Boarding School: Flex
By Cory Russell
In the last episode of Boarding School we looked at volume, a relatively new and contentious way of quanitifying surfboards. For this entry we’re gonna stay on a modern tip and discuss a new, but also kinda old, surfboard characteristic - flex.
It was George Greenough who initially got me thinking about flex in surfboards through his spoon design as well as his love for riding inflatable surf mats. George’s motivation was always about fitting the wave and harnessing its energy.
The flex properties of a surfboard are often discussed amongst surfers and shapers with various experiences and opinions established yet little in the way of objective analysis.
The truth is there's been very little research done about how our surfboards flex during use. We're able to document the specifics of each material used in surfboard construction but the assessment of the final product is difficult to calculate. It's even harder when varying ocean conditions are added to the equation; a surfboard may require different amounts of flex owing to a person’s weight and the waves they surf.
I don't have all the answers to understanding flex in a surfboard but I'm determined to know more. In an effort to explain the flex properties of a surfboard I've provided a list of the defining factors.
The foam used in the majority of surfboards is either polyurethane (PU) or expanded polystyrene (EPS). For PU, a mixture is created and poured into a mould by hand. Attention to detail can't be underestimated as the goal is to produce a consistent density of foam throughout the blank. Failure to achieve this will result in a blank with varying densities which creates unpredictable flex and unwanted soft spots. PU blanks are available in various densities from heavy to ultra-light and they all have different flex properties. In Australia, we're blessed with a number of PU blank manufacturers that consistently produce great blanks for shapers.
Expanded polystyrene is created from small pellets injected into a mould and expanded using steam. In the past, shapers have manufactured their own blanks from blocks of EPS but I find the producers are unable to maintain a consistent density. The best EPS for surfboards is created in specific moulds under pressure. Unfortunately, there are no local manufacturers for these types of blanks - they must be imported from the United States. I've used many types of EPS blanks and Marko foam is definitely the leader in this area. They're constantly improving their products for manufacturers.
The stringer or lack of is the single most important factor in determining a surfboard’s flex capabilities. The type of timber, number of laminations, and thickness all determine the amount of flex. Surfboard manufacturers have experimented with the stringer structures for decades and the two main stringer constructions are traditional and paraboilic.
The traditional use of a stringer down the centre of the board provides structural strength and resistance from nose to tail, yet it allows the board to flex in a twisting motion. The use of multiple stringers spread across the width of the board provides more strength and reduce a boards twisting flex.
There has been a variety of methods applied to the traditional stringer construction including the tapering of the stringer as it travels from nose to tail (see the work of Jed Done). Also, I recall Rusty Priesendorfer making boards for the Hobgood brothers many years ago with a ¾ length stringer to increase the flex in the tail.
Parabolic timber stringers have been explored a lot in recent years. The motivation for shapers to use this type of construction is to improve a surfboard’s ‘torque’ by increasing the stiffness around the perimeter of the board.
I refer to ‘torque’ as the efficient transfer of energy between the rotational force applied by a surfer compared to the response provided by the surfboard. A parabolic stringer limits the amount of twisting flex. The amount of flex in a surfboard can slow the board’s response time as it bends and twists. More flex means more lost energy.
I recently spoke to Elliott Bemrose from Core Surf Industries who explained the combination of the stringer to foam density is equally important as they should compliment one another. He told me there are some manufacturers that specifically request different tensile strength stringers but often it is more about aesthetics or what the pro’s are riding.
The fibreglass cloth we use on surfboards has changed a lot since we first started using it over sixty years ago. At the time the fibreglass cloth available was not specific to surfboards and was particularly heavy. In the first issue of Surfer magazine (1962) an ad for Dave Sweet Surfboards advertised 20 ounce cloth. The standard these days is 4 or 6 ounce!
Aside from weight, surfboard manufacturers must consider the actual fibre strand, weave, clarity, and whiteness. The weave is the main factor when talking about flex. Standard fibreglass (E class) cloth is made with fibreglass strands running nose to tail as well as running at 90° from rail to rail. There is also ‘S’ class cloth available which has an increased number of fibreglass strands running from nose to tail which reduces longitudal flex. The tighter the weave the stiffer the board, while loose or open weave will provide more flex.
These days there are so many different fibreglass cloths available as well as the introduction of fibreglass cloths that have been knitted. The additional knitting of the weave in fibreglass cloth reduces flex while providing more strength. This type of cloth is popular on stringer-less boards.
The use of carbon fibre on surfboards has reached such high volumes that it can be hard to find a surfboard that doesn’t have any carbon fibre on it. Carbon fibre is extremely strong and was designed to provide structural strength, yet contrary to what people think or the marketing spin, when a piece of carbon fibre tape (as found on the tails or along the centre of most surfboards) is laid flat on a surfboard it provides very little strength if any to the structural integrity of a surfboard. It can alter the way a surfboard flexes the same way an additional equivalent piece of fibreglass would.
However, when carbon fibre is wrapped partially or completely around the rail of a surfboard it can provide structural strength. Stringer-less surfboards with carbon fibre rails rely on this structural strength and they create a reduced amount of ‘torque’ compared to the timber parabolic stringer construction. The stringer-less carbon fibre rail construction flexes a lot more and this increased flex makes these boards a great option for waves under 3-4 feet. The reason is due to Newton’s third law of physics that ‘for every action there is an equal and opposite reaction’. A board with greater flex will react and respond quicker compared to a stiffer board. This is ideal for short, quick turns not long, drawn out ones.
There are two types of resin used in the construction of surfboards: polyester and epoxy. Polyester resin has been the most widely used but things are definitely changing. As a general rule, the tensile strength of a polyester bond will be around 20 percent weaker than epoxy. Epoxy resin has greater resistance to wear and which is sometimes referred to as the ‘Barcol’ factor. Years ago epoxy resins were difficult to use, they weren't clear, and had inferior UV inhibitors so they would go brown quickly. Nowadays, manufacturers have created excellent epoxy resins for surfboards that are easy to use and comparable to polyester resins. Surfboards constructed with epoxy resin are usually post-cured in an oven at high temperature and ready to surf soon after they come out. For the first time ever the market place is demanding more and more boards be constructed using epoxy resin.
As you can see there are so many elements that go into making a surfboard that it's almost impossible to re-create a magic board. The amount of flex and where a board flexes is a key element. When flex is isolated to a specific area of a board it can be a great enhancement to the design. Mitchell Rae from Outer Island surfboards has been committed to understanding flex in a surfboard and using it to our advantage. Mitch’s flex tails are well known and revered around the world. His dedication to his craft is of the highest standard and the application of knowledge, design and materials makes him an expert in my opinion. Mitchell doesn’t follow market trends but creates what works from the feedback he receives from his customers.
A few years ago I started talking with Guy Walker from Geoblanks and was immediately impressed with his dedication to isolating flex patterns in a surfboard with his EPS blanks. Guy uses a series of hexagons with various densities arranged and fused together to determine the flex in a blank. He's taken a short hiatus to improve the technology but they'll be back on the market shortly.
Nick Miles is a passionate shaper on Sydney’s northern beaches and is dedicated to creating a better product. Approximately 18 months ago Nick created a blank design called X-Core that uses carbon fibre stringers placed on an angle and in a butterfly shape in an EPS blank. This design aims to provides strength and resistance where you need it but also increased flex through the tail. Nick is always thinking about how he can keep improving surfboard construction and an exciting guy to talk to.
As surfers we generally prefer a surfboard to be more rigid between our feet but an increase of flex in the tail of the board allows for tighter turns at high speeds as well as a level of forgiveness. If a board has too much forward flex the board will push water when it bends in front of your front foot.
At the beginning of the article I mentioned there's been little scientific reserach done on flex in surfboards, yet there is an area of science that covers it. Scientists refer to the ‘Modulus of Elasticity’ to measure an object's resistance to being deformed elastically. I have no doubt the rigours and demands of surfing will see the technology improve and, just as volume has become a measurement, so too will a modulus number.
That's all for now,
Cory Surfboards / Stretch Boards Australia