The two-triangle bike frame is pretty hard to improve upon…there are little tweaks but for the most part everybody uses the same design because it works. I’ve always loved Retrotec and other frames that incorporate unique lines but try to make a rigid frame less rigid where it matters. I know there’s very little evidence other than subjective opinions on the effect of bending tubes on making the ride quality more supple…but if done right it works and adds a cool look to some frames. If nothing else, it’s a way to be creative by trying to re-create the wheel — it looks nice, but may do nothing except make the frame harder to build. Sometimes, it may make it even weaker.
Anyways, I can’t bend the big main tubes yet or create big radius bends on small tubes, and I prefer to figure out how to build a ‘standard’ frame first before experimenting with harder designs. The one exception to this is the wishbone seat stays. I just really like the look of wishbone seat stays and believe they are a bit beefier/stronger than traditional stays. I have no evidence for this other than the tubes I use are beefier than regular seat stays which should mean they’re stronger and stiffer if it’s all built up right. [I’ve been using 0.9mm wall 4130 tubing that’s 25.4mm diameter at the seat tube, 19 or 22mm at the crown, and then 16mm at the main stays, whereas the ‘normal’ stays builders use tend to be 0.6mm wall and 16 or 19mm at the top and tapering to around 12mm at the dropout.] Honestly, I don’t think many would notice the difference in stiffness or strength of either design – it’s likely more of a matter of ease-of-fabrication or ‘do you like to bend tubes or cut & miter them?’ It’s mainly just another way to distinguish your work from others – and i think they look cooler.
I’ve tried the unicrown/wishbone seat stay on frame #1 (above on the old Bontrager) but the pre-bent wishbone stays are pretty thick and heavy. I may bend my own someday and see how that goes. This style is a bit easier to build but turned out too heavy and less aesthetic than I like. So i tried something else – inbetween what Sycip, Nobilette, and Dekerf make.
The Sycip wishbone stay is really just a segmented fork for the rear end.
The Nobilette design is pretty cool. I’ve never seen it before and it has smooth lines. It has the option to combine tig welding and fillet brazing to join the lower legs to the crown. It also is potentially easier to fabricate (if you have the tools to bend the main crown piece).
The Dekerf design is unique and seems like it’d be the easiest to fabricate – or at least fixture while you weld it together. This design is really nice, and similar to the Nobilette design in that you can micro-adjust the wishbone legs up or down if you need to before you weld/braze them to the dropouts. [One leg may be a tiny bit longer than the other which would pull one dropout up and mess with your rear triangle alignment and wheel spacing.] This means you wouldn’t weld/braze together the wishbone “fork” until you fixture it up in the frame jig and see if one leg needs to go up or down. Once all looks good, you could then either do it all in the frame fixture or just tack it and connect it all outside of the frame fixture. Another way to do this is make the wishbone stay like a fork, then file or slot the lower seatstay legs to fit the dropouts after…not sure if that’s better or worse though.
I think why many builders use, or have used, wishbone seat stays is due to the potenial ease of fabrication and the ability to do ‘production runs’. I don’t agree that they are easier to fabricate (when created one at a time) than fastback seat stays, even if you bend the fastback stays, because there is a lot of cutting and mitering and fixturing going on with most types of wishbone seat stays. But the one thing I’ve heard which makes more sense to me is that builders (like Bontrager) have many of their stays pre-fabricated (many created in production runs) and then they just need to miter the wishbone tube to the correct length and angle to match the frame being built. So you can use the same exact wishbone seat stay on a 20″ frame as on a 16″ frame just by cutting it to a different length.
Below are some shots of what i’ve tried on the last two frames. I’m pretty sure I’m going to copy Nobilette’s wishbone for upcoming frames, as soon as i get a bender all set up…
Next, just have to weld the stays to the bike, add the disc brake mount, slot the seat tube collar, ream/face, paint…and then done. Hopefully, Heyride will be riding this in the CB100 in a couple of weeks!