K75 fork identification

[Navigator]

Hello men and women of knowledge...   
This is my 1st post on ANY motorcycle forum, but probably not the last.
I have tried in more than 40 years of motorcycle ownership, riding, breaking and fixing, to amass knowledge via reading, observing and experimentation (and more breaking), without bothering other users.
However, I am finally humbly asking for assistance, because my brain is not coping with the following...

I have some bikes, and some questions about those bikes (in particular their forks)
Bike 1 (of this conundrum) - 1994 K75S
Bike 2 - 1994 K75RT (ex West Australian Police bike)

I have read every forum, manual, specification, etc., and thought I had an understanding... but then I measured, and discovered I knew little (and/or The Motorcycle God laughed...again)
I thought that my K75S (3 spoke wheels) had to have 41mm Showa forks due to late build, and my K75RT probably (?) the same.
Actual fork tube measurement K75S 41.3mm, K75RT 41mm
I think that means the K75S does not have Showas' and the K75RT does?
Attached (hopefully) are photos of the offending items

K75S fork lower


K75S fork upper


K75RT fork lower


K75RT fork upper


1. Can anyone definitely identify the fork type/manufacturer?
2. Is their an educated opinion of the better fork, in both standard form and/or which is better if modification is possible? (The K75S is remaining a road bike with some engine mods and K1200RS wheels, the RT changing to an adventure/GS intention)

Again, thank you in advance for your assistance, and I hope I haven't run afoul of any forum rules already

p.s. I currently have 25 bikes, but have been owned by north of 130 so far... so my illness/addiction is probably permanent 

[Martin]

  My 92 K75s has the same forks as the ones shown as the RT. Mine are definitely Showas but yours are missing the plastic cap that fits over the filler plug.
Regards Martin.

[Laitch]

I thought that my K75S (3 spoke wheels) had to have 41mm Showa forks due to late build, and my K75RT probably (?) the same.
Actual fork tube measurement K75S 41.3mm, K75RT 41mm. I think that means the K75S does not have Showas' and the K75RT does?

Correct! In the world of old Bricks and their limited supply of parts, some riders will replace damaged forks with whichever undamaged set is available regardless of manufacturer if they need to get back on the road.

Your K75S fork is either a Brembo or an F&S. Brembos  have the company imprint stamped on the wheel side of the slider somewhere. If you clean up those sliders and can't find one then likely it's an F&S because both those manufacturers' tubes measured ~41.3mm.

[Navigator]

Hello Martin, thank you for the Showa confirmation... I had taken the plastic cap off the fork to assist in identification.

And hi Laitch... K75S wheel out and they are confirmed Brembo forks. From this I'm assuming they aren't the reduced travel 'sport' forks usually fitted to K75S :( (visuals indicate similar stance when on their wheels).

Do either of you gentlemen, or any other motobrickers, have any information on upgrading these respective forks?
Does one or the other (both/neither) have an upgrade path...  springs/internal mods/emulators... for improved action?
One of the front ends will go on an improved K75 road, the other on a more dual purpose (GS type) K75. I'm ready to swap the front ends if necessary.

[Laitch]

From this I'm assuming they aren't the reduced travel 'sport' forks usually fitted to K75S :( (visuals indicate similar stance when on their wheels).
Do either of you gentlemen, or any other motobrickers, have any information on upgrading these respective forks?

The challenge to identifying forks is that these old motos have been subjected to parts substitutions by various owners over the years.

A Sport fork will have an S stamped on the spring retainer next to the filler bolt. Whether that is the original part on the original fork tube or a scavenged part is unknown. The Sport fork will have a brace. The way to verify what you have is to disassemble the components in the tube and compare them to diagrams in a parts fiche or one of the Brick workshop manuals.

As far as upgrading, somebody else should answer that query. 30%–40% of my riding is done on gravel roads of variable quality. The Showas on my moto have progressive rate springs and the guts get cleaned every couple of years. It's all down to fork oil viscosity after that. That is a personal and circumstantial choice. These work well enough. I might have a smoother ride with a moto like an R1200GS or an Africa Twin that has more travel length and adjustment in its suspension, The only change I'd make to my moto is installing an interior floating piston shock instead of an emulsion shock but it gets me where I want to go as-is so I spend my money on fuel.
 

[Martin]

There are spring upgrades available. However mine have stock springs, but I did play around with oil weights and oil capacity. After playing around with different weights and levels I went back to the stock settings. I do change and flush it every couple of years.
Regards Martin.

[The Mighty Gryphon]

I've had 5 different K bikes, and all had the stock forks.  So far, all I have done is change fork oil every two years, lately using automatic transmission fluid in the specified amounts. 

The stock springs are okay for fairly aggressive riding, the bikes feel nicely planted in turns.  My only complaint is the long travel forks on my K75RT.  Somehow, they are able to combine a lot of brake dive with harshness over the concrete slab joints that are so uneven on older highways in New York State.  You would think that with all that travel they would be able to provide a nicely plush ride with good control.

[Navigator]

Thank you all for your replies and information.
I've now got the part numbers from my K75S Brembo forks (1 454 325 & 1 454 326), and they don't appear on any of the BMW microfiche sources available (they can be found on general net searches though as non-specific model K75 forks)

I also noticed while looking at my forks that the axle is slightly offset behind the fork tube centreline, yet steering axis to fork offset isn't extreme. This is not common, most are inline, or in front to reduce trail for quicker steering while keeping wheelbase longer for stability.
BMW do things differently, don't they?

I'm fitting K1200RS wheels and Brembo 4 spot calipers  to my K75S, so reduced overall diameter will change geometry slightly.... lots of new numbers to run around inside the brain cavity. It's also amazing and great that the much newer calipers mount on the same fork lugs as the earlier ones.

[Laitch]

Thank you all for your replies and information.
I've now got the part numbers from my K75S Brembo forks (1 454 325 & 1 454 326), . . .

Where were those numbers located?

[Navigator]

Where were those numbers located?

Inside of lower fork leg (just below front fender lugs), visible after wheel removed