Monoshock rear conversion

Starting this thread to post WIP pictures of the monoshock rear conversion I'm doing. Won't go into reasons why, as they are entirely cosmetic. However, I expect the performance to be very good. Shock is from a BMW Montauk, which weighs exactly the same as the XJ750 Seca I'm working on. Position is as shown in the photo, which gives the required leverage ratio (3.4 - 1) which was computed both by finding spring rates and doing the math (double ratio = 4 times spring rate) and by measuring photos of a Montauk and figuring out the rate they use.

In the rear, this will be acheived by welding a plate to the swing arm that lays directly on top of the final drive housing (so it can't flex down). The mounting rails for the shock tabs will bridge across this weld, so it can't flex up. Metal will be same thickness as the final drive mount plate.

In the front, there will be a square tube running up from the cross-member above the swingarm to a new upper cross member. Braces will also run back from where the square tube joins the lower cross member (which is maybe 2" from the desired upper shock mount) to where the upper rail cross member intersects the frame (on on each side, somewhat asymetric) so the (very hgh) force of the compressed shock does't twist the frame and (at least partly) gets transmitted to the frame members originally intended to deal with shock force. I figure this bracing is a good measure against frame flex in general as well.

Since it's single sided, I'll need to MASSIVELY brace the swing arm against twisting. Again, not a bad thing from a general standpoint. To make room for the shock, I also have to bump the left shock brace (the one that holds the fender) out a bit. Not a big deal, since on my bike its not really gonna be handling anything except rider weight. Still, I plan to leave it as strong as stock.

Interestingly, these changes aren't so radical that I won't be able to just pull the new shock off, mount up the old ones, and ride away. Which is cool, because it means I can test both and see if the new version is actually as good / better.

Photo shows the desired shock position:

 

I think your going to need to change the angle of the monoshock so it will operate correctly. Either raise it up and create a upper swingarm frame to bolt to or raise the front of the shock to the backbone.
My suggestion would be to look at several monoshock style bikes to get a good idea of what will work. The current angle appears to limit the spring travel at that low position.

 

I know its different from most existing monoshock designs, but I did not just base that placement purely on aesthetics and guesswork and where I could easily jam it in the frame. There was also a f***ton of math and experimentation involved.

"Limited spring travel" seems to me to be another way of saying "very high leverage" - which is exactly what I want. This spring has a rate of 2000lbs per inch, and a throw of maybe 1.25" max. If it had a normal amount of spring travel (and hence a lower leverage ratio) it couldn't use most of it, and the ride would also be insanely harsh.

The stock seca units are 100lbs per inch each. Since it is 10 times as strong as both combined, I need to have 3.16 times the leverage to make it work. The stock seca units compress 23mm for each 25.4 mm of wheel travel; I need to cut that down to 7.25mm of shock compression per inch of travel. Which is pretty much exactly what the position in the photo gives. That's not a guess- it is verified by placing the frame in a jig, supporting the swing arm with measured blocks, and measuring the span where the shock would be using a calipers.

 

Well I hope it works out for you, will be following this build. Anything looks better than those original side shocks in my opinion. Keep the pics coming. 8)

 

So, you're essentially just running a sinlge spring/shock on the shaft side at a steeper angle and tying the swing-arm to the axle side with some sort of brace?

Assuming I'm understanding your plan (I'm pretty tired so this is a big assumption, haha), you don't have any tire clearance issues or whatnot?

I'm certainly going to be watching your progress. Seems like an interesting plan.

 

Corrupt Rev - yep, pretty much as you say. TBH, I haven't put the tire and shock both in the frame at the same time to see if it fits. It looks to do so, but the clearance may be pretty slim.

The main issue limiting clearance is the tube running from the swingarm pivot to the original top shock mounting point. If I chop that out, there's plenty of room (the shock runs through where it used to be). A concern (but a solvable one) would be re-building something equally strong (so I can still run the original shock setup for comparison) that doesn't poke out into any uncomfortable places. I can gain an easy inch just by welding a pipe along side it, and cutting out the bridged section, which is probablyh enough, but I may want more in case I upsize the tire or some such.

 

Did some of the prepratory work yesterday. Had to bump out the tube running from the pivot to the left shock mount to ensure I'd have wheel clearance. With the new suspension design, I could actually do without that tube, but I wanted something equally stout in place so I can run the stock setup if I feel like. Also lets me cross brace to the new shock mount, which will be taking HUGE loads, so I want to distribute them over as many well triangulated locations as I can. I may have over-done it.

 

hmm. its all such deep info to relay but makes so much sense when in front of you, I have been contemplating a similar suspension change,which i plan on modeling after my KZ550 All-trac, hopefully I can get it more aestetically pleasing than Kawasaki did, considering this will be a bare bones, single rider chop job of a bike i should be able to go a bit "minimal" comparitively. I got my monoshock from a three wheeler frame at the junkyard, and i must admit i haven't punched the #'s and done my homework yet. I think its pretty awesome that your actually figuring it out mathematically, will definitely be following this build to see how that works out for rideability! cheers!

 

When I first posted, I didn't realize that you were mounting the mono-shock on one side of the swingarm. Have you ever seen a mono-shock mounted this way before? I've never seen one. The reason for that is, a mono-shock has to give even stress relief on both sides of the swingarm. If it's mounted on one side, the other side will absorb more stress and be in danger of twisting or bending. Just looking out for you.

 

My BMW and several other bikes only have one side
On the swingarm, they may be a little beefier but still.....

 

I see a fly in the ointment -

The BMW (I bet) has real bearings in the swingarm - you're putting a TON of force on one bushing, like throwing the weight of a car on it ??

(Other than that, how was the play, Mrs Lincoln??)

 

There should be bearings there now.
sebwiers, did you figure spring rate compressed or extended, there should be quite a difference.
I think that's why they started using "dog bones"
To keep the shock from getting close to horizontal and spring rate consistent.

 

I'm concerned with the longer and thinner side. Maybe if you strengthen that side it'll be alright, just wouldn't trust it as is.

 

Oh yeah, swingarm would be getting as HELL of a lot of bracing, would end up like one of those huge fat sportbike swingers, made of steel. Extra swing arm weight isn't a good thing, but extra stiffness is, so it might balance out. Certainly no worse than a hardtail!

Spring rate stays pretty consistent through travel; it drops a bit near the sag point, and spikes up a bit at the very end, making it slightly progressive in both directions. Really nice match, from what I could tell. I've got the numbers at home in my notebook; I think it varied between like 9 and 7mm compression per inch of wheel travel in the actual zone of travel. Linkages probably give better control over the exact character, and also allow better shock positioning; there's obvious drawback to putting the lower shock mount shock so far back on the swingarm, then laying the shock down, from both an ergonomics and mass centralization angle. Start running a fat tire, and you back end would get very wide indeed.

The force on the pivot is potentially a real concern. I suppose I'll play around a bit, see if I can find something that pushes "down" more and "back" less. Funny how I thought real hard about the effects on the frame twisting at the pivot and and shock mounting points (pretty easy to brace for that), and ignored the pivot itself.

Good news is, a couple days back I bought a second swingarm from a junkyard (figured I might have a welding mishap, or use it for my mockup) so maybe I'll just give it a shot and replace the modified swingarm if / when the bushing / bearing go south. The setup is such that I can still run stock shocks, so if I find there's bearing issues, I can go back to the stock setup / look for a different solution.

 

Hope it goes well, good to have an extra just in case.

 

looks like it could turn out pretty cool, keep it up!

 

Just a thought....once you have everything in place would it be possible to put a ratchet strap on each side of the frame and after taking a few measurement cinch it down til it bottoms out, then check the same measurements for any flex. Don't know if you'd see anything by "eyeballing" but if you did you know that it would be worth looking into (pardon the pun) and adding bracing etc.
Say from centre of axle to a point on the rear most part of the frame or the furthest point back on the swing arm and point on the frame, maybe, kinda. :?

 

Even if I can't figure out any useful measurments to take, that's a good idea! Lets me test that all the important parts can take (at least for a limited number of times) the forces that would come into play on a "bottom out" sized hit, and also lets me ensure via actual testing that the wheel won't be slamming into anything during such a bump. (Woho, no crashy tire lockup death!)
Probably worth repeating that test every 500 miles at first, and making it a regular maintenance inspection even if it looks good. Can likely do the same thing on the front end (Hossack system) where failure would be much, much worse. Since I know my spring rates, I could tell how much load I'm simulating pretty easily.

 

OT: Sebwiers, did you see the private message I sent you?

 

Bump to avoid falling below the fold.

I ordered a spare swing arm from a junkyard, just in case. I couldn't get mine off, so the idea was to make it easier to template parts, and to have a spare in case I wreck mine.

Spare cam complete with bolts; looking at them, I see the instructions I had were either wrong, or I was confused by them.

Anyhow, after looking at the bearings, and thinking a bit, I decided to go for it, with my original design. I looked at some other setups, and they would all be just as bad, in terms of load on the swingarm pivots.

Did some welding today. Didn't template as much as expected, more just freehanded with tubing. Think its coming along, and it looks ... wierd. In a good way. Asked a former moto racer if he thought it would be stiff enough and he said "hell, with all those funny angles in there, I honestly can;t even guess." He did make the useful suggestion of spaning some open spaces with sheet metal, to turn it into more of a monocoque structure.

Should finish it up next week, get some photos.

 

Swingarm is getting close to ready for mounting the shock to. Added a prodigious amount of bracing in order to prevent twist, what with all the shock force being applied on one side out near the pumpkin.

 

What's the story with this baby? Any progress to report?

That thing looks like it'll support a couple of dozen XJ's lol (put it this way, if that sheet/plate is as thick as it looks, it ain't gonna twist IMO)

I'm wondering how much it would even "want" to twist anyway, seeing as our wheel is mounted on splines to the pumpkin and may not transfer a lot of force to the right end of the axle once it's bolted up, I'd expect this setup to make for a pretty stiff swingarm from the get-go (kinda like having a brace at the open end)

Now there's a thought, your monoshock design, but cut the other side off for a single sided swinger, VFR style... 8) Wonder if that'd work?