Some riders have had a bad experience with a creaking bottom bracket and want to be sure their next frame doesn’t creak. For that purpose, they prefer a threaded bottom bracket.
But be sure to choose the best system for the type of frame you have, because not all threads are created equal.
What’s the bottom bracket problem
Bikes used to have threaded bottom bracket shells, and they sometimes creaked but not that often. When frames became carbon, it got complicated. Putting threads into carbon is complicated and fraud with problems, so the only viable solution is to make threaded metal inserts and glue them into the frame.
But engineers don’t really like gluing metal inserts into a carbon structure. Insert “oh but in airplanes” comment here. Of course sometimes it’s unavoidable, but if there is an alternative, that’s usually preferred.
Galvanic corrosion is the issue most people know about in this regard. But even more general, reducing complexity is the best way to make a structure more reliable. So when given the choice between gluing inserts in or not, most engineers will choose the latter. So they looked for alternatives.
PressFit bottom brackets
This came in the form of PressFit, where the bottom bracket cups slide into the frame without threads and the crank arms basically hold them into the frame. That got rid of the threads in the frame, so the BB shells could be made out of pure carbon/epoxy without the needs for inserts.
One problem solved, but the issue of creaking bottom brackets became more prevalent. So while PressFit didn’t “invent” creaking bottom brackets, it surely “popularized” them. This is true in particular when frame makers didn’t properly understand the tolerances required for a PressFit BB shell.
There were also other reasons some liked moving to PressFit. It is cheaper to produce a frame without metal inserts than a frame with metal inserts as you save the cost of making and installing the inserts (Economics 101).
There’s nothing wrong with doing something that is less expensive and saves material resources in the process. But the result should not be inferior (and hopefully even superior). Which brings us to the third reason engineers were trying to get rid of the inserts: weight. What isn’t there doesn’t weigh anything (Physics 101), so if you can eliminate the metal inserts, you can make your frame a bit lighter.
Proper PressFit frame, still creaking
While proper manufacturing of the frame and cups can prevent creaking in most cases, the problem didn’t completely go away. Most importantly, as a consumer, you can’t easily know if a particular frame model will create a problem or not. So just as “insurance” even more than a desperate need, threaded bottom bracket solutions are gaining traction. Solutions appeared in two directions:
Threaded BB shells & bottom brackets:
This is certainly a great solution for metal frames (which have also made somewhat of a resurgence). If your whole BB shell is already metal, you may as well cut threads into it. That just leaves the question of which system, and since many cranks have gone oversized to 29-30mm diameter, it makes sense to use an oversized bottom bracket system like T47.
Thread-Together bottom brackets:
Sounds similar but completely different. For carbon frames, we don’t want to go back 15 years and glue threaded metal inserts into the frames. All the disadvantages still exist.
But we do like to thread the bottom brackets to avoid any chance of creaking. With the thread-together bottom bracket, the parts thread into themselves instead of threading into the frame. So the frame has a smooth, full-carbon BB shell for PressFit. And the bottom bracket consists of 2 or 3 metal or reinforced plastic pieces that thread together.
So as the left and right cup thread together, they align the left and right bearing, and they slide into and clamp around the carbon BB shell. In almost all cases this solves a creaking BB. And best of all, the frame maker doesn’t have to glue any metal inserts into the frame.
The beauty of thread-together bottom brackets
Thread-together bottom brackets avoid all the problems of traditional PressFit, keep all its advantages and add three more:
- Avoids the need for metal glued-in inserts that can potentially come loose
- Doesn’t add additional cost to the frame (although it does add some cost to the bottom bracket, since threads need to be added there)
- Keeps the frame light, although it does add some weight to the bottom bracket. So it weighs a little more than PressFit, but less than a setup with threaded glued-in inserts.
- Optimizes bearing alignment. Both bearings are part of the same structure, the best way to ensure they’re aligned. And proper bearing alignment means the lowest possible drag.
- Proper engineering. It puts the threads in the material (the bottom bracket) designed to be threaded and doesn’t introduce threads into the part that isn’t suitable for it (the carbon frame).
- If the threads ever get damaged, the damage is contained to an inexpensive, easily replaced part (the bottom bracket cup). That’s a lot better than ruining the threads in an expensive frame.
What does 3T recommend?
We’ve had good success with standard PressFit bottom brackets and thus we can recommend them without issue. But we do understand some people are nervous about them – especially if they have had a bad experience.