyDeleteThe limit of your braking power is based on two things. First, how much force you can apply without skidding. Once you begin to skid, you have much less braking force than the moment just before skidding (sliding friction is lower than static). Skidding also means you lose steering control. But this is why a brakeless fixie can never stop as quickly as a bike with two handbrakes. The best they can do is slowing the rear wheel just short of the force that will cause a skid.
The front brake is more powerful because as you slow, more of your weight drives into the front wheel, increasing the amount of friction it has. This si why it is nearly impossible to skid a front tire on dry pavement. Here, the other factor is how hard can you brake without flipping the bike. In this instance, you can greatly impact your stopping power by shifting your weight back a little or even scrunching down to lower your center of gravity. I think most people who flip over their bars probably did so not because their brakes are so powerful. But more because they hit the front brake hard and didn't brace themselves at all. As the bike slows, they slide forward, moving the center of gravity farther and farther forward on the bike until they just fall over the front.
I really recommend everyone, even those of you riding lovely 40lb city bikes at 8mph do an occassional practice stop. Find a nice clear parking lot, get up to speed and experiment with stopping as fast as you can using just the front brake, just the back, and both. I think you may be surprised how quickly you can stop and it really may save you sometime.
To the original question, I agree. Anything much above 10-12 when you're dealing with traffic, parked cars, pedestrians, etc. is probably pushing it. The only time I'll generally go faster is when I'm taking a lane and acting more like a car. In that situation, I think you can make the case that higher speed is safer because there is less speed differential between you and the cars around you.
I am surprised reading opinions that it's difficult to endo the bike with the front brake.
ReplyDeleteI think it is in fact very easy, particularly with modern road bikes and their powerful brakes, but not altogether difficult with older bikes.
The whole bike pivots over a locked front wheel with grace and ease. Forester writes about this, but you can also see these air helmet testing videos (object in front wheel): http://www.youtube.com/watch?v=CwF-ZbJ_txw
For a transportational cyclist it's only a matter of time before someone jumps out in front of one's path. It will happen sooner or later. My front hub brake in the Pashley is not powerful enough to lock the wheel like in that video, and I am actually kind of "okay with it," having been in several situations where I had to brake suddenly. But I am _not_ advocating weak front brakes on transport bikes, just merely trying to tell more people that endoing doesn't require particular talent or effort.
The photo linked in jan heine @ 10:36 shows exactly what a maximum hard stop looks like. What maxutility and others here are talking about. Great photo. The rider looks very fit, looks good in kit. You do not have to look that good to do what he does. It's pretty instinctive to do just what you see in the picture when the situation arises. The pictured rider would be otb or too close to it if still on the saddle.
ReplyDeleteHardly anyone really practices hard stops. You could even race or messenger a long time without being forced to learn. Practice does not mean taking chances or pushing to the edge. More like learning the safety envelope is bigger than you thought it was. And from then on you cycle more confidently all the time.
It's very easy to go otb when the CoG remains high and forward, as demonstrated by the video.
ReplyDeleteJan's pic, though I've disagreed with him about causes and effects on the same subject, show how to properly move the CoG lower and back. In this case I'm sure the rear wheel was lifted for dramatic effect.
The point is, with practice and proper form, it's an intuitive move with good bike set up.
Typing this with a busted wrist from a high speed stoppie gone wrong.
I agree with MDI about the ease of endoing. If there is a front brake at all than it's surprising how easy it can be to carry the rear wheel, intentionally or not. You can shift your weight forward simply by standing up or if the rear wheel catches the lip of a pothole or something at the wrong moment etc. etc. Shift enough weight up and forward and I think your average pitiful Rod-brakes would get the job done nicely. Bikes have more potential for this kind of dynamic change in center of gravity than just about any other means of getting to the bakery that I can think of.
ReplyDeleteWatch even a moderately skilled kid on a BMX bike practicing endo's and you'll see just how little it takes to get the wheel up, especially at low speeds. Sometimes it all comes together when we don't expect it.
I like doing endo's. They aren't hard and you develop a feel pretty quickly that also helps you get the most out of whatever brakes you have at hand. Like trackstands, they're pretty satisfying when done well but awfully humbling when they aren't.
Spindizzy
" I think Velouria was really asking about traffic condition influences on speed rather than the physics of bicycle braking."
ReplyDeleteI found the comments about the physics of hard braking interesting, and I don't disagree. Physics are physics. Hard braking is indeed a skill that can be learned (thought I would not go so far as to call it universally instinctive, just because it is instinctive for some of you hard-core bikey gentlemen).
Having said that, there is a difference between theory and practice. My observation has been that typical transportation cyclists are unable to stop in a safe and timely manner when they are surprised with an obstacle with only milliseconds to react. That inability may be at least in part psychological and not merely physical - for instance, it may be due to delayed reaction time followed by panic. But nonetheless in practice cyclists very frequently go over the handlebars, swerve into traffic as a gut reaction to avoid a pedestrian or swinging car door, or crash into the pedestrian/door in front of them because they cannot stop in time. Among cyclists I know, I would say that more accidents have been the result of this than of other types of collisions. So, make of that what you will. My personal conclusion is that the surest way to prevent this sort of mishap is to control one's speed in chaotic urban traffic conditions.
V ^ You're observations and my observations are MILES apart, though I do agree that maintaining a sensible speed goes a long way towards keeping one safely on the bike.
ReplyDelete"thought I would not go so far as to call it universally instinctive, just because it is instinctive for some of you hard-core bikey gentlemen."
ReplyDeleteIt's not like I was born able to do stuff - lots of practice.
"My observation has been that typical transportation cyclists are unable to stop in a safe and timely manner when they are surprised with an obstacle with only milliseconds to react. That inability may be at least in part psychological and not merely physical - for instance, it may be due to delayed reaction time followed by panic."
Situational awareness, practice and experience can over come this.
"But nonetheless in practice cyclists very frequently go over the handlebars, swerve into traffic as a gut reaction to avoid a pedestrian or swinging car door, or crash into the pedestrian/door in front of them because they cannot stop in time."
They can't stop in time because they're not used to it and/or their bikes aren't set up to do so. I asked how fast the traffic was going - with the right bike you take yourself out of the bike lane, which apparently is too narrow.
Speed attenuation is a must for most, for sure.
This notion that folks blame their bikes, terrain, others for mishaps may be true, but from what I see the vast majority of people riding don't approach it from a skill-building, bike handling perspective.
"My personal conclusion is that the surest way to prevent this sort of mishap is to control one's speed in chaotic urban traffic conditions."
ReplyDeleteI'll second that. And just to carry on the physics discussion I'll point out that speed and stopping distance are a non-linear relationship. Double speed means quadruple stopping distance. So it takes four times as long to stop from 20mph as from 10mph.
On top of that, the distance covered in your reaction time (before you start braking) is affected by speed as well. That only doubles with double speed. But added to stopping distance means that it likely takes 6 times as much distance to stop from 20mph and it does from 10mph, all other things being equal.
Back to your original question, I'm not sure what my personal speed limit is or how exactly I adjust it but I certainly find myself doing it. I've never thought about it in these terms but I will say that I have noticed that when I'm riding more aggressively in that sort of chaotic environment, my top speed will be higher, but my lowest speed will be lower as well.
ReplyDeleteI think it might be because I'm more engaged and my concentration is much more focused. I'm more likely to perceive a threat and react defensively than when I'm "just riding along". I don't believe that I'm creating more sketchy situations(I'm not jacked up on meth or Red Bull and blowing stoplights or anything like that)and I'd like to think that my judgement is operating at a higher level when I'm really working at it.
It seems like you've identified a situation where your judgement encourages you to slow down to the point where you feel in control and comfortable. I wonder if the same mechanism is at work when I'm pushing harder(let's be clear, I might be going harder than I normally would but I don't think anyone would be astonished at my pace these days), my mind decides the situation requires more focus and concentration so that I can discern and react sooner/better... Maybe I'm kidding myself but I know I've gotten more "fingers" and honks when I've just been cruising than when I'm "Flying".
Is this what you were thinking about?
Spindizzy
I found this an intriguing topic and have been reading the responses with a great deal of interest. My own commutes are suburban/rural and while my very early morning rides (5 am) are relatively car free, the clutter of traffic - or rather, the miles long parking lot that passes for a highway as it bridges the urban area where I teach, the rural transect that I commute, and the bedroom community in which I live - can be a capricious hydra of automobile madness at the best of times ... and downright dangerous when you factor in the combination of rush hour with freshly released high school students. I ride the rural segments at a moderately fast pace - perhaps 18 to 20 mph on average and slow down appreciably when I re-enter the civilized world and encounter lines of obtuse drivers. I found the advice of Jan Heine/Bicycle Quarterly to be instructive with regard to body position when braking: butt back, body low, arms forward. I can brake quickly and use my body to moderate the braking simply by shifting forward or backward and using my front brake as the primary. This allow me to add a significant element of control and provides me with a degree of confidence I might not otherwise have when encountering traffic.
ReplyDeleteOK, I'm back.
ReplyDeleteI've been thinking some more and want to say that there while the phenomenon of increased awareness and focus as speed rises that I was being so evangelical about still appeals to me in a creepy/irrational way(the Lady Gaga of goofy concepts) I realize that that approach, even IF valid, requires some sort of limits, otherwise the safest drivers would be the ones willing to drive the fastest(and even I'M not willing to make that assertion).
I'm sure that this comes down to judgement and since V's example of speed limiting in traffic seems to demonstrate more of this than a lot of the rest of what we've all shared than maybe I should be thinking about how to do that more consciously instead of looking for examples that validate what I've been doing all along.
I'm usually not this thoughtful but Christopher Hitchens just died and I'm affecting an intellectual funk.
Spindizzy
No, I don't have an urban speed limit.
ReplyDeleteAll this micro-scrutiny and general faffing around concerning bikes and bike bits and pieces only makes cycling sound like some sort of skill or competence.
But it isn't.
Just get a bike and ride it.
pete - Now that's just crazy talk!
ReplyDeleteI've been thinking about this topic quite a lot recently, as in my city (Wellington, New Zealand) cycling infrastructure - even painted bike lanes! - is non-existent. I've made it my motto to cycle to avoid collisions. To do this here you need to be constantly alert and able to respond to everything around you. I reckon one of the best things to do is slow down. It kind of sucks that you have to, but if it means you'll be able to stop before you run into the car that's pulling out of a side street right in front of you, it's worth it.
ReplyDelete12mph is a good benchmark, when traffic gets dicey or if riding at night where there isn't enough room for a bike with parked cars and the lane i'll hop onto the sidewalk where I'll likely stick to 5mph max for caution of cars coming out of driveways or pedestrians
ReplyDelete@V - a bit more on stopping and speed. The stopping deceleration of a bicycle is discussed in Bicycling Science. On an "normal" bicycle, you can get about .5 G with the front brake (before flipping) or .25G with the rear brake (that is the skidding deceleration). You can't really get much more by doing both brakes because front braking reduces the rear wheel's force on the ground.
ReplyDeleteAnd yes, I have practiced emergency stops, and used to be pretty good at them. I've done the ABS thing where you break till the rear rises, and then pulse the brakes to keep it mostly on the ground.
Tandems can stop harder because their center of mass is further back. A loaded, longtail cargo bike, can stop harder, because the load holds the bike down.
A cargo bike, vs a car door, might turn out badly for the door. A guy at wheelworks told me about someone he saw in Somerville coming down a hill, with a "paperboy", actually fully loaded with papers. Someone doored that bike, and it took the door off the car. Xtracycle owners sometimes "tag" (lovely euphemism) things with their wideloaders or their cargo, and the experience is that they just keep going. I've done this twice, though I did bend my wideloader a little bit. The big bikes are very stable.
I believe the good doctor is referring to Bicycling Science by Whitt and Wilson, MIT Press. My Second Editon is 1982. Chapter 8 is Braking.
ReplyDeleteI thought I'd read every word in that book 5 times and now I have to do it again.
Zero point five gravities is a lot of braking. A lot of available braking power. Newer Porsches and such have 1.0g or so, ordinary sedans may have 0.8g. Most cars will go from factory to scrapyard without ever approaching that braking limit. If you're ever a passenger in a Corvette and the driver offers to demonstrate his brakes, say no. A full gravity of braking force is terrifying. The braking power a bike has at the limit is more than drivers of cars normally use.
The big caveat is braking traction on a bicycle changes with the least ripple in the pavement. Whitt and Wilson discuss that too and I must pause to reread that.
To the point now raised a couple times about hitting pedestrians: Don't. If it's really about to happen and you can't stop, can't swerve, you should drop the bike. Bicycle falls rarely hurt much, rarely cause major injuries. Risking unknown harm to a pedestrian is not a good thing. Drop the bike. And sure, never coming close to this alternative is a good reason to slow down.
My Dutch bike doesn't really go faster than 12 mph, so yeah, I'll say that's my self-imposed speed limit. :)
ReplyDeleteI'm an urban cyclist in Los Angeles - there almost always traffic on some part of every ride. Sometimes intense volumes, sometimes high speed, sometimes just lost...
ReplyDeleteThe League of American Bicyclist teaches a maneuver called the "Quick Stop" in it's Traffic Skills classes. Adding that technique to my skills lets me stop in about 12 feet from full speed.
I generally cycle and occasionally get up to 24 mph (according to the cyclocomputer) on a modest downhill. I don't need to slow much to adjust to traffic conditions. I'm just ready to do a quick stop if I have to.
The maneuver take practice: you have to slide back off the seat to move your weight back toward the rear tire as you apply both brakes. Feather the rear brake to maintain maximum traction. You'll have to look this one up to see how to try it. It's great thing when you need it. I recommend practice!
So, traffic isn't anything that I change my riding speed for. Nonetheless, if there are a lot of people in the bike path or in the bike lane, I slow way down. Peds are pretty unpredictable and quick.