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What is this "made up" word? It is two words (stick and friction) that
have been put together. The friction is great enough to cause the forks to
stick until forces break it loose and then it jumps to another location and
sticks again.
First test for stiction
The first test for stiction is very easy, but takes two persons. Take
the bike off of the stand. Grab the front brake and push forwards and pull
backwards. The front end will telescope in and out. The fork springs
try to return it to a common place. While holding the front brake push
down and let it return. You need a way to mark that return spot.
Your helper can use a tape to measure the length of the rubber boot, or gator.
Now pull up and backwards and let it return. Again measure the gator
length. It should return to the same spot, no matter if you are
compressing the forks or pulling up on them. The difference in length
should be less than 1/2" or 15 mm. If it is in the range of 1" or more the
problem is serious enough to need correction. I have seen them stick
wherever one stops it.
Second test
This can best be done on bikes without fairings. A few fairings will
allow this test. Find a fairly smooth straight road without traffic.
Take it up to about 30-50 mph, 50-80 km/h. Reach down with your left hand
and grab the gator. Squeeze it just above the lower leg casting onto the
tube itself. Your hand is now located on the fork tube to sense the lower
leg coming up and going down.
It might help to have found this spot in the gator
before you get up to a thousand miles per hour and reach over to grab the forks
and almost fall off of the bike on a blind curve.
A normally operating fork will just about never stop bouncing. It will
seem as it the lower leg is very busy. It might move 1/8" to 1/2," but the
important thing is that it is constantly moving up and down. It is maybe
better described as a vibration. If it only moves about once every 1/2 or
1 second and moves a bit more distance, maybe 1/2" to 1" then that is stiction.
If it seldom moves more than once in a few seconds and always moves an inch, or
more, that is serious stiction.
If the bike fails either stiction test, try a friends BMW, try everyone's BMW
at the next club meeting. This is the best way to learn what is about
correct and what is wrong. If you test 5 /5 or /6 bikes you will probably
find one sticking.
Here is some theory about how it works.
The whole telescopic fork is a structure. The structure is made up of
two parts that telescopic together, an upper and a lower.
The upper structure
The two fork tubes should be parallel and the same height. They are
held in place by two parts, the top clamp and lower clamp, often called a triple
tree. The fork tubes must be straight and not worn to a smaller
size.
The lower structure
This is made up of the fork legs. The fork legs should also be
parallel. They are held in place by two parts, the fork brace (upper
fender mount) and the axle.
The resulting structure
Structurally, one must consider the fork tube and leg as one part. A
telescopic fork has two of those parts, one on the right and one on the left.
They must be parallel. To hold them in alignment are 4 parts; top clamp,
lower clamp, fender brace and the axle. If this structure were perfectly
rigid, then if any one part gets bent, all parts must be bent. In real
life it is possible for some parts to get bent in a minor accident, but not all.
Real life parts
It should be obvious that the two upper clamps must locate the fork tubes
exactly parallel. In real life it isn't so simple. One hundred
percent of the top clamps have the outer two holes punched in the wrong place.
It is easy to fix. That is to say that the distance between centers of the
holes isn't the same as the centers of the lower fork clamp holes.
The expensive part, the lower clamp, is usually good when new. They can
also be fixed if bent. For over two years, about 40% of the fork tubes
that we got as spare parts weren't in spec. They were bent. Always
test fork tubes, new or used. They can be fixed. About 1/3 of the /5
fork braces weren't straight. We had to tweak, shim or lap them to use
them. Axles were straight from the factory, but easily got bent in an
accident. They can be straightened too. To some extent a bent rear
fender brace can affect alignment.
The largest legitimate reason for friction is the fork seals. If the
seals were left out there should, in theory, be little friction.
To buy all new parts and just assemble them and have them in alignment is
impossible. In 72 we finally had a full understanding of the huge factory
production problems. We disassembled the forks of all of our new bikes and
aligned them to much better than factory specs. This was about a 4-5 hour
job per bike. We recovered no warranty on any of it.
This is a tedious job and takes some special tools, knowledge and lots of
patience. The good news is that I have talked a couple of novice owners
through this procedure, via email, and they got it corrected.
More about fork alignment tools
For the best article on BMW motorcycle fork
alignment
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