CJ360T/XR600R, Possibly Monoshock "RoadRunner"

Alright everyone,
I've been chipping away at this project for a little bit now and I want to start a build thread not only for all the questions I know I'm going to have but also to create some accountability so that I actually finish this monster.

The project started as a 1976 CJ360T that I bought already "Cafed" out. I rode the crap out of it, commuting 100 miles every day to and from work.



I came to love riding the thing, how it vibrated so much, how it was a pain to start on cold mornings, how the seat was so thin my backend got sore, however there were a few things I didn't like.

The biggest issue I had was that for such a small bike it was still heavy and the 360 motor was a little anemic. So originally I was planning on doing the piston swap to get closer to 400cc but I stumbled upon this https://www.hondatwins.net/forums/members/2239.html#/topics/9003?page=1.


Basically someone had dropped an XR600R motor into the 360 frame. So after a casual stroll on Facebook Marketplace I found a guy selling an XR600R for $150. I bought the whole bike and started ripping into the motor. I'm in the process of replacing most of the bearings and seals on the bottom end. I decided if I was gonna do this I was gonna go all out so I sent the top end out for oversize valves, porting, and a 660 big bore kit.

In the meantime, I've been looking at some monoshock builds and I'm inspired. I really like the clean look of a monoshock and a brat seat. However, whenever I google how to do a monoshock on a bike all I get is comments along the lines of "you'll build a deathtrap" or "if you have to ask you shouldn't do it" and my personal favorite "that frame will crumple up like aluminum foil, spontaneously catch fire, and kill half the people on the planet."

I understand that there is a lot of math and engineering that goes into a monoshock and I want to learn, and if that story about Thanosing the planet is really true, then I'll scrap it.

A huge inspiration came from CompoundCycles' Doris, and Kanticoy's Gretta, as well as a lot of learning in both those build topics.

While those bikes are both gorgeous, I'm hoping to achieve the look of this bike with a more vertical shock and thus a cleaner look.



Rather fittingly, I also really like the looks of this bike which itself is an XR600R.



Basically is there anyway to make the near vertical, hidden monoshock work safely an effectively on this frame?

Also for all the help I'm sure I am going to get, I want to say thank you.

of course. depends how well you want to do it.

the predominant issue is that with the shock at the pivot end of the swingarm, any force resisting the rear wheel travel is much greater at the mount points than with the shocks at the wheel end of the swingarm. so you need a much stronger swingarm to handle the much larger bending load it will see, plus the now unrestrained twisting effect.

the frame also needs to be stronger at the swingarm pivot, which is where the twisting force is now handled. the frame also needs to be stronger and possibly modified in shape at the top shock mount, and even up to the steering head.

if you are using a linkage system, you either need to design your own with characteristics as you desire, or if you remove one from a bike you need to replicate the various pick up points to keep the system working like it was designed.

if you have an xr600r, look at the rear of that and how the frame goes to the front. you could use the xr600r bits and have someone restrict the shock travel.

the main issue with any of this is that someone will declare their intention to make this happen, then buy parts from various bikes on the assumption that if it worked on x,y or z it'll work just fine on their cobbled up piece of shit, and go ahead. for instance, someone here had a linkage setup which someone else modelled and showed them how, if it compressed toward the maximum allowed, it would go over centre and collapse. that sort of stuff. it's not that hard, it's just geometry really.

brad black said:

of course. depends how well you want to do it.

the predominant issue is that with the shock at the pivot end of the swingarm, any force resisting the rear wheel travel is much greater at the mount points than with the shocks at the wheel end of the swingarm. so you need a much stronger swingarm to handle the much larger bending load it will see, plus the now unrestrained twisting effect.

the frame also needs to be stronger at the swingarm pivot, which is where the twisting force is now handled. the frame also needs to be stronger and possibly modified in shape at the top shock mount, and even up to the steering head.

if you are using a linkage system, you either need to design your own with characteristics as you desire, or if you remove one from a bike you need to replicate the various pick up points to keep the system working like it was designed.

if you have an xr600r, look at the rear of that and how the frame goes to the front. you could use the xr600r bits and have someone restrict the shock travel.

the main issue with any of this is that someone will declare their intention to make this happen, then buy parts from various bikes on the assumption that if it worked on x,y or z it'll work just fine on their cobbled up piece of shit, and go ahead. for instance, someone here had a linkage setup which someone else modelled and showed them how, if it compressed toward the maximum allowed, it would go over centre and collapse. that sort of stuff. it's not that hard, it's just geometry really.

Click to expand...

Thank you for all the information Brad!
I assume simply making mounts on the swingarm in front of the tire and mounts on the frame for a near vertical shock is not the right answer?
Honestly, that was my primary plan as I had seen it done from that picture in my original post.
Looking at the XR600R swingarm, the only real difference is that it is made of square tubing vs the round tubing on my cj, and already has a mount for a linkage on the top.
Similarly, the backbone extends a little bit to allow for the shock to have an upper mount.
I assume the linkage is what is making this possible?
Again, thank you I appreciate you taking the time to help.

Attached is my frame with a "photochop" of what I wanted to do, a picture of what I'm trying to emulate, and what the setup on an XR600R looks like.

I've done a monoshock swap, and the best way IMO is to start with a monoshock swingarm. This way you know the swingarm can take the stresses, and if it used a linkage set up grab that too. Bonus if the bike you are grabbing parts from is close to the weight of what you are converting.

You'll need to make sure your upper mount is reinforced properly as well as anywhere linkage connects to the frame. But done right it's not bad to do. I started with a FZR400 swingarm and linkage with a R6 shock. For me that was perfect, about 1.5" longer than stock and a shock from a bike fairly close in weight to the XS it was going on.

Measure 3 times, expect to tweak it, tack it and remeasure, then maybe stare at it and check it again before making anything permanent. Swingarm angle, and rake/trail up front are critical for a bike to do well and not have issues.

Re: CJ360T/XR600R, Possibly Monoshock "RoadRunner"

Thanks for the input guys! Would never have though of using the swingarm from the XR.

Getting the swingarm off the XR was an adventure itself, but after getting it off, I mocked it up on my frame.

Looks like all I need is an upper shock moount, and a mount for the linkage, and maybe a shorter shock.

Any thoughts on how to make the upper mount and a linkage mount?

Thank you!

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Re: CJ360T/XR600R, Possibly Monoshock "RoadRunner"

Watching. I started an xr600 in cb360 frame project a few years ago. This makes me want to get it back out and finish it.

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Following along.

Re: CJ360T/XR600R, Possibly Monoshock "RoadRunner"

Hurco550 said:

Watching. I started an xr600 in cb360 frame project a few years ago. This makes me want to get it back out and finish it.

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Click to expand...

Re: CJ360T/XR600R, Possibly Monoshock "RoadRunner"

Starting to plan out the geometry for the monoshock while the crank halves and covers are being blasted, and the top end is having some work done.

I basically mocked up the swingarm, threw on the wheels, and tossed the shock in there.

For the upper shock mount I want to try an do something similar to the XRs mount. (My second picture)

My third and fourth pictures are what the CJs frame is looking like from the top and bottom respectively.

Im thinking if I chop off the two smaller tubes that form the seat loops, and open up the backbone I can wedge in some square or round tube, weld it up then use that as the upper shock mount a la the XR.

Any thoughts?

Thanks again for the help!


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Here's my linkage mount. Something similar, but pointing up instead of down would probably work for the linkage you have.

Re: CJ360T/XR600R, Possibly Monoshock "RoadRunner"

Rat_ranger said:

Here's my linkage mount. Something similar, but pointing up instead of down would probably work for the linkage you have.

Click to expand...

Thank you!

Is that just two mounts welded onto that bar or did you do any reinforcements?

Also do you happen to have a picture of your upper shock mount?

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It is 2 tabs welded on, with a gusset running across the front edge and angled down to the frame. This makes sure they can't fold to the side while giving extra support vertically as well. You can see my upper mount in post 3.

this guy starts doing a "shock between swingarm and frame" job about page 21. i start upsetting him on page 24 i think. jpmobius gives lots of good info. it's all geometry really.

http://www.dotheton.com/forum/index.php?topic=44804.0

ktm do it on some of their dirt bikes. search ktm pds vs linkage. if you look at their shock, it's not vertical. read karloss's thread for why not to make it vertical (depending on lower mount point).

the original swingarm is not strong enough. if you cross section the original and the xr600 arm the xr600 will be stronger. some arms taper larger in cross section the closer they get to the shock/linkage mount from the axle, due to the greater bending moment as you get closer to that point.

rat ranger's set up is what you need. little brackets welded on with bracing. pay attention to replicating the xr locations. maybe find out what the xr linkage does through the travel, it might get very progressive toward the short end. you might be better off with a road bike rear - maybe midsize 80's or 90's model - cb or vfr or the like.

remove the spring from the shock, and then you'll be able to work out what sort of shock length and travel you want. depends how much you want to spend, but getting someone to do a shock for you will really make the final result.

So real life got in the way, and work took me away from home for a month or so.
While I was gone, the topend from the XR came back. It had a oversize valves installed and lapped, new seats, guides and springs, and bored out to 660.
I put the topend on the empty crankcase for now to check fitment, and it looks good. No pics of the engine in the frame, but the CJ's top motor mounts line up and the motor hangs nicely.
Front mounts are going to be crush tubes in the downtube and the XR mounts.
rear and lower mounts need to be made up.

In order to prepare for the rear and lower motor mounts, I started chopping into the frame. I removed the lower bar and the rear bar. While doing that, I got a little carried away and totally committed to the monoshock idea by chopping off the seat loop, shock mounts, and their reinforcements.

I've also mostly given up on the vertical shock idea I was so stuck on earlier. After looking at "Doris" and the Wilkinson Bros CL360 Scrambler, I've decided on the triangulated lower shock mount idea found on those two builds.

Sorry about the double post, but I forgot to actually post the questions I had, that I wanted to ask about.
Basically, I'm in the process of cleaning up the frame and removing all the extra material that I don't want and get the frame back to being mostly tubular.
I also am trying to find the right spot to relocate the lower bar so I can accommodate the new motor and I noticed that the lower frame rails are not on the same plane. The right rail is a bit lower than the left, so having a bar going from one to the other would mean this bar would not be level.
Would it be okay to simply mount the bar going directly from one rail to another, having it on an angle and simply using different length motor mounts to ensure the motor is level?

And on another note, while grinding off all the excess welds, and material I don't want or need, I realized how easy it would be to accidentally grind into the tubular frame and possibly compromise it's integrity.
Is there a suggested method on grinding off excess material without affecting the frame itself?

And my last question needs a little background. My welding and fabrication skills are very...amateurish and I wanted to make sure all the welds and frame work were done perfectly or close to it so I'm having an experienced fabricator help me out. Would it behoove me to simply have him make a pretty much new frame from the downtube to the spine and remove the differently sized frame rails, while also ensuring that no grinds affect the integrity of the frame?

Is that something easily and safely do-able or should I just leave it as it is, and work around it?

Thank you for all the help guys!

Attached are some pictures of extra material I am trying to remove and hopefully not cause and issue with the frame.

If I were you, I would get that bottom end in, or even better, a shell with the top end, and do some frame planning with your mounts. You're going to have to account for how motor mounts, shock mounts, bracing, your carburetor(s) and air cleaner all interact. Not sure which XR engine you have- the RFVC or the single ports.

irk miller said:

If I were you, I would get that bottom end in, or even better, a shell with the top end, and do some frame planning with your mounts. You're going to have to account for how motor mounts, shock mounts, bracing, your carburetor(s) and air cleaner all interact. Not sure which XR engine you have- the RFVC or the single ports.

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Thank you for the suggestion Irk! Right after work I put in the shell with the top end and started doing some planning.
The top motor mounts off the CJ should work just about perfect, just need some longer bolts and spacers on either side.
The front motor mounts will be crush tubes on the downtube.
The lower front motor mounts should work with some new tabs on the existing bar.
Lower rear mounts need a new bar welded on with tabs, as does the top rear.

However, I'm still wondering about the frame rails not being level. If I weld on a new bar in front of the old one for the rear lower motor mounts, it will be at an angle. Do you think that will be an issue?

And I'm still wondering if it would be worth it to pretty much have new frame rails installed.

Attached are some pictures of the XR motor in the frame, and a marking of what I'm thinking of having replaced, and a new photochop of what I'm thinking but it has the XR swingarm because I don't have a picture with the CJ one.

360 engine is asymmetrical. I would just make the necessary adjustment with brackets and not worry about the frame tube.

irk miller said:

360 engine is asymmetrical. I would just make the necessary adjustment with brackets and not worry about the frame tube.

Click to expand...

Thank you Irk! So basically, the bar would look something like this?

Also any tips or tricks on how to grind off the tabs and stuff without compromising the frame itself?

So real life got in the way of this project for a little while but I am back and making some headway.

A buddy of a buddy of mine is going to help me do the frame work, motor mounts, and the monoshock conversion.

So right now I need to figure out what shock to use.

I know that the Wilkinson Bros used a Ducati Monster shock for their CL360 conversion, and CompoundCycles used a RD350LC for Doris.

Is there a rhyme or reason to choosing a monoshock? Or do y'all have any suggestions for which one to use?

Thank you