Fat guy and speed wobbles on a Trek 5000

The front end of my 2006 Trek 5000 scares the living crap out of me at 35+ MPH and I don't know if it's because I'm a lard monkey (6'-1" 190 lbs.) or my frame is a TCT carbon noodle, but I've got to figure something out. I've upgraded the wheels to Race Lites, they run true and the spoke tension had been checked. After searching though various threads on speed wobbles there seems to be a variety of causes so it might be simpler to ask if there are any known weaknesses of the Trek 5000 frame and fork. Would a stiffer frame or fork help?

And I've always been curious about the TCT carbon... In what ways is it inferior to OLCV carbon? I don't mind extra weight, but I want a stout frame... Is TCT a bad choice in my case?

 

I'd better let some of the smarter folks help you diagnose the problem, but I seriously doubt that it's your weight that's causing the issue.

A bike should be able to handle more than 190lbs.

 

Ahhhhh, speed wobbles. They can be enlivening, can't they? There most fun when they happen going through fast downhill corners!

First, you're not really a lard monkey. Your certainly not overwhelming your frame. As you may have read or guessed, all sorts of things can lead to speed wobbles. Those things excite a harmonic, and things start to get playful. A loose headset could lead to a speed wobble. Loose quick releases can do it. An overly flexible fork can do it if loaded the right way. And sometimes the weight distribution of the bike/rider system happens to be just right so that forces put into the frame excite a harmonic (If you're unsure of what a harmonic is, go to YouTube and find video of the Tacoma Bridge failure.). A quartering wind can cause oscillations. FYI, every frame out there has a harmonic, but most of the time a bike's harmonic exists outside the range of normal operation.

What to do? Check your bike over to be sure that everything is torqued as it should be and is in good condition. If you're not comfortable with that, take it to the LBS and have them do it (but watch them so that you can learn). Check the wheels for balance. Unbalanced wheels can cause speed wobbles. If they need balancing, balance them. If none of that works, you might want to consider changing something.

Make sure you're not gripping the bars, on a descent, with a death grip or with rigid arms. Stay relaxed. Relaxed arms and body can damp oscillations. If you get the wobbles, squeeze the top tube with your knees (another way to damp oscillations) and don't jump on the brakes. Light application of the brakes can change your speed slowly, taking the frame out of the range of speeds for which the bike is sensitive to oscillations (This is dependent on road surface, weight distribution, wind, and etc.).

 

I totally agree with my man up here. Get the bike THOUROUGHLY checked at your LBS and then don't try to HIT 35 mph. I crashed at that speed on a speed wobble and scared the s**t outta me. It's taken me 6 months to get back to that speed again. Just try to get down slow and with confidence and push the envelope slowly. It will make you gain confidence on your bike. Also one more thing that my man above did not mention is that cold can cause speed wobbles. Now if you are coming down at 35 mph you obviously climbed a hill to get there. Obviously it gets colder up above than down at the bottom of the hill. Now consider lack of sunlight if you are riding down a hill in a gully, and the wind factor of riding of 35 mph. It gets really cold. Now what the cold does is make you clamp down of the handle bars or even worse start to make you shiver ever so slightly. If the shiver matches your bikes harmonic frequency, boom there you have it, a speed wobble. So it might not be your body, might not be your bike, it's just plain ol' cold. So dress warm and don't speed up if you feel cold going down hill. It's a concoction for danger.

Lastly, I don't know if someone's told you this. The way to cure a speed wobble is to start pedaling and to accelerate. It changes your bikes harmonics. I know it sound idiotic but it's logical, but once the wobble's started I know you are already scared so your brain doesn't think straight. So remember NOT TO CLAMP ON THE BARS, and try to accelerate out of it.

Good luck and ride safe!

 

Thanks for the input guys... I think one of my problems may be the death grip I have on the bars when the bike gets more responsive at the higher speeds. I've been hyper sensitive to it ever since I had a heart stopping near crash at ~40 MPH on a freshly chip-sealed road. I came flying down a hill and got into a bit of the loose stuff at the bottom. The tires started skipping, sliding and wobbling all over the place, I don't how on earth I kept it on two wheels. I wasn't really afraid of bicycles until then. Now days I feel safer during my interstate commutes to work on my Honda. How sad is that?

 

Here are a couple of links and a video that might be informative:

Dave Moulton: Bike Tech.
CalfeeDesign
[ame=http://www.youtube.com/watch?v=xODNzyUbIHo]YouTube - Bicycle Shimmy or Speed Wobble[/ame]

 

Hopefully that's what's causing your wobbles. Don't rush working back up to silly speeds. Let the speeds rise with your confidence. FYI, for the majority of riders, fear is the limiting factor and not tire grip. Work on relaxing. When cornering, look through the corner: the bike will go where you're looking. Looking just in front of the wheels is bad. By looking through the corner you also help avoid target fixation, i.e. when a rider ends up staring at what they're afraid to hit or where they're afraid they might end up........and end up doing what they're afraid of.

 

Good video. Believe a lot of bikes tend to do the same thing at speed with no hands on the bars. I've experienced the same thing a couple of times when sitting up on a long descent to stretch or remove the vest, but never in a "normal" descent position. It certainly points out the role of weight on the front end. And even with hands on the hoods, sitting upright at 40mph+ you've got a few more pounds of drag force acting to unload the front wheel.

The OP's "near crash" experience may have been scary, but to me it again validates the inherent stability of our two-wheeled machines at speed. Even if a tire slips or moves around for an instant, it can recover: everything "wants" to keep going in a straight line. Believe the more the rider can relax and "trust the bike" during descents or through rough patches, the less prone to crashing we become. On fast descents, a good tuck position to keep maximum weight on the front end feels the most stable to me. From that position, it feels easier to lean through the bends by using bodyweight rather than the thinking about steering via arms/bars. Also the low sightline just feels safer when really leaning hard.....not as far to fall

 

That's right. A low position also allows you to start sliding or tumbling earlier! It's best to get things over with quickly.