It looks like the point has at least a small radius, which should be plenty to avoid a major stress riser in steel. I'd have to do some math that I haven't done in a LONG time, but I believe steel is strong enough that the bottom bracket itself should be fine under almost all circumstances. The tubes are weaker, so the stresses are transferred to them, and they break first.
A Hydraulic tube with the same Outside diameter and wall as a bottom bracket shell could probably take about 3,000 Lbs per square inch as a working pressure. The rated burst pressure would be about 4 times as much. This sort of load is similar to the load on the bottom bracket shell while pedaling, but only similar. If the cut out had a point under the bearing cups, or if the shell was made very thin, it might break at the edge of the shell, but I can't picture anyone making it that way.
Steve Birmingham Lowell,Ma
Date: Wed, 7 Apr 2004 10:46:23 -0400 (EDT) From: wheelman@nac.net To: classicrendezvous@bikelist.org Subject: [CR]Inquiring minds want to know Message-ID: <49696.146.152.216.1.1081349183.squirrel@webmail.nac.net> Content-Type: text/plain;charset=iso-8859-1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: list Message: 4
I have somewhat of a mechanical background, above average I like to think. I know you have all seen these photos before of the track Confente bike. I just got fixated on the BB photo. Particularly on the spade cut out on the bottom of the BB. Looks like a lot of material removed to me. The BB takes quite a bit of the load when riding a bike. Each pedal stroke does flex the frame at the BB back and forth. I can only think that the BB itself is under a lot of stress. Now this cut out of a spade comes to a nice point in the center of the BB and to me makes it a natural stress point in that metal shell. I understand that the other tubes are supporting it. I also have seen BBs separate from the tubes like most of you have. Has anyone seen a BB fail at a cutout point. If you put this BB in a tensile/compression machine and applied pressure slowly, I guarantee it would fail at the point of that cut out. Not that I am worried about that, just something to open discussion on.
http://www.serotta.com/
Ray Homiski
Elizabeth, NJ
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