This is kind of what I'm thinking...

Could it all be marketing hype? Could the modern compact frame be stiffer than the traditinal steel frame because it uses huge oversize, shaped tubes, an oversize bottom bracket, etc and not because of the compact design itself?

And please, let me restate, I'm NOT trying to argue one is better than the other. It would not matter to me anyway. I ride a traditional frame for essentially the same reason women wear high heel shoes - it looks better (IMO anyway, horizontal top tube and women in high heels ) and not because I think it is better, whether it is or not.

Simple: because the Marketing Department says they are.

But a longer unbraced tube (i.e., the longer seatpost required by a compact frame) is more flexible than a shorter tube.

presuming other variables are the same (tubing material, thickness and diameter, etc), this is true...

Basically shorter tubes are stiffer... (as dit mentioned)

How is this not for lower standover height on uneven terrain?

This is so filled with bad information I'm not even going to try and correct what it says. In fact, I can't even tell what the third point below even says.

"most modern bikes are made with oval aluminum tubes"

"aluminum isnt as flexy as steel is"

"where as steel flexes quite easily and can endure the stresses of bicycle riding. so the tubes are circular, which allows it to flex more than oval aluminum tubes."

I like your argument best so far caterham. Not sure I agree with everything but it seems very well thought out.

The marketing department of most every major bicycle manufacturer and damn near everyone over on the road forum

Now there we disagree - maybe the clamp on top of it is an external part but the post itself is there to allow you to fine tune the length of the seat tube which is part of the frame.

But think about what you are implying there - hypothetically if we could build a frame of some new design with some unobtanium new material such that it had absolutely zero flex but the seat post still flexed a great deal, you would ride that bike and still think it was flexy. You cannot separate the seat post from the frame outside of a laboratory.

Most folks are looking for a frame that's stiff in the bottom bracket. Flex in the seatpost doesn't adversely affect ride quality for most riders. Ridiculously long seatposts are heavier, but a couple inches longer than required for a horizontal toptube frame ain't no big deal.

I repeat, I don't care how much my seatpost flexes as long as it don't break!

I doubt you could really get a seatpost long enough to get it to flex that much, since they are made of such thick-walled aluminum. My 15" compact geo mtb frame has about 3" more seatpost showing than a 17" mtb, it actually feels stiffer in the pedals.

The flex that is undesirable in bike frames is generally not related to flex in the seatpost.

I take it you're not in the market for a Trimble.

Let's put it this way. If you're putting extreme lateral forces on your seatpost while just riding down the road, then:

A. You're not pedaling properly.
B. You need a different saddle (narrower nose, perhaps)
C. You'd likely start a fire if you wore corduroy.
D. All of the above.

According to that link. Aluminum has 1/3 modulus of elasticity of carbon steel, therefore aluminum actually flex more than steel not less.

But when we talk about bicycles, we are only talking about inches or fractions of inches - so is the difference in stiffness measurable? and if it is measurable, what is the bottom line to the rider? Am I faster? If so by how much? What are the measurable benefits to a compact frame?

work = force x distance.

if we define work as the flexing of the frame, it takes a certain force applied at a certain distance to generate the work. consider distance to be the length of tubing on a bike frame. the longer the tubing, the more flex is introduced to the tubing by a given force.

if one considers the increase in resistance to deflection or torsion for a singular tube in isolation, then the net effect would be insignificant... however almost the entire frame structure is affected in a compact/sloping frame design. the incremental increases in resistance to deflection of each shortened tube becomes accumulative and compounded when assembled as a structure. not only are the toptube and seatube shorter & stiffer but the seatstays as well. virtually every attachment/bracing angle becomes more acute and the structure as a whole becomes squater,and more efficiently triangulated .

If you look at the bike in total, maximum stiffness and strength between any two loads with minimum mass is reached with a deeper than shallower triangle shape.

AFAIK if you put a load in the middle between two fulcrums, the optimum structure to support it is an equilateral triangle. With similar beam thickness, a shallower triangle with a pole sticking up or string hanging down would be weaker, because of Pythagoras' theorem. And a deeper frame would be stronger but it would also be heavier.

Think of it this way, if you have two attached points on a wall on top of each other and have to support a load one meter from the wall, you can do with a lot less strong tubes if the points are separated more vertically than less... The top tube will be in tension while the bottom one will be in compression. In fact the strength goes to infinity as the points get closer to each other.

I could draw a free body diagram.

Since the front wheel has to turn you have to connect it at the top, and there is also a load both at the seat and the bottom bracket. And the rear hub. So you connect these all together with as close to equilateral triangles and take into account the shape limitations as you can and you get the best weight for strength.

Also, small frames look really ugly with their long seat posts, girl style angled top tube construction. They are not waiting to shoot ahead like road bikes should look, more like bogged down. They are not elegant, they look lazy. Like a tractor vs a sports car. I never understood the fashion to make nonhorizontal top tubes.

Now, if you want to make a large bike with small wheels, (long head tube) I have an idea how to do it stiff too - the classical arrangement moves from triangle to a parallelogram and that's bad!

Springs are made of special heat treated steel alloys due to modulus of elasticity and fatigue life. With the right alloy a aluminum spring could be made but it's life would be very short and allowed deflection pretty minimal. Aluminum has about 1/3 the bending stiffness and strength of steel the same diameter. Take a 1/2" aluminum bar and a steel one and the aluminum will bend much easier. Read the frame materials discussion it "Bicycling Science" third edition, published by MIT Press. It is available on Amazon.

Stiff aluminum frames are due to very oversized tubing being used as compared to normal steel tubing used in frames.

In addition to stiffness racers like compact frames due to their lowering the frame center of mass enough so that when out of the saddle the bike feels lighter and more responsive.

Mavic tried some wooden protos back in the eighties. Serious stuff, latest laminate tech, glues, modern hubs with 7 speed units... It didn't work, the things delaminated. Probably loss of knowledge about wood, or they called in some overeducated whizzkid engineer to design the things with a computer... Anyway, to get back to alloy, having found quite a few old bikes, I can attest to the fact that aluminium ages really badly: I've pulled spokes out of two different back wheels, ruining beautiful wheelsets, and I don't develop much wattage.

Totally agree with whoever said that the scariest thing after CF is aluminium. Aluminium ages, CF has a catastrophic failure mode even when spanking brand new. And it fails often. But what can you expect from a compromise construction of stiff fibers with random alignment in a medium of glue, 'coz that's all it is. Remeber, a chain is only as strong as it's weakest link.

Hmm, on closer inspection it seems on first approximation that the length minimizing of members in compression is of great importance to frame weight minimization. Stuff buckles in compression at much lower loads than it breaks in tension, and length exacerbates this. ie buckling strength is proportional to 1/Lē.
That would mean focusing on the rear triangle's seat stay and the main triangle's top tube. (Probably the seat tube minorly.)

This would result in a straight angle between both the seat tube and the seat stay, and the seat tube and the top tube. But then you'd get problems in the very shallow rear triangle again.

That actually would be low bike. Much depends on the angle of the seat tube - a lower frame for more angled seat tubes. A straight top tube for a vertical seat tube, the rear triangle would be a compromise.

I gotta check back on this later somewhen, probably have to write some scripts and iterate it...

Oversized tubing is the key to stiffness. The old Vitus frames = flexy. Cannondale's are known to be stiff.....they have oversized tubing. Tubing sitffness increases in multiples as the the diameter increases. Diameter has more affect than wall thickness.

I doubt that this is true. I have never seen a new frame set sold with a seat post. If the seat post was flexy it might contribute to a "softer ride" but it would not cause the chainwheels to rub on the fd or cause the chain to jump cogs. Flexy frames generally show up when hammering out of the saddle.

Too many variables to generalize to a definitive statement. Those of you who are mechanical engieers who have taken classes in structural engineering and mechanics of materals courses can identify the ignorant statements and those with scientific validity. The rest of you will just have to judge on your own.

Of course there is always the element of, what will sell. That does not necessarily jeapardize the safety liabilty constriants on design. If you like compact frames, manufactures and others will give you enough information to support your decision to buy that frame and vice versa.

If you are not an engineer with experience and the tool set to understand and you want to understand the difference, either get a degree or talk to an experienced engineer. The forum is not a vehicle that will adequatly answer this question.

No offense, but to paraphrase a similar, popular saying...

This post is worthless without pictures.

I'd agree with "stiffer" (on topic), but perhaps not "stronger." Strong is the ability to withstand dynamic forces without permanent damage. To use seat tube/seatpost as an example, a frame with a shorter seat tube and longer seat post may be stiffer, but in terms of ultimate stress, I think that a frame with longer (and more flexible) seat tube may be able to take more than a less elastic combination like short seat tube + long seat post.

In thinking about failed frames that I see on this forum, it seems to me that more of them are of the medium/shortish ilk than the large frame sizes. Now, since larger and heavier people generally ride larger frames, you'd think that most of the damaged/broken frames we see would be of that type, if they were inherently weaker. Doesn't seem to me that it's the case. I think that it's heavy bike-punishers riding those "stiffer" short frames that kills them.

The longer the post, the more likely it is to break. And remember, aluminum has no fatigue limit, so when it fails, it does so without warning.

JohnD, typical compact geo exposes maybe 2" more seatpost. I don't fear SP breakage on my 15" mtb frame anymore than on my 17" mtb frame. both have the same saddle heights.

Sure glad these aluminum bars bent before failing without warning...