r100 81 fork

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Phil_J
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Location: uk

r100 81 fork

Post by Phil_J » Sun Dec 31, 2017 5:25 pm

Hi
I will be replacing the fork seal soon and cleaning the internals as the forks take a bit of force before they start to move "sticktion". I have read an article that there were some BMW updates to the forks internals until they moved to the monoshock bikes. I was wondering if any one fitted the modified parts and was there any improvements or has ridden the 81 and 83 bikes.

Thanks


Phil_j
Phil

R1200RT R100RS

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SteveD
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Location: Melbourne, Oz.

Re: r100 81 fork

Post by SteveD » Sun Dec 31, 2017 8:28 pm

There were some changes from 12/1982. You can see the differences via review of the fiche.

9/1980 to 10/1982 R100T. ........................................................from 11/1982 R100T.
Image Image

http://www.realoem.com/bmw/enUS/showpar ... Id=31_0316
http://www.realoem.com/bmw/enUS/showpar ... Id=31_0316

Stiction however might be more about the forks not being aligned well enough. Can you easily reinstall the front axle? If that doesn't slide in very easily then you may have some alignment issues. Google Randy Glass fork realignment but grab a mug of hot coffee first. It's quite a read.
Last edited by SteveD on Mon Jan 01, 2018 3:52 am, edited 1 time in total.
Cheers, Steve
Victoria, S.E.Oz.


1982 R100RSR100RS supergallery. https://boxerboy81.smugmug.com/
2006 K1200R.

barryh
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Re: r100 81 fork

Post by barryh » Mon Jan 01, 2018 2:32 am

The update clearly shown on the right hand side in the fiche is replacement of the long solid damper valve body with a shorter sprung valve body. The spring is the same part number as the topping out spring so they just raided the parts bin. Not sure about this specific year but at some point the steel valve washer itself was changed to plastic with increased thickness and a chamfered top and reduced clearance to the damper rod. This was done to increase rebound damping after complaints of a topping out clunks. If you do have a steel valve washer the shorter travel of the new plastic washer reduced the dead band between compression and rebound damping so the forks should be a little better controlled but the main benefit was noise reduction.
barry
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PAS
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Re: r100 81 fork

Post by PAS » Mon Jan 01, 2018 9:38 am

Over the years I have made all the improvements on my 81 R100RT forks to improve the clunking noise..I still have the noise and have never been able to cure it. The spring loaded valve body was done to eliminate the need to shim the valve body. Last year I ordered a set of the plastic valve washers and they arrived without a chamfered top! Max BMW now states the part is no longer chamfered, I wrote and told them about it so they changed the description. I would not chase the forks internals too much, just make sure they are aligned properly and have all the parts.

barryh
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Re: r100 81 fork

Post by barryh » Mon Jan 01, 2018 12:05 pm

I made my own plastic valve washer and reduced the leakage past the damper rod sufficiently to create proper rebound damping over the last bit of travel which eliminated my clunking completely. If you want to do the minimum mods to these forks and are prepared to experiment then getting the valve washer dimensions right is the key.
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Valve washer.png (39.78 KiB) Viewed 723 times
barry
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Phil_J
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Re: r100 81 fork

Post by Phil_J » Mon Jan 01, 2018 2:57 pm

I found that the fork sliders would not drop separately under there own weight before I putting in the spring and oil or spindle and mud guard. This is why I wanted strip them down and see if there are any improvements around or any tips on putting them back together or anything prone to wear. Is it the valve washer 13 has been changed from steel to nylon. Looking on BMW-Classic.de that valve washer 13. spring 11, ring 20, and lock ring 21 can be ordered so should be available but the valve housing 19 does not show as available so I shall check with my local dealer on its availability. If not I my just update the valve washer as they appear fairly inexpensive .


Phil
Phil

R1200RT R100RS

barryh
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Re: r100 81 fork

Post by barryh » Mon Jan 01, 2018 3:31 pm

You will need to check your dimensions but if you replace a thin steel valve washer with the thicker plastic one but retain the old valve body then it will need the recess in the valve body machining deeper. Note my drawing above was a custom made valve washer in the original depth valve body.

The measured dimensions with metric conversion I have for the new plastic washer and valve body from a very trustworthy source are:

Valve washer Thickness: 0.155 Inch (3.937mm)

Valve body recess Depth: 0.170 Inch (4.5466mm)
barry
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England

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ME 109
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Re: r100 81 fork

Post by ME 109 » Mon Jan 01, 2018 4:25 pm

As PAS already mentioned, check the fork legs for straight while the sliders are off. Use a piece of quality flat glass and place the glass across the fork legs. The glass should not be able rock diagonally. If it does, find the 'Randy Glass' article and learn what the basics of alignment are.
Importantly, you want a torque relief bar to tighten/torque the three top nuts without inducing a twist in the forks.

Fitted right side to tighten, left side to loosen.

https://photos.smugmug.com/Airheads/i-g ... 2671-M.jpg

There's nothing wrong with '81 forks imo, they've past every test I've thrown at them. There is that occasional clunk tho. :mrgreen:
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Phil_J
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Re: r100 81 fork

Post by Phil_J » Tue Jan 02, 2018 4:17 pm

Thanks for all your advise I think I shall start by stripping them down an clean every thing up fit some new BMW seals as they are were probably the cheapest about when they were replaced and rebuild checking the alignment is correct as I go. Also thinking about I never noticed any problem for the many years I only notice after getting the bike back in use after using my r1200 for a couple of years and perhaps make sure they are working correctly as they are before making any update.
Phil

R1200RT R100RS

Wobbly
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Re: r100 81 fork

Post by Wobbly » Wed Jan 03, 2018 6:29 pm

I concur. The place to start is with the fork stanchions not being absolutely parallel. A minor bump could twist the fork assembly in such a way that the wheel does not steer where the bars are pointed. That is to say maybe 1º or 2º either to the left or right. That can be caused by a simple pot hole encounter. This type misalignment can be fixed by loosening the triple tree pinch bolts and knocking the front wheel in the opposite direction.

Along the same lines, but slightly more trouble is a bent fork stanchion. Fork tubes will typically bend just below the triple tree, but this can go undetected for years. The easiest way to spot this condition is to remove the fork tube/ fork leg assembly complete. Clamp the bottom of the fork leg in a bench vice and spin the tube in-place. If the stanchion "wobbles" or shows any run-out, then the tube is bent.

Hope this helps.
After 20 years as a professional bike mechanic and 30 years as an engineer I know just enough to be dangerous !

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