the knowledge you must be know about the bicycle bottom bracket

With the continuous development of bicycles, a variety of medium-axis standards have emerged on bicycles, which has dazzled many riders. Today, we carefully comb the bicycle's central axis standards and their compatibility with each other.

First of all, one thing we have to know is the five-way sprocket wheel mounted on the frame (what is the five-way? The five-way is the narrow tube that is perpendicular to the plane of the sprocket platter). Equivalent to a conversion interface that allows the frame and crankset to deliver our energy in the way we want. Therefore, the central axis is not only about the crankset, but also about the frame.

This supplements the next basic concept that will be used later - Q factor, referred to as QF. There are players in China who call the Q value, which is the distance between the outer edges of the two cranks on the left and right sides of the value. In the game, the smaller Q factor can help the player's power, as well as reduce the windward area and reduce the wind resistance to a certain extent.

Then let's talk about the standards of the five links on the frame:


1. Thread British Standard BSA

This is one of the most traditional five-way standards. Its thread features one positive and one reverse. The left side is a positive thread and the right side is a reverse thread. The left side is clockwise and the right side is counterclockwise. The thread has an outer diameter of 34.798 mm and a tooth pitch of 1.37*24 TPI. This five-way specification is most common on everyone's road and mountain bike frames. The width of the five-way is mostly 68 and 73 mm (this five-way width will be longer in DH models, but this article will not be discussed). If you want to know the width of your five-way, the easiest way is to take a ruler with a ruler.

2. Threads

This type of thread is only found on old road bikes in Italy. These threads are characterized by a positive thread with an outer diameter of 36 mm and a tooth pitch of 36*24 TPI. This kind of five-way is very rare, and it needs to cooperate with the special axis. Now the rules are hard to see. If a rider hits an intended road frame, he or she can buy the corresponding axis of the toothbrush according to the instructions. Mountain bikes generally do not have the axis of the Italian standard, and the majority of mountain bike users can ignore this thread.

3. Press-in type BB90

This type of five-way is named after the width of the five-way. In fact, the frame of the BB90 has two widths of 89.5mm and 92mm. The inner diameter of the five-way is 41mm. In fact, the BB90 central shaft adopts a press-in mounting method.

4. Press-in type BB86

This kind of five-way is developed by Giant on the basis of BB90. The difference from BB90 is only 56.5mm width. The original design of this system should be to reduce the width of the five-way to use the smaller Q value of the crankset, but In fact, the current big-name companies have not introduced the corresponding crankset. Most of the systems still use the common one-piece crankset, so it can be said that this system does not make much sense for the improvement of QF.

5. Press-in type BB30

Such a five-way system is a central axis system jointly promoted by FSA and Cannondale, 30 represents the diameter of the axis of the crankset - 30 mm, and the common one of the integrated cranksets represented by Shimano has a diameter of 24 mm. The five-way has an inner diameter of 46 mm and a width of 68 mm. Did you think of something? The common width of the traditional inch threaded five-way is also 68mm. What does this mean? Look down slowly!


The picture above shows Race Face BB30

6. Press-in BB386 EVO

The 386 also represents a three-jaw crank on some of the FSA's discs, and there is also a BB386 system in the mid-axis system. As early as the BB30 and BB90 systems appeared, some experienced riders were expecting a shaft with a thick BB30 sprocket wheel and a sprocket wheel with a BB90 system width. This makes the five-way rigidity increase extremely extreme. The weight reduction of the crankset and the frame is also significant. So FSA and BH developed such a system. The five-way feature of this system is the same 46mm inner diameter as the BB30, and the same 86.5mm five-way width as the BB86 system. This is also the name of its BB386. Of course, such a system has very few manufacturers, and the use of such a five-way frame is extremely rare. For road vehicles, this system is undoubtedly the most lightweight and most perverted, but this system can not be used for the full suspension mountain bike of the three-piece crankset (because of the influence of the turning point and the small disk space position), so this system Success or failure, it is hard to say that at least for nearly 10 years there should be no hope to become mainstream.

Next, let's talk about the classification of the shaft system in the crankset: In general, the central axis is divided into three categories: square holes, splines, and one body (ie, the external bearing Bottom Bracket). Speak it out.

Square hole

The square shaft of most square holes is the same and can be used universally. However, please note that the square hole of the campagnolo square can not be used with the other axis, because the taper of the socket is not the same. Use British or Italian thread (as with the five links, the rules are very rare). This system has been completely eliminated in the mid-to-high-end sports bicycle market, and it is gradually being eliminated in the low-end market.

2. Shimano's splined central axis (eight keys)

This system is the central axis corresponding to Shimano's own splined sprocket wheel. Can only be used with Shimano splined sprocket. At present, it has also been eliminated, but this system is still technically advanced than the square hole axis.

3. FSA is represented by a series of manufacturers of splined central axes (ten keys)

This system is the product of the FSA and other manufacturers in the past, in order to counter the technological monopoly of Shimano. Can be used with all non-Shimano spline sprocket wheels.

4.Shimano is the representative of the plug-in Palin integrated central axis

Many people call it the GXP axis, but in fact this is completely wrong. GXP is the name of the sram's plug-in Palin axis system, similar to Shimano's plug-in Palin axis, but the two are not General, I can't find out how Shimano named their own plug-in Palin axis, including not seen in Shimano's own technical information. I only saw Shimano in the introduction of 980 crankset as Outboard Bearing. Bottom Bracket and Literal Translation are external Palin axles. Why do I use a type of central axis system represented by Shimano here? Because of FSA's Mega Exo system, Raceface's X-type system has many other external axis based on the external Palin axis (except SRAM) that is compatible with Shimano's central axis. However, according to the domestic well-known version of the light measurement FSA's Mega Exo system, the inner diameter of the shaft is 0.1mm larger than the inner diameter of Shimano's central axis, which leads to the FSA's Mega Exo system disk mounted in Shimano's plug-in Palin. It will be tight on the shaft, and sometimes it will take a lot of effort to load the crankset, but in fact these systems are fully compatible, so you don't have to care too much.

5.SRAM GXP system

This system is still in the category of External Bearing Bottom Bracket. However, it is not compatible with the central axis system of companies such as Shimano. GXP is written by Giga X Pipe, literally translated as 100 billion X tube. It can be seen that this can not be related to the plug-in Palin or the integrated axis. Why is Giga used? Before SRAM acquired Truvativ, Truvativ had the splined shaft of the Giga series, so the word Giga is only a mid-to-high-end mid-axis series at SRAM or for Truvativ. So please don't arbitrarily call the plug-in Palin axis called GXP axis. If you want to speak English, then say External Bearing axis, if you miss the abbreviation? Then I am here to claim to be called the EXB axis. If the majority of riders can accept it, then let's just say it. The EXB axis is collectively called all the plug-in Palin axles.

6. Externally mounted Palin central axis

The old-fashioned road-shifting champion Campagnolo's plug-in Palin axis, I really can't find Campagnolo's plug-in shaft technology information, so I can't determine whether it is common with Shimano's center-axis system. Let's tell everyone about this rider test or find technical information.

7.BB30 system

The system features an increased 30mm axis and a narrower five-way width for easy reduction of the Q value of the crankset while reducing system weight for greater rigidity.

As a rule of thumb, the central axis of the CP should not be compatible with Shimano.

Finally, let's tie the frame to the center axis:

First BSA British standard thread 68,73mm five-way: can use the square hole center shaft, spline shaft and plug-in Palin middle shaft, many riders can not understand the length of the square hole shaft and the spline shaft, generally come Said 68mm five-way minimum selection of 108mm square hole or splined shaft, but this also depends on the design of the sprocket wheel and the rear fork, here is not to say, choose this situation is too little, indeed encountered If you want to change it, change it to a longer axis. Then the shaft of 113 can be used on a 73mm or 68mm five-way. If you use this 68mm five-way, you need to add a 2.5mm thick spacer on the left and right sides.

If you are using a plug-in Palin shaft, if you are installing on a 68mm five-way, you must add two spacers on the right side and a spacer on the left side. If the frame is 73mm, you only need Add a spacer on the right side. In addition, the traditional 68mm BSA British threaded five-way can also be installed with the BB30's crankset. What you need is a BSA30 central shaft developed by Rotor. This central shaft allows you to install BB30 teeth on the traditional BSA five-way. plate. Therefore, the center shaft can be mounted on all the square holes, splines, and integral sprocket wheel of the sprocket wheel. It can be said that this is the most versatile kind of five links.

Italian thread five-way: This stuff is difficult to see in the millennium, as long as you pay attention to buy the axis of the supporting plate of the tooth plate, and there will be a manual with the axis to teach you how to install it, so I won’t go into details here. .

BB90 and BB86 five-way: use the indented BB91 central axis, or the push-in GXP central axis, as well as the CP BB90 central axis. The specific installation method also needs to be installed according to the actual width and description of the five links. In addition, this system supports all non-BB30 integrated cranksets that can be used on traditional inch threaded five-way. Simply match the corresponding press-in center shaft. The reason why this system cannot use the BB30 is that its five-way width is 86.5mm and 89.5 and 92mm, while the BB30's five-way is only 68mm.

BB30 system: It is also possible to install a variety of sprocket wheel, first of all, of course BB30 sprocket wheel. Secondly, many companies have introduced the BB30 directly into the center axis of the BB90 system with Palin. The CP also introduced the center axis of the BB30 direct-installed sprocket wheel. In addition, the BB30 system can also be converted into an ordinary BSA five-way through the conversion seat of the BB30 to BSA, and then all other sprocket wheels can be installed. At this point, you can refer to the safety classification of the first BSA five-way.

BB386 system: From the technical specification analysis, BB386 compatible with BB90 system and GXP's crankset is completely no problem, and will definitely appear in the conversion seat in the future, but the BB30 crankset should not be compatible with this system, given that this system has just been released this year. I don’t know much about it, so I won’t say more here.

Finally, a small tip is added. The center shaft used in the integrated sprocket wheel usually has a plastic sleeve in the middle. In fact, the purpose of this sleeve is to waterproof the inside of the outer lining of the central axis. So when you use the carbon frame, because the carbon frame's five-way is closed, you can not install the set. The first closed five-way makes the cover lose its meaning, and then does not install it. Reduced the weight of a few grams.

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