I hear people commenting frequently about thier frames being "flexy" or "noodly", or something to that effect.
I understand them to be discussing the frames lack of stiffness, particularly when hammering hard in a big gear.

I've not hammered too many bikes, 2 actually, both steel.
1- Early 90's Specialized Sirrus - lugged steel frame
2- 2002 Bianchi Vigorelli - steel frame (not lugged)

I don't know what a "flexy" frame feels like. How would I know if it was flexing? I guess I would know if the chain was rubbing differently, when hammering on the pedals. Then it's pretty obvious.

I've been experiencing something with the Bianchi, that I never noticed on the Sirrus. When I am in the "big ring-little cog" combination, and really getting some serious effort into the pedals, the back end of the bike starts to feel a little like the tire is sliding around on the road. Almost like I'm on ice, or on a flat rear.
The first time I noticed it, I was in my idea of a TT mode, crouched low, trying to see how fast I could get the new bike moving. I've had the Sirrus up around 31-32 mph (with a nice little tailwind) and it felt solid. On the Bianchi, as I got up around the 28-29mp area, I thought I had a flat, so I slowed down, only to find the tire was fine. I've noticed it a few times since.
Is it just me?
Is it my wheels? (Mavic Cosmos, I had CXP23/Ultegra on the Sirrus)
Could my frame be torquing and causing this feeling?

What do you all think?

No response?

I have never experienced frame flex and I don't think I will ever experience it. I am a light rider, 55kg. However, if you are putting in serious effort out of the saddle, the bike will move around beneath you. I think there is nothing strange about it. It is weird though that you have the feeling of a flat tire but to find that it is fine. Did you remember to pump up the tires? I believe a flame flexing happens at the bottom bracket area where both triangle tends to collapse inwards. Hope you will get some response.

Impossible to describe in writting.

I weight about 50kgs. and I have had frames flex. Have an old Batavus that I would say is dead as well as another older steel frame.

Basicly you feel the bike not jump when you sprint.

The bikes really has to be ridden for many many miles and years in many cases. A new bike ridden about 100 miles a week will not show flex for years if ever.

Frame flex is noticeable in some of the situations you describe, but most noticeable on short hills where you might climb out of the saddle in a big gear and in corners and coming out of corners, especially coming out of corners when you may sprint back up to speed in a big gear. You'll notice a flexible frame will feel like its difficult to keep in a straight line or that the bottom bracket seems to be swaying back and forth alot under your pressure. Very flexible frames have been known to shift on their own in corners and on hills because the flex and resulting misalignment of componentry. A flexible frame will "wallow" on bumps and in corners, feel like it's giving in instead of "dancing" on them or gliding through them.. An overly stiff frame will feel like its hammering you up the arse and thru your wrists on the same bumps.

The latent benefit of a flexible frame is that it usually is more comfortable over the long distance.

The perfect frame, and I know this is subjective, feels solid and accelerates in a straight line when pedaled hard, feels resilient on the bumps, and smooth when pedaled under normal conditions.

You may want to have a pro shop tension true your wheels with a tensionometer, or better yet, buy a Park tensionometer for a little more than you would pay the shop, and go to the Barnett's section in the Bicycle Mechanics section, look up wheel truing and do it yourself! That could eliminate the wheels as a variable. Then see how it feels and get back to us.

Any frame that has had too much heat put in it during the manufacturing process or for some other variable(light tubing, weak or poorly built wheels) can feel flexible right out of the box. New is not necessarily right.

A rider can adapt to a flexible frame. The best way to adapt is to learn to ride lower gears at higher rpms. One of the greatest rider's of the modern era, Sean Kelly, rode a very flexible frame, a Vitus Aluminum frame, for most of his career. Many great victories have been won on "screwed and glued" ALAN aluminum frames, a real flexy flyer. The riders just knew how to ride.

I think a frame ican be called "flexy", if you make your chainring rub on the derailleur whenever you "stomp" on it.

Ride Stiff
Pat

a "flexy" frame simply flexes. it bends left and right when you put power into the pedals. this causes some of your leg power to translate into bending the frame as opposed to translating all of it into forward motion, so the bike will feel sluggish compared to one that doesn't flex. a bike will also feel flexy if your wheels aren't tensioned. (which is obvious when the rim starts rubbing on the brake pads when you accelerate)

flex is most obvious when accelerating hard off the spot in a medium-tall gear, not so much when you ride at high speed. steel bikes are naturally flexy, but new steel bikes, like your vigorelli, which are built for racing and have a tight racing geometry should feel just about as stiff as aluminum bikes... i'm pretty sure that what you were experiencing wasn't frame flex...

that's actually because you're going so fast already that there isn't enough downforce to keep your rear wheel on the ground!!

seriously though,
i get the same weird feeling when i ride my road bike, only i'd describe it more like it feels like the rear cogs are slipping into place...like when you put a new freewheel or cog on a single speed or fixed gear, then go out and really torque on it with the pedals, you take off for a second, then it slips, then it's fine. it feels like something just shifts for a quick second, just like if you had a flat or hit a small patch of ice. i think it comes from the rear hub, and i think it has something to do with cassette hubs, beause i've never noticed it on an older road bike that uses the freewheel-type rear cluster (unless said cluster has been freshly installed and isn't tight yet). it might be some weird quirk with how the hub is engaging or something like that. or it could be the chain stretching a little, or more likely, settling into the tiny cogs when you apply more force.

i've also felt frame flex and it doesn't feel like this. the first time i felt it i thought the shop hadn't tightened the freewheel all the way, then i realised i was riding on a cassette hub. then i thought the cranks were tearing away from the chainrings, but it wasn't that either, it could have been that the chainring bolts weren't tight enough...it could have been anything... i'd check the bike over, making sure all the chainring bolts are tight enough, make sure the rear wheel is secure, make sure your chain is the proper length. then take the bike out again, and if you still notice it, take it to a shop and explain it. they'll probably have a creative explanation for you if they don't really know the cause.

EDIT!!!! i just noticed you said it happened on your bianchi. my previous frame was a bianchi eros frame with a 105 7 speed cassette. this was the frame that i noticed that feeling the most. i haven't ridden my newest frame (a pinarello) enough lately to say for sure if i feel it on that too. maybe it's a bianchi thing?

I'm 190 and dont know what it feels like.I have to say that i'm in my seat 99% of my rides.I wouldnt worry much about it unless i race.

Thanks for the replies so far.

Just to clarify, this is NOT when I am "out of the saddle", because I almost never get out of the saddle. Just hammerring flat out, big ring, little cog.

Not when I'm accellerating, or going thru a turn. Just going in a straight line, it feels like the back end is moving side to side, like it might with really low air pressure. But the tires are fine.

Loose or worn wheel bearings can cause that feeling,so can loose
or too flexible spokes. An out of alignment frame too.Take it to a good bike shop.A loose quick connect on the wheel might do it.
A too flexible frame, and a broken frame,look for small cracks.
Try to get a good mechanic to ride it if possible.It could be many things,that are hard to find.