Descending - The Wobble
 

I have been searching for an answer to a problem situation I have encountered a few times now. When descending, often I feel as if my rear-wheel begins to wobble and the stability of my new carbon frame and campy zonda wheels come into question. I have noted this on technically simple descents (straight downhill without a single curve or bump) and real mountain passes (now 'oh-****' as opposed to 'yippie' moments). As one travels downhill, the downward force of the bike increases and leaves one mathematically more stable, yet I feel as I am playing in the cycling equivalent of the Tacoma Narrows bridge. My question is simple, am I hallucinating or is my perception of a real phenomenon for which I should take some corrective action?

It's a real thing. It's a speed wobble. If you're going fast enough for it to happen, you probably shouldn't be pedaling anyway, so get in your aero tuck and grip the top tube with your knees. This will make the wobble go away. Loosen your grip on the bars too, death grips on the bars can make it worse.

It's normal. In order to build a good race bike, there are certain harmonics that at high speeds will become pronounced. Not much you can do about it, except grip the top tube.

Relax your arms and put your knee on the top tube.

+2

prior to my new bike (a Pearl Dice), I was on a Cannondale and then a Trek. On both those bikes I never noticed any sort of speed wobble despite bombing hills like a kamikaze. The feelings is strange and disconcerting to say the least....

i've never had speed wobble before.

and i hit 50 on a descent once. scared the **** out of me.

If all else fails, your fork may be out of alignment. Have your LBS check it out.

It's a harmonics thing so the conditions have to be just right. I'm much more likely to get it with my 46mm carbon tubulars than with my training wheels, especially if there is any kind of wind. It is not necessarily a speed thing, I've had a wobble in the 40s and had no wobble well into the 50s.

A custom builder claims that fork blade length (dropout to crown) can cause problems. I stumbled upon that only a couple weeks ago so I can't verify or deny that it's true.

Regardless, wobble is not normal. I've gotten well into the 60s without a wobble.

cdr

it is not "correct" but it is not unusual for people to get it.

I get it ocasionally, but certainly not frequently.

it is the "not correctness" that gets to me. 'not correctness' shouldn't, in my opinion, be part of a well constructed bike. i have a strong aversion to death and the feeling of being in life-threatening danger. new frame - new experiences

We are getting into semantics here. It doesn't mean that there is something wrong, but it is not really supposed to happen either... but it does happen. Is your new frame much lighter than your old one? In my experience weight makes some difference too.

My buddy has it happen to him on his Cadd, I got on it to see...and nada. But when he rides it it is there.

So, unless he is talking out of his assz, it must be a combo of the bike and rider???

This may (or may not) help.

http://www.calfeedesign.com/forksymmetry.htm

the bike is monocoque and 1100 grams. in other words, it is a bit heavier than my old madone but made from better material and an integrated saddle. i got it in april/may of 2008 and have 20000 km on it. i just got the wobble again yesterday, though last year in an Ironman race in Weisbaden, the wobble almost dropped me at 107 kmph. as i said, new frame - new experience - and had trouble finding info. thanks for the tip with the top tube and the grip/fork suggestion (though I did not think that the fork would have that great effect on the rear wheel).

Yep, the weight of the rider plays a role, as well as center of mass and probably 300 other variables.

If I take my hands off my bars, my bike will start wobbling at about 25 mph. Most of my friends who have descended at 60+ all say it's normal, and you just grip the top tube and it goes away.

I'm not sure that anyone really knows precisely what causes it. I read a study someone did on it a while ago, but I can't find it right now (read: I'm too lazy to look for it).

Whether it's a flaw, or not a flaw, I don't really care. It's easy to deal with.

Did you check the wheels to see if they were true? That would be the first thing I would check. I'm not doubting there are other possible causes, but even a slight lateral problem on a rim could become a BIG problem at high speed. Especially if this is a new problem for you this year, I'd look at things that might have changed like spoke tension, headset tension etc...

it sounds like its your front wheel that is shimmying. It is my rear wheel that is giving me fits.

for the front shimmy, check this link out

It can feel like it's coming from the front or back but is likely the same thing.

There is a known cause it's a harmonic sympathetic vibration. Now as mentioned above, there aare about 300 variables at play, and which one in particular needs changed in aparticular case is another issue.

+3 on knee on the top tube.

And since it is a harmonic,dependent on frequency, one answer is pedal faster, and get past the speed where the harmonic vibration occurs.

I've used the same wheels on three different frames, or at least the three frames that are working now. One of them is pretty much scary at 45 mph (Giant TCR carbon, stock), one is fine at the same speed (Giant TCR AL with a Reynolds fork, and I have two of those frames), and one is fine at any speed I've hit (SystemSix).

The SystemSix is lightest at 15.x lbs, the carbon TCR is about 19 lbs. But I think the front end of the TCR seems much less predictable than the SystemSix.

I've rarely had speed wobbles (actually I can't remember ever having them really bad), but when a bike feels unstable it's not good. A bike's wobble harmonic ought to be high, high enough that the speed is not normally seen.

I descend in the same way on the different bikes - knees to top tube, hands next to stem, face inches away from the front tire, butt in the air. There is such a big difference between the Giant and the SystemSix that it's pretty shocking. What's equally shocking is how much better my AL Giants were when I put on the Reynolds fork (hence the second frame having the same fork, and they both had the same original aero Giant fork). The carbon TCR seemed to have a stiffer fork but it's still not as good as the Reynolds.

cdr

Quick aside... Speed wobble caused the scariest moment of my cycling life about 2 weeks ago during a descent I've done a thousand times before. It's about 1/2 mile long, and I don't know the grade but I hit speeds in the upper 40's every time.

As I'm approaching top speed, I start getting front end wobble at about 45 mph. I look down to see traffic approaching, and I'm drifting into the other lane as I can't control my direction. I had to consciously relax, slow things down mentally and make clear decisions, release the death grip from my bars, and gently apply the brakes... none of which are easy when you think your about to hit the deck and slide under a minivan.

I didn't know at the time to put my knees into the top tube. I'm glad it was mentioned above should this bib-dirtying experience happen again. :D

Pretty sure it happens because your wheels are out of balance, not true. That valve stem, well it's heavier than the rest of the wheel. Pick the bike up, spin the front wheel, watch what happens. Fascinating huh?

No, it's not an issue with the wheels at all. You can generally correct it by shifting your weight forward or backwards. I have a bike that has a little more rake, and it happens when I sit and ride no hands. Not fun. My racing bike will occasionally do it going downhill. I suspect it also has to do with moving my weight back. The really scary oscillations involve the head tube moving back and forth rather violently.