Bicycle Mechanics - Another question about rims

I have a trek 4500 with stock rims. When i ride down the road my tire almost looks like its not all the way in the rim and my rim is perfectly straight, so why does my tire move back and forth?

This ain't no reply its a ?

Perhaps your tire is not very symmetrical? It obviously works or your tube would immediately blow.

For safety, make sure the tyre bead is seated on the rim. Also, you may find that some tyre brands fit your rims better than others.

tires can be slightly "off", especially when they get a bit old and stretched; and by "old" I don't mean "years"; even after a few months of pounding they can lose some shape. If you've seen those tires with "stripes" on the tread, you'll see that the stripes often don't spin in perfectly straight line.

Check that it is mounted properly.

What sort of tire is it?

As some else said your tire/tube would blow if the tire was not on the rim or seated. The tire going back and forth can only be 3 things, either the tire is not perfectly round, or there is a bulge in the tire, or the rim is not true. Even if the tire was not mounted properly, the bead was sitting higher on one side then the opposite end, that would be a up and down motion not side to side; and it would be nearly impossible to mount the bead higher on the right side then the left because one side would set the bead the same way the other side is automatically...at least I've never seen this happen. I also never experienced a tire losing it's shape as it aged, other than the fact the tread is wearing down. I think you either a tire that was not made well or the rim is out of true-which you say is straight so that just leaves the tire. But do yourself a favor and flip the bike upside down and using your brake pads as a guide, spin the wheel and watch to see if the rim gets closer and farther from the brakes as it spins watching both sides of the brakes.