is using mild steel erw and 4130 moly on same chassis legal ?,(example would be mild steel frame rails and 4130 roll cage)
I would not think that it would be a good idea to mix the two types of steel in the same structure. The mild steel will need to yield in a crash situation, i.e. bend, stretch, compress, or do whatever the situation requires. The higher strength steel will not yield (bend, stretch, compress) as much before it reaches it's breaking strength. I think the result will be that all the load will come onto the stronger members before the mild steel has had a chance to it's job, which is absorbing energy. The welds would probably fail, and the entire structure would no longer act as a unit.
A similar example of this is the use of too much reinforcing steel in a concrete beam. A beam with the proper amount of reinforcing steel, will actually bend, stretching the steel (which becomes stronger as it stretches), and hence the failure mode will be slower and noticeable. A beam that has too much steel, will literally explode, as the concrete exceeds it's compressive strength, and fails catastrophically before the steel has reached it's yield strength. Think about that when you're driving beneath all the overpasses on the way to the races!
Tom, B.S.M.E.