Static Fork Stiction

[bobbydyabb]

Can't seem to resolve it.  Have tried glass plate alignment and bounce-tighten-assembly procedures multiple times, and I still get static stiction.  When I sit on the bike, the front end doesn't sag (am about 210 lbs) ubless I plop on the seat.

Have started disassembling the forks and am posting my findings here in hopes that something odd jumps out to the experienced eyes on this list.

First the fork springs.  503.24mm long, 66 coils (7 closely wound at each end), 4.34mm wire diameter, orange/brown pain dab on one end. This doesn't seem to match what was mentioned in the "Parts for forks" thread.  Is this perhaps the "heavy duty" version?

In a fully assembled fork, it takes 30lbs to start the slider to move with a hint of jerkiness.  And it takes 80lbs to move the slider to about a 2" sag equivalent. Probably normal except for the slight jerkiness at around the sag area.

The oil seal:  With the slider (damper rod disconnected, oil drained) assembled to the stanchion, it doesn't fall off on its own. Oil seal stiction? Normal?  Is there now available, a low friction oil seal?

The 3 damper piston rings: Slight drag and jerkiness sliding it around in the stanchion. Normal?  The rings are worn flat and with sharp edges. Time to replace? Are they still vailable? Are there any teflon ring versions maybe?

Well, that's all that I have for now. Your comments please ...

---
Bob D, 83 R65, 80K miles, Cerritos, CA

[Tony Smith]

Bob

I have no answers, only suggestions.

Firstly have the fork legs polished if they are not positively smooth as a bay's bottom.

Fit either genuine seals or the "Ariette" brand flogged by Motobins - Replace the rings..

The last simply because if they are worn enough they will "cock-over" and will jam the forks - only ever seen this once, a long time ago with a very high mileage R60, but I filed it away - they looked OK to me, just worn flat, sharp and bright. A now deceased BMW whisperer called Don Wilson told the owner to replace the rings and that cured the problem.

I asked for an explanation and the above is what I was told.


There is a Teflon ring available for other model but it requires the damper piston to be machined, I bought some and then measured up the amount needing to be machined and wasn't comfortable that there was enough 'meat' left, you may have a different opinion and there was a user here a couple of years ago who did it and I have not heard of him suffering an accident due to his forks falling off so it may be that my opinion is/was overly conservative.

[Tony Smith]

Bob

To which I will add, I was going to clamp the Teflon strip between two sacrificial pieces of metal and then run them through a mill to make the strips narrower, BUT, the reason I was goign to do this was that the rings were NLA for some time, then suddenly Motobins had them again (and still has them) so I simply fitted new ones.

One thing I forgot to mention  was to have a look at the inside of the fork tubes, obviously any corrosion, wear etc could easily account for your problems. I removed mine by making up a length of plastic plumbing tube, sealed at one end - placed fork tubes inside and added swimming pool acid - that cleaned the surface corrosion off and I then gave them a bit of a polish with a "flap wheel" secured to the end of a piece of dowel.

Alternatively, Forking by Frank becons, or you could do what I may well be doing in the near future and bolt a K100 or K75 front end in place.

[Barry]

Your description of the springs makes them sound stock in every respect except one. The stock springs have a wire diameter of 4.25 mm.  The wire tends not to be accurately circular top the nearset 4 thou so careful measurement in different places may give you 4.25mm.

On the stiction I would try to divorce alignment stiction from internal stiction and remove the whole fork leg to try and gauge if it's seal or damper piston rings. You could always leave out either the seal or the rings to help diagnosis.

There are stories on the net of replacement rings being oversize with insufficient end gap and advice that originals  should not be replaced.  

The best option might be to do what a guy on another forum did.  Unable to resolve similar stiction problems he put the damper pistons in a lathe a turned the ring gap wider to accept the later white plastic wipers. It resolved the problem.

[Barry]

I remembered it was chasbmw on advrider that fitted plastic wipers.

Tony,  

Plenty of meat on these pistons but I take your point as ours must be half the height that these are.

[Tony Smith]

I remembered it was chasbmw on advrider that fitted plastic wipers
Plenty of meat on these pistons but I take your point as ours must be half the height that these are.

I wish I could remember more of that story!
Are those pistons R80ST? If so, they would probably fit the r65 damper rods.

I wonder if they are still available...... Answered my own question - not for Motobins.


Also, I've fitted 2 complete sets of new damper piston rings sourced from Motobins - they fitted perfectly and were dimensionally identical (as near as I could measure) to the 1 NOS original I had.

The only person I've ever heard of complaining about inconsistencies in dimensions was Snowbum, and he was complaining about genuine BMW supplied rings.

The ones supplied by Motobins do not come from BMW as BMW has them NLA.

[georgesgiralt]

Hello Tony,
I replaced my fork rings in October 2013 with this P/N  31 42 2 333 753 (there are officially for an R80/7)  bought from BMW.
I just checked, they're still available new from your favorite BMW dealer.
So far, they are still perfectly functioning.
P.S. : As far as I know, the rings were never listed on the R65 fiche. One of the numerous mistake from Mr BMW part department.

[Tony Smith]

Hello Tony,
I replaced my fork rings in October 2013 with this P/N  31 42 2 333 753 (there are officially for an R80/7)  bought from BMW.
I just checked, they're still available new from your favorite BMW dealer.
So far, they are still perfectly functioning.
P.S. : As far as I know, the rings were never listed on the R65 fiche. One of the numerous mistake from Mr BMW part department.

The ones I used from motobins were -

Part No.29000
BMW Part No.
31 42 1 232 059

[georgesgiralt]

And RealOem gives my P/N as a substitute ...
http://www.realoem.com/bmw/enUS/partxref?q=31+42+1+232+059+

[Barry]

I wish I could remember more of that story!
Are those pistons R80ST? If so, they would probably fit the r65 damper rods.
 

They were from an R90/6.  There is a better write up somewhere if I can find it but this is where the pic came from.

http://advrider.com/index.php?threads/fork-stiction-ate-forks.924902/#post-22485795

[bobbydyabb]

Thank you for the responses.  As time permits, I will do trial assemblies and diagnose exactly where stiction is coming from and report back.

Re the springs, Barry, I re-measured them and they are really 4.34mm wire diameter on coils mid length on the spring.  Could I have BMW HD springs?  An increase of only .004" (or approx .09mm) in wire diameter apparently results in a 10% increase in spring rate. But, we'll see if it matters when I do my test assemblies.

Re the damper piston rings, thank you Tony and George for pointing out that they are still available both from Motobins and BMW. I understand why they should be replaced for damping purposes.  But shouldn't worn rings create less stiction? Unless the worn but sharp edges sort of digs into the stanchion insides instead of merely sliding along.

By the way, there are slight half circular ridges on the inside of the stanchion at about the 2" sag target area. I'll try and smooth them out by using Tony's "flap wheel" idea.

Interesting, the plastic wiper ring idea. And the K75.K100 front end transplant. Maybe someday ...

[Barry]

An increase of only .004" (or approx .09mm) in wire diameter apparently results in a 10% increase in spring rate.

Yes 10% doesn't surprise me. Spring rate is so sensitive to wire diameter that you have to allow for paint thickness on rear shocks.   I don't have rates for R65 HD springs but going off other airheads I think they might have been more than 10 % stiffer. For example on the early type 247's the HD springs were something like 40% stiffer than stock.

The bottom line for whether your springs are OK should be that you get sufficient loaded sag to make the forks work without topping out.