It has been my experience that the 80 Meter bandwidth will not be affected much by the existence of a trap.
I have a trap dipole for 15, 40, and 80 Meters. The traps are made from RG8x on PVC forms. I was just doing some adjusting yesterday and did some testing. I found that the antenna covers the entire 15 and 40 Meter bands with a SWR of 2:1 or less. And it covers about 110 KHz of 80 Meters with a SWR of 2:1, or less. That's about the same bandwidth I get with single band dipoles.
Overall the 15 Meter traps shorten the 40 Meter sections by 2 feet, each side. And the 40 Meter traps shorten the 80 Meter section by about 5 feet each side. Overall the savings should be around 14' of total length, or more. The savings will be less if all your doing is making a 40/80 Meter trap section. But it is still a substantial savings.
There is a bit of loss, due to the trap, but it hasn't bothered me much. It is best if you have some kind of Grid Dip Meter handy. The trap should be checked for resonance before it is installed on the antenna. Once the trap is attached, first adjust the length of the 40 Meter section, then adjust the adjust length of the 80 Meter section.
73, Martin, K7MEM
Ash Fork, AZ
In my area, it seems that every pickup truck or SUV comes with one or more dogs. It's so common that I can only assume that the dog(s) must come with the vehicle. So logic tells me that, if you want to keep the truck for a long time, go for the multi-dog option. Otherwise, if the dog dies, you have to buy a new truck. I have five dogs (4 dogs as of 4/4/2013, RIP Katie), so I'm set for a few years.