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Discussion Starter #1
Okay, first off: I'm a college student. I don't have a ton of money, so progress may be rather slow at times.

Secondly, and likely more importantly, I realize that the CM series of bikes garners absolutely no love from the masses. I know the forks are soft, they aren't fast, they can look pretty awkward, and aren't worth anything, among all their other problems. If you have suggestions on how to go about fixing these shortcomings on a budget, please let me know.

I picked up this pair of bikes on craigslist for $600. Call it a bad deal or a good one, but I've been happy with the purchase. Both cm's were runners, although the custom is missing gauges and lights and the like now. It also doesn't have a title, so it's basically a parts bike, and will either be parted later on, or the carbs will be reinstalled and sold as a runner in need of TLC. By now you are probably getting bored of reading, so here are some pics of right after getting them home:





I have a fair amount of experience with cars, and so far the bikes don't seem too complicated. The black bike is the titled runner.

The (rough) plan:
Install lights
Fix gauges
swap exhausts (done)
weld holes in exhaust (done)
rebuild/upgrade forks
get it running like new (done)
replace the hideous bars (looking for suggestions; leaning towards superbike-style bars)
modify seat padding for better position
lower-profile rear lights (suggestions?)
custom rearsets, because I'm tall
lighten the bike, where possible
more power (carb jetting, velocity stacks, exhaust, the like. I don't expect much)


So far I've given the bike a pretty good onceover. New oil and filter, reclocked and tensioned the balance shafts, rebuilt the tach (still working on the original speedo-has a fractured bushing) valve adjustment, retorque head bolts, new plugs and filter, new battery, tension cam chain, tension drive chain, reinstall factory exhaust, weld stock seat brackets back on and install factory seat, and cleaned the carbs.

The bike is mostly just for transportation, but I need to it be twisty-road-worthy. It only has 9k miles on it, and the inside of the engine looks basically new. It runs wonderfully and doesn't make any odd noises. The power isn't stunning, but that's what I expected.

So any recommendations? Eventually I'll probably end up selling it and getting something with more power, but it fits my needs pretty well right now and it is really easy to work on. If I can get it to handle the way I want, then I'll probably keep it a few years.

More recent pics to come; gotta get them hosted.
 

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Discussion Starter #2
Current pic, and a pick of an inspirational bike of sorts.


 

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Discussion Starter #3
Current pic, and a pick of an inspirational bike of sorts.


 

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I liked the flat black bike as it was for a starting point. The worst parts of it were the bars and short rear shocks.
Do you have any welding/fab skills or access to it? As you have figured out, the CM400 is not going to be a classic cafe, but it can certainly be better than stock. I think it is more of a bobber/tracker kind of bike. I kind of liked the straight pipes, too but they were a little too long. I think you have a good plan so far.

Ken
 

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I liked the flat black bike as it was for a starting point. The worst parts of it were the bars and short rear shocks.
Do you have any welding/fab skills or access to it? As you have figured out, the CM400 is not going to be a classic cafe, but it can certainly be better than stock. I think it is more of a bobber/tracker kind of bike. I kind of liked the straight pipes, too but they were a little too long. I think you have a good plan so far.

Ken
 

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Discussion Starter #6
Those short rear shocks are actually the factory shocks. And I will eventually be getting some longer ones, probably 1" or 1.5" longer, but these still seem to be in good shape, amazingly.

I have fab skills all the way up to running a CNC machine and handmills, but I don't have a welder. There are a couple of local shops that do good work for cheap though, so that shouldn't be a problem.

I kinda liked the lower seat and that fender, but the way it was all on there before was just dangerous. The seat mounted to the plastic inner fender liner.... the tongue of the seat also didn't fit right, so occasionally the front would pop out while riding. Not fun. I'm also rather tall, so having a low seat just didn't make sense. I'll probably either modify the one I have or sell the good one and buy a junk one to modify.

And the straight pipes.... still have them, but the bike is ridiculously loud (duh, they're straight pipes) and I live in a neighborhood. They also didn't have any mounting points; they were held on solely by the exhaust clamps on the head. Not good, hah. Eventually those pipes will probably be modified into a nicer exhaust and heat wrapped. In fact, I already have header wrap from my turbo car build, so all I need is tune the exhaust length, figure out where I want the exits, and get some mufflers...
 

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Discussion Starter #7
Those short rear shocks are actually the factory shocks. And I will eventually be getting some longer ones, probably 1" or 1.5" longer, but these still seem to be in good shape, amazingly.

I have fab skills all the way up to running a CNC machine and handmills, but I don't have a welder. There are a couple of local shops that do good work for cheap though, so that shouldn't be a problem.

I kinda liked the lower seat and that fender, but the way it was all on there before was just dangerous. The seat mounted to the plastic inner fender liner.... the tongue of the seat also didn't fit right, so occasionally the front would pop out while riding. Not fun. I'm also rather tall, so having a low seat just didn't make sense. I'll probably either modify the one I have or sell the good one and buy a junk one to modify.

And the straight pipes.... still have them, but the bike is ridiculously loud (duh, they're straight pipes) and I live in a neighborhood. They also didn't have any mounting points; they were held on solely by the exhaust clamps on the head. Not good, hah. Eventually those pipes will probably be modified into a nicer exhaust and heat wrapped. In fact, I already have header wrap from my turbo car build, so all I need is tune the exhaust length, figure out where I want the exits, and get some mufflers...
 

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Since you have fab skills and access to welding, I would change the upper and lower shock mounts to accommodate the more common eye to eye shocks. Then I would go shopping on e-bay. My new favorites are the oem Honda shocks for their shaft drive bikes. They seem to have stiffer springs and some have adjustable damping. They come in a variety of lengths, but the ones from the V45 magna, V65 and CB700SC range from 12.5 in to 14 in. Just a thought.

Ken
 

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Since you have fab skills and access to welding, I would change the upper and lower shock mounts to accommodate the more common eye to eye shocks. Then I would go shopping on e-bay. My new favorites are the oem Honda shocks for their shaft drive bikes. They seem to have stiffer springs and some have adjustable damping. They come in a variety of lengths, but the ones from the V45 magna, V65 and CB700SC range from 12.5 in to 14 in. Just a thought.

Ken
 

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Discussion Starter #10
Okay, I'll keep that in mind. The rear shocks are actually in good shape, so for now they stay. The front forks are a little more pressing, as they are a bit leaky...in my eyes, they have so little damping it is a safety hazard. I have new seals for these (came with the bikes) and the shafts aren't pitted at all, so either I will rebuild these with 15w fork oil or something heavier and cut the springs and add spacers (hack, I know, but I can't find stiffer springs for these forks, and they are ridiculously soft. Yes, I will check for coil bind and make sure droop and compression are still correct. I'm not going to lower the bike this way either. I like suspension travel.) to get more stiffness, or upgrade... forks are 33mm. For my off-track use and probable future upgrade, I don't want to spend a ton to run emulators and such, at least not at this point.
 

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Discussion Starter #11
Okay, I'll keep that in mind. The rear shocks are actually in good shape, so for now they stay. The front forks are a little more pressing, as they are a bit leaky...in my eyes, they have so little damping it is a safety hazard. I have new seals for these (came with the bikes) and the shafts aren't pitted at all, so either I will rebuild these with 15w fork oil or something heavier and cut the springs and add spacers (hack, I know, but I can't find stiffer springs for these forks, and they are ridiculously soft. Yes, I will check for coil bind and make sure droop and compression are still correct. I'm not going to lower the bike this way either. I like suspension travel.) to get more stiffness, or upgrade... forks are 33mm. For my off-track use and probable future upgrade, I don't want to spend a ton to run emulators and such, at least not at this point.
 

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I don't see how cutting the spring then adding spacers will help? If you want stiffer just add shims. maby 1/2" at a time? The problem is the origional spring has lost some tention from sagging. Adding spacers will take up any slack and add preload. This is a temporary fix, and may or may not help, and you can only add so much till the spring coil binds at full compression. If you could switch to 35mm forks (cb450,cb750, xl250, etc.) You mighe do better. Then if you got, say cb450 forks that are 29" long and looted the springs from a bike with 31" forks you might get much stiffer? It takes some looking and tinkering (and time) but it's cheap if you are patient enough to get the right parts. I don't know specificaly about the cm400, but I have a lot of different cb450-750 35mm forks I Could measure for you if you are interested and want that info. Those bikes are both much nicer than anything I usualy start with, nice score!
 

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I don't see how cutting the spring then adding spacers will help? If you want stiffer just add shims. maby 1/2" at a time? The problem is the origional spring has lost some tention from sagging. Adding spacers will take up any slack and add preload. This is a temporary fix, and may or may not help, and you can only add so much till the spring coil binds at full compression. If you could switch to 35mm forks (cb450,cb750, xl250, etc.) You mighe do better. Then if you got, say cb450 forks that are 29" long and looted the springs from a bike with 31" forks you might get much stiffer? It takes some looking and tinkering (and time) but it's cheap if you are patient enough to get the right parts. I don't know specificaly about the cm400, but I have a lot of different cb450-750 35mm forks I Could measure for you if you are interested and want that info. Those bikes are both much nicer than anything I usualy start with, nice score!
 

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Discussion Starter #14
Adding shims won't make a spring stiffer. All that does is change the location of the fork in the travel. If you add enough preload to the point where there is little to no droop, then it will feel a little stiffer, but that is far beyond optimal.

I'm not sure if the factory springs are progressive or not, but cutting them does actually make them slightly stiffer (note: if the spring gets hot, you will start to anneal the metal, and the spring will lose stiffness. Cut with a hacksaw or a cutoff wheel). A coil spring is basically a torsion bar twisted in a circle. The stiffness of a torsion bar made of a certain metal is determined by its diameter and the length of the bar. If you cut some length off of a coil spring, you are essentially removing some length from the torsion bar, thus making it stiffer. This is why if you take two equal length springs, with the same wire diameter, but one with closely spaced coils and one with coils spaced further apart, the closely-spaced coils will always be softer. If the spring is progressive, cutting the springs has an even greater difference because you can cut specifically on the softer end of the spring, and the endmost coils are often 'dead', at least in cars. This will stiffen the spring and shorten it with no coil bind issues. Hard to explain this without pics... The trick has been around in the hotrod/muscle car crowd for a long time. Not to say it's the best way, but it does work.

I know about the coil bind issue-I'll be checking for that and if my plan is even workable once I get it all apart. I know the spring has lost tension, but I'm just working with what I've got, hah.

I'm going to do some more research and see if there are any triple clamp options that would fit easily on my frame that would allow 35mm forks, and then I may be emailing you. Again, though-longer springs from other forks will raise the front end without a rate change, unless static compression is significantly greater. What might work, though, would be springs from, say, a CB900 or something that are likely stiffer from the factory due to the extra weight. Like you said, lots of time and tinkering haha.

Thanks for the compliment; I wasn't expecting the bikes to be anywhere near this good, or even really running well, when I went to look at them. Needless to say, I was pleasantly surprised!


EDIT: look http://en.wikipedia.org/wiki/Spring_%28device%29, under 'theory', and there is a good equation for spring stiffness. spring force has negative association to number of active coils.
 

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Discussion Starter #15
Adding shims won't make a spring stiffer. All that does is change the location of the fork in the travel. If you add enough preload to the point where there is little to no droop, then it will feel a little stiffer, but that is far beyond optimal.

I'm not sure if the factory springs are progressive or not, but cutting them does actually make them slightly stiffer (note: if the spring gets hot, you will start to anneal the metal, and the spring will lose stiffness. Cut with a hacksaw or a cutoff wheel). A coil spring is basically a torsion bar twisted in a circle. The stiffness of a torsion bar made of a certain metal is determined by its diameter and the length of the bar. If you cut some length off of a coil spring, you are essentially removing some length from the torsion bar, thus making it stiffer. This is why if you take two equal length springs, with the same wire diameter, but one with closely spaced coils and one with coils spaced further apart, the closely-spaced coils will always be softer. If the spring is progressive, cutting the springs has an even greater difference because you can cut specifically on the softer end of the spring, and the endmost coils are often 'dead', at least in cars. This will stiffen the spring and shorten it with no coil bind issues. Hard to explain this without pics... The trick has been around in the hotrod/muscle car crowd for a long time. Not to say it's the best way, but it does work.

I know about the coil bind issue-I'll be checking for that and if my plan is even workable once I get it all apart. I know the spring has lost tension, but I'm just working with what I've got, hah.

I'm going to do some more research and see if there are any triple clamp options that would fit easily on my frame that would allow 35mm forks, and then I may be emailing you. Again, though-longer springs from other forks will raise the front end without a rate change, unless static compression is significantly greater. What might work, though, would be springs from, say, a CB900 or something that are likely stiffer from the factory due to the extra weight. Like you said, lots of time and tinkering haha.

Thanks for the compliment; I wasn't expecting the bikes to be anywhere near this good, or even really running well, when I went to look at them. Needless to say, I was pleasantly surprised!


EDIT: look http://en.wikipedia.org/wiki/Spring_%28device%29, under 'theory', and there is a good equation for spring stiffness. spring force has negative association to number of active coils.
 

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The stock springs are well known for looseing tention over the years wether they are progressive or not You'll get something from shimming them, possibly close to what you'll get if you can determine what end of the spring to cut off. The deal is, if it dosent work, you can take the shim back out. Can you weld the spring back together if it dosen't work when you cut it? I'd personaly try this first, if I'm wrong I'll have to drink two or three extra beers to get over my shame. If you're wrong you'll be buying parts. Just saying...
 

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The stock springs are well known for looseing tention over the years wether they are progressive or not You'll get something from shimming them, possibly close to what you'll get if you can determine what end of the spring to cut off. The deal is, if it dosent work, you can take the shim back out. Can you weld the spring back together if it dosen't work when you cut it? I'd personaly try this first, if I'm wrong I'll have to drink two or three extra beers to get over my shame. If you're wrong you'll be buying parts. Just saying...
 

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Discussion Starter #18
True, shims are reversible. Once I get it all apart I'm going to check the rest length (I know it isn't tension but its as close as I can get in my garage) and see how close it is. I'll have to see what it looks like and then go from there on shimming/cutting. The ride height and sag are about right though right now, and the last thing I want to do is raise the front end more with shims. But yeah, you're probably right. I may just try shims as first and see how that goes. I can always take it apart again later.

And no, you can't weld a spring. The heat will cause it to lose tension, thus making it not much of a spring anymore.
 

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Discussion Starter #19
True, shims are reversible. Once I get it all apart I'm going to check the rest length (I know it isn't tension but its as close as I can get in my garage) and see how close it is. I'll have to see what it looks like and then go from there on shimming/cutting. The ride height and sag are about right though right now, and the last thing I want to do is raise the front end more with shims. But yeah, you're probably right. I may just try shims as first and see how that goes. I can always take it apart again later.

And no, you can't weld a spring. The heat will cause it to lose tension, thus making it not much of a spring anymore.
 
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