From time to time, riders post here about bicycling accidents and generally other (reasonable, wise, helpful) people advise the Original Poster not to put details online, which has always seemed prudent.
Today I saw evidence of why that's good advice, in a story about a SF bicyclist who struck and killed an elderly pedestrian and then posted about it in a bike forum.
Cyclist Who Struck Pedestrian At Castro: "I Just Plowed Through The Crowded Crosswalk""
In a message posted on the Mission Cycling Club forum, member C--- B---- apparently identified himself as the cyclist responsible for the crash in an incredibly detailed post describing his early morning ride from San Francisco to the Marin headlands and back. ...
The news media, the police, the dead victim's family, and the litigants all have copies of the bicyclist's initial posts before they were deleted - and they also got copies of his GPS-app data from Strava.com that showed him doing 35 mph descending into the intersection (apparently it auto-posts).
So it's good not to put anything online that you're not willing to see on the front page of the newspaper or the drudge report, and - chances are - the first few hours after an event, you shouldn't be publishing anything. And my respects to the folks that have been saying that, all along.
cheers, V.
"way too committed to stop" == "riding too fast for conditions".
Of course, since he's a cyclist there will be outrage and he might even have to face some consequences for his actions. Certainly he deserves to if the facts are as presented, but I'd imagine if he were in a car this would just be another routine pedestrian killing we'd never hear about.
That cyclist made a whole flotilla of bad choices. But wow, there are a lot of assoles in SF, and I'm not talking about the cyclist.
"...and he might even have to face some consequences..."
I certainly hope he will. And not because he is a cyclist, but because his actions resulted in that person's death. The cyclist here is not being singled out - except by himself!
While I agree that motor vehicle v. pedestrian deaths are common enough to be considered "routine," that does not mean we should accept them, nor accept it when a cyclist does the same.
@Salty "way too committed to stop" == "riding too fast for conditions".
+1
True for bikers. True for cars (particularly tailgators), and even true for joggers. Too fast for a timely stop is too fast.
That being said, there will probably be charges filed and widespread anger - and there would not be those things if the driver was in a car. It would just be one of those things.
See the "No charges were filed" thread.
i don't know, i think anyone who plows into a crowd of people, whether it's bike, car, etc, will garner a huge level of outrage.
Automobile drivers seem to be under the misapprehension that they will be severely prosecuted under the law for an infraction against a cyclist, this is a recurring meme in any comments section of an article of this type. I say this is a myth we do not want to bust. Let the sad and desperate auto defenders whine all they want. Imagine if they knew the reality.
35 mph in a city. That's pretty impressive/stupid.
Well Liberty Ave has a speed limit of 35 m/h. As well as West Liberty Ave. And the later one has a long descent if ride towards tunnel and you can get up to 35 m/h on it. So it depends.
That's not the point.
How long does it take you to bring your bike to a complete stop from 35 mph in a panic stop (without endo-ing and splattering yourself on the pavement)? I bet it's double if not triple that of what it takes your average car.
SINCE it comes up, panic stops are an excellent thing to practice in a safe, off-street, non-emergency situation. Find an empty parking lot or alley, get up to speed, and practice numerous full-on, both hands, hard-as-you-can stops, maintaining control. A good way to be ready when it is needed.
You should be able to stop your bicycle in the same distance as a car would take to stop from the same speed -- regardless of the weight of the cyclist etc, and assuming the same reaction time for the driver and cyclist.
The reason for this is that the frictional force needed to stop and the energy possessed by the car or bicycle are both proportional to the weight of the vehicle. So the two factors cancel out, as does tire pressure, and the number of tires. All that counts is the tire material, how much friction the brakes can apply to the wheels, and, of course, the road surface conditions and the vehicle speed.
Of course, in a panic stop you might lose control of your bicycle and end up flipping or falling over, but that would probably shorten your stopping distance further, because of the loss of energy in the crash.
This came up in a thread a year or two ago. Upright bicycles in the best conditions on level dry pavement can brake at a max of about 2/3g. At higher deceleration than that, you'll flip over the bars. Most cars on dry pavement can brake at close to 1g.
Wet pavement or a looser surface changes things a bit, but generally cars, esp. with anti-lock brakes, are better because it's easier to get right up to the limit.
@John, that seems reasonable. So, a car at 35 mph can stop in about 82 feet, where a bike would take 123 feet. Either way, that SF dude was going way too fast.
What John said is consistent with my experience. Other than a truck with a heavy trailer behind it, I've never operated a vehicle that stops as poorly as my road bike. Also, I'd agree that panic stops are well worth practicing. It's a skill worth having, so you don't find yourself "way too committed to stop," like the topic of this thread.
@john What about a fixie with no brakes? Skid vs no-skid.
With stopping it all comes down to the tires contact patch. The bigger the patch the more friction you can generate. So tire width is probably more important than rider weight in most instances. But distance equals velocity squared divided by twice the acceleration so stopping distance is most affected by your speed in real world situations.
Also, race car designers do all of that fancy suspension stuff to keep the tire in contact with the pavement. So in a panic stop on a bike you want to throw your weight rearward not just to prevent going over the bars but to keep the rear wheel in contact with the ground. I’ll let someone with better knowledge and experience than me explain how to actually perform the task.
@marko82 Actually tire width doesn't matter at all. The tire will always deform so that the area of tire in contact with the road, times the air pressure in the tire, equals the vehicle weight. So the total friction equals the vehicle weight times the frictional coefficient of the tire material and road, independent of tire shape etc.
@dmtroyer On a fixie without brakes you're stopping with the rear tire only and as you brake the weight shifts forward onto the front, non-braked, wheel and the back wheel starts to skid. So you have a tight limit to how much you can brake and I would imagine you can't get near the 2/3 g force @John mentioned.
With rear wheel-only braking the best you can do is about 1/3g -- if you can manage not to skid the wheel. Skidding reduces friction even more.
never underestimate the biggest factor in any cycling characteristic - the rider (skills and attentiveness). I think that's the biggest determiner of stopping distance.
But if we're going to brush that aside and get technical, it's the friction of your brake pads on your (bike part - disk/rim) that will actually slow you down, unless you're suggesting you've locked up your wheels with your brakes.
If your wheels are still rotating, brakes matter second most. If your wheels are no longer rotating, your tires matter second most. Just wanted to be clear there.
I don't like panic stops. And that's one of the reasons I don't like going fast.
@jonawebb, reference??
@marko82, my head. I figured this out myself, from basic physics, while on my commute. The downward force from the tires has to equal the weight they are supporting. So they must deform until the area times the tire pressure equals the vehicle weight.
FWIW, when I need to stop a road bike in a big hurry, I will sit up/lean back to transfer more weight to the back wheel (which also lets your upper body act as an air brake), and tighten my arm muscles so that I stay there.
Not sure that this furthers the discussion, but to add to Marko's comments, some race cars use a "balance bar" master cylinder setup from Wilwood Engineering or a similar setup which uses dual master cylinders, one for the front brakes and one for the back, and a "balance bar" between them which lets you manually adjust how the pressure is distributed between the front and back brakes. The basic idea is to match the brakes' power with how the car's weight is distributed front to back, so you can get all 4 wheels as close to max braking as possible. We can do this a little easier with bicycles and motorcycles since the front and back brakes are mechanically separate. The problem is we have to adjust the weight bias "manually," by shifting our bodies on the bike.
Amontons' Second Law: The force of friction is independent of the apparent area of contact. (Source, the Wikipedia, and a hazy memory of high school physics.)
@edmonds panic stops are an excellent thing to practice...
what he said.
@Jacob, I think maybe they want the tread to last longer. As the weight shifts to the front, the front tire deforms to take it and the total friction stays the same (except that I guess the sidewall might then be in contact with the road. Maybe that is what they are worried about.)
On a bike I don't think this matters that much. I'd guess the best strategy is to keep your weight as low as possible so it takes greater force to flip you over the front wheel, and you can therefore get closer to maximum braking without flipping.
I don't know the physics of friction well enough to debate it one way or the other, so I'll take your word on that aspect of this. I do know that the balance bar setups are not designed to affect tire wear, they are designed to let you dial in as much brake pressure as a wheel can take given a vehicle's weight distribution, weight transfer during braking, and probably other factors. The "To win ..." books written by Carrol Smith, I've been told, have some good discussions on this.
I'd agree that weight distribution on a bicycle is not a huge deal, but when you are at the absolute limit of braking traction, as I found myself recently when a car unexpectedly panic stopped in front of me, every little bit counts!
I don't know the minutia of the physics of this either, but my experience with road bikes, and about 40,000 miles of superbikes on the street, is that all else being equal, sitting up and getting your upper body into the wind will slow you down better than getting low and bracing yourself against an endo. If you watch a Grand Prix motorcycle race on TV, the racers sit up, not tuck behind the windscreen, when they approach the corners. I imagine the same holds true if you watch the descents in the Tour, but I'm not certain. Anyway, interesting discussion, as always.
Jacob, in simplified model friction depends on pressure (taking into account that other stuff is equal for bicycle, car, motorcycle). Pressure is force per square unit. That is why, as Jonawebb pointed, width of tire is canceled. Aerodynamic forces start to play a significant role if speed reaches 18-20 mph. For a bicyclist at the speed of around 25 mph around from 75% to 90% of all energy bicyclist puts into pedal is spend to overcome aerodynamic forces -- depends on position on the bike. And you are right that at speed 35 mph aero breaking is very efficient. I am using it all the time. 
E.g. if take Stanton toward Butler you can up two 28-30 mph pretty easily (I can get more due to my weight -- I've tried) sitting in aerodynamic position. But as soon you sit high, one leg extended, knees out, elbows out, 24-25 is pretty much a limit. But you have to feather your breaks now or you get over the bar easily. Center of Gravity is much higher.
I would hesitate to count on too much similarity between GP motorcycles and bikes. In those races the riders are bringing their speed down from 170+- mph to around 50 - 60 for corners, and in that range aero braking is MUCH more significant than in the bicycle range of <25 mph.
Also the rider body weight is much less of a percentage of total vehicle weight on an MC than on a bike, so sitting up has less of an effect on the location of the CG.
In a panic stop on a pedal cycle I would rely much more on getting my body low and back than I would on sitting up and aero braking.
@edmonds59 In a panic stop on a pedal cycle I would rely much more on getting my body low and back than I would on sitting up and aero braking.
+1.
Aero braking is fine for gently slowing on hills and such.
No use in a panic stop - unless you are in the process of getting off the bike, anyhow (The imfamous "skin-on-asphalt" panic stop. Very efficient for stop-or-die situations, but sometimes dying seems preferable.)
I stopped reading after Heffner posted but he's right about bicycle accelerations. Cars can stop faster. Some jerk that buzzed me Tues (on a bike) got a quick lesson in physics and karma shortly thereafter.
http://eecycleworks.com/VNJune%20BrakeTest.pdf
Pretty interesting data in that test although we kicked around the pros and cons of it before.
Interestingly, the panic stop decelerations they report there mostly exceed the 2/3 g value @John reported earlier -- 8.69 m/s^2, 8.52 m/s^2, 7.6 m/s^2, etc. The 2/3 g value sounded reasonable to me. I doubt you can get to 88% of g (the 8.69 figure) on a bike, or that your ability to reach that value depends that much on which brake you are using.
That's what happens with blanket statements.
The point is EVERYBODY can bring their car to a stop within XX number of feet, whether you're a brand new driver or a professional racing driver. On a bicycle, to get in the same ballpark of that stopping distance, you need to have a LOT of skill.
35 mph in a city with traffic and pedestrians is suicide (or homicide as in this case).
