Bicycle Frame Flex

This is a good article/podcast from the CyclingTips.com. 

While bike frame flex and surfboard flex may not be technically parallel concepts, there's a lot to be learned by studying both areas...

CyclingTips Podcast

Does Frame Stiffness Matter?



 
It has long been a goal of the bicycle industry to make road bike frames increasingly rigid in an effort to convert a greater percentage of a rider’s efforts into forward motion. Few of us can deny being enticed by that immediate snappiness of a super-stiff bike when you first push down on the pedals. Surely if less of your power is going into flexing the frame, more is going into making the rear wheel turn, right?

The idea that some degree of frame flex can be beneficial is hardly a new one. Steel bike devotees have long advocated the idea that the side-to-side flex witnessed at the bottom bracket isn’t wasted; rather, it behaves more like a spring, storing and returning energy in sync with your cadence. Nevertheless, it wasn’t until late 2014 that someone coined a widely accepted phrase to describe the theory.

Jan Heine, publisher of Bicycle Quarterly and head of Compass Bicycles, tagged the phenomenon as “planing” in his popular blog, referencing a term from the boating world when a watercraft starts to rise out of the water, barely skimming the surface and reducing hydrodynamic drag. Heine admits that it’s hardly a direct analogy, but the phrase has stuck nonetheless.

According to Heine, planing isn’t just a theory; he’s actually measured its positive effects with dramatic results in a specifically crafted, double-blind test. Almost without fail, riders in his trial were faster on a more flexible bike, and slower on a stiffer one.

Not surprisingly, that flies in the face of what the bicycle industry has touted for ages. Perhaps no one should be more resistant to Heine’s findings than Cannondale road engineering manager Damon Rinard, whose lengthy (and storied) background in the business includes lengthy stays at CervĂ©lo, Trek, and the early days of bicycle composites pioneer Kestrel. Rinard has spent much of his career trying to translate what riders feel into measurable numbers, and while he falls short of fully supporting Heine’s planing theory, he does feel that after years of studying the topic, bicycle frame stiffness isn’t a simple matter of linear progression; if some is good, more isn’t necessarily better.

Given how long bicycle engineers like Rinard have worked to numerically characterize bicycle performance, why is so hard to prove (or disprove) the idea of planing? If some flex is good, how much is too much? Is there such a thing as an ideal level of stiffness, and is that sweet spot the same for everyone? Are there different kinds of frame stiffness? And if so, how does each one affect how a bike feels on the road?

We tackle all of this and more in this week’s CyclingTips podcast, and some of the answers may surprise you.

Transition Era Effluent

During the transition era, there was a window of about a year when board builders, especially in California, were shaping shortboards out of leftover longboard blanks.

This led to several unintended consequences.

One: Early shortboards often had very flat, evenly distributed rocker...


Two: There were a lot of left-over end portions from the oversized blanks that were too big to throw away in good conscience.

At least one manufacturer -- The Greek -- started gluing these chunks of foam together and making belly boards and small surfboards out of them!

 

The Joker: Early Hull Adopter ???

Little known fact about The Joker: Even though he was holed up in a dingy Gotham City basement with his crew of merry gangstas, he was also a leading edge transition-era surfer! Depicted here with a stringerless, flexfin roundtail. Looks like a mid-length from 1968 or so...

More McTavish

(This is the board McTavish is riding in the Burleigh Heads sequence of Innermost Limits.)

"This is my new surfboard. 
It's lovely. 
My favourite waves are tubing ones. 
So I got my board specially to ride my favourite waves. 
And it does (with me on it, ha!). 
It's seven-foot long, twenty-one inches wide and that's plenty, and pretty thick. 
The rails have a breakaway edge on them, all the way around. 
The rail line is top at the tip and bottom at the back.
It's only really got lift in the front end, but, as it's a tube speciality and as it's got to be able to perform in the hollow curvy face, it isn't really straight in the back. 
Just natural curve.


The plan shape is interesting.
It has all the trimmings cut off.
The widest point is one third from the front, as is the thickest point.
Yes.
It's a foil.
This lack of nose allows tremendous freedom deep in the curl.
For instance, a take-off can be so late that the board drops near vertical and will not pearl, but just pulls into a turn in the face.
And quite often the wave throws over in the turn, so it's a full-on turn in the tube!
And that feels good!
 
It has the same turning curve in the plan shape as a medium-range pintail (minus the pin). 
So it cuts a neat arc. 
The area in the tail is enough to feel strong in a turn without fighting back more than my ten stone can handle.
As it's so small, and as it's got that natural curve in the bottom section, it doesn't really hiss along unless it's in the face. 
Any face that's a bit hollow will do, but when the wave starts sucking and the curl gets a foot thick, it's wailin' ! When that day comes, and it's six to ten and very hollow, I'll be thankful I have my Mongul special, 'cause it'll let me stand up in the tube, climbing and dropping to hold back in. 
And pulling up into the top little throwout and getting inside instead of going round the bottom. 
And just sit up in there trimmed out at curl speed.


Oh, Yes!
It weighed eight and a half pounds new.
Bennett gun blank, single six dry glass job finish resin fill coat, sanded back to furriness, then wet and dried with 180 and 400.
(Six deck patch, too.) That's as light as you can go without boring big holes in the shaped board and glassing over it.
That's what Buddy Boy did and his weighs six and a half.
Buddy Boy weighs about thirteen stone.
He's on a diet.
The first day I rode my board it had cured for two days.
About 60%, I think.
It got a buckled deck from my right foot big toe pressing through single six into light foam every turn.
After surfing it for a couple more days, it felt too light for its volume.
A bit of a cork through turns.
So I slapped six more ounces on the bottom.
Now it feels better.
And so do I.
'Cause now the bottom will preserve itself.
And I want it to be around for a while.
I like my surfboard, I like it. I like it."

Bob McTavish. November, 1968 

Stringerless Sunday

Don't know much about the history of this board. It has a "Dale" label, and is said to be shaped by John Otton.

8'3'' x 23''

It's a stringerless "tracker" style shortboard. Very progressive for its era. The outline, fin placement, and hull set up all look pretty close to ideal. Maybe a bit flat, rocker-wise...but better too little than too much!