reducedfatoreo

Today I had the pleasure of checking out a funky forked seat tube racer from Grebart Cykler, a Danish bike builder. Don't know much about it as it's not mine, but if there's interest I can try to get more pictures. Curious if anyone here knows anything about this type of build.

SJX426 

Seen this before. This is an example of an approach that does not required bending the seat tube to shorten the wheelbase. IIRC this is for TT purposes.

Reynolds 

It's to get the rear wheel nearer to the BB, as said above. The Italians call it "bici corta" IIRC.

smd4

That's one of the shortest wheelbases I've ever seen. The amount that the chainwheels overlap the rear wheel is astounding!

Rocket-Sauce 

Bet it's great for popping wheelies!

Catnap 

Check out Mecacycle bikes from France. They commonly used a split seat tube in the same manner.

Chombi1 

Reminds me of the French "Mecacycle Turbo". Could it be just a rebranded frameset?

reducedfatoreo

I think Grebart is a custom builder, so it's most likely his own welding, though the idea and design could definitely have been lifted from the Turbo if they were first. I believe he started in the 80s.

Trakhak

Here's a page featuring a Rigi bike built in Italy in 1978.

Chombi1 

You are most likely correct as the subject bike does not seem to have a brazed-on FD mount (To work with a Huret FD.), as seen on the Mecacycle Turbos.
I did not notice any tubing decals on the subject bike. Mecacycle built the Turbos out of Supervitus 980 tubing (At least all the obes I have seen.) if that might help.....
..

T-Mar

Bifurcated or paired tube seat (and down) tubes are a very old concept. They date back to the first bicycle boom, over 125 years ago, in the 1890s. The concept is periodically revived. During the last half century, bifurcated seat tubes have been used primarily to allow for minimum length chain stays. This stiffens the rear triangle for more efficient power transfer. Consequently, such machines were generally used for speciality events, such as time trials and hill climbs. Alternate approaches for shorter chain stays include steeper seat angles. cut-away.indented seat tubes, S-shaped seat tubes and eliminating the seat tube.

However, the origins appear to lie in the desire to produce a lighter, more comfortable riding bicycle. A single tube down has equal stiffness in all directions. By replacing a single tube with two smaller diameter tubes, the designer can increase the stiffness in one plane while making it more compliant in another. Typically, the tubes are aligned side by side to create a triangulated structure which increases lateral stiffness, while supplying more vertical compliance. By tailoring parameters such as thickness and outer diameter is it is possible to trade off stiffness between and weight. For instance, one could design a bifurcated frame that would be lighter than a standard frame, yet still have equivalent lateral stiffness. The biggest drawbacks for bifurcated frame designs are increased manufactruring costs and non-compatibility with some components.

​​​​​​Paired tube frame design arguably peaked in the late 1890s with the introduction of the Dursley-Pedersen bicycles. The space frame design was comprised of reportedly 57 individual tubes and incorporated a hammock saddle! Claimed weight on a standard Pedersen was 22 lbs (28" wheels and mudguards), which was quite light for the era. There was a Military folding model with 24" wheels at 15 lbs and a special lightweight racing model (again with 24" wheels) that was reportedly just under 10 lbs!

However, the oddest bifurated design that I've encountered is the 1890s Bronco. Pedal drive is at the rear hub, with the saddle directly above, creating a 90 degree effective seat angle. This allowed the wheelbase to be shortened to what I estimate to be about 32"/ 81cm. have been unable to find any weight claims but this looks like it would have been quite light. With the extreme aft rider position, upright riding position and short wheelbase, it would have had a very light front end and may have been the ultimate wheelie bicycle.

Lattz

Luciano Paletti also built quite a few frames with split seat tube although the "gain" in terms of shortening the wheelbase does not seem that extreme big, like with the bike on OP's photos or the Mécacycles. Although he is considered as an innovator, also an artisan builder, so only he knew why building with 4 more parts and 6 more brazings worth it.But I think, the the result is stunning.