I've been putting together an MS Excel workbook for designing twintips up to 140cm. Its not exactly slick but it gets the job done and has a decent number of parameters you can play with. Its still a work in progress but it produces the outlines and the rocker/ concave jigs templates that can be printed off 1:1 scale.



Some more pictures here:
http://myvirtualshed.blogspot.com/2011/05/off-table.html

I'v christened it 'BoardOff' design tool. If you want a copy of it (including the very beta flex profiling model) there is a link on this page where you can download it off google docs.

http://myvirtualshed.blogspot.com/2011/05/my-first-plagarism-free-board.html

Cheers
Matt

Thanks for sharing this spreadsheet.
Look forward to playing with it and will let you know if it "fits" boards I've built...

Your welcome. I'll be looking forward to hear if you find it useful.

Also, any tips on why the flex modelling is so far off the mark would be very welcomed

Matt

Epoxy systems, cure rates, fiber saturation, material density, material orientation. As you can see there are so many variables at play I don't think our meger home setups allow for much consistancy. Our homemade boards are literally one of a kind items which is good I think

The real goal of developing a model for the flex etc is to be able to put a bit of rigour around what changing a particular parameter will do to the board 'all else being equal'. For example, if you add x mm of concave what could you expect to happen to the stiffness, if I reduce the core thinkness by 10% what is the impact, is having a step profile at the tips better than graded and why, if I use thinner s-glass in place of e-glass what is the impact flex and strenght.

I'm with you on the issue of there being a few variables at play but all the ones you've mentioned and more have been modelled pretty extensively in the analysis of beams and composites that is pretty plentiful on the web. I think that the thing that will make it make it v. difficult to predict the actual flex of the board in advance (rather that the 'all else being equal what happens if I change ....' ) is how variable the properties of the materials are from batch to batch. If I calibrate the existing simple model to a test point on my current boards and take into account that I squeezed all the concave out of it by clamping the b'jesus out of it to test then the agreement when placed under other loads is not too bad. Already its helped me get my head around the issue of reducing core thickness vs reducing the amount of glass.

The thing that I'm not sure about because its been a long time since I did any continuum mechanics (20 years+) is whether there are affects that I have left out of the model that will have a significant impact.

Cheers
Matt