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Question(17 posts)

Sep 25, 2003 9:08 AM
I'm still very new to all this, I have a question about a term I read here all the time. What does "flex" mean when being talked about a frame. I here alot of your referring to it.
Thanx in advance
touchy subject around here....funknuggets
Sep 25, 2003 10:21 AM
Well rubendc19,

Flex for newbies:

Flex is 'typically' due to characteristics of materials and build and refers to the relative "bending" of the part under applied pressure. Generally flex discussions refer to frames... but could also be applied to stems, wheels, forks, cranks, or handlebars.

Different materials generally have specific tendencies. Take an Aluminum frame for example... generally it does not flex at all, where a steel or titanium frame would tend to flex more. Now, there are frame designs that can eliminate that flex and there are different types of ti, steel, or carbon that will be more "flexy" than the other. This difference between frames and materials often make "flex" a category of differentiation when discussing the ride characteristics of one bike to another.

Generally speaking... a flexy bike will ride less rough. Lastly, the most common flex is the relative 'swaying' of the bike in the bottom bracket as force is applied to one pedal or the other. The frame... if flexy will bend away from the side of the downward force, as the handlebars are positioned to pull the bike away from the downward force. This causes the frame to "bend" slightly, but pops right back.

There is great debate as to whether this helps or hinders the rider... thus the subject line... and I do not want to even bring that up. But this is the general explanation of flex and how it is used around here.

So sayeth the Funk,
...and some people say that flex is all in your head... nmMShaw
Sep 25, 2003 11:09 AM
Interesting, but wrongKerry Irons
Sep 25, 2003 5:08 PM
The flex of a frame is ALL about design and virtually NONE about materials. While many Al frames are designed with large diameter, thick wall tubes that make them stiff, it is a function of design, not material. If you were a biker 25 years ago, you would have incorrectly concluded that Al frames were flexy, because in those days they were made of small (like steel) diameter tubes. The stiffness of a frame comes from design, not from the material chosen. Full stop.
You seem to know alot about this.Kristin
Sep 25, 2003 5:39 PM
Is what I feel when I rest my foot on the chainstay, frame flex? I also can get trapped in the big ring when climbing if I'm not careful. Does this happen to everyone? If I get onto 5%+ grade I can not shift from large ring. It can hear it try to come off, but when I bring the pedal back around it just stays put. That is also a flex issue, isn't it?
I agreebimini
Sep 26, 2003 5:39 AM
I have to put on my old engineering hat on on this one. The modulus of elastisity of steel is about 3 times that of aluminium. This means it takes 3 times as much force to deflect a piece of steel the same amount as a piece of aluminium of the same dimensions and thicknesses.

I think aluminium has the reputation of being stiffer because the builders had to go to thicker wall tubes and larger tube diameters to stiffen up the bikes and reduce the fatigue problems of aluminum. (fatigue is a form of metal failure. If you place repetative or cyclic loads on a piece of metal it will break after a certain number of repetitions. The amount of force, number of cycles, tensile strength and Modulus of elasticity all come into play in determining the fatique life of an item) Aluminiums fatigue properties are pretty poor compared to a quality piece of hardened and tempered steel. You need thicker sections of aluminum to spread the forces over a larger area to reduce or eliminate the fatigue problem. This can be done in aluminum without sacrificing weight since aluminum is so much lighter. If you notice on the high end racing aluminum frames and handlebars they rarely offer a lifetime warrenty and some even recommend a finite life of the frame or handlebar. This is due to fatique life.

I'm not saying one material is better than the other. Either one is a good material as long as the frame is designed well for the material and purpose of the bike.

For sprinting and pounding the bike - stiff is good. Energy is force x distance. If you are stroking extra distance due to frame and crank flex you are wasting a small amount of energy since your feet and legs are covering a little more distance that you need to.

PS: Where does that woosh - woosh - woosh sound come from during the sprints? Wheel flex? Frame Flex? Change of speed of the spokes? (love that sound, only comes for a few seconds when you are maxed out)
Sep 26, 2003 11:00 AM
go get your self a standard 1 inch diameter 3 ft section of (6061-T6) and another of the same dimensions of your favorite steel, say 853 or whatever. You secure one end and then tie a weight to the end. Check to see which one bends more. The aluminum will be... in the region of 15% less defelection than the steel. So, please MATERIALS DO matter, I never said it was ALL frames, I used it as a general rule and it CONTRIBUTES to the flex of a frame.

Reread what I said and check your facts before calling me out like that. Punkazz.

If you want to experience bike flex, do thisKristin
Sep 25, 2003 11:20 AM
On your next ride, unclip from one of you pedals and rest your uncliped foot (carefully) on the chainstay. Then continue to pedal with the other leg. You will feel the frame flex. Of course how much will depend on what type of frame you have. When I do this on my lugged steel frame, I easily feel it.
Unless it is a stiff frame....funknuggets
Sep 25, 2003 12:53 PM

What kind of silly noodle of a bike do you have... a freaking Softride or something? From a general seated position and just sitting and spinning, I have no clue how on Earth you would flex your frame, much less your rear triangle. Help me to understand what you are saying it is you feel. Im not doubting you, Im just not sure what it is you are feeling. I have a carbon, an aluminum and a "squishy" steel 525 and I really want to compare your "flex" test.

I don't know what to tell youKristin
Sep 25, 2003 12:58 PM
I have a Columbus Aelle lugged frame. 53cm. Small tubing. 2 years ago when I was fighting ITBS, I would sometimes need to unclip my injured leg and I would rest it (carefully) against the chain stay. Its hard to describe what it feels like outside of saying it feels like frame flex. Have you ever ridden with your foot on the chainstay???
Sep 25, 2003 1:10 PM
only on the trainer when I do one leg intervals... but that is interesting. Neither my Fondy or my Look 381 will flex at all, I know that for sure. My Lemond might, however. I do know of flex in the BB, that you get when mashing or stopped and stand on the pedal in the 6 oclock position. I just have no clue how or why a bike would flex simply seated doing ONE LEG pushes. That would tell me that you are likely feeling something else especially if it is pronounced enough that you can feel it through your SHOES.

That is all I am saying. I hope the knee has healed up nicely!!!

Flex is your friend. nmSpunout
Sep 25, 2003 11:40 AM
Sep 25, 2003 12:28 PM
Yeah see you can tell I'm new, Cause I don't have an expensive bike (me being a newbie and all) But I don't feel any flex, maybe it's becuase I have an aluminum. I suppose once I get another bike, which I will next summer becuase I really enjoy road biking. Is when maybe I will be able to to tell these types of things
You can see the frame flexovertrained
Sep 25, 2003 12:52 PM
when you step on the pedal in the 6 a clock position while stopped. Or just watch the big guys frames during group rides.
Not surprised you don't feel it.Fez
Sep 25, 2003 1:22 PM
Reasons why you may not feel it:

You don't generate a lot of power
You don't get out of the saddle a lot
You don't really notice it, even if it is flexing

Ride behind some guys who are hammering if you want to see it
Borrow an bike generally regarded as very stiff and hammer away or climb on it. Then go back to your bike and ride the same way. Compare.

Put your bike in a trainer and ride away. Probably more noticeable in this environment.
Stand up on a trainer and watch the bottom bracket.(nm)dradrian
Sep 25, 2003 1:00 PM
aluminum doesn't flex?Sao
Sep 25, 2003 3:31 PM
Dang, maybe my bike isn't aluminum after all. 'Cos it has been flexing like a noodle lately. Not sure what is causing it but it has been unnerving.

I have become a stronger rider this year, but I doubt that that is the reason, to be honest. Jan I ain't.