Well, if you got some 1020 tubing made with the same wall thicknesses as an SLX set, that frame would not serve you for as long as the SLX frame.

I'd be afraid to ride it with any vigor.

And that's precisely why they spend so much time developing new alloys of steel and aluminum and many other metals. So you can have strong tubing that doesn't weigh as much as the old tubing.

And if you arrange them in smaller triangles, the resulting frame will be stronger than one made with larger triangles.

Keep in mind that "stronger" is a relative term. Any bike you ride is going to be strong enough to not fall apart on you while you ride it unless it has been in a crash. Ride what you like.

An armored truck will be stronger than my Honda but I'm not driving an armored truck to work everyday. Just ain't gonna happen.



But the question "Why are compact frames stiffer?" was answered long ago with the simple "smaller triangles are stronger than larger triangles".

Quite true! I was thinking of going the other direction.

No, the stiffness of any steel tube really only depends on its diameter. But a high-tensile steel tube will not be as strong as a chrome-moly or manganese-moly tube of the same diameter, so it is likely to have thicker walls to compensate, and therefore will be heavier.

You do realize that this statement is inherently incorrect, don't you?

If the question is about stiffness, you cannot answer it by discussing strength. You must respond in the context of stiffness - rigidity, resistance to deflection, high natural frequency. Strength really has nothing to do with it (stiffness).

Any bike that you can buy has already been designed by someone out there who had to do the design studies, make the trade-offs, elect the materials, build a protoytpe and test it, refine the design, ... Then they had to make further compromises as they moved into mass production. They've already done the stiffness and strength work for you.

Have you ever watched an aluminum wing on a large jet. If they don't flex it must be an optical illusion. Don't you think it is more about design than material?

Precisely so.

However, aluminum is notch sensitive. There's no doubt at that. Still, even notch sensitivity can be dealt with during the design process.

Okay, Mike, we'll do it your way: small triangles are stiffer than larger triangles.

Close, but not quite. For a given torque (axial) a tube of a certain length will twist a certain amount. If you put the same torque on a longer tube, the twist will be greater. This assumes the twist is measured by how much the two ends are moved out of line with each other.

It's not really work, though you could say energy is stored in the tube when it's twisted, if it's metal. If it's carbon, a significant amount is dissipated as heat. Still tends to resist twisting.

So in a compact, the top tube is part of the system that resists twist due to pedaling forces and due to hand forces as the rider pulls on the handlebar. Those two couples impart a twist that tends to pull the head and seat tubes out of plane, by twisting the downtube and the top tube. Downtube and seat tube stiffness are important. The top tube is shortened in a compact because it approches perpendicular interface with the seat tube. That would represent the shortest top tube. As an additional stiffness benefit, the seat stays get shorter. A compact will be stiffer than a conventional frame, even if they share the same materials and construction, assuming they have stiff seatposts.

Excuse me? You can exercise your Constitutionally-given right to blabber as if you know some science all you want, and join the masses of Americans who know NO science but speak as if they do, but there's no reason to take gratuitous shots at people who ACTUALLY know something about science, and without whom you'd be shivering in the fricking dark, having trouble surviving the winter!

Unless by "overeducated" you mean "unseasoned" or "inexperienced."

No. The inherent stiffness (strain to stress ratio) of high tensile steels is the same as that of CrMo and MnMo. A set of frames brazed up out of those three materials with the available variations in wall thickness, will show load, shock, and torsionally-driven flexing commensurate with the differences in tube wall thickness and outer diameter. This assumes the joints are of equal rigidity in all frames.

What's different among the steels is strength. Better allows, such as Reynolds 853 or Tange Infinity, are stronger, not stiffer. This allows making tubes with thinner walls. Thinner walls make tubes more flexible, so to restore suitable frame stability (not TOO much flexing), the outer diameter is increased.

Interesting discussion thus far. One thing I started thinking about as I was making some adjustments to my wife's mixte is why the designers decided to run the twin diagonal tubes back to the rear dropout rather than attaching them to the seat tube or just using a single sloping downtube. According to what many are saying here, a frame should be stiffer by positioning the top tube lower on the seat tube. But in the mixte design (at least the Peugeot that I'm looking at) the twin diagonals go from the head tube to the rear drop outs and aren't connected in any way to the seat tube. So those designers apparently felt that it is not necessary to brace the seat tube at all as it's only attached at the bottom bracket and the thin seat stays. Now granted mixte frames aren't likely to be put through the same level of stress as a racing frame but still, it does make one wonder about the argument that the top tube is there to somehow reduce flexing of the seat tube. Apparently its primary function is to support the loads on the head tube.

You mistake my perspective on this. I can make a large triangle that is stiffer than your small triangle. Stiffness doesn't necessarily have anything to do with size. In one way, larger is stiffer.

Who says stiffness is the be all and end all of bicycle design, anyway?

Kommi, go back to the mixte thread started by Veloria earlier this summer, we went through all of that. My reference for believing the top tube is involved is basically my knowledge and physical insight as an engineer, but it's shared by a physicist/frame builder/author, Tony Oliver, and explained very well in his book, "Touring Bicycles; a practical guide." I provided a complete reference to the book in that thread. Sorry to pass you around, but I have made this case a number of times on BF.

The fact that Peugeot and others did not choose the solid top tube design does not mean it is not beneficial, or better with respect to torsional stiffness than the (let's call it) floating twin lateral design. I still don't fully understand the purpose of the conventional mixte frame design.

Maybe it doesn't have a strong enough effect for the expected mixte riders. Maybe ... it is just style, a nod to the space frame concept - aircraft, Bucky Fuller, the Birdcage Maserati, Raymond Leowy, I don't know. But engineering-wise a large (say 25 mm or more) top tube in line with the rear axle and the top of the head tube or all the way up to hirizontal will make a frame that better resists twisting around its longitudinal axis, as might be created by pedaling and handlebar reaction forces.

We also need to consider what kind of stiffness we are talking about. longitudinal torsion? Vertical deflection due to road shock and rider load? or one of the many other ways a frame or its elements could twist or flex?

It is understood that the triangles would be made of the same material, differing only in the lengths of their sides. This also means the angles of the triangles would be the same.


"Why are compact frames stiffer?" - By having smaller triangles, all other variables being equal, the frame is stiffer.

"Who says stiffness is the be all and end all of bicycle design, anyway?" - Professional racers and those who aspire to be like a pro racer. People who can appreciate a bike with a quick, responsive feel. That's not the majority of bicycle riders but it is why a great deal of research time and money is spent on designing new frames and frame materials.

So the ultimate in stiffness would be to get rid of the seat tube and top tube completely, right? An infinitely small triangle should be infinitely stiff.

I could be wrong, but I think the stiffness a racer is after is out of plane bending stiffness "below the lines". One "line" being the line between the bottom of the head tube and the rear axle. The other "line" is from the head tube to the front axle. These are the the portions of the frame which would flex under load and tend to sap energy.

It is also important to have some in plane compliance, to soften the ride.

You'd also want a low hysteresis material so it will spring back after flexing. I would also bet hysteresis can be adversely affected by poor workmanship.

If the seat tube flexes a little, I'm not sure it matters in terms of energy transmission.

I don't think so. I think it is still limited by the modulus of the material (modulus of elasticity).

Let me rebutt a few of these statements, just for clarity. I believe there is a lot of incorrect concepts and incorrect statements in this thread, so I think a little clarity of thought is in order.

I don't think it is a good a-priori assumption. Large frame versus small frames, new designs versus older designs, etc. There are too many variables to just assume everyone understands this as a precondition. It is not unheard of for larger frames to be made of different tubing than a smaller frame of the same make/maker. Now, if you explicitly state this as a basis for your argument, then I have to agree.

There are many racers who intentionally introduce flex into their frames in order to improve their performance. Part of that "responsive feel" you mention is low hysteresis spring back.

I seem to recall the Paris-Roubaix (?) racers all wanted a more flexible frame because a significant portion of the race was over cobble stones and other poor road surfaces. A super-stiff frame was hard to control and beat the racer to death. Mountain bikers also introduce LOTS of compliance into their frames (front and rear suspension) for the same reason.

I think we all know we want a little compliance in between that front wheel and our hands, too. The fork stiffness, handlebar padding and gel inserts in our gloves is how we get it.

I'm not sure the OP was restricting the question to just racers, either. What about a touring bicycle?

The long tubes welded to the seattube might result in an overly stiff rear triangle. Might result in a rough ride, possibly a dead-feeling frame. It's possible that after experimentation, that tying the head tube and the rear dropouts together was the best configuration.

on a mixte, the twin longitudinal diagonals attach at the rear dropouts,where the bike has its widest structurally stable girth to triangulate the headtube/steerer from lateral & torsional deflection, making the bike safer & more precise in handling and stronger & more reliable than by simply attaching at the seat tube. the mixte design's intended user was expected to value comfort & convenience over performance & efficiency .using a semi-rigid attachment of the diagonals at the seattube or in the extreme case of the peugeot,bypassing attachment altogether, allows a greater degree of vertical compliance for absorbing road irregularities by utilising the full length of the tubes as spring members functioning under compression.

bingo!

one must always consider handling & road adhesion factors. the driveline & steering must be held locationally stable to provide precision,consistancy & efficiency but the very same tube structures must also provide suspension travel & shock absorbtion for comfort & road contact. that was my point of contention back in my first response to this thread-
"to me, the real question is whether or not all the added stiffness in a compact design is necessarily consistantly beneficial and without trade-offs . in my experiences, the thorough, well-considered, goal-oriented implementation of any given design philosophy is the real key to any superior and satisfying result."

I almost forgot to mention the compliance beneath the butt bones. A little compliance there is a good thing.

Are you saying the softer the cushion, the better the pushing?

Very good discussion thus far. I'll throw another question out there just for grins - if the compact frame is so much better for the stiffness required of a racing bike and given that frames resembling the compact design have existed since the late 19th century, why did the horizontal top tube design become the standard for the first 1 1/4 century of the modern bicycle? Especially since there were no oversized carbon tubes and oversized bottom brackets with outboard bearings and all that...given the limitation of their materials you'd think they would have wanted the stiffest frame design they could get.

It seems you're thinking of road bikes, right? If so, then:
I'd guess MTB trickle down effect.
Not everyone wants the stiffest frame they can get and frames should generally be stiff in some ways/areas, yet compliant in other ways/areas.

A very good discussion. Stiffness does not equate to "better" necessarily. I think we like a conventional steel frame because it is not brick stiff. The same reason that I don't personally like cannondales...they are brick stiff. But there are times that I do like a klein, especially in steep downhill corners. Wiggly top tubes are perhaps not dangerous, but they are scary.

Why the compact frame? Not because of increased stiffness. Not because of weight. Because the manufacturer can fit 95% of the world with three or four sizes, saving a boatload of capital on their production costs. A Cinelli SC or a DeRosa used to come in 1/2 cm increments...making just-in-time and inventory management almost impossible for retailers.

They are ugly, IMHO. I have four that I ride regularly, however. Once you are on 'em you don't know how ugly they are, and they ride just about like a bike.

I'm supposed to be setting up the trainers (snowed today), and I'm here drinking scotch and blathering.

Good night!

imo, compact design wasn't totally viable and largely unexplored until after "oversize" and shaped tubing became readily available. with "standard" tube diameters, lateral stability in locating the headtube/steerer was too compromised. oversized top & downtubes created a much more stable steering geometry which could be exploited without serious compromise. it's my personal belief that there's an additional factor to consider regarding longitudinal stiffness and handling/cornering qualities- ie: that any sideways frame deflection exhibited should ideally be even & progressive and occuring along the entire frame's length, from steerer/headtube to the rear dropouts or else a "hinging" behavior will occur where one or more segments of the frame deflect at a different rate or load than the rest of the structure, leading to unbalanced vehicle dynamics, loss of adhesion and unpredictable cornering behaviour.

This is getting old. Most "compact" frames come in the same sizes as non-compact. The only frames that come in XS, S, M, L, and XL are mostly low priced ones. That's the way low priced frames have always been.

1/2 cm increments are ridiculous. If you need something that precise, buy custom. you might get a 1/2cm in seat tube length that hleps, but is the top tube what you wa nt? Or the seat tube angle? It's crazy.

i have a question/request.

why is it that after 4 pages of considered responses, rabid rants, secondhand folklore & speculations that not one person, including the OP, has asked of the owners of compact bikes what they feel are the benefits (if any); their experiences & potential downsides to the design?
i have my own thoughts but given the huge scope of the topic haven't yet had a manageable framework to reply to. i'd like to hear others opinions on the merits,issues & reactions to compact/sloping bikes based on their personal observations gathered from actual usage.

k