How do you measure rocker?

KneeBumps · Post by **KneeBumps** » Thu Mar 01, 2007 7:48 am

I keep getting into these circular discussions with shapers here on the east coast about rocker- everybody has their own way of measuring it. We usually wind up comparing several boards laid flat on the floor. It is of course a complex equation, because shapers deal with fluid design, curves and very different board lengths.

So- has anyone come up with a grand unified theory of measuring rocker? Something like- pick a point on the board (for example dead center/midpoint) and measure rise in each direction to points 1/3 of the length from the nose and 1/3 of the length from the tail?
I can already see the problems- best example would be contained in the thread on nose shapes- but seems worth trying to come to consensus.

Or, am I just out of the loop, you guys already have a system?
I think its important because we know so much about how "flat" plans glide through mush vs "rockered" plans pivot and turn, etc.

Beeline2.0 · Post by **Beeline2.0** » Thu Mar 01, 2007 8:57 am

rick mottola · Post by **rick mottola** » Thu Mar 01, 2007 10:28 am

here is a good discussion from swaylocks, Rocker Apex Revisited

Headwax. · Post by **Headwax.** » Thu Mar 01, 2007 2:24 pm

relative or absolute rocker?

realtive is the easiest

above pic of two boards of course

but in shaping

mark up the board you want the rocker from in 200 mm increments on the rail

after cutting out planshape mark up blank same way

superimpose rocker pics during the shaping process by using "layers" in your software so you can see one rocker at the same time as the t'other

compare marks on blank and board

you can extrapolate the roccker to a longer board by hiding one board within the other... so to speak

red · Post by **red** » Thu Mar 01, 2007 4:29 pm

From Miki, inventor of the APS3000 (but no longer involved with that company):

'To scan a board precisely you need one of those expensive laser scanners or touch scanners (please make me an offer, I have one for sale). But if you want very good results try the following:

To scan a board you first have to have a good look and check it out for anomalies like changes in the fluent curves (kicks) or irregular decksflow (S-deck) and so on. Keep these places in mind as you will have to create a set of extra measure points around those areas.

I use my measure frame but it should not be a problem to create something similar with a kitchen table. The frame has holes every 50mm to enable very accurate scans if desired but in general it should be sufficient to measure 7 points per curve + some extras for the
irregular stuff. I lift the positioning shafts to the board, as many as I like, and to keep things simple I use always the same X positions (X starts at the tail end with 0). Before doing anything else I mark a few horizontal measurements on the board so I can later use these to scan the
outline.
http://i119.photobucket.com/albums/o155 ... r_jig2.jpg
http://i119.photobucket.com/albums/o155 ... re_jig.jpg

Then I have a good look and determine the apex of the curves and I measure nose and tail rocker and thickness and determine the length (straight line, X max). It is now time to calibrate the calipers. I feed it through the top bar to the bottom bar and press zero. Every measurement I take now from the top is actually a measurement from the bottom, xactly what you need. I take a series of decks measurements, note them
down and take the board out. Next I measure down to the positioning shafts to get the data for the bottom curve and note these down too. These data I insert in the program as 􀂳GUIDE POINTS􀂴 and bring the curve through.

To measure the decks roll I use my top secret decksrollmeassurer together with my trusted calipers. And here I need some mathematics as well as the board thickness to create the guide points. You learn these things at year three."
http://i119.photobucket.com/albums/o155 ... easure.jpg

Failing all this put the board on a 2x4 (use the 4" for height), then you can get under it to measure it. You can never see the subtleties of inflow and outflow on a table. I secure the 2v4 so I can access it from the bottom and fix the board in place with nails (joking). I use a digital vernier caliper (you can see one in the 3rd link above - it's that vertical ruler thing) to measure from the bottom of the 2x4 to the stringer. I then subtract the thickness of the 2x4. I find this more accurate because I can rest the vernier on the wood and not estimate where I should start measuring.

Forget 'photos, projections onto walls, etc for relative rocker - they're all a waste of time. Get the measures right, put the boards into design software and compare the rockers directly. You can even re-size boards to get accurate comparison. Trust me, it's quicker than trying to get accurate enough photos.

Hint: the top deck is probably flat for most of its length, so if you set it up level and the 2x4 is level, then you can be sure of getting an accurate measure of nose and tail rocker.[/img]

frankfqr · Post by **frankfqr** » Thu Mar 01, 2007 5:25 pm

Thanks Red on trying to clarify, but still boiling it down to "laymens" terms, your pics. utilize a glassed board with stomp pad applied. Since by simply removing the pad and hence the weight than you will increase the tail lift and decrease the nose lift. Establishing the center point of reference I don't percieve from your pics. I guess I could gleen thru the 3 pages on sways and see what the consensus is, but the ungiven of weight distribution (possible affected by thickness, boxes, no. of fins,glass schedule, etc.) seems to come into play. Expound if you would please!! thanx, frank......

Headwax. · Post by **Headwax.** » Thu Mar 01, 2007 6:40 pm

red looks like the thing hutch used to use ... but it only had one horizontal component

bit mean to dismiss the photo way though

out of hand

frankfqr · Post by **frankfqr** » Thu Mar 01, 2007 7:10 pm

Well I'm not savy enough with the crosslink push button transpose tech.application, it does spell it out for me. I can see it plane as day. Not sure why Red dissed it so redily. I always went for the overall flow of the curve, foil as it were. No given point to measure, all up to the eye.

Headwax. · Post by **Headwax.** » Thu Mar 01, 2007 7:31 pm

red's way looks good but accuracy determined by the increments of measurementn and the straightness of the pointy probes ...

as opposed to overall comparison of curves lets you use the mark one eyeball... shows you eg where the rocker flattpoints are - lines between points aren't always a curve - except in the statitician's orifice ... oops... I mean office

of course the photo method only shows the rocker highpoints and not eg the stringer rocker

Frank

photo soft ware programs usually support what are called "layers".

Eg In real life you have a photo of a board.

You take another photo of a board.
And place it over the first photo. You now have two sheets of paper (layers) one over the other.

Of course you can't see the bottom photo any more unless the top photo (layer) has some kind of transparency like tracing paper.

In software you can adjust the property of a layer so that you can make it more or less transparent.

That way you can see the photo beneath it.

If the top layer is semi transparent you can move one photo (layer) till the curves are the boards are neatly superimposed.

That way you can measure the congruence, incongruence.

cheers

PS hope this is clearer than mud.... but in real life muddy water is the fresh water

Headwax. · Post by **Headwax.** » Fri Mar 02, 2007 12:30 am

Psa they are both 5 ten... obviously one man's 5 ten is another man's five nine (or eleven)

red · Post by **red** » Sat Mar 03, 2007 5:34 am

HW,
Thought you might be hurt by the 'photo dissing. Don't take it personally. I went through the whole gamut of methods before I did it properly. I now have a 12' frame that lives in my "measuring room" and can get accurate rockers (plus thickness) in minutes.

Frankfqr
As I mentioned, the top deck is horizontal. This means that low point of rocker is fixed - you can tilt the board (by adding weights, for instance) but need to keep the top deck horizontal. Then you'll get accurate nose and tail rocker.

But nose and tail rocker are pretty limited components of board measurement (like wide point, 12 back, 12" forward) - it's possible to build a limitless range of boards from those 5 dimensions. You can drop the rocker 1/4" in the intake and change the board to one that pushes water (by the way, that's why the photo method is not accurate enough. I use photos as underlays in design software and they are not accurate enough - I still have to set the rockers up by hand).

red · Post by **red** » Sat Mar 03, 2007 5:52 am

lines between points aren't always a curve

I think flat spots are over rated, but that's personal
Put a light behind the jig and the subtle curves are easy to see. My rockers only touch at one point - the intakes and outflows are very subtle - you wouldn't pick it without the light.

RE accuracy determined by number of measures and straightness of pointy things.
Number of measures - of course - that's the benefit - take more measures where you need them.
Straightness of pointy things - not if you only use them to hold the board in place (top and bottom) - and use a carpenter's square to ensure measures are vertical. Using this means that you can set it up square to the wood base, push the ruler up to the bottom of the board, tighten the holding screw, remove and read off the height.

frankfqr · Post by **frankfqr** » Sat Mar 03, 2007 4:55 pm

O.K. red.. seriously, thanks heaps, I percieve that I must throw away the pursuit of the center point or flat point to measure rise in either direction. If I take it further with your device than I can throw away your terms utilizing "horizontal" because the method can be used mounted verticle. I sense that the primary concern is the over all curve which brings it back to weight distribution???, i.e. glassing schedule, no. of fins, boxes, pads?? For that mater our weighting and unweighting, thickness etc. etc. Oh man I'm spiraling out of control. You state that changing the nose kick 1/4 inch will push water, but won't I achieve that by increasing the the tail flotation? O.K. kneebumps you started this, is it all crystal clear for you? Elighten me please......

Beeline2.0 · Post by **Beeline2.0** » Sat Mar 03, 2007 5:01 pm

red · Post by **red** » Sat Mar 03, 2007 5:51 pm

Frankfqr
Weight distribution is important - this is why shapers try to get equal thickness 1' back, 1' forward, etc - to keep the board balanced. I wouldn't worry about pads. cameras on the nose, yes, but not pads or a couple of ounces from fins.
Rocker is far more important than weight. You can easily adjust for a pound or so by shifting body weight fractionally, but you can't compensate for poor rocker - you end up stalling the board trying to "add" nose lift by leaning back - even if the fat tail will support you in this position. Board design is about balance. Introduce one strong element and it has to be balanced by another. Keep everything subtle and the balancing effort is that much simpler.

Beeline
If you measure both top and bottom, then all positions become relative. This is good enough for board design because the lift and thickness needs to be fitted to the blank, not just the bottom.

In general:
Rocker low point doesn't matter within an inch or so - it's the relative intake and exhaust that's important - if they are right (and balanced) then the board has a good chance of working - regardless of outline or ultimate lift at the extremes