Skip to content

Lower fork leg anatomy of a /5 BMW motorcycle

2011 December 13
by Dirty Fingernails

A photo diary of the internal components of a /5 BMW motorcycle fork leg.
I plan on rebuilding these forks entirely.  Good thing too.  The rubber is completely rotted in some spots and it has over 10 years of fork oil sitting in these legs. NASTY stuff.

I snapped these photos partly for the blog, but also for my own memory when it comes time to replace many of these parts and reassemble everything.  Please excuse the beat up linoleum floors in my garage.

Feel free to comment or ask any questions. I didn’t plan on this being a ‘How To’ post but more of a photo of the pieces and parts involved in the forks. But the post ends up being a bit of both…

Please note, i removed the entire fork assembly from the frame.  The fork rebuild can be done while still attached to the bike. When i go through the reassembly, and mount them to the triple tree, i will have to perform a strict alignment procedure to make sure the forks are installed properly.  A misaligned set of forks can cause some harmful results to the rider.  I will be sure to post my realignment procedure when the time comes.  In the meantime, for reassembly, please refer to Duane Ausherman’s site on fork alignment:
http://w6rec.com/duane/bmw/forktool/index.htm

Also, please note that i drained all the fork oil prior to starting this procedure.

I removed the upper chrome fork cap with the BMW tool

BMW fork cap pin wrench.

I then removed the upper fork bolt that secures the fork leg to the top triple tree and maintains pressure on the internal fork spring.  I believe it is a 36mm nut and it can often be secured very tightly. I recommend getting a 38mm socket and a breaker bar for the removal process.  The BMW fork wrench is often too small and you can not get the leverage needed to get that nut loose. Before i had a 36mm socket, i remember removing these nuts extremely difficult.
I snapped the following photo so i can remember that the nut had a washer, and the size of the spring spacer.  Some models do not have a washer between the nut and the triple tree. And some spring manufactures like Progressive supply a PVC piece of pipe to cut down to size for spring tension. A longer size will equal more tension. I will probably stick with the spacer that the bike has on it.  It’s a nice aluminum spacer vs PVC.

Upper fork cap removed and upper fork bolt removed.

 

The pinch nut for the lower triple tree needs to be removed and then the fork tubes should slide out from the triple tree.

Lower triple tree pinch nut and cable hook on right side fork leg.

 

 

The springs were taken out and the fork legs removed.

Fork Legs removed.

 

 

You need to grind a flat edge on either side of a 13mm socket.  Then install a hex (Allen) wrench in the hole of the socket.  Hold the Allen wrench in place and loosen the nut.  This will separate the chrome fork tube from the lower fork leg (lower fork slider).

Special filed down socket in which an hex tool can fit through and a wrench can keep the socket secure. Also in the photo is the 30mm socket for removing the bottom cap from lower fork leg.

 

 

I really need to invest in a good circlip tool.  I have bought a few and they never seem to have the correct tension for the precision needed in removing and installing circlips.
The circlip needs to be removed from the bottom of the chrome fork tube.(FigureA)
Then you need to unscrew the lower threaded ring from the base of the fork tube. I used needle nose pliers to fit into the small holes of the threaded ring. (Figure B – though the /5 ring looks different)

Image taken from Clymer Manual. BUY ONE!

 

Here is a shot of the first stage of my fork disassembled.

First photo of fork anatomy.

 

 

I already unscrewed  the threaded plug from bottom of the dampener rod.
This photo shows how to remove the upper dampener piston.

Internal fork actuator disassembly.

 

Photo of entire fork components… in order of their removal.

Entire fork disassembly.

 

Close up of the lower dampener rod’s threaded plug, spring and ball.

Close up of lower nut, spring and ball.

 

Close up photo of the orientation and order of the upper dampener rod and the upper piston for the dampener rod.

Close up of upper fork piston.

4 Responses leave one →
  1. Rodney Mcdoe permalink
    December 22, 2011

    Thanks for helping out, superb info .

  2. tim casey permalink
    April 13, 2012

    I have a question for you about the orientation of one of the parts in one of your pictures. I am servicing my forks and found that the hat bushing in your last picture “close up of upper fork piston” are different from one fork to the other. One was oriented like you have it pictured and the other one was where the hat bushing was turned around, where part of the bushing was inside the spring. Can you tell me which is correct? Or how both of your forks were oriented? Being an older bike they may have gone through several rebuilds, like mine. Your picture is the clearest that I have seen on the internet. The reason that I am being so picky is that where the part of the bushing inside the spring, actually covers up some hole in the damper rod when it moves up and down. Don’t know whether this helps in the dampening process or not. Obviously if you have had no problem, then I will install them both as you have them pictured. Thanks Tim Casey

  3. Dirty Fingernails permalink*
    April 13, 2012

    Hello Tim, I’ll look at the actual fork tonight or this weekend but i think the details in the photo of the dampening rod assembled are a bit fuzzy. The rod has a ‘top hat’ like ring that is part of the rod. Then the spring goes onto that part of the rod. Following the spring, goes the actual ‘top hat’ washer/ring that goes on with the thick side touching the spring. Then the small part of the top hat fits into the piston.
    I’m 90% sure it is the correct procedure. I have an assembled rod that i can easily access in my garage when i get home from work. I will post back and confirm. And hopefully take more detailed photos to replace these shots.

  4. Dirty Fingernails permalink*
    April 13, 2012

    I went home and pulled out a dampener from another fork leg. I was correct. The spring goes on the rod first, then the top hat washer, and the small part of the top hat goes into the piston. Just like the final photo in the blog post. Hope this helps. -J

Leave a Reply

Note: You can use basic XHTML in your comments. Your email address will never be published.

Subscribe to this comment feed via RSS