Not meaning to upset anyone, just friendly forum conversation here.

I actually flat-spotted my 24 inch BMX cruiser rear wheel from a bad jump landing, so that it looked exactly like my illustration. I've also seen this on MTB downhill wheels as I meched for a couple of pros that like to ride with low tire pressure even over rock gardens. About 4 nipples at the flat spots are sunken and can be easily turned by hand (thank goodness they're double-walled rims so they didn't quite reach the tubes) and the spokes are loose and wobbly. I can move them side to side by about an inch so they might as well not be there. They didn't break. Also out of necessity, the guys had to continue to use their wheels in this condition until we got back to the workshop, and in my case until I saw the flat-spot while loading up the bikes. Every time the hub was directly over the loose spokes, I didn't notice any hub "drop," in fact the wheels worked just fine, no sounds, no creaking... still true side to side.

Obviously the wheels had to be rebuilt because the "load" was then being shared by a lesser number of spokes and some would be over-taxed (like a rugby team with several players in the sin bin).

I don't understand the principle of "negative pre-tension" that produces lift. This would mean the lesser the pre-tension the greater the lift? But isn't the tension up top increasing as the hub pushes harder down the wheel?

We also raced in Australia up at the Mount Buller Ski Resort during summer some years ago where the national downhill track was literally head sized rocks all the way down. After several practice runs the vibration was so bad that ALL the spokes on the front and rear wheels had loosened to a point where they resembled a wheel that had just been hand laced prior to tensioning (like you can move the hubs from side to side). Needless to say, those wheels were rolling with the hubs literally hanging from the top of the rims. We tightened spokes after every run after we caught that.

Now I've been working on bikes long enough to know that there are a few weird things that ARE the opposite of what they seem, so I'm open and willing to learn. So I have a feeling that, the fact that I haven't grasped the other theorem would mean that none of what I said really matters?

So what am I missing? I'd really like to get my head around it if I can.

Cheers - Pocko

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