I see on ebay that there are tank mounts for cell phones, and am going to get one, but they have metal plates that come up that look like there's something that needs to be screwed/fastened to, and i'm not getting the full picture...anybody with experience doing this that can help a dummy out? I have an S6 i want to tank mount for the GPS function....

You might want to consider a Ram Ball Xmount. I got one from Amazon, I use it for a Garmin or my cell phone.

http://www.amazon.com/gp/product/B00C7B2NV4?keywords=x%20mounts&qid=1445127940&ref_=sr_1_2&sr=8-2

Thanks, SZ...that looks good for holding the phone, yes...what does the 1" ball on the back go into? In other words, how does it mount to the bike?

Never mind, got my answer with a little more investigation based on your link...thanks a mil!

You can fab up your own stuff too...

Go to the hardware store and buy some heavy duty velcro.  My phones have rode for about 25000 miles on my C10 and now the C14.  Works great!  I do replace the hook side (on the bike) each year and the loop side (on the phone) twice a year to keep a good positive connection. 
Just over $4/ year.

I have an S6 i want to tank mount for the GPS function....

You can do what you want but here is something to think about.

I use a GPS unit on my bike and keep my cell phone on me via a clip on my belt.

If you are ever in an accident and become separated from your bike (which is a strong possibility) and your phone is on your bike you
will not be able to call for help.  This is most probable in an accident where no one else is around and maybe you took a curve to fast or
a critter jumped out in front of you or oil on the road and you slid off the road and over a hill/cliff etc.  Especially if you were not able to walk back to your bike
because of injuries from the crash.  Not to mention if you hit something solid your phone might very well leave the bike and you can't find it. 

By keep your phone on you, your chances of calling for help are greatly improved.   JMO.

^^^ with the above post, you might even consider a smart phone app called CRADAR (there are lots more out there).  It can auto-send messages and report you position - or at least last known.  I use it on longer trips.  It initiates by impact force settings.

You can do what you want but here is something to think about.

I use a GPS unit on my bike and keep my cell phone on me via a clip on my belt.

If you are ever in an accident and become separated from your bike (which is a strong possibility) and your phone is on your bike you
will not be able to call for help.  This is most probable in an accident where no one else is around and maybe you took a curve to fast or
a critter jumped out in front of you or oil on the road and you slid off the road and over a hill/cliff etc.  Especially if you were not able to walk back to your bike
because of injuries from the crash.  Not to mention if you hit something solid your phone might very well leave the bike and you can't find it. 

By keep your phone on you, your chances of calling for help are greatly improved.   JMO.

This is excellent advice...much thanks to you and RFH87 for thinking this the rest of the way through for me...