I recently wrote a post concerning frame stiffness and a phenomenon I encountered in which some of the bikes I’d ridden on dirt road descents would begin to oscillate if the surface was particularly rough in turns. Based on the feedback I got from several smart people I know in the bike biz, the culprit appears to be a lack of sufficient stiffness in the frame.
I’ve had a few questions about why increased frame stiffness always seems to be everyone’s answer to a better performing bike or, alternately, what is wrong with a bike that lacks stiffness.
The first point I want to make here is that there is no one correct answer to frame stiffness. Provided the bike isn’t inherently unstable due to a poor design, an experienced cyclist can ride a bike with very little stiffness, and do so well. The simple proof to this is Sean Kelly. For a great chunk of his career, one of the finest riders of his generation rode a Vitus 979. I’m willing to bet a Ben Franklin that the Vitus 979 was the least stiff bike ever ridden in the Tour de France (caveat: in the modern era). And yet, despite the fact that the bike was extremely flexible, King Kelly won stages of the Tour de France, and great classics, such as Paris-Roubaix, where that softness did him favors on the cobbles, but conferred no benefit in the final sprint.
Kelly is proof that any rider can get used to most any bike. With experience, a rider can learn how a bike flexes in a sprint or on a descent and muscle memory simply incorporates that experience into their expectations. And that’s the key, right there, expectations. When a rider expects a bike to react to input in a certain way, and the bike doesn’t do that, that surprise can be unnerving.
Case in point: Some time in the late 1990s, I was on the Donut Ride in Palos Verdes and at the bottom of a ripping descent into San Pedro the road bends left. What I didn’t know prior to entering the turn was that my front tire had a slow leak and its pressure was somewhere south of 40 psi. I entered the turn and the sidewall of my lightweight clincher began to collapse (known in the mountain bike world as tire squirm) and in that instant I could feel the bike begin to oversteer as a prelude to low-siding.
On a scale of 1 to 10, my pucker factor was about 34. I can still feel the jolt of adrenalin as it shocked my body.
I straightened the bike out to get the tire back under me, hit the brakes and then, because I was headed for a stone curb, began to ease back into a turn even as I continued to brake. I fixed the flat and then chased the group (never caught them, natch) with a couple of friends.
It was on that same stretch of road perhaps a year before that I was riding a frame I knew with an especially light carbon fiber fork from Look. I expected the bike to enter that turn and I’d carve through and get back on the gas ahead of a thundering sprint effort. But the feedback I was getting at the handlebar seemed off, like I couldn’t quite tell exactly what sort of arc the bike was carving, and then the bike began to oscillate; I braked until the oscillation stopped and found myself at the back of the group. That Look fork was absolutely the most flexible fork I ever rode, though I’m aware of a titanium fork that I’m told was even softer. Ugh. Could I have become accustomed to that fork and the way my bike handled with it? Sure. But I’m not sure I’d ever have been able to attack descents in the same way because the way the fork handled conflicted with my need for a precise sense of what the front wheel is doing, where it is pointed.
A corollary to this is the high-speed wobble; I got to speak with Ben Serotta about this issue many years ago as I was working on a review of a Serotta. Other colleagues reported about this as well: Serotta did research into what causes a bike to enter a high-speed wobble. The problem is simple enough: at higher speeds the front wheel begins to oscillate left and right and as speed increases the rate of the wobble increases as well. The folks at Serotta never did pin down an absolute cause, but when their customers reported a speed wobble in one of their bikes, they would ask them to ship the frame back and they’d simply remove the top tube and replace it with one that was stiffer. A stiffer top tube eliminates a speed wobble.
Stiffness is, if I am to be fair, not the be-all, end-all of frame design. If I lived in a flat place and wasn’t racing criteriums, a flexible frameset wouldn’t be much of an issue because I wouldn’t be putting all that much force into the bike in turns. Never once, in all the years I lived in Memphis, did I break 40 mph on a bike, and the closest I got to that threshold came while I was going perfectly straight.
A listener to the Paceline brought up Jan Heine’s taste for more flexible frames (thin-walled tubing with a traditional 1-in. top tube and 1 1/8-in. down tube); when you add in his preference for 650B wheels with tires pumped to relatively modest pressure, it becomes apparent that the Bicycle Quarterly publisher does not find a lack of stiffness to be an issue. A rider who has lots of experience on more flexible frames can move from one to another without much issue. However, moving from a stiffer bike to a more flexible one, or vice versa, will cause many riders some confusion.
The phenomenon that occasioned my investigation—oscillation in a turn on a rough, unpaved road—is a simple fact of a more flexible frame. Could I learn to live with it? Certainly. So what are the consequences? I simply wouldn’t descend as aggressively. What’s the problem with that one wonders? Well, I like going downhill fast. That’s where the flow is.
So, lest a reader think I’m advocating that more stiffness = better, my larger point is that a bike needs to handle in a predictable way. A stiffer frame and fork can yield a bike that handles more consistently. But predictability comes in a few different forms. Consider my observation about the slow leak I had on the descent. The front tire behaved in a way that surprised me. The unexpected = no bueno. That returns me to another important point: pumping a bike’s tires up to the same pressure before each ride is key to developing critical muscle memory.
So stiffness isn’t the goal itself, but a characteristic that serves a goal: predictability. The more that a bike’s handling fits with the rider’s expectations the more assured a rider will be, and if a bike doesn’t respond as anticipated, then muscle memory can’t take over and without muscle memory guiding a rider’s actions, it’s impossible to feel confident on the bike.
Predictability contributes to a rider’s experience in what is one of the most important aspects of a rider’s relationship to the bike. That feeling of becoming one with a bike happens when feedback from the bike is instantaneous and conforms to a rider’s expectations. At a certain point that approaches a flow state, but may not be exactly correlated to flow, a rider’s parietal lobe begins to shut down. It’s a phenomenon known to happen in meditation, some religious experiences and mystical states. When the parietal lobe goes on break, our sense of where we end and the universe begins gets blurry. At its least extreme we experience things like being one with our bike, feeling the road surface while driving our car or, perhaps, typing with such fluidity that we simply see words appearing on the page.
Ultimately, it’s that seamless experience of merging with the bike that leads to our most rewarding rides.