Originally Posted by
Point
So, why is press fit “optimal”? There has been no correlation shown between any measure of stiffness and efficiency. Numerous threads on this site have been dedicated to that one. Weight maybe, but we’re talking about pretty small amounts when the bike and rider are taken in account as a whole. A 100 gram savings for a 160 lb rider/bike combination is a 0.137% advantage, and then only really on climbs. A BSA bottom bracket is limited to the diameter of the axle as you stated. A 24 mm axle is stiffer torsionally than say an old 17 mm square taper (higher “J” value), but any amount of “wind-up” (very small) in the axle will be returned with essentially no loss of energy during the pedal stroke. As for external bearings, they don’t add anything to stiffness since that is a function of the frame construction, not the location of the bearings in the BB area.
Setting aside stiffness and lightness claims (both of which are real but in my opinion likely not very meaningful in the real world) the main benefit of press fit ironically has to do with reliability. Manufactured correctly, press fit will result in better bearing alignment than threaded. Raoul at Leuscher Teknik and Hambini both have videos explaining this:
Calmer but less complete explanation:
Annoyingly trying to be edgy but has a complete explanation starting at 11:50
To summarize what they say, threads need a bit of "slop" to work properly. Press fit does not need this slop. By removing this slop, it's much more likely for the bearings to be well aligned. Bearing misalignment is by far the most common cause of bottom bracket issues. As a result, if you manufacture a BB correctly it is theoretically more likely to run smoothly and less likely to have problems. It's the "theoretical" part that's the issue because press fit relies on higher manufacturing tolerances than threaded. Note that the manufacturing tolerances required by press fit aren't unachievable by any means, but they are tighter.
I think the main benefit of threaded BBs is that they're easier to work on. I can replace the threaded BB with a $10 tool if you have an issue and simply lubing the threads will quiet most BBs. On the other hand, properly pressing in new bearings if necessary isn't that hard either. Also, it's worth nothing that press fit is the standard world wide for every bearing. Threads are a holdover from an older age of manufacturing. The underlying engineering and tolerance requirements of press fit bearings are mature and well understood. Bikes are more weight sensitive than many other bearing application which introduces some difficulty, but this is a solvable problem.