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How to Measure Bottom Bracket? Choose the One That You Need!

What Is a Bottom Bracket

In our blog articles, we have already talked about many elements of a bicycle. But one thing we haven’t talked about is the bike’s ​​bottom brackets and their variety. Often when choosing a new ​​bottom bracket, we are faced with the problem of choosing a type and a size. Another question to be answered is how to measure the bottom bracket? Well, it’s time to close that gestalt as well.

What Is a Bottom Bracket?

There is one important part of a bicycle that is responsible for pedaling – the bottom bracket. The carriage or bottom bracket is the component of a bicycle that is used to connect the bike’s crank to the frame, allowing it to rotate independently of the frame itself. It is usually screwed in, but sometimes also pressed into the frame housing – the bushing connects the tubes to the front triangle of the frame. Without the bracket, your bike’s drivetrain would not be able to spin. Carriages use bearings to allow rotation and are usually considered maintenance items. After intense usage, the bottom bracket is suitable only for total replacement, but not repair.

What Is Included in a Carriage?

All bottom bracket systems are fundamentally similar in their construction, and consist of:

  • the body, with mounted bearings and an axle;
  • the cups fixing the mechanism in the bottom bracket. These cups can either be pressed in or screwed into the frame – it all depends on the type of mounting of the whole mechanism;
  • the axle threaded through the body, which can rotate freely. The ends of the axle are attached to the bike’s connecting rods.

The place where the entire carriage system is located and secured is called the bottom bracket shell.

What Kinds of Bottom Bracket Are Available?

What Kinds of Bottom Bracket Are Available

The variety of bottom brackets for bicycles is incredibly large. They can differ in bottom bracket measurements, threads, and the way the connecting rods are attached. They can be demountable – cone cups, bearings, and not demountable – the mechanism, which is collected at the factory and pressed inside the bearings. The latter variant differs also in that there is less accumulation of dirt in the system, and hence a longer service life.

Also, brackets are divided into several types, namely:

  1. Cartridge-type bottom bracket – have a demountable body, bearings, and shaft.
  2. Threaded – screwed directly into the bike frame.
  3. Outboard bottom bracket with a hollow axle;
  4. With solid-metal axle;
  5. Press-fit – pressed into the frame, not threaded.

When choosing a carriage, it is important to pay attention to two dimensions, namely the size of the body of the carriage and the length of the axle. The body width of this bicycle part can be 68 mm, 73 mm, and 83 mm. In some cases, 100-120 mm. The most common bicycles have a 68 mm wide bottom bracket. Knowing the dimensions that will fit your bike will help you decide exactly which one to buy.

Standard Connecting Rods

The next thing to look for when buying a bracket is the standard of the crank. There are lots of options, but here’s a look at some of the most common.

  • Square connecting rods are the most common and allow the carriage shaft to be bolted to a square crank arm housing. The shape of the shaft can resemble a cut-off pyramid and expands closer to the frame. The most common carriages with this system have a shaft that is 110-126 mm long.
  • Hollowtech 2 is a system with an axle integrated into the right crank and with outboard-type cups.
  • Mexa Exo – also has axle integration and outrigger cups, like Hollowtech 2, so in many cases, it is quite possible to replace one system with the other.
  • Octalink, namely two types V-1 and V-2, are the system with 8 splines. It is worth noting that each type of system has its own size of splines.
  • ISIS – this connecting rod standard has a major difference, namely that the shaft has the same length regardless of the type of connecting rod.
  • BB30 – incompatible standard with all others, as it is brand new. What distinguishes this variant is the diameter of the integrated axle, which is much larger and amounts to 30 mm.
  • Power Spline is a standard connecting rod that has 12 splines, and there’s probably some overlap with the FiveD and Iso Flow/connecting rods.
  • HOWITZER – the connections are rigid and are made using a spline. The system also has a self-contained axle and externally mounted bearings.
  • Wedges – a variant used in simple and cheap bikes. The crank arm locking is soft and parts need to be replaced often.

How to Measure Bottom Bracket Size?

How to Measure Bottom Bracket Size?

Since it is always in use, it sometimes fails and has to be replaced. If you think that all you have to do is walk into a bike store and ask for a carriage, you are wrong. There are many pitfalls to choosing this part, and the carriage for one bike may not work for another. Well, how to understand what size bottom bracket do I need?

Now you know what types of bottom brackets there are and what they are designed for. Now it’s time to figure out how to find out how to measure a bottom bracket, so you know for sure that it will fit your bike.

The first thing to do is to get the right tools to measure it because, without them, you can’t do anything. In general, you need a caliper or ruler (metric). These two tools are designed to measure external and internal dimensions. You will also need a special axle around which the pedal rotates. This is called a spindle.

Steps required to measure bottom bracket:

  1. The first thing you should do is prepare for the possibility that the bracket will need to be replaced at some point. To do this, you need to know the exact size of the parts and understand that the bottom bracket is measured in mm. This is what you will need a caliper or ruler for.
  2. After preparation, it’s time to do the bottom bracket measure procedure itself. The first thing to do is to turn your bike over and place it on the saddle and handlebars. Then you should put a caliper or ruler against the body of the bike. Now you can measure the length of the hull (start at one end and go to the other end). These measurements do not include the length of the spindle, which is inside the shell. As we have written above, most brackets come in standard sizes, namely 68, 70, or 73 millimeters.
  3. The next step is to measure the spindle. First, remove the connecting rods. You will not be able to measure the bottom bracket spindle with them. When you have unscrewed all the connecting rods, you will need to measure the length of the spindle. It is important to remember that if the connecting rods are connected by nuts rather than bolts, you should not include the connected parts of the spindle in the measurement.

We hope that after reading our complete guide, you have figured out the process of measuring lower brackets. Our job was to help you find the answers to your questions. Now you can buy new parts for your bike on your own.

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