Caps on tailpipes of trucks?!?

[Danish]

I was driving the other day and noticed that a bus had a cap on top of its tailpipe. The cap was hinged at one end of the tailpipe. The cap would open and close as the amount of exhaust rose and fell.

My question is, why are these caps on the tailpipe? Wouldn't they simply increase backpressure and rob the engine of power? And if they are made to prevent things from falling in the tailpipe, why have I seen so many trucks/buses without this cap??

(not the best description, i know)

[AutobahnSHO]

I think they're to prevent rain from getting in, but I don't know why all vehicles don't have them.

[MX793]

Those caps are typically only found on vehicles with exhaust outlets that are pointed straight up and are indeed there to prevent rain water and other falling debris from getting into the exhaust.  A lot of rigs and buses use exhaust pipes that bend 90 degrees at the top so that the outlet is pointed rearward or to the side, which keeps rain and other falling debris out without needing a cap.  Quite a few vehicles with vertical pipes don't bother with the caps and I've personally never heard of any great problems arising from the caps not being on there.

[S204STi]

I think that big chrome flappers look perty.

[TBR]

I suspect that has as much to do with it as anything