cyclofiend.com - peripheral thoughts & notes

May 2024
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I’m Too Flexy…
Filed under: general
Posted by: The Cyclofiend @ 10:57 am

Been thinking about flex a fair amount this past week, for a variety of reasons. And the only way to expunge the “I’m Too Flexy” reworking of the Right Said Fred song from my brain is to write about it…

Bicycle frame flex is a monsterously misunderstood phenomenon, made much worse by the claims of marketing departments and the placebo effect. Most riders have been told that the frame they’ve selected, or hopefully will select, is “vertically compliant” and “torsionally stiff” - that it, the frame will resist the torque generated by the pedal stroke so that all the power will be directed into the drivetrain (that’s the “torsionally stiff” part) and presumably, rocket-like forward movement.  Additionally, as your bicycle begins to color-shift as it’s moving so quickly, it will absorb bumps in the road surface (that’s the “vertically compliant” part) as if it had knees and elbows, allowing the cyclist plush and fluffy comfort while scorching over the nastiest of conditions.

While this may not be complete and utter hooey, very few of the bicycle companies have been able to demonstrate this vertical deflection in any substantial way, or explain how their bicycle would be adaptive enough to, say, know the difference between the road-shock-absorption needs of a 126 pound bird-boned climbing fool and a 275 pound crank-snapping endomorph.

Frame stiffness starts to become street-corner mysticism.  I remember a bike shop manager who talked endlessly about his S-Works M2 Stumpjumper.  The M2 (metal-matrix - introducing ceramic to the basic aluminum structure) frame had just a little more of a “supple feel” than the regular aluminum (at that time) Stump did.  As near as I can tell, he was dead wrong from a materials point of view, but it didn’t stop him from believing that, or firmly implanting that image into the mind of several customers. Cannondale staked a big part of its marketing on the oversized swaged downtube being this immovable object through which massive irresistable forces could be directed. It didn’t hurt that the Lion King himself went very, very fast while riding their bicycles.  If you didn’t have a stiff frame, your efforts were akin to walking through thigh-deep mud - you worked extremely hard for very little forward motion.

One of the glossy bike mags had a frame testing rig, I think called “The Tarantula” or something like that, and (suprise) they tended to downgrade bikes that exhibited more flex.  Everything that deflected was seen as “inefficient”. 

A couple of issues ago, Bicycle Quarterly (formerly Vintage Bicycle Quarterly) ran an article looking at the stresses into the frame. In this article, it pointed out (suprisingly enough) that when you torqued something, it tended to torque back.  To simplifly a bit, when you honked on the pedals, the pedals honked back when you finished pushing. This idea, along with editor Jan Heine’s experiences on a variety of frames, led him to articulate the idea of  “planing”.  Planing is the idea that a bicycle has an inherent speed and cadence at which the flexibility of the frame (or perhaps more accurately - the bicycle as a whole) serves to maximize the effort - in Jan’s analogy, similar to when a boat gets up on plane - rising up out of the water so it moves over it rather than through it.  Having spent a little time around boats and on the water it makes sense to me - you can quite literally throttle back at this point and feel the boat moving forward with less effort. For a bicycle, it literally means higher speeds with less effort. This gets discussed with varying levels of intensity on the iBob list, and there’s a lot in the archives if you want to puruse it.

Now, I’ve actually not been able to feel planing on a bicycle in an articulate way. I seem to have good days and bad days on any of my bikes.  But, I am hopeful, and try to remain open to the experience. 

Although, it does seem like the stiffest bikes I have are the ones I ride the least.  That’s an interesting statistical anomolie.

It does potentially mean that by tuning the flex, planing can be optimized for a given rider. I think this is why custom built frames can ride better. Whether by design or intuitively, a builder doesn’t need to design for the lawyers - who want to make sure the bicycle will withstand forces from anyone likely to ride it - and instead can build for the rider.

Let’s divirt to an example of fly rods, which flex by design. They are rated for certain line weights, so you use a #4 line on a 4 weight rod, and the weight of the line (well, specifically, the weight of the forward 30′ of the line) in the air flexes the rod.  When the angler uses appropriate movements and timing, that flex is returned in the opposite direction, which carries the line forward to the casting target. Now, there’s also a bit of fudging in the fly fishing industry, and as such, line manufacturers increase the actual weight of some lines, so that your 4 weight line would be the equivilent of a 4.6 weight line (line weights are given in full numbers, but the allowable weight is actually a range).  Most rods will accept this “overloading” and return the energy as more casting distance.  You think you are a better caster and like Line “4.6″ because it casts better. This causes the other line company to pump theirs to 4.7 until you get to the point where the lines are really overloading the rod, which behaves sluggishly.  (Of course, the rod manufacturers keep using more advanced graphite fibers and sophisticated layups, which cause their designs to need these heavier lines to load properly. This has lead to - suprise - stiffer rods. Nope, no parallels there…)

My printer ran out of paper this past week, and so I lugged home a couple reams of 24 pound stock on the Zeus.  Now, the Zeus, to put it mildly, is a reasonably “supple” frame to begin with - I can deflect the bottom bracket easily with pedal pressure when just sitting there, and I suspect that a high speed following camera could record movement during hard riding (hmmm…maybe I can pitch a Mythbusters episode…).  I’d secured the paper to the rear rack, so there was maybe 10 pounds of dead tree mounted up and back behind me. Didn’t give it a second thought until I came out of the saddle on the incline out of town. I’m pretty sure it wasn’t just a change in the frame weight, nor the perception of a good deal more mass at the tail of the bike - that tail  began wagging me and I could feel the bike moving very differently. When I negotiated a barely-wider-than-the-bars space, a resonant frequency set up as I steered, corrected, steered and corrected, the bike feeling like a small scale example of the Tacoma Narrows Bridge.  It’s a short alleyway that I generally glide through without a thought, but the load had considerably altered the performance. This is when I started chanting, “I’m too flexy for this load, too flexy for this load…” to the rhythm of a really annoying one-hit wonder.

So, in this case, I’ve got a system that really isn’t quite stiff enough to handle load, which is sort of laughable for a bike which is frequently used for errands.  Now I’m tempted to see what would happen with similar loads on my Dawes, as a comparison.  But the point is that like the over-lined fly rod, it was sluggish and responded poorly - in certain circumstances at the edge of controllability.

I’m not sure there are any great truths or considerable conclusions from all this.  All I know is that I spat out a corner of a molar the other morning.  I’d been having some serious “zingy” pains from my tooth for a day or two, and went to see the dentist - really hoping that it wasn’t happening underneath the cap I’d had to get a few years back. Turns out it was not the capped tooth, but the one behind it, which was cracking as a result of an old filing and the way I bite, which tends to flex my teeth.  We scheduled a follow up appointment for what he figured would be another cap, and told me not to be suprised if it let go before I got back in.  He was right.

Teeth are relatively hard, don’t you know, and when they have a hardened glob of metal in the middle of them that force gets directed along the stress risers. These days, they use a bonding filling, which tends to help hold things together.  But, my old-skool work meant that the back corner of the tooth got more stress than it wanted, and because it was stiff, the stresses found the weak point and kept working it until it failed.  Oh well, it gives my tongue something to do while I type…

One Response to “I’m Too Flexy…”

  1. Jim G Says:
    Mythbusters! Yeah, let’s get them to get to the bottom of this “planing” thing!!! Great idea!