Originally Posted by
Kommisar89
On the other hand, I suspect (though obviously I have to hard proof) that the majority of flex comes from the areas around the head tube and bottom bracket. Chain stays are very narrow and are often ovalized such that they would better resist flex in the vertical plain. Seat stays are just as narrow if not more so and are so close together at the top that I doubt they offer any significant resistance to lateral forces whether the frame is traditional or compact. And while an oversized bottom bracket and stiffer (whether from thicker tubing, larger diameter, whatever) seat tube and down tube will offer increased stiffness, I don't see how angling the top tube down such that it intersects the seat tube at a lower point would increase stiffness. If anyone read Jan Heine's recent tests comparing frame stiffness (all traditional horizontal top tube frames) he theorized that a stiffer seat tube was more likely to cause rubbing under heavy loads because the bottom bracket shell itself deflects. I've had similar experiences with my bikes - my old Bottecchia Special with thick hi-tensile steel tubing would rub like hell and I'm sure I could actually see the flexing when sprinting out of the saddle. My Gran Turismo never rubs and I don't notice any flex even though it's made of thinner Columbus SL tubing.
Honestly though, if I were a young person buying a bike today and had no prejudices either way, I think the compact frame offers sufficient advantages to make it a viable choice, maybe even a better choice regardless of whether it's actually stiffer as a direct result of the design. As it stands however, I'm an old coot who thinks traditional horizontal top tubes just look so much better (along with lugs and wheels with lots of spokes, and polished aluminum doo-dads and chrome, lots of chrome) that I would choose a traditional frame anyway. But I do like to keep up with the latest technolgy and understand why it may (or may not) be better, hence this post.
BTW - Jan's tests also indicated that a stiffer frame was not necessarily better at power transfer but you'll have to read his article and make up your own mind.
When talking about the advantages of compact geometry versus vintage classic geometry bikes its easy to say that modern aluminum frames with compact geometry are obviously 'stiffer' and thus the design (the compact geometry) is the reason.
This clearly isn't the case. Vintage lightweight steel bikes with classic geometry, when ridden today, can not be reasonably be compared to modern aluminum bikes, with shaped and ovalized down tubes, oversized tubing, larger head tubes, and superior engineering. The truth is, for all the ranting from the 'steel is real' crowd, a vintage steel frame just can't begin to compare to a modern frame. Its completely inefficient in terms of transferring wattage to the ground, the amount of bottom bracket flex and chainstay flex is unreal.
However, you don't have to compare vintage steel lightweights to learn that. When Cannondale and Klein changed the paradigm with oversized aluminum bikes it completely rewrote the standards for frame weight, stiffness, strength, and efficiency. Bicycling Magazine used to have a 'tarantula' test jig for measuring frame stiffness. The Cannondale 3.0 frame was the stiffest frame ever measured.
People enjoy riding steel bikes. People certainly enjoy riding vintage steel bikes with period correct kit and components. Another great way to enjoy cycling. However, no one rides steel bikes because of the 'performance' they offer. Quite simply steel bikes were cheap to produce, required almost no specialized labor to craft (unlike Aluminum and Carbon fabrication), and resulted in a very compromised ride.
Comparing a vintage steel classic geometry bike to a vintage aluminum classic geometry bike (Klein or Cannondale) will reveal the inherent disadvantegeous of the steel bikes, and reveal why any discussion of compact geometry versus classic geometry absolutely should not involve any consideration of steel bikes, vintage or otherwise.
However, all that being said, compact geometry bikes are stiffer than classic geometry bikes. All other things being equal (shaped and ovalized downtubes, identical frame tubing, identical chain stays, etc.) a compact geometry bike is going to be lighter, stiffer, and stronger than its larger classic counterpart.
Its simple engineering really.
Years ago GT came up with their triple triangle concept. It was mostly marketing, but not entirely.
The larger the frame size the more significant the benefits of compact geometry (as compared with classic geometry). Very small frame sizes have such small triangles to begin with that the benefits are minimal in comparison with larger frames.