phoebeisis

Right-you can design a tire with .86 "friction adhesion"-for a bike or a car-heck "rubber is rubber" (yes it isn't rubber anymore)
But that doesn't mean a BIKE rider can actually brake to the limits of it.
ANY IDIOT driver with ABS can pretty much brake right to the limit of adhesion.
.8g for me means 145 lb forcing my hands into the bars-a stiff sudden bench press(some of it would be pedals some of it the seat-but most would be a bench press with me having a tendency to pivot up and over the bars)- yeah very very few riders can brake like that.

And bikes are more sensitive to road surface-once again-ABS adjusts for grit and slippery crap(and water) on the road surface
and ALL roads have crap on the surface-a bike riders adjusts to it by losing traction- falling-scraping along the tarmac

I-Like-To-Bike

I used to have one; it only slept 20 hours a day.

gsa103

Coefficient of friction for cars varies widely by tire type.

Racing slicks (at temperature) have coefficients significantly >1.0, because the tire effectively "sticks" to the road. An F1 car can accelerate at ~1.5g.
At the other end of the spectrum you have low-cost, low-rolling resistance tires which are probably at or below bike tires.

The stopping distance of cars also varies widely as it does with bikes. A good racing bike with Al rims and dual pivot calipers might be able to out-stop a cheap commuter car with poor braking, but will lose badly to a BMW.

At the end of the day, the only bike that can consistently out-brake cars would be a downhill MTB with the seat lowered. An average upright road bike will endo at ~0.6g, while even a Toyota Corolla with mediocre brakes (30-0 in 33 ft) can stop at ~0.9g.

phoebeisis

Downhill MTB-with lugged tires-will brake "not great" on concrete-
Probably brake GREAT on softish surface-maybe over 1g-digging in with those lugs-plowing its way to a stop.

Dragsters-almost 350 mph in 4 seconds-
0-500+ fps in 4 seconds- 125fps/s acceleration- OVER 4 GS
Yeah-obviously there is some molecular bonding going on-not just gravity sticking those surfaces together.

dougmc

It's not quite that bad.

Not all your weight would be on your arms. Most, perhaps, but not all. (As you already said.)

You're ready for it (well, you should be), as you're the one who pressed the brake levers. Bracing your arms should begin even before you start pulling the levers.

If you're strong enough to do one push-up and hold yourself at the top for a few seconds, you should have enough arm strength to handle maximal braking. And really, it's not the pushup that matters, but holding yourself at the top of it, because if your arms are fully extended when you start, you're not actually doing a bench press because all you need to do is keep your arms straight -- you're not lifting the weight from your chest, but instead starting with it already above you and keeping it there with your arms locked. And only for a few seconds. If your arms aren't extended when you start, you'll want to get them extended as soon as possible, which will mean lifting yourself against the force of the braking -- which would basically be a bench press, though you can control the weight that you're lifting against with how hard you brake (by reducing your braking for a bit) if needed if you're skilled enough.

Most riders seem to ride most of the time with their arms mostly extended, though TT bikes seem to have their arms bent pretty significantly most of the time. If you're out of the seat, you'll have to push yourself back into the seat (and maybe beyond) so in that situation it would be a lot like part of a bench press.

Now, certainly, not everybody can do one pushup and hold it for three seconds, but I imagine most can. Some inexperienced riders do get surprised when they try to stop quickly, let their arms buckle, they fly forward, moving their CoG forward which makes them even more likely to endo in addition to risking injury to their face, front or crotch from crashing into something on the bike, but with some experience people can avoid this.

Really, people should practice hard braking occasionally and get used to how it feels, learn what their own limitations are by starting out small and ramping up the braking rate as you get used to it. If you don't get your arms locked quickly and don't reduce the braking force when you realize that your arms aren't locked ... it's likely to be ugly. Better to get a feel for how to do it before you need to do it in an emergency.

Just to be clear, ABS doesn't really make the car stop faster. It just helps keep the wheels from locking up. A skilled driver can stop a car without ABS as well as or slightly better than a car with ABS. But where ABS really shines is when used by an unskilled driver -- they can just mash the pedal as hard as they can, and the car will stop as quickly as possible (or very close to it) without losing control.

Race cars usually don't have ABS -- it adds weight, and it's generally not needed because the drivers are skilled enough to not need the assistance.

mconlonx

Yes you are. You are claiming that the friction coefficient is "risable and needs no refutation as it's completely unrealistic."

What's a more realistic coefficient of friction for bike stops? What about car stops? Shouldn't we be comparing the two at the same levels of friction involved for an apples to apples evaluation? If not, what friction coefficient values should we be using for each?

Merely stating "You're wrong!" and not providing alt data doesn't really add to anything here...

How 'bout this: there's no way one may take all considerations into account, so there's really no realistic comparison between the two.

RobertHurst

This is an easy experiment and I'd encourage anybody who's been proclaiming on the subject to give it a try, or else cease their proclaiming. Bicycle, calibrated speedometer, tape measure. Maybe put a piece of tape on the ground. Do a bunch of runs to account for slop. Even do it on a slight downhill slope to account for slop. Use different riders You will see that .85 - .9 g's is entirely realistic for a decent rider on an upright bike. You will see immediately that claims of .6 - .7 g for a max stop are completely ridiculous.

This discussion is really frustrating for me as the long-standing claim of .6-.7 g max stops is obviously way off, as shown by a very simple physical experiment, which, apparently, nobody but me ever bothered to perform. When writing my book Art of Urban Cycling (now called Art of Cycling) about 10 years ago I gathered several skilled riders to test the traditional claims of minimum bicycle stopping distance, as propagated by Forester and others, using calibrated speedometers (roll-out test) on a slight downhill slope in an alley in central Denver. Everybody came in about .85-.9 g as a max controlled stop (you might be able to stop shorter but wouldn't be on the bike at the end of the stop). It was more than obvious that a .6-g stop is nowhere near the max deceleration of a bicycle. A .6-g stop is an extremely easy, casual maneuver. A max stop is a serious, almost violent maneuver by comparison. I suppose not everybody can do it. As a result of these experiments Joe Riel revisited the long-held assumptions and came up with his .83 number by moving the rider's mass back, which you'll notice still doesn't quite describe what happens in real life, perhaps because it ignores the movement of the rider's body mass, perhaps because he didn't move the body back enough in his formula.

In any case, try the simple experiment. Please. Or quit the proclaiming.

g = (velocity in mph)(velocity in mph) x .0333 / stopping distance in feet

Keep in mind that stopping distance on a bike, as opposed to a car, has a great deal to do with rider body movement, not to mention brake feel. The max stop is an athletic move. Therefore it is more accurate to speak of a bike-and-rider's minimum stopping distance -- it's not the bike that performs the maneuver. Totally different than the car, where the maneuver is performed through power brakes and steering and the driver's body movements have no effect.

rekmeyata

I will and can prove this, the disk brakes you have won't stop any faster than caliper brakes if all things are equal on the bikes and on the road.

By the way, cars stop faster, a performance bicycle stops at about .5G's while a performance car is around 1G to 1.5G for the best street cars while a Formula 1 car can stop at around 5G's. Due to their high center of gravity and short wheel base a bikes main stopping force is on the front wheel, as braking force is increased the bike will either skid (potentially losing balance) on the front wheel or flip the bike and rider. If the rider can think fast and react properly and slids his rear off the seat and puts more of his weight on the rear wheel, and modulates the brakes precisely that the rear wheel is about to lift off the pavement results could go up to .8G's. What's odd about all of this is that a bicycle no matter if it's a $350 bike or a 15,000 bike will both stop darn close to same distance at the same speed with the same rider skills applied and the same tires and road conditions, whereas in a car the low end car won't stop as fast as a high end sports car.

Sixty Fiver

There has been much discussion on braking on the FG/SS forum and I have cited similar numbers and a few dedicated individuals even posted some non scientific G force reading from portable accelerometers. This showed that once you passed .9g ejection was pretty certain and one fellow even got a reading that was over 1 G which resulted when he crashed and figures this was measured when he hit the ground.

So what this indicates is that under optimal conditions a bicycle will be able to stop as fast as the average car... in real life riding is is fairly apparent to most of us that if we hammer on the brakes in traffic we need to worry about the cars behind us and the drivers not responding quickly enough.

gsa103

On anything over than dry road or a very very low traction surface, an ABS equipped car will stop shorter, END OF STORY. Dry pavement is forgiving for skidding, in wet conditions and trained driver can't approach the ABS performance.

As for the arguments about weight distribution. Yes, if I executing a perfectly timed stop I can drop my weight and lower my CG. In a panic stop situation that's very very challenging. Especially on a road bike with a very high CG and no way to drop off the back-end.

I will speak from personal experience, at one point during a descent I had a rabbit run in front of my bike. I slammed on my bike as hard as I could safely. I felt the back tire start to lift. At the last second the rabbit turned away (I think my tire touched his tail). I can assure you my car would've have easily stopped in half the distance.

0.6g is about the point where a roadie (in standard riding posture on the tops) goes over the handle-bars. It might be as high as 0.7g, which puts it at or below the worst performing cars. If you want to compare the best setup bike with an expert rider, clearly the comparison should be against a sports car on Z-rated tires with an expert driver. In which case the car wins even better. Cherry-picking the best bike and the worst car is completely misleading.

Yes, you absolutely have to worry about a car rear-ending you if you slam on the brakes. Bikes don't have brake lights. People don't know we're stopping until we're stopped. Its not a matter of distance, its reaction time.

Sixty Fiver

The best road cars can exceed 1 g in a stop... F1 cars can pull far more than that and generate enough G force that normal human beings cannot drive them for long as our neck muscles cannot sustain the repeated high G's.

Reaction time for drivers is about 2 seconds between observation and action.

RobertHurst

Please stop spreading baseless lies about bicycle riding.

gsa103

You mean compared to the random internet wisdom, of "I knew a guy who could stop at 0.95g..."

Since you wanted evidence. Here's a study where they explicitly looked at bike braking on hybrid and basic mountain bikes.
https://www.beckforensics.com/CMRSC14BeckBicycle.pdf

They measured a number of <0.5g.

I have yet to see any real evidence of a bike stopping at anything close to 1g. Just because you can plug 0.85g into a calculator doesn't make it reality.

gsa103

I found a test!
https://www.eecycleworks.com/VNJune%20BrakeTest.pdf

They actually did a real stopping distance test (average of 10 stops). The only caveat is that the rider was told to brake a specific mark and would therefore have shifted weight according. This situation is probably only representative of braking power during a descent.

The winner was Shimano 7800 at 0.98g, the low value was 0.57g. So clearly there's a lot of variation.
Shimano 7900 -- 0.87g
SRAM Red -- 0.78g

Sixty Fiver

The test also got a plus 1 G result from the 7900 brakes.

gsa103

Ah, interesting find. I didn't read the full text of the models.

Above 1g, but that's definitely the limit since on a few other attempts the rider started to endo.

Sixty Fiver

When you start to endo that is when you know you have hit your limit and where skills and good modulation keep you from going OTB.

gsa103

The other interesting note was the picture on the very last page. There's an image from the brake test. The rider is completely off the back of the saddle, and has his butt basically over the rear axle. While that definitely a good test of brakes, that's not exactly standard riding posture.

Sixty Fiver

That is how you panic stop a bike.

I-Like-To-Bike

As long as the cyclist has sufficient advance warning in time to shift position before the panic stop is initiated.

RobertHurst

No, the max stop is performed by throwing the weight during the (short) moment of actual braking, to counter the force that is trying to throw the body forward. If you start with the weight already back and keep it there it won't work. Over the bars with you. Body movement is the key to the max stop.

Check out the chapter on panic stops in Art of Cycling and maybe go practice a few times to get the feel of it.

Sixty Fiver

I used to give braking classes for the hipsters at the shop, many who felt that a brake was a needless accessory on a fixed gear bicycle and even those who ran brakes could not believe how fast you can stop a bike when you know what you are doing.

It takes a lot of practice and even longer for this to become a reflexive action on your part.

dougmc

It's rarely the end of the story when somebody says it is ...

Wikipedia seems to disagree with you -- it says that skilled drivers have a hard time matching what some ABS systems can do on dry pavement, fine, but also that they tend to beat ABS on gravel, sand and snow (which I guess would be your "very, very low traction surface", though I'd reserve that description for ice or especially wet ice, personally.)

A skilled driver can come very close and possibly beat some systems by a small margin, and beat it by a significant margin under some conditions (such as gravel, sand and snow.)

But ABS still wins overall because it makes relatively unskilled drivers almost as good as the best drivers at braking.

If you practice it, you can throw your butt off the back of your seat even during a panic stop if you start at a somewhat high speed. But certainly, you want to practice it first.

I think you're confusing my posts with somebody else's or something. I've been picking standard upright bikes (that will generally endo at about 0.7 g's) vs. standard passenger cars (will skid at around 1.0 g's or so under ideal conditions.) (Comparing to F1 race cars is a bit unreasonable.) My position from the beginning has been that cars can stop faster than bikes.

Even if you pick a bike that won't endo for whatever reason, the car will still beat it by a small margin because the car can more safely approach the point where the wheel will skid -- if the wheel skids a bit, all that happens is that you don't stop as fast. If a bike front wheel skids a bit, you tend to crash. Also, cars tend to have more rubber on the road per pound, having lower pressure tires (around 30 psi) than most road bikes. Rubber+pavement doesn't exactly follow the ideal "coefficient of friction" rules -- for tires, having a larger contact patch per pound (30 psi = 4320 lbs/ft^2 of rubber, 80 psi = 11,520 lbs/ft^2 of rubber) will give you somewhat more traction, where if it simply obeyed the simple "fixed coefficient of friction" rules the tire size and pressure wouldn't matter.

To be clear, I don't think that anybody here has suggested that it's smart to brake check a car while riding a bike. The thread tile is "bike braking distance vs. car braking distance" -- braking distance may or may not include the reaction time, depending on what exactly you want to cover, but if reaction time is counted, you should count reacting to the same thing for both situations.

RobertHurst

What is the deal around here. Is this thing on??

RobertHurst

Yah, if you're thinking about how to do it it's too late.

"Only when it is known in the mind can the body know it; but knowing with the body is superior to knowing with the mind." -- Wu Ch'eng-ch'ing, 19th c. tai-chi master, could probably brake like a mofo