Rear wheel finally swapped.

[ZOA NOM]

 

[FJmonkey]

Some good observations and points have been made. I currently don't have a lot of free time to work on the bike. I will inspect what I can with the arm in place, and later I will remove the swingarm (when I replace the rear tire) and inspect the arm, bracket and modification for signs of deformation.

[ribbert]

Perhaps I should have chosen my words better. When I said I agree with everything you said, the inference was that I also understood everything you said, no further explanation or example was necessary.

Haha, I also did the course but it was 50 years ago and I have no idea what I got.

When I said I didn't believe it would compromise the integrity of the section, I was saying that such a modification would not exceed or even get near the factor of safety, which would be considerable.

However, on the point of greater strength on the compression side than the tension side, I don't agree. Yes, it is the case on solid tube, as per the several examples you gave, but not on hollow section.
If you apply a load to a hollow section until it deforms, it is the compression side that fails, it buckles and folds in on itself, if the swing arm was solid, it would fail from the underside as you suggested.

And then there's that great theoretical leveller, empirical evidence. There's Marks bike and there's my bike and probably many more. I drilled two holes longitudinally, approx. 10mm in diameter about half way down on the underside of my swing arm (for another purpose, not the brake arm) I doubt there is an FJ here that has done more miles, with heavy loads over worse roads and routinely bottoms out the rear end more than mine. Those holes were drilled over a 100k ago. There is no hint of deforming nor would I expect any.

The safety margin built in to these sort of components is probably something like 5-10 times the calculated maximum load.

We surely must have an engineer here that can weigh in with some professional cred? We need one smart enough to know the answer but not smart enough to stay out of the discussion.

Noel

[MACHV]

This is the beginning of a bracket I am working on that will bolt up to both footpeg holes. This is 1- 1/4 inch scrap I had laying around the shop. When done, there will be more meat on the back side (welder has it right now) and the total strength should be substantial enough, and yes, clear everything. The torque arm will ride between the two furthest holes and the bracket will double the sheer plane of any bolt put through there. .... because after the years, the money, the time, I'll be dammed if I am taking the swingarm back off, drilling holes in it or, buying/rebuilding another caliper, wasting a custom brake line, etc.

Pics when done.

[Bill_Rockoff]

We need one smart enough to know the answer but not smart enough to stay out of the discussion.

Noel

HAH! That's my favorite bit from this site since the last thing I read from Klavdy. I definitely meet the second criterion, don't know about the first.

Someone with a bit more time on their hands could certainly calculate what it would take to bend the swingarm there, in both unaltered box-section and drilled-hole c-channel versions, to determine what it would take to fail the altered swingarm in terms of rear wheel load. If you've really put 100,000 km on yours (and those recent pics look amazing, I'm very envious) heavily loaded to no ill effect, I can bring myself to believe that the holes don't weaken it enough to matter in that type of use.

I had assumed that 1) the aluminum in the swingarm has an equal modulus in both tension and compression, and 2) that the box-section is symmetrical, and that therefore 3) the top and bottom elements would fail at the same time, which would mean that weakening the bottom at all would weaken the entire assembly. Then again, a friend who went on to get a PhD specifically in materials told me 30 years ago "in grad school you learn how to solve the equations when none of the assumptions made in undergrad are true." Very likely, the strength you'd lose drilling a hole in the compression-side would be less-than-fully made up by replacing something in that hole.

MACHV, I don't think your problem with using the footpeg bolt is due to the footpeg bolt being too weak to withstand expected torque-arm force due to braking with the rear caliper. I think your problem has nothing to do with braking forces at all, and everything to do with the fact that the caliper moves with suspension movement, while the footpeg doesn't.

[MACHV]

Why would that matter? Both ends of the torque arm pivot and the front is on a ball pivot. If all parts are installed near perfect parallel to the driveline (which they will be), all axiis involved will be in the same direction. I feel the problem is entirely brake force related. The bracket I am making "should" ask the bolt to do "at best" half the work by allowing it to bridge a gap rather than hang in space.
Will go through some rigorous stops in a protected area, followed by close inspection before I feel comfortable.

Additionally,  I have to wonder if those who mounted the arm to the center stand bracket have experienced a similar issue. Granted, the bolt there IS larger but if the problem lay in "how" the arm is mounted... should be the same problem in that location.

[ZOA NOM]

Jesus christ, just weld the fucking tab on the bottom of the swingarm.

[MACHV]

Jesus christ, just weld the fucking tab on the bottom of the swingarm.

I can't weld aluminum. I don't know who does. In order to get my swingarm out, I have to remove my Penske. To get that out I have to remove the airbox, the carbs the gastank... BS. Suzuki didn't deem it necessary to mount to the swingarm, so now that I see this, I feel better about my solution.

[ZOA NOM]

My post wasn't aimed at you. The "engineers" above however... 

As for aluminum welding, take a look at these.

[MACHV]

My post wasn't aimed at you. The "engineers" above however... 

As for aluminum welding, take a look at these.

Ah. No problem. I skipped over those anyway because I have to work with what's on hand and at the hardware store at this point. Not spending any more $$$ on this conversion, other than the 12 pack I promised to the guy welding the steel for me.

[Bones]

This is the beginning of a bracket I am working on that will bolt up to both footpeg holes. This is 1- 1/4 inch scrap I had laying around the shop. When done, there will be more meat on the back side (welder has it right now) and the total strength should be substantial enough, and yes, clear everything. The torque arm will ride between the two furthest holes and the bracket will double the sheer plane of any bolt put through there. .... because after the years, the money, the time, I'll be dammed if I am taking the swingarm back off, drilling holes in it or, buying/rebuilding another caliper, wasting a custom brake line, etc.

Pics when done.

Looks like that'll do the job just fine, I might knock up one myself.

[Bill_Rockoff]

I would buy the guy TWO twelve-packs, and have him weld a tab on the swingarm and cut/weld your mounting arm to length so you can connect to that tab instead.

I don't remember pulling the carbs or airbox to R&R my rear shock, just the tank and the battery box.

It *does* look like Suzuki made theirs work on those early (mid-80s) GSXRs by using a really long caliper arm, kept nearly parallel to the swingarm, and with its forward mount far enough below and in front of the swingarm pivot to mimic its rear mount location relative to the axle - a four-bar linkage. I don't remember those breaking anything, so it must have worked okay. Yours broke something, so I'd put some serious thought into why Suzuki's worked and yours didn't if I were you. You want to make sure it doesn't bind anywhere during the swingarm's motion. Until you figure out what went wrong, I would be worried that a stronger bolt (or a sturdy plate at the front) won't keep you from breaking anything from now on, it would just make sure that something different breaks next time.

For what it's worth, later GSXRs mount the front of that caliper torque arm to a tab welded to the swingarm. The caliper moves with the swingarm that way, and stays stationary relative to the swingarm unless you're in the process of moving the axle to adjust the drive chain. Everybody uses a tab on the swingarm these days; everybody does it that way for a reason.

I understand the appeal of trying to make it work with whatever happens to be on hand. But if you don't have the right things on hand, it's just not going to work.

Shoot, Noel says he hasn't had any trouble with a swingarm full of bolt holes, maybe you could bolt a tab to the swingarm like he did. I'd still go through the side plates with a bolt running left-right, rather than going through a top or bottom plate with hardware running up-down.

[MACHV]

Mine hasn't broken.  I just finished the modern haven't even test driven it yet only to find out that after years of reading about this thing and finally doing it that bolts were getting broken.  And I am not pulling the swing arm back out of the bike until it's time to re grease everything in 15000 miles.  I already know this idea  Is going to work. I just hope the prototype becomes the finished  Products in time for me to actually test ride the thing before it starts snowing around here.

[Pat Conlon]

....In order to get my swingarm out, I have to remove my Penske. To get that out I have to remove the airbox, the carbs the gastank...

No you don't.

[ribbert]

Jesus christ, just weld the fucking tab on the bottom of the swingarm.

Rick, I believe I already said that at the outset (but not so eloquently) -

...."just weld a tab where it needs to go on the arm and move on to the next job, there's nothing to think about".....

Noel

My post wasn't aimed at you. The "engineers" above however... 

Thanks Rick, I'm flattered, I'll add it to my CV.

Noel