Forks issue - not happy

[peteremc]

Well, following the recent completion of custom restoration of my 82 LS, it's been on the road for a whole 3 weeks and, as expected, there were a few bugs to iron out. O-rings leaking oil from final drive in to drum brake, no circuit to charge to battery, ageing regulator... all fixed. Rode it on Saturday and all felt fine, but now?

Just as I thought I was getting to the end of these, I dropped the bike off the centre stand yesterday morning all prepared for a ride on a truly beautiful day and the forks seized! Right there and then. No more up ... and no more down. Just sitting there, refusing to budge. What the?

I had these forks apart a couple of months ago and they were surgically clean and lovingly reassembled with absolute attention to detail and everything new that was needed and all was going well.

Finally found time to have a decent look late today. Step 1 dropped the oil. Left fork first and I drained 200 mls. Perfect. But what a surprise - well, it may not be to some, but it was to me. There were obvious signs that the lovely new, recently installed, red bump-stop rubber which is under 300 klms old was breaking up (photo 1).

Drained the right fork. Yep 200 mls and flecks of red.

With the front wheel removed I discover that the left fork works fine, but the right will not budge. OK, so let's pull the fork slider, so off comes the disc brake and front guard (fender for some), undo the bottom screw and remove the slider and look inside and think I can see more residue (it's dark in there), so I up-ended it onto the bench and the damper rod support in the bottom came out looking like photo 2. Unbelieveable!

Like I said, under 300 klms since these forks were put into service following a complete overhaul. This is crazy.

With the slider off, the damper rod moves, but not freely, so looks like we go all the way again with dismantling. This is a pain anyway, but even more so because I have Tarozzi clip-ons and a headlight mount which clamps to the fork tubes with rubber guards firmly stuck to the fork tubes between the upper and lower fork braces. With this in mind, can I undo the snap ring/circlip and remove the piston assembly from the bottom and leave the fork tubes mounted on the bike?

And what do I replace the bump-stop ring with? Another one that will fall apart in 3 weeks???!!! Is it essential and can I just leave it out?

Not happy!

[georgesgiralt]

Hello Peter  (is it Peter ?)
I've had the very same problem. At first, I found cubic bits of very hard white plastic with the oil drained.
Then i replaced the broken damper stop with the black version from BMW. This transformed in a black goo clogging everything in the forks. I then installed the red supple version BMW sells now and I bet there is something in the fork oil that eats this plastic rapidly. I'm on my second set of damper stops.
The first one was eaten away by Bel Ray oil. I've changed oil but i fear I'll found red goo in the oil at next oil change.
I talked to my dealer about this and he said he does not have the problem (he refuses to work on old bikes...) so do not expect too much help from BMW...
Next time I'll try BMW sold fork oil to see if it is the oil or the material...

[montmil]

What a mess!

With fork caps off and springs removed, yes, you can remove the fork lowers, leaving the stanchions in situ.

As to the disentigrating rubber, I have no clue other than to ask where you bought the bumpers and what fork oil you are using. Others will likely chime in with additional thoughts.

Never seen nor heard of this problem occurring so quickly.

[Barry]

I can't remember if it was on this forum or another but someone else recently had this exact same problem with nearly new red rubber topping out bushes.
 Has to be a bad batch.

If you use these from Motobins I promise you they won't dissolve but they won't be as compliant as the red ones either.  You can't have everything.

[georgesgiralt]

Barry,
I had some of theses in my fork. They did not dissolve like the red plastic, but they disintegrate in very tiny cubic parts (like a Securit windshield when it broke). It was not a part from Motobins but from BMW. It was in hard white plastic. I bought them because the  original one had dissolved and where evacuated with the oil...

[Barry]

The ones from Motobins are a a tough nylon type of plastic and don't look as though they would break up.  Mine have been in for many years.  


Peteremc,

Scroll down the thread list on this board to "another fork problem" to see pics of a red rubber extruded through the perforated valve plate. The rubber has to be too soft or is made soft by something in the fork oil.

[montmil]

The ones from Motobins are a a tough nylon type of plastic and don't look as though they would break up...

Bought my 'rubber baby buggy bumpers" from Motobins and have had no issues. The OEM bumpers in my 1981 R65 had turned to sludge.

[Bob_Roller]

Well, I can tell by the color of the oil, that it isn't OEM BMW 7.5 wt .

The BMW oil is purple .

I've had the red rubber stops, installed in my '82 LS since Oct 2010, no issues yet .

I would try the BMW oil with new stops .

[Tony Smith]

Peter

In answer to your question - remove the big circlip and the valve body from the bottom of your forks and you can push the damper rods out that way - in fact that is the CORRECT way to put them in as well as taking them out. Trying to feed damper rods in through the top of the fork legs has driven more than one owner quite made with frustration and been the reason for the buoyant market in replacement damper rod piston rings.

If I may diverge for a moment, when you put the damper rods back in through the bottom the missing valve body gives you more room to "c0ck" the entire assembly so that you can insert the rod sufficiently to get half the piston rings started in the bore, by then gently "rocking" the assembly you will eventually get all the rings started in the bore and can re-assemble the valve body and circlip - here in hands a tale, the constant "clicking" that drives some owners to distraction emanates from the valve body moving up and down against the circlip - there are shims available.

Last comment - one of the reason I use Castrol fork fluid is that it doesn't eat the rubber stops.  I use a 50/50 mix of 5 and 10 wt fluid, wife runs straight 5wt. - she weighs 65kg I weigh around twice that.

don't sweat it too much if you can't get new stops - the damper will hydraulically lock before metal to metal contact, the rubber stops were NLA for many years and nobody had any problems with their absence.

[peteremc]

Georges, yes, it is Peter and my main aim here is to avoid having to replace these parts again real soon. Certainly not in the next couple of weeks!

Thanks Barry. I found the bushes you are showing at an Australian BMW parts supplier for $4 each ($1 cheaper than the rubbish that just melted in my forks!) when I called them this morning. Supposedly genuine part for later models and each type was measured for comparison while I was on the phone. The nylon ones are slightly smaller inside diameter to the parts for my bike, but otherwise pretty much identical measurements and better material. They will fit the damper rod no problems.

I also looked up the photo of the red rubber extruded through the perforated washer. Eeeewww, this is probably what mine was like in week 1 of riding the bike.

Bob, it is certainly not BMW fork oil I was using. I actually Googled BMW fork oil and had no luck locating it. Can I name the brand that I used here, that I think is the main culprit in this drama? Hope so. I was using Motul 10W. I have heard from several reliable and experienced sources today that Castrol fork oil does not have this effect. I don’t think this is any fault of Motul, specifically, just a warning not to use it with these bushes.

So, my final fix (I hope) will be to order the nylon ones as suggested by Barry and change to either BMW fork oil if I can find it or Castrol fork oil as suggested by Tony. Wish me luck!

[georgesgiralt]

Well, Motul is a reputable brand as is the Bel Ray I used into my fork. I think the culprit is the swelling agent they use in fork oil.
I wonder if my dealer stock 7.5 W  BMW fork  oil  and if it will make  any difference.  This oil is made for  modern BMW bikes not our dinosaur... So  it may run fine ....  or not....
Las but not least, I bet BMW oils are re-badged oils made by the less costly oil maker. So no warranty of composition, fitness, etc...
Add to this that environmental regulations are different in Europe, USA, Oceania... So you won't know if the oil which is good in England will be good (provided it is sold there) in Australia or USA. So we are in total darkness here.

[stubmw]

I also looked up the photo of the red rubber extruded through the perforated washer. Eeeewww, this is probably what mine was like in week 1 of riding the bike.

This was from my topic a week or so ago.
So. I fitted the red rubber buffer from a aftermarket bmw supplier and the photo was the result after pumping the forks a few times with NO oil in the forks, I could feel it changing over a dozen or so pumps so I pulled them back to pieces. I guess with NO oil they took more greif (no damping) but even so that shouldn't happen.
I have now fitted some more direct from bmw, I shall lopk into them in a week or so. They looked and felt identical to the first pair.
I also definitely had metal to metal topping out  which is why I pulled them apart. The red ones in at the moment have stopped that.
Tony Smith....do you use castrol synthetic (gold container) or other (grey container)?
Thinking Barry's white buffers are the way to go.
Stu

[Barry]

don't sweat it too much if you can't get new stops - the damper will hydraulically lock before metal to metal contact, the rubber stops were NLA for many years and nobody had any problems with their absence.

That's what the forks are supposed to do once the rebound orifice drops below the valve washer but mine had no perceptible hydraulic bump stop effect at all due to excessive leakage between the damper rod and the bore of the valve washer. From what I've read over the years it's not an uncommon problem on the early models which will be why BMW modded the damper valve several times. The problem got fixed in several ways. The plastic valve washer was thicker and had reduced clearance to the damper rod which will have reduced the leakage and the whole damper valve body got sprung so that if topping out did occur the short spring would absorb the shock. That spring incidently is the same part no as the topping out spring used in other forks like the R80st. A topping out spring is what our forks should have had in the first place. I've often wondered if you could modify the forks to fit them by using the longer damper rods from the R80st.

The history of the topping out bump stop is something of a saga. The original ones were black rubber which eventually became hard and broke up and now we have red rubber ones that are too soft. The white ones I've been using from Motobins are not even intended for our forks or at least Motobins don't think they are. But they fit and they are durable so they seem to be the best option.

[Tony Smith]

Bob, it is certainly not BMW fork oil I was using. I actually Googled BMW fork oil and had no luck locating it.

You will not find it either - the oil specified for the R65 was an obscure Aeroshell hydraulic fluid. I located what was alleged to be the last drum of it in Australia and the refueling agent was kind enough to give me the 20lt drum that he had in stock.

It was a major disappointment - my forks topped out horridly and had grossly insufficient damping.

Out of curiosity I made inquiries some years later as to why it disappeared from the market and was told that it was of a not frequently specified grade of hydraulic fluid and that there was a permissible replacement that was much cheaper. The permissible replacement was Dexron II or Dexron III transmission fluid.


BMW habitually under damp their forks, you will get no improvement by using their fluids.


Castrol fork fluid is sold at SuperCheap Auto bye the way....

[montmil]

Bob, it is certainly not BMW fork oil I was using. I actually Googled BMW fork oil and had no luck locating it.

...The permissible replacement was Dexron II or Dexron III transmission fluid.

Confession with a back story:

When I acquired my 1978 R100S, the front forks were extremely stiff; hardly any action in the damping and rebound department. Time to open them up and see what's what.

First thing I found was a 1.50-inch, thick walled PVC spacer. As an experiment, I cut the spacer to 0.75-inch and reinstalled. After draining the fork oil, I recalled a long past note from Tony about using Dexron ATF juice. Rolled the dice and poured in the manual's stated volume.

Big difference in front end performance... For the better. Still running the short spacers. Don't feel the need to experiment further. BTW, the Dexron "looks" to be close to the BMW 7.5wt I had on-hand.

An economical experiment.