[戶外運動] Braking and Turning Your Bicycle

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[戶外運動] Braking and Turning Your Bicycle

文章 #1  未閱讀文章HG » 2007-06-23, 01:31

Source:http://www.sheldonbrown.com/brakturn.html

Braking--Front, Rear or Both?

Since your bike has two brakes, one for each hand, if you want to stop as safely as possible, you need to pay attention to how you use each of them.

Conventional Wisdom

Conventional wisdom says to use both brakes at the same time. This is probably good advice for beginners, who have not yet learned to use their brakes skillfully, but if you don't graduate past this stage, you will never be able to stop as short safely as a cyclist who has learned to use the front brake by itself.

Maximum Deceleration--Panic Stops

The fastest that you can stop any bike of normal wheelbase is to apply the front brake so hard that the rear wheel is just about to lift off the ground. In this situation, the rear brake cannot contribute to stopping power, since it has no traction.

Won't I Go Over The Bars?

The rear brake is O.K. for situations where traction is poor, or for when your front tire blows, but for stopping on dry pavement, the front brake all by itself provides the maximum stopping power, both in theory and in practice.

If you take the time to learn to use the front brake correctly, you will be a safer cyclist.

Many cyclists shy away from using the front brake, due to fear of flying over the handlebars. This does happen, but mainly to people who have not learned to modulate the front brake.

The cyclist who relies on the rear brake for general stopping can get by until an emergency arises, and, in a panic, he or she grabs the unfamiliar front brake as well as the rear, for extra stopping power. This can cause the classic "over the bars" crash.

Jobst Brandt has a quite plausible theory that the typical "over-the-bars" crash is caused, not so much by braking too hard, but by braking hard without using the rider's arms to brace against the deceleration: The bike stops, the rider keeps going until the rider's thighs bump into the handlebars, and the bike, which is no longer supporting the weight of the rider, flips.

This cannot happen when you are using only the rear brake, because as soon as the rear wheel starts to lift, there is not more braking force generated by it. Unfortunately, though, it takes twice as long to stop with the rear brake alone as with the front brake alone, so reliance on the rear brake is unsafe for cyclists who ever go fast. It is important to use your arms to brace yourself securely during hard braking, to prevent this. Indeed, good technique involves moving back on your saddle as far as you can comfortably go, to keep the center of gravity as far back as possible. This applies whether you are using the front, rear or both brakes. Using both brakes together can cause "fishtailing." If the rear wheel skids while braking force is also being applied to the front, the rear of the bike will tend to swing past the front, since the front is applying a greater decelerating force than the rear. Once the rear tire starts to skid, it can move sideways as easily as forward.

If you don't believe me, perhaps John Forester can convince you...see his Entry in the rec.bicycles FAQ on Front Brake Usage. (Unfortunately, the maintainer of that site has a habit of breaking links, so you may need to go to the rec.bicycles FAQ index to find the article.)

Learning to Use The Front Brake

Maximum braking occurs when the front brake is applied so hard that the rear wheel is just about to lift off. At that point, the slightest amount of rear brake will cause the rear wheel to skid.

If you ride a conventional bike, the best way to master the use of your front brake is to practice in a parking lot or other safe space, applying both brakes at once, but putting most of the effort into the front brake. Keep pedaling as you brake, so that your legs will tell you immediately when the rear wheel starts to skid. Practice harder and harder stops until this happens, so that you will learn the feel of stopping fast, on the edge of rear-wheel liftoff.

Some cyclists like to ride a fixed-gear bicycle, that is, a bicycle that does not permit coasting. When you brake hard with the front brake on a fixed gear, the drivetrain gives you excellent feedback about the traction situation at the rear wheel. (This is one of the reasons that fixed gears are favored for winter riding.)

If you ride a fixed gear with only a front brake, your legs will tell you exactly when you are at the maximum brake capacity of the front brake. Once your fixed gear has taught you this, you will be able to stop any bicycle better, using the front brake alone.

If you find the fixed-gear concept intriguing, I have a major article on Fixed Gears for Road Use on this site, and also a page of Fixed-Gear Testimonials from happy converts.

When to Use The Rear Brake

Skilled cyclists use the front brake alone probably 95% of the time, but there are instances when the rear brake is preferred:

  • Slippery surfaces. On good, dry pavement, it is generally impossible to skid the front wheel by braking. On slippery surfaces, however it is possible to do so. It is nearly impossible to recover from a front wheel skid, so if there is a high risk of skidding, you're better off controlling your speed with the rear brake.
  • Bumpy surfaces. On rough surfaces, your wheels may actually bounce up into the air. If there is a chance of this, don't use the front brake. If you apply the front brake while the wheel is airborne, it will stop, and coming down on a stopped front wheel is a Very Bad Thing.
  • Front flat. If you have tire blowout or a sudden flat on the front wheel, you should use the rear brake alone to bring yourself to a safe stop. Braking a wheel that has a deflated tire can cause the tire to come off the rim, and is likely to cause a crash.
  • Broken cable...or other failure of the front brake.
  • Long mountain descents, when your front brake hand may get tired, or you may be at risk of overheating a rim and blowing a tire. For this situation, it is best to alternate between the front and rear brake, but not to use them both at once.

When to Use Both Brakes Together

Generally I advise against using both brakes at the same time. There are exceptions, however:

  • If the front brake is not sufficiently powerful to lift the rear wheel, the rear brake can help, but the best thing to do is to repair the front brake.

Typical rim brakes lose a great deal of their effectiveness in rainy conditions, so using them both together can reduce stopping distances.

  • Long or Low bicycles, such as tandems and long-wheelbase recumbents have their front braking limited by the possiblity of skidding the front wheel, since their geometry prevents lifting the rear wheel. Such bikes can stop shortest when both brakes are applied.

Tandem caution: when riding a tandem solo (no stoker on board) the rear brake becomes virtually useless due to lack of traction. The risk of fishtailing is particularly high if a solo tandem rider uses both brakes at once. This also applies to a lesser extent if the stoker is a small child.

Which Brake Which Side?

There is considerable disagreement as to which brake should be connected to which lever:

  • Some cyclists say it is best to have the stronger right hand (presuming a right-handed cyclist) operate the rear brake.
  • Motorcycles always have the right hand control the front brake, so cyclists who are also motorcyclists often prefer this setup.

There are also observable national trends:

  • In countries where vehicles drive on the right, it is common to set the brakes up so that the front brake is operated by the left lever.
  • In countries where vehicles drive on the left, it is common to set the brakes up so that the front brake is operated by the right lever.

The theory that seems most probable to me is that these national standards arose from a concern that the cyclist be able to make hand signals, and still be able to reach the primary brake. This logical idea is, unfortunately, accompanied by the incorrect premise that the rear brake is the primary brake.

For this reason, I set my own bikes up so that the right hand controls the front brake, which is not the norm in the U.S.

I also do this because I'm right handed, and wish to have my more skillful hand operate the more critical brake.

See also my letter to Bike Culture magazine.

Leaning in Turns

To turn a bicycle, you must lean inward toward the direction of the turn. The faster you are going, and the sharper the turn, the more you must lean. You have no choice about this, for a given speed and turn radius, the center of gravity of the bike/rider must be moved sideways a particular amount or the bicycle will not balance.

What you do have control over is whether you lean the bicycle more than, less than, or the same amount that you lean your body, to get the overall center of gravity to the place that it has to go.

  • Leaning the bicycle sharply while keeping your upper body more upright

This approach is popular with beginners who are scared to lean over sideways, and who feel less disoriented by keeping their bodies more upright.

This technique is recommended by some racers and coaches as offering the possibility of recovering from a skid, but I don't believe it.

  • Leaning the upper body sharply while keeping the bicycle more upright

This approach is popular with riders who are afraid of striking a pedal on the road. This is a particular concern for riders of fixed-gear bicycles, since they cannot coast through corners.

This technique is also recommended by some racers and coaches as offering the possibility of recovering from a skid, but I don't believe it.

  • Leaning the upper body and the bicycle together, keeping them in line as when riding straight.

This technique has the advantage of keeping the steering axis, tire contact patches and center of gravity all in the same plane. This preserves the proper handling characteristics of the bicycle, and makes a skid less likely. You can verify this yourself by performing an experiment suggested by Jobst Brandt:

"Some riders believe that sticking out their knee or leaning their body away from the bike, improves cornering. Sticking out a knee is the same thing that riders without cleats do when they stick out a foot in dirt track motorcycle fashion. It is a useless but reassuring gesture that, on uneven roads, actually works against you. Any body weight that is not centered over the bicycle (leaning the bike or sticking out a knee) puts a side load on the bicycle, and side loads cause steering motions if the road is not smooth. Getting weight off the saddle is also made more difficult by such maneuvers.

"To verify this, ride down a straight but rough road standing on one pedal with the bike slanted, and note how the bike follows an erratic line. In contrast, if you ride centered on the bike you can ride no-hands perfectly straight over rough road. When you lean off the bike you cannot ride a smooth line over road irregularities, especially in curves. For best control, stay centered over your bike."


A letter to Bike Culture:

The following is a letter I wrote to Bike Culture magazine in 1995, addressing some of the points covered above:

In Bike Culture #6, Marek Utkin decries "Stupid Standards" in the bicycle industry. He begins with a wonderful exercise in guilt by association "Throughout history, despots have tried to regulate every aspect of their society..." He goes on to link Hitler to the D.I.N. and Stalin to G.O.S.T. standards. Unaccountably, he forgets to mention the strong linkage between Napoleon and the Metric system--perhaps he likes metrics, and doesn't want to bring up its despotic antecedents.

He is specifically exercised by new E.U. regulations that specify that the front brake should be controlled by the right hand. "The trouble begins when a bike with right lever to front brake is ridden...in any country where traffic keeps to the right. The left hand signal, across traffic, is much more hazardous than the right hand signal. With the right hand...front brake the potential danger increases. Stopping the front wheel only, with one hand on the handlebars, can cause front-wheel instability. Also the upper part of the cyclist's body moves forward as the bicycle abruptly decelerates, causing further pressure on the right side of the handlebars."

I have to attribute his attitude to a common misunderstanding about brakes. With bicycles, as with virtually all wheeled vehicles, the front brake is the more important and effective brake. The front brake by itself will stop a standard bicycle twice as fast as a rear brake by itself. The front brake by itself will stop a standard bicycle as fast as both brakes used together, except on very slippery surfaces.

Unfortunately, many casual cyclists and non-cyclists have the mistaken idea that using the front brake is dangerous, and that you are likely to lock up the front wheel, pitch over the handlebars and crack you skull. This type of accident is extremely rare, and unlikely on a bicycle that is in good repair, ridden by a cyclist who has learned to use the front brake sensitively.

The danger is more real for bicycles with damaged rims, or mis-adjusted brakes. The danger is even greater for the cyclist who habitually relies on the rear brake alone when suddenly faced with the need for a panic stop. A panicky rider who is unused to the front brake may indeed grab it full-force as a last resort, and may take a header.

If you will forgive an automotive analogy in these green pages, a driver who has never driven a car with power brakes is likely to skid a few times the first time he or she tries driving a car that has them. This does not mean that there is something wrong with power brakes, however, it means that the driver needs to learn how to use them.

I frequently ride a fixed-gear bike with a front brake only. This is an excellent way to learn subtle control of the front brake, as the fixed gear gives very good feedback of the traction available at the rear wheel.

In the early '80s, I became infatuated with mountain bike riding in the woods, and completely re-adapted my braking style to cope with the loose surfaces common on woods trails.

In 1988, I moved to France for a year, and got back into road riding. Near my house was a wonderful bit of road down the side of a valley, called La Route des Sept Tournants. It is a series of sweeping switch backs, beautifully paved, very well engineered. I used to descend it regularly on one of my favorite loops. The problem was, I could never really go fast down it, I always felt that I was on the verge of losing traction with my rear wheel and spinning out. After a few months of this, I was beginning to conclude that I had just become a coward as I reached middle age. I remembered I used to go faster around similar bends on my old fixed gear with no rear brake----wait a minute, maybe that's it! The next time I went that way, I decided not to use my rear brake unless I felt I really needed it--I would just go slowly at first, only as fast as I felt comfortable with using the front brake alone.

Mirable dictu, I found that I was my old self again! It had indeed been the rear wheel that was on the verge of slipping, and only because I was using its brake. Without the drag of the rear brake, the rear wheel was in no danger of slipping. The front wheel, thanks to the weight shift caused by the braking I still had to use, had plenty of traction as well.

Many people will tell you that it is dangerous to use your front brake in a turn; I would respond that this is so if your turning/banking technique is incorrect. The center of gravity of a bicycle/rider must lean into a turn; this is required by the laws of physics. There are three ways you can do this. One way is to keep the bike more-or-less upright, but to lean your upper body into the turn. Another is to keep your body more-or-less upright, and lean the bicycle under you. The third, and usually correct technique is to keep your body in line with the bicycle frame, lean the bicycle and rider together as a unit.

Leaning the bicycle and rider differently messes up the handling of the bike, by moving the center of gravity sideways from the plane of the wheels. If you apply the front brake while doing this, the braking force exerts a steering force through the now off-center headset. Jobst Brandt has an excellent way of proving this to yourself: try riding down a straight, but bumpy stretch of road while leaning the bike to one side and your body to the other. If you are brave, try applying the front brake very gently.

I must admit to a bit of ambivalence as to whether a government body should tell people how to set up their bicycles. In the U.S.A., de-facto government regulation has made left-front all but compulsory for new bikes to be sold. I strongly object to this. I once had a near-accident as a result: I was riding an unfamiliar bike that was set up left-front, even though I am used to right-front. I came to an intersection, a car cut me off, I instinctively grabbed with my right hand. Since this was the rear brake, I was only barely able to stop in time!

I will add one further reason for preferring the right-front setup: Most people are right handed. I think we can all agree that the front brake requires more skill than the rear; therefore, it should be assigned to the more skillful hand.
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前制動理論測試

文章 #2  未閱讀文章HG » 2007-06-23, 01:35

整體理解,單獨運用前 brake 呢個技術,從物理學角度是正確的。

  1. 根據力學定律、在制動時原本速度使重心由平衡點移向前方,導致前後輪磨擦系數發生變化:前輪在重力轉移下相應增加、後輪相應減低
  2. 單獨運用後 brake,因磨擦系數減低:在相同的制動力下制動效率變低
  3. 單獨運用前 brake,因磨擦系數增加:在相同的制動力下制動效率變高
  4. 同時運用前後 brake,要视乎制動時原本速度去計算力距角度變化。7/3 比例未必在計算上正確、關鍵在制動時的速度、制動到靜止時間、重心轉移滯後時間等等因數。現實當然不需要小數後幾個位迼樣精確,使用模糊邏緝已經足夠。

結論:前 brake 技術需要個人經驗、器材質數較高及穩定的條件下運用;否則,運用此技術只會增加不必要的風險。
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