Paging Greg Loehr: on the subject of modulus

General Discussion
1

After having played with surfboard construction of various styles for a while now I’ve been trying to adjust build schedules in search of a poppy flex that so many have talked about. While I’ve made some really fun boards I haven’t found anything that would really be classified as too drastically different in the flex department. I’m aware of what are talked about as the major influences in flex of a compsand such as core thickness, shape and overall structure thickness. My question of the day is about modulus. RR offers resins of different flex and I’ve done a board with 2020 bottom and rails, trying to free up the fiberglass ‘shell’ so that flex and return rely more on the wood. While the board went well I don’t really know that the desired effect was achieved. So my question is:

Do these resins work in this way? As in controlling flex of the laminate.

-OR-

I’ve heard the analogy about modulus and how it relates to cloth and resin, in regard to matching modulus of resin with cloth:

“If you hang 10’’ rubber bands from a hook (cloth - higher modulus) and another set of 16’’ rubber bands from the same hook (resin - lower modulus) and then hang a weight from the end of that (stress) it becomes obvious that the larger rubber bands do little or nothing to support the weight.”

By this description it would seem that cloth/resin combos with differing modulus wouldn’t be as structurally sound because either the cloth or the resin is supporting the majority of the load. Do the alternative resins we use (RR 2020 for me) compliment fiberglass well enough that the structure is supported enough by both or would it be better to find another fiber that is better for 2020 or 2040 resin?

I’ve seen the ideal fibers for each resin on RR’s web site, mostly just wondering if using 2020 with fiberglass will achieve a more flexible laminate without asking for trouble. While an answer to this question would be great, I’d really like to learn more about modulus and it’s importance in choosing resin/fiber combos for surfboard construction.

I found this site with some interesting stuff on modulus in relation to composite fishing poles:

http://www.flyanglersonline.com/cst/cst010801.php

Go into as much detail as you care to, an eager young mind awaits.

2

I’m not Greg Loehr but something I have heard many times is surfboards get the majority of their strength from the fibreglass, not the resin. No doubt, resin helps but if you’re really serious about all this sort of stuff try checking out the ASM composites manual volume 21. If you’re prepared to swallow the price of the book that is. Apart from that, I think what you might be trying to achieve or the way you are trying to achieve what you are after might be complete overkill for a surfboard and quite possibly accomplishable in much simper ways.

3

A high modulus epoxy will achieve roughly double the stiffness of a low modulus epoxy.

A high modulus epoxy is twice as stiff, once laid up, as fiberglass.

If you mixed different moduli laminations in parallel on the same plane (lengthwise strips

on the hull for example), then you would achieve a significant shearing effect - the high stiffness

strips would bear the load.

2000 vs 2020 is not really that big a difference, though.

1980 vs 2040 would be a big difference.

Greg used to mix CF in the hull with a high modulus epoxy and fiberglass

on the deck.That’s a 10-fold stiffness difference. Stiffer on the bottom.

4

Now that’s some good stuff… from what I’ve read here - the bottom is often what gives in a buckle/snap. Always thought about that… maybe one of these days… I’ll get into that… Oh, so many ideas, such limited time and money…

5

The resin and its properties are very important in transferring shear loads between the fibers. When you have bending, such as in a surfboard, the rails for example experience a lot of shear due to compression on the deck and tension on the bottom. The 0/90° weave of the fibers (in standard cloth) does accomodate some of this shear, but just like the resin has an extensional modulus, it also has a shear modulus which will contribute to how much the different resins will stiffen up a board.

6

This reminds me - relates to the thread a while back about the 45/45 cloth.

I took Greg L’s suggestion and layed patches in that orientation, and will do so on my current board… As time tells, I’ll report.