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by Carl Brewer last modified 2007-09-13 01:13

Keith Bontrager on stiffness

Stiffness and the Sean Kelly Paradox

Stiffness. You mash down on your pedals. The bike snaps forward, leaping by your structurally deficient competition. Stiff is good, stiffer is better. What more is there to say?

Even though it seems downright simple to almost anyone, I am not convinced. I'd understand it at this point if you thought that I was a bit thick, but please keep in mind that I'm getting paid for this, and at least hear my case.

Stiffness relates the deflection of an mechanical structure to loads that try to distort it. The applied loads must be small enough to avoid permanently altering the structure's shape or we have to start talking about strength. Stiffness and strength are not the same. If the structure is stiff, its shape doesn't change much when the loads are applied. If it is not stiff (flexible), its shape will change quite a bit when loaded. How much "doesn't change much" and "changes quite a bit" actually are depends on the structure and the application; we're talking bikes here, eh? Simple.

A coil spring has stiffness, and it's referred to as it's spring constant if you push in the right direction. Push a little, and the spring squashes down. Push a bit harder and it shortens a bit more. Stop pushing and it returns to its original length. While a bicycle is much more complex than a coil spring, the analogy is useful. To think about the meaning of it all we need to figure out what the loads on the bike are like, what complexities there are in its structure that might spoil the analogy, and how the bike's deflections affect its performance. I'd like to start this rant with a simple basis for distrusting the common version of modern stiffness dogma, the version out on the streets. I'll talk more about the physics later.

First of all, I'm suspicious, in principle, about anything technical that is common knowledge and therefore considered obvious by folks without a technical background (like magazine staffers). I know, I know. I'm a skeptic; what can I say?

Here's a good reason for this. Solid, resilient, lively, dead, responsive, mushy, snappy, supple ... all the adjectives that are commonly used to communicate thoughts about a bicycle's stiffness are imprecise, and are very subjective. This makes it difficult to communicate a connection between the stiffness of a bicycle and performance in a quantifiable way. But this is how most of us have come to respect the attributes of a stiff bicycle. I find it troublesome to trust the opinions that I hear or read (except in MTB Pro of course).

I'm not completely senseless though; I can't ignore sensory inputs. I agree that a very stiff bike can "feel" quicker when you sprint. And I agree that, at first glance, it should accelerate infinitesimally better in principal. While I've felt this, I haven't observed a definite, consistent change in actual sprinting or climbing performance. I know I'm slow, but I'm not just making excuses. A snappy feel is not the same as a real, significant improvement in mechanical efficiency. Sorry for what seems like hair splitting; I hope the difference will become less murky later.

My introductory paragraph, the one that seemed to confirm what you knew all along about a stiff bike, the one that had you nodding contentedly while you were reading it, was rife with bad assumptions. I believe that our perceptions of performance, gotten while riding, are not always very accurate (back to the snappy feel is not the same as real changes in efficiency comment). I picked up this invaluable observation while working on motorcycles. My job was to develop fast motorcycles. I would go out to a track with very skilled riders, world championship caliber riders, and test with parts and tuning combinations in order to determine whether they could go faster. We tried the setups and solicited opinions from the rider, asking him how the bike worked and whether he thought he was going faster than the last time around. Of course, I would measure the rider's performance with a stopwatch as well. So I had two indications of performance, the rider's opinion and the clock's version of the story.

When the results from the two methods were compared, the rider's view and the clock's account often differed. Test configurations that seemed fast to the rider weren't always good according to the clock, and parts that made the rider think he was slow could actually be making fast lap times. The stopwatch earned its keep.

Comparing the rider's opinion of a given part's performance characteristics and what he thought should happen proved interesting as well. If the rider didn't have any hints about the upcoming test, his description of the result often differed considerably from what I actually did to the bike. The riders couldn't always tell what was happening from what they felt as they rode. On he other hand, if the rider knew beforehand what should happen in the test, his report was much more likely to be in agreement with the changes we performed. A good way to get a uniform, consistent, but potentially useless evaluation of a part was to discuss the anticipated result with the test rider before the test. Sound familiar?

I know my methods were imperfect in many ways; we were under time constraints. But I believe it supports my point that even a skilled rider's perceptions are not always reliable and can be influenced by what they think before the test. Objectivity is tough to come by, maybe impossible, and I think that this is more or less the case when it comes to evaluating the relative merits of stiffness while riding, especially when the opinions come from a magazine writer or acquaintance.

Of course, our image of the off road bicycle is not often affected by the unbiased and insensitive (with respect to designer's, advertiser's or rider's feelings) criticism of the stop watch. But until it is sorted out, you know where I'm putting my money. Of course, if anyone has the money and desire to sort it out, you know where to find me.

And whenever the subject comes up in a discussion, I'm haunted by images of Sean Kelly in his prime, shooting by a handful of thick legged Italian and Belgian sprinters on his wimpy glued aluminum Vitus, to win a sprint in a classic. If ever there was a contrary example, this is it, and it sticks with me.

Detractors will quickly point out that Kelly (while not a sprint specialist himself) was simply much stronger than all of the sprinters that day. The fact that the frames involved in the sprint could have differed in stiffness by a factor of two or more, to Kelly's disadvantage, would just be one more noteworthy aspect of Kelly's remarkable physical superiority. I revere Kelly, but I don't buy it.

So, at a minimum, I should have called the popular dogma on the subject of stiffness into question in your minds. Think of Kelly's wet noodle of a Vitus frantically carving out a drunken sinusoidal path through the pack whenever you waiver. In coming months I'll discuss how stiffness can affect performance, a very crude method that you can use to evaluate how the stiffness of individual parts can affect the stiffness of the bicycle overall and what is likely to happen due to stiffness variations in the most important specific load cases on a bike.

Until then have fun in the mud; I'll be thinking of you from sunny Santa Cruz.

Stiffness 2 - More ranting about stiffness . . . if you haven't already had enough.

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