What Size Bottom Bracket is Needed for Fixie?

If you are building your own fixie bike, one of the crucial decisions you need to make is the size of your bottom bracket. The bottom bracket is an essential component that connects the frame to the crankset and allows the pedals to rotate smoothly. Choosing the right size will not only affect how your bike rides but also how compatible it is with other components.

What Size Bottom Bracket for a Fixie

What Size Bottom Bracket id Needed for a Fixie?

Understanding Bottom Bracket Dimensions

Most threaded bottom bracket measure around 68 millimeter, but many cross country or trail Mountain Bikes measure 73 millimeter. The inner diameter is equally important, and a standard size is 34.8mm.

It’s also worth noting that both the left and right threads go anti-clockwise.

In addition, modern road frames have a dropout spacing of 130mm, whereas older track frames measure 110mm, and newer ones measure 120mm.

Square taper bottom brackets are the most common type, but there are other options, such as the Miche bottom brackets, which are ideal for track and fixie bikes.

Width Options: 68mm vs 73mm

The two most common bottom bracket widths for bikes are 68mm for road bikes and 73mm for mountain bikes. However, it’s essential to note that when it comes to fixies, the width chosen is dependent on the frame’s sizing. The frame’s width dictates the bottom bracket width.

Generally, fixie bikes range from 68mm to 86mm in width.

Given the various widths, it’s crucial to find the right size for the bike’s frame to ensure a proper fit. Once you know the frame’s width, it is then possible to select the appropriate bottom bracket width, with a wider bracket needed for a larger bike frame.

Inner Diameter: Importance of 34.8mm

When it comes to bottom brackets for fixie bikes, the inner diameter is an important factor to consider.

Specifically, a 34.8mm diameter is the most common for threaded bottom brackets. This means that the bottom bracket shell on the bike frame should have an inner diameter of 34.8mm in order to properly fit the bottom bracket.

The importance of this inner diameter lies in the compatibility between the bottom bracket and the bike frame.

If the measurements don’t match up, the bottom bracket won’t fit or function properly, leading to potential issues while riding.

It’s important to note that not all bike frames will have this size inner diameter, so it’s important to check the specifications before purchasing a bottom bracket.

In addition to the inner diameter, other factors to consider for fixie bike bottom brackets include width options, thread direction, and axle length. It’s also worth considering compatibility with older frames, as some may have different specifications than modern frames.

Thread Direction: Left and Right, Anti-Clockwise

Another important aspect to consider when choosing a bottom bracket for your fixie bike is the thread direction.

Both the left and right threads have an anti-clockwise direction, which means that you need to turn them in the opposite direction than you’re used to when tightening or loosening them. This is essential information to avoid damaging the bottom bracket or other components of your bike.

Make sure you follow the manufacturer’s instructions carefully and use the right tools to avoid any mishaps.

Remember that the correct thread direction is especially important when dealing with fixed-gear or track-bike hubs, which include special threads for a lockring that tightens in the opposite direction compared with the pedals.

Dropouts Spacing: 130mm for Modern Road Frames

In order to determine the proper size bottom bracket for a fixie bike, it’s important to understand the dimensions of the frame.

One key factor is the dropout spacing, which is the distance between the dropouts on the rear wheel and hub.

For modern road frames, the standard dropout spacing is 130mm. This means that the bottom bracket needs to be compatible with this spacing in order to properly align the chain with the rear wheel.

It’s important to note that different types of bikes may have different dropout spacings, such as 110mm for older track frames or 126mm for rear 6 and 7-speed road bikes.

By considering all of these factors, riders can ensure they have the right size bottom bracket for their fixie bike.

Bottom Bracket Axle Length: No 1 Standard for Track

The No 1 standard for track bikes is usually 165mm or slightly longer, measured from the centre of the bottom bracket to the centre of the pedal spindle. This length allows riders to achieve greater power transfer and efficiency.

However, it’s important to note that longer axles don’t always translate to better performance. The ideal length may vary depending on the cyclist’s riding style, leg length, and bike fit.

It’s always best to consult with a professional or experienced rider to determine the ideal bottom bracket axle length for your track bike.

Square Taper Bottom Brackets: Most Common

Square taper bottom brackets are the most commonly found type in fixie bikes. This type of bottom bracket has a square shaped axle that connects the crank arm to the bottom bracket, making it easy to install and remove the crankset.

Plus, it’s a cost-effective option that has been used in bikes for many years.

A square taper bottom bracket comes in different sizes, but the width options are usually either 68mm or 73mm, with an inner diameter of 34.8mm.

This type of bottom bracket is compatible with various cranks, like Shimano or Campagnolo, and is easy to find replacements for when necessary.

How to Find Your Bottom Bracket Size?

Once you have understood the different dimensions of bottom brackets, it’s time to find the right size for your fixie bike.

1- First, determine the width of your bottom bracket shell by measuring the distance between the inside faces of the frame’s bottom bracket shell.

2- Next, measure the diameter of the shell to confirm it fits the standard 34.8mm size.

3- Then, consider the crankset you plan to use as it dictates the required length of your bottom bracket axle.

Marketed sizes may vary between 103mm to 127mm, but it is essential to select the right size for proper chainline and pedal clearance.

If you’re unsure of your bike’s bottom bracket type, consult the manufacturer’s specifications or bring your bike to a bike shop for expert advice. Remember, proper bottom bracket sizing will ensure efficient power transfer and a more comfortable riding experience.

Considerations for Older Frames: Bottom Bracket Compatibility

When it comes to older frame bicycles, bottom bracket compatibility is a crucial consideration.

1- The bottom bracket shell width is likely to be different from modern bicycles, so it is important to measure the shell accurately to determine the right size.

2- Older models typically use a threaded bottom bracket while newer ones use press-fit or threaded systems. It’s essential to identify this difference to ensure compatibility with a new bottom bracket.

3- Some older frames may not be compatible with common bottom bracket sizes, so it’s best to consult with a bike mechanic or do thorough research before making a purchase.

By considering these compatibility factors, one can ensure a smooth installation and optimal performance of their fixie bike’s bottom bracket system.


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