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- 1978 Cx500 w/ an 83 CX650 engine
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Discussion Starter · #1 ·
Hello all, this is technically my introduction as I’ve only lurked in the dark around here. Anyways I have a 78 cx500 that originally was just going to be a mono shock conversion then I picked up a complete cx650 for $400 so of course I had to swap the motor! This projects been on the back burner for a while but I just got around to building and tack welding my rear mono shock mounts and addressing that god awful ugly area when you cut off the rear subframe. I see a lot of shops/people just cut it off and put little caps on them which to me is an eye sore! I wanted mine to look “factory” so here’s what I came up with. Obviously there is still a lot of welding and grinding/blending to do but let me know what you guys think or any recommendations. Also just to add the mono shock is from a newer bmw r nine t and was built for use without a linkage system.
 

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- 1978 Cx500 w/ an 83 CX650 engine
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5 Posts
Discussion Starter · #3 ·
Not yet, I have a cx500 output shaft sitting on the shelf, I just haven’t got around to putting it in; it’s on the long list of things to do. To be honest I’m a noob when it comes to wiring and ignition system things so I’ll be relying on this community to steer me in the right direction.
My hope was to use the factory system for this year. Plan a full tear down next winter and upgrades where I skimped this year.
 

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The stock CX650 ignition is quite robust. I wouldn't hesitate to continue using it.
Make sure to check the automatic tensioner. Replace the cam chain if the tensioner is near the end of it travel.
 

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I agree, use the cx650 ignition. Need the 650 coils, spark units, kill sw and ignition sw, (all different) and some wiring changes, or the 650 harness maybe? Shift lever?
 

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- 1978 Cx500 w/ an 83 CX650 engine
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Discussion Starter · #6 ·
Recline thanks for the heads up! I’ll be sure to check it when I crack it open. I do have the complete 650 wiring harness as I had a complete donor bike that was buried in the back of an old man’s shop for nearly 20yrs. However I just said to hell with it and bought an x21 from NWT.
 

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'84 CX650E that is evolving into a GL500
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Welcome to the forum. Please add your location and your bike's model and model year to your profile so that you don't have to remember to tell us every time and we don't have to keep asking when you forget (see Forum Settings link in my signature).

And welcome to the world of antique vehicle ownership (they own us, not the other way around). Your bikes are about 4 decades old and may or may not have had all of the maintenance necessary to keep them safe & reliable so it is highly recommended to download the Factory Shop Manuals for your models (available through the CX Wiki - link in my signature) and go through all of the service procedures (at least on the parts you are going to use), regardless of whether the bike has reached the specified mileage.
I also recommend looking on all rubber parts with suspicion because rubber does not age gracefully. Before you think about using the tires that came with either bike check the date codes on them and if they are over 5 years old replace them no matter how good they look & feel (old rubber simply cannot flow around the irregularities in the asphalt well enough to grip, especially if it is cool or wet).
The same goes for the original rubber brake lines. They should have been replaced every 2 or 3 fluid changes (= 5 or 6 years) so I recommend shopping for modern stainless braided ones (they last practically forever and double the life of the fluid).
And don't forget things like the rad hoses and the boot between the engine and swingarm (they can crack on the bottom where you don't see it).

It may be too late but the best advice anyone can give you about customizing any vehicle is to get it safe & reliable in more or less original condition and use it for a while before you start making any changes so it can tell you what changes it needs to make it do what you want/need better. That approach almost always results in something you actually want to keep and use but making changes based on style or on what someone else (who may or may not really understand how the changes affect the way it works) has done often results in a piece of expensive yard art that you can't stand sitting on for more than a few minutes and might even be dangerous.

Personally, I think you would have been better off starting with a GL650 to end up with a 650 and a monoshock but I understand that you are trying to work with what you have.
People with more experience than me have described at great length why converting a CX500 swingarm that was designed to be supported by a pair of shocks to a single shock near the pivot point is not a good idea so I won't get into that (you can do your own research on the advisability of it).
But I will advise you that it is easier to make sure there is enough clearance between the tire and the frame now than it will be to change things after you hit a bump and find out the hard way. Before you go any farther set the shock at minimum preload and use a couple of ratchet straps between the frame and the swingarm to fully collapse the suspension and make sure that the tire won't hit the modified frame. Don't forget to leave room for a fender and to allow for a larger tire if you expect to use one (although fatter tires never handle as well as skinnier ones).
 

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- 1978 Cx500 w/ an 83 CX650 engine
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Discussion Starter · #8 ·
Sidecar bob, thank you for the heads up. I got around to checking the clearance tonight. Looks like I’m in the clear with a 120/90/18 tire. I’ll be switching to a 140/70/17 this week so I should definitely not have any issues with clearance. As to the mono shock- I can totally understand how it would be deemed “not advisable”. With the dual shocks, they are so close to the rear axle that there really isn’t any pressure on the swing arm at that point. The swing arm just holds the wheel in alignment really. My lower shock mount does extend all the way across the swingarm however I’ll still probably look into welding supports to strengthen the actual arm portion. Thanks again for the tips!
 

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'84 CX650E that is evolving into a GL500
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From what I've read the swingarm's arms (or at least the one on the side without the driveshaft) should have some bracing to prevent bending.

BTW: It is well known that narrower tires handle better because they are easier to lean into turns. The only reason to use a wider tire on a bike is if it needs a larger contact patch to handle higher power, which is not the case for these bikes so the wider tire is one of those things that are done purely for style at the expense of performance.
 

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Not really. Though these don't tend to spin the wheel in a straight line I've pulled out into traffic sideways quite a few times.

Leaned and turned these happily spin the rear wheel without trying very hard.
 

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I gotta agree with Bob on this one. On a professional sports side of things you only see the fat rear tires at the drag strip. I’m not saying you can’t put fat tires on your bike and not enjoy the twisties, but the bike will handle the twisties better with the tire size recommended for your bike.
 

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It's nearly a moot point. You can go from 110 to 120 without mods on the standard. The difference will be negligible in either instance. I have them with both. I have 500Cs with 130s. I can't tell the difference. Maybe it's just me.

The 140 though may make a difference.

And yes, Bob is correct on this.
 

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- 1978 Cx500 w/ an 83 CX650 engine
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Discussion Starter · #13 ·
I definitely don’t own a CX for its performance. Anywho, forgive me as I’m fairly new to the street motorcycle world but in my head I’m thinking -the difference between the 110 and 140 is 1inch max difference. Which split that in half is 0.5inch wider on each side. That doesn’t seem like much. But then a tall sidewall has to have some ill effect on cornering right? If I’m thinking clearly- 120/90/18 has a sidewall height of 4.25 inches and the 140 is 3.86in. The difference is 0.39in. (Not even factoring in that the 18in wheel radius is already 0.5” further away from the axle). That has to account for something? I mean, tell me we are just splitting hairs here? I know modern sport bikes are running 190 tires which is a huge leap from 110-120.
 

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'84 CX650E that is evolving into a GL500
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The difference between a 110 and a 140 is 27%. That is a big difference.

Modern sport bikes have ridiculously wide tires partly to handle the power and partly because they are fashionable.

I thought my first bike handled pretty well with the tire sizes that were on it when I got it so I replaced them with the same a couple of times. Then as I was ready for another set I read a magazine article that explained the physics of why it is harder to make a wider tire lean and recommended using as close to the sizes the manufacturer specifies so I figured it would be worth trying and I couldn't believe how much better the bike handled with tires a couple of sizes narrower.
 

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'84 CX650E that is evolving into a GL500
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True, but as I said that wasn't the first time I had replaced the tires on that bike and the previous tire changes didn't make anywhere near as much difference as when I changed to the correct sizes.
 

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It's about left to right roll in both cases - ignoring compounds having gone off in a replaced tyre.

With a wider tyre it is how much the contact patch moves from the centre line with lateral roll. Small amounts make little difference. While true it is often overstated. As I've said, I can't tell any difference between my bikes with 110,120 and 130 section tyres. Some may be more sensitive than me though.

As to the replaced tyre, unless you live on the tail of the dragon you have flatted the centre line of the tyre. It may look round across the section but it is not as round as when it was new. It gives a funky roll transition which can cause instability through sweepers where you end up running on the ridge between the straight ahead 'facet' and the remaining roll of the tyre. This I do notice. I tend to wear tyres out through summer and get new for winter. I'll use them in dry weather with a trace of tread up the centre line so a big difference on replacement.
 
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