Topic: Front Fork Suspension Clunk on Rebound

[zeech]

I have this strange issue with my CB650 build where on the front suspension rebound, it makes this awful clunking noise, it's like there is no dampening. It seems like the short spring that goes on the pipe bottom (that's what Honda calls it) isn't doing anything and it's just clunking hard against whatever is in there. Video below—I don't have the fork springs in here, don't need them to replicate what the problem is—when I ride and go over bumps it makes this exact same awful clunk/chattering noise as there is no rebound dampening I am guessing. Do you know what would cause this?

Maybe I don't have enough oil? I believe I have 4-5oz. I did a full rebuild and saw Hondaman say in a post you may need more after them being dry. But I think if I add too much my suspension won't move.

I have progressive springs with a pvc spacer. All Ballz tapered bearings. I fully rebuilt the bike but this is the last problem.

I am doing something slightly unorthodox with the forks, but it should still be working fine I would think. I'm using a 1979 CB650 fork tube inside a 1980 CB650 fork case.

[Flyin900]

A couple of thoughts.

How do you know the 79/80 parts are interchangeable? There could be differences between the fork tube lengths in the two years. You changed to Progressive springs and a spacer, so is the spacer length correct and how did you determine the right length with the 79/80 fork tube swap?

There maybe some play with the internal components if the parts are not correctly spaced internally, so is the bottom fork bolt in the lower fork leg fully tightened down? That sound you hear in the video is the fork tube hitting the internal slider bushing/washer and the fork seal area where the upper slider tube and lower fork leg join.

Lastly the fork oil seems too low. I realize the Progressive springs are slightly different size wise, yet the larger Honda forks I have rebuilt normally take much more fluid than 4-5 ozs, that seems too low. What does the shop manual call for, or did you do a in fork oil level measurement?

[calj737]

I would also strongly suggest that you check the spanner nut on the top clamp, and verify it is tight enough to compress the bearings, but allow full lock-to-lock rotation of the forks. I have experienced this exact issue only to discover that some knucklehead (the man in the mirror) never had the nut tight enough. Drove me nuts trying to diagnose the problem and discovered it upon my effort to remove the trees to commence full disassembly.

[zeech]

Flyin900

The internal tubes are different lengths but I don't think that would matter, it's the same as if I chopped the '80 ones shorter. As far as the spacer goes, I determined it based on the compression and feel, it's about 2" long or so and locks up tight when the top nut is on. The compression is just fine and the forks work good, it's the rebound nasty clunk that is the issue.

The bottom fork nut is completely tight yes. When I put the spring and top cap on but don't put the bottom on yet, there is little to no play, it seems to be just how it should be. Wouldn't it seem that the short spring should be taking that shock of the rebound them I pull down? Unless the spacing internally isn't right, perhaps I could add a small pvc spacer below the short spring so it makes contact, instead of right now which it might not be even making contact because the internal spacing is off. I wish I had an xray of what it looks like inside.

I used this amount based off the manual, it seems low but that's what it says. Too much oil and the forks don't rebound.

I have no doubt the issue stems from mixing the parts (although the fitment is fine), I'm just trying to figure out a fox for it.


calj737

It is definitely tight and clamped down firm—the bottom of the tree is tight against the top of the bearings and neck. As you can see in the video the fork itself is making the noise, not the triple tree or not being loose.

[Don R]

 Oil is cheap, try more. If it's too much, take it out. It doesn't seem to be damping at all there.

[zeech]

Yea I will try that Don R — Keep in mind in the video there is no oil, but I am getting the exactly same sound even with oil.

[Jerry Rxman Griffin aka MuthaF'er]

Someone recently said the 150ml is for a refill not a rebuild. Think 225ml comes to mind??

[przjohn]

Too light an oil will do that.

[zeech]

I'm using Honda spec fork oil, what else can I use instead?

[Flyin900]

Are the Progressive springs made for your particular model of CB 650 or just some springs you picked up? The correct length of fork tube and lower oil lock piece is definitely important. You were able to screw the cap bolt back into the oil lock piece in the bottom of the lower fork assy?

Progressive normally supplies the info in the box on the correct length of spacer to use based on the model of bike. I have rebuilt many of the later DOHC honda forks and sometimes used Progressive springs. You normally remove all the old Honda springs either the one long spring, or the two on models that use a split dual spring design.

There is usually an oil lock piece that extends down to the bottom of the lower fork that is part of the upper fork assy and that a cap bolt that screws up into the hole in the very bottom of the fork and locks down the upper fork tube and connects everything together. This has a small spring as part of the assy and maybe the one you are referencing in your post. That one needs to remain on the oil lock piece.

There is clearly something wrong with the mix and match set up, yet without looking at the parts vs the exchanged parts it is hard to determine.

[zeech]

As dump as it sounds, it looks like adding more oil may have fixed the problem...

Flyin900

I'm not sure on the springs, but I know I needed spacers because the top fork bolt would not even compress the springs, so I played with the length a bit till it felt good. Yes the bottom fork oil bolt goes in nice and tight as it should. yes I have the oil lock and spring you are describing.

I don't think it ended up that something was wrong, I ay still perhaps need a small pvc spacer down on the piece that connects the two forks together where that short spring is to clean up some of the slop, we'll see.