No, it wasn’t this bad before. I started driving in 2001. Since then I have noticed in the past, like, 8 years, drivers in the main become far more sloppy and distracted and, worst of all, slow. I can’t believe how goddamned slow so many drivers are off the line when the light turns green. I routinely get trapped between two red lights because some wanker up ahead waits three seconds before accelerating to 20-odd miles per hour (holding up the entire line of cars) because he or she was on their phone instead of paying attention to the traffic signals.
I do agree with most of what you said, or the intent.
Related to front tire lockup, it is far more possible that people would expect. Here is an example.
Related to friction and area, and related to the fact that the bike in that video was a GS, with hybrid tires (and no endo)You are referencing the classical physics Amonton’s paradox, where friction does not relate to area. Note that this does not apply in the case of stick and slip as in the shear forces generated at the tread–road interface are typically modeled as springs similar to bristles that cause the slip, and not static friction. Remember that a tire is a bending balloon and not a static surface.
As an example.
https://pdfs.semanticscholar.org/3594/f444fa9156ee9fd9e96ca156c3ca394dcebc.pdf
While there are factors like material choices and construction impacts this, my understanding it is typically carcass deformation that results in the initial skittering.
While the construction and the shape of a motorcycle and a car tire differ greatly, having a wider contact patch provides more “virtual springs” to spread this shear stress and greater weight to stop does impact this.
But the materials are quite similar, and Amontons’ 2nd law as it doesn’t account for the deformation that results form the shear force or the “true contact patch” but only the “apparent contact patch”.
Here is another link that demonstrates the complexity in modeling this.
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.816.6605&rep=rep1&type=pdf
Note that Amonton’s paradox has been resolved, as that classical model accounted for the apparent contact surface and not the actual or effective contact area.
[
](https://hal.archives-ouvertes.fr/hal-00394988/document)
For more information you can search for “Bowden and Tabor” and their papers from the 1950s.
Unfortunately, unless you are a graduate student, work in the field, or obsessively read papers there would be no reasonable way run across this updated information with the intentional hard split between classical physics and modern physics in the educational system.
But thanks for the correction.
Cool! I happen to agree with pretty much everything you wrote.
I should point out that I specified static friction and included the phrase “to a first approximation” on purpose, as I didn’t want to get bogged down in excessive detail.
You’re definitely more up to speed on the resolution of Amontons’ paradox than I am, and I appreciate your extended response. I happen to be in a position to appreciate all of the complexity involved in modeling friction in tires. Like, eerily so.
Funny you should mention those things. I was a grad student in engineering physics about fifteen years ago; my coursework included a graduate-level vehicle dynamics class and I did some research in a nanotribology lab, though that’s neither here nor there.
It happens that I do “work in the field,” as you put it, as long as the field is mechanical engineering. Mostly, I do finite element analysis of nonlinear structures, including, occasionally, hyperelastic/viscoelastic ones similar to tires (I haven’t modeled tires, though, because I don’t specialize in them; tire modeling is a career unto itself).
I’m getting more than a whiff of snark here…am I imagining that?
I guess I can see why someone like yourself might bristle at being told he held a common misconception about this subject, which is exactly what I told you. I didn’t mean to offend you (if I did), but in my defense, I had no way to know what you knew based on what you wrote.
I could tell you were probably a motorcyclist (as am I) but on the other hand, you mis-spelled “braking,” which didn’t immediately suggest a serious interest in the relevant theory. I don’t care about spelling or typos, but really, I had no straightforward way to intuit the depth of your knowledge here.
You had no way to intuit mine, either, except for maybe looking at my public profile. I don’t fault you for not figuring it out; maybe you could cut me some slack too.
Or maybe you didn’t intend to be snarky and I’ve misunderstood. Either way, I’m digging the ASME paper you posted. Thanks!
P.S. You’re right, of course, that a nontrivial number of motorcycles on the road are limited in braking by friction, not pitchover. I concede that I was thinking of sportbikes and things like GP motorcycles, which aren’t exactly prone to skidding their front wheels in straight-line braking. But that thought doesn’t jibe with the OP’s question or the post (by you) to which I initially responded.
Another reason I wrote what I did about pitchover because there’s currently a slightly silly debate in the bicycle world about road bikes with disc brakes. It’s common to hear people claim that, while mountain bikes are well suited for disc brakes, road bikes aren’t because “the greater braking force would overwhelm the smaller road bike contact patch.” Any decent road or mountain bike (save recumbents and tandems) is braking-limited by pitchover, not friction.
So I reflexively responded to something you didn’t say and did so in an excessively narrow way. Sorry about that!
To be clear, no snark at all. I personally take finding out when I am in error a happy event as I learned something.
I don’t know everything and no matter how much I do know it will always be eclipsed by what I don’t know.
I apologize if it came across in that fashion, the only snark in that post was directed at myself, because I tend to read papers when others seem to have more healthy hobbies.
(I have two custom bicycles with disc brakes btw, but that is because I commute in the rain and don’t like to wait for the pads to wipe the rim clear before they start to work)
This depends very much on the bike. a 600-cc sportbike (i.e. relatively lightweight) with sport (i.e. grippy) tires and a short wheelbase (i.e. center of mass relatively high and close behind the front tire’s contact patch) will be prone to pitchover.
Heavier bikes, longer wheelbases, and less grippy tires (i.e. touring or sport-touring) mean that front wheel lock-up is more likely to be the limiting factor.
I have a BMW R1200RT, which is more in the latter category. On clean, dry pavement, very hard braking engages the ABS on both wheels; the rear wheel gets very light, but I’ve never encountered a situation where I felt it becoming airborne.
As for why I ride the way I do:
If someone is tailgating me, I’m going to leave a very long gap in front of me so that I can stop gradually enough to avoid having my tailgater hit me. Even if nobody is tailgating me, I’m going to leave a long enough gap so that I can stop without much drama if the guy in front of me slams on his brakes. I also want lots of warning in case the car in front of me rolls over an object that could take me down. A 2x4 laying in the road won’t make a car crash, but it might make a bike crash; I’d rather go around something like that than over it. (this is part of why smart riders ride in the wheel tracks instead of the center of a lane: the wheel tracks are less likely to have problematic debris in them.)
If you’re talking about stop-and-go traffic jams, there are two reasons:
#1: your clutch hand gets tired of repeatedly pulling the lever (and holding it while stopped). Same goes for a trucker’s left foot.
#2: at very low speeds (less than first gear, i.e. clutch pulled in), it’s a bit of a self-challenge to see how slowly you can ride without having to put your feet down. Basically a way to pass the time under an otherwise boring riding condition.
Here’s a question for you: are you leaving a large enough following gap when you’re driving your car? Depending on who you talk to, the recommendation is 3-5 seconds. On the bike, since crash outcomes are worse, I’m going to tend toward the latter.
Regarding comments upthread about weaving back and forth in the lane, when you weave you cover a greater distance than when traveling in a straight line. Bikes that are geared tall have a hard time creeping with slow traffic without slipping the clutch excessively. I do this routinely right up to the point where I turn around and go another route. Or lanesplit!
One other thing that I haven’t seen mentioned yet: There are still a fair number of air-cooled motorcycles on the road (fewer and fewer in the US, as EPA regulations make it more difficult to meet pollution limits with an air-cooled motor). Sitting still in traffic severely limits the cooling ability of the fins on the outside of the cylinders, causing the bike to heat up. This can result in overheating. Sometimes, if traffic is stalled for extended periods, you may see bikers pull off the road and stop, or start filtering in-between cars. While filtering is also often done out of impatience with traffic, I believe it was originally intended to provide bikes with a way to keep air moving across their engines.
In second (or third) to the posters above, it’s also about trying to prevent getting pancaked by a read-ender and it’s kind of a fun skills challenge to try not to put your feet down. When we ride in groups and hit traffic, there is often a spontaneous “slow-roll” contest.
Hold on. It sounds like you’re saying most bikes have the same nearly-useless style of brakes as their pedal-powered siblings!!
I know the first motorbikes where just regular bicycles with an engine attached, but that was DECADES ago. But, surely, they’ve added some sophistication since then.
That was an off topic thread, and was only related to the post I was responding to, where the subject of disc brakes was mentioned.
With Bicycles there are two serious issues with rim brakes if you ride in the wet daily.
-
Soft rubber pads collect road grit, and this grit wears away at your breaking surface in an accelerated fashion.
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Due to lower pressures, softer materials, and higher speeds rim breaks need to wipe a layer of water before they become effective where disc breaks have.
a) more pressure
b) great distance from major water sources
c) stiffer materials
This results in about half a lane of distance difference in stopping in my non-scientific tests. People buy road bicycles pretty much based on what the bike racers use, irrespective of if it is better for them or not. While they have been working on getting disc brakes on road race bikes there is a problem about unequal breaking in large groups and potential dangers of discs working like a bacon slicer. This is changing right now in this time of history. (Note it also allows for wider tires which are more energy efficient BTW)
But on the subject of motorcycles, yes I use to commute on vintage motorcycles. 1967 Triumph Thunderbird and a 1969 Moto Guzzi Ambassador to be specific, for years. I gave up all of those bikes and even more modern ones for a bike with a slipper clutch, traction control, modern suspension and ABS. Those vintage bike’s brakes were marginal at best even with upgrades to modern materials.
Things have improved in the motorcycle world, but unfortunately they have improved more for automobiles. ABS and Traction Control can be far more aggressive on a 4 wheeled car even ignoring contact patch area etc… simply because they can mediate the lockup of two wheels per end, and can risk on wheel sliding for a greater time than would be allowed for in a motorcycle.
All right. I’m impressed.
Motorcycles are also required to have their headlights on at all times. Basically, for any bike built since the 1970s or later, if the key is in the “on” position, the light is on. We can’t turn the light off without shutting the bike off. Many motorcycles don’t put out enough charge when they are sitting still or crawling in stop-and-go traffic, so aside from overheating, you can also end up with a dead battery.
Same for me. I don’t want to get pancaked, and I like the challenge of trying not to put my foot down.
I have two bikes, a fairly lightweight Honda that takes off like a bat out of hell and stops on a dime, and a much heavier Harley that definitely does not stop on a dime. I can probably stop the Honda in close to the same distance as a car can stop, but definitely not the Harley. Even with the Honda, though, I’m not going to risk it. If the rear wheel locks it’s not that big of a deal, but if the front wheel locks, you absolutely have to react very quickly or you’re going to be sucking pavement.
Neither of my bikes has ABS.
motorcycles had mechanical drum brakes for a fairly long time, then like cars switched to hydraulic discs up front first. later many switched to hydraulic rear discs, but some (typically Japanese cruisers) still employ mechanical rear drum brakes.
What keeps a bike upright is a surprisingly complicated question but a fascinating one.
I get the OP is on about motorcycles and not bicycles but the same physics principles apply (I would think) to any two-wheel contraption when it comes to its balance.
(Someone provided that link to me here some time back when I asked about keeping bikes upright…I looked a bit but did not find it again to give proper thanks for the source here.)
An additional reason I followed further behind cars was because you never know what you are going to find on the road. (This is a variation on the previous comment about potholes). If a tin can or a piece of wood or an oil slick appears on the road, I can only see it after the car has passed over it, and I have a very short time to change my position. If I am in a car, I don’t worry much: other cars have already tested the obstacle, and the worst it’s going to do is damage the car.
I rode much further back in the wet: if I hit something on the road, the car behind me would be unable to stop in the wet, and was going to drive right over me. In the dry, I would mostly worry about the car in front of me stopping suddenly: I have to assume that the car in front was completely unaware of me (and wouldn’t care much anyway), as frequently demonstrated.
I stopped riding around 1990. I’m surprised at the consensus that “cars stop faster than bikes”. If that’s the case now, the stopping distance calculators and documentation on the web haven’t yet caught up with reality. An additional reason I could stop faster than a car was because I rode with my hand on the brake. It used to be that car drivers drove with the brake foot on the accelerator, and had to move it across to brake, which, particularly at high speed added a big step to their braking distance (even when trained and motivated). On most cars, that still the case.
Yeah, me too. My RZ stops so hard, my eyeballs hurt.
I don’t know that it’s really a consensus. It seems to be a hotly debated topic on the internet.
IIRC, the Motorcycle Safety Foundation does say this in their basic rider’s course.
I have seen a test of a car and a motorcycle, both with ABS. The motorcycle had the shorter stopping distance at both lower speeds and at highway speeds. My memory is a bit fuzzy but I think the two speeds they tested were something like 30 mph and 60 mph.
To be fair, it was one bike and one car. There are all kinds of different bikes out there with all kinds of different stopping distances, and the same goes for cars.
I can add the two more data points that I already posted upthread. My late 90s Honda Nighthawk stops in about the same distance as a car, and my Harley (also late 90s) doesn’t. Neither bike has ABS.
IIRC, ABS first debuted on a motorcycle in the early 1990s; it’s becoming more common, but only in a relative sense; I think most bikes on the road still don’t have it. Motorcycle tires do tend to be grippier than car tires, so if you have a very experienced rider, on smooth, clean, dry pavement, in a non-panic situation, he may be able to exceed what a car can do, even if the car has ABS.
That’s a very rare set of circumstances. Most of the time the pavement isn’t perfectly smooth/clean/dry, or the rider hasn’t practiced emergency stopping techniques, doesn’t know the limits of his machine, or has outright panicked. If he’s thinking clearly, he’s probably going to underbrake the front (where most of your stopping power is) and skid the back; if he’s in a panic, he’ll probably lock up both wheels and fall over (or go over the bars if it’s a grippy sportbike).
So I think the assertion that cars can stop faster than bikes is, on average, true: take an average driver in an average car, and put him next to an average rider on an average bike, and the driver will have a shorter stopping distance than the rider.
both my bikes have ABS, but I’ve only ever triggered the rear ABS on my Dyna, during a class.
I’m scared when a motorcyclist is on the road near me, scared for their safety, and try to avoid being in front of one. I hope they are operating safely. I got this way because of the time a biker did not operate safely and got hurt.
I was accelerating from a stop at a light, my left turn signal on, because I was going to turn onto the left on-ramp to the highway just ahead. It was dusk and I had my lights on. In my rear-view mirror I saw the headlight approaching, accelerating through the green light but then not slowing down, though I was accelerating at a lower rate. It all happened in 2 seconds. I felt the impact of his wheel on my bumper like a kiss and saw the headlight topple sideways. The crashed motorcyclist was lying in the gutter, semiconscious, groaning in pain and bleeding from his head. There was no helmet. I called 911 and waited until the ambulance and police came and I gave a report. It was on a Friday night in Twinsburg, Ohio.
I was not at fault. He hadn’t been paying attention where he was going. He chose to ride without a helmet and got his head bust up. Still, though it was over 30 years ago, I cautiously keep as much distance from motorcycles as I can.
To the posters on Amontons: I can’t put my finger on what is paradoxical being discussed in Amontons’ Laws discussed above. Rather than unduly stress the OP here, I have bumped
** Did I just lie about Galileo to a fat kid on a sled?** which is essentially on that topic as much as this OP is, but I think at a level better suited to “whoah, huh?” posts, as opposed to those on first-order approximations and their alternatives.
Rather than x-post, I ask here my “
” and if any help can be posted to that fat kid thread.
The online debates concern training and technology.
When I took lessons in 2002 they were teaching that motorcycle riders with track experience could stop in a shorter distance than cars.
However this was comparing riders who had been trained on how to blip their brakes to the verge of locking up, compared to a driver who was locking the brakes and skidding to a stop.
Since then ABS in cars has become the norm (although I don’t believe it’s mandatory in US or Canada) and cars so equipped on a flat surface will be stopping in a shorter distance than a motorcycle.