Old, but good - so you could describe a square axle. It is somewhat out of fashion, but many older bikes and especially children's bikes are still on the road with it. However, bottom brackets, just like most other moving bike parts, are wear parts. If your bottom bracket creaks, squeaks or cracks, you need a new one. Here you will find all possible square bottom bracket, and we tell you why they run round despite their four edges.
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Square bearings are robust and do not cost the world... On new bikes they are nevertheless installed less and less in recent years. Why? Quite simply: they are quite heavy, today a real Abturner when it comes to bicycle components. In addition, manufacturers have put a lot of development work into lighter, modern bottom brackets. No matter how long they have proven themselves, the old components are then only carried as spare parts. Today, square bearings are only found on inexpensive, simple bicycles and children's bikes, or they are used as spare parts for retro bikes.
How a square bottom bracket works
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In a square bottom bracket, the axle that connects the two crank arms is part of the bearing. It is held in a completely sealed sleeve (also "cartridge", hence the name "cartridge bearing") by two bearing shells.
The square end of the crankshaft fits into the also square hole of the crank and is bolted there. To the ends of the crank arms go the pedals, and you're ready to go!
There are two particularly common standards when it comes to the dimensions of the squares:
- ISO is the European standard
- JIS is most commonly used by Asian manufacturers
The two systems are not compatible! If you want to buy a square bottom bracket, you should first find out which system your crank has, with the naked eye the two standards are difficult to distinguish.
The dimensions of a square bottom bracket
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Square bottom brackets come in different housing standards, which means they have different diameters and threads. You can easily sort them here in the online store using the filter on the top left. The housing standard determines whether a square axle fits into the bottom bracket shell and can be bolted there. In almost all cases, square bottom brackets are bearings with thread. BSA, ITA or FRA are different thread types, they differ in the thread pitch (TPI, number of grooves per inch) and in the direction of rotation of the thread.
In addition, they are available with different axle lengths from 102mm to 127mm, again you can simply trouble our filters.
The surest way to identify the right spare part is to look at your old bottom bracket, there is usually the width, diameter, thread standard and a hint whether you drive so far ISO or JIS.
Will you buy a square bottom bracket?
If you need a new bottom bracket, you've come to the right place here at BMO Onlineshop, because we have a wide selection of bike parts for you. But do you even need a square bottom bracket? Quite simply, if you remove the crank and then come across square bolts, you have a square bearing. If there are more corners, it's a many-tooth inner bearing. If the crankshaft hangs tightly on one of the crank arms, you have a two-piece crank and a hollow axle.
You can easily filter by the different standards and sizes, so you can quickly get to your new square bottom bracket. If you need more information, you can surf past the bottom bracket, there is a particularly detailed buying guide.
