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But it's not enough to have an element in a system out-of-balance. You need the system to be flexible, that is, you need a lack of stiffness, or a lack of damping, or both. (It's not your shock absorbers that suck up the imperfections in the road; it's your car's springs. Your shock absorbers "damp" the system, keeping your car from bouncing.)
These are formal devices made specifically as dampers. You can build damping into a system. Carbon frames are stiff, so they resist oscillations associated with speed wobble, and they are damped, because of the nature of the material. Layers of carbon weave act as natural dampers.
An imperfection in a rotating element can set an oscillation into motion. But it doesn't have to be a rotational imperfection. The famous Tacoma Narrows bridge collapse was caused by a kind of speed wobble initiated by an "aeroelastic flutter" (at a certain speed even the wings of an airplane oscillate – oscillating wings: good for birds; bad for planes).
Speed wobble was occasionally a problem in the "old days" when bikes weren't very stiff, and when they didn't have much in the way of a damper. Steel bikes were fairly flexible, undamped, springs. Once a particular speed was reached the frame started to shimmy. In recent years I have found this to be a non-problem. Extinct. Ever since the advent of modern, stiff, monocoque carbon bikes, especially road race bikes, problem gone. But, not gone for many of you.
What causes speed wobble? One theory is that it's simply the resonant frequency of the "system," such system having or lacking inherent stiffness and damping, made worse by weights that sit particularly high or distant from the center of mass (more on that later). The system, at a particular speed, might be subject to a wobble if excited or initiated, maybe through your body shivering on a chilly descent, or via "sewing-machine leg" (tuck on a long descent, you'll know it if this happens to you). All might be prime or possible actors. Speed wobble might not require an "agent" at all. Simply the requirement by every rider to lean and steer his bike (the very act of riding is an oscillation) might be enough, once a particular speed is achieved. And even that might not be required.
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An alternative theory (or maybe these theories aren't mutually exclusive) is that speed wobble is an example of a "Hopf Bifurcation" (no, Hopf is not a typo). Simply put, a system that is stable, operating normally, and as it should, can cease operating normally when one parameter changes, the parameter in the case of speed wobble being velocity. At that point the system becomes unstable, and a "periodic orbit" comes into existence, such orbit, as I understand it, expressed as the oscillation in your bike during a speed wobble. The aeroelastic flutter described above is a kind of Hopf Bifurcation.
What difference does it make which theory is correct? None, really. Except that, as I understand it, the Hopf Bifurcation is almost a requirement when a specific speed is reached. There is no energizing agent (shivering, for example) that prompts speed wobble. Most observers do believe rider input can be an agent in speed wobble.
I asked members of our reader forum for examples of speed wobble they've experienced in the newer generation of carbon bikes. Here are some examples, you can peruse them or skip past them, as you choose. We will refer back to some of these by number further below.
1. 2009 Kestrel Airfoil 47, stock front end setup (90mm stem, no spacers, profile T2 cobra aerobars). Wobble at 34mph on a descent.
2. 2009 Felt B2R, stock bayonet stem and bars, 80mm front wheel, wobble on a fast descent.
3. 2012 Felt DA4, stock bayonet stem and bars, 35mph (replaced the frame with another DA, same parts, high-speed wobble problem gone).
4. Specialized Shiv, Size M, Zipp Vuka Bull, 1 spacer, 40mph, speed wobble in 2 races, both races had cold temps.
5. 2014 Trek Madone 2.1 H2 geometry, bars 40cm-wide Zipp Service Course 70, stem OEM + 10mm, +7° pitch. Speed wobble.
6. BMC TM01, Medium Large, speed wobble at 45mph.
7. Scott CR1, 110mm stem, 10mm spacers , hit bump @33mph, (I caused the speed wobble by clamping up and remaining so after it initiated).
8. LOOK 695, 110mm stem, 10mm spacer HED Stinger 4, 55mph, same rider as above (again tightening after side wind + rough section of tarmac, I caused a very severe speed wobble by tightening up, ceased when slowed to 25mph). Shat pants. Understandable.
9. Speed Concept 7.0, Flo 90/disc. Happened twice, 50mph each time.
10. 2014 Speed Concept. Rear bottle setup. Positioned extremely far forward (too far forward) on the bike from the saddle to my aero bars, wobble at 30mph. Moved everything back, so far no speed wobble.
11. Cervelo 2010 P2, 56cm, 110 stem, slammed, Vision bars, Zipp 404 tubulars, Hill decent during a race, started at about 35 mph. Added counter weights to off set the valve extender weight on my Zipps and never experienced the wobble again.
12. Canyon Aeroad 2013 Model - Size XL 100mm stem, Handlebar width 44mm, 1cm spacer. 50mph wobble, if I pressed the brakes to hard the wobble got worse. Eventually rolled to a complete stop, and felt like throwing up.
13. Felt DA4 56, stubby stem cocked up about 45 deg angle, 40mph wobble, worn out front tire. Never happened again once I got new tires.
14. 2011 Madone 6.2, 58cm H1 geometry. 110mm x -7deg Bontrager RXXXL stem, 42cm Syntace carbon bars, 15mm of spacers. A chunk of steel equivalent to a fully loaded 2-pack of 28oz fluid + other accessories cantilevered out behind the saddle as part of ride evaluation. The bike went into speed wobble almost instantly when I removed one of my hands from the hoods early in the descent. The same bike, sans the simulator, has never speed wobbled with or without hand movement in any other condition at speeds in excess of 55mph.
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If this happens to you, are you just screwed with that particular bike? Not necessarily. You've got to change something about the system. It might be as simple as a new set of tires. Note this from one of the examples above. Tires. I've seen experts say that increased tire pressure can add stiffness to road tires, but the knobbies on MTB tires don't subtract stiffness, rather they add damping.
It might be the rider. I'm not being glib. One rider experienced speed wobble on a descent on a bike I loaned him, but that bike never exhibited it for me, or anyone else who rode it.
Weight leveraged far in front or back of the system seems a trigger. Note examples #10 and #14 above.
Weight perched high above the triangle seems a problem. I had one bike that wobbled at speed, it had 20mm of spacers between the head tube and the stem. I took out 10mm and, no wobble. It wasn't a carbon bike, rather an old round-tubed aluminum bike. Kind of flexy. These bikes were more prone to wobble than the new, ultra-stiff carbon frames. But, look at example #5 above. This is a bike that had its "factory" spacers in place, and by that I take to mean at least 30mm of spacers and probably more. The factory stem was replaced by a stem 10mm longer, and it was flipped up into its +7° pitch. This means the bars were way above the frame and some distance out in front of it. This was Trek's H2 geometry and, in truth, seems like a bike one size up and in the H3 geometry sounds like a better fit.
This bike above is an example of why today's modern fit systems are helpful. Would this bike wobble if there was more main triangle between the bottom bracket and the handlebar? And less steer column, spacer, and upward turned stem? The only way to know is to test it, but my instinct says yes, because even if the stiffness of some of these appendages that stick up and out from the frame (steerer, stem) test sufficiently stiff in one axis, I'm not so sure they're adroit at handling all the twisting multi-axis forces that frames are better at resisting. In other words, could this be a case of a frame that is stiff, but a system that is not stiff, because of the distance between the frame's terminus and the rider's weighted hands?
This is also the reason for triathlon geometry frames. When I designed frames in the late 1980s specifically for riding in the aero position, it wasn't to "create" a position; rather to build a frame for the position we were already riding in. It wasn't that we couldn't get into today's modern triathlon positions using our road bikes, rather that that we were far out in front of our road bikes once we retrofitted them for the position. Today's tri bikes have more front/center than today's road bikes, and that means we have less weight cantilevered in front of the steering axis. We ought to be less prone to speed wobble on bikes like this.