Originally Posted by
The Golden Boy
In the past, I've thought the 620 was slightly more heavy and dense- but I'm wondering how much of that perception is colored by "legendary Trek 720" reputation.
The 720 is 531C. I'm not sure I entirely understand the "C" in there- my 78 Trek 730 is full 531 frame/fork. My assumption is that it's 531C- it was intended to be a "race" bike. However, it is much lighter and more... "compliant" than the 720- which, as you noted, is a good thing for a touring bike- but the difference in the feel/weight are more than geometry can explain- To my mind, 531C is a tube set with a given thickness and butting. It doesn't make sense to me, but the only explanation of the difference between the 730 and 720 is that it's different thicknesses of the 531 tub sets. But it doesn't sit right with me that "531C" "Competition" is a heavier/thicker tube set than other 531 tube sets.
It has never sat right with me, either, but the Trek tubing charts (I believe it's the front few pages of the 1983 catalog) shows that the three main tubes are the same set for the 531CS and what Trek calls the 531C. That's estalished by what Trek said, if they did not misprint 35 years ago.
Now the question is, what are the stated tubing specs for 531C (or other 531 sets) as sold by Reynolds to other major (or minor) customers. I remember Tony Oliver's book on frame building indicating "531 Competition" downtubes being 28.6 mm OD and .9/.6/.9 IDs, and the "531 Super Tourist" downtubes being 28.6 mm OD and 1.0/0.9/1.0 ID. The only real inconsistency is that Trek seems (again, it could have been a misprint) to have specified the 10/9/10 tube for all it's Reynolds frame building.
Trek had to be one of the biggest frame manufacturers in the US at that time, so I assume they had some leverage with Reynolds especially if they paid their bills on time (I have been a supplier, it is not surprising that large and consistent customers can get "something special.").
I agree, it doesn't seem right, but i think it is plausible.