Looking for feedback- I have the opportunity to get a good deal on a 1992 Serotta Legend Tri frame/fork. I do plan on doing Duathlons/Triathlons this summer. I currently have a 2004, 17.5lbs steel road bike with comfortable geometry. Is there any advantage to the tri geometry or the 650c wheels that would make this preferable to race over my current road bike? I enjoy repairing/building bikes but I would like to have something somewhat useful. What are people thoughts?

Is there any advantage to the tri geometry or the 650c wheels that would make this preferable to race over my current road bike?

650 wheels spin up faster but don't roll as well, all other things equal. The fact that 650s have not taken over the TT bike market should tell you that there are tradeoffs and that the design is not outstandingly better. Tires are harder to find, but not that hard. Assuming the frame fits you well, there can be an advantage to a TT bike in that it allows you to get lower/narrower and still keep an open angle between your legs and torso. A full out TT bike is typically worth 3-4 minutes an hour vs. a standard road bike, but that includes the wheels and aerobars along with the better position.