It sounds like Jackie is talking about high speed vibrations in the front or rear tire that can lead to a "tank slapper". What happens is your front or rear wheel is out of true when you are cruising down the road. Normally its the front tire that causes this, but not always. This defect could be because of a tire balance issue, defective tire (e.g. broken cords, bulge, missing tread, cupping), improperly loaded bearings, bent rim, bent or mis-aligned spokes, improper alignment of the rear wheel, incorrect rake setting, etc....<br /><br />Everything that rotates has a natural balance point where the rotation is smooth and even. This is why they add tire weights when they balance your wheels, they are trying to achieve this natural balance point where you have a smooth turning of the wheel. Because something has altered this natural balance, you end up with a greater load on one side and as the wheel rotates, it tends to pull towards that load. Since the wheel is turning, that unloaded point moves so what happens is it pulls at different points which makes the wheel want to move back and forth - the wobble. <br /><br />Now we add one final element into the mix, matching frequencies. Lets say your tire is out of round and has a high spot. When the tire rotates, as the high spot comes in contact with the ground, your bike is lifted by the amount of the high spot - lets say 1/2 inch. Lets now say that this happens at 40 miles per hour your bike tire does 6 revolutions per minute. So every 10 seconds your tire goes up and down - one cycle. Lets further say that the dynamics of your bike and the pavement you are riding on also have a frequency at 40 miles per hour of 6 cycles per minute and 1/2 inch. Picture the pavement as having low spots, high spots, low spots, (this should be easy to picture in Regina...). Well, if you are lucky, when your tire's high spot is coming to the bottom, you also just happen to be riding into a dip and the two exactly balance each other out. If you are unlucky, your tires high spot hits the pavements high spot (matching frequencies) and now you have 1 inch of movement. This makes the wobble. <br /><br />A tank slapper is just an extension of this. The wobble we just went over sends a corresponding movement into your handlebars and front end. Well, guess what, they also have a natural frequency that they move at. So, if the wobble being sent to your handlebars also matches the frequency of your handlebars, things start shaking really hard, really fast. It can get so bad, each end of the handlebars hits the tank, left grip, then right grip, etcs - the tank slapper.<br /><br />Most of the time, if you keep you hands on the handlebars, you'll never encounter this because your hands being on the bars changes the bars frequency of oscillation and the simple act of moving your arms also changes the resonant frequency on a constant basis.<br /><br />If you do ever get into a tank slapper, and you still have your hands on the bars, try to give the bike a bit more gas - move yourself at a different frequency - and the oscillations will stop. Braking is generally considered a no-no since your front tire isn't making full contact with the grouod and that leads to all sorts of issues typically ending in the bike and rider taking a spill.<br /><br />Factory bikes have engineers who spend inordinate amounts of time calculating rake, trail, tire dynamics, etc to offer you the safest ride possible. While rare, most tank slappers on factory bikes are caused by improper tire care. Running your bike on tires without enough air causes them to cup - a condition where you end up with ridges on the tires (remember the up and down example - then think ridges). It can also damage the side cords and now you have stiff and weak spots on your tire which can also cause the same up and down. <br /><br />On non-factory bikes, if the person creates a front end simply for looks, you can end up with some really interesting dynamics that can easily lead to a tank slapper - a not uncommon issue in the 70s when chopping bikes was cool.<br /><br />Anyway - off my soap box. If you have any additional questions on this, please let us know.
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