Jim’s Tech Talk
by Jim Langley
The new T47 bottom bracket comes to the rescue! I’m joking with this week’s title, but only a little. I’m poking fun at the bicycle industry, which in my opinion – and many other two-wheeler trade pundits’ views – took an ill-conceived detour about 12 to 14 years ago. That’s when most of the major bike makers started to adopt new designs for frame bottom brackets, which the T47 BB now looks to solve.
What’s a Bottom Bracket?
The bottom bracket is the part of the frame where the crankset is installed. The crankset is the mechanism that the pedals are attached to. The crankset turns when you pedal because it’s held inside the bottom bracket of the frame with a mechanism containing bearings.
Unfortunately for anyone learning bicycle terminology, the mechanism that the crankset turns on – the one that holds the bearings, is also called a bottom bracket. So, to try to keep things straight, the frame’s bottom bracket (an integral part of the frame), is best referred to as the “bottom bracket shell.” To simplify a little, the bottom bracket is often abbreviated as “BB.” So you can say “BB shell.”
And, the mechanism that is installed into the frame bottom bracket shell, is best referred to as just the “bottom bracket” (or, say “BB”). Confused? Sorry. Please visit this bicycle anatomy page on my personal bike website and look at the labeled picture. Also, scroll down just a bit and read where I explain and show the bottom brackets: https://jimlangley.net/wrench/bicycleparts.html.
Briefly, what happened back then was that companies went away from the tried-and-true threaded bottom bracket. Instead, they switched to a myriad of different press-fit systems where the bottom bracket bearings are held in place in the frame’s bottom bracket shell primarily by friction, not threads.
Depending on the specific design, sometimes bearings fit inside cups that are pressed into the frame. Other times the bearings are held in place with thread lockers or special carbon glues.
There are reasons why these designs came along and why these types of BBs are still found on the majority of bicycles. Mostly it was because carbon frames require metal inserts in order to provide threaded bottom bracket shells (cutting threads in carbon weakens it). So it was easier and lighter to stick with just the carbon shell without threads.
Noisy, Loose or Binding BBs
But, unfortunately, as often happens with new ideas, there were and still are lots of problems with some of the designs and how they were implemented. It’s not just the design, either, problem can occur from mismatching cranks and bottom bracket parts and improper assembly techniques, too. Because there are so many different designs, it’s easy to make mistakes.
The issues usually were/are caused by bearings and cups in the frame that don’t stay fixed and begin to move slightly. It’s easy to understand how this can happen when you consider the massive pedaling forces bottom brackets have to withstand.
And, when cups or bearings move, it can make a bike click or tick or clunk with every pedal stroke. Or, if the bottom bracket part moves enough, it could even cause worse problems. And, it’s possible to have binding with problem bottom brackets, too.
Tip: If you’re having problems like this with a non threaded BB, they can usually be fixed by a mechanic with enough experience with your brand and model of bottom bracket. So, for example, if it’s on a Specialized, you would try to find an ace Specialized mechanic. Failing that, there are a few innovative companies that provide aftermarket BB solutions. A few examples are Wheels Manufacturing , BB Infinite and Praxis Works. (Disclaimer: I work for Praxis.)
The T47 Threaded Bottom Bracket
It’s these problems with the non-threaded bottom brackets, that has me excited about the relatively new bottom bracket type, called T47 (T for “threaded,” and 47 for their 47mm nominal cup diameter). Finally, we will get back to bottom brackets and cranks that get installed once and run trouble-free.
And, at 47mm, the T47 cups are significantly larger than the roughly 35mm diameter threaded cups on the old standard BBs. So this new threaded standard accepts the oversize axles on modern cranksets, which is arguably the best thing to come from the industry’s detour to no-thread BB designs.
Already some major players are using T47 BB shells in their bicycles, such as Trek and Specialized. They’re not on every bike model, so if you’re shopping, look carefully at the specifications to make sure.
If you’ve ever had a problematic bottom bracket, this new back-to-the-future standard should be welcome news. In fact, I just bought an Ibis Hakka MX partly because it was the only bike of the ones I was considering that has a T47 BB.
Ride total: 9,459
Jim Langley is RBR’s Technical Editor. He has been a pro mechanic and cycling writer for more than 40 years. He’s the author of Your Home Bicycle Workshop in the RBR eBookstore. Check out his “cycling aficionado” website at http://www.jimlangley.net, his Q&A blog and updates at Twitter. Jim’s streak of consecutive cycling days has reached more than 8,000. Click to read Jim’s full bio.