I am designing an underslung swingarm. I have a couple of reasons for doing this.
1) I want to convert to chain drive and would like to use a sprocket brake. 2) It would really clean up the rear end by having the sprocket and brake both on the same side. Sprocket brakes are designed for softtail or hardtail frames and will not physically fit a straight sportster swingarm. The design I am working on will resemble a hard tail in appearance but I will still have the shocks. I will probably add some drilled gussets to a couple of stress points but this is basically what it will look like.

I will probably mock it up out of copper since copper is much more workable before I try to make a prototype.

Does anyone have any comments? Give em to me, either positive or negative, I can handle it.

 

Do you have an example model of a sporty so that we could see how this fits up to the bike? I'm into 3d modeling. I kinda see where your going with this and it seems good, but without giving your model a chromic material and putting it on a actual model, or even a side view of a sporty with your model super imposed over top the 2d pic would be great as well.


Just need to see this thing on the bike in somefashion to know if it will look right. :-D

 

I think that would look awesome. My only concern would be dragging the low part on a turn.

 

I have searched the inter web for some type of 3d drawing or model of a sportster frame but have been unable to find one. I think maybe I can take a picture of my bike and paste the side view of the swing arm onto it. Basically I have measured the existing swing arm and used those dimensions. That is the reason I want to make.a mock up before going much farther.

 

That won't be a problem. My 3d model was drawn using 1.5" tubing but I need to redo it using probably 1.25" x .120 (I think). I don't remember the exact thickness. It is intended to have the approximate same configuration as the lower part of a hardtail so if I corner that hard I will already be on the pavement.

 

That won't be a problem. My 3d model was drawn using 1.5" tubing but I need to redo it using probably 1.25" x .120 (I think). I don't remember the exact thickness. It is intended to have the approximate same configuration as the lower part of a hardtail and the pivot point has to be in the same place as the original so if I corner that hard I will already be on the pavement.
Overlook that last post. I screwed up and posted twice. This doesn't work to good on a SmArT phone.

 

I envision a lot of flex in that concept (handling issues) for several reasons.

The first is that the length of the arms are considerably longer than normal. Deflection (bending) is generally related to the CUBE of the length of the swing arm. (The length of your swing arm design is not simply the distance from pivot point to rear axle - it is the total length of the tubing before bending.)

From an eyeball estimate of your drawing, looks like the length would be about 1.5 times the normal swing arm length which would make it over 3 times more flexible. (Imagine making a banked turn at speed and having the rear wheel flex in a manner that doesn't track the direction you want to take.) Any irregularity in the road would also affect handling and would require your rear shocks to have increased damping to make up for the additional "springing" of the swing arm.

Round tubing is equally rigid - or equally flexible - depending on one's point of view, in all directions. Normally, rectangular cross-sections are used for swing arms because they are stiff in the two directions that are maximally loaded and most important. In order to obtain the same stiffness from the round tubing, you might have to double the diameter of the tubing, adding weight, cost and possibly increasing the required length of the axle.

If your intent is style (e.g. street chopper) rather than handling ... well, it's your decision

 

However he did say that he'll probably add some drilled gussets to a couple of stress points. Also keep in mind that HD used round swingarms on the earlier Sporty's and Big Twins. So no doubt that it can be done successfully.

Big Twin Swingarm



Sportster Swingarm

 

Certainly it can be done but please note that the earlier swing arms were short compared to the sketch of the OP. As I noted, stiffness is related to the length of the swing arm to the third power. Besides old Harleys were not exactly notable for good handling in turns - except for flat trackers

In fact the swing arm in the photo posted may even be even shorter than a modern rectangular cross-section swing arm (perhaps due to smaller tire diameters ?). Just visually estimating the length, it seems about 25% shorter which would more than double the stiffness.

P.P.S. Looking again at the old swing arm design above I noticed that the length of the round tubing is only about half the length of the swingarm - beefy steel forgings at both ends make up the rest of the length. That's a typical design example of the fifties and sixties, as I recall and my two British thumpers of that era had similar swing arms. The short length of the tubing kept flexure to a minimum - probably three or four times the stiffness that I originally estimated.

Gussets MAY help with stress although stress really isn't a problem anyway providing that the tubing is of adequate diameter and wall thickness. The problem is flexing as related to handling. BUT gussets, unless they are applied properly to the appropriate areas, frequently INCREASE stress. (Do a search for "stress risers".)

Modern swing arms, or any old ones that I can recall, didn't use gussets in the design except for the axle fishtail; with the possible exception of horizontal gussets on the "U" section at the front of the swing arm.

As mentioned previously, if the intent is style rather than handling then it's strictly a matter of personal taste and the amount of risk the rider chooses to take. I simply point out the problem - not to create conflict but because the OP requested constructive comments.

P.S. These are probably good reasons why a configuration like the original sketch hasn't been successfully done before -

 

The reason for combining the sprocket & rotor is so both are on the less visible left side of the bike.
Are you going to move the kickstand & front brake to the right side?