Honda CB400F Cafe, first attempt

I waiting on steel to be laser cut to add in, going to add it under the seat at a 45 degree angle


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You may wish to consider adding some tubes rather than gussets here. Your suspension arrangement will send most of the load more or less vertically up to where you have your shock mount welded to the crossbar of the frame. Since the seat mounting area is sort of stuck out in space, the suspension loads will create a large moment around where the seat structure attaches to the main frame. Tubes are very bad at this and while gussets will spread the forces over a broader section, simply placing another tube between where the cross bar meets the seat support down to the swing arm pivot will be an order of magnitude stiffer (not to mention vastly less prone to fatigue cracking) for a small (depending on your gusset plan)weight penalty. Rider weight on the assembly is also a factor and while those forces are more or less opposite to those of the suspension, it is unwise to design as if they cancel each other in any way. Not saying gussets won't work, as your design so far looks pretty robust, but gusseting is generally reserved for where you want to add strength and stability to a connection and there is not a practical way to do so otherwise, which is not the case here. Adding tubes will somewhat obscure the shock and linkage aesthetically - not sure if this is the driving force. Also, be advised that the steering neck to backbone gusset in your picture is a textbook example of very bad practice (though it does look nice) so do not follow it if your intent is sound structure. Pic shows rough idea of a typical good option. Gussets welded to face of tubes, not between tubes. One on each side of the structure instead of one in the center. Also, gusset to gusset connection making a "T" as in the original scheme is a bad idea. The gusset between the front down tubes is a not a bad idea (not that great either) but loading it up in the center with the other gusset in its weakest plane is renders both much less effective.
 

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@jpmobius, thanks for all the advise, could you please maybe draw out where you think I should put the tubes?

I was thinking of bracing the tubes with guuests, the vertical one I'm the middle of the frame.

Would you suggest two vertical tubes like goal posts in the middle of the frame?


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75hondacb400 said:
@jpmobius, thanks for all the advise, could you please maybe draw out where you think I should put the tubes?

I was thinking of bracing the tubes with guuests, the vertical one I'm the middle of the frame.

Would you suggest two vertical tubes like goal posts in the middle of the frame?


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First off, reading the thread and looking at your frame progress I want to suggest you might want to add a tube gusset as suggested by jpmobius (lower orange line). Add it so it's tying the upper shock mount with the swingarm pivot, preventing the upper shock mount (red) to move upwards. Another thing to take into account is the way you designed upper mount. When the suspension travels it's trying to make the brace that connects with the seat (left red line) to get away from the one that goes to the crossbrace above the swingarm pivot (right red line), as the green arrows show. I'd suggest you add an additional brace that triangulates the upper mount braces (upper orange line). This will also make the seat stiffer.

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As for the seat I'd follow jpmobius again and suggest this

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Then how long and the tilt angle of the seat brace it's up to you.

Hope this helps some! Cheers bud, keep it up.

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hi guys,

thanks for all the advise, i would take for the shock to come loose under my a**.

do you think my shock angle is to steep? should i have a better angle to it?

would you suggest i do similar to the pic attached,
‘69 Honda CB 350 – Cognito Moto
 

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It's not thats too vertical or too horizontal. This is something you had to have sorted before committing to welding suspension points on the frame.

You are not looking for some "shock angle". We can not give you solutions just looking at your bike and guessing if the shock is correctly positioned related to the swingarm.

This measurements, angles, and positions for the shock, swingarm, lower link attachment, and upper shock mount, etc, where data that you should have collected from the original donor bike in order to try to replicate the response and behavior of the suspension (vf400), on your bike.

P.S.: that bike serves nothing as a starting point for your shock. That build his a ducati swing arm and the motion links for the shock/swingarm are completely different from yours.

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The shock tower looks to be fine for the most part, the only thing being that the shock top is now located far forward of the original two shock mounts that really were support for the rider more than just suspension, keeping a balanced CG (center of gravity), now the ride position is cantilevered behind the shock top mount actually lightening the front some and even more so in high speed turns or rapid changes in elevation i.e. bumps and dips in the road. The shock (vertical) angle is always scrutinized but no angle has been left off the table by any manufacturer. I don't think any added tubes will change or move the direction of inertia... It's up and into the backbone now, just not at the optimal angle, the only way to change it is to change the angle.

As for gussets, you can read all you want, but unless you do a good job engineering them you may just be moving weak spots without actually building in any added strengths. And for the shock to work possibly a little better, a small tube from behind the ride position to the frame angled directly at the foot peg would help to balance the CG some.

Gusset Guide

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If I am wrong... see my avi sig :eek:
 
I'm thinking of adding some tube in this way please give any advise.
1.
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2.
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Or add gusset around where I have welded the tubes in, comes to around 6. I will try photoshop them in and send it to you.

Here are some extra photos
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Much better pictures. I didn't realize the shock location had such an offset. Since you are not that far along, I think I would redesign the upper shock mount a bit. But first things first. You need to determine the range of motion for the swing arm in order to decide on the proper upper shock mount location. Without getting into a broad kinematic discussion about these types of linkages, the primary goal of the original design is to provide a mechanical rising rate of motion to the spring and shock. The idea behind this is to have a softer and more compliant action for small deflections, but ever increasing shock/spring effort with larger and larger deflections. That way when you hit tiny bumps, the suspension easily gives way, but when larger ones are encountered, the suspension is not overwhelmed and bottoms out. This effect is provided by the large change in angle between the various linkage components as the suspension travels through its range of motion. For simplicity let us focus on the angle between the bellcrank and the shock. See the first picture. As you can see, your current angle is close to 90 degrees already, and as the suspension compresses, it will come up to and then pass 90 degrees. As it passes 90 degrees, the suspension will become softer and softer - exactly the opposite of what is needed. You need to remove the spring from the shock so you can move the suspension through its complete range of motion.

Unfortunately you have removed the original suspension and no longer know what the original top out and bottom out angles for the original swing arm were, so you will now have to estimate those positions and work from there. Don't underestimate how very important this is to do. Take the shock off and put a block under the swingarm to locate its lowest position, and then lift it up to its highest position and measure. Fixture the frame and blocking so this can be repeated over and over reliably without changing. Then you can put the shock back in (without spring) and see where you need to go. Obviously you want the shock to be fully compressed and fully extended with the arm at each end of its travel, but there is rather more to it which is where that bellcrank angle comes into play.

For your linkage, you will want the the angle between the bellcrank and shock to be at 90 degrees when the suspension is fully compressed. At 90 degrees, the shock has the greatest mechanical advantage over the linkage, and therefore resists further compression with the greatest force. As the suspension is relaxed, the angle of the bellcrank becomes more and more acute, and the linkage has more leverage against the shock which has the effect of making the shock easier to compress. This is what you are after. At normal ride height with the suspension 2/3 or so uncompressed, small bumps will easily compress the shock and spring because the linkage has a lot of mechanical advantage over it. As the suspension travels farther and farther and that angle gets closer and closer to 90 degrees, the mechanical advantage gets lower and lower making it ever more difficult to keep compressing the shock.

Once you figure your swing arm displacement, you can adjust the bellcrank to have the right orientation to the shock (to get that 90 degrees at full compression) by adjusting the length of the lower link and the upper shock mount location. Yes, this is a pain to do as there you pretty much need to change everything at once and subtle changes have rather a big effect. The good news is that you have everything mocked up so you have a starting point to help you determine where you need to go.

To illustrate, let us assume that your swing arm displacement is currently perfect where it is. You would want to take the spring off, and put the shock back on. With the suspension in its lowest fully uncompressed position, look at the bellcrank and lower link. You want the angle there to be 90 degrees. See picture 2. Then raise the swing arm to its max upward travel position and fully compress the shock. You will see that the bellcrank to lower link angle has become much larger than 90 degrees. The larger this angle becomes, the stiffer the suspension will be, so moving the links mounting point and changing its length will have a profound effect on the suspensions behavior. Now you can determine the upper shock mount location. Fully compressed, you want the shock to bellcrank angle to be 90 degrees.

Be advised that this is just a starting point, and that the behavior of the system is affected potentially dramatically by seemingly very tiny changes in any of the components or their mounting locations. But this should get you fairly close. And yes, this is a very fussy and time consuming process and likely you will have to mock everything up several times to get it right, but I am not sure how else you can proceed unless you can acquire an original bike to simply measure and copy exactly - which is not all that easy either.

As far as the structure goes, I don't think you need any extra shock mount tubes, but since I am sure you need to move the location, I suggest some different tubes. See pic 3 and 4. I would replace the tubes you have and angle them to meet the intersections of the existing cross and side tubes. A bit trickier to make, but not as bad as you might think. The way to do it is to get your shock positioned in the right place and clamp it here with something and bolt the upper fixture into it. Then just cut and try each of the four tubes one at a time. Once you have them all fitted up you can tack them in place and unclamp the shock and check the action by moving the swingarm through its travel.

As far as the side bracing goes, I would simply add one on each side similar to the original factory tubes. The strongest arrangement is in red, but you can alter the location rearward if you want a particular look or not obscure the shock from view.
 

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Thanks for all that advise. I will be printing it and reading it piece by piece.

OK so I got a buddy with a vfr 400, same swing arm. I got a bunch of measurements off this bike to set mine up.

Swing arm pivot = 420mm
Front end angle = 24 degrees

I have set my rear swing arm at the same measurements and front end angle. I will take the spring off the shock and move it up and down,and take more photos of angles. I have about 3cm play on full extension down. And have about 20cm going up, this is before the shock touches the swing arm.

I'm will take off the spring and play some more and add the tubes in like you suggest. Also speaking to another guy about cnc me a new dog bone to have more options of extension.

I was thinking of gusset to the frame on the front facing mount to the frame and then under the seat another set. Also on the side, but will draw it up and get it to you.

Thanks again for the help, if you were in South Africa I would bring you a case of beer.

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that is unfortunate
fella if you want a single shock bike buy one that came that way from the factory
you just dont have enough know how to make this work
god love ya for tryin man but if you want to design weld and fabricate you gotta learn how to do the basics take a community college class or 10 learn from an instructor in a shop setting first hand
but this single shock mania going on is a fools plunge down a dark mossy street with no lights
you cannot put a single shock in an old bike in that simple sense .,the old bike must be redisigned completely
a builder is far better off building a monoshock frame for the vintage bike from scratch the futility of doing it any other way (as you are attempting) reels the mind
there were and are companies making hi tech frames for vintage iron elgi vincents,spondon did tremodous work as did the rickman brothers bimota it goes on the frame,champion, redl;ine,cheney, trackmaster building cottage industry has faded but if you guys with this desire ,if you guys would actually educate yourselves you ma\y be the next frame builder with his shit togehere
get the tony foale software
read about those builders above mert lawill another the armstrong gang more brits in cold damp stone sheds filling miters in tubes with rocks
build a goddam scratch from raw stock frame !! study the greats as you learn to fabricate ,you arte not evern treading water,your drowng man, SWIM SWIM FOR IT !! get educated to where you can do it without sourcing strangers on the internet,before you start slamming a board to your face
bescause that is my only advice for this frame debacle
go outside pick up a rusty board
slam it into your face until the dumb goes away once you loose the dumb which is nothing to be ashamed of unless you do not recognize it, i been there pal believe me
once you loose it you can attempt to keep it at bay
thats all
 
xb33bsa said:
you just dont have enough know how to make this work
god love ya for tryin man but if you want to design weld and fabricate you gotta learn how to do the basics take a community college class or 10 learn from an instructor in a shop setting first hand

This is really all that needed to be said, but xb tends to get abrasive. I agree for once though, I only added a rear hoop as a modification and I paid a guy to do that, and even that guy hated mono shocks and refused to do them for customers.
 
Wow, I think you may need to step back and take a moment and see where you are going. You've really take on a biggie.
 
Tune-A-Fish said:
Stop welding man! you are not skilled enough to weld this frame,

I see no welding skill at all!

Your life is going to depend on the quality of the welding and frankly it looks like you have a death wish.
 
Hope you will not be put off your project. Yes, generally it takes a pretty fair bit of experience/training to make satisfactory welds on potentially life threatening assemblies, but you certainly can get everything tacked together yourself so someone more skilled can weld the critical stuff until you learn. Make as small and minimal tacks as possible. Anything not "just a tack" will have to be completely ground off so a proper weld can be accomplished, so just get everything stuck in the proper place. The ability to ask questions, make mistakes, and being ok with altering your plans to get it right is far more important than having expertise in every field at your fingertips. I have rather a lot of experience teaching people to weld, and without question, your project requires someone highly skilled at welding things like race car chassis. Should be pretty easy to find someone. They will be able to help you with fabrication as well, even if they are not motorcycle chassis designers. As long as you are patient and willing to seek out help in areas you are unfamiliar with, you should be able to successfully build what you want. Putting a lot on your plate means you may have to spend more time than expected at the table - it does not mean you have to scrape your meal into the garbage.
 
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