swing arm mod

[chiz]

Just a quick question regarding a swing arm mod.. this would be a steel swinger with twin shock. They say an under arm strengthening mod is always better because the mod will be in tension. Modern swing arms seem to mostly have most of the beef below the axle as well, anyway stupid me... what the heck does in tension actually mean? As an after thought would not a strengthening cage welded to the FJ swinger be of benefit?
Chiz 

[JPaganel]

Your main problem with going to twin shocks will not be the swingarm, it would be where the other ends of the shocks go.

The tail of the FJ is a separate subframe that really isn't all that strong. It's only meant to hold up your butt, not you and the bike. Putting the stress of the whole thing bouncing around  on two points of that frame seems a little dubious.

[Tuned forks]

Just a quick question regarding a swing arm mod.. this would be a steel swinger with twin shock. They say an under arm strengthening mod is always better because the mod will be in tension. Modern swing arms seem to mostly have most of the beef below the axle as well, anyway stupid me... what the heck does in tension actually mean? As an after thought would not a strengthening cage welded to the FJ swinger be of benefit?
Chiz 

But to answer your question Chiz, in tension means when something is being pulled apart.  In compression is the opposite when something is being compressed.  (Yeah I know, duh, but I wanted to be clear).  In shear means when something is experiencing stress that wants to cut it, like a bolt being sheared off.  Those are the main three stresses fasteners experience.

Joe

[chiz]

Your main problem with going to twin shocks will not be the swingarm, it would be where the other ends of the shocks go.

The tail of the FJ is a separate subframe that really isn't all that strong. It's only meant to hold up your butt, not you and the bike. Putting the stress of the whole thing bouncing around  on two points of that frame seems a little dubious.

You misunderstand me sir I would not add twin shocks to the FJ just a strengthening cage to the bottom of the arm. At present I am talking about my XS 850.

[chiz]

Just a quick question regarding a swing arm mod.. this would be a steel swinger with twin shock. They say an under arm strengthening mod is always better because the mod will be in tension. Modern swing arms seem to mostly have most of the beef below the axle as well, anyway stupid me... what the heck does in tension actually mean? As an after thought would not a strengthening cage welded to the FJ swinger be of benefit?
Chiz 

But to answer your question Chiz, in tension means when something is being pulled apart.  In compression is the opposite when something is being compressed.  (Yeah I know, duh, but I wanted to be clear).  In shear means when something is experiencing stress that wants to cut it, like a bolt being sheared off.  Those are the main three stresses fasteners experience.

  So how are the bits welded to the bottom of the arm being "pulled apart"?

Joe

[Tuned forks]

Viewed from the right side of the rear wheel, when you are moving forward, the wheel rotates clockwise.  When you apply braking pressure, the caliper wants to follow the direction of the rotor/wheel which is clockwise as viewed from the right side.  Therefore, the caliper puts the torque arm under tension when the torque arm prevents the caliper from rotating with the rotor.

Joe

[Pat Conlon]

Think about the swing arm as a beam.

[Millietant]

Your main problem with going to twin shocks will not be the swingarm, it would be where the other ends of the shocks go.

The tail of the FJ is a separate subframe that really isn't all that strong. It's only meant to hold up your butt, not you and the bike. Putting the stress of the whole thing bouncing around  on two points of that frame seems a little dubious.

You misunderstand me sir I would not add twin shocks to the FJ just a strengthening cage to the bottom of the arm. At present I am talking about my XS 850.

That's an interesting idea Chiz, as Pat notes, the under arm bracing helps resist the tension forces that apply to the lower end of the swinging arm. But also, in many of the newer bikes, the bracing is always more substantial on the drive side, presumably because of all of the torsion and tension from applying all of the drive forces "off centre" to the rear wheel, and in many cases, the real "bracing" is along the top of the swinging arm on the drive side (as per Aprilia RSV pic below) - presumably because the top of the front sprocket is effectively using the chain to try to pull the rear sprocket/wheel forwards (whereas the chain has "slack" on the underside of the swinging arm).



In the case of your XS, as it has shaft drive, I'd guess the way the drive forces act is slightly different, given the bevel drive in the rear hub and the drive shaft running through the swinging arm tube. Withhe larger diameter tubing and thicket wall thickness, I'd guess your swinging arm is already much stronger on the drive side than on the brake side, so the addition of bracing might be more of a cosmetic benefit than a noticeable performance benefit - but I do like cosmetic benefits, would be nice to see pictures if you ever get it done.

[Millietant]

If you want any ideas in how you could modify your XS, this photo is something I'd suggest you use as a target 

My favourite XS 850 of all time.

[chiz]

Certainly is gorgeous but I am supprised to still see the shaft usually by this amount of work the builder has gone to the expense of ditching the shaft.
  But notice the large round
chrome cap covering the swing arm pivot shaft.... I is not a shaft at all but a short pin threaded on the outboard end that is screwed in to the frame and locates in the swing arm bearing the opposite side is the same. The adjustment of these pins locate the swing arm central to frame. It is one of my beliefs that this arrangement is not nearly as stable as a through shaft'
Chiz

[Millietant]

You're probably right, but if the end bearings are tapered rollers and everything is properly bolted in place and adjusted, then I'm sure there won't be anything amiss that you'd ever notice in normal road use.

Looking forward to updates on progress with the bike