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Discussion Starter #1
I really don't like the look of the stock CB350 engine cradle. I do like the look of the CB77 engine cradle or lack thereof. I also don't like having to lift the engine in and out. So, what would need to be changed if I wanted to remove the front down tube and lower frame tubes and use the engine as a stressed member. I think that the top motor mounting plates would need to be chnged out for something that is more rigid and connects to the top of the engine at more than one point and wider out from the centerline of the engine. I would not use this on a race frame, but would it be adequate on the street?
What are your thoughts?
Ken

AHRMA 412
Vintage racing - old guys on old bikes
 

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I have a 78 cb400 that uses the engine as a stress member there is really not much to it .Monday I will sand blast and paint it so i will post it on tuesday so you can see how it was designed . George



Edited by - LiLBull on Nov 03 2005 06:46:40 AM
 

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Is this a bike you intend to ride on the street or one you intend to race? do you have to keep the original frame to meet some sort of class rules?

The smartest thing to do in this situation would be to start fresh with a trellis wishbone design slimilar to a ducati. Hopefully your welding and fabrication skills are up to snuff.

If you are adimant about keeping the original frame here are some things to consider. Most bike frames from the 1970s are not the most rigid. removing sections of that frame will only reduce its effectivness. you will probably have to add more metal to the frame than you are subtracting so if this is for weight savings, then forget it.

The quickest way to crack a frame is to stress the motor mounts in different directions. That being said you are basically going to hve to find a driect route to tie all the motor mounts together. The easiest way would be to build a halo around the top part of the engine, with lots of connecting cross braces and incorporating the original mounts.

Just my thoughts on the subject
 

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I've built a few frames.....think REAL hard before you consider that a viable option. You'll need excellent welding and fabrication skills, a good jig...I use one based on a 4' section of large I-beam.
Ask Frank C. what's involved...he's made about a thousand of them.

Also...the vertical model Honda engines aren't designed to be stressed members...the right angle orientation of the cylinder and cases is not a sound design for use as a stressed member. The Honda models that used the engine as a frame member have sloped cylinders that place the top engine mount closer to the steering stem and the 30 degrees slant of the cylinders provides a more stable transfer of forces from the steering neck to the swingarm pivot.

The best approach would probably be a frame with a very strong backbone that "hangs" the engine from the frame. Ala Ducati like Geeto suggests. A new fabricated frame is always a last resort for me, it can be a tremendous pain in the ass.

Course it would be kind of cool to find a late model Duc Monster frame and put a CB350 engine in it :)
JohnnyB
 

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Discussion Starter #5
Well,
I would not be tryig to race this frame. I want to build a caferacer for the street out of leftover parts from race bikes. I have the fenders and external spring front fork from 1st year 350s and thought they would make a col loking old timey cafe. I thought getting rid of the engine cradle would add to the look. I guess I need to give this additional thought before I do somehing really stupid. I like the idea of hanging the engine from the frame but that would involve a lot of extra bracing, but it is an extra frame and its not like they are hard to come by if it doesnt work out. I might just see what happens and see what flexes or breaks first. I think I will reinforce the rear motor mounts and replace the upper mount with a tubular structure welded to the steering head and go from there.
Ken

AHRMA 412
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Hell, I'm sure you know enough about how a bike should handle to tell if it's right or not. Would be a cool experiment...do what you think is right, take it out and see how it works. Even in a really bad frame you almost never see catastrophic failures, something might bend, or tweak but it's not like it will explode or anything :)
JohnnyB
 

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Discussion Starter #7
JohnnyB, You are right about it probably not totally failing. It cant be as bad as some of the evil handling junk that I have tried to race over the years, so it should be safe enough on the street to try some stuff out. It doesn't have to be right the first time. Plus it can't be worse handling than a SR500 with a broken swingarm at Road Atlanta. I just think that keeping the steering head from twising after the front down tube is removed is going to be the major concern. If I triangulate from the outer cylinder stud area to the top motor mount on the frame and the lower steering head, would that be sufficient? Is the stamped frame backbone stiff enough if the seams are welded to not overly stress the cylinder studs and cases of the engine? Time will tell.
Ken

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Getting a triangle with the base across the top of the head would certainly help, and another along the axis of the frame from the bottom of the neck to the top of the head.

The bottom of the back bone on many honda twin frames are open about 2/3s of the way back from the front. I usually cut a 16ga plate to fit this gap and TIG weld it in so it's not just a three sides. Adds a ton of rigidity to that particular component of the frame.
JohnnyB
 

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here is not the best shot of the frame the four hundred has a rectangle center frame rail and a elongated ,one side open box section that cradles the motor mount in the rear.

http://www.georgescustomcars.com/cb400.html

Looking at both the cb360 and the cb four .I could make a frame that would mimic it to fit the 350 but I have to ask my self why Id wantto? Reading your reasoning Im sure u could tinker with a few production frames but that would be if you had access to a bike salvage yard to sift thru the various frames. If from scratch because the small shop doesnt have metal stamping machines ,it have to be "very Ingenious" to create on light and stiff ...but Im the last one to tell you cannt do it so .I wish you good luck !Be glad to shoot ideas at you !!!
 

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Guys, I can't contribute a whole lot to the frame geometry discussion, but...I'm a little surprised to hear you seemingly dismiss or diminish the neccesity of a good handling frame for street use versus race track use.
I'd be more concerned about good predictable handling on the street, where there are many more (dangerous and hard) variables than the track.

When Yamaha added the SR500 to the TT/XT line, they built the SR frame almost 30lbs heavier than the XT, which is a dual purpose frame. Honda did the same when they introduced the XL variant of the XR500. In both cases, some of that was beefing up critical stress areas (joints) to improve the handling on the street.

Don't mean to sound like yer grandma, but I know how I ride on the street....

FR
 

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But what I read from his initial idea of an easy accessible mounted motor and using spare parts , the initial design premise .Seems to me he was just kicking around the idea ,and just didnt seem he was going to do more then tool around on his creation ...I dont think his design Idea was to chase modern rice rockets around beside alot of beefing up street frames by a manufacturer is ,they have to over engineer not for stress in its symplist meaning, its because they need to build for the masses not knowing what wieght rider will be on it,they have to facterin "time" of being on the road. Factor in rust degeneration and general wear ..so they say strees but the stress of producing for all kinds of riders in all kinds of around the globe usage .Is a total different kind of stress facters in a race bike of known wieght of driver,longevity of use a few hours or minutes on track.
 

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Well, you'll put more stress on a frame in one race than you will in a year of street use.
I've repaired quite a few street frames that have cracked under race use despite the fact that the bike was 20% lighter than in stock form.
Why else the bracing of OEM frames, the increase in front fork size, the bracing of swingarms, the increase in spring rates? Racer's learned a long time ago that in all but the "stock" classes, OEM frames won't hold up in the long run.
How often on vintage street bike do you brake from 90 to 40mph in about three seconds with the front end fully compressed and the back end skipping over bumps? How often do you lean the bike into a corner dragging the pegs at 90mph? Well...on a race bike figure about 50-60 times a DAY at typical race meet. How often do you crash on the street? Figure a couple times a year racing. Believe me, a vintage race bike with probably 50% more hp than stock, a front brake with twice the stopping power of stock, being ridden under conditions way outside the envelope allowed for OEM bikes....needs all the OEM frame strength plus some.
My 205 lb 175 will stand on the front wheel with a 200 lb rider, and lift the wheel changing into second gear. It's got 23hp at the rear wheel compared to about 14 for the stock bike, tops out at about 100 mph compared to about 80 for the stock bike. Believe me the frame is under more stress in one race that it would see in a year of street riding.
Thousands of racers for decades have been increasing the strength of rigidity of OEM frames so they can race them...they ain't doin it just for fun.
JohnnyB
 

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went out to the garage last night to move some stuff. Started looking at the derelict dream sitting in the corner of the shop. realized that the motor was about 305cc, was physically only slightly smaller than a cb350 motor, and is suspended from the frame without any downtubes. Of course the bike is also a pressed frame bike so the engine is not exactly a stressed member. so I got to thinking....


Why not put a cb350 twin in the dream frame? A little reinforcing with plate steel, new mounts, maybe some tube frame reinforcement in the hollow sections, it looks like it might work. you could build a psuedo replica of the old nsu sport max racer, one of the few race bikes I know of with a pressed frame, and the shape is similar to that of the ca77 dream. plus you can get whole good condition dreams for under $1000 if you look hard enoungh. the hardest part I could see is that the 305 has a top mount while the cb350 has a front mount, so getting the front mount to mate to the frame may take some creative tube bending, and some welding.

here are some pics of the nsu and one of the dream. I know a guy in New Orleans with a sportmax replica (without the fairing), but I am having a hard time getting in touch with him for pics.

http://www.lump-proof.com/CLASSIC/RACER/fb/

http://www.classic-motorrad.de/bendix/NSU-Web/nsu.html

http://www.honda305.com/forum/
Edited by - Geeto67 on Oct 31 2005 06:19:11 AM

Edited by - Geeto67 on Oct 31 2005 06:23:23 AM

Edited by - Geeto67 on Oct 31 2005 06:28:00 AM
 

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The CB350 engine being a verticle cylinder model would require moving the engine down at least an inch or so in the frame. The Dreams/Superhawk/CB77 had a 30 degree incline on the cylinders. When you stand them straight up like a CB350 the engine is taller. If you move the engine down in the frame the drive sprocket will no longer be in the right place...ie the chain will pass though the same place as the swingarm pivot. You'd have to fabricate intakes because the carbs will move backwarks about 3"...probably interfering with the pressed frame.
And, as far as I know a pressed steel frame by no means is an indication that the engine is not a stressed member. It just means it's a cheap to produce frame.
Check out the size and strength of the engine mounts on the inclined cylinder engines....compare it to the CB350. The CB350 top engine mount (head cover) would last about a week before it broke or started popping off head nuts.
I don't think any of the vertical engines are going to stay in place without their massive front engine mount attached to something.

If you want the cool Dream/Superhawk exposed engine look....might be easier to just get a Dream or Superhawk. I mean if you can get decent examples under $1000...just do it and spend all the time and money making the bike look and go a bit better. It would get a lot more attention and be a nicer ride than a hacked up CB350.
JohnnyB
 

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quote: The CB350 engine being a verticle cylinder model would require moving the engine down at least an inch or so in the frame. The Dreams/Superhawk/CB77 had a 30 degree incline on the cylinders. When you stand them straight up like a CB350 the engine is taller. If you move the engine down in the frame the drive sprocket will no longer be in the right place...ie the chain will pass though the same place as the swingarm pivot. You'd have to fabricate intakes because the carbs will move backwarks about 3"...probably interfering with the pressed frame.
And, as far as I know a pressed steel frame by no means is an indication that the engine is not a stressed member. It just means it's a cheap to produce frame.
Check out the size and strength of the engine mounts on the inclined cylinder engines....compare it to the CB350. The CB350 top engine mount (head cover) would last about a week before it broke or started popping off head nuts.
I don't think any of the vertical engines are going to stay in place without their massive front engine mount attached to something.

If you want the cool Dream/Superhawk exposed engine look....might be easier to just get a Dream or Superhawk. I mean if you can get decent examples under $1000...just do it and spend all the time and money making the bike look and go a bit better. It would get a lot more attention and be a nicer ride than a hacked up CB350.
JohnnyB
from personal expirence, the cb77 superhawk is a stressed member engine in a tubular frame, the ca77 dream does not use the engine as a stressed member because of the design of the backbone of the pressed frame. You are right that a pressed frame is not any indication of whether the engine is stressed or not, but from my ow expirence of taking a few dreams apart the engine is not being used to strenghten the frame.

forgot about the engine angle, that would be a problem since tilting the cb350 motor forward tends to expose the rear tranny gears which then begin to break down. too bad because the cb350 twin is a nicer motor than the boat anchor 305.


Looks like you put a lot o thought into this swap johnny, were you considering it at one point?

Edited by - Geeto67 on Oct 31 2005 06:47:42 AM
 

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I don't mean angling the engine. I mean the angle of the cylinders on a 305 compared to a 350. The 305 cylinders were angled so they would provide a straighter path for forces from the next to the SA pivot. and to provide a connection point closer to the steering neck. Of course the 350 engine would have to mounted in it's intended orientation...which provide a slew of clearance problems, not to mention I think it would break the top mount right off the head. The 350 engine is probably 4" shorter in horizontal length and 1" taller in vertical length too. It would make for very rear heavy bike in a 305 frame...as it is most of the "sloper" Hondas were already rear heavy. One of the reasons racer extend the swingarm.

Never really put any thought into that swap...but I've taken apart and built whole lot of 77's, dreams, CB's, CL's etc.
JohnnyB
 

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I agree with everything you said, I was just thinking that in order to get the 350 in there you would have to lean it at an angle similar to that of the angle of the cylinders of the 305....ok, it sounded way better in my head, just didn't come out cleary in typing. I don;t have a 350 motor for comparison, and to be honest I haven't owned very many of them.
 

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You ain't missing anything. They are a solid dependable racer but they don't inspire a lot of love :)

Not sure why...but just like Ken noticed...those angled cylinders and lack of front down tubes makes the 305's look pretty sweet.

And...why are we talking about this when we should be talking Ken OUT of doing it :) I seem to get stuck on "what if" questions just like you.
JohnnyB
 

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been running a 350 on the track for four seasons now. no matter what i do to the damn bike. i just can't get over how freakin lame the frame is. don't bother cutting up the cb frame... just build a new one.

mt
 

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Matt,
What I never understood is I don't recall seeing even one CB350 in SuperVintage running a custom frame.
A frame could probably be built 15 lbs lighter and stronger to boot.

Do you know anyone running one?
Maybe we should get together someday when I have a year with nothing to do and build a frame.
JohnnyB
 
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