I am designing a seat mast for one of my customers and this is the first time I have done this style and was hoping for some input as I want to be sure this has sufficient strength. I wanted to get some different ideas about testing methods to find out what type of load the seat tube can withstand under the weight of a rider. What type of formula would I use to calculate the amount of stress on the seat tube? I am gonna be using a 73 degree seat tube angle with an externally butted seat tube. Any info would be appreciated.

I don't know about formulas, I work off scantlings and weight budgets. If someone else is doing it it has to weigh a reasonable amount, and it has to come from about one of 3 wall sizes if it is/could be 4130. I don't bother with calculations, though it would certainly be reassuring if I could. I know enough seat of the pants engineering to know what happens in gross terms when I mess with tubing diameters or wall thicknesses, but I can't "run the numbers". If I could I would still need some fairly difficult to arrive at info for the structure and the loads it is going to be subjects to.

Two other things are how the stuff works, and how much of an extension one is working with. Is it a seat post less design, or say something like a Bike Friday. And what does it interface with from basically nothing in the former example to ferules on the low and and seat posts on the upper in the BK case. These issues can affect sizing also, and often allow one to reverse engineer/copy a part. My guess for the former would be .035, and the latter something as heavy as 0.58, depending on whether that would feel like a crowbar or not. If it felt like half my weight budget, I would drop down to .049.

I understand most of what you are saying Peterpan. I am not a numbers guy or engineer either, I have learned most everything by trial and error but I also like to hear from different people and weigh the lot of it, but in the end you are back to the actual test. You lost me a bit with the numbers and what is BK case? All I can tell you is that I am planning on using an externally butted seat tube and am building an aluminum seat mast/seat cap similar to the Ritchey stubby seatpost. I don't want the frame to ride like a jackhammer but its better than a failing seat tube. This is my first design of this product and I have to rely on my customers for feedback and I can alter it accordingly. The reason I asked about formulas is that a bike mechanic told me he had one for determining stress under a similar circumstance but couldn't remember it at the moment. There are probably so many variables that playing with numbers is hardly worth trusting, back to trial and error.

Sorry BK meaning the4 »Bike Freiday extended post which allows their frame to fold and have a long seat mast. It is a very long extention compared to the odd bike one sees that is `cool`and doesn`t have a seat post just an extension of the seat tube. Not familiar with your project, but I can say that Aluminum often ends up about twice the wall thickness of steel, so if that were part of your problem you could reduce accordingly. Of course these are crude eztimates, but then we don`t have an infinite range of tubing dimensions to sellect from. Oc course, if one is using all he different tensile strengths it thickens the choice a lot...