Topic: lowering a cb750 fork

[750four]

Trying to lower a cb750 fork with no luck. I was going to put a spacer under the damper, but I can't even get the fork apart. I took the top cap off, oil drain , drained oil, removed c clip from fork seal and whoala nothing. Still in one piece. Does the seal need to come out with the upper? Is there a trick to it? I've searched with no real luck. Help

edit: got it done check bottom of page for the play by play.

[mystic_1]

Remove the center bolt at the bottom of the fork lower, inside the axle clamp.  This holds the damper rod to the fork lower, damper rod goes through center of fork upper and retains it in place.

Be careful not to strip the allen head bolt.  If the damper rod spins inside the lower as you're removing the bolt, use a long wooden dowel to hold it.

mystic_1

[750four]

Yah, I thought I had it out. The broom stick worked great. thanks, I'm an idiot.

I'm putting a 2" spacer under the damper spring ( short one). I know it calls for the long travel spring to be cut down. I'de like to keep it firm up front so I think I'll try it with the spacer and full spring first. Will the uncut spring cause any binding?

[MCRider]

Trying to lower a cb750 fork with no luck. I was going to put a spacer under the damper, but I can't even get the fork apart. I took the top cap off, oil drain , drained oil, removed c clip from fork seal and whoala nothing. Still in one piece. Does the seal need to come out with the upper? Is there a trick to it? I've searched with no real luck. Help

It depends on the "K" number. early style forks, you just remove the C clip, and forcefully "pop" them apart. The friction of the old seal is the only thing holding them together. K2 or 3 and later, you do as Mystic says.

[MCRider]

Trying to lower a cb750 fork with no luck. I was going to put a spacer under the damper, but I can't even get the fork apart. I took the top cap off, oil drain , drained oil, removed c clip from fork seal and whoala nothing. Still in one piece. Does the seal need to come out with the upper? Is there a trick to it? I've searched with no real luck. Help

It depends on the "K" number. early style forks, you just remove the C clip, and forcefully "pop" them apart. The friction of the old seal is the only thing holding them together. K2 or 3 and later, you do as Mystic says.

Mystic, I see your bike is a 1970, so am I wrong about this?

[mystic_1]

Yes, I have a K0.  On those, there are valve sleeves and retaining rings on the end of the upper fork.  The upper rebound valve sleeve (actually its retaining ring) bears against the fork lower bushing when installed, to remove you use the fork upper like a slide hammer and the bushing comes out with the upper.


mystic_1

[MCRider]

Yes, I have a K0.  On those, there are valve sleeves and retaining rings on the end of the upper fork.  The upper rebound valve sleeve (actually its retaining ring) bears against the fork lower bushing when installed, to remove you use the fork upper like a slide hammer and the bushing comes out with the upper.


mystic_1

That's what i thought. But does the allen in the bottom still have to come out in the earlier forks?

[mystic_1]

Parts book shows damper rod at all on the early forks, IIRC, hence no bolt, but I could swear mine has them.  I may be mis-remembering.

mystic_1

[MCRider]

Parts book shows damper rod at all on the early forks, IIRC, hence no bolt, but I could swear mine has them.  I may be mis-remembering.

mystic_1

No you remember correctly, I just went thru this with my project, long story don't ask. But you're right the fiche doesn't show the damper or the bolt, but they are there. They just don't have to come out, ever, so the fiche doesnt show them.

I thought I had the wrong parts, my fork having been apart for many years and just laying in a box, I thought i had somehow mixed newer parts in with older, as i have the early fork, but went ahead for some reason and took the dampers out.

[MCRider]

Yah, I thought I had it out. The broom stick worked great. thanks, I'm an idiot.

I'm putting a 2" spacer under the damper spring ( short one). I know it calls for the long travel spring to be cut down. I'de like to keep it firm up front so I think I'll try it with the spacer and full spring first. Will the uncut spring cause any binding?

I don't really know for sure. But play assumption: if the fork travel was 4" (that's close to reality), and the spring was near coil bind at that point, and you put a 2" spacer below the spring, then 2" of travel would put you close to coil bind. Pretty firm and bumpy ride.

[mystic_1]

Putting a spacer under the damping rod is equivalent to always having your forks partially compressed.  Coil bind shouldn't be a problem, you're just starting out half way there.  You will have very stiff forks indeed though as you're adding quite a bit of preload to the springs, and the fork caps will be hell to reinstall.

mystic_1

[MCRider]

Putting a spacer under the damping rod is equivalent to always having your forks partially compressed.  Coil bind shouldn't be a problem, you're just starting out half way there.  You will have very stiff forks indeed though as you're adding quite a bit of preload to the springs, and the fork caps will be hell to reinstall.

mystic_1

I didn't think of that. I'd say the fork caps will be impossible to install.

[750four]

Putting a spacer under the damping rod is equivalent to always having your forks partially compressed.  Coil bind shouldn't be a problem, you're just starting out half way there.  You will have very stiff forks indeed though as you're adding quite a bit of preload to the springs, and the fork caps will be hell to reinstall.

mystic_1

I didn't think of that. I'd say the fork caps will be impossible to install.

You'de be right, got one fork done, I cut a little less than 2" off the spring. If I did get it on my guess would be I'de lose an eye if I ever took it back off! Overall I can say this was a great mod. Free, and pretty simple if you're not doing it at work on the clock and in a hurry..lol

[MCRider]

Putting a spacer under the damping rod is equivalent to always having your forks partially compressed.  Coil bind shouldn't be a problem, you're just starting out half way there.  You will have very stiff forks indeed though as you're adding quite a bit of preload to the springs, and the fork caps will be hell to reinstall.

mystic_1

I didn't think of that. I'd say the fork caps will be impossible to install.

You'de be right, got one fork done, I cut a little less than 2" off the spring. If I did get it on my guess would be I'de lose an eye if I ever took it back off! Overall I can say this was a great mod. Free, and pretty simple if you're not doing it at work on the clock and in a hurry..lol

Good deal!

And Hey Hey just noticed, another Hoosier, ton of us here.

[750four]

Here was the just of it all.

I had my forks off, but I don't see why you couldn't leave them on. Just remove the front wheel.

1. I started with removing the top cap and pulling the spring. Dumped out old oil.
2. Remove the oil drain located on the lower ( on the side behind the axle )
3. Pump the fork a bit to get the remainder of the oil out.
4. Remove the Allen head bolt from the bottom of the lower. This bolt is holding the upper and lower together and can be a pain in the rear. It threads into the valve in the fork and it's free to spin. The trick I found to work well was the broom stick. I had a friend push on the broom stick through the fork and onto the valve to keep it from spinnig. The bolt should then back out with out the valve rotating.

Now it's apart:

Clean up everything as good as possible. Check seals, etc..
Now you'll see the valve has a spring under the top of it. This is where you be adding you spacer. I used a piece of stainless milled to .90 OD, .75 ID, and 2" long. The lenght with give you the same amount of drop. Put the spacer in between the spring and valve.

Back together now:
1. Put the valve assembly including your new spacer back into the uppers.
2. Put the upper into the lowers, use the broom stick again and tighten the bottom bolt into the valve.
3.Cut the spring up to the length of you spacer. I knocked off 2". You'll have to cause if not you'll never get it back together.
4.Replace the oil drain plugs and put some new oil in it. My forks called for 7 oz's of 10 weight fork oil.

Feel free to add this was just the way I did it. Worked very well and cost was only for oil!
5. Put the caps back on.

[mystic_1]

You can also source some extra rebound springs and use those instead of a spacer, i.e. double up the rebound springs.

Early forks, K0-2, have no rebound spring.

mystic_1

[Bill/BentON Racing]

Please take video of fork cap installation!!!!!!!

[1974CB750rider]

I've heard that it's possible to use CB550 forks on the 750 to lower the front end. Has anyone done this?

[750four]

Please take video of fork cap installation!!!!!!!

obviously you must have witnessed this before.
It's a pain.

This mod is so cheap you'de have to be crazy to source a set of cb550 forks. Why buy forks when you have the tutorial to lower them for free?

[Bill/BentON Racing]

How far and fast can a cap fly? FRUSTRATING,for sure and down right funny if its not yours or if you are watching!! ,Bill

[stealthy]

is the procedure the same for my 73 cb500?

[754]

You could have slid the forks up an inch and a bit to lower (depends on bars)

 450 & 500/550 forks are a bit shorter but probably have lighter springs.. might be around 3/4 in shorter so you could probably get you uncut 750 springs in.

 NEVER use a wrench to remove fork cap, use a ratchet and extension or t-bar, so you are holding it when it comes out (having bike on c-stand helps)

 Funny you mention doing fork work or seals with tubes still in trees.. I thought that can save a bit of time, but a few went off on me for even mentioning it (they figured you only saved a few minutes). I would only do that on a front end I know is good. this K4 I am working on, I would rather pull them completely apart and check for straightness/damage/wear & straighten if needed.

 Got to ask why you lowered that much? bobber or street racer?
 Stiffer fork is a lot better for reaction times at the strip BTW..

[ratty750]

Here was the just of it all.

I had my forks off, but I don't see why you couldn't leave them on. Just remove the front wheel.

1. I started with removing the top cap and pulling the spring. Dumped out old oil.
2. Remove the oil drain located on the lower ( on the side behind the axle )
3. Pump the fork a bit to get the remainder of the oil out.
4. Remove the Allen head bolt from the bottom of the lower. This bolt is holding the upper and lower together and can be a pain in the rear. It threads into the valve in the fork and it's free to spin. The trick I found to work well was the broom stick. I had a friend push on the broom stick through the fork and onto the valve to keep it from spinnig. The bolt should then back out with out the valve rotating.

Now it's apart:

Clean up everything as good as possible. Check seals, etc..
Now you'll see the valve has a spring under the top of it. This is where you be adding you spacer. I used a piece of stainless milled to .90 OD, .75 ID, and 2" long. The lenght with give you the same amount of drop. Put the spacer in between the spring and valve.

Back together now:
1. Put the valve assembly including your new spacer back into the uppers.
2. Put the upper into the lowers, use the broom stick again and tighten the bottom bolt into the valve.
3.Cut the spring up to the length of you spacer. I knocked off 2". You'll have to cause if not you'll never get it back together.
4.Replace the oil drain plugs and put some new oil in it. My forks called for 7 oz's of 10 weight fork oil.

Feel free to add this was just the way I did it. Worked very well and cost was only for oil!
5. Put the caps back on.

first, where could i get a piece of milled stainless that size? and second, where would it go? i'm new to a lot of this and am not sure what the valve is. are you talking about the top filler plug? i would appreciate any help. thanks.

[754]

If you want to assemble with excessive preload, machine aluminum pins to fit into fork  cap and into spring, .. might keep them from popping out.. use a socket or T bar..to install nut..

[mystic_1]

first, where could i get a piece of milled stainless that size? and second, where would it go? i'm new to a lot of this and am not sure what the valve is. are you talking about the top filler plug? i would appreciate any help. thanks.

First, some explanation.  Here's the diagram for the K3 fork:




The valve is item #8 aka the "damper rod" or "damper tube".  #6 is the rebound spring.  It slides onto the damper tube and then both slide down the middle of the upper fork tube #5, passing through a hole in the bottom of the tube so that the shaft of the damper tube is sticking out of the bottom of upper fork tube.  The rebound spring #6 sits between the flare of the damper rod, and the inside of the upper fork tube.  The bottom of the damper tube sits inside the seat, #9, which is bolted to the fork lower by bolt #19.

The damper tube is called a valve because as the fork upper tube moves on it, it covers or uncovers the small side holes to restrict oil flow, this is where the fork's damping action comes from.

So, the idea of this mod is to increase the effective length of the rebound spring #6 by adding a spacer.  This forces the fork upper to sit lower on the damper rod and therefore lower in the fork lower itself.  The spacer needs to have an OD that will fit inside the upper fork tube, and an ID that will fit around the damper rod.  It's length will determine how much drop you get.

You can have something made, or you can find some additional rebound springs and use those as spacers, if they're the height you need.

I've heard of people using PVC as a spacer but I cannot attest to it's suitability as I've never tried it.


As pointed out above, if you only need an inch or so you can just slide the forks up with the triple tree.  This mod is good for larger drops but you're sacrificing fork travel.

mystic_1