Thanks for all the replies and suggestions, all of which will be taken into consideration.
Fatigue failure in motorcycle frames
Sitting in my sunroom today looking at whitecaps on the lake, with ice pellets blowing horizontally and small birds flying backwards, I was not encouraged to finish disassembling my frame for sandblasting in the shop, so I wrote this probably rather boring analysis of my frame failures.
The frame I built for my bike is made from 1" x .060" wall 1018 CREW steel tubing. The yield strength of this material is approximately 34,200 psi and the ultimate strength is about 51,300 psi. It's elongation may be as low as 10%. The area of the steel tube is .177 square inches. Two years of landspeed racing, or less than 6 hours of wide open throttle run time has resulted in 5 cracked or broken frame members.
One of the tubes has now failed twice, and it should be fairly easy to analyze the forces acting on it.
Analyzing this tube would show that the force required to cause it to fail in compression or tension would be approximately 6053 lbs. The endurance limit of this tube, if it were polished and had no other physical defects could be as high as 4590 lbs. which is 50% of its ultimate strength. That is, if a load of less than 4590 lbs is applied, the steel tubing should last indefinitely. There are many other factors that reduce the endurance limit of steel including size, temperature, differential loading, reliability, and most importantly, surface condition. Polished gives the 50% endurance limit; corroded, notched, welded, or otherwise imperfect surface conditions can reduce the endurance limit. The lowest I've found is about 20% of its ultimate strength. For the above tubing, that would result in a minimum force of 1816 lbs to induce early fatigue failure.
The static forces acting on this tube are only that of about four feet of tubing at .6 lbs per foot, one quarter the weight of the gas tank equal to about 4 lbs, 1/2 the weight of the fender which equals 3 lbs, and 1/2 the weight of the tailpiece, or about 10 lbs. Total weight is less than 20 lbs. There would be no force induced by the acceleration of the bike, as that is transferred to the axle which is supported in the well trussed main frame members below. Not even the force of the air friction should be transferred to this member, because the tailpiece simply rests on the tail end and is not fastened at this location. (It's fastened thru the use of Dzus fasteners to a crossmember attached to the mainframe, and to the forward bodywork. There is one Dzus fastener into the right side of this sub-frame, but none on the left side.) There is also a chainguard connection as shown, which is one of three such connections to the frame. A fourth chainguard connection is located further back by a simple link to the sub-frame above. I can fathom no other loads on this tube except those that may have been induced by the welding of the frame together into a rigid unit. So how does a 20 lb load become magnified to a force of over 1800 lbs required to initiate a fatigue failure?
I believe the answer lies in the load factor known as impact. Most impact factors used in construction rarely exceed two, which is obviously not high enough.
I found an interesting study on the internet on the failure of a three wheeled motorcycle frame here:
http://products.asminternational.org/fach/data/fullDisplay.do?database=faco&record=1410&trim=false(It was a 500cc motorcycle built in 1982, so it couldn't have been a BSA!)
It made use of a finite element analysis, but I have no such software.
So my theory is that the moving mass in my case is the above mentioned 20 lbs, and its velocity is related to the up and down movement caused by the rotation of the wheel, which is not exactly round (.070" out of round as measured at the outside of the tire.) I also believe that the .070" of actual movement probably results in much greater movement of the parts because the deflection of the tire. Hence the rotating wheel, at about 1900 rpm at 135 mph, creates a vertical high frequency vibration. The tube in question and the weight above it is moving up and down at the same frequency, which means that it cycles at about 32 times per second. Because this mass changes direction 32 times per second, it must be accelerated and de-accelerated at the same rate. I believe that since force is equal to mass times acceleration, if the acceleration can be determined, the force can probably be determined.
Can anybody come up with a suitable formula to determine the appropriate impact factor to help determine the actual forces generated? This would help greatly in the re-building of this frame.
Tom