850 Forks

I have received some great tips on fork dampening from this forum.
The issue I have now is how to remove the air from the forks. The symptom is the forks bottom on minor bumps and the front end sits lower on the 75 850 than the 74 850. The 74 never bottoms out but the 75 always does. I have made washers .1mm less than the piston without the flats and added an additional ounce of fork oil. I have also added an additional spacer in the rod to preload the New Progressive springs a little more. It is better but while I was adding the additional spacer I noticed that it was very difficult to hold the rod while adding the spacer. There is a vacuum pulling the rod down. My conclusion is that I have air in the chamber that is sucking the rod out of my hand.
I am at a loss as to how to remove this air apparently trapped in the fork chamber. Any tips would be greatly appreciated........Jerry

I have no idea what is going on here, but have you tried removing the little fork drain plugs at the bottom of the forks, to see what comes out ? It should be oil, which needs to be added back in....

P,S. Did you measure or compare the lengths of the progessive springs against the originals ?
It sounds like they could be too different ?

850bruno said:

The symptom is the forks bottom on minor bumps and the front end sits lower on the 75 850 than the 74 850. The 74 never bottoms out but the 75 always does.

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There shouldn't be any noticeable difference in ride height (unless that is a result caused by the progressive springs) or damper operation between the two models as the damper parts should be identical?

850bruno said:

There is a vacuum pulling the rod down. My conclusion is that I have air in the chamber that is sucking the rod out of my hand.
I am at a loss as to how to remove this air apparently trapped in the fork chamber.

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If there is an apparent vacuum under the damper piston which is sucking the rod back down then I can't see air being the cause of the problem, only that the oil is not entering the damper tube when the piston rises as it should and thus a vacuum is developing below the piston which sounds as if the holes at the lower end of the damper tube may have been blocked off?

Even forks which have no damping at all shouldn''t bottom out more easily, provided they have springs fitted in them ?

P.S. The tele Roadholders that Nortons used prewar reportedly had no damping system, and they won TT races...

Jerry,
You said you made new washers for the damper piston, is that correct? From my recollection the rebound damping, (pulling the damper rod up) is controlled by the clearance in the end cap, (oil escaping past it) and the washer on the piston sliding down to sit on the pin, to allow oil to flow past. Maybe your mod to this washer has created a kind of pump down to occur, thus bottoming out. Also it is typical of the progressive spring to feel softer than standard, a Roadholder as standard has minimal if any compressive damping. Jerry I tried doing some mods to the piston washer and and also made new tighter pistons. But I was never satisfied with the results, in the end I bit the bullet and purchased a set of Landsdowne forks dampers, used the progressive springs, fitted Honda fork seals and made my own teflon insert fork bushes and bottom stanchion bushes to reduce stiction. The result is forks that behave like modern high end forks, they now react to minor imperfections with movement, rather than jolting back through the bars and if you hit some corrugations or a bigger bump mid corner they just soak it up beautifully. The end result is much improved comfort and confidence in the front end.
Cheers Richard

Thanks.....The washer mod I made definitely increased the rebound dampening. But I'm used to the rod gently dropping down in the tube when it slips while you are trying to put the top nut on. In this instance it is being sucked down, as if by vacuum.....This led to my suspicion that there is air trapped in the cylinder......Thanks......Jerry

About 12 years ago on joining other lists I began to hear complaints from anyone over 175-80 lbs that the off the shelf progressive springs bottoms on normal lumps and on much brake use. Unless you have custom springs, likely same issue I've heard time and time again. Solution is go back to factory single rate or fab up your own multi rate.

Any air trapped on assembly or filling under the damper valve would not stay trapped more than a few seconds. It you could lift against any vacuum it should still draw in air or fluid pretty fast so would lose the sense of damper rod sucking back in about the time you stopped trying to raise it and just hold. As damper valve raises it should tend to draw in fluid via the bottom damper holes, which are pretty big an area to only resist fast powerful flows, not mere hand speed. One would think if holes were very restrictive, they'd tend to help springs lessen the bottoming on normal lumps. In road use normal forks pump down air pocket pressure, but that's above the damper tube. Only thing that could exert a force in an opened forks is a spring compressing or hung up tensioning. Is it possible your spring is coiling around the rod somewhere to tug back?

Something is amiss here, It's impossible to cause a vacuum in the damper, there is four 1/4 holes in the damper body base, unless they have been blanked off as Lab as already suggested.
That would cause the damper to "pull" a void.

john robert bould said:

Something is amiss here, It's impossible to cause a vacuum in the damper, there is four 1/4 holes in the damper body base, unless they have been blanked off :!:
That would cause the damper to "pull" a void.

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There should be two 1/4" holes in a Commando (061888) damper tube (drilled through the tapered section) .

Two or four ,check them out before anything else.Plus dont bother adding any spacers to the main spring, they only increase the ride hight, bottoming out is the spring becoming coil bound, the commando damper as more travel than the springs compressed length.

L.A.B. said:

john robert bould said:

Something is amiss here, It's impossible to cause a vacuum in the damper, there is four 1/4 holes in the damper body base, unless they have been blanked off :!:
That would cause the damper to "pull" a void.

Click to expand...

There should be two 1/4" holes in a Commando (061888) damper tube (drilled through the tapered section) .

Click to expand...

john robert bould said:

bottoming out is the spring becoming coil bound, the commando damper as more travel than the springs compressed length.

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I agree that the damper has more travel than the length necessary to coil-bind the springs, however, in my opinion, the stanchion should bottom out in the slider before the standard spring reaches coil bound.

post124541.html#p124541

Pre-war tele Roadholders? Is that a new term for Girders?

Rohan said:

Even forks which have no damping at all shouldn''t bottom out more easily, provided they have springs fitted in them ?

P.S. The tele Roadholders that Nortons used prewar reportedly had no damping system, and they won TT races...

Click to expand...

john robert bould said:

Pre-war tele Roadholders? Is that a new term for Girders?

Click to expand...

Norton used undamped two-way sprung telescopic forks in 1938 (Harold Daniell winning the 1938 Senior TT on a tele-forked Manx) however, I'm not sure they were called "Roadholder" until after WW2- or perhaps you were thinking of a different war, John? :mrgreen:
http://suite101.com/article/hl-danielln ... art-a99916

In Cdo factory Roadholder the bushes collide about 1/4-1/8" before spring coll binds. The clank sound is the bushes clashing on bottoming or the damper valve clashing on underside of damper cap on topping. My method for weak spring rate was cut out a ~1.5" section of weak factory spring to stiffen it and then add some real anti-bottoming force by placing a valve spring in the gap. There's a cheap kit to get 6" full travel with soft-indefinite top and bottom stops with progressive dampening and spring rates or more expensive kits that all seem designed to only help the dampening issues, which is a very good thing of course, just not the full scope of what can be done for fullest fork joy. Further threading of top of damper rod can give a bit of spring sag level setting too w/o a spacer.

hobot said:

In Cdo factory Roadholder the bushes collide about 1/4-1/8" before spring coll binds.

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Maybe take a little time to think about that some more?

I cannot think of how a vacuum could be in the tube but I used to use a magnet to pull the rod up after I dropped it. This time the magnet would not work so I used a "grabber" and it was difficult. I've done this at least twenty times on these two Nortons (heck, I did it 15 times yesterday) and I've never had this problem before......Maybe it's just my imagination and I'm just older and weaker....Who knows......Thanks for the tips.
I've owned English cars/bikes since '68 and have relied on them as my only vehicles and have found that there is good news and bad news. The good news is that they are designed so you can work on them yourself.......the bad news is that you have to. I still love them....more than anything. But, I have to admit I have a GMC pickup and a '93 FXDL that I just get in/on and go.....no issues.,,,,,but no personality, no thrill. There is something to said for that. Being almost 65 years old I still appreciate the days when motoring was an adventure and I still have the vehicles to experience that, and I use them.....Thanks again....Jerry

Ugh LAB good catch, bushes collide on extension, at ~6.12" travel, only after clashing damper valve is out the way. Bottoming noise is stanchion bush hitting damper tube end.

As I picked up confidence and places on factory Trixie I get all the top and bottom noises complained of, Heck sometimes just lifting up to move backwards. With 20-50 oil inside, her brake dive no longer limits/scares me, which is evidence of damping alone helping like extra spring. But it makes forks more jarring to ride rough stuff and definitely aggravates tire chirping/bouncing squeals on pull downs by factory brake. This last thing is a new phenomenon/issue to me this spring as prior thin oil and old hard tire was her limiter. Ms Peel cobbled Roadholders didn't do this or I'd not been able to attempt stoppies that ended up sliding front a few bike lengths w/o forks snatch out. Puts a spike of fear right up my joint I tell you, but had to find out on my terms as not vintage craft I was dicing it up with on Peel. Trixie can control squeal/howl till skips on chirps. I see fork ends hopping, ugh. When I get video mounted will show what I mean.

Three-4 times now I've gotten in over poor Trixies head diving into creek bed or even crossing my routine ones and 'Great Escapes' flings onto pastures cratered with partially filled in goofer holes. Pure factory Cdo's suck in comfort on rough roads and are dangerous on handling and braking when pushed to their meager limits. So let down after last week's max out I may never take Trixie over 90 mph again in opens yet that's where her pull gets even stronger. Peel spoiled me for anything else in all ways in daily life or special flings. Other wise a plain Combat is my 2nd favorite to ride, sanely.