My Hydra Glide Forks Pounding on Rebound *******Solved******

[51Hog]

Several years ago I bought a pair of after market fork tubes that had "All of the internals Ready to install in the sliders, fill with oil and put into service".

I knew that the bushings in my sliders needed to be replaced so I did not expect the forks to perform as new. I did expect them to have the capability of at least some dampening effect.
They may have had some dampening on compression, but not on rebound.

I tried extra oil, Marvel Mistery oil, HD fork oil, different weights Motor oil....etc...
It seemed the heavier weight the oil, the more the forks dampened---Mostly on compression.

It appeared that the problem with the hydraulics was in the sloppy fit of the lower bushing to tube.
Looked like the oil was bypassing the valve and shooting through between the bushing and tube.

I built a fork tube bushing puller and driver pulled the bushings fitted new bushings to my tubes, then pressed them into the sliders. They felt a little sloppy so I pulled the bushings again and sent them to Bills. When they came back to me, they were no tighter fit than the installation that I did. One tube has a little side to side movement (I can hear it when I move the tube side to side inside of the slider. If I had to guess, I believe there is possibly .003 - .005 side play in the upper bushing.)

I disassembled the tubes for inspection and found that they were missing the baffle assembly that screws to the bottom of the top tube nut.
Also missing were the shims HD # 45826-49 that adjust the gap between the Compression and the Rebound parts in the hydraulic valve assembly.

There is supposed to be no more than .004 clearance between the bottom of the valve assembly and the cir clip in the bottom of the fork tube. Mine had approximately 1/16-3/32" .
This gap DIRECTLY transfers to the center of the valve, letting the two halves separate too much.
The small springs that are between the valve metering disks and the center slider were slipping out of their little channels and blocking the center slider of the metering valve from traveling fully to the metering disk. This lets too much oil through the valve and it looses its dampening effect.

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IMG_1477.JPG

This explained the reason the valves intermittently function correctly.

The holes in the bottom sides of the dampener tube assembly allow fluid to bypass the valve completely, as does the clearance between the slider bushing and the fork tube.
There is also fluid that bypasses the valve between the shaft of the valve assembly and the valve assembly center disk.

I carefully bent the two springs in the assembly to where they are forced to ride inside the reliefs built into the valve metering disks (Diameter of the springs is smaller and now easily fit the reliefs.)
I shimmed to less than .001" between the cir clip and the bottom of the valve assembly.

I installed the tubes into the sliders, added 7 oz Dextron III automatic transmission fluid to the forks through the drain hole while on the bench and put the drain plug in.

I have not installed them onto the bike yet as the baffles have not arrived yet.
I have bounced with my 260 lbs weight on the individual forks. I can now hear the fluid being forced through the valve assembly where I could not hear it before.
I can slowly compress one leg almost completely and abruptly turn it loose.
NO SLAM ON REBOUND!!!!
I definitely hear the oil being forced through the valve.

Wonder if the weight of the Wheel/Brake assembly will over power the dampening effect when the bottom drops out from under the front wheel-----Time will tell.
I will post the results when fully tested.

Two things should increase the dampening effect.
1 Heavier oil.
2 Making the bypass holes in the bottom of the valve assembly tubes smaller.

Dale

[john HD]

dale,

i got my forks to act just like you describe the last time i had them apart.

i did not pay much attention to the springs.

let us know if this is the fix. if it is i'll take mine apart again!

john

[VintageTwin]

Anyone know what the clearance should be between the fork tubes and the slider bushings?

I still have pound on re-bound, but I think Robert Luland, nailed the problem (yrs. back) when he suggested that he and his buddies thought that the smooth surface of hard chromed tubes allowed the tube to rebound on compression too fast (past the honed bushings). He said that the cross-hatching surface of OEM and Forking by Frank would cure the problem.
No I don't know the clearances between the tube and upper and lower bushings, but Robert was right in my opinion. Slick tubes won't help the top-out clunk problem and I sure wish somebody would get the cross-hatched tubes from Frank and apply Roberts theory. http://www.frankmain.qpg.com/
But here's the continued article on FHP. The search stopped with the last post.
http://flatheadpower.com/tech/viewtopic.php?f=13&t=9750
Synopsis as of 3/23/09:
Nobody's popped for cross-hatched fork tubes from Frank yet. Something as important as this, I would be on the phone asking Frank about the cross-hatch theory and cure this problem, if possible.
If you call him, tell Frank we're using his parts over at http://hydra-glide.com/

The above post from 51Hog seems to say he used eliminated the gap between the cir-clip and lower damper bushing distance and used thicker oil and the forks didn't slam on rebound.
Adds some creedence to the Robert Luland application and theory of needing a 0.001 - 0.002 ?? clearance between the bushing i'd.'s and cross-hatched correct o.d. steel fork tubes.
Wonder what Frank's opinion would be?

[fourthgear]

With that kind of play ,I would send them back to Bill's . They did a fine job on mine ,and they did rough up my tubes where they go into the bushings . not sure if it was a cross hatch pattern or not ,but the chrome was sanded or some thing. Mine had no wiggle at all between the bushings & tubes . I would at least call Bill's & tell them what you found.I know they replace both bottom & top bushings . Did you send your tubes with the lower legs to them ?
I had some problems with my springs also ,If I remember correctly.

[51Hog]

I believe that the post referenced is talking about Show Chromed tubes being the tubes that are too slick.
Hard chromed tubes are not slick---At least mine are not, nor is the surface as hard as the shiny chrome.
I dinged one and polished it off with 600 grit wet dry. Looks like cross hatching....
Panacea bought a set of Franks tubes according to that thread.

The transmission fluid appears to be close to the same viscosity as the HD fork oil.
It may be a little thinner.
My opinion on the gap between the cir-clip and the bottom of the valve is ---the less the better.
Just need some so that the clip can be inserted.
Dale

[VintageTwin]

I'll call Frank....tomorrow - see what they say. Total scoop from Frank or no scoop.
http://www.frankmain.qpg.com/
There has to be a reason. Harley fork oil has worked in Glide forks for 60 yrs. Forks before V-Twin did not clunk. What is the reason? The cure couldn't be using different fluid. That's a stop-gap band-aid isn't it? Are we of "precision fit between two surfaces" people or are we "fluid remedy-er" people

[51Hog]

I did send the tubes with the sliders. I know that they replaced the upper and lower bushings because I pulled all four of them before sending the forks down to him.
I am goin g to try the forks on the bike as is.
If I can't get them to work properly, I will probably send them back down to him.
With his reputation, I have no doubt he will take care of it.
I have not contacted Bill about this yet, but I will.

[51Hog]

I'll call Frank....tomorrow - see what they say. Total scoop from Frank or no scoop.
http://www.frankmain.qpg.com/
There has to be a reason. Harley fork oil has worked in Glide forks for 60 yrs.
You can be relitavely certain that the Harley fork oil has changed during the last 60 years.

Forks before V-Twin did not clunk.

Mine Pounded before I bought the v-twin tubes--They were worn out.

What is the reason?

The reason my forks were pounding is listed in my first post in this thread.

The cure couldn't be using different fluid.

My putting Tranny fluid in the forks was not an attempt to cure anything. I was thinking that if the forks worked with the thinner less expensive oil and didn't leak, then they would work even better with the thicker oil in them.
Replacing slider bushings, re-forming springs, and correcting clearances were the attempted cures.

That's a stop-gap band-aid isn't it? Are we of "precision fit between two surfaces" people or are we "fluid remedy-er" people

Some of us are "parts changers", some of us are "Mechanics", Some of us "depend on others" to do the work for us, and some of us "keep on trying until we get it right". I believe that most of us do fall into the both catagories that you listed.Stop-gap band-aid?? Where does one draw the line on using non Harley parts? If it takes after market oil to make an aftermarket part function properly, so be it.

[VintageTwin]

You can be relatively certain that the Harley fork oil has changed during the last 60 years.

I do not agree. They would have added a "-A" to the number

I'll see if Franks' in this morning and report back what he says.

I called Frank's. I need to speak with their head machinist Marty. He'll be in after 3 p.m. today.
" I'll be back..." - der Plumbinator

[fourthgear]

51hog
You may be on to something about the cir-clip,I bought after market assem. that were completely assembled ,so I did not mess with them . Maybe I should have . I finaly went to 30# (heavy) fork oil & it took care of my rebound slaming problem to this day. My tubes were show chromed ,I believe they took off all that or most of it before they fitted them with the bushings & mine were a tight fit.

I think you are right about mixing parts & what it may & may not need to get things to work right . We do not know the tolerances of after market parts . If I ever have to remove or inspect the Damper assem. I will check that gap, But as long as it works ,I'm not messing with it .

[VintageTwin]

I took Marty off the lathe, and apologized for that, but told him that everyone here at http://hydra-glide.com/ would appreciate his opinion.
He told me if we wanted to use the stock Harley-Davidson® Fork Oil, then there were a couple of areas to took at for pounding:

"Of course the tighter the clearances (of O.D. tube and bushing & damper piston and I.D. of tube), the less chance for misalignment on compression & rebound."

Problems to look for:
1. Excessive oil (overfilling the tubes). I mentioned that some people had found a heavier wt. oil stopped the pounding which might lend credit to heavier oil taking up space created by a too loose fit between metal parts.
2. Excessive clearance between damper piston and i.d. of tube wall. (Marty said, "Clearance is an issue here. All you have is the steel piston in there, and it's close-clearance to the wall of the tube is important - too loose and it won't be able to hold back oil on compression. There wouldn't be any compression. Ditto for slowing down the spring-action on rebound". He said he's seen AM tubes in the past, where a machinist counter-bored the tube up 10" as a mechanical "correction" at the factory (to be able to fit the entire damper assembly into the tube, (but no more than 0.004" past the cir-clip groove, as would be needed).
So, right there you have suspect i.d.'s of all AM tubes (all but Frank's® that is ) that need to be checked for i.d against the piston's o.d. when isolating the fork pounding problem(s).
3. Tube's o.d. fit into honed i.d. slider bushings.
He said the micro-finish of their tubes is either "15 or 20".
Frank's stock '48-77 tubes are 1.624" o.d.
Their '48-77 tube has an i.d. of 1.312"
Their '48-77 tube is 20-3/8" long (without the plug installed).
4. If you have the damper cir-clip to lower bushing gap less than 0.004" as is called out in the OE manuals, that's sufficient. [Note: My V-Twin® forks were 0.002" clearance and still pound, (but I never measured the i.d. of the V-Twin® fork tube, or the o.d. of the V-Twin damper piston.)]

With the (constant) Frank's® tube o.d and tube i.d. sizes, now someone could call Bill's and ask them, based on the tube dimensions, how close would they size the upper and lower slider bushings and what is the side-play allowed between the damper piston and tube i.d. wall
Be sure and tell Bill's that your from hydra-glide.com/

Thanks Marty http://www.frankmain.qpg.com/

[51Hog]

I spoke with Bill Jr. and asked him about clearance between slider bushing and tube.
He said .001 - .003 ".
Told me if there was too much clearance in my newly installed bushings, to send the legs back and they would check them out. Another month and another $100.00 in freight--- Don't think so--- I'll repair them myself.

I asked him about causes for dampening problems. He said "clearances" He is going to check with others to see if they have any ideas.
Says he assembles forks with the parts as recieved and never has any problems. He does use 30 weight fork oil.
Next time I will speak with Bill Sr.

[VintageTwin]

Thanks. "001- .003' tube to bushing is attainable. Thanks to Bill's Jr. and Sr. - steady onward with willing parties, it's fun .
Still we wonder:

what is the side-play allowed between the damper piston and i.d. tube wall

I have an old OEM damper piston to measure, but no old OE tube for an i.d.

[51Hog]

I just measured my OLD USED UP Tubes and valve assembly

Tube OD in several locations = 1.620"
Tube ID at valve area= 1.438" - 1.441"
Valve OD = 1.435"

[VintageTwin]

So the gap between the piston (valve) and i.d. of the tube is .003 - .006" ?
.003 sounds about OK, but a .006" gap will let a lot of fluid go past. Ya think?
Maybe the clunk is something we'll live with and use 30 wt. oil to act as "Motor Honey®" or STP® to the pounding Glide forks
Might be why Bill uses 30 wt. - too hard to get a consistent hone without the stone walking? 30wt. makes up for in consistencies?