# Leaf Springs

Greg Loehr mentioned leaf springs on YouTube (thanks Greg). My understanding of leaf springs is that that are not welded together, rather bound together by some sort of metal band. I also believe that they are independent of each other but have the strength of all combined. Is this what you compsand guys have figured out?

I have no idea how this is relates to comp sands, but I can tell you about the mechanics of leaf springs. They are layered arcs of spring steel that are bound together (not welded), and have a friction effect between the leaves. As with any compression spring, they have a “rate” which is how much resistance the spring will give relative to how much it is compressed in distance; like 5 lbs per inch. Because of the layering of longer to shorter arcs in the leaf spring, the rate of the spring is quite variable. Specifically, you can compress it a greater distance than a wound coil spring (where the coils would smash against each other). There is another type of leaf spring called a parabolic where the leaves are thicker at the center than the ends, and the ends of the stack are welded together. The main purpose of leaf springs is to distribute the load of whatever they support over a larger area than a round coil spring would. To make leaf springs work, the load most be applied over the ends, never directly on the leaves.

In surfboards, the only relationship I see with a leaf spring is if you turn it upside down (so that only the nose and tail are supported) and stand on it. Mechanically, any surfboard is a composite solid; there is not enough elasticity to consider it a spring because the rate would be so small. However, if we’re trying to describe some sort of twisting effect, that’s different. This is torsion, and a surfboard can be considered a torsional spring to a small degree (or a wood plank for that matter). That has nothing to do with leaf springs however.

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Greg Loehr mentioned leaf springs on YouTube (thanks Greg). My understanding of leaf springs is that that are not welded together, rather bound together by some sort of metal band. I also believe that they are independent of each other but have the strength of all combined. Is this what you compsand guys have figured out?

Can you post the link? I’d like to see that.

Leaf spring theory is simple enough. Individual springs of increasing length working together as one spring pack to give a progressive spring rate – flex/reflex.

(The metal band just keeps them together and from fanning apart while on the vehicle.)

Greg, I hope I didn’t go so far out of context that I have athlete’s tongue.

Think of the leaves as being foam, veneer and fibreglass. Each on their own can be strong but are much stronger when bound together. The binding is not achieved with a metal band but by being wrapped with fibreglass. The friction effect you speak of might be the equivalent of the different materials involved in a compsand shearing against each other. I’m not sure how a compsand would go without the veneer being glued to the foam with a layer of fibreglass and resin underneath (more independent from the foam without this layer of fibreglass and resin underneath?) and only bound together with an outer layer of fibreglass.

It’s mentioned in one of the three parts.

I can understand what Greg was trying to convey, but the leaf spring idea isn’t a good analogy. There’s been much discussion of flex on this forum, but it doesn’t relate to how much a board bends rail to rail or nose to tail. That is two-dimensional flex (height and length) and the only aspect that could be related to a leaf spring.

The flex that Greg was describing is torsional flex (twisting). As he says, if you have a stiff perimeter, the board has less flex in torsion. Engineering-wise, flex/stiffness has to do with bending moments, which is the amount of force needed to cause rotation (force x distance from axis) like a lever. Normally this is analyzed as a simple beam. What is wanted in a flexy surfboard is to have a large force near the end of the beam (rail) without causing rotation. The way this is done is by allowing the beam itself to absorb the load and bend before rotation starts. By having a beam that is thinner or more pliable at the ends is one way to do this. However, the perimeter stringered WMD design really doesn’t work quite that way. The foam density, stringer spacing, thickness of the wood veneer, and the glass control how much it will bend before it rotates, which has no relation to a leaf spring. Because of the curved stringers, it bends in a twisting way, and differently each time depending on where the load is applied. Flex in a “traditional” compsand is the same as a WMD except for the rail material used. The actual “spring” of a board only applies to how quickly it can recover normal shape after flexing. Compsand or WMD, the composite structure moves and bends as a solid, nothing moves independently or it would fall apart.

It isn’t my intention to seem like a know-it-all, or to be disrespectful to anyone like Greg who has given so much in design and materials. If you come up with a design that has favorable characteristics, that’s an accomplishment. And, if you have the technical understanding how it does what it does, then that’s a real accomplishment. It concerns me when anyone (especially the name companies) makes technical statements to a majority of non-technical people that aren’t accurate or just plain wrong. If you don’t know, find out; don’t guess (or say nothing) and let everyone think you do know.

I’m not sure how a compsand would go without the veneer being glued to the foam with a layer of fibreglass and resin underneath (more independent from the foam without this layer of fibreglass and resin underneath?) and only bound together with an outer layer of fibreglass.

Deanbo

the below board is 10’ long 3.5" thick, stringerless, .o45 veneer top and bottom bonded directly to eps foam, 4oz glass with epoxy

weighs 18 lbs with fins, works good,

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I'm not sure how a compsand would go without the veneer being glued to the foam with a layer of fibreglass and resin underneath (more independent from the foam without this layer of fibreglass and resin underneath?) and only bound together with an outer layer of fibreglass.

Deanbo

the below board is 10’ long 3.5" thick, stringerless, .o45 veneer top and bottom bonded directly to eps foam, 4oz glass with epoxy

weighs 18 lbs with fins, works good,

Veneer bonded directly to foam? With only 4oz over? Or 4oz over and under the veneer? What weight foam did you use? And how does the board stand up to deck denting? No stringer at all? Not even in the rails? And where did you get a 10’ long 22" wide piece of veneer? One more question- .045 is the same as 1/40" standard veneer thickness?

I think it was a 2lb blank with one center glue cut

veneer directly on foam

I think I did a double 4oz on the deck

and a single on the bottom

I think 1/40 is the same as .045 or close to it

its so thin that it has a paper backing to keep from spliting

It holds up well from pressure dents

I did accidently cave it in in a few spots trying to suck down a heat bubble repair (too much vacuum)

The veneer on that board is Poplar peeled off one log in a full sheet, no seams.

my bro owns a veneer factory/shop so I can get whatever I need

heres a board I did for him with the same cut of veneer

I remember seeing that photo not so long ago. That’s a tidy rig Ken. I think I’m going to have to buy a venturi thingee for my air compressor and make one myself.

Dean, if you’re thinking about getting into compsand/vacuum bagging, don’t bother with the venturi set up. I tried. it’s not very efficient. Invest in a good vac pump. I have a Gast, with bleeder valves and gauges, rated 24/7. Turn it on, dial in the vac desired and walk away. Quiet too.

Kensurf- sweet boards. I’ve never seen poplar that dark in color.

Those two boards were pretty the first time around, good to see them again.

llilibel and afoaf both use these great little pumps… Check eBay periodically and you should be able to find something like this, but prob for cheaper. As llilibel mentioned Gast is good.

I wouldnt do the venturi method for a few reasons. Unless you have a very high volume per minute compressor, you will not be able to pull a continuous vacuum, compressors are loud, and if you are going to shell out money for a venturi valve (Arent they about \$100?) I would just spend a bit more and get a pump designed for continuous use with a bleeder valve. It works extremely well and is stress free.

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It's mentioned in one of the three parts.

Thanks. I understand how he was relating to it now. Made sense to me. I guess interpretations are subjective.

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I guess interpretations are subjective.

I wouldn’t disagree.

MD

ya know my bro was supose to hang that board on the wall in his office

Darn thing is still in a board sock tucked away at his house

The leaf-spring analogy is pretty apt when you think of the shear forces between a top and bottom skin on a board, as it flexes That was one of my reasons for going with kevlar (over carbon fiber) as a rail material: to allow for some shear movement in the kevlar/epoxy schedule, so the skins aren’t over-stressed.

How did you get on with the Kevlar rails? The longboard I infused was epoxy and Glass / kevlar hybrid cloth. It really is a pig to repair the kevlar . The various dings ended up being ground back and several layers of glass bagged on so I had something to sand without hitting the “kevlar fuzz”.

Just about to start a stringerless polystyrene epoxy longboard and was going to put carbon rails on it. Feedback appreciated.

Very cold Rik

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How did you get on with the Kevlar rails? The longboard I infused was epoxy and Glass / kevlar hybrid cloth. It really is a pig to repair the kevlar . The various dings ended up being ground back and several layers of glass bagged on so I had something to sand without hitting the “kevlar fuzz”.

Just about to start a stringerless polystyrene epoxy longboard and was going to put carbon rails on it. Feedback appreciated.

Very cold Rik

Kevlar’s not much fun to work with, that’s for sure! A really clean, tight lamination, butt-joining seams, rather than overlapping them, and top layer of fiberglass along with a thick top coat of resin helps, though, to prevent sand-throughs and the dreaded fuzz.

DEan, keep an eye on Trademe - Gaast pumps come up now and then - got mine for \$150.