I am buying a new frame and am going to need a new bottom bracket. The information supplied with the frame tells me i need a 68mm bottom bracket. Bottom brackets also have another measurement relating to the axle length. I need to know how to measure this so can buy the right sized bottom bracket for my new frame. (I already have my cranks but am just unsure of how to measure the axle length.)Thanks in advance
There are only three measurements to a crankset. The first is the shell width and is going to be either 68mm, 73mm, or 83mm. 68 and 73 are easily the most common with the 68mm being the more common of those two. 83mm is pretty much limited to downhill bikes. The axle width can come in anywhere between 100mm to 150mm and is determined by your crankset and your required chainline. Cheaper cranksets will often have a concave feature on the crankset spider, where the small chainring is closer to the bottom bracket shell than it is to the crankset spider. For those, you'll need a wider axle to offset that spacing and maintain a straight chainline. On nicer, more expensive cranks, the crankset spider is actually closer to the bottom bracket shell than is the small chainring and therefore needs a shorter axle to maintain chainline. The only real purpose of the varying axle widths is to set your chainline parallel to the centerline of the bike. Of course, there's a formula, but to use it you have to take measurements that are not practical to get precise enough for decent results. Basically, with the bike in the middle chainring and the middle cog in the rear, your chainline should look parallel to an imaginary line drawn from the center of the headtube through to the center of the rear axle. As a general rule, if the frames are pretty similar, you probably wont have to change anything. For example, I upgraded a Klein Palomino to a Maverick ML8 and didnt have to change a thing. If, on the other hand, you're stepping up to a bike with a 150mm thru-axle in the rear from one that had 135mm qr axle, you're going to run into issues and will need a wider axle.
Bottom Bracket Axle Length
The 68mm figure is the width of the shell on your frame.so to start with, you need a bottom bracket that will fit 68mm. The good news there is that virtually every bb on the market will fit that. The other common shell width is 73mmmost bottom brackets will do either size with the addition or omission of spacers on the cups (road and mtb typesbmx bb's are a little different). There are a few that are made specifically for either 68mm or 73mm, so just be sure that yours is made for the 68mm shell that your new frame has. The axle length is determined by the cranks that you're going to use. The vast majority of mountain bike cranks are made for a 113mm axle, but it just depends on the cranks you have. You can visit the manufacturer's website and maybe get that info, or just about any bike shop can consult a reference book to tell youor if you post it here maybe one of us can tell you. Give some specifics about the frame and cranks if you can. I'm sure you already know this, but you'll also need to match the type of axle splines to what your cranks use. Hope this helps some, post up again if you need more. :o)
