Certain items just don’t work with classic diameter steel very well.
A prime example of this is disk brakes for road, road race or cyclocross use. I look forward to the day we have great hydraulic disk brakes for road bikes but in the mean time there are maybe more disadvantages than advantages.
Steel road fork design currently dates back at least 60 years and they were not engineered with disks in mind. If you visualize a traditional caliper or cantilever brake you can see that the braking forces are equal on each side and that they are closest to the strongest portion of the fork (the crown) Disk work differently. The braking forces are on one side only. The force is as far away from the crown as possible creating a large leverage ratio. The forces are also typically pushing straight towards the rear of the frame as well. In the worst scenarios this can cause the fork to twist permanently and essentially it is damaged beyond repair.
In order to alleviate this one would have to use fork blades that are much thicker and therefore heavier, stronger crowns and disk brake mounts. This is next to impossible with traditional forks that have a brazed crown. It is possible with unicrown steel forks but there is a weight penalty as well and it does not fit the aesthetic of a traditional lugged frame. Even carbon forks optimized for disks tend to carry a fairly hefty weight increase for the needed strength, around 150 grams. Many of these new carbon forks are of the newer taper standard and this requires a huge head tube on a steel frame which is once again heavy as hell (at least a 100grm penalty) and no lugs are made to fit them which means by default you are welding or fillet brazing a frame.
Now onto the disk brake. The only one that currently works well for road/cyclocross is the Avid BB7 and although admirable it has its issues. Disk rub and squeal for one. Weighs a lot itself, add in the beefed up fork and downtube and it weighs more. They interfere with rack placement and fender attachment as well. The only thing I think one could really claim as being superior is alleviated rim wear but good ceramic coated braking surfaces will last at least 3 years under the hardest of use and honestly don’t cost a lot more than a new disk so I consider it standard wear items. Maybe someday we will have much better options but heavy, noisy, and mounting difficulties make them not the best choice in my book for Randonneur or cyclocross bicycles.
Now, you may ask, “Hey Dave, I have seen students who had disk brakes on their builds at your shop.” Yes, that is the case but you may also notice that the build method was different. I will do this but typically the frame must be fillet brazed to take advantage of the tubing that can handle this sort of thing. The frame will weigh more. Usually I suggest using nice Paragon drop outs for the rear which incorporate a disk mount and they cost more and are more difficult by far to work with.
You may also say, “Hey Dave, my Google-fu shows me that lots of other builders have done just such a thing.” Well, I am open to many different concepts with framebuilding and many ideas are great ideas. Although in my opinion most of these builders doing this are just plain wrong. Many do not have the technical expertise to make a reasoned decision on this. Second, most of these bicycles are city bikes and the like which really will not experience high loading factors over the units life, like a mountain bike or a road race/touring bike. It is very different if you really plan to use it to a high degree. This singular choice pretty much dictates the design and construction methodology of the entire frame/fork so please keep these issues in mind when deciding to use a disk or a traditional brake.