I figured since I had the data, I'd throw it up here. I'm not that familiar with steels since most of my work deals with aluminum and composites, but I understand the mechanics.
From MMPDS-01 (FAA released material properties)
I could only find 1025 Carbon steel so that will have to be close enough
Property (1025 / 4130)
Ultimate Tension Strength (55ksi / 95ksi)
Tension Yield Strength (36ksi / 75ksi)
Ultimate shear Strength (35ksi / 57ksi)
Modulus of elasticity (29msi / 29msi)
density (0.284 / 0.283) pounds per cubic inch
So, to summarize, they are nearly the same density, but since chromoly is 73% stronger, you can use thinner walled tube to get the same strength, making the structure lighter. The MMPDS doesn't even provide Fatigue data for 1025 which leads me to believe it has very poor fatigue characteristics, evidenced by the low tension yield allowable. The stiffness (modulus of elasticity) of the two materials is identical. Therefore, if you were to build two frames of equal strength (thinner chromoly tubing=lighter frame), the chromoly frame would be MORE flexible. If you built two frames with the same size tubing, they would have equal weight and stiffness, but the chromoly frame would be stronger. Obviously, a chromoly frame would be better, but the driving factor to use one or the other would be cost.
Now, I think the original discussion was about frame flexibility. Again, I am by no means a motorsports frame expert. My expertise is in aircraft frames which are designed to different criteria. But a motorcycle frame must maintain a certain amount of rigidity in order to maintain predictable handling characteristics. The factory spends a great deal of time perfecting the stiffness of a frame such that it can handle numerous riding conditions. As with aircraft, there would also be some safety factors built in. Although there are some areas of a frame which can be lightened because they are overdesigned (usually in order to keep manufacturing costs down) additional bracing added to the frame should be maintained unless it is obviously for something that is no longer needed (i.e. frame rails that extend back to support fender/luggage rack). If key parts of a frame are removed, the load distribution within the frame changes, increasing the loads in some locations above what they were designed for.
Unless you are doing trick jumps off ramps or offroading your CB, you will probably never experience an ultimate failure of the frame material. The most likely failure to occur would be due to fatigue cracking. Cracks form near stress concentrations (welds, holes) where the stress is higher than the surrounding material (like when someone puts square holes in their brake rotor.
) Fatigue damage builds up over time at the grain level as the material is loaded past a critical spot on the stress-strain curve (see
Miner's Rule) until a crack forms. If the frame is adequately stiff, the material can not be stretched to the point where it will reach this critical strain, and the fatigue damage will not accumulate.
Alright, I know I'm not explaining this well (I'm not a teacher) so I'll just say that flexible frames will crack before stiff frames, given the same loads.
BTW, material will not fail in fatigue due to shear, cracks form in tension fields. If something in shear fails due to fatigue it is because of local bending that induces tension on one side, like prying of a bolt head.
I plan on cutting off an arch just behind the shock mounts on my bike so my seat will fit, but I'm going to try and replace it with something.
Ride safe.