Let’s assume that CS ovalization is necessary where the tire is closest to the CS.
Let’s further assume that, in case of ovalization, the degree of ovalization at the bottom bracket end would be the same as the ovalization where the tire is closest to the chainstays.
My thoughts:
Pro ovalization of CS at BB end:
Maybe the ovalized and dimpled area where the tire is closest to the CS is stressed less because the chainstays are a bit more “flexy” (in torsion and lateral bending) near the bb.
More vertical stiffness might help reduce stress at tt-st-ss cluster. I guess that this effect is insignificant.
Pro round CS at BB end:
Laterally stiffer near the bb end.
Higher torsional stiffness near the bb end.
More vertical compliance. I guess that this effect is minor.
The dominant characteristic of the entire rear triangle is the truss structure; the individual tube shapes have very little to contribute. The shaping of the stays are:
Mostly for aesthetic reasons
For practical reasons
An oval CS does not touch the ST. Otherwise, that is a very tricky miter
It is easier to dimple an oval stay
You can’t just transition from oval to round suddenly
That the CS is oval where it’s closest to the tire is out of question.
Are you saying that what you can see in the second picture in my post above (round CS at the bottom bracket and oval where the CS is closest to the tire) is significantly more difficult to manufacture than if the CS is oval all the way to the bottom bracket? Or are you saying that it is the abruptness of the change that is problematic?
Those chainstays in the pictures above will crack eventually. You have a very stiff structure transitioning into thin section stays. The abrupt change concentrates the stress. The red marks are the most likely spots. Ovalising is generally ok in a truss structure but dimpling will invariable create the weak spot. They would have been better to run the oval section into the BB shell and had the guard mount as a spigot from one CS.
May I ask you about another type of “sudden change”? Not to much thinner, but a rather abrupt/sudden bend in the chainstay. Excuse the very rough sketch, I guess it’s more clear on the drive side. It’s essentially another effort of trying to maneuver the CS around the narrow clearance, but here I try to bend things around instead of reshaping.
