I've inherited a frame my father used to ride and was planning on getting it blasted and coated.
Checking it over it all seems to be in fair condition (its decent 531 Reynold tube), some rust, but not deep. Except the tube behind the bottom bracket shell between the chain stays (what do you call that bit?) which has a large rust hole.
How bad is this? Safe to ride? Fixable?
If it didn't have sentimental value I'd probably chuck it, but it does so I'd like to use it if possible.
When building frames high quality tubing, such as Reynolds 531 is often only used on the main tubes (i.e., main triangle) and if you are lucky the rear triangle. Less critical tubes, such as the cross-brace, will typically use a lower quality steel, such as a mild steel, which is more susceptible to rusting and will show earlier signs of deterioration. Therefore, there is still a chance that while the cross-brace has severely deteriorated, the main structural frame may have remained relatively intact.
There is of course no way to know for certain until you strip off all the components and paint and really have a close inspection of the main structural tubes. Once the paint is off you can can see how deep the surface rust goes. You can also try tapping the tubes lightly with a small hammer to see if you hear tone changes (i.e., severe thinning due to interior rusting). Careful inspect the chain and seat stays as some frame builders use cheaper steel here, which may not have faired as well. Finally, the cross-brace was often used for fender mounting and was usually drilled, paint damage at this point is also common which could have further accelerated the rusting.
Even if you do all this and the main frame seems reasonable intact, I would still be cautious in use as there is no way to determine with absolute certainty the safety of the frame without pressure testing it (i.e., ridding it). This is a delicate proposition as you want to ride it as lightly as possible until you see how it reacts and slowly build more confidence as you use it in differing situations. One of the things working in your favor is the failure mode of steel is a lot better than other frame materials such as aluminum and especially carbon, which you should generally avoid riding if you have concerns over structural damage.
How bad is this?
Bad. Real bad.
Safe to ride?
No. I sure wouldn't.
Fixable?
No, not in my opinion. That's what you can see. What does the inside of the seat tube look like? Or the insides of the welds that hold the steer tube to the top and down tube?
Just look at the peeling paint on the bottom bracket. And water doesn't generally pool nor collect and remain on the outside of the bike...
If it has significant sentimental value, blast it, clean it thoroughly, and then see what you find. It might be salvageable, but I wouldn't wager too much on that possibility. That cross brace is completely rusted through.
I would say that the frame is probably not safe as it is. For one, I don't know how important that connection was for the structure of the frame. But most importantly:
Water is likely to have entered the chain-stays through the hole created by the rusted connection. The consequence is, that you cannot trust the chain-stays without checking them. And chain-stays are a vital part to the security of your bike. So, you don't want to take this lightly.
Depending on how far the water (and rust) entered the frame you can have either of these outcomes:
No hole in the chain-stays => the frame can be salvaged by replacing the connection.
Hole in the chain-stays and water/rust entered them => you may need to replace the chain-stays.
Hole in the chain-stays and water/rust also entered the bottom bracket from that direction => well, looks like you need to replace the bottom bracket as well. I guess this kind of repair could get expensive due to all the welding that would need to be done.
I guess, it should become clear which of these it is once you have fully removed the rotten chain-stay connection. Look at the state of the chain-stays themselves at the point where the connection was welded on, and proceed accordingly.
