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.
That spot at the chainring is a hard one on mtb and gravel bikes with bigger tyres. You cant really do much other than squash tubes to get them through there…or use flat plate.
On my mtb yokes I have quite a bit of a notch but I also have an internal rib and I’ve increased the wall thickness by a fair bit. Ive also elevated the CS slighlty to pass through and area with a bit more clearance.
Larger radius bends are ok generally as they usually dont produce sharp corners.
Yeah I want to avoid both of those but then of course the other choice is some fairly sudden bends…But I guess mine isnt too bad in this case then?
And really cool looking yoke btw! I have never seen similar…Unique and so much more aesthetically pleasing than those flat plates (bit of pet peeve of mine but may need to use them on a hybrid winter fat bike / MTB build eventually)
For the noob, is this an engineering perspective weak point, as it will crack there first, as all things have a point where they will crack first?
Or should I avoid building like this as it will be prone to failure (when not done well?).
Stress, like water, air or energy, which is what is going into a structure, will always go from high resistance to low resitance. Water from gravity, air from pressure. Energy from…hmmm…not sure how to phrase that. Basically though you have a really stiff structure going into a realtively weak tube/structure. The bending is always going to start from the weakest part that the forces act upon. If you have a sharp edge or drop off in stiffness, through either material volume or geometric shape, the stress concentrates at that boundary. It pulls and tears at the material until it fails, a crack usually.
For that chainstay structure in the above example if they had a butted tube that was nice and beefy at the bridge and a transition away from the bridge, there will be a much better load bearing as the stress would be spread over a section of the tube as it gets thinner and the bend would start in the transition from the butt and not at a tight corner or edge.
To be fair we are picking on this one example and we don’t know the make up of the chainstay. For all we know it could be well supported and the builder already knew the issues at play. Everything else I see in those pictures shows a builder is pretty good at what they do.
Another factor to take into account here is the squashing/dimpling in this instance acts like a butt transition would. It’s full section and stiffness at the bridge but the gradual shape change to a much thinner section transversely will mean the stress spreads over the tapered area. It’s possible this is a deliberate layout that has taken in account all the things we have mentioned in this thread.
In summary though, just avoid sharp corners and abrupt thickness changes. Especially when you are using thin wall tubing.
