Fun story on the chainstay bridge of the bike of my avatar. TiCycles built me a rather unusual fix gear based on a trashed Peugeot I saved from the dumpster. That circa 1990 sport Peugeot was a blast to ride fixed but the drop, while horizontal, was very short. I could ride a double sided hub if I kept the cogs to within 2 teeth of each other, 3 with a half link and the right chainring. So I designed this new bike to have a really long horizontal dropout (not a rear facing track end which I despise for road bikes). Dropout opens to the front, then a right angle turn down. This allows the wheel to be slid all the way forward with the biggest fix gear cog made, almost touching the seattube and still make for easy and fast wheel removals for gear change flips.
Fine and dandy, except the builder didn't quite think through what happens at wheel changes with the biggest planned tire size. Tire hit the bridge before the axle was far enough forward to reach the opening, Deflate tire to do anything. Oops! Gave him a call. He admitted his mistake, then thought up a rather neat idea. Ti frame so the bridge was a ti tube rather larger diameter than a typical steel tube. So he cut a "fishmouth" out of the tube and welded in an oval piece cut out of a larger diameter tube. And drilled a hole through for the fender with a countersink for a FH thead screw on the back/wheel side. Ingenious, looks like it was planned that way from the start and for the fender, works really well. Fender comes down forward of the bridge. I had to trim for both bridge and chainstay. I run a SS steel strap down from the fender and the FH screw from the tires side, through the bridge, strap and a nut. The area forward is just big enough to get one finger in to start the nut easily and slide in a skinny open end wrench. 3mm Allen form the rear does all the work.
And while I am on chainstay bridges - their effect structurally is to move chainstay failures to right behind the bridge. My UO-8 broke there (at 19,000 miles and way into double digits on crashes). The sport Peugeot I picked up 20 years later had been hit from the side by I'm guessing an SUV. Took it to the coop to straighten out the rear triangle and reset from who knows what to 120 OLD. Way to easy! Both chainstays were about to break. Cracks 2/3rds DS and 1/3 NDS right behind that bridge. But first ride "this bike is fun! In fact, I haven't had this kind of fun since my racing bike long ago." What do I do?
Well, I am Ben, a former boatbuiled with real fiberglass skills. And someone long ago gave me a couple of feet of carbon fiber boat hull material. I always have boat builder grade epoxy resin on hand. So, time t dive in and get messy! I did and learned the this CF stuff didn't wet out and go soft and piable like fiberglass cloth. Instead, it sprung away from all the tight turns of the ST, DT, BB, chainstay and bridge junction. No gentle curves there at all. So, disaster ready to set up and become permanent! Instant thought: Wrap it with innertube! Working lightening speed, I cut innertube strips and wrapped the whole mess tight. Resin squeezed out everywhere. Well, damage ios done. There's nothing more I can do so I cleaned and washed up and left the bike to die of epoxy in the garage. Came back the next morning knowing all that innertube was now solidly bonded to that sweet riding frame. Oh well.
Start peeling the innertube off and it all came off clean! The confounded CF had laid back down, had ll its excess resin squeezed out, all the weave showcased beautifully and looked for al the world like a high quality vacuum bag job. OMG! And the bike went from a cracked wet noodle (stiff is an adjective never applied to any Peugeot I am pretty sure) to probably the stiffest Peugeot ever.
That wrap started an inch or so up the DT, maybe a half inch up the ST, around the whole BB, chainstays and bridge to about 4 inches behind the bridge on the DS and 2" NDS. (I had to file a little off the DS to clear the chainring but there is so much excess strength and stiffness, hardly an issue.
So chainstay bridges - they are another thing to get wrong, they save chainstay/BB stress crack issues (probably the main reason they are there) but push those failures a little further back. And yet another place inventive DIYers can get creative. All in addition to being a nice place to land a fender.
Now, in this world of declining mental health, we are seeing that bridges are tempting suicides like never before. Bicycle manufactures are taking heed and eliminating them before the lawsuits happen.