Honda CB750K frame cutting

[Geesus]

Hi!

Probably a really stupid question but I'm new to this and trying to get a grip of everything. I recently got my hands on a Honda cb750k 1980 and are now trying to decide on the style to go for... My question is about cutting the frame.

The frame looks like this

But I prefer the straight angled seat like this

Is it too much work to remake the frame or should i go try to find a new straight frame?

Thanks!

 

[cinqmars2]

You'll have to remake the whole subframe, the one in the photograph is an earlier SOHC model, yet the frame has been modified specifically for that build. The rear-end is easy enough to make yourself, you can buy pre-bent hoops for frames from places such as Dime City Cycles:

http://www.dimecitycycles.com/vintage-cafe-racer-caferacer-chassis-seat-parts-rear-steel-hoop-section-tail-dcc-cb550750diytail.html

The hard part is relocating the shock mounts to fit underneath the seat line, at the moment they are sitting in-line or slightly above. They have to be even and dead straight with your swingarm. Have you got any experience welding?

 

[Geesus]

Thanks for your reply!

Thats what i thought.. Is it an option trying to find a earlier SOHC frame and fit my engine in? I bought a rear end hoop from wrenchmonkees earlier.

I havent got ny real experience with welding and I plan to give that task to a pro. Just considering if its easier and more cost effective to get a new frame than to have a pro relocate the shock mounts and straightening the lines...

 

[Geesus]

I guess this is an option, to raise the seat frame rail as much as possible and have a less high angle at the end.

 

[cinqmars2]

As far as I can tell, you can't just swap them out, a DOHC engine like yours won't just slide in to a SOHC frame, you'd need to make new motor mounts all round and you'd still have to figure out chain line too. When it comes to customs a lot of it is about how much you're willing to fabricate to get the look you want. Changing the rear end of a frame is really not the most demanding task (even though it may seem daunting) and I would suggest trying it yourself! I learnt to weld through playing with bikes, it's much easier than you'd think, much harder to make it look pretty though!

If you can find a pranged SOHC cb750, something that's been in a head-on but is straight at the back, you may be able to just swap out the entire tail section and whack that hoop on whilst you're at it. I'd imagine the width of the two bikes would have been fairly similar, but don't quote me on that...

In my opinion, I like the stepped seat on that bike, nicer than that bench on the Wrenchmonkees one.

 

[Bert Jan]

The "horizontal" tube on you donor frame is not horizontal when bike is assembled. If you want to make a horizontal seat, you'll have to mod the rear. you could chop the two "horizontal" tubes and weld new ones in between, leveling it out.

Quick photoshop

EDIT - Geesus also suggested that

 

[notlob]

If I was doing it and wanted a straight seat base and hoop I would replace the entire top section with one U shaped tube and fabricate the shock mounts to suit.
If your getting a shop to do it that would be easy for them than piecing bits together then trying to clean it up.

 

[Hoosier Daddy]

Not trying to sway your build method but you can do a lot with the seat shape to hide the dropped section of the frame.
If you plan your seat pan right you can come out with a flat seat on the stock frame. Here's mine.

 

[Bert Jan]

If I was doing it and wanted a straight seat base and hoop I would replace the entire top section with one U shaped tube and fabricate the shock mounts to suit.
If your getting a shop to do it that would be easy for them than piecing bits together then trying to clean it up.

true. but i seemed to me he wants to do it himself? And in that case its best to modify as less as possible, to keep things straight and strong.

 

[notlob]

true. but i seemed to me he wants to do it himself? And in that case its best to modify as less as possible, to keep things straight and strong.

Or not

I havent got ny real experience with welding and I plan to give that task to a pro. Just considering if its easier and more cost effective to get a new frame than to have a pro relocate the shock mounts and straightening the lines...