Topic: Purpose of a Seat Pan

[Ellz10]

My build is a '77 750 and I'm at the point where I'm about to weld on my rear hoop/loop. What follows is the seat and I am terribly confused about the purpose of the seat pan. I know it should be 16 gauge and then rolled to fit where the seat will be. What I'm confused about is whether or not it gets welded to the frame, bolted to the seat when finished or is it just a template sort of thing? Also, where should I start placement with the pan right below the 2 notches I've indicated in the picture? Sorry to sound like such a noob, but I am one. Learning as I go.

[754]

Without knowing what us between seat rails, how can we advise.. ?
 Sounds like you are maybe not ready to do this..

[Ellz10]

Without knowing what us between seat rails, how can we advise.. ?
 Sounds like you are maybe not ready to do this..

Electronics panel, battery, and oil tank

[754]

 Yes well ....how high they sit matters.. how well wirimg is done matters.. , what you want seat to look like matters..
 I am not going to guess at how /where you are mounting things... nope not  me..

[Ellz10]

Yes well ....how high they sit matters.. how well wirimg is done matters.. , what you want seat to look like matters..
 I am not going to guess at how /where you are mounting things... nope not  me..

Perhaps you misunderstand what my question is. I simply want to know the purpose of the seat pan.

[jamesw]

What you'll need to do before going forward is to mount the tank you're going to use to mock up how much and where to cut the frame.   After your hoop is in, and with the tank in place, you can make your pan.   

 The seat pan is not welded to the frame, or else you couldn't access anything afterward. 

I welded square brackets (that have a hole in the center) to the frame rails, and my seat pan has threaded bolts going through them, that align to the holes in square brackets for securing.

You need a seat pan because the foam in your seat has to rest on something.

[Bodi]

One of the stranger questions here.
So you seem to be building a cafe bike?
Before you get to the seat, you need to have a tank mounted (defining the front end position of your seat)and decide how long a seat you need. It will probably be fairly long, giving a bit of space to change posture. The seat length dictates where you trim the frame for a rear hoop. Once you have that done you have to measure where the seat goes to design a pan, and figure a way to attach the seat pan. I suppose a steel pan could be welded on but that makes access to what's below rather difficult. I would either bolt it to tabs welded to the frame of fabricate a hinge and catch mounting.
The seat pan supports your butt. The cushion foam sits on the pan and the cover is attached to the pan. So you need some way to attach the cover, pretty easy if it's plywood but a steel pan takes some thinking.

[Ellz10]

What you'll need to do before going forward is to mount the tank you're going to use to mock up how much and where to cut the frame.   After your hoop is in, and with the tank in place, you can make your pan.   

 The seat pan is not welded to the frame, or else you couldn't access anything afterward. 

I welded square brackets (that have a hole in the center) to the frame rails, and my seat pan has threaded bolts going through them, that align to the holes in square brackets for securing.

You need a seat pan because the foam in your seat has to rest on something.

Okay THIS is what I was looking for. Thank you very much. I'll be copying this exact idea for brackets.

My next question would be this: when I finally cut the metal for my seat pan after all is mocked up, where should the alignment of the pan be dead center of the frame rails or is it personal preference at that point?

One of the stranger questions here.
So you seem to be building a cafe bike?
Before you get to the seat, you need to have a tank mounted (defining the front end position of your seat)and decide how long a seat you need. It will probably be fairly long, giving a bit of space to change posture. The seat length dictates where you trim the frame for a rear hoop. Once you have that done you have to measure where the seat goes to design a pan, and figure a way to attach the seat pan. I suppose a steel pan could be welded on but that makes access to what's below rather difficult. I would either bolt it to tabs welded to the frame of fabricate a hinge and catch mounting.
The seat pan supports your butt. The cushion foam sits on the pan and the cover is attached to the pan. So you need some way to attach the cover, pretty easy if it's plywood but a steel pan takes some thinking.

What kind of hinge and catch mounting? Do u have any examples? When you say "cover", are you talking about the leather upholstery?

[kerryb]

I'm going to jump in here b3cause I have some pictures that may help.
1.  A seat pan is the framework that forms the seat.  One example shown here is a stock 78 seatpan with a "hump"  on the back that will use the stock mounting hinges and latch.  It is kinda big and quite heavy...but this is how we learn.  So this seatpan needs paint, foam, and a Vinyl cover to make it complete.  A side feature of this seat is the "glovebox" under the "gas door" which is functional.
2. To form the actual seat for the bike pictured, first the tank is mounted, and the electrics tray is placed between the framerails, this tray will be held in with screws on the inside.  After the electrics were mocked up to find space needed, a pair of hoops were formed that will sit on top of the framerails.  One hoop in front also has a tab to fit into the original seat mount bracket from the stock seat.  The rear hoop has two studs welded to it to fit holes in the frame crossbrace  near the shock mounts.  What is not shown is the two hoops joined by a metal spine down the middle for structure.  The metal frame will be covered with a fiberglass panel to hold the seat foam and blend into the typical "hump" that will end at the top of the hoop.  I think I have a picture of the framework before it went to my co-builders house to receive it's fiberglass pan and hump.  These have been formed over Styrofoam to the desired shape.
Hope this helps.  I got this far by reading all the project threads that dealt with my needs/interests.

[kerryb]

I just realized there wasn't a decent picture of the frame with hoop, so here it is.  The bike was originally an '82 cb650sc  which did not have a straight framerail for the seat
mocked up electrics to determine space needs.
proposed shape of the "hump" to aid design of the hoop.
finished seat spine before fiberglassing, which is done but I haven't seen it yet.