It could have been caused by anything or nothing.

My first bike used to do it to me. I finally figured it out after a few to many near misses exactly like yourself. There was a very slight bit of play in the headset. When i held in the front brake and rocked the bike you could feel the movement.

Lotta times if you get a speed wobble in the 40s, take the speed up to 50mph or more and it'll smooth right out.

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http://www.jt10000.com/

Speed wobble is, unfortunately, a consequence of the entire bike+rider system. Geometry, frame flex, and the rider factor into it. A bike that wobbles horribly for one person might not wobble at all for a different person.

Hence, the safest solution is to sell the bike, and get a different bike.

It's frequently suggested that bearing play can cause shimmy. I'm not really sure if this is true, but you might as well check your headset to make sure that there's no play, because that's a good thing to do occasionally anyway.
It's also frequently suggested that using a headset type with high bearing friction can help to damp shimmy. Heck, Soma suggests using a needle-bearing headset with their Grand Randonneur frameset due to concerns of speed wobble. If you can find an affordable roller-bearing headset for your bike, trying it out wouldn't be the worst idea in the world, probably, maybe.

Did you unweight the saddle when shimmy occurred? That's a common recommendation.

It used to happen to me a lot. Changed the front wheel and it’s never happened again..... I’d try this before selling the whole bike....

My speed wobble happened a couple times when using aero skewers that were difficult to clamp down.

I haven’t had a problem using stock skewers.

I feel more stable in gusts and have hit much higher speeds without issue. I am much better about clamping down on my front after the couple of scares I had.

PS - I was ready to get a new bike if I couldn’t figure it out.

Good you are ok. I only read about it on bikes but I know about driving a Jeep. Just happened lately and what I know from solving that it sums up by a lot of things. Just google it for understanding.
I just make a summary for the bike.
Tire / wheel not 100% true, spoke/s loose , skewer not tight enough
Not proper tightened Headset
Too much flex in the components

Instead of speeding up (like one recommended in pink) slowing down is the safest option.

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-shoki

Speed wobble used to be very common, but the intro of extremely stiff carbon frames has made it much rarer. The bike is probably fine, but something in the system is loose (headset, wheel bearings, etc.).

The other possible culprit is an out of balance wheel. Put the bike on a repair stand and spin the back wheel up to speed. See if the bike shakes. The front wheel is a bit trickier, but can be spun up using contact from the rear wheel of another bike. If out of balance, they can be rebalanced with a bit of epoxy putty on the rim.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

I can tell you that I had the same issue on my Speed Concept. Wobbled badly and I was able to stop. I had a rear flat at the same time so I attributed it to that. A couple of weeks later I had a bad wobble descending and crashed. Both were during a trip to Boulder where I had aero extensions changed at a LBS at the start. It turns out there were two screws within the headset that were not fully tightened. The shop was a little cagey afterwards, not wanted to take on liability. I have not had an issue since then, so perhaps just make sure everything is tight. I did replace the three screws holding the base bar on as one of them sheared off during a ride (that was an interesting experience trying to stop.). Stainless steel now. Good luck, and I know it’s unnerving until you know what’s going on.

In the past, this bike has been rock solid on descents. This was a first. I am pretty sure the headset was good and tight as I checked when packing the bike for travel. But, there is always the possibility I missed something.

I tried slowing down with the back brake. But, with the road bending to the left and my really not being able to steer fully the ditch became the option.

I would like to try and figure out a possible culprit as it was quite unnerving. Makes getting up to speed mentally harder.

There's a ton of info on here about speed wobble. Maybe even slowman has written an article or two. Beyond the mechanical items already noted... It can happen on any bike, even 100% mechanically sound ones. The best theory is that its an inverted pendulum interaction between the rider and bike. Once the action gets started, it's self perpetuating as the coupling between the frame, saddle, rider, arms, and steerer creates a positive feedback loop... Exacerbating the steering input from the arms.

The number one method for quieting a speed wobble is to get your butt off the saddle. That breaks the coupling.

The knee into the top tube isnt usually very effective at STOPPING a wobble. It's better at preventing it. My practice is always to keep a thigh against the tube once you spin out, and keep the pedals level. If I feel a wobble starting, immediately unweight the saddle. It doesn't take much.

At the end of the day, your confidence likely won't come back until you learn to control it and come back from it... And know that you can. So, check your mechanics off the list and then practice recovering from it.

Just a guess but a couple of things to consider. Are you sure it was speed wobble? If it was a turn and windy that may have triggered the control loss? Also, I am assuming when you mentioned putting the leg against top tube you were squeezing the top tube with both legs, which is the technique to use as a damper. Another thing to check is tire pressure. If running really high that could cause some additional instability. Glad your ok.

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I miss you "Sports Night"

Yes, it was speed wobble. It was not really a corner, the road just swept left in that it wasn’t a straight road.

And I guess my method was a single leg against the top tube and not to squeeze it between both legs. I was sitting on the seat and not unweighting the bike by standing up.

Any chance wheel stall played a role in this? I've had a few momentary, and very scary wobble incidences, in swirling winds, with my 90mm Flows. I've never experienced it with 50mm Rovals, and the stock aluminum rims, on the same bike.

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Athlinks / Strava

Got it. Yeah, I stay seated and squeeze the top tube between my knees.

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I miss you "Sports Night"

Too many times on my speed concept-sold it, never on my PR6...

Earlier in the same ride was going 43 downhill in aero. Solid as a rock. Just the slight drift left or right if the wind pushed or when a semi passed you and created the hole in the wind.

My wheels are in the 50mm depth range. Specifically brought the Aeolus rather than my 808 to keep the depth a bit shallower for the potential winds.

Speed wobble most likely caused by the rider. Gripping the bars too tight, then gripping even tighter as the wobble begins and worsens...

Prior to the wobble, had a simple light grip on the bars. Once the wobble started, yeah my grip tightened up as I tried to slow the bike down. But prior, nope. Hell, I would have been in aero had the descent been a bit shorter or less steep. But nine miles at 7-8 percent grade was a bit much for my taste of top speed...

My fix was replacing that bike. Rider and everything else was fine, as is likely in your case. Though with a Trek SC, and it's many parts in the steerer, I'd have a Trek LBS evaluate it, and tell you if they can find anything loose.

The bike took a good hit in the crash. It needs a quick look over on the frame to make sure it is all good. I will have them check out the headset steerer area too.

Others have hit the nail on the head, there are mechanical causes for speed wobbles, but the rider is also a component... It's also known in bike racing circles as the white knuckle wobble (and the old rule of thumb is that if your knuckles are white, you're holding the bars to tight)...

In general, most problems, barring equipment failure, that you encounter on the bike can be handled by relaxing... it's counter intuitive, but the number of times that I have avoided crashes racing crits just by relaxing and letting my body make simple adjustments is pretty high... Probably the most extreme was someone inside of me drifting into my line in a corner and getting her drop bar inside of mine (my first though was I'm going to crash, my second thought was to gently feather her breaks, which got me enough separation to get clear and stay upright, and as soon as we exited the corner, I hammered my way up to the front of the pack to steer clear of that rider), but it also works for bumping, wheel touches, correcting a lean when entering a corner too fast, etc.

I have had many different road and tri bikes and have tried half a dozen more in various roads, events, and configurations (wheels, aero bars, etc.) and the only one that ever wobbles is the ...

Speed Concept.

Now, as many have stated here, there are many contributing factors to a wobble but for some unknown reason I get it really bad with my SC. I have tightened all the screws, used different wheels (303, 404, 808, Aeolus, etc.) and replaced the stem, base bar (UCI versus non UCI), aerobars, etc. many times over and I still get that wobble once I hit a certain Speed and a certain angle.

Compare that to two of my Colnago road bikes, which is sooo stable at downhill they are an absolute joy to ride.

I will sell my SC as soon as I can.

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Others have given good input on external variables, but having owned a few SC's and well built several others for folks, I would go through the front-end and pay particularly close attention to steering axle assembly.

Low torque of top screw (2NM) with lower lock bolt and patterned tightening of stem (fronts first then pinch bolt). I have also seen stems that will not fully seat due to interference with frame stock (near front) which leaves a slight gap. Although the steering axle is the main source of compression having the stem fully seated is good insurance and reduces its ability to act independently of steerer. I may have some pics of this problem as well.

If these steps are not followed it will leave some slop in the front end. Of if over cranked (often see this) bearings will get housed fairly quickly. This is before we get to the bar mount bolts.

http://trek.scene7.com/...t_Service_Manual.pdf

Again, not giving assignment to above for your issues, but time well spent nonetheless and something you need to rule out.


Cheers!