If your Harley Davidson's rear wheel is shaking, there are a few possible causes. The most common for this is unbalanced tires. You can easily fix this by taking your bike to a tire shop and having them rebalance the wheels.  Another possible cause is that the spokes on the rear wheel are loose. This is a more serious problem that will require a trip to the mechanic. Lastly, your rear wheel would shake if the bearings were worn out. This is also a serious problem that will require a mechanic's help. If your rear wheel is shaking, don't ignore it. Fixing the problem in time will save you from more serious issues down the road, literally.