The stiction test for BMW motorcycle telescopic forks
|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.
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 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.
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. 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.