KZ550 Cafe Rear Loop

I've got a build thread going on in the projects page, and after I cut off the rear of the frame, a couple members commented that I need to weld on a tail loop to make the bike rigid and avoid having an unintentional IRS. I don't think that's the case, but I figured I'd make my argument and see what everyone's opinions are.



Here's the 1981 KZ550 frame. For reference, this is before I cut the frame. The place I cut the frame is immediately to the right of the first yellow line, just beyond the rear shock mount gusset.

The green circle shows where the rear shock mounts. The red arrows show all the tubes that support this mounting point. You can see by the green point that the mount is directly below the meeting point of two sides of the triangle that support the weight and distribute the forces. There is also a crossbar welded close to this mounting point.

Now lets move to the rear I cut off. There are only two pieces of metal that connect the two sides of the frame in the rear, both marked in yellow. When I cut off the rear of the frame, I only removed the amount of rigidity these two pieces give to the frame.



Here's the first piece from the side. It's some fairly thin bent plate. It's made more rigid by the embossing of the edge, but less rigid by the large almost 90° bends in the entire bracket. It provides a mounting point for the rear fender, and I think some support for the seat.



This is the other metal, on the far left. It's also just thin plate steel, and full of holes. It provides a mounting point for the tail accessories.

In order for the shocks to move independently with this frame (with or without the rear cut off), the force would have to overcome the strength provided by all the frame part circled in blue. Based on the shape of the rear frame pieces I cut off, it seems really unlikely they were providing any rigidity against the kind of torsion that would cause that to happen.

The rear shock points are fully box gusseted, not just plate. You can see that in the photo below (post cutting).



So, I believe the frame was designed to fully support the loads of the rear shocks without the assistance of the rear of the frame. I think the rear of the frame was there to support the weight of the passenger and provide mounting points for the accessories. I don't see a tail loop as being necessary, or adding any required strength to the frame. What are your thoughts?

Basic physics. The long lever arm of the subfame lets thin piece of sheetmetal control the frame tubes sufficiently. The stamped tail plate keeps the tube spacing and keeps the frame more rigid. The little bridge shaped piece can do neither since it is not stamped into a reinforced shape by any significant amount. The bridge shaped piece is just for mounting plastic parts etc.

Think of it like a tuning fork versus a closed square. Which is more rigid? You currently have a tuning fork type setup. Tuning fork shape is chosen for it's ability to vibrate (move). That is not what you want in a motorcycle frame, at least up to a point. In the 1990's frames got to be "too rigid", so they backed off of the frame support, but a 1980's Kz doesn't come close to being "too rigid".

If it's just going to be a slow cruiser, then it's not an issue. If you want better handling, you need to put in a substantial loop to connect the rear of the frame tubes. It's not that big of a job, and will look much better too. If your worried about the tail being too long if you add a loop, you can cut the frame shorter, then attach a rear loop (with some detail work to connect the open pieces).

Those pieces you did away with don't provide a lot of strength independently but combined with The others they do. It would be wise to weld a bar across to make the two rails one.
Steve

If you don't weld in the piece you will have re-invented the Flexible-Flyer. There are plenty of forces at work back there when the bike is in motion, and you don't want the frame and shock geometry changing while you ride, so weld it in. Ed

Weld the loop in .Better to have it and not need it than to need it and not have it.

All right, all right, I'll weld in the tail loop. Rather, I'll have someone do it for me. Welding is one of the skills I don't yet possess, and mandrel bending requires tools I don't own. I was hoping to minimize the work I had other people do to the bike. You guys and your safety first mentality. :laugh:



Then here's how I'm thinking of designing the tail loop. I want to maintain the existing rear tire clearance, so my options are to go up or back with the loop. I don't want a long rear (trying to keep it close to centered over the rear tire, so I was thinking of using this shape (in orange). It would fairly closely follow the geometry of the fender bridge I cut off, but would be canted somewhat to the rear. I'd have it made from 1" DOM tubing, mandrel bend it, and weld it to the frame. That shape would integrate well with my design vision for the bike, while giving the wheel the travel it might need.

Any structural reason that won't work?

I like it, if it can be done. Sharp bends would be almost impossible with that type of tubing, so it will probably be several sections welded together. That would also help keep the frame short. Any decent welder should have no problem with it. The angular loop is more fitting with the styling of the bike in general, too.

Heres my frame with a shortened arse and the loop welded back in. Its not beautiful (Im not a welder but I thought Id have a go as I also got tired of farming all the work out to other people) but its strong.
Theres also a plate welded in under it to tie everything together

I normally like the factory tail section as is, but that homebrew tail looks good to me. (Good project to try out welding too, since it's not super critical on stresses.) I like it more than the typical rounded loop tails I've been seeing. To me they look a little bit too much riding an ironing board.

Well, for some reason, all the fabrication shops I call in town seem confused when I use the term "mandrel bending". I haven't yet found a place that can do this rear loop. It's getting a little frustrating. I don't want to cut up a bunch of small pieces and have them all welded together unless I have to.

Davido, what kind of shop did you have do yours, and if you id it yourself, how did you get the tight bends?

Agreed above, it is like taking one side out of a cube so it loses rigidity, Lou exlained it quite nicely. Convertible cars often weigh more and have reinforced floors and sills to compensate for the missing roof, again think losing one side of a box.

Don't get hung up on mandrel bending, you could do this just with a thick sheet steel plate
bridging the two subframe 'stumps'. Think ahead here, you need to mount a numberplate and rear light.

Personally i hate chopping the tails off old kz's the duck tail kz or later chiseled swage lined GPz tail is one of the nicest design features of these iconic bike imo, also gives you handy storage for tools etc. Just because you cut it off it would not be too hard to weld it back in. Good luck to you either way.

i have cut the center out of some old handlebars to use for a rear loop on a project before.
might be a thought.

leon