Cripes. People keep saying this. It is wrong. There is no need to make an aluminum frame stiff to preserve frame life.

... and still it would mean nothing because of people's self-reporting bias, selection bias, need to please the test administrator, variations due to their health, etc.

You keep thinking that if you control for enough data and variables that the people part will naturally fall into place. That's a fallacy.

Our brains only process an approximation of actual events, our memories are faulty and have predictably unpredictable oddities, and what people say is never what they mean. If you require any information that they filter then it's unreliable. People just aren't rational, at the core. That's why marketing is more about emotional appeal, presentation and manipulation than objective information clarity.

Now, if I misunderstood you and you just want to measure frequencies and pop them into a spreadsheet, sure, go for it. But that still won't tell you anything about people.

A) Testing frame materials alone would be impractical to the point of impossible and/or meaningless.
B) You can characterize a huge number of ride characteristics with this type of analysis if you want.
C) It wouldn't be all that expensive (<$10k), especially for a manufacturer.
D) Cars are much, much more difficult to characterize and it's done every day.
E) Car chassis construction plays a huge part in ride characteristics and NVH concerns even with a suspension. Modal analysis of a car chassis is a big part of the design and development of any car.
F) Tires are a huge component. The compound is less important than the construction of the carcass, but they're both players.
G) Rider impressions aren't perfect, but they're a good starting point and when correlated with numerical data can be surprisingly accurate in a qualitative sense.
H) Any manufacturer that isn't doing this isn't serious about building a good bike. Having said that, I don't think it's common practice.

It may be a fallacy, it may not. You can record a person's opinion on ride qualities against the vibration frequencies. If you have a large enough sample size you can statistically determine if there is a correlation between the opinions and frequencies or if there is not.

Your hypothesis is that there is not a correlation. The opposite of your hypothesis may be true.

More like an impossible analysis to do properly.

Your suggestion for an analysis of comfort as a function of material assumes that there is such a function that is based on the inherent material properties. But the evidence is that such an assumption is wrong. The example given earlier of the Vitus 929 vs. Cannondale CAAD frames is instructive in this regard.

The Vitus frames were almost universally reviewed as being very flexible and giving a cushy, highly comfortable ride. Racers liked them for the light weight but some claimed that the flex made them inefficient. Nevertheless, they were raced successfully, so the efficiency can't have been all that bad. [And, BTW, the Vitus frames disprove the idea that Al frames must be stiff to be durable enough for bicycle use; some did have failures but those were almost all at the bonded joints.]

The Cannondale (and Klein) frames got exactly opposite reviews. Praised for being extremely stiff with no wasted energy going into flexing the frame. But some claimed that the lack of compliance made them too harsh riding and uncomfortable. Nevertheless, many were successfully used on long endurance event rides, so apparently the comfort can't have been that bad for everyone.

So two frames, made of the same material, can have diametrically opposite evaluations on the qualities of comfort and stiffness. Good luck on finding that 'comfort function' that's determined by choice of frame material.

...and you know this how?

Almost every Al frame in the marketplace uses thick large diameter tubes to reduce flex in the interest of reducing frame fatigue. Are you suggesting that almost every Al frame builder/tubing manufacturer/OE builder doesn't know what they are doing?

Dr. Cox: I heard "I know I'm being annoying," and then...white noise.

exactly

Quantifying ride quality for whom? For the measuring device (the engineer) or for the discriminating rider who may be a pro rider on a race. In the Tour, a rider may have 3 different bikes. On the Paris Roubaix, the bikes may change out forks for different wheels because of the cobblestones.

In the end, its not about a debate, its about the riders and their race situation. The two engineers who started a bike company in Toronto started a new team and called it Cervelo Test Team. Guess what they were testing and who do you think they were asking to test them?

I'd like to see a list of relevant parameters. It will be a very, very long list.

The OP is attempting objectively quantify ride comfort. (his words, not mine)

To see the fallacy in this approach one need not look any farther than a standard mattress. What one finds comfortable is painful for some one else.

"comfort" is purely subjective. You can not measure it, at least not in manner that will meet everyone's standards. You can reach a median in most instances, but there will always be outliers that are not happy.

And your answer is to just give up and never try?

A lot of driving enthusiasts like how a BMW rides and drives. Those people buy those cars. Same deal with a bike. Make it suit your target buyer better that the other guy, and you'll likely sell them. Will it be all things to all people? Of course not. It was never meant to be.

Because aluminum frames with thin and flexible tubes have been manufactured in their thousands; see above for the example of the classic Vitus aluminum race frames, but there are dozens upon dozens of other examples. You see, part of actually knowing what you're talking about is having a historical and informational context against which stupid generalizations (like yours) can be tested.

As I said before, this is false, incorrect, a foolishly ignorant statement, however you want to put it. Most of the aluminum bikes that riders have experience with are road bicycles, typically race bikes. And most of these are built with large-diameter tubes because riders of these bicycles like a stiff frame. However, there are also aluminum frames out there built with different design goals in mind, and those... frames... flex. Sometimes a lot. Is this getting through, yet? Rigidity is NOT necessary for aluminum frame life.

No, I'm suggesting that you don't have the slightest clue of what you're talking about.

One of the european mags did a test of a new carbon Pinarello (I think) vs. a vintage steel one, using same wheels. Had several pros ride the both, and timed ascent, measured power, etc. The new bikes were marginally faster, but the pros really disliked the flexibility of the steel frame. I'll try to find the article.

I am familiar with Vitus frames. A lot of them broke at the joints, quite likely due to excessive flex.

Simple fact is that aluminum has no fatigue limit which means the material is prone to cracking. Aluminum bicycle tubing manufacturers and aluminum framebuilders know this which is why they try to reduce flex in the frames. If you don't believe me try going to both Easton and Columbus's websites and look at the Al tubing they offer; all of it will be large diameter so flex is reduced in order to preserve frame life. That is not to say that a flexible Al frame is guaranteed to fail, just that there is a higher probability than one with stiffer tubes. The major builders don't take unnecessary risks, thus the stiff tubes.

You can choose to disbelieve these simple engineering facts, and insult me, but that won't change these basic truths.

Just curious. Are you an engineer? You're using some engineering terms, but you're also using layman terms. If you are an engineer, you're a bit mixed up on what's the chicken and what's the egg.

I am an engineer and I'm not mixed up. The chicken came first.

Ok...let me call your bluff. What are the most comfortable tires and at what pressure are they the most comfortable. And don't come back with a "it depends on the bike" answer. According to you, it shouldn't.

In fact, breakage at bonded joints is a common problem for older frames joined in this way, irrespective of material. Old bonded CF bikes have the same problem, but because no one thinks that CF fatigues (though of course it does), they blame the real culprit: the joints. You are grasping at straws.

You appear to know about as much about materials science as the ancient Egyptians. "...no fatigue limit, so the material is prone to cracking." Okay there, smarty-pants. One of these things is not equivalent to the other. Aluminum is used all the time in applications under which it undergoes considerable flexing forces. Ever look at the wings on your airliner when passing through turbulence? Now multiply that by near-daily flights over service lifetimes of often 15-20 years and tell me again that designing aluminum structures to be stiff is the only way to be certain that they will not fail from fatigue.

And please - waving an engineering degree around to shield your ignorance from criticism is crass. Being an engineer doesn't mean that your gut feelings are more likely to be correct than the average joe's.

As for the tubing sets sold by manufacturers and used by builders, there's a much more parsimonious explanation for their tendency toward being large in diameter, thin-walled and stiff. High-end aluminum is in the greatest demand for racing bikes. The frames that most people want for most of these applications are light and stiff. And so the tubesets are built accordingly. But there are other uses for aluminum, and lots of non racebikes have been built with it. And they are built with tubing that is thicker-walled and smaller in diameter. Lo and behold, it flexes AND is plenty strong.

That, by the way, is your answer, though you've been too busy waving your hands and diploma around to see it staring you in the face. The lightest possible aluminum frame does indeed require stiffer tubes in order to have sufficient strength. Though it should be said that even this basic rule isn't as clear-cut as you might think. The 1,000g Spooky Skeletor has smaller tubes and a much better ride than the old CAAD3s, but it is still much lighter. But there are a lot of bikes out there made out of aluminum that are not race bikes, and they need to ride softer. At the same time, weight is not as much of a concern. So these bikes are made with smaller tubes with thicker walls. But the reputation of Al as a material was built on Cannondales CAAD racing bikes.

So what I said was true: most people think that Al is harsh because most people experience it as a road racing frame. Road racing frames are supposed to be stiff and as light as possible, and Al is well-suited to this, so you get a lot of stiff, light frames. That doesn't mean, though, that there is any need to make an Al frame stiff in order to make it safe. It is not at all necessary. Any claims to the contrary are ignorant storytelling.

Yes, an aluminum frame can be made to flex but about 90% of the aluminum frames are built to be stiff to hedge against breakage. Your racing frame reasoning for using stiff tubes is not correct, there are lots of low end aluminum frames, such as for kids bikes, and most have large diameter stiff tubes.

And I'm not waiving my engineering degree around. Fat Boy asked if I was an engineer and I answered.

Knowing that you are an engineer means we can discuss this somewhat differently that if you weren't.

Vitus frames, which I know only in passing, didn't break due to excessive 'flex'. If they broke, they broke due to excessive stress. Stress relates to load and the amount of material that is made to take that load. Strain is a measure of the deflection, flex in your terminology, produced by the load. Stress and strain are not interchangeable, so you muddy the water when use them as you have.

Easton and Columbus offer large diameter tubing to reduce deflection and that is why higher quality frames are less prone to breaking you say? With respect to what? If we are comparing to a steel frame, then the modulus of elasticity of aluminum is roughly 1/3 that of steel. To produce an aluminum frame with the same deflections for a certain load, then the respective moments of inertia need to be increased to compensate. Keep in mind, this does not mean that we have to use 3 times the material as a steel frame. Aluminum race frames are generally a bit lighter than similar quality steel race frames. Depending on how we've chosen our materials, our aluminum frame may very well operate at higher stress levels (if not in absolute terms, then certainly as a percentage of yield) in localized areas than a steel frame even though it's overall deflection (flex) is less. The deflection is related to the stresses, but as a byproduct. Flex or the lack thereof doesn't keep our frame in one piece or cause failures.

And now we get to the oft-revered endurance limit. I don't know what stress levels steel frames are built to, but from what I've seen, steel bikes are built to see stress levels _higher_ than the materials endurance limit. This is a bike, not a bridge. While an endurance limit is a quality that steel has, it's only of importance if a designer keeps stresses low. If a designer chooses tubing that is hefty enough to keep them that low, then the bike turns out too heavy. Heavy bikes, specifically heavy road bikes, don't sell well. So, designers keep the tubing thin, the stresses a bit higher (but below yield) and they get the opportunity to actually sell the bikes they are designing.

Now this is all statics. This is the easy stuff.

The fun part is when we think about how these big tubes act when we start pumping road vibrations through them. Big diameter, thin-walled AL tubes have high natural frequencies. When road input transmits the right vibrations into the frame, those tubes start to dance around and the infamous 'road buzz' rears it's head. Skinny, dense steel tubes have a lower natural frequency. They get excited at lower frequencies that aren't as objectionable to many riders. Steel doesn't 'damp' anything, it just resonates lower and people assume that the higher stuff is damped. No, it's just not transmitted as efficiently.

Figuring out the particularities of these issues sounds like a hell of a fun geek-fest to me. I'd really enjoy doing it. So far I haven't had any takers.

I'll bite. Schwalbe Big Apples at 35 psi.

You forgot to also mention temperature, road surface properties, frame dimensions and angles, lots of wheel parameters, fork parameters, lots of material properties, etc. I'm a ME and do a lot of multivariate, nonlinear analysis and modeling. As a practical matter, I don't think the OP's thesis holds water. A workable approach would be to build (solids, FEA) computer models of bikes+components with some physical testing to calibrate the models, then put people on bikes and poll them, and then correlate the poll results to the properties of the bikes. One would expect clusters based on how people use bikes, e.g., touring vs. racing, but who knows.

From all the quantitative and qualitative customer testing that I've participated in, it isn't so simple. People's perception of sensations isn't very accurate - for example, if they're holding objects of different weight in each hand, they need a 5-7% difference to perceive anything. With the subtleties in ride quality we're talking about, I'd bet the test administrator's behavior would have more bearing on the results of the test than the material.

My position isn't that it's not able to be determined - it's just that it really doesn't matter when so many other factors get conflated in people's minds so much. You could probably make an extremely stiff bike 'feel' softer just by using more rounded tubing.

Sorry about the crack on your degree.

I still don't buy it. So there are low-end kids bikes built with big stiff tubes - so what? There are also Huffys with drop bars. The use of Tubing A in a kid's bike doesn't really imply that it was necessarily built that way to guard against breaking... though of course, in this case, we're probably talking about pretty crummy aluminum alloy. Tube sizes used for aluminum bikes are mostly a matter of style. Increase the wall thickness and you can flex all day long with no problems on an appropriately designed aluminum bike. You still haven't addressed the use of aluminum in structures that are subject to high-force oscillations that are used for decades, nor the fact that the failure of Vitus frames at bonded joints does not suggest that the material used was problematic.

ETA: It is reasonable to suggest that most people don't want to bother with aluminum if the result is a frame that is whippy and not all that light. And that would be a good point. The lightest possible aluminum frame is going to be pretty stiff, for the reasons you suggested. It would be surprising if aluminum bikes made with more flexible, heavier tubing were more widespread. The point I'm making is that the use of mostly stiff, large-diameter Al tubesets does not imply that this kind of tubing is necessary for a fatigue-resistant bike. They're not. There's just not any demand to speak of for flexy aluminum bikes.

I've forgotten to add, by the way, that the question of aluminum ride quality has gotten a bit lost in this discussion. I think that the "harsh ride" of aluminum is way overstated. My aluminum race bike doesn't ride any harsher than the average racing bike I've ridden, built of any material, big tubes and all.