CB750F DOHC - Seat Pan/Mounting

[jcharny]

So I've asked several members here about their experience making a brat-style seat pan for my '81 CB750F but wanted to bring it into one topic. The bike's subframe has an up-swept tail section that has been giving me quite a bit of trouble. There are two curves I need to match, highlighted with arrows in the pic below. I tried using 0.19" aluminum sheet to make the pan, but couldn't bend it adequately. I'm considering switching to 16 gauge steel as it is thinner and will respond better to heat (I think). Does anyone have any tips for making the pan?



Thanks,

JMC

 

[J-Rod10]

I welded up an entire new sub-frame on mine to fit a specific seat.

Easiest way to do what you are wanting to do, stock seat pan. Narrow it up to your liking, thin the foam, and have it reupholstered.

 

[J-Rod10]

Lot of seat choices for the SOHC. Frame is flat, and easy to work with.

 

[J-Rod10]

You won't find a lot in the café world that mounts to stock hinges. Cross member is fine on that bike with a number of seats, however.

 

[jcharny]

Easiest way to do what you are wanting to do, stock seat pan.

J-Rod- I'm going to look at my stock pan tonight. My concern is that it was raised significantly near the tail light as there used to be a mounting bracket (see below). Have you been successful modifying a stock pan?

 

[J-Rod10]

Never tried on this particular bike. I hate the frame on them. It has been done, though.

 

[Calebduncan10]

Running into a similar problem with my 82 450
Curious on what you have in mind

 

[jcharny]

I last tried to make it out of 0.19" thick aluminum and it was a total disaster trying to put two bends in it in close proximity to each other. Has anyone made something similar with 0.1" thick aluminum? Even thinner? I'm not planning on adding any strengthening features (e.g. rolled edges) to the pan, but will weld 1 or 2 additional cross braces to the frame as mounting points.

If anyone has an engineering background, is there a convenient calculation for the tolerance of weight distributed over a certain area for a given metal?

Can't wait to finish this seat pan and move on..

 

[Djfob]

That bend in the frame looks awkward to work with. Maybe try working with 16 gauge steel.

First try a straight bend against a flat surface, then round bend with it c clamped to a 2" tube

 

[o1marc]

J-Rod- I'm going to look at my stock pan tonight. My concern is that it was raised significantly near the tail light as there used to be a mounting bracket (see below). Have you been successful modifying a stock pan?

You say "used to be" , I assume you cut it off? While it is a mounting bracket, it is not it's only purpose. It is also a brace to keep the side arms from flexing and it sits so high for tire clearance under compression. If you plan on having a flat seat pan there you may have issues with the tire hitting it under load.

 

[MattCB750OT]

I cut that back bracket out, and welded in an upset hoop. I've accepted that the seat will be done custom. The seat pan I cut and then shaped with a hammer to curl the edges. Added a small bend down the center to add strength. It's probably overkill but will see when all done.

 

[cqyqte]

I recently built a '79 CB750 which has the same frame and I suffered the same pains searching for a cafe seat. In the end I made my own from fiberglass and wood sandwich, similar to the hull construction of boats. If you draw a line from the point where the fuel tank meets the frame back to the just behind where the rear shock top mount and the frame rails go straight, that line nearly reflects the bottom contour lines of the tank.

I cut waterproof plywood shims (wedges) that filled the gap below that line and the notched frame that sat below that line. My plan was to sandwich these wedges in a wrap of fiberglass and lay them to the underside of a flat seat pan made from mahoganey plywood sub flooring. At the point on the seat pan where the rear shocks attach to the frame was where I added the angled round arch which formed the flat front of the cafe hump. I chose to use wood and fiberglass but the same could have been constructed using steel, where you would have cut the wedges from square channel tube and welded them to the bottom of the seat pan.

The wedges serve as the resting point for the seat pan, and take the weight of the rider and apply it to the frame rails directly. You can see in the picture the basic construction and the finished piece.

 

[esmoojee]

You can buy a seat that is already done http://www.ebay.com/itm/112048389454?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT It's made to fit our bikes. The other trick is lots of clamps along the subframe.