SysResearch

It is known that maximum braking occurs when the front brake is applied so hard that the rear wheel is just about to lift off. Skilled cyclists use the front brake alone probably 95% of the time.

However, improperly modulating the force applied on the front brake will bring the bike to a sudden stop, rider and bicycle's weight shifts to the front thus generating a momentum (force) that will send the rider "sailing" forward over the handlebars and eventually the bike will flip.

Q1): Does anyone know what is the name in physics of this generated force.

Q2): Based on speed, bike/rider mass-height how can this force be calculated.

Any links?

Thank you anticipated

CaptainCool

Why do you want this, when you don't know much about physics to start with?

You can do it with a free body diagram with inputs of mass and position of the center of gravity relative to the front wheel. Even then, you'll only find the maximum braking force; a force significantly greater than this is required for flight.

bcart1991

Moment of (polar) inertia may be the term you're looking for.

damnable

This is incorrect.

Which force in particular are you talking about, as there are a few at work?

wahoonc

Biology major here but IIRC wouldn't the basic mass in motion tends to stay in motion theory and rules apply And FWIW I have yet to do an endo using just the front brake if it is used correctly, and on at least one of my bikes that is the only fully functioning brake

Aaron

Cyclaholic

Actually, maximum braking occurs when you spot the sign that says "free beer" with an arrow pointing down a driveway you just passed..... or when your brakes fail and you stop by hitting the side of a building...... and when your parachute doesn't open (but that generally will only ever happen to you once).

Q1): That would be the "Oh*****I'mAboutToDie" force. It's one of the most powerfull forces in the universe. It's so powerfull that survivors often report a slowing down of time and everything happening in slow motion while under its influence.

Q2): You can't actually calculate it based on just your speed and bike/rider mass-height as Newtonian physics assumes time to be a constant, which clearly it is not when under the influence of the Oh*****I'mAboutToDie force.

Stacey

StephenH

"Skilled cyclists use the front brake alone probably 95% of the time." Did not know that. Then again, I don't have a front brake.

" thus generating a momentum (force) that will send the rider "sailing" forward"
There is not a force sending the rider forward. There is a force slowing the rider down. But it is not applied at the center of mass of the man/machine, and therefore also tends to rotate the assembly.

Q1): Does anyone know what is the name in physics of this generated force. The force is "friction", slowing the brake down. The force is applied eccentrically, causing a "moment" or "torque". The polar moment of inertia doesn't enter into it until you start calculating how fast the bike rotates over.

Additionally, some of the rotational momentum of the wheels would be transferred to the bicycle, but it's probably a minor issue. But if you had a bike floating in space with the wheels turning, and then locked the brakes, the whole bike would then rotate but much more slowly. "Conservation of Angular Momentum" at work there.

Q2): Based on speed, bike/rider mass-height how can this force be calculated.
Not really. I mean, you can calculate it, but not with any accuracy because you don't know the data you need. What's the coefficient of friction between tires and ground in a particular case? Where's the center of gravity of a person and of a bicycle? The fact that a person/bicycle combo is not a rigid assembly also complicates things enormously if you are trying to get an accurate result.

cooker

In theory if you knew all variables you could calculate the forces required to flip you over the bars, but there are so many variables: the force of braking, the coefficient of friction on the road (which prevents the front wheel from skidding) the location of your centre of gravity (which will move forward as you brake), the angle between your centre of gravity and front tire contact patch, the angle of your arms to the handlebars, the wheel length of the bike since a long bike is less likely to endo, any load you may be carrying on the rear rack, whether your feet are clipped in or not, etc., etc.

Velo Dog

But has anybody really BEEN over the bars on braking force alone? I'm sure you can manage a front endo (in fact I've been there), but in braking the rider will shift his weight to the rear, and if the back wheel lifts, he'll ease up on the lever a little.

cooker

Yes, and it has resulted in fatalities. Link to follow.

rmfnla

Darwinism at its finest.

Riding a bike? Hold on.

Duh!

CastIron

What. The. Hell?

Abneycat

If you're riding a normal road bike, the force required is proportionate to the materials being used in construction of your rig: carbon will immediately flip, owing to its immaculate loftiness. Aluminum will flip you out of spite, as it wanted to be born as a mountain bike. Steel will base its punishment on the amount of lycra you're wearing, and if you're wearing a bright neon jersey, steel will wait patiently until you need to brake suddenly when surrounded by broken glass.

cooker

Some over the handlebar experiences:

https://www.cyclingforums.com/t217749...very-time.html

"Managed to avoid being killed by a blind motorist this morning, but my effective panic-braking technique saw me go flying over the handlebars and landing on my shoulder and head."

https://query.nytimes.com/gst/fullpag...=&pagewanted=3

" few years ago, a bee flew down Sheri Taylor's shirt while she was cycling. She slammed on the front brake, which locked the front wheel, and she flew over the handlebars. "

There is a thread on BF which I will find when the search function is restored, about a guy who died due to panic braking and endoing when cut off by a truck.

StephenH

"Skilled cyclists use the front brake alone probably 95% of the time. This is incorrect."

I've read that something like 80% of the braking capacity is in the front tire- as the rear lifts & skids under heavy braking.

But then I'll see statements that 80% of the braking that is done is with the front tire. I think that is likely an incorrect interpretation of the capacity statement. Unless people just choose to brake mostly with the front, or unless they routinely brake so hard they are forever skidding the rear tire.

On something like a long slope, where you need prolonged braking but not hard braking, the capacity of the two brakes should be about equal, based on heating, rather than skidding.

CastIron

The process is simply too dynamic to assign one static value. I adjust my braking accordingly.

rmfnla

Modulation is the key.

d_D

I've done it while drunk. I guess the alcohol caused me to pull the lever stupidly hard. Then my face hit the ground before my impaired reactions had a chance to kick in.

I think if you have good brakes and really yank the lever you don't have a lot of time to react. The key is to know the limit so if you do have to brake in a panic you are less likely to exceed it.

grolby

Both of you are off somewhat. The fact is that up to 100% of your braking capability comes from your front brake. When you are decelerating as fast as possible, the rear wheel contributes 0% of the braking force, since it will have virtually no traction under maximum braking (if your rear wheel has enough traction to make the rear brake useful, you're not all that close to maximum braking). Under dry conditions, there is no need to bother with the rear brake, and no sense in doing so. When the roads are wet or conditions otherwise make for the possibility of a front skid preceding the rear wheel lifting off, I use both brakes.

Emergency stopping "techniques" that suggest the coordination of front and rear brakes on dry roads aren't much good, because (again), if your rear brake is useful, you could be decelerating harder.

Not braking too hard and going over the bars is a matter of practicing a skill that requires the development of some degree of muscle memory, not a matter of braking hard with the front brake being inherently unsafe. If you aren't braking so hard in an emergency that your rear brake is useless, it's also possible that you could end up under the wheels of a truck. It is undoubtedly true that many people have been killed because they weren't able to slow their bicycles fast enough, likely more than those that died as the result of an endo. Not to resort to hyperbole, or anything, but being able to stop in the shortest distance possible is an important safety skill, and the only way to do it is to master the front brake.

StrangeWill

^Only if your weight is shifted forward, at no time should you be using the traction of only one tire.

While it is right that you get a huge amount of braking, you'll never get 100% unless you're riding your front wheel like a unicycle. As long as that wheel is on the ground you have some sort of friction and braking power available to you.

However, the need for a rear brake isn't exactly huge, being as you do a lot of stopping with the front.


Generally I use my rear for slowing descents slightly. I'll mix with front more and more depending on how much more braking I need. You could easily do it with a front brake only. However I have this idea that if I lose traction due to bad road conditions (dirt, water, etc), having the back slide is going to be much easier to control than having the front slide.

Then again, in theory while that may be nice, in reality it would be quite rare.


Anyway, I've gone over the bars once, panic situation that if I had more experience I would have avoided, gripped the front brakes hard as hell, considered it a lesson learned. I was also down low and quite forward on the bike, if I was sitting in a more normal cruising position I would have never gone over the bars. Whats really fun is in this position you're going FACE FIRST with your arms under the bars.

Abneycat

Well, on a serious note instead of inputting more silliness, like CastIron mentions, this is one of those things that simply has far too many variables to take into account to really be easily calculated.

The one endo i've done as a result of braking was as a kid, on my mountain bike with moderate quality indirect pull cantilever brakes (not exactly the pinnacle of stopping power). Normally, I wouldn't even consider these brakes potent enough to do such a thing, but they were apparently

AndrewP

People go over the bars when they dont brace their elbows for braking. They apply the front brake, tha bike stops and they fly forward because their arms are not holding them back with the bike. Their body then hits the bars and their momentum carries them over.

SysResearch

I still believe that it has to be a formula that this force (Angular Momentum I believe) can be calculated taking in consideration the speed, height and weight of the bike, rider's weight and gravity center.

Its like this; A bike with a mass of 15Kg, having a rider with a mass of 80 kg hits a parking concrete barrier at 5 km/h, where hitting the barrier equals with brake force.

In this example, at this speed, chances are the rider will not fly over the handlebar.

Yet in the same equation, if we change the variable "speed" with 15 km/h, chances are the rider will start sailing thru the air.

Now the case scenario, only that the rider would have 120 kg. Most likely the bike wouldn't tip over. (due to rider's mass)

Now if we take this last rider's mass of 120kg but we increase the variable "speed at 40 km/h, chances are the bike will tip over

The question is how do I calculate the force of the impact occurred at 5km/h and at 15 km/h knowing the speed, and the mass of both rider and bike.

Thank you!

SysResearch

My dear friend, dismissing something without contributing with a replacement theory, thus substituting the previous one, does not make you more knowledgeable, nor demonstrate that I am wrong and your are right (or me incorrect and you correct), and certainly you are not entertaining a constructive discussion.

Your answer would have been more regarded if you would have say: "This is incorrect. This is what I believe to be correct.. .. etc, etc... based on ... . etc, etc"


But thank you for your insight anyway.

By the way, do you have any "correct" theory that you'd like to share with us?