Sounds backwards -- why would a sloped TT be longer than a horizontal TT on two bikes of the same size and geometry?

I'm sure you're correct.

I was thinking of a sloped top tube that goes very far down, near the bottom bracket (a ladies frame).

A Rivendell isn't a compact geometry bike at all. A compact geometry bike has the head tube in the traditional position and the top tube slopes downward creating smaller frame triangles. A Rivendell is sized at the absolute largest frame one can possibly stand over, including with some 'discomfort'. The top tube does not run down from the head tube, so much as the top tube runs up from the seat collar to an elevated head tube. Much like what a bike with an add on Serotta Heads-Up looks like. However, almost all Rivendell bikes go out the door with beautiful lugged quill stems, or Nitto Technomic quill stems. The founder of Rivendell definitely is NOT a fan of threadless.

Zinn definitely incorporates compact geometry principles into his Project Big bikes. He also uses elevated head tubes. Zinn stocks carbon forks with 450mm steerer tubes. However, the reason Zinn utilizes compact geometry has nothing to do with the extended head tubes. The compact geometry allows the fabrication of very large frames without having to use as much frame material, granting all the benefits of a smaller compact geometry frame, only with increasing returns of stiffness and lighter weight. The elevated head tube has nothing to do with the use of compact geometry. You can use an elevated head tube on a classic geometry bike (Riv does, Zinn did, and does).

You can extend the head tube of either a compact geometry bike or a classic geometry bike with a head tube extension (either a Serotta Heads-UP or a similar product).

At the end of the day an extended head tube (or not) has nothing to do with compact geometry.

Ever heard of the Pythagorean Theorem?

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.

This is kind of funny.

With tandems some 'racing' tandems get rid of the additional tubes (lateral tubes) to save weight. Removing lateral tubes on a tandem saves about 400g on the bike. The average racing tandem weighs between 12000 and 18000 grams. The really funny thing is that the couples and tandem teams that own these 'open' design tandem frames swear (from their seat o' the pants meter) that the bikes are just as stiff (if not stiffer) than equivalent tandems with lateral tubes. Apparently when you spend between $6k and $12k on a tandem, its not easy to acknowledge that you bought something that doesn't make sense on paper. However, there isn't really a competitive advantage in having a stiffer more efficient tandem in racing, because the tandem racing cult all seem to buy 'open' design bikes.

Bill McCready of Santana Bicycles has spent serious time and money researching frame designs, and testing comparable designs (overbuilt frames sans lateral tube, standard tandem design, etc.) and determined using benchmark testing and real metrics that the loss of the tubes results in removing "a tandem's lateral tubes to make it lighter, because the resulting bike will have a lower level of pedaling efficiency, it will also be slower. While lighter or weaker teams might not be able to detect the loss of performance---repeated scientific testing shows that no competitive tandem team is so light or weak that they should prefer a frame without laterals." (Source)

The irony being that no one has done more tandem frame testing than Santana, and they still produce frames that are significantly less efficient and noticeably flexier than other tandem makers.

Triangle size is very important on tandems, where the frames are subject to twice the drivetrain forces of a single bike. You can actually see a steel tandem torque itself laterally from the wattage the team produces.

However, there are unusual bikes that have open designs, that are effective. Bikes like Co-Motion's Periscope, and Bike Friday's Twosday. McCready acknowledges why these 'open' designs work: "Other tandems lacking laterals...are frames of exceedingly small stature. In this group I include Bike Friday's "Two'sday" and Co- Motion's Periscope. What these smaller "open" frames have in common is that there isn't much room between their top and bottom tubes." (Source)

So what is all this talk about tandems in a thread about compact geometry? Its relevant empirical proof that compact geometry makes sense, and allows builders to make lighter, stiffer, and stronger bikes with compact geometry than they could with otherwise classic geometry.

Tandems are the ultimate torture test of bicycle frame design. Every production tandem made utilizes 'compact' geometry, or some variant geometry to a great extent. Tandem frames utilize significantly longer seatposts, and typically higher stems than do normal single bikes.

There is a reason for this. Smaller triangles.

The frames of UCI pro roadies are some of the stiffest bicycle frames made. They are about pure efficiency, getting the produced wattage onto the road, with minimal compromises. Racers absolutely do not want to introduce 'flex' into their frames.

What they do want is bikes that don't compromise efficiency for the sake of comfort.

A steel bike, aside from the nonsense you'll hear from the 'steel is real' cult, is just generally inefficient, and just generally flexy. The benefits of comfort come at the cost of pedaling efficiency and stiffness. If you don't believe this to be true go test ride a Santana steel tandem some day then go hop on a Cannondale tandem and the difference will be beyond evident.

A modern race bike can utilize a carbon front fork, a carbon seatpost, a carbon stem, and carbon handlebars to contribute to racer comfort. The frame itself is all about performance, not comfort.

However, carbon frames can be laid so that there is vertical compliance without any compromise of lateral compliance or drivetrain efficiency. Essentially building into the bike 'micro' suspension for the cyclist isolated from the drivetrain. This is possible, to a lesser degree, with titanium frames as well (ti can be made stiffer than carbon or aluminum, or made merely to be 'better' steel).

As for Mountain bike suspension frames. These are the stiffest and strongest frames of any bicycles. The frames are designed oversized and overbuilt, and are specifically engineered to not flex, but rather, to let the suspension absorb the riding surface irregularities and impacts. The frames on these bikes are off the scale in terms of how small the rear triangles are, how stiff the frames are, and how strong they are.

You can use inferior frame materials like steel, and make them less inferior with compact geometry. You can also take epic frame designs made from Titanium, aluminum, and carbon and make them even more legendary.

He was right and I was wrong. I was shooting from the hip but had a conceptual error, probably the same one you are making. The head and seat tubes are canted relative to the horizon. The top tube is shortest when it intercepts those two tubes at a 90 degree angle, not the 72 degrees typical of many frames.

Yes, actually, I have; have you ever used it in a real world example?

correct. i offered my elaboration because your disjointed & confused name dropping of Zinn & Peterson for the sake of feining authority made no sense in the context of this discussion.
It's also noted from your most recent posts that you haven't sobered up in the interim.