BIKEPGH MESSAGE BOARD ARCHIVE

I'll let a properly-qualified mechanical engineer comment on this; my own knowledge is (loosely) in acoustics. Resonances will occur under pretty specific conditions. Most designs try to counter these in various ways; I assume the Specialized people understand this and build accordingly. What's odd about your experience is that vibration continued as you slowed down; in as far as it was speed related it should have gone away as you went 30->15 mph (otherwise it would have been there from 15mph on up). My guess is that something was wrong with the bike (as in, damaged) that only surfaced on the fast downhill (and, oddly, persisted as you slowed down). If you need to blame anyone, it should be the rental people. But, better yet, I have personal experience: A long time ago I had an accident. (Yes, I collect accidents. Why do you ask?) All was good except that my bottom tube had a dent and it was bent slightly out of true. I had no trouble riding the bike afterwards, except on downhills when it started vibrating (which would dampen as I slowed down). I asked about a fix but no one would touch it (back in the days when Pro Bikes was this dinky shop on Murray with a reputation for snootiness(*)) so into the basement it went. How hard could it be to replace a double-butted 531 tube? But I digress. My guess is that you had a damaged frame that the shop didn't bother to pull aside. Or maybe a wheel out of true. (*) re Pro Bikes snoot: I don't recall being mistreated, I even remember seeing them treat some people with a kid really nice. But maybe I never got served by certain individuals. [This is not an endorsement, or snark.]

I was obviously pretty inclined to blame the shop / bike, but it really appeared to check out when I brought it back (somewhat miraculously the ER was within walking distance, and I was pretty ambulatory at first, before my hip contusion "matured" -- note to self, ice that more next time...). I fully expected to find either a flatted front tire or a completely screwed up headset). The only damage to the bike was scratches on the shifters, and there was no sign of damage, slackness, cracks, etc. I had ridden the bike the night before at the same pace though only on a maybe 3% grade, and it felt totally stable. The obvious difference is that at twice the grade I probably shifted farther back. I was also not braking hard and the thing started out as a rough sensation of vibration -- thus my initial reaction that I had flatted. The oscillations certainly got worse as I slowed (this all took 3s) but from what I read online it is possible that I tensed up and hit a resonant frequency of me (the wobble is 15 Hz or so). I have no recollection at all of trying to actively steer other than trying to hold the bars straight, but I was also pretty focused on braking...

Bodies don't do that; they're too complicated (no dominant resonances). Sitting back and keeping your arms straight is the right thing to do, otherwise you're asking for a header. I've experienced braking that sets up vibration. So it's possible you had different things going on. Still sounds like something was wrong with the bike. A well set up bike shouldn't be doing any of that stuff. Remember, it doesn't have to be obviously visible to be bad.

I found this video which shows a speed wobble in action. https://www.youtube.com/watch?v=VfngbsIUSj8 Not a clue what would cause it, but unclipping and putting his foot out seemed to control it. Edit: Sheldon article says to unload the saddle without standing up. http://sheldonbrown.com/brandt/shimmy.html

I'm glad to hear that the crash wasn't worse. This is for what it's worth: I've never had "speed wobbles" on a bicycle but have experienced essentially the same thing on motocross and road racing motorcycles. The problem seemed to be caused by (1) hitting some surface imperfections; (2) while the bike was not weighted ideally for the conditions; and (3) without having enough grip or weight on the handlebars. It should be noted that the bikes were in good operating condition: correct tire pressure, nothing loose in the steering components, ride height set properly, etc. Some bikes, like my Ninja 600R and CR250 (two bikes noted for quick steering) felt a little more prone to it than my 900SS and YZ250 (bikes noted for high speed stability and less precise steering at low speeds). I believe the former bikes had steeper head tube/fork angles than the latter. Regarding bicycles, of all the bicycles I've ridden, my Specialized 58cm Tarmac feels like it is the most prone to speed wobbles. I imagine it has about the same geometry as the 58cm Roubaix. The bike steers very precisely at lower speeds but does not have a terribly stable feel at higher speeds. I believe it would wobble if I hit the right surface imperfection at the right speed and without sufficient pressure on the handlebars. I've been about to about 60 mph on the bike without a problem, but that's with my hands "in the drops" and pushing forward with plenty of pressure to stabilize the front wheel/fork/handlebar assembly. At that speed I am more worried about a gust of wind than speed wobbles. Bottom line: some bikes seem to be more prone to it than others due to their geometry, and if your bike is prone to it, applying pressure to the handlebars at high speeds should help.

How were your bars setup? Were you on the hoods or the drops? I agree with more weight up front. Get your cg low and forward, but I come from a motorcycle point of reference too, I've never had it on my road bike (it happens just about every time I get on my motorcycle though).

I was in the drops. It was such an ordinary situation, just starting to slow a little for a U bend that was a few hundred yards ahead, but for speed and grade it was like Hazelwood Ave but straighter, and a different bicycle...

And yes I came across that Wintergreen video; it looks very much like what I experienced.

Sorry about your experience. Unfortunately it's very common and will happen with almost any bike. And the feedback loop is viscous too since tensing up makes it more prone to happening. Arms loose, and clamp the top tube with your knees.