R853 vs. R851? Not much difference, IIRC, but I'm not sure I've ever seen a bike of R851. Now if you're talking 531 vs. 853, then you've got a decent jump in strength to weight ratio and stiffness with the 800-series.

7000-series Al is slightly stiffer and has a slightly higher strength to weight ratio over 6000-series.

Aluminum is generally stiffer than steel and transmits more "road" to your hands and butt, so carbon forks and seat-posts are way nice to have with aluminum frames. An R853 bike is very nearly as stiff as a 6000-series frame, though, so at that point, it's kind of a wash. Steel frames are usually a tad heavier than aluminum frames, if you look at frames in the same general price range.

Both materials follow the same general quality curve. So, if you buy a $150 aluminum frame, you're getting a $150 aluminum frame and a $1500 steel frame is an odds on favorite to be a better frame. Although, in many cases, high quality steel frames will cost more than a high quality aluminum frame, since most of the former are hand-made in Europe (although those Italians and Dutchmen have branched out into aluminum now, as well).

[reply]R853 vs. R851? Not much difference, IIRC, but I'm not sure I've ever seen a bike of R851. Now if you're talking 531 vs. 853, then you've got a decent jump in strength to weight ratio and stiffness with the 800-series.

7000-series Al is slightly stiffer and has a slightly higher strength to weight ratio over 6000-series.
[/reply]

With all due respect, this is simply not true. There is no difference in stiffness between different types of steel. The stiffness is constant for all steels regardless of the exact composition. The same goes for Aluminum, regardless of the exact alloy, the stiffness is identical. It is molecularly impossible for there to be a stiffness difference. Material science 101.

As for strength, since all frames eventually fail at the weld, the material strength is not nearly as important as the weld strength. And when it comes to that, weld design and execution is the driving factor, not the material.

Case in point, there are plenty of frames in Ti (6-4 ^ 3-2.5), Alu (6000 & 7000 series), carbon and steel that fail fatigue tests, and there are also frames in all four materials that pass them. And when you examine where they fail (as for example EFBe does independently from manufacturers), they always fail either due to a weld imperfection or a braze-on, like cable stops or a derailleur hanger.

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Gerard Vroomen
3T.bike
OPEN cycle

Maybe we have different definitions of "stiffness" in mind, but when you alloy a metal you affect the molecular crystalline matrix of the solid which drastically affects mechanical properties. For instance, since you bring Ti into the picture, CPTi is much more elastic than Ti3Al2.4V, which is a more elastic material than Ti6Al4V. They also become harder from an impact-resistance and maleability standpoint through the same order. Now, the differences between CPTi and alloyed Ti are more drastic than those between the two alloys (hence you don't see much CPTi outside of the occasional medical implant...it's still bio-inert), but the difference is still there. The Poisson's ratios of various metals may be the same to several decimal points for each base metal (steel to steel, Al to Al, Ti to Ti), but the elastic moduli are very different and I would have to believe that that affects ride quality from material to material. You may never notice it unless you build two identical frames of different materials, which, as a consumer, I doubt I'll ever see, so it probably becomes a moot point.
I absolutely believe you that tube shape, construction technique, and finished structure play a huge role in how the bike feels and how much stress and fatigue it can handle. And I whole-heartedly believe that structural connection points, be they welds, bonded joints, threaded joints, or compression fits, are the weakest points of a frame.

[reply]elastic moduli are very different[/reply]

No, they're not. When you look at he E-moduli of various Alloys, they are virtually the same, since it is a function of the molecular distance which barely changes. It changes a bit, resulting in very small differences in E-moduli, but that is hardly worth mentioning (6061 is 10.0 Msi, I believe 7005 is 1-2% higher). And for example annealed 6061 has the same stiffness Modulus as hardened 6061, the only difference is that the yield strength is much lower in the annealed state. I'll have to look it up for Ti.

So there may be differences in the plastic phase, but there are hardly any inthe elastic phase, and that is the phase that applies to a bike frame (except for the alignment proces and for when the frame breaks).

That may be where the confusion comes from, since you also comment on hardness which is a typical plastic deformation, not elastic.

In the elastic range (where the Elastic modulus applies), the ratio between load and deformation is the same for all alloys (give or take that 1-2%). But there is a difference in the yield strength, i.e. where the elastic deformation changes to plastic deformation. There the "stiffness" will drop off from the E-modulus, but again, a bike frame never gets to that range in normal use.

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Gerard Vroomen
3T.bike
OPEN cycle

Any material can be made to ride stiff or flexible. One of the most flexible frames ever made was Vitus' attempt at early aluminum, 5000 series I believe, they were whippy and unstable, by the same token, before advanced composites designs, STEEL frames were the stiffest, often approaching 8 pounds for the framesets, the top track riders in the world would ride steel when the "soft" aluminum bikes just wouldn't do. They still use steel handlebars. To be honest, if you buy frames that are advertised to be stiff, and they are fairly expensive +$1000, they are likely so. I've owned a terrible steel frame, a Bianchi EL/OS and a great one, a Croll Custom 853. Same with Aluminum my Coppi Galaxy was super harsh and heavy, but my Felt F1R is smooth and compliant. My only ride on Carbon was a Colnago C40 and I have nothing bad to say about that except the price is 4X what I can afford.

Shop around, mentoin specific models here and I'm sure you'll get (often conflicting) feedback.

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https://www.kickstarter.com/...bike-for-the-new-era

mjpwoo. what SORT of differences are you inquiring about? what do you like in a frame, ride-wise ? do you need a frame that will shoot out from underneath you as you pedal? one that will give the smoothest ride? (how do you define smooth? ) what can you live with, price, longevity, and weight-wise ?? i am of the " there are differences and they are groupable by material school, myself. you can tell the difference between a bike from each camp with nothing more than a ping of your fingernail on the top tube - i do not for the world see how pwoplwe claim you can't then feel it on the road. but that is neither here nor there. what are you loooking for exactly, and where do you come down on the many tradeoffs seen in the engineering of a great bike in any material ?