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Discussion Starter #1
finally getting around to starting the frame for my cb750, "sketched" it out in 1/4 rod, any thoughts? the top tubes are going to be 1-1/4" along with the two tubes going down to the front engine mounts, the rest will be 1", the black foam is where mounts will be welded to the valve cover. the subframe will either be aluminum tube or possibly carbon fiber, in which case the whole seat/tail section would be one piece of carbon. the swingarm will possibly be connected to the shock through a linkage where the shock is mounted to the underside engine mounts

IMG_1591.JPG
 

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Forget using the valve cover as engine mounts. It's dumb and unnecessary on so many levels. The old heavy cases should be plenty strong enough to use as a stressed member.

Draw the bike up in real scale on a wall in chalk. Plenty of famous frame builders did it that way back in the day. Base your geometry on a fast stable bike like a GSXR1100K. Stable for the road, motoGP flickable is for the race track.

You might not have the ground clearance to use a Buell type shock using a 4 : 1 pipe. Do something different, twin shocks like a TT rotary Norton maybe?

Check out Google pictures on how Egli uses big alloy frame plates at the rear of the motor. It's strong and pretty easy to fabricate. It's also far easier to change the frames design if you have F*&^ed up totally at first.

Make the seat subframe in mild steel tube to begin with, and make it so you can easily bolt alternative ones on later. Bear in mind all the café fashion seat subframes are only poor copies of motoGP ones, and they are nowhere near strong enough and safe enough for road use for fat old Gino from Jersey to ride. Triangulate or die.

Danger, is my business."
 

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The carbon subframe thing works out great.... too bad CB750's are such heavy lumps of crap.
I have no love for inline 4 cylinder motorcycles.... they sound like sewing machines.
 

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Discussion Starter #7
The carbon subframe thing works out great.... too bad CB750's are such heavy lumps of crap.
I have no love for inline 4 cylinder motorcycles.... they sound like sewing machines.
yeah, I would love a better engine, but it's what I've got at the moment, and it's already built to 900 so I'll keep improving the handling and shedding weight. you think the carbon subframe idea isn't crazy?
 

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how is it dumb and unnecessary??? I can't believe I'm talking to a bot.
Well if Egli, Seeley, Rickman, Bimota and Moto Martin didn't think a head steady was necessary on a sohc custom sports frame, I believe them.

There are plenty of reasons why your idea of using the valve cover as a mount is daft. I'll tell you if you want.

You need to study Moto Martin frames design.

Danger, is my business."
 

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The carbon subframe idea works. I did this on one of my race bikes back in 2004.
It was super light and strong... I would visit this again.... however I am working on other things right now
 

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I would imagine they didn't spec one because they used off the shelf engines and the frames were designed around that fact, using the head to help triangulate the neck is used quite a bit, but if you've got a ton of reasons I shouldn't, then go for it
The valve cover and it's screws are too weak to have chassis stresses applied to it.

The valve cover will flex and cause it to leak oil.

The motor will expand when hot and that may cause the head gasket to fail.

A bolted on valve cover is a maintenance nightmare.

It would be far better to design the bike so you can easily pivot the motor forwards and downwards on the bottom back engine mount like a CBX1000. Then you can get the head off in minutes for maintenance.

A valve cover motor mount on a sohc is about zero benefit for a big box of HeadF^%$s.

Danger, is my business."
 

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either .08 or .06 on the 1.25, and .06 on the 1"

I'll box in the neck area with sheet gussets
Gussets? Why on Earth?

If you have good mitred joints and good welding they are completely unnecessary. In fact they will concentrate stresses and may cause cracks. Also all that welding will weaken the tubing and cause everything to flex all around the place when welding it up.

Bike frames are not bridges or hotrod chassis.

Danger, is my business."
 

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Unfortunately I have to agree that a SOHC valve cover is not a good place to attach mounts. The thin die cast alloy won't take it and you would have to allow for the expansion and contraction of the cylinder and head, which would defeat the original purpose of the mount.
When you do get around to fitting and welding the tubing, I would recommend using the swing arm pivot mounts as datum.
 

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I like the idea - carbon fiber for the rear - very trick. I'd make sure that valve cover & Head cover can be removed with the engine in the frame. I do like the Martin Frame..
 

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Rock, here is my concern:

where is the swingarm pivot?

If you follow the prevailing notions of what makes a good handling frame the overwhelming consensus is that you need to tie the swingarm pivot and the headstock in the most direct way possible. Look at that Moto Martin frame whit posted, or any japanese wish bone frame and you will see: either a direct connection to the pivot or the most heavily reinforced area is where the pivot meets the backbone spars. Its in tonys book too.

now, the other reason I am curious about the pivot is, there is this other prevailing. Opinion that the pivot should be as close to the front sprocket as possible. Why? To counteract driveline forces acting on the chassis.
 

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Oh and I agree, the valve cover, however strong they are is not a good place for a mount on a cb750. Now the head....you could drill vertically through all the cooling fins, insert spacers in between them and have a pretty good mount.....maybe.
 

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Discussion Starter #19 (Edited)
cb750 frame-Layout1.jpg

surprisingly the neck needs to be that high with 30" forks and a 17" wheel, it's deceptive looking but when you go and measure things out, that's where it's gotta be

i don't particularly like the morini frame because if you really look at them they're wide as hell, the tubes wrap around the engine, so instead of triangulating the neck with short stiff tubes, you've got long gangly things hanging around, as far as the swingarm, it's going to be built off the rear 4 engine mounts, which tie into the top tubes which go to the neck and valve cover mounts, so all the tube section going from the rear mount to the valve cover is doing is adding additional stiffness to the engine structure
 

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There are a bunch of interesting frames out there.

Check this place out.
Old site, but good information to start more research.
Satanic Mechanic - Bakker

Also I would seek out Tony Foale's book on frame design.
I know that you are not looking to make a replica of a frame, but looking at what others have done and reading about how it worked will give you a leg up.

Way done the line, but have you thought about quantifying the results of the new frame?
I have always been impressed with your body work and thrilled to see you trying out frames. I actually dont even have a good idea as to how to show if the new frame is better, worse or indifferent of the stock frame though.
Just thinking via my fingers.
 
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