Flex Measurement

[MikeDaniel]

As I’ve discussed on the “show us your flex” thread, at Coil Construction we use a simple jig

to measure flex in different portions of the board. I’m offering up it’s design so that the builders

of flexible boards everywhere will have a means to compare their flex rates accurately. I’m

open to input on improvements, but we need to reach agreement on a design so that consistent,

repeatable data is obtained…

This is NOT an ASTM technique, rather it’s a simple, easy to make garage-style device. We can

go the high-end route if anyone wants to, but that requires submitting boards to a testing facility,

paying for the tests, etc.

Wish I could post pictures, but I’m still trying to figure that one out. That re-sizing thing has got me stumped.

This “jig” is so simple that pics aren’t really needed, anyway.

Start with a baseplate of plywood or heavy masonite about 4’ x 3’. Attach 4’’ x 4’’ beams on the long

sides, one on each edge, the same length as the base. Cut two more pieces of 4’‘x4’’ to span across

from beam to beam. Attach ONE of these securely at one end of the jig. The other one can be unsecured

as you will see. That’s it for the ‘‘building’’. If your boards flex a lot you may need to raise the crossmembers

to get more of a gap to the baseplate.

To measure flex in the center of the board, say the middle 3’ of a shorty, place the moveable crossmember

3’ from the fixed one, and set the board across them (like a rack) deck up. Place an ‘‘auto-stop’’ tape measure

on the base, extended to meet the bottom pre-load. Place the load in the center (we use 40-50 lb bags of

mortar) and the flex will push the extended tape down. Subtract the loaded measure from the pre-load and

you’ve got a measure of flex per whatever weight you chose.

To measure tail flex you don’t need the moveable crossmember. Slide the board (deck up) under the fixed crossmember

with the tail sticking out the end and most of the board resting on the baseplate. Pick your distance to be flexed

and locate the board so that the crossmember contacts the deck that distance up from the tailblock. (Some

distribution of load may be needed at this contact point.) Now you pull UP on the tailblock to flex the tail. The

attachment to apply this force can be as simple as a C-clamp and some shims or as complex as you want. To compare,

though, it needs to be a consistent point, say 1’’ up from the block. Find the starting point (your unflexed ""zero’')

and pick the distance you’re going to flex. A helper is kinda needed for this one, he measures while you pull up

on the tailblock. We use a fishing scale between the puller and the tail to measure the force needed to produce

a certain distance of flex.

The tail flex can be done two ways; one isolating the flex, one allowing the middle of the board to flex at the same

time. If you want to isolate the flex, secure everything forward of the crossmember to keep it from bending on the

fulcrum created by the crossmember. If you want to let the middle flex, only support at one point forward of the cross

(and this needs to be consistent for comparibility)

Now we’re open for discussion.

Mike

[peteuk]

hi mike,

interesting subject flex, i must admit i don/t know too much about it all really , i have been making some balsa compsands and in general are stiff , what are your conclusions regarding flex and has your knowlege benefited the boards you have built, my own thoughts based on where my son tends to break poly boards , is flex is needed in front of the forward fins , probabaly from the middle to the fins is where most of the stress is and in short he overloads this area , what i am thinking of doing is making this area able to flex more but have a linear control over it , maybe some pre glassed carbon strips in the centre of the board or springy wood so that there is initial movement but the more it flexes the more force is required to flex it further, pete

[MikeDaniel]

First off, I do believe my boards have benefited from our knowledge of flex, but then

you’d really have to ask the riders. I know we have a lot less broken boards, for sure.

We had a good discussion recently about improving the break strength of "conventional’’

boards (hand-lammed poly or epoxy w/center stringers). I’m not sure if you’re asking

about reinforcing your son’s poly boards or are you building him cored skin stuff?

While I’m on the questions, what are the dims on his boards? Is he breaking them on

air landings or more typical circumstances?

I’d be very wary of any ‘‘spot’’ reinforcement, as it just tends to create a focusing of

load on the end of the ‘‘spot’’.

Some of the cored-skin guys can chime in here with advice on how to make those more

flexy, that’s not really my area of expertise.

Mike

[peteuk]

hi mike , my son has broke 3 poly boards in the last year ,two from were he hits the lip and his back foot over stresses the board and the 3rd one on a floater on a small wave ,that board was only about a month old , he rides 6ft 1 in 18 1/4 x 2/14 , i have been making balsa cored skin boards just to try and get some more life from his boards but in general they are stiff or so they feel to him , i have made one with very thin skins and minimal glass and that has been the best so far but i am getting back to a durability issue hence my thoughts on allowing the board to flex in the area in front of the fins but controlling or limiting the amount with internal controls,

how do you view how a board flexes , i can only imagine them flatening and then spriging back to the original rocker ,i can/t see a board taking on more curve , just my thoughts, pete

[MikeDaniel]

Pete, we touched on that flattening flex in the "show us your flex’’ thread that

spawned this one. But most boards break from a load on the deck side that increases

the rocker until the structure fails. That’s what’s happening on your son’s boards.

(but I’d have to see the breaks to be positive) Kids sure can break 'em, can’t they?

Flexing to take on more rocker in turns and then releasing to a flatter trim rocker is

the goal of many builders. That’s the type of flex we usually measure. If you want to

first-hand feel this flex get towed behind a boat or PWC or kite on a surfboard. Put

your feet close together in the center of the board and you’ll feel some flex! We don’t

experience these kinds of loads in normal surfing.

I do know enough of what the cored-skin guys do to tell you that if you want a strong

skin on a flexy board the core needs to give(in order for the board to flex) and/or the

skin has to have the ability to shorten on the deck-side, and lengthen on the bottom.

Trying to induce flex in a particular area to keep a board from breaking seems sketchy

from what I know about composite structures in general. But as I said, one of the cored-

skin guys can give you specific advice on that application.

Mike

[f1f859bda2f2a3ab344c]

mike

i need a picture please

excuse my inability to get it

pm if you like

hmm not many views

and not many interested in demonstrating it either?

cmon josh and bert

i know yours do!!!

pete

if you want a hard to break board that flexes

maybe it might be time to try a different build method

btw boards that snap from airs

the deck can fail in either tension or compression

though it most likely compression

[MikeDaniel]

I’m sorry about my inability to post pics. My explanation was way more

complicated than the device itself. The mid-flex test is just like setting

the board on a shaping rack and putting a load in the center. The tail-flex

test uses one of the "racks’’ and the ground to pin the board down while

you pull up on the tail. Does that help?

The lack of views is probably my fault. Should have given it a sexier title.

Anything with ‘‘China’’ in it seems to be popular.

Thanks for your participation and showing us your cool work.

Mike

[pierrek]

Yes, photos of the system and a demonstration (photos again) would be great to make things perfectly clear… Someone could post the photos for you, I could try if you like (I can give you my e-mail by private message).

[meecrafty]

The lack of views is probably my fault. Should have given it a sexier title.

Anything with ‘‘China’’ in it seems to be popular.

or you could claim that your flex measurements allow you to do 37MPH in headhigh beachies

that’ll get things goin…

Not sure if there’s a jig there Mike…Ive been using two 4x4s spread apart a given distance for supports for a while. Just slap those puppies on the floor and test anyway it makes sense. What’s really weird is that the difference between a stiff board and a flexible one is very small, but it makes a huge diff. Not only turning performance, but also ride feel via shock absorbtion and dampening. But like you said, too much can be slightly problematic.

[MikeDaniel]

OK, I’m going to work for a while and I’ll take some photos. PM me your

email and I’ll send to you. Sorry I’m so helpless on these machines. Thanks

in advance for your help.(if I’m successful in emailing them)

craftee, you’re right there’s not really a jig there. That’s why I enclosed it in quotes

when I first made reference to it. The idea is to have something very simple, but

consistent enough that comparisons can be made by builders everywhere…

Mike

[KKsurf]

I like it!

Good on ya for putting some hard data to something that most people avoid.

Flex is definitely something that has elements of both qualitative and quantitative nature.

As part of experimentation with a test rider(s), it would give some valuable information.

Of course, a certain flex pattern would mean different things to different people in different waves, which is where the qualitative aspect comes in…

If I had started measurements from my first composite board, I might have had some decent data by now. oh well.

The great thing about composite boards is the consistency and repeatability that you can achieve, control of variables, allowing you to really nail down what is contributing to flex.

It’s a long road to really knowing exactly what is going on, and thats a path i’m drunkenly stumbling down at the moment… a few wrong turns here and there, get distracted at the pub on the corner for a while… but on the right track

Having a decent testing and recording method definitely will shorten the development cycle.

[f1f859bda2f2a3ab344c]

hello dave

i was thinking more along these lines for “hype”

http://www.youtube.com/watch?v=ghv43OoTGa4

lol:)

might try the same methods to market my compsands in NZ as well

i guess no ones posting/viewing cuz no one gets it??

anyway

i get the idea mike

what say we decide on a few measurements to place the members as reference points

3 measuerments

above the front foot ( maybe maybe put the beam at center and 12 inches from the nose)

the middle (24 inches from each end)

and the tail ( at 24 inches from the end)

anyone want to improve on those suggestions?

[dougirwin13]

Pardon the edit. After posting I decided to rewrite and include a little sketch Pictures, 1000s of words and all…

I’ve been reading this thread, but hadn’t commented yet. Now seemed like a good time

I posted a simple idea based on a set weight, set supports and measuring deflection some time ago.

At a high level that’s the standard way of measuring a lot of different archery gears’ flex (bows, arrows, etc).

Very simple and works very well.

You should be able to do the same thing with surfboards by having two supports, say, two feet apart. Simply place the section of board you want to measure flex for across these two supports. Pop a 20kg (@44lbs) weight on the centre and measure how much the centre of the thing deflects downwards (in millimetres or 1/1000/inch). That measurement itself can easily be taken in a number of ways. Marking something in the background with the assistance of a laser level would be one simple approach.

Anyway, here’s the quick and dirty diagram:

One thing easy to miss will be how readily it recovers. with many materials 90%-95% of recover is “instant”. But many constructs continue to recover somewhat over the following few minutes. The more reduced that follow-up recovery the “springier” the construction is considered to be under subjective use. With bows this is often used to estimate hysteresis and internal stress and is measured over time.

Now, as for the “how can you measure a feeling” train of thought. Of course you can’t. But you can get an empirical measure of certain characteristics. And other areas of sport, hobby and research tend to find the same thing over and over again. Given enough of these profiles taken people tend to find they have their own preference for what works well with them. In archery the “standard” for a bow is about 50#@20" of draw. Personally I am quite comfortable with 55#@22" and do well enough up to 65#@22". With hard training 70#@22" becomes comfortable for me. Those numbers probably don’t mean anything to anyone on here, but they would give an immediate image of what I shoot to any other experienced archer. And the same applies time and again in many other areas.

But we’ll never know how that applies to surfcraft until people start measuring the flex in their favourite boards and posting the results. Because it’s only from there that you’ll be able to start comparing flex.

Anyway. Just my AU$0.02 and YMMV… Considerably in some cases.

[joshrastafari]

Hey Mike, I am interested in some form of system to make consistent measurements, I am tired of standing on my boards upside down trying to “feel” the difference. I can feel it in the water, but measurement and analysis would make me feel like I was doing something much more scientific than I am.

If you’d like, you could email or pm me some pictures, and I could post them. Or… I could come by the factory and take some shots, look at some top secret vac bag coil stuff. I believe my ex-neighbor–Eric is it?–is the man behind the coil stuff. I stumbled upon his operation, and he said I already know too much, but not in a mob you’re gonna get wacked sorta way…

Anyway, I have a pretty good image, but a visual would be better. I would post the results of the boards I still have. I think a collection of build schedules with flex measurements and rider reviews would be a helpful resource.

Josh

[MikeDaniel]

Glad we’re getting some interest going here, thanks to all. I have some photos but

still can’t get them to upload.

josh, since you’re close and onto our trail already, I’d be glad to (try to) email you

the shots if you’ll post them for me. PM away.

Mike

[pierrek]

Here are your photos Mike, I had to compress them so they could upload :

Photos 1, 2 and 3

[pierrek]

Photos 4, 5 and 6.

[pierrek]

Photos 7, 8 and 9.

[MikeDaniel]

Many thanks, Pierre.

I think the mid flex measure should be easy to see. That’s the first three pics.

The tape is extended to meet the bottom pre-load, as the board flexes it pushes

the tape in. I keep saying I’m going to get one of those laser measures but

haven’t gotten around to it. That’s a 50 lb/22.8kg bag of mortar.Should have

put 2 bags on there to really bend it.

Pics 4,5, and 6 are the tail flex.The movable crossmember doesn’t come into play.

That’s the fishing scale attached to the C-clamp. As you pull up on the scale, the

baseplate and rear crossmember pin the board and the tail begins to flex. A helper

reads the square and/or the scale.

The last three pics show the ‘‘jig’’ alone. Full garage tech. I use cedar because its

stable, and kinda soft(easy on the boards). Plus the bugs won’t eat it and it makes

my shaping room smell good.

Sorry about all the delay and a public thanks to the others who offered to help(josh & doug)

I think this can be useful for comparisons OR just keeping your own records for your

builds. And there are variations both in testing and set-up of the jig we can discuss.

More later.

Mike

[f1f859bda2f2a3ab344c]

very good mike

i believe craftee was playing with a similar idea a while back

well done