Flexible vs Stiff
 

What is the meaning of flexible and stiff on bicycle frame? I heard steel frame is flexible. Can it bends and retains to normal structure? What about aluminum and carbon fiber? These don't bend at all?

Everything bends if enough force is exerted. Aluminium doesn't have a fatigue limit, which means that if you repeatedly flex it, even a little, it will eventually fail. Steel, on the other hand, has a distinct limit beneath which it can be flexed indefinitely without failure. So alu frames are typically "stiffer" than steel frames so they flex very little.

People talk of "stiff" and "flexy" or "compliant" frames very loosely, it isn't always clear what they think they mean. Usually they are referring to the ride quality of a specific bike, but that is influenced by a lot of things other than frame material. Forks, geometry, wheel, tyres, all play a role.

You'd be better off asking your question in the frame builders sub-forum, there are lots of posters there who have real metallurgical and other expertise.

its about ride quality. an all aluminum bike frame (given no suspension) will be a much harsher ride, you'll feel all the bumps. steel absorbs much more shock and is much more "comfy".

carbon fiber has what's called "dampening" qualities. its stiffer than steel but can still be a very smooth ride. at least i think thats the jist of it, i don't have a full carbon bike.

This is misleading, at best. How "stiff" steel is relative to other materials depends on the tubing used, among other things. Some steel bikes feel very stiff indeed, some aluminium bikes ride beautifully. The idea that frame material, in and of itself, determines ride quality is a myth.

It ain't just the material it is how you work it.

But what tire size and pressure you use trumps either in ride quality.

Not to mention frame geometry.

What is your current riding experience? A little bit of this type of information may help to explain the meaning.

Vitus and Alan were two companies who produced aluminum framed bicycles in the 70's. They were known to be quite flexible.

Yes, they used "standard" diameter tubes epoxied into lugs, which with aluminum meant that the frame would be more flexible than a steel frame using the same diameter tubing. Klein and Cannondale used welded oversize tubing, ostensibly to make the frame stiffer, but also to increase the weld area to reduce the risk of joint failure. Some people prefer a stiffer frame, others do not.

Carbon fiber, especially in its modern monocoque type construction, offers the advantage that reinforcement can be added explicitly to the areas expected to benefit from it, and reduced in other areas to minimize weight. Other frame materials are less accommodating in this manner, although modern hydroforming methods may be changing this.

[rather than Opine] Science! It is something you can set up a facility to test.

Like : Measure the Deflection distance under a measurable force .

dont forget that the wall thickness of a smaller diameter tube can be increased to make it stiffer .. (smaller ID)

though a Larger diameter tube can be using thinner wall , so to have lower weight data to help sales

They Do.

I heard some people are talking it. I just don't quite understand. I have a steel framed bike with front suspension. I haven't notice my bike flexes.

Yup. Lock a bike in a stationary trainer.

Back in the 60's or 70's, Bicycling Magazine had Gary Klein build them a fixture to measure bike frame flex. It was called the Tarantula. It had dial indicators to measure movement with a measured force input on a crankarm.

There is a theoretical optimum relationship between tube diameter and wall thickness. In general a tube resists bending as the diameter increases but you eventually reach a point at which a thin walled tube becomes too susceptible to physical damage (beer canning).

Frank Berto was Into making test fixtures, and wrote for the magazine
.. Scanned material - Frank Berto

Klein of course wanted to demonstrate his 'My Frames are stiffer' selling point.


In aluminum and steel there are different characteristics in each alloy mix ..

The next question is: How much does it matter?

But that effect tends to be much smaller than tube diameter. That is, with the same total amount of material, a wider tube with thinner walls is stiffer than a narrower tube with thicker walls.

Hence why vintage steel frames with narrow tubes are sometimes flexier than modern steel frames with wider tubes, even though the material's stiffness is the same, and even though the newer frame's tubes might be made from less material (and thus lighter). Diameter has such a big effect.

Double butting thins the middle .

Not Sprinting to the finish, I dont obsess over stiffness

though my 2 side by side .75 .049 wall top tube touring bike didnt 'plane'

like the 1.125" single tube Specialized expedition , flexing and spring back with every pedal stroke .

Also keep in mind that one may not want different amounts of flex in different parts of the frame.

So, if you look at the Specialized Roubaix, it is specifically designed to have a stiff center triangle (bottom bracket to seat to head tube) for performance and accurate shifting, while having moderate flex in the chainstays/seatstays, and fork for comfort.

Standard old guy response to an old, old discussion in cycling:

I've been riding and racing bikes for decades. The longer I ride, the more confused I am about the physics of stiffness. Frankly, bike stiffness, especially when measured in extremely fine gradients like bike magazines do, has started to smell of snake oil to me. Despite all the pseudo-science applied to this topic I've never seen any hard evidence published anywhere that watts are "lost" with a flexy frame.

It seems intuitive: any pedaling force flexing the frame is not being used to drive the rear wheel.... but is that really true? Can someone explain how exactly is energy "lost" through lateral deflection? Any energy expended is likely returned as the frame flexes back, what happens to that energy? Does that returned energy help on the next pedal stroke somehow or is it just "gone"? Are any of these effects measurable? How would one even measure it?

One thing I'm clear about: a stiff fork and stiff head tube is really important for good handling. To me, that's not up for debate.

There are endless possibilities, I was just digging around and I came up with old batches of photographs ... In the old days, the Frame was Steel and the Fairing was glass or carbon fiber...
https://www.flickr.com/photos/116299...57639939606343
Now, everyone wants to buy carbon, but we still haven't considered that carbon and glass (fiberglass) are both "Composites" . and are made with another element (compound) "Epoxy"...
Example of a Fiberglass Ladies Bicycle, the Frame and Fairing were cast in one piece... which is an oddity, but then , most Monocoques are oddities...
https://farm2.staticflickr.com/1596/...382dd49f_z.jpgFiberglass Ladies Bicycle by AviationMetalSmith, on Flickr
The Designer wanted more FLEX in the Seat area, but since this is a Prototype, and I weigh a lot more than her, I changed the design a little.... The Frame Tube, Or Frame Rail, is a Box Beam, 4 inches wide by 6 inches deep...

Now, I wish to build more Prototypes, with Glass Fiber as the main component, but with the same Epoxy, and a little-itty-bit of Carbon Fiber, which carries six times more weight, on a tensile strength - to - weight basis...

Keep in mind, in Compression, Carbon is NO stronger than Fiberglass... ALL of Carbons advantage is in the TENSILE strength test...

Anyway, this photo shows a "stiff" frame, with a partial Fairing, which is also unique that it's a Cargo Fairing... No one else has built a cross-breed between a Velomobile and a Cargo Bike, this is it, it's unique,
Dubbed the Fiberglass Ladies Bicycle
.
And this picture shows a semi-recumbent, the seat tube is huge to offset the Cantilever- Leverage, a normal seat post would bend if it was at that angle...

https://farm6.staticflickr.com/5683/...0dfb422a_z.jpgFiberglass Ladies Bicycle by AviationMetalSmith, on Flickr

That bike was sold off for twelve hundred dollars in 1999 , and since then , I've been riding a few other Prototypes, but trying to keep it low-key...

I hope a newer generation of Engineering Student can learn from what we done, and build something evolutionarily better ...

You could probably change this Frame Design , so the Frame Rail is 3 inches wide, by 5 inches deep, and made of Carbon, put Carbon Monocoque Panniers on the rear, leave a Cargo Platform in front, but without the Fairing,

add a Coroplast™ Fairing later and enter it in a Velomobile Race... But don't take it too seriously , it is a Cargo Bike/Velomobile, the proverbial "jackass designed by a committee ".

Good Luck.

I prefer flexible

So many variables would make it difficult to objectively evaluate frame stiffness/flexibility:

• Materials
• Joining
• Geometry, along with trail and fork design
• Wheelbase
• Give or play in the bottom bracket, crankset and steering doodads
• Wheels
• Tires
• And, subjectively, rider weight and strength

I still love the look of gently curved steel forks and am convinced straight aluminum or carbon forks must be horrifically uncomfortable because they don't meet my aesthetic standards. So there's that. :rolleyes:

thank you for this. i would actually like to learn a lot more about how frame geometry effects things. i know steel can vary a lot.

i guess what i was touching on more is that generally these days you see a lot of cheap aluminum bikes which are often not very comfortable to ride. whereas with steel its a pretty sure bet you wont have the same problem.

my son has a Specialized hard rock aluminium .... I was aghast when he stood alongside it, held the handlebars with one hand and the seat post with his other hand and pressed his foot against the crank with slight pressure.... the bike bends considerably (sideways).

They all do that.