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I know that cutting material out makes a bike lighter, but at some point there must be a crossover between weight and aerodynamics. Since tri-bikes and track bikes have more oval/flat shaped tubes, you'd think that closing off the entire front triangle would improve aerodynamics even more. And making it out of carbon fiber wouldn't add too much weight. See also the use of enclosed wheels.

So, outside of the obvious reasons of weight and the loss of a bottle cage, why wouldn't you want to do this?

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Same reasons for not using disc wheels. Plus it would likely not improve aerodynamics much (if at all). –  Daniel R Hicks Jan 29 at 21:02
    
because a big sheet of material would be easily dented or ripped. Better to taper the "tubes" like an aero frame does, and if you look at those extending the tapered section until it met the next tube would mean you'd have one sheet. And obviously that sheet would have more aero drag than no sheet. –  Mσᶎ Jan 29 at 21:39

4 Answers 4

up vote 11 down vote accepted

Much of this is due to UCI regulations. They specify what shape and size of frame can be used. It is not allowed to add extra parts just for improving aerodynamics.

See the UCI document Technical Regulations For Bicycles - A Practical Guide To Implementation (PDF) which covers most of this.

As it says for Article 1.3.020, the frame elements must be tubular, with a maximum transverse dimension of 8 cm and a minimum transverse dimension of 2.5 cm, in a maximum ratio of 1:3. Also Article 1.3.024 "Protective screens, aerodynamic shapes, fairings or any other device that is added or forms part of the structure, and that is destined or has the effect of reducing wind resistance, are prohibited."

It could be possible to make a more aerodynamic bike that was not allowed by the UCI. But you would be restricted by what races it could be ridden in. A few brands have mode non-UCI compliant race bikes, mostly designed for triathlon. eg some versions of the Specialized Transition or Shiv, which have extra aerodynamic tubes etc.

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Actually TT and TTT road bikes do have some aerodynamic elements. –  alex Jan 29 at 23:01
    
@alex: But they are still diamond frames. The speed and 24h records were all done with fully faired recumbents. –  arne Jan 30 at 7:38
    
@arne how about climbing on recumbent? Is it rocks? –  alex Jan 30 at 17:42
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The question isn't limited to UCI bikes, so I'm not sure why the OP accepted this answer. –  Mσᶎ Jan 30 at 21:22
    
@alex Climbing with a recumbent is rather painful, that's true, but the OP was about aerodynamics which do not play such a big role in climbing. –  arne Jan 31 at 6:59

The primary reason is weight. Round tubes are very strength to weight efficient. Putting a lot of material where no strength is needed is not efficient. Also for most Tour de France road riding aero is not as important as weight. The other factor is a large flat panel can be affected by cross-winds, very dangerous in a peleton. Full disc wheels while popular once are less common for this reason.

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While I accepted vclaw's answer, I like what you said about the crosswinds, I guess I hadn't thought of that as much! –  Aaron Jan 30 at 14:49
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It's not necessarily true that weight is a factor. Most professional level bikes are so light that the mechanics have to stick weights inside the frame to meet the UCI's minimum weight requirements. The points about cross winds are valid though. –  jimirings Jan 30 at 21:26

The aerodynamic is not the only the issue in the road race bike. The most of the time in the road race the racers ride inside the peloton protected from head wind and have no aerodynamic problem at all. Moreover, at the mountain parts of the race aerodynamic plays no role in the race, but weight and stiffness of the frame do. You can find the TdF racers used light low profile non-aerodynamic rims on the mountain stages. The road race bike is a compromise of the many problems, where aerodynamics is not the most important.

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just chiming in to say that the pros climb fast enough that aerodynamics still play a role. that role is diminished compared to flat portions of the course, but it's still there. –  Paul H Feb 5 at 19:08
    
@PaulH Yes, the people like Wiggo or Froom needed protection on the most of the parts of the climbs they did, but in general for speeds below 30 km/h you not need something specially aerodynamic. The racers riding in upright position. The aerodynamics at lower speed mostly neglectable. –  alex Feb 5 at 20:54

I'm surprised that nobody has mentioned the Lotus Type 108. This was a bike made with a carbon monocoque frame instead of the traditional tubular double diamond frame. The bike was very successful, breaking several records and earning Gold in the 1992 Barcelona Olympics.

This type of frame is no longer allowed by the UCI.

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But that frame doesn't have a triangle at all. –  Daniel R Hicks Feb 5 at 12:21

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