BMX Drivechains can be quite complicated if you don't quite know the sizes of things or the easiest way to assemble. This drivechain guide has been put together to solve just that. Here we cover cranks and the different size axles or crank arms lengths, bottom bracket sizes external and internal, sprocket, chains and everything else in between. Take a look through and feel free to ask any questions.
BMX Drivechain Explained
Cranks - Many cranks are bought in sets which include the crank arms, axle and the bolts needed to clamp it all together. Some sets now come with the supplied bottom bracket which is usually for frames with a Mid sized BB.
Crank arms - Crank arms can be bought separately if you’re in need of a replacement but usually this is for aftermarket cranks. Crank arms do however come in various lengths i.e 160mm, 165mm, 170mm and 175mm being the most popular. Most street riders tend to have smaller length crank arms whereas dirt/ trail riders and racers may want larger for more stability and quick acceleration when pedalling.
Axles - These vary depending on the crank arms that you’re running. Most entry level complete bikes have an 8-splined axle. Some of the more aftermarket cranks may have 48-splined crank arms or their own shape axle like the Odyssey thunderbolt crank which uses an wedge cluster to hold the crank arms on the axle.
Bottom Brackets (BB) - There are 3 main sizes for BB’s which are 19mm, 22mm and 24mm. This size refers to the inner diameter of the bearing. The outer diameter of the bearing has a name in BMX and they are - Mid, Spanish, USA and Euro. Mid BB is the most popular in BMX today as most frames take a mid sized bearing. Spanish BB’s are less popular now as the bearing’s outer diameter is quite small and with a 22mm inner diameter the bearing can’t take much load/pressure before breaking. USA BB’s are fitted on many entry level BMX bikes as when kept looseball these are cost effective. Sealed USA BB’s would be much heavier than a MId and therefore aren’t used that much. Euro BB’s are mainly used on Race frames and thread into the frame.
Sprockets - These come in a number of sizes and should be matched with your rear driver. The following are standard sprocket ratios - 23T-8T, 25T-9T, 28T-10T or 30T-11T etc. Usually when buying a sprocket it will come with two sprocket adaptors for 19mm and 22mm cranks. For 24mm cranks you can fit the sprocket straight onto the axle. Some cranks run a slightly different system and will use socket or spline drive. For these cranks you will need to purchase the relevant socket or splined sprocket.
Chains - Usually come in one length which has many spare links allowing it to work with all size sprockets and drivers. The difference between most chains is the quality of the links and the thickness of the plates. For example, KMC 410 and KMC 510 chains are made by the same brand except the 510 chain has wider plates increasing the strength of the chain. There are two different types of chain the standard which we’re all used to seeing and halflink chains. Halflink chains allow you to adjust your chainstay length by removing just half a link instead of the usual 3.