dougmc

As the "Edit:" probably suggested, that wasn't there originally. I went looking for your memo, and I guess I found it.

It's not obvious to me. Can you explain how this movement of the body mass will help one stop faster than somebody who's butt is already back?

Could you start by explaining what sort of motions exactly you're talking about here? What does this cyclist do and when? I don't see a lot of room for moving around to improve your braking performance (beyond throwing your butt back at the start, of course, and maybe hopping up at the last second so you can squeeze the brake a little harder) but you seem to have something in mind.

As I see it, you can move your body around, but not that much -- and once you've moved it all you can in one direction, it has to stop, and that stopping has the opposite effect (on the bike) of whatever starting the movement was. So a sudden jump at the very end could help a bit, but beyond that ...

Max sustained deacceleration, or max instantaneous deacceleration? Have you actually measured somebody slowing down from, say 30 mph (fast) to 0 and averaging 0.9 G over that interval? The math says they should be able to do 0.8 G it just by moving their butt back, but did anybody get more than that?

Good point, I hadn't considered going sideways. WIth the bike at the right angle, it won't endo or fall over. It's still dynamic friction rather than static friction, but your braking isn't unloading the back tire like it does if you're just going straight.

Indeed. If you're skidding your back wheel sideways, you're somewhat out of control. Make any mistakes, and you're on your face or you put the entire bike down. I wouldn't want to rely on that in traffic! Still, 0.5 Gs from that, even with everything done just right, seems too high. Did you actually measure that?

Nah, I imagine it was fine, given it's assumptions. It just assumed people stayed in their seat, and I'm still not sure about these movements you're talking about. Moving your butt back, that changes the center of gravity, so the same formula would be used -- you'd just alter the geometry somewhat. Not sure about the movements.

In this particular case, I imagine he did the math for a specific geometry, assuming that somebody kept their butt in their saddle and didn't move around. The figure seems to be in the right ballpark, anyways.

sudo bike

I think, the point is, a skilled fixie rider who rides within his limits is, of course, able to ride safely. But as far as vehicle code is concerned, do we really want to start from the position of "If a skilled rider can do it..". I think the real question is what the average rider/driver can do, and what requirements should there be with that in mind... from my personal experience the average brakeless fixie rider is at a huge disadvantage, in terms of safety, as the average cyclist with a brake. Certain safety standards for vehicles on the road is perfectly normal and reasonable, and I think having a brake is certainly within that realm.

In terms of this conversation, the braking discussion between dougmc and RobertHurst has been more interesting to me than anything else, actually.

GrandaddyBiker

A guy name Evel Knievel made money back in the 70’s doing daredevil shows. I got to thinking I could make some money too. I could get me a fixie with no brakes and then publicize an event and sale tickets. People would pay money to see an old man like me ride a fixie with no brakes, don’t you think?

AlmostTrick

As long as you're jumping busses or canyons, sure!

dougmc

It's not entirely clear how many bones he broke during his career, as there's some disagreement -- but however you count it, it's a lot. Any money he made, he earned the hard way.

https://www.stevemandich.com/evelinca...elinjuries.htm

delcrossv

Agreed, it's geometry limited for a standard bike. Some recumbents (lowracers particularly) can pull more g's as they don't endo. There, disc's make a lot of sense-you can brake both wheels very hard and just stop- really fast.

RobertHurst

I'm not entirely sure, but I bet someone who remembers physics class could put a name to it. Consider how the mass of the body dominates the bike-rider system. You are not riding a machine -- you are the machine.

In any case, I think if you go to a safe location and practice stopping as quickly as humanly possible on your bike (while not crashing), you will find yourself heaving your body back with respect to the bike. It absolutely will become obvious to you. Because, that's how it works. Everybody I know seems to do it roughly the same way, just like they turn in roughly the same manner, and bunny hop using roughly the same technique, etc. You have to move the body in certain ways to make the bike to certain things.

Putting your weight back, keeping it there, then hitting the brake will not bring a similar result, I assure you. The movement needs to occur during the deceleration.

Practice, and the timing of the move will become automatic, just as the moves needed to balance the bike are automatic.


Well, I don't know. Try it. Feel it. Certainly you don't brake hard while sitting on the seat? Or keeping your body in one static position? I hope not.


Without an accelerometer, I guess you could estimate instantaneous acceleration along the curve using calculus, given the g of the stop. I'll leave that to somebody else. We had several riders doing around .9g from 20 mph, which is an easy speed to reach and maintain. Of course such an experiment is still hard to control, and the slop makes a big difference -- there is an exponent in the formula. But if you get lots of dudes doing about the same thing with a calibrated speedometer, over and over, and all of them having no problem whatsoever with .8g and going over that mark you start to get the idea. (And slop works both ways in this case, sometimes the riders would be over 20 mph, and the slope was slightly downhill as well...) I would say given the slop in the experiment that several of us could do near .9. Around (over?) .9 and control is difficult to maintain, at least for me (back wheel waving in the air, jackknife, scary, etc.). It gives a fair idea of where the real number is. I wondered how Forester, et al could be so wrong, and saw that the old formula assumed a static seated rider; and that is not how people who know how to ride use the brakes.

I mean, who rides like that? Hard braking while seated? And sitting still like a statue? I shudder to think.


I measured but can't remember the exact numbers for the track guys as I was more interested in max g and they were more trying the experiment for novelty purposes. We didn't have any world class track riders that day but they weren't ridiculously far off the mark. None of that .1-.2 stuff as claimed in this thread, much better than that. But skill level has a big effect on what they can do.

Folks probably don't realize exactly what's going on out there these days in terms of 'skilled' track bike riders. Lots of people on track bikes can whip a controlled 180 or more pretty much on a dime. They flip a fast 180, with the rear tire on the ground the whole time, feet on pedals, then ride out backward. It's a common trick now. Check out the popular hipster video MASH for more on that. Under very hard braking on a track bike, that's pretty much exactly what the bike wants to do anyway. The back end comes around.

Another thing that people should realize -- panic stopping on a fixed gear will tear up your knees. I mean, literally.


In terms of giving us a number for max deceleration on a bicycle, the formula is an airball.

It's a perfect example of how false assumptions will render a fancy formula completely useless. Information is not the same thing as knowledge.

delcrossv

In less words, you're moving your center of mass back to keep the center of gravity behind the front axle. I'm not really convinced that in a panic stop an average rider could pull off that motion fast enough and far enough to provide of a .9g deceleration. (without going over the bars)

RobertHurst

Me neither. Skill level, body control, eye-hand coordination, whatever, has a lot to do with it. It's not like stopping a car.

In the real world, all that skill won't do much good if the reaction time is poor.

dougmc

I do remember physics class -- I have a degree in it.

Of course the cyclist mass dominates. But if you heave your weight backwards, that will push the bike forward. But you can only heave your weight backwards a few inches, and then the heaving stops. And this stopping puts exactly the opposite force (well, technically the opposite affect on the momentum) on the bike, pushing the bike back.

Sure -- you need your CoG as far back and as low as possible, and unless you were already riding like that, you need to get like that as soon as possible if you want to maximize your braking potential.

I absolutely see the benefit of having your butt back off your seat -- it moves the CoG where it should be for maximum braking. But any moving around beyond that, I'm still not so sure.

I don't see how you'd do this estimate, beyond simply taking the "g of the stop" and saying that's your estimate. (And really, if you're moving around, the question becomes "instantaneous acceleration of what?", because every part will have a different answer.)

Without an accelerometer, how are you measuring acceleration? Looking at braking distance and then doing the math? This could be tricky to measure -- at 20 mph, the stopping distance at 0.8 Gs is 4.5 m and at 0.8 Gs it's 5.0 meters. 20 inches difference. But at 20 mph, the bike covers 20 inches in 0.06 seconds, so the rider has to be more precise than that when he starts braking at the line or it'll mess up your results by more than the difference you're looking for.

As I see it, some sort of accelerometer would be very useful here. Something that can record the acceleration many (50 or more?) times a second for analysis later. (Higher speeds would reduce the need for extremely high sampling rates.) Mounting it to the bike would probably be best rather than to the rider, as the rider can rotate and the bike probably won't (if it does, that won't be a test we'll want to use.) Modern cell phones often have accelerometers, but I suspect they're not fast or precise enough.

I don't really doubt the 0.8 G figure -- if that's what the formula says for one's butt thrown back, it seems reasonable. Certainly, it doesn't even require that much skill to keep the brake right at the cusp of doing an endo (as long as you're not distracted by other things, like traffic or death.) I'm just sceptical that moving around will make a significant difference, beyond that initial "get your butt into position" shove.

I guess I'll have to try it -- though the difference is small enough that I don't think I'd be able to accurately detect it without the accelerometer I mentioed earlier.

RobertHurst

It seems that the 'throwing the bike forward'/throwing body back is just a way to exert a more intense counteractive force on the force which is pushing the body forward during deceleration, and therefore a way to apply more stopping power and achieve a shorter stop.


You are the one who brought up instantaneous deceleration. I have no interest in it whatsoever. The formula for g-force is a simple matter of stopping distance and initial velocity.

The stop is measured by the front wheel, of course. Measure the stop by the front of the front tire, or hub if you want. That's going to be the most forward point of the bike-rider system in controlled stops.


That's exactly what I said. Slop can screw up the measurement. But the slop works both ways. And the slope was slightly downhill too, which is also a major factor working the other direction (lengthening stopping distance). Add a lot of repetition and you can get a pretty good read on where the real number is, despite the slop.

g = (v)(v)(.0333)/stopping distance

It's a really simple experiment. I personally have measured, I don't know, at least 100 of my own stops, and lots of other people's too. Also, practicing actual hard stops is a lot more useful for your safety than talking about them on the internet. Although it will jack up your back tire (if you're doing it right).

You don't know how many times I have had this exact conversation.

You don't need the precision of an accelerometer to see what basic differences in technique will do. The difference is measured in feet of stopping distance.

In any case, if you had an accelerometer, all you would see is that different riders have different skills, and different stopping distances from the same speed under the same conditions. It won't tell you what the real max deceleration is or give a more accurate estimate of that threshold than a simple experiment with speedometer, tape measure and repetition.

The only reason .83 "seems reasonable" now is because of the physical experiments that I performed, which led Joe Riel to create a more careful formula. Before that, the results of my experiments were met, universally, with haughty derision. I was informed, in the most condescending tones imaginable, that .8 was not only unreasonable but completely impossible, due to the laws of physics. The world needs more people like Riel, who, instead of entrenching himself in old assumptions, took another look. But Riel's formula is still too simplistic. Why am I so sure? Because of the physical experiments that I performed.

Doesn't require that much skill to ride the front wheel under hard braking?? That's a tricky move imo. And it's mostly about body control, not brake control. I once worked with a guy who could ride around corners and up curbs on his front wheel. I can't do that. Very few can, but it is possible. Super insanely relaxed body control. If the back tire is on the ground or near the ground, you are nowhere near the 'cusp' of an endo.

You won't even have to measure the stops I suspect, or check em against marks on the pavement -- the difference in feel and control should be obvious. Please be especially careful though when braking hard with a static body position. That's a recipe for disaster.

Bikeforumuser0017

Sorry if I piss anyone off (who shouldn't be pissed off) for me bringing back an old thread, I just wanted to give my say.


I am an experienced cyclist from San Francisco who has 3 bikes, a geared road bike, a mountain bike, and a brakeless track bike. I think it's easy to tell who's riding a "fixie" just for the looks, and who's riding one for different purposes. I will admit, I kind of do ride brakeless for the aesthetics, but as a daily commuter who pretty much goes everywhere with a bike, there is more to be said about riding brakeless.

I learned how to ride a bike on a mountain bike (as pretty much everyone else does, right?) when I was maybe in first or second grade. I never really biked for fun/transportation until I got into Highschool when I got my first road bike. There I would do 50+ mile rides with the friends who brought me back to biking on the weekends and then less than a year later I got a track bike, brakeless OF COURSE, right? The first few rides I had on a track bike were VERY intimidating and really did believe I needed a front brake, but already having thousands of miles of experience on my road bike and the legs required to stop a brakeless track bike, I quickly adapted to riding this new kind of bike.

I bought a track bike because I needed something theft resistant, mechanically reliable, and "inexpensive" bike. I put "inexpensive" in quotations because I have already invested around $800 in upgrades to my track bike to the point where the only stock parts are the frameset and headset, but this bike was intended to be a beater. Already putting several hundred miles on my track bike (as it is my daily commuter), I have learned many skills. One thing is efficiency, on my track bike, over time I have learned the exact speed to go so I rarely have to suddenly brake (as I use to always do this on my road bike). I have also paid more attention to traffic lights so I know if I can or can't make a green light a block ahead of me. By far, the most important skill I've learned from riding brakeless is maneuvering around things instead of braking. Yes, I will admit using your legs to brake takes longer than using rim brakes, and that usually means having to go around things at speed. This has made commuting for not only faster and more fun, but also more dangerous and it's obvious why one would wan brakes on a fixie. But why ride a fixie with a brake? Are you training for the Tour de France (some riders used to train on fixed gears)? Do want the "zen" of riding fixed (this is really BS)? Why have a brake on a fixie if you can have a brake on a freewheel single speed? A TRACK BIKE WITH A BRAKE AND FREEWHEEL SINGLE SPEED WITH A BRAKE LOOK EXACTLY THE SAME.


So there, sorry for ranting a hit at the end as well as getting off topic. To make a long story short, I ride a brakeless track bike for the reliability, theft resistance, but also just because it was different from my 2 other free wheeled bikes.

DX-MAN

CB, most of the time, I agree with you; this time, sorry, but no.

It DOESN'T satisfy the legal requirement; there's no brake mechanism involved, like the very similar coaster brake does have. But, really, more telling -- why does someone have to DIE before it becomes an issue? The family of that first dead person would ask you the same question.

There's a lot of noise locally (for me) about "spice", the synthetic drug; the current coverage doesn't talk about any deaths, but focuses on the problem of addiction and criminal interstate trafficking. Many would say using such crap is a choice, no one else is being hurt (like you point out here), but I'm all FOR some enforcement on that. THAT is the part of "the war on drugs" that I support -- get the dangerous "designer" synthetic shat off the streets; people still get 'mickeyed' regularly. I have an adult niece who was almost committed because of the effects of a mickey (a dose of Ecstasy that was, my sister was told by the doctors, fatal for TWO people). She didn't knowingly take it, didn't ask for it, and has lost a big piece of her life because of it.

You and others may resent this analogy, but I feel it's right on.

LesterOfPuppets

We get plenty of people in the shop that have less braking ability than a brakeless fixed gear.

Lotta kids running brakeless freewheel BMXers.

I doubt many people really care about enforcing bike brake regulations outside of a handful of big cities. NY, SF, Chi, etc.

-=(8)=-

Fixies are the Stiletto hi-heels of biking.
You can find a lot of people who say stiletto's are great for running in, also

JoeyBike

Most beach cruisers can't operate a coaster brake without a chain either. Kids bikes? Oh, the carnage!

squirtdad

I think you answered you own question.....because it is safer. I see lot's of fixie riders (mostly teens) and some have mad skills others can just barely skid the rear wheel and as often as not lose control.

My teen age son rides a fixie with a front brake. I will say that I gave him no choice on having a brake...that was not negotiable (and as I built both of the fixies he has they were delivered with a front brake) But he likes the feel of riding fixed, being able to do little things like long track stands and going backward. He also has a torn ACL that can't be fixed until he quits growing and did not want to be strapped into the pedals.

so he gets the fixie feel and capabilities, but with added control and safety of a brake, without having to strap in.

dougmc

Why have a brake on a fixie?

Fixies can do things that a bike with a freewheel can not. Mostly these things are about "tricks" (riding backwards, etc.) and forcing you to pedal rather than letting you coast (which I'm told has training advantages.)

A brake on a fixie, especially a front brake, can stop it better than backpressure on the pedals, and it's far less rough on your tires, so it's certainly an advantage there.

If by "freewheel single speed with a brake" you mean a coaster brake, then they don't look exactly the same. In particular, the coaster brake almost always has a reaction arm.

If a cop hassles you, you can probably just say you have a coaster brake -- pedal back to stop -- and unless he knows bikes he won't know the difference, unless he asks you to demonstrate. But if he does know bikes, the lack of reaction arm will be obvious, and he'll have noticed the lack of it already before he starts hassling you about not having brakes.

Reliability? Is it really more reliable than the same bike with a front rim brake added?

As for theft resistance, does anybody really believe that thieves can't ride a brakeless fixie? They'll be more likely to crash, sure, but then again ... so are you.

If you want a bike that the inexperienced have a hard time riding, try a tall bike, or a SWB USS recumbent. Or a swing bike.

Now, that reason I have no arguments with.

JoeyBike

This thread reminds me of all those "anti-red-light-running" threads here. A lot of folks with absolutely no point of reference ASSUMING something is inherently dangerous because they have no understanding of that thing. Or just flat-out trolling for trouble.

gcottay

My few urban fixed gear miles were on a borrowed bike with a front brake. The owner called it an "emergency brake." As a decidedly novice and wholly non-expert fixie rider I made multiple stops, but never had to use the brake. I fully expected using the it, after all, I always had. Bikes need brakes. I quickly found, however, that riding a fixed gear bike is a remarkably different experience. No matter what our stopping systems we can get ourselves in trouble, but I don't share your assumption of necessary hazard. In some environments even this inexperienced fixie rider may even be safer than on my usual rides with their disk brakes.

Bikeforumuser0017

If you're doing tricks on a brakeless fixie, you shouldn't be doing it in the streets (I'm assuming barspins and keos), so the advocacy part of this is out of the picture.

dougmc

Not at all.

First, 1) people *do* do tricks on the streets, and 2) even if you're a responsible bike rider and don't do tricks on the street, you still have to get your bike to where you're going to do your tricks. So do you put it in your bike rack and drive there, or do you ride your bike there? Or do you mount a brake, ride over, then remove the brake once you're at your destination?

If you don't ride your brakeless fixie on the road, then I'm not suggesting that you ought to get a brake. But I'm assuming that your brakeless fixie *does* make it onto the roads, and once on the roads you'll possibly encounter 1) situations where the braking ability of an actual brake, especially a front brake, might come in handy, and 2) police officers who feel that your legs are not a "device" and so your bike is not equipped with a brake and will ticket you for it.

You certainly can have a brake on a fixie and still do barspins (and keos are even easier -- standard brakes are OK for that.) You could use something like the BMX gyro (and that's not the only option that allows a front that can be spun) and still have your brake on the front or back wheel, or you could mount the brake handle to the frame or seat post rather than the bars and have it brake the rear wheel. Not as effective or as convenient, but 1) it keeps the law satisfied, and 2) still gives you an emergency brake if you prefer to do most of your braking in the usual fixie ways.

squirtdad

I have tried my son's fixie....it is really different, kinda cool, and my knees simply can't handle it. Or maybe it is can't teach old dog new tricks. (I still hop on it every so often to see if it works for me with the general result of a laughing teenager)

Any other comments are based on observation, which is around here the average fixie rider is 15 to 20 and for every one that has really good skills and proper foot restraints so that they can ride in control are about 10 that don't have the skills.

Separate from the skills is, that probably due to the age group/hip/cool factor or whatever. many local fixie riders tend to be more noticeable when they are not in control because they salmon/ninja/run reds/ and otherwise act like teens.

The extreme form of stopping for fixies is a skid, which by definition is loss of control. The mad skils guys are impressive in their ability to handle this, the less skilled guys not so much.

San Rensho 

Fixie on the street, a bike that is in the wrong gear 95% of the time, that you can't go around corners fast. Not much else to be said. If you want to limit your riding so much, go right ahead. I have a track bike that I ride on the track, for what its designed for.

By the way, fixies are so 2000snds. You are so far out of style.

rydabent

Well I will post it. Bikes without brakes are stupid and dangerous!!!!