If you spend enough time reading old archives of the framebuilder's email list (from back in the dark days of usenet) or any of the current or defunct framebuilding forums on the web, the topic of chainstay bridges (and to a lesser extent, seatstay bridges) comes up repeatedly. There's quite a bit of debate on the topic of whether or not the bridges are necessary; and what effect they have (usually with regard to lateral stiffness).
There are two camps:
-The butter-side-up folks say the bridges are mandatory, and that frames without them will be noticeably less stiff as well as more fragile.
-The butter-side-downers argue that bridges are a historical artifact. They were first used, the argument goes, to mount kickstands and fenders and simply became de rigeur on all frames; the vestigial tails of the bike world (unless you want to mount a kickstand or a fender, of course). They often go on to claim that if anything, the bridge serves as a stress riser and makes the assembly *weaker*.
If you're really bored, you can google it up and read on and on.
Of course, like most internet arguments, the question never really got resolved. I generally don't use chainstay bridges unless specifically asked, so I guess if anything I'm the "it doesn't matter" camp, but I'm hardly a serious partisan when it comes to bridges, so when I recently realized that a natural experiment had occurred, I was pretty pleased.
The root cause of this was my own laziness - I recently built a super-fun cargo bike and was, as usual, too impatient to really *finish* the frame before riding it. I left a lot of things unfinished but notably for the last several weeks I was riding the bike around (with some pretty big loads, too) with *no chainstay or seatstay bridges*. The bike rode, well, fine. It's long and gangly and doesn't get around tight corners well, but when mashing the pedals to haul dogfood up the hill to our house, I didn't notice any untoward flex.
Fast forward to Wednesday, when I installed a HUGE (2" wide 1/8" thick plate!) chainstay bridge and a seatstay bridge as well so that I could park the bike and not get my butt all wet (both good things). At the time I wasn't really thinking about their potential effect on the riding characteristics of the bike - I just wanted to be able to park it. So theoretically, I was pretty unbiased or at least had minimal expectations going in.
Did I notice a great improvement in lateral flex? A now rock-solid ride? In a word, no. In fact, the effect (if any) was so far below the threshold of detection that until I started thinking about the issue this evening I did not even realize that I'd just done what might be (short of some kind of true double blind with hundreds of different riders) the perfect experiment. Consider:
-The bike weighs 60 pounds and often has me (150#) and a decent load (80-100#) onboard. There's a LOT of force trying to make every part of it flex under those loads.
-Other than the bridges (and fender/kickstand) there was no change in ANY other aspect of the bike's construction or geometry. Just one variable changed.
-While I'd never claim to be unbiased, I wasn't doing this intending it to be an experiment and didn't really have any conscious expectations going in.
Bottom line: at least in this case, bridges don't matter much. That's not to say they never matter, or that it's somehow wrong to want them on your frame. But next time you get into beef with someone online about the subject, send 'em here.