Starting over...

In an earlier post I mentioned how my unwise use of a dremel tool with a burr caused a few nicks in the tubing around the seat mounts. Initially I wasn't worried about this, but the more I thought about it, the more it bothered me. I'm worried about stress risers and the loss of strength caused by the thinning of the tube. I ran some numbers based on assuming that the rest of the bike was perfectly rigid (obviously false) and hitting a 2" bump at 50 mph. Granted this was not an FEA approach or anything so sophisticated but it should give a nice upper bound.

I found that the maximum stress was right at the seat mount (no big shock there) and with the full wall thickness I was right at the limit of 4130 steel. Thinning the wall by .005" increased the stress by close to 200MPa which is WAY over the limit. Now I realize that these numbers are way high since it assumes that the rest of the bike is totally rigid, when in fact the tires take up a lot, the fork flexes, the wheel deforms slightly, etc. All of the above dramatically reduce the maximum stress. What the numbers do tell me is how much relative strength is lost by thinning the walls at that location.

Because of this I decided to put that frame aside and start on a new piece of tubing. Realistically the original would probably never fail, but this is my first frame and I just don't have the experience to make that call. I might finish the original frame yet for someone lighter and/or less aggressive than me. 50mph downhills are an everyday part of my commute.

The good thing about all of this is that I still have several pieces of tubing left and the only thing I needed to re-order was the headtube. I took the day off work on Friday and spent some time working on the bike. I am basically back up to my stopping point on the first frame. I'll go over the details in future posts.