Topic: Pin2Hot's 73 CB750

[calj737]

Plenty strong! And you'll have an easier time welding that tubing to your stock frame.

[Pin2Hot]

I'm meeting with a local guy who's willing to take a look at the rear hoop so let's keep our collective fingers crossed!  He seemed optimistic that the design I'm after is feasible which is an even bigger bonus.

In the meantime, I was able to solve a couple problems I was facing.  I got some DOHC swingarm spacers in to help position the rear wheel and line up the sprocket.  So far things appear to be working out quite well.  The new chain shows up in a few days so I'll be able to see if it truly lines up.  I ended up using the brake side spacer from the DOHC swingarm and the stock K3 drive side spacer (which is thinner than the DOHC drive side spacer) on the drive side.  That put everything nicely in line.  IMPORTANT NINJA EDIT, that did NOT put everything nicely in line.  I'm posting an update now with my issue and possible solutions.

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I also got some xylene and wintergreen oil and mixed it up 70% / 30% (respectively) to soften up my carb boots.  Man it works like a charm.  I left them in the solution for about 8 hours and it softened up the connection points just enough for them to slip on the cylinders and the carbs.

Please ignore the dusty engine:


I also got the carbs all put back together although I'm missing one #120 jet and I'm going to replace the small machine screws that hold the slide stems in place.  It's possible, dare I say it, to fire the engine up soon.  When that happens, I assure you it will be recorded in video for allllllllllllllllll to seeeeeeeeee.

[Pin2Hot]

So things aren't lining up on the rear end as I'd hoped.  I'm remembering a certain calj737 say "hey, chopping that swingarm like that probably isn't the best approach".  Welllll turns out I think you're right.  I found this out as I was double checking my measurements for my monoshock hoop and realized just how offset things are.  I kept noticing that the monoshock wasn't pointing at the center of the tire...

Closer measurements of the wheel position in the swingarm show that the DOHC brake side spacer and the K3 sprocket side spacer don't actually put the wheel in the center.  It's close but not great.  BUT what's worse is that the entire wheel is not on the center line of the bike.  In fact, it's 3/8" off to the right.  Moving the wheel to the center line of the bike does two things:

1) offsets the wheel in the swingarm
2) moves the rear sprocket to the outside (further left) of the drive sprocket

Possible solutions:

1) Chop off 3/8" off the left side (sprocket side) of the swingarm pivot to re-center the swingarm to the centerline of the bike.  That then requires a 3/8" spacer on the right side of the pivot to keep things tight.  But this doesn't solve the sprocket problem.  I know there are 3/8" offset sprockets from both CognitoMoto and CycleX but I can't tell if it offsets it to the correct direction (I need it to offset further out).  Possibly a 77-78 sprocket carrier will fix the issue?

2) Get a new damn DOHC swingarm and start again and do it correctly.  Still doesn't solve the sprocket problem but maybe the offset sprocket or the later model carrier will fix it.

I'm gonna dig through the forums and see if anyone else has run into this before but damn this sucks.

[calj737]

A 3/8" offset sprocket moves the teeth away from the motor, so its the correct direction for your issue.

Post up some pictures again of the swing arm and how things sit currently (too lazy to look back through the thread). Maybe some other ideas will surface.

[Pin2Hot]

I'll post some new photos tomorrow.  Here's what's keeping me up tonight though: how is my sprocket offset when I haven't changed anything related to it?  It's the same cush drive and rear sprocket with the same drive sprocket.  It should line up!  I've got to have something flipped around or something dumb.

[RAFster122s]

Did I miss something...you have a DOHC swingarm rather than the SOHC swingarm that is what is different and it changed your offset on the sprocket.

I'll post some new photos tomorrow.  Here's what's keeping me up tonight though: how is my sprocket offset when I haven't changed anything related to it?  It's the same cush drive and rear sprocket with the same drive sprocket.  It should line up!  I've got to have something flipped around or something dumb.

[Pin2Hot]

Did I miss something...you have a DOHC swingarm rather than the SOHC swingarm that is what is different and it changed your offset on the sprocket.

What was bugging me last night, and I probably wasn't clear, was that the rear sprocket wasn't lining up with the drive sprocket when the wheel was placed on the center line of the bike.  I expect there to be an offset within the swingarm but not in terms of chain drive and wheel center line.  Both items are stock and have not changed, i.e. the width of the sprocket/carrier hasn't changed.

BUT it turns out in my haste/anger/hot-ass-Missouri night, I wasn't measuring properly.  I checked this morning and it appears the sprockets ARE, in fact, lining up.

Sprockets lining up:


Offset within the swingarm:


Gap on the brake side:


Overall look of the rear-end with wheel on center line:



I still have a 3/8" offset within the swingarm itself which requires a solution.  I need the wheel to be in the dead center of the swingarm because the monoshock hoop will visually amplify any offset as it'll be very close to the tire itself.  I'm think I'm going to remove 3/8" from the left side of the arm pivot to bring it back in line.  I have another backup backup plan but I hope it doesn't come to that.  Stay tuned.  Thanks for everyone's insights.

[calj737]

Shortening the pivot tube length changes the bolt length, and then the arm won't fit between your frame.

Slow down, and think through the dominos of your changes.

[Cafemoto]

Awesome work so far!


Sent from my iPhone using Tapatalk

[Pin2Hot]

I've definitely been slowing things down a bit to ensure I get this right.  I got myself a nice little laser to make sure I know where my centerline is.  I measured it in a few different ways, putting the laser close, far away, etc and came up with a good mark on the swingarm for where the centerline SHOULD be.

Frickin lasers:


I then used a small airsoft BB to find the true center of the swingarm (I'll probably use a heavier ball bearing to double check this measurement):


Annnnnd finally, a new DOHC swingarm (on bottom) to modify once I get it right.  I know my current swingarm is pretty much useless at this point so I'm going to make my correction on that one, check fitment and then transfer the right measurements to the new swingarm.


One thing I've also learned from this huge fckup was that my rear hoop is also not in the center!  So that's gonna get corrected during this swingarm swap.  It's pretty amazing how satisfying and infuriating this process can be.  But I've definitely learned how to deal with my problems one at a time and focus on the problem in front of me. 

One side note (while thinking of problems I've encountered) can I just curse the tiny screws that hold the carb slider stems in place?  I purposefully got new M3 10mm screws to replace those because they cause such a problem and IN THE PROCESS OF CHANGING THEM, I ruined an entire slide by trying to remove a stuck screw.  I got a replacement and everything is great but come on!  /rant

[RAFster122s]

Need the micro blue wrench when working on the small stuff.
You are not familiar with the blue wrench?  It is your torch and its blue flame, in this case a small one to have a fine jet of flame for more precise application of heat.

[Pin2Hot]

I've been messing around with the swingarm spacing using my now-useless swingarm.  Let's just say the DOHC swingarm modification is NOT as simple as "just cut all off from one side" if you're looking to be symmetrical on all fronts.  That is, center the wheel to the swingarm and the swingarm to the frame.  Because I'm converting to a monoshock suspension, I need everything to be centered relative to the backbone of the frame.

After millions of measurements I think I got it sorted.  It'll end up requiring custom wheel spacers but the wheel is in the "visual center" of the swingarm, the swingarm pivot is along the centerline of the frame and the sprockets line up.  I ended up cutting off 0.385" off of the chain side of the swingarm pivot and that placed things where I needed them to be.  I haven't performed surgery on my new donor swingarm because I'm waiting for my monoshock hoop to come back from my guy.

Speaking of which, the hoop is also asymmetrical due the shape of the swingarm.  Long story short, pretty sure my fab guy hates me now...  I'll post pics when I get it all mocked up. 

Here's the useless arm fitted in place, note the gap on the right side:


I also go the rear sets were mounted this weekend.  They're not fully shaped and/or pretty but the rear set mounts are tapped and things appear to fit.  Of course, now that these are fitted, it's opened up a whole new world of problems but I'll deal with them.

Rough shape with CBR900RR sets in place:


Both sets mounted:


Full shot of the bike with rear sets mounted.  Note the custom cardboard seat.  Sitting on the bike has made me think the upward angle of the seat is a bit much.  It pitches me a bit farther forward that is comfortable so I'll revisit the angle once I get the rear suspension on and the bike sits normally instead of on jack stands:


Finally, the shift side of things.  Here's where it gets interesting.  I could make up some crazy linkage system to get it working or simply flip the shift lever around from its stock position and get it under my toe.  This obviously shifts the pattern to GP style, 1st is all the way up, 5th all the way down.  I'm willing to entertain this idea simply because it's an easy fix and it's my only bike.  If I had multiple bikes this may cause some confusion and not be good.  The old peg / engine mount bolt will be shortened so ignore that.

[LokisTyro]

I really like the upward angle on the seat. Maybe some extra foam up front would help, but that may become uncomfortable having your bits mashed into the seat. Careful shaping might work unless you're going for a flat seat. Just a thought. Nice build so far.

[Pin2Hot]

I really like the upward angle on the seat. Maybe some extra foam up front would help, but that may become uncomfortable having your bits mashed into the seat. Careful shaping might work unless you're going for a flat seat. Just a thought. Nice build so far.

Thanks dude.  The seat will be pretty flat, essentially embedded within the seat frame itself.  It won't be the most comfortable seat in the world but I'm used to riding a supermoto so it'll be a step up.  Plus, this ain't gonna be a long haul machine so an around-the-town trip will be about the limit for the seat.

Got the rearsets figured out last night.  All it's gonna take is a simple modification to the shifter arm on the transmission itself.  I need to add 3/4" to the bottom of it so that it reaches further inward to get the linkage arm away from the shift lever, see below:


Overall:


I've always kept the parts that come off of bikes and from parts I've replaced from being an idiot.  I kept the broken shift lever from my first big crash on my old Wee-Strom so I'm going to cut part of that off and use it to lengthen the shifter arm.  Hopefully no one gets too upset that there's a tiny bit of Suzuki on this bike...

[Pin2Hot]

My guy delivered the bend to me last night and I got busy mocking things up.  This allows me to finish a lot of outstanding problems I've been facing such as exhaust final position / mounting, seat angle and monoshock placement.  Lots of things moving quickly now, getting exciting!

Monoshock mounting:


HOOOOP DREAAAAAMS 


I'm not certain where the bottom of the monoshock will get officially mounted.  Obviously I want it directly on the end of the hoop directly in line with the shock but there may not be enough room.  Once I cut down the hoop to size, I'll be able to formally fit it up but I mayyyyy end up having to put the mount 90 degrees out of line on top of the hoop.  We'll see.


Preparing the new donor swingarm for final fitment


I also got the shifter arm mofidied.  I'm almost embarassed to put this on the internet . . . look, aluminum is hard, OK?  I'll shape it up all nice and neat soon but it works!


Lots more to do but it may be rolling very shortly.

[Rnobx67]

Nice job on your bike, you have done great work on it

[calj737]

I also got the shifter arm mofidied.  I'm almost embarassed to put this on the internet . . . look, aluminum is hard, OK?  I'll shape it up all nice and neat soon but it works!

We all can't be Instagram welders! Good on you for showing the ugly side of fabrication! And yeah, aluminum is hard. Cast aluminum is even more "harder"  Strong is far more important than pretty when your fabricating because a grinder and buffer can make pretty from ugly. But it can't make strong from weak 

[SOHC4 Cafe Racer Fan]

I'm sure everyone would like to make "weldporn" dimes but sometimes you get posted on @thefabpolice. I'm no one to talk.  I don't know how to weld, yet.

[Pin2Hot]

I'm sure everyone would like to make "weldporn" dimes but sometimes you get posted on @thefabpolice. I'm no one to talk.  I don't know how to weld, yet.

Frantically googles @thefabpolice . . . . ok good, I'm not there. 

[SOHC4 Cafe Racer Fan]

I'm sure everyone would like to make "weldporn" dimes but sometimes you get posted on @thefabpolice. I'm no one to talk.  I don't know how to weld, yet.

Frantically googles @thefabpolice . . . . ok good, I'm not there. 

You are nowhere close! But it is an entertaining IG.

[Pin2Hot]

It rolls.  For the first time in 9 years, this bike rolled on its own two wheels.  Ok it actually fell off the jack stands as I was putting the rear wheel on TO roll it but I caught it in time.  I was then stranded in my garage holding it up until my wife heard me yelling to come help.  I should probably figure out the kick-stand . . .

I spent all weekend working the rear hoop to get it into place.  I needed a way to find the center 10mm of the hoop tubing so I printed a little jig to help me measure it.  I placed it over the tube and marked the center channel to remove so that the hoop will slip over the vertical swingarm . . . arms.

Jig on tube:


That allowed me to cut out the proper channel using my trusty 4" cutting wheel.  It was a lot of take a bit here, measure, take some more, measure again.  But eventually I got it.


On the bike with shock in place:


From the rear:


Wheels:


I also spent a lot of time fixing some out of whack tacks I had.  Basically, the entire bike is straight and aligned.  I was able to sit on it and feel the suspension work which didn't compress nearly as much as I thought it would so that's good news.  Gives me more range of motion than I expected.  I'll be gusseting the rear hoop with three, maybe four support strut things so don't worry, I'm not leaving it hanging out there like that.

I'll get the exhaust hanger figured out next and make all of my final welds as a lot of tacks are holding things in place at the moment.  It feels like I've turned a corner on this project and I'm already thinking about disassembly for paint.  Exciting stuff.

[calj737]

Before you install any hoop struts, plan for the chain alignment to avoid any interference on the left side. 2 struts should be plenty from either side of the shock bosses.

[SOHC4 Cafe Racer Fan]

Pin,

For those slow folks like me, can you explain what considerations and "math" you have done to figure out the angle of the rear shock mount.  Since it's movement is not in a direct line to the compression of the shock, how does that impact those calculations? Also, are you using an off the shelf shock or having one custom built for your application?

[Pin2Hot]

Pin,

For those slow folks like me, can you explain what considerations and "math" you have done to figure out the angle of the rear shock mount.  Since it's movement is not in a direct line to the compression of the shock, how does that impact those calculations? Also, are you using an off the shelf shock or having one custom built for your application?

I honestly don't have much math to back up my design here.  But since the angle of the shock is slightly off the straight line by about 10 degrees, most of the force is going in the horizontal direction.  ForceHoriz = totF * cos(10).  ForceVert = totF * sin(10).  So let's say totF = 2500 Netwons, ForceHoriz = 2462 N.  ForceVert = 434 N.  So while there's definitely some "up/down" force there, a majority is along the length of the shock.  Is this the right way to do it?  I have no clue.

The angle itself is a result of the length of the shock I used and the length of the hoop.  I'm using a rear shock from a 2007 CBR600RR.  I was hoping for a more in-line angle but I would have had to go with a custom solution which project budgets (my wife) won't allow at the moment.  I believe you can swap on the spring on this particular model if I need a larger/smaller spring rate.  I don't claim for any of this to be the "proper" way to do a monoshock conversion but I do believe there will be enough strength in this design to keep me alive.  Famous last words?

I was very happy with the amount of sag I experienced sitting on the bike.  I'm not really sure how that will translate to real-life road riding experience but it seems I have a lot of travel in the bank for big holes.  A downside to my design is that I'm locked in to this shock.  I've seen some other designs that allow for some adjust-ability but I ain't got none of that.  I just wanted a wide open space back there and I think I got it.

[calj737]

What you might find is your design can work but would require a shock that allows both low and high speed compression adjustments. Given the range of motion, one or both of those adjustments could bring the rear to heel with your modification.