7

Most biking accident stories (ignoring collisions) that I hear about involve squeezing the frontbrake too hard and flipping over, often when biking on a downhill.

Read, for example, this fellow's story:

I commute to work a few miles on my bike in the city. Today someone walked out in front of me and I squeezed the brakes too hard and went over the handlebars. I suffered a mild elbow fracture because I put my arms out in front of me and the palms of my hands are destroyed.

In my own experience, I've also pretty bad accidents with the front brake:

I was biking downhill (perhaps too fast), and hit the front brake. I then went flying Superman-style, while watching my bike turn at least three complete somersaults out of the corner of my eye. Not fun.

So, the question is -- why do we need front brakes? Since my accident, I've rarely (if ever) used my front brake, and I bike just fine.

I've also been very puzzled to rent bikes somewhere, and find that the bikes only had front brakes -- in my opinion, back brakes are much safer to use.

So what do we need front brakes for?

  • 5
    Front brakes are much more effective than back brakes. I do like to have the possibility really brake if I have to. Just learn how to use them and they are save. Not using your front brakes means you won't fly over the bars, but just crash into the obstacle instead. – linac Nov 27 '14 at 19:57
  • 3
    I cant imagine riding without using the front brake. It basically does all the work, take your bike down a hill (slowly) sometime and try different braking methods; front only (obviously do this lightly), back only, both. You will feel the difference. – W1ll1amvl Nov 27 '14 at 20:42
  • 1
    Have found a similar question here, should be helpful. – W1ll1amvl Nov 27 '14 at 21:03
  • 4
    Short answer - Because of physics. – ShemSeger Nov 27 '14 at 22:38
  • 1
    As with all useful tools, it's possible to do the wrong thing with front brakes. If this class of argument had any merit we would have banned cars a long time ago, because while useful they are extremely dangerous. – Móż Nov 28 '14 at 0:09
12

I think you need to separate operator error from optimal mechanical functioning.

Mechanical advantage

By your own anecdotal evidence you have demonstrated how powerful front brakes can be. In short we have front brakes because they are the most powerful brake. When a bike (or any vehicle) decelerates weight is shifted to the front wheel. Because bicycle tires work through static friction, and static friction is proportional to the force applied, under braking conditions the front wheel will have the most traction (assuming no additional forces, such as turning are applied). By contrast the harder you brake the less force is applied to the rear wheel and the static friction of the rear tire decreases. As a result the harder you brake, the less traction the rear wheel has, which reduces maximal stopping force (a bit of a negative feedback loop going on here).

The problem as you identify is not with the physics behind the inclusion of the front brake but operator error.

Correct Operation

As your anecdotal stories indicate that many bicycle operators are unfamiliar proper body position during emergency braking. If you learn to move your weight back (ideally behind your saddle) during strong braking then going over the handle bars is nearly impossible (except under very steep hills). On the flat, with correct body position, it is possible to break traction with the front wheel before going over the handle bars.

The issue then becomes one of education. In essence you are asking whether front brakes should be removed because sufficient education on how to operate a bicycle is lacking for many. That is a political question and is unclear if a "right" answer exists.

Physics fun

Event though a wheel is rolling traction is governed by the laws of static friction. When part of the tire touches the ground, it is not sliding across the ground (usually). From a physics perspective this can be considered a body a "rest" – as strange as that sounds.

The resistive force observed in a body at rest is defined as

Fsμ * Fn

Where Fs is the resistive force, μ is the coefficient of friction and Fn is the normal force. The normal force is the force applied perpendicularly to the measured resistive force, on an incline the normal force is less than on a flat surface. The resistive force (Fs) can range from zero to a maximum of μ * Fn.

The resistive force is what does the braking or turning. If braking or cornering exceeds the resistive force your tire slides. The coefficient of friction is related to factors such as tire compound, tire pressure and road surface. The act of braking will not change it (unless you exceed the resistive force).

Under braking Fn increases for the front wheel and decreases for the rear wheel due to the weight shift. As such the maximal braking force that can be applied increase for front wheel and decreases for the rear wheel the harder you brake.

As long as you are not throwing other variables in, such as rider weight distribution (i.e., too much weight up front), changing road conditions, and exceeding a tires maximal resistive force (doing so no the front will make you crash). The front wheel is the best place to focus braking forces.

  • 2
    It's not the first time I've thought we really need MathJax, like Math.Se uses. At least we could code μ and the subscripts so that they render nicely. – andy256 Nov 28 '14 at 2:13
  • @andt256... Could be argued if you need them, the question belongs on matt.se as its more about math than bicycles..... This question probably belongs on sceptics.se though - the premise of the question is the back brake is safer, which is the opposite of the collective wisdom of the cycling community. – mattnz Nov 28 '14 at 2:59
  • 3
    @mattnz - why can't we have math and bicycles? Most Bicycling related math is relatively simple and would bore most mathematicians, but can be relevant to cycling questions. – Rider_X Nov 28 '14 at 3:17
  • Thats F_s max -- if there is no force acting in the direction perpendicular to the surface normal, the static frictional force is zero. – Batman Nov 28 '14 at 17:13
  • @Batman - I updated the formula to be more technically correct. Note that I had defined it as "resistive force" rather than "static frictional force", which implies the maximal force possible. – Rider_X Nov 28 '14 at 18:41
8

So what do we need front brakes for?

We need them for maximum braking efficiency and better control of the bike. Your question is flawed in the sense that it only has anecdotes from unskilled riders. Let's see some similar examples of equipment misuse:

  • why do we have a rear brakes? They are not efficient and last time a friend of mine used it, the bike skidded and lost control of the bike, fell on the ground and broke his foot.
  • why do we have shifter levers? Last time I used them (while standing and pedaling very hard) the chain broke and I went over the bars landing teeth-first on the ground.
  • why do we have axle bolts instead of having the axle permanently fixed on the fork and chainstays? My brother went on a ride but his front axle nuts were lοose. Mid-ride his front wheel came off and he crashed badly injuring his nose and his Top Gun Aviator Sunglasses.
  • why do the drivetrain chainrings have teeth? My fiance rode my bike while wearing her wedding dress and it got jammed between the chainring and chain, ripping it apart. This cost me $2000.
  • why do we have... (ok, you get the point)...
8

There are more factors than just the front brake that contribute to the flipping accident.

I myself got into the accident once. It happens so fast that you never have time to lean your body backwards and provide more tractions for the rear wheel like other have stated.

I should list some of the factors that contribute to the 'flipping'-style accidents:

  1. Braking : like other said, if you slam on the front brake, the momentum you have built up moment before the accident is still there. This momentum is transferred mostly to the handle bars. There is a net torque that flip the bicycle (assuming wheel is stationary at the moment of flipping, which is true, because that's how you flip a bike), the horizontal force from your momentum to your handlebar force the bike to rotate. In common sense, you can try this by apply front brake, and push the bar horizontally and see how easy it is to flip the bicycle.
  2. Cycling positions : aggressive position (racing) will contribute more torque on the bicycle at the moment of flipping because your whole body can only be stopped through the handlebar. On mountain bike, the momentum partly transferred to seat and pedal as a result from a more up-right pedalling position. Remember that your body is about 3-5 times the weight of the bicycle, and they are on top of the bike. So the higher you are from the ground, the easier for you to toppled up. Note: this has something to do with the handlebar position as well.

  3. Handlebar position : believe it or not, the height and angle of the handle bar from the ground also determine how easy you can flip your bike. The more backward, the better; the lower, the better chance for not flipping your bike.

  4. Speed : obviously it is related to your body momentum.
    Diagram of force and torque on bike when braking

So you see:

  1. Rear braking: When braking, with only rear brake, it loses traction. The reason is when you decelerate, your momentum carry forward into the handle bar, slightly lifted the bike (flipping forward). That is why the rear always loses traction first when you decelerates.
  2. What's next? When you lose your braking on the rear wheel, only braking in the front helps you escaping catastrophic accident.
  3. What cause the flipping? Your momentum still carry forward, and is resisted by wherever you can hold on to your bike. Unfortunately, it is the handlebar mostly. So when you decelerate hard enough (or more accurately, the bike decelerate enough), the torque from your remained momentum wins over the torque produced by your weight (Note: pivot is at the contact between the ground and front wheel). That is your rarest moment of experiencing the being superman, flying over a road of Kryptonite. Make a good memory of it!

Side story

There are more underlying causes for the flipping. For example, my most recent accident, I was braking fine, until the car still decided to make me swerve even more. So braking with swerving is pretty much asking for a "failed aero-flip". You lose traction of the front wheel, you lose the balance of the whole bike, you .. well, pretty much have nothing left to hang on to except "Let it go". Prepare your mental for the shock, check your fingers if you can still feels it, then your toes; bring your finger and move around your visions to make sure you have no double-vision. Ask for helps! Oh gosh, I just want to hug whoever have a flipping bicycle accident if I have a chance.

  • 1
    Thanks for the moral support at the end of your answer; if it makes you feel better, that accident I wrote about in the question happened at least 10 years ago :) – AnotherUser Nov 30 '14 at 15:10
7

We have front brakes in order to stop.

In an emergency stop there is hardly any weight on the rear tire, and the rear wheel has very little traction. In each of these cases, the bike would not have stopped where it did, and there are certainly situations where rolling further would be more dangerous.

There is a proper technique which is get back and low, bracing with your arms. This reduces the chance of a header.

On pavement the other trick is to use 90% front 10% back. If the back starts to slip then you are getting close to flipping, and should ease off the front slightly. Don't just slam on the front brake, modulate it. The more you use the front the more practice you'll have at controlling how hard you brake.

2

Aside from the other answers, there is one big one: legality. For example, in Germany, you are required to have two independent braking systems. The easiest way to achieve this (esp. on bicycles which have freewheels) is to put one brake in the front and one brake in the back.

  • 2
    The reason for the laws most likely is redundancy. If one of the braking mechanisms malfunctions, you should have a second one to use. Imagine going down a big hill and your brake cable snaps. There would be no good way to stop your bike. With a second brake, you can just use the other one. it's good to keep both brakes in good working order, because in an emergency situation, you won't have time to switch to the functional one if the one you're using breaks. – Kibbee Nov 28 '14 at 17:10
1

Go to YouTube and search "essential mountain biking skills". Its the best technical skills tutorial I've watched. They'll also teach you how to break properly going downhill.

For my personal opinion? 60% front 40% rear, and never lean forward. Keep your head high, body low, heels dipped, flex you knees, then slowly press the rear brake following the front brake, and both (again 60% front 40% rear). Feel your center of gravity, and balance it out using your body and brakes. Always relax and breathe going downhill and ONLY use the brakes if needed. As much as possible, get your technical skills on, and view the tracks ahead while going downhill to prevent unnecessary use of brakes. Always prepare 3 meters in advance, and only brake coming in to roll-offs, drops, and very tight turns.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.