I’ve never like the look of dimpling nor have I felt comfortable in doing it. Wondering what everyone is using for CS to clear a 45-47mm tire without the use of dimples.
Thanks.
Depends on the CS length and on the chainrings you want to be able to run.
If you have a CAD program you can simulate this.
My take on dimples is that it can decrease durability if the dimpling method is not executed well. So needs to be done right. But all else equal a dimpled chainstay is laterally stiffer than a non dimpled chainstay that achieves the same amount of space / clearance between tire and chainrings just by ovalizing the chainstays. That’s because with dimpling you only reduce the width of the chainstay at a short section of the chainstay. That in turn focuses the stress to that dimpled area which intuitively isn’t good for durability. But empirically it just doesn’t seem to be a problem in practice. It seems that that this local stress concentration isn’t big enough to matter enough in practice to be a problem.
4 Likes
Conversely, the largest number of frame repairs I have done are dimpled chaninstay cracks at the end of the dimple or across the thinnest section. If I was designing a bike that needed aggressive tube manipulation I would be going for a long squash of teh tube so the narrowing cross section is more gradual or just go straight to a 3D printed insert between BB and full chanstay section beyond the chain ring.
4 Likes
I strongly recommend CAD for this. That’s where you draw the plan view of the tire, stays, BB and chainrings, full size, on a piece of cardboard. It does require you to have a pencil, straight edge and eraser (optional, “not making any mistakes” is one alternative to erasers).
It’s possible to design without the aid of cardboard; for example the Great Pyramid was designed using only papyrus. But ever since cardboard-aided design was perfected, the modern framebuilder will need to learn how to use it to stay competitive.
I use Windows, but drawing outside also gives great illumination, weather permitting. Some prefer a Mac, in case of rain, but most CAD packages are not compatible with a Mac, because the cardboard isn’t waterproof.
Also, watch out for overstrained metaphors and tired old jokes.
13 Likes
Ahh now I understand why you thought so of the Fairlight’s sudden shape change in the chainstays.
1 Like
I’m using Columbus ZON114OV425133CR without dimples for 45mm (2X) and 47mm (1X), with a 86.5mm BB.
2 Likes
My go-to chainstays for tires that wide are Columbus Zona CX bend, Columbus part number ZON114OV425133CR. I have made these work nicely with 650B x 46 (actual width) tires, a 68mm lugged BB and Sugino OX2-901D 46/30 crankset with no indenting and good clearance. Rear was 130 OLN. Chainstay length was 450mm, so pretty long. As others have said, draw it. BikeCad is great, but not super accurate on chainstay clearance, as it assumes things about the shape, especially on the oval end. Any time I have a tight application like this, I draw it with pencil and paper, if possible with the chainstays and dropouts in my hand so I can map the exact shape onto the paper. I have a set of chainstay “templates” I cut out of paper so I can try different shapes by laying them on a drawing of the rear end of the bike. Sometimes just scooting the stays forward or back a couple of cm and trimming the ends accordingly can make all the difference. And of course keep in mind the numbers on the sidewall of tires are not always accurate, especially in larger sizes. I’ve seen “38mm” tires that are 44mm wide! So get the intended tire, inflate it and let it stretch, then see what it measures. Chainring size is also a key factor. Fortunately, very few riders are running big rings these days (53/39, I’m thinking of you!). Some crank manufacturers publish good enough drawings that you can design with confidence. Others, not so much; I often ask for the crankset so I can measure it to make sure the bolts and spider will clear. Finally, sometimes it’s necessary to tweak the chainstay bends a bit to get everything to clear as best possible.
5 Likes