Just checked mine, it's probably about 1/4 of the length of the one thrasher572 posted, but it's cracking at a slight angle (to the left). Is this something that absolutely needs to be fixed if I'm doing gentle riding? Maybe somebody with more experience with materials can tell us how likely the frame snapping would be based on a crack like that?

 

The engine is solidly mounted in the frame. This engine develops a high frequency vibration that puts your hands to sleep around 5500rpm. That vibration is transmitted through the frame. Ride it below 5000 rpm it is smooth, ride it above 6000rpm it smooths out. 5500 rpm with stock gearing in top gear is approx 60mph. If you or the previous owner have put up with the buzz on the highway at the speed limit (or close) for an extended period that high frequency vibration has to more than put your hands to sleep... Metal will eventually fatigue and I think that is what we are seeing. Early in my riding career I didn't like the buzzy feel at the 5500rpm range so I did a few things to avoid running the enginge at that speed. Drop a tooth at the countershaft (front) and the result is 500rpm higher at 60mph. The other thing was to cruise in 5th rather than 6th gear. Or become a habitual speeder. My frame is intact and it has been under my use and care since new. I have 3 hurricanes an 87 i bought new , and two I aquired in the last couple of years plus I maintain a fourth one for a friend. The one I bought new is good... The other three have cracks... one repaired. My friend has continued to ride his copy as is. I will end up with that bike when he is done with it and then the repair will be made... until then I am still monitoring it to see if it gets worse.

 

I've got one frame that is cracked in exactly the same position, and it seems to be a fatigue crack. I don't know the frame's history.

I have a theory that the crack might be related to (or contributed to by) the engine mounting through bolts not being fully tight. When I changed the engine in my other frame (which had no cracks!) I noticed that the 2 through bolts weren't very tight. Also,one aluminum spacer on the LHS (could have been the top one but I'm not sure) was worn, indicating that it had been loose enough to have been rubbing between the engine and the frame. I checked the spacers from the cracked frame, and one spacer was even more worn. If one or both through bolts weren't tight enough, the frame could flex more, or vibrate more (see Thrasher572's comments on vibration), which might add to the stresses on the mount. So I'd suggest checking the torque on all the engine mounting bolts (36 ft-lb or 50 N.m), especially the through bolts.

 

The bike in the picture has not been disassembled for repair yet... I will check the tension on those bolts to see if they are loose on that one... Thanks for the feedback.

 

The 88 I picked up in December alsocracked there (about the same length too). I'm planning to weld it up as it sits for now (grind the area clean and disconnect battery) -I'll look tearing it all apart in the fall to do a full repair and paint the frame.

 

My bikes fine no cracks, had to check it out right after i read this post. Looked over everything.
I have 45,000 miles
Plus the bikes been down before at slow speeds (Previous owner, my friend, stunted it alot)

 

I shoul've adopted the same attitude...

Damnit! Why did I read this thread? [:@]

I got 'ze crack', and it looks identical to that in the posted image.

It got me thinking and I'm wondering, this is a 20yr old bike, at what age or after how many miles would this crack typically start appearing? If left untouched, does it expand and eventually grow long/large enough to split the frame? Did anyone hear about it breaking off completely? If not, assuming the crack has been there for the past 10 or 15 years, it tells me that this is about as bad as it will ever get and people been riding the bike as is without any other consequence(s). If a repair is absolutely necessary, can it be done with the engine still in the frame?

I'm just throwing out my thoughts on this. I'm pi**ed off that I didn't notice it when the engine was out and don't feel like doing it all over again, especially since it just started being nice enough outside to ride...

 

IDK, I'm still putting my money that these cracks are caused by stunting, wheelies in specific, the force of coming down on the front wheel seems like it would stress upwards consistent with the cracks.

 

But why only on the left side of the frame and not the right?

 

I didn't realize it was only the left side. Was that on your bike alone, or what the OP said? its been awhile the details are beseiging me.