Flexing... BB vs crank vs...
 

Tonight I found something interesting. The POS patrol bike I am forced to ride occasionally is really really flexy side to side. If I push down on one side of the crank the whole frame flexes from the BB area. The seat tube is not that thick either and the frame is cromo (steel). Now that got me wondering how do we actually measure stiffness, BB side to side or front to back etc? Also how can you isolate where the flex is coming from (i.e. BB or crank arms or seat stays, etc).

If the chain rubs on the front derailleur, the bb is moving. It's also not one tube that's responsible, especially the seat tube... it's the whole package, particularly the bb shell design and chain stay/downtube shape, daimeter, thickness, etc... Cromoly comes in many many varieties. Some were/are very stiff (Pegoretti Big Leg Emma), Stiff/Medium (Colnago Master Light), and soft...most mid-level road bikes (Bianchi Veloce, etc...) I doubt that any bb's are going to move front to back. I think there's a whole generation of younger or newer riders who've never ridden a really good steel frame. That's not a judgement, just an observation. It's not where the market is these days. Rider weight also makes a big difference with particular tube sets. You're too heavy for a top quality EL OS tubeset, but I'm certain couldn't budge the Pegoretti B.L.E. or and old columbus MAX bb. All said and done, a mediocre bike is just that regardless of the materials, and yes, manufactures can measure bb flex. Personally, I really liked the old Columbus MS (multi-shape) tube sets. A bit heavy, but a good smooth ride that climbed well out of the saddle...guess the age is showing, eh? :)