Swingarm Truss - 71 XS650

[Punkskalar]

Finally got some time to mess with Tevan's Resto-Mod Cafe' project. Since he is using a 71' Chassis, we knew would need to stiffen some things up. We will focus on the rest of the frame later, but for now, here is what we did to the swingarm..

I've been showing Tevan how to bend and notch tubing, but it was finally time to let him try it out for himself... Armed with some 7/8" OD tubing, an angle finder and the bender, he managed to get the upper truss/brace bent up in one shot...

The notching took a while to make it all fit properly, but his patience has paid off... I think he may have practiced on about 4 feet of tubing before he was finally comfortable with his notches...

Careful consideration was given to proper chain clearance and shock clearance. We had to double/triple/quadruple check the frame clearances as we went...

All welded up... TIG on 40 year old steel can be a real challenge, but we managed...

The factory had some fairly gnarly looking welds, so we cleaned up what we could... Not perfect by far, but much better than it was

We are cleaning up the old frame now, and might even get some time to make the tail section mounts here today...

 

[moparpat]

I'm planning to stiffen up my 72 so I'll be following this thread closely.I've compared the frame drawings of both the early and later bikes and plan to add gussets to as closely as possible duplicate the later frames.

 

[Punkskalar]

I'm planning to stiffen up my 72 so I'll be following this thread closely.I've compared the frame drawings of both the early and later bikes and plan to add gussets to as closely as possible duplicate the later frames.

Making it resemble a later frame won't be doing it any justice, as the later frames are uglier too

We'll stiffen up the motor mounts, add some bracing under the backbone, and a few other little tricks to keep it looking "original" and clean...

 

[ProSimex]

NICE, I like, I like

 

[Yamaha_chop]

Those are some pimpin' lap welds!

Swingarm looks great, very clean and functional.

 

[XSLeo]

Don't most swing arm braces go under the swing arm? Adding a tube across the frame just above the swing arm pivot is a good idea, also across theframe at the top shock mounts.
This will stiffen the frame where it flexes around the swing arm pivot.
Leo

 

[Punkskalar]

Don't most swing arm braces go under the swing arm?
Leo

If the brace is the same design, the Top/Bottom mount doesn't really change it's strength.. We chose to top mount it, as it will compliment the lines on the tank and tail section. Top mounting is not the easiest though, There is alot more opportunity to mess up as you have to clear the frame throughout the full suspension travel, clear the chain, clear the shocks, etc...

 

[moparpat]

Making it resemble a later frame won't be doing it any justice, as the later frames are uglier too

We'll stiffen up the motor mounts, add some bracing under the backbone, and a few other little tricks to keep it looking "original" and clean...

Keeping the frame looking original while improving its stiffness is my goal.Most improvements will be hidden by the tank,sidecovers,seat etc.Some will be more in the open like the front motor mount but should not be too obvious.

 

[Tomterrific]

Steel is hell for strong in tension but not so much in compression. I think this might make the engineering of a stiffening brace on the bottom of the arms lighter weight for the same amount of extra stiffness. I'll ask the chassis list for an opinion and get back.

Tom Graham

 

[Tia813]

Thats very cool, Hugh! Welds look great too. The early swing arm that flimsy to truss?

 

[Punkskalar]

Thats very cool, Hugh! Welds look great too. The early swing arm that flimsy to truss?

The early swingarms are actually shorter by 1.5" or so than the later model ones.. I'd think that the shorter length would make it a bit stouter since there is less leverage on it...

As for the brace being in tension or compression. IF the swingarm was trying to twist, one side would be in compression and the other would be in tension, regardless of top/bottom mount... I'll put it this way, I don't think its gonna be any less rigid than before Plus, it looks great in the chassis!

We poured the fiberglass last night for the seat pan... This thing is really coming along

 

[Tomterrific]

Top or bottom, probably a moot point.

A braced swing arm really does make a performance statment. Look! A fully developed old chassis. Nice work Hugh.

Tom

 

[pregrid]

Beautiful work as always Hugh. Following this with interest.

 

[Tomterrific]

Response from the motorcycle chassis list is that a swingarm brace can be top or bottom. One fellow modeled it with structural software. Tony Foale noted that a top brace was a bottom braced swingarm turned upside down. :^) But another comment was that, in GP racing, a bottom brace, being closer to the tire contact patch, is thought to give better traction under hard accelleration.

Tom

 

[1974jh5]

Nice work! Makes my welds look like mud dobber nests.

 

[mitchp]

Im not a factory engineer, but if top mount equals the bottom mount in all ways then its always better to keep weight/mass as low as you can get it. I have always admired Erik Buells motorcycle design. Mitch

 

[54inches]

Any updates?

 

[Punkskalar]

Any updates?

Not much to update, the swingarm is mounted, chain and everything clears nicely, now it's just waiting on us to finish the bike...

Hugh