Topic: CB550 swing arm shock position relocation

[squirley]

So I have finally had time to get some welding done on my 550 frame.  I was making a rear wrap around hoop, and I think that I may have made the back a little to short, especially by the time I get my extended tank on there.  Anyhow, I am looking for opinions with the position of the rear shocks.  I have decided that they look better following parallel to the frame bar, and would like the top eye mount to be located at the brace and seat intersection for support reasons.  With the upper mount up so far, I need to move up the mount on the swingarm; I am wondering what aspects of the riding condition may change if I move the bottom mount up to somewhere around 3 inches from the factory location.  Any input/ideas are welcome and I will post pics as soon as possible, just realized I can't post up as I am not at my personal computer.
Jens

[squirley]

Got a pic

[Bob550four]

as you have it now, I think, but not 100% that it should work alright.   as long as you're not bottoming out your back shocks,(and you shouldnt be anyways) that configuration ought to work fine.

[FunJimmy]

Jens

Changing the lower shock location in relations to the pivot center only changes the leverage ratio of the suspension. That being said, spring rate is the significant change from the current location. If your shocks where too soft, then they will be softer after that modification. Solution: stiffer springs. Dampening will also change effectiveness from what it is. Time to order some custom valve Works Performance shocks.

Like to location though.

[crazypj]

For same shock travel your going to have a significant increase in wheel travel. You may also find swing arm flex is more of an issue as you now have bending load in 'middle' of swing arm. Changing to heavier springs will only make things worse.
 Just my opinion
PJ

[squirley]

Just to clear some things up, the shocks will be shorter than the ones in the pic, so the force of the swingarm will be level with the bike, and not in the negative arc as shown.
Upon first thoughts, I would think that a stiffer spring would be the opposite of what I need if I was choosing to keep roughly stock spring rate.  By moving the spring up, I am reducing the ability of the swing arm to absorb small disturbances, so wouldn't I want a less stiff spring?

Crazyjp, I am wrapping the swing arm in a brace, I just haven't decided if I want to wrap above the swingarm or below, there shouldn't be a problem with bending after that.

[crazypj]

You moved the 'fulcrum' (the lower shock position) Originally it was near axle so the leverage on swing arm was minimal.
 Its probably going to take quite a lot of bracing to keep it rigid
Even with a shorter shock you will need a lot heavier spring to maintain 'stock' spring rates.
If you moved it forwards by 3" a starting point would be whatever percentage of total swing arm length you have (eg, 18" swing arm, 3" = 16% heavier spring) As stock 550 springing is way too soft (I fitted CB750 shocks/springs to mine) about 20~25 % heavier would probably be a good staring point.
 Looks cool though 
PJ
 

[TwoTired]

Anyhow, I am looking for opinions with the position of the rear shocks.  I have decided that they look better following parallel to the frame bar, and would like the top eye mount to be located at the brace and seat intersection for support reasons.  With the upper mount up so far, I need to move up the mount on the swingarm; I am wondering what aspects of the riding condition may change if I move the bottom mount up to somewhere around 3 inches from the factory location.  Any input/ideas are welcome and I will post pics as soon as possible, just realized I can't post up as I am not at my personal computer.

It looks like you are going to put more force on the swing arm pivot as the swing arm moves up under G loads.  This is because the swing arm upward movement does not directly compress the spring, but is angular to the swing arm mount arc plane.  Mark a point where the lower shock mount on the swing arm is in at it's uppermost and lower most positions.  Draw a straight line between these two points.  The shock position on the bike should be in the same plane as this line.  If not spring compression forces will be transferred to the swing arm pivot bearings.  In other words, as the spring is compressed, spring forces try to move it rearward as well as downward.  What keeps it frrom moving rearward is the swing arm pivot.  Expect faster swing arm pivot wear.

Moving the shock pivots closer to the swing arm pivot gives more mechanical advantage to motion/forces at the axle point.  Therefore, a stiffer spring and higher damping strength of the shock will be required to counteract those increased forces.  Also, there will be more flexing of the swing arm between the lower shock pivot and the axle, and no bracing to keep each arm side flexing the same amount.  This will allow one side to flex more than the other under variable down force and side force loadings.  The stock shock mount position was very near the axle, plus there were stiffeners on the arm to negate this flexing differential side to side.  The stiffer spring required and moving the fulcrum of bending forces closer to the swing arm pivot will tend to make the swing arm bend near the lower shock mount on the swing arm.  You are going to need a much stiffer swing arm if you plan to do any canyon carving.  However, it will work fine sitting on the floor of a bike show.

That's my opinion, anyway.

Cheers,

[squirley]

Not trying to revive a worthless thread here, but I scraped the shock position moving up on the swingarm.  I think that I am going to go the route of the picture below.  Please feel free to add input and/or advice/comments.  The chain side is gonig to be a real challenge in order to make room for the chain.  The brackets holding the shocks are just temporary to get an idea of positioning; some nice, strong ones will be made for the final.  Now, the rear set brackets need to be redone because they just don't match the frame

[FunJimmy]

Wow!

You really dive in don't you?
Looks very interesting. Can't wait to see it come together.

FJ

[turboguzzi]

regarding your first mod

agree with most comments here. moving the lower shock mount forward was done in seventies 'crossers to INCREASE wheel travel for a given shock.... pretty sure that is not what you'd want in a sport road bike.
More serious / dangerous seems to me the repositioning of the bracing tube, I mean the one running diagonal from the top shock mount to the middle of the rear down tube, joining it near the motor mount ear. It's not good practice to load a tube sideways in the middle (that tube is going to pull under shock loads), hope it doesnt brake on ya.

Regarding the second option,
looks more sound but I believe that your leverage will still be too soft / give too much wheel travel. A good idea to check this would be by removing the springs from the shock and checking how much wheel travel the set up gives you. wouldnt go for more than 4" at the wheel axle. Another thing is that the angle between the shocks is going to introduce side forces to the rear shock mounts. I think you should add a cross memebr to tie the new two curved tubes.

My 0.02

TG

[squirley]

I agree that moving the shock up the swing arm was not the best of ideas, but as you can see I am constantly changing ideas, some for better and some for worst unfortunately.

As for the repositioning of the diagonal tube, I can't see how there would be enough forces in any direction to alter the state of the tube.  It may be side loading the engine down tube, but there's not enough forces to actually do any deflection, especially if I reposition the shocks to tie in to the main frame.

For the new set up, side forces are inevitable, but believe me when I say it will be fully gusseted, and the tubing that I am using is .120 wall dom left over from a roll cage project.  I will take the springs off and see how much travel this system gives me as you have suggested.

[crazypj]

I like that a lot better. You'll still have to mess with spring rates but all the loading goes directly into steering head, the top tube is in compression.
 The CB400F I did with similar set up (using RD350 swing arm) handled beautifully
PJ

[turboguzzi]

I believe you

:-)

And BTW, take a look at the new Kawasaki 650 twins for inspiration.

TG

[DarkRider]

just so im clear on the second set up...you are now making the bike a mono shock in the rear? if so..why not look at using a sport bike rear shock..because right now you have a mounting simular to them...actually from what i see it looks a lot like the CBR954 mounting scheme..

[CrisPDuk]

It looks to me like it will stay dual shock, but he's only mocked up one side so far, the 'easy' side

[squirley]

Yes, just the easy side right now, I am still playing with bends on the other side to see how I can miss the chain.  So, I would like to do two shocks as I am a man of symmetry, and it looks very cool from the back with two.  As of now it has a very "mechanical" look to it.
However, I still haven't decided if this is the route that I will go, as it has been stated before I normally do vintage style rebuilds, and I feel that this one may be getting out of hand.  It is easy to take a nice looking stock bike and get sawzall happy, I need to learn to tell myself no.  So I am not looking at this from a one sided perspective, is the bike loosing its looks/abilities?

[squirley]

crazypj, do you have any pics of the 400 you did?

[crazypj]

Unfortunately I don't have any.
I did take an awful lot at the time during the build and when finished.
They were in Britain, my friend who had them was killed in a car crash 2 yrs ago. ( she had married so don't know if they were even kept?)

PJ

[FunJimmy]

Sorry to hear that PJ.

[dusterdude]

damn that sucks

[squirley]

That is a bummer pj, sorry to hear that.
Do you have any helpful hints, I am trying to cad the swingarm to see how I have to bend the tube to clear the chain.