measure fork stiction

The stiction test for BMW motorcycle telescopic forks

by Duane Ausherman

Start by reading this article.

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 them loose, and then it jumps to another location and sticks again.

The 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, push forward, which compresses the fork, then gently release the brake.  The front end will telescope in and out.  The fork springs try to return it to a commonplace.  You need a way to mark that return spot.  Your helper can use a tape measure to measure the length of the rubber boot or gator.  Now pull up and backward, which extends the fork springs, 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.  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 so badly that is it stops anyplace.

The second test

This is more easily 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 and onto the fork tube.  Your hand is now located on the fork tube to sense the lower leg coming up and going down.

A normally operating fork will just about never stop bouncing.  It will seem as if the lower leg is very busy.  It might move 1/8″ to 1/2,” but the important thing is that it is continually 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 a serious amount of stiction.

If the bike fails either stiction test, try a friend’s BMW and everyone’s BMW at the next club meeting.  This is the best way to learn what is correct and what is wrong.  If you test several /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.  Click here to see actual sketches.

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 the lower clamp, often called a triple tree.

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; the top clamp, lower clamp, fender brace, and axle.  If this structure were perfectly rigid, then if any one part gets bent, all parts must be bent.  In real life, some parts can 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, the distance between the 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 only a 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 better than factory specs.  This was a 4-5 hour job per bike.  We recovered no warranty on any of it.  There is much more to this story.

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.

Updated 30 March 2023