This topic is rather strange.
I've ridden a few bikes here or there now, and I've got to say, most anything that was a production bike in this era and even after had spaghetti forks up front. The bigger the bikes got, the scarier they were to drive. IMHO the CB750 solidly falls under that class of "flex like you wouldn't believe" bikes. The CBX was actually a lot worse though
I think through all of this, it can be accepted (at least empirically) that A REAL FORK BRACE, offers a substantial improvement to the bike. As does a GL front end, due to the larger diameter tubes (which actually are not that much bigger internally, but mainly thicker wall). Other things such are boring for a bigger axle, steering dampers, and horseshoe braces really help on these old bikes.
As for the idea of the stock fender brace? As an Mech Eng, it's my opinion that the rubber mounted fender mount will do next to nothing nothing. I'll also say that the stock solid mounted fender brace would not provide a significant amount of stiffening. When you think honestly about the cornering forces these bikes endure and where the primary areas of twist are, it should be fairly apparent that the bending stiffness of the stock solid mounted brace is not significant (in all by one primary direction, the brace provides completely negligable stiffness). Also, there is no triangulation in any technical sense going on with the braces.
Now, as for the effects of the brace, this is where it gets interesting. It does have a notable effect. This was noted by many people (as can be seen in Hondaman's post) and from generally riding with or without a brace on most bikes with a similar front end. The fact of the matter is that when you have forks up front that are bendy like you wouldn't believe, a little means a lot. You have to consider how little stability is there in the older geometry and construction of the front end, and how fine a line it is between cornering hard and going down hard. Every little bit you have in terms of stiffness on these front ends is rather significant, and I wouldn't doubt that the brace makes a large difference in handling. Imagine yourself going into a corner and you throw yourself in slightly too hard, causing you to cut your available front tractive forces down to the limit. In this situation, even a slight oscillation of the fork would have a large effect, and may even cause you to loose what you have left in terms of tractive forces. All it takes is a fraction of a second of stability loss to cause a bike to try and buck you off. It's impossible (or close to it) to model how these old bikes respond when being pushed, and it was done partially from a design standpoint, but also partially through trial and error on many of these older designs. The amount of flex in each bike might even have been different enough that you can remove the brace on one bike and have an almost unnoticeable effect, but if you remove it off another it's night and day.
For the honest truth though, The brace is really nothing special to make, and a stiffer than stock brace could easily be made using no more than basic hand tools. Even an older horseshoe style brace could be made at home and from parts from your local hardware/plumbing store. Arguing whether or not the stock brace is effective is probably moot, because FWIW, a proper brace is something that the bike could definitely benefit from anyways, and in the time it takes to argue, you could just make a decent brace
Hope that didn't offend anyone, and I just offer this as my $0.02 (canadian of course).
Cheers
Disclaimer The contents of this post are purely informally written, and should be taken with caution. No opinions expressed above should be used to remove or modify any suspensions components without proper knowledge and guidance. Read at your own risk!
