Promised swingarm mod

[jmnjrpa]

This mod will allow running a tire up to 5.6" wide.
It will be close but will completely clear RWL.
My camera does lousy close-ups but here it is.

Tools needed

and one good bench vise

Remove all races, bearings and seals.
Take a look at your swing arm. Note the concave structure where the tire would rub.
This mod leaves that area flat when complete.

Measure your swingarm in two places per pics.
The first dimension (1.56+) is where we grind from and second details the width of the material (.156) - our goal is .07 to .10 removal.



With the paint marker mark up like this. with the marker rolling over the edges about the width of a quarter.


Clamp firmly in the vise


First grind the angle down about a quarters width deep. Keep it even as possible and keep the grinder moving. Less is best in grinding. if the metal starts to discolor you need to move faster with less pressure.


Score the face several times with the edge of the grinder using the quarter for guidance. The paint is there for a visual. Caution -You can always grind off more later.


Start on one end and grind out iislands one at a time til you are finished.



Smooth up and blend angle with good radius.
Keep close watch on the measurement you are looking for =1.46/1.49"



Now blend other end with a good radius.
Make sure all is smooth with no gouges.


Finish up by grinding a smooth radius on all edges




HINT- do a good job marking and keep an eye on the marking and it will be a lot easier!

Now all that is left is strip and paint!

OBTW a couple points of interest- You have to remove or swivel the fender out of the way to install a 140 tire and you will have to give it a good push to get the tire past the brake dampening lug/final drive. This is the widest tire you can mount without major mods.

[jmnjrpa]

Other mods

Hopefully before summers out I will have the supersize tank mod and the podded airbox mod posted. Got a couple more going but they are not completed yet.

[XS1100 Newbie]

The mod is interesting, but i wonder how much that grinding weakens the swingarm. It would be rather nasty to have it break.

[jmnjrpa]

Industry standard

Is a minimum 100% safety margin. Typically that translates into -if it takes 10 lbs force to break something then it is built to withstand 20 lbs force. I maintain between 1/16 to 3/32 thickness. I modded mine 3-4 months ago and have put it thru hell. You take an acceptable loss in strength on that one side. You retain at least 85% overall strength or basically 70% over the failure rate at industry standards. Additionally, I have noted that Yamaha used a 200% safty margin on these bikes. That is the reason they are "bullet proof".

[crazy steve]

John, I'm surprised you didn't grind through; you don't have as much metal there as you think.

I did a similar mod, but I just cut that area out deeper and welded in a 'patch'. The interesting thing was, Yammy didn't use the same metal thickness throughout the whole swingarm. While the stock used for the 'u-joint' surround is .156", for the driveshaft 'tube' it's only .100" thick. So if you ground .070" or better off that, there's not much left. Here's pics of what I did...

[IMG][/IMG]

This pic shows the arm with the patch in, but before I 'covered' the exposed shaft. The shaft is actually sticking out about .050". Sorry, no pic of the open hole as my camera wasn't working at the time. I cut this a bit over .250" deeper than the stock relief, and fitted the patch 'inside' the hole, not 'on' it. Not wanting to run the shaft exposed and leaving an opening for dust/dirt to get in, I fabbed a cover from some .030" sheetmetal and welded it over the shaft, leaving minimal clearance between the shaft and the cover. All this gained me .210" additional room, so I figure you can fit up to a 6.15" wide tire. I've got a 160/80-16 I intend to fit; and yes, getting it in there will be interesting...

Did you weaken the stock arm? Undoubtably. But if you've been running it with no issues, I guess that's your proof that it works... but you might want to watch it.

'78E original owner

[jmnjrpa]

Steve,

Went back and carefully remeasured. You are correct in the center piece being less than .156 however I got repeated results of .118. I may have measured too fast on the first swingarm attempt (difficult to get to) and assumed it was the same. Could there be a difference in swing arms? I have done 2 now with no issues. The first I took down to 1.45. I don't like the welding idea because of potentual stresses and warpage however tacking a bar on top and bottom should not be an issue and will beef it up quite a bit. I was going for the simplest method that most anyone could do. You still have the limitations of interferance between the brake dampener lug and final drive for tire mounting so 140 is the limit. If following your method, you remove the final drive to mount the tire you might be able to get a 150 in there but that's a lot of extra work. Also if you grind out as you did I would consider tacking braces on the top and bottom. Though the real strength of the swingarm is most likely far understated and it will be just fine.

OBTW- hate to tell you this but a 160 measures 6.30" so you would still have to find a way to shim it over. Good luck and keep me posted!

[crazy steve]

John, I've only done the one, but I mic'd the piece I cut out and it was .1". Could Yammy have used different thicknesses in different years? Who knows... I've found enough strange little changes and oddball parts that it's possible. I still haven't figured out where the two oddball brake rotors I have came from (.050" difference in the 'dish', otherwise look identical).

The big issue with welding on these is they do warp a bit. I did carefully measure before/after and did have to 'work' the arm a bit to get the alignment back, but it wasn't a big deal. The 'filler' piece I used is .125" stock, so I'm not concerned about strength. I don't figure the welding process hurt anything as these are a welded assembly to start with.

Getting the tire actually on there... I haven't tried that yet. I have swapped the rear caliper to a modified 4-piston Harley unit, so I may not have the same issues as a stock bike. If I have to pull the FD to get it all on, well, that's a good time to lube the driveshaft splines. As to the actual tire size, I've got it in my grubby little hands and it mics out at 6.15", including a extraneous 'lip' that could be sanded off with no harm. This is a older Gold Wing size, designed for a 3.5" wheel; I figure the 3" wide special wheel will suck the sidewalls in a bit and keep it from swelling out too wide. I haven't mounted it yet, as I'm 'detailing' the wheels; this has gotten out of hand... the original plan was just to 'clean' them up for paint, but the castings are so rough and lumpy that by the time I got them reasonably smooth, that I'm now going to go for a full polish Another month and they should be ready to go....

'78E original owner

[jmnjrpa]

Yeah,

[QUOTE][ this has gotten out of hand/QUOTE]
I understand completely! You are right about the tire changing width somewhat when mounted. Don't know what direction cause it may depend on the tire. I do think you may be able to shift the tire some by shimming the drive but I haven't investigated that yet.

[crazy steve]

I think it will all fit as-is, but shimming the wheel/FD is the last-ditch alternative...

I'll note that this tire size is supposed to be 6.15" wide, so you can imagine my relief when it showed up and was that size.

'78E original owner