ranger5oh

I had my R700 up to 47mph and it was pretty darn twitchy. Honestly, it probably isnt safe at that speed if you ask me. 35mph feels fine though.... just my .02

Eatadonut

My Pinarello Prince is unstable at high speeds. Above 35mph, he's been known to kill a man with no provocation.

carlfreddy

The fastest I've ever had my bike ('07 Lemond Tourmalet, 59cm with stock Bontrager Selects) was 42mph, on a descent of course. At that speed the bike exhibited no "unpredictable" behavior; no shakes, no speed-wobles, no twitch, no flex and no float.

I descend in the drops, pedals horizontal (unless I'm still mashing), knees in (can't say I've ever "clamped" the top-tube though), and butt off the rear of the saddle. I have no idea if that's the proper way to descend on a road-bike, I just translated what I learned from mountain bikes and it works.

marqueemoon

I don't think "stable" is really the word for it, but I don't feel like I'm not in control of the bike. Quite the opposite. My bike is happy at speeds that scare the **** out of me.

patentcad

Weight shouldn't be a factor. My bike weighs about 16.2 lbs, very light. The bike is quite stable.

grebletie

Fastest I've taken my bike is 52 mph. It felt stable, though I considered during the descent that now would be a horrible time for a catastrophic part failure.

Dogsled

Well, thank you for input, I especially enjoyed Moultons blog. A lot to think about but not sure where to start. Several posts describe what to do when it happens. I would prefer to pursue making the problem go away by making adjustments if it is possible. I have heard different things like, shift body forward, seat post, have fork checked (I have read the Calfee article and it does make sense) try different wheel set, tubes, tires. I have been contemplating getting a pro bike fit and I wonder if I should start there or is it a component thing and I should start there?

CerveloNW

Sorry, I had a typo: I meant to say I was going 42 mph uphill. Now don't tell me that's not fast....

aroundoz

My Custom Co-motion is not....but it's better than it was. Long story short, after three years owning the bike and complaining about wobble at high speed, they finally determined I was given the wrong fork. Prior to realizing this, they replaced the frame which didn't help. It's better now but still not great.

kleng

Could it be something to do with fork stiffness, I notice the Roubaix has the zertz elastomer inserts, maybe there is more fork flex going down hill with the change in weight distribution.

My Giant TCR advanced team has super stiff forks that beat you up while riding
but are solid when descending.

sac02

I think what is happening in a "shimmy" can be explained without too much engineer-speak. A shimmy is like a resonant frequency - some sort of perturbation starts a vibration at just the right frequency and it continues to grow unless something is changed.

In the simplest terms, the natural frequency of a system (w) is defined as

w=sqrt(k/m)

where k is the stiffness of the system and m is the mass.

So to eliminate the shimmy you have two options:
1)change speed to alter or eliminate the input signal (vibration) that is causing the system to resonate at its natural frequency
2)change the natural frequency of the system by either altering its stiffness or mass

Examples of changing the stiffness and mass have already been given in this thread:

When you clamp the top tube with your knees, you are effectively stiffening the system. (If you want to play semantics, technically you are not "damping" any vibrations; you are stiffening the system and changing its natural frequency so that it no longer resonates in response to whatever input it is experiencing)

The member who posted about seeing a motorcycle shimmy so hard it bucked its rider off, then continued straight (without shimmy) for quite a distance gave us an example of changing the mass: when the rider was bucked off, the mass of the system changed and thus so did the natural frequency, and the shimmy disappeared.

I didn't staty at a Holiday Inn Express last night, but I did take a few mechanical engineering grad courses in dynamics and vibrations...

Mac

R900

Both my Madone and Cannondale CAAD 8 are solid. I've had both up to at least 48 mph (never cracked 50). No shimmy or other problems.

terezajoy

Has anyone else experienced a shimmy on a Specialized Tarmac Comp?

c64

my commuter bike- a Stumpjumper M2- is stable at 52mph.

Sorry, I just wanted to say that. There was one hell of a tailwind that day.

DrPete 

My F4:13 actually feels MORE stable the faster it goes. I've had it in the high 40's/low 50's a couple times with no problems at all. That's with bladed-spoke Fulcrum 1's.

v1k1ng1001

My old 1997 S-Works M2 is stable at 50 mph off-road! I love that bike.

My new Lemond Buenos Aires is stable over 50 mph, much more so than my old R1000.

shabbasuraj

Gone up to 122 km/h... no issues... STABLE.

jhglaw

My Specialzed E5 is absolutely stable in 45 mph downhill switchbacks. My Lemond Zurich (carbon/steel) is spooky in the same switchbacks at 38.

eippo1

Ha,
"That's right, Iceman, I am dangerous..."

Dubbayoo

i haven't crashed but i rarely exceed 40mph

eippo1

I actually hit 35 on rutted dirt on a Battenkill-Roubaix downhill on my aluminum Bianchi. Not sure why the heck I was looking at the computer, but damn was that fun. Passed a bunch of riders b/c they probably thought I was crazy. I later found that someone flipped at the bottom of that hill when they hit a rut dead on.

ILUVUK

My bike (Windsor Fens) has topped out so far at an apparently extraordinarily pedestrian 39.7MPH so far. The bike was very stable at that speed.

AlanS

I think you are on to something, here.

Stems under 100mm feel unstable on very fast downhill runs. The reason for the twitchy behavior is obvious.

timmyquest

Sigable

deskjockey

Hmm....this thread gives me some insight into the thread I posted the other day. I have a new Merlin Works (compact) and had some scary shimmy with the wind at my back on a flat road. I had it up into to the low 30's while pedaling (and wind ) and as soon as I adjusted my hands on the drops, the shimmy started. Freaky. I suppose it may have had to do a bit with my hand movement, but it seemed controlled. Very frustrating esp. when my much less expensive and non-professionally fit older bike never had this problem.