In my K75C -> cafe racer restoration project I found one fork tube has rust pitting deep enough to concern me and I've been looking for a replacement tube. My search has turned up several offerings on ebay but most sellers won't return my e-mail questions. Concern is that I get the correct size. My measurement of the existing fork tube gives me 41.35mm OD. The brakes are marked as "Brembo" and are single pistons. The tube has an "S" stamped in the top plug which holds the springs in and next to the hex socket screw which you fill the tubes through.

I've searched the archives but haven't found a good description of how to identify the range of offerings in tubes for the K75 other than one comment in Showa Front Fork. Which I believe was intended for other purposes:

if your (Sachs or Brembo) fork works rough - throw it out
get a complet Showa (incl seering because of 41.0 mm instead of 41.3mm)

Which leads me to believe I have the Brembo tubes.

So can anyone educate me on the differences in fork tubes for the K75 and the K100 and their interchangeability? And BTW, is a 38 mm OD fork tube worth anything? That was my first error.
thanks
J.
P.S. Rick G has a nice chart in Chart of tubes and Oil quantity that gives me the idea that there were three basic types.

I also have the dimentions chart thanks to K & R

Hello Rick G,
That's impressive and looks like part of what I was looking for. BUT (sorry), this bike seems to fall into some sort of hole in the chart. By the VIN as taken from MAX BMW's website and from the title: From MAX BMW website K75C(K569) Produced: 03/90 USA. I'll make a copy of the chart for future reference. But can anyone describe how to identify the various forks? And what does that "S" stand for? Sport? And if it is sport does it actually mean anything?

Thanks for the help.
J.

AFAIK the S is for sport and came on the K75s and one of the K100 RS models for a short time as a special edition.
The forks have a much shorter travel, 125mm as against 187mm I think.
I don't know if the K75 and K100 S forks were the same diameter.
But the big problem we now face is that many things get changed around from model to model due to crashes and replacement or just because the owner likes them better and just general wear and tear.
My wifes K75 RT has forks that look identical to a K100RT but are 41mm and not 41.3mm as the K100 forks are.

for clarities sake read the R/H side of your rear frame tube above R/H footpegs. it will say K75 something and you should also have a date there usually 08/87 etc but if it is a K75 it should have a sports fork set in it, ready to see if the forks have a bridge/brace and a two piece fender set, usually signifying a sports fork with 135mm travel. i believe you will need the numbers on  the inside of the fork tube right next to the tyre, so you have to remove the wheel to read them, god bless those little bike builders

RicK G wrote:AFAIK the S is for sport and came on the K75s and one of the K100 RS models for a short time as a special edition.
The forks have a much shorter travel, 125mm as against 187mm I think.
I don't know if the K75 and K100 S forks were the same diameter.
But the big problem we now face is that many things get changed around from model to model due to crashes and replacement or just because the owner likes them better and just general wear and tear.
My wifes K75 RT has forks that look identical to a K100RT but are 41mm and not 41.3mm as the K100 forks are.

From the thread on the SHOWA forks that I cited above was the bit of information that the Showa is 41 mm as opposed to the 41.3 mm for the Brembo and Sachs. Course I have no idea if that poster knew what he was wribling about. What was curious is that the table you posted for me had no reference to K75C after '88 while my bike was built in '90 and sold in '91 as a K75C. Guess we aren't the only ones confused.

K75cster wrote:for clarities sake read the R/H side of your rear frame tube above R/H footpegs. it will say K75 something and you should also have a date there usually 08/87 etc but if it is a K75 it should have a sports fork set in it, ready to see if the forks have a bridge/brace and a two piece fender set, usually signifying a sports fork with 135mm travel. i believe you will need the numbers on  the inside of the fork tube right next to the tyre, so you have to remove the wheel to read them, god bless those little bike builders

The bike does have a bridge/brace and a two piece fender set. But one of my "improvements" was to bead blast and powder coat the frame. You wouldn't believe the amount of rust. But powder coating fills in some of the stampings as well as removing the factory VIN sticker. I'll look at the fork tubes (I believe you mean the lowers) in the morning.
thanks

I could be wrong and it wouldn't be the first time but '90 sounds too late for the C model.
Better heads than me will chime in to confirm. My K75C has 41.3mm forks no S stamp.

Most sources show the K75C ending production in 1990. From that point onwards the naked K75 is just that, listed as K75.

The easiest way to separate the two is by the rear brake (drum for K75C and disc for K75) and rear wheel. The 'C' has the drum cast into the wheel.

MikeP wrote:Most sources show the K75C ending production in 1990. From that point onwards the naked K75 is just that, listed as K75.

The easiest way to separate the two is by the rear brake (drum for K75C and disc for K75) and rear wheel. The 'C' has the drum cast into the wheel.

The first gen of K75 standards (86-89) also had a rear drum brake. In 90 they got the rear disc.

The last K75C was built in March of 1990.

duck wrote:

MikeP wrote:Most sources show the K75C ending production in 1990. From that point onwards the naked K75 is just that, listed as K75.

The easiest way to separate the two is by the rear brake (drum for K75C and disc for K75) and rear wheel. The 'C' has the drum cast into the wheel.

The first gen of K75 standards (86-89) also had a rear drum brake. In 90 they got the rear disc.

The last K75C was built in March of 1990.

 
There are some sources that list K75 production from 1986 to 1996. They appear to be contradicted by BMW who list the K75C as 07/85 to 03/90 and K75 from 07/90 to 09/95.

There does not appear to be a 'K75' model designation prior to 07/90 (no matter what national importers may call them), if they were neither K75S nor K75RT, they were K75C.

MikeP wrote:Most sources show the K75C ending production in 1990. From that point onwards the naked K75 is just that, listed as K75.

The easiest way to separate the two is by the rear brake (drum for K75C and disc for K75) and rear wheel. The 'C' has the drum cast into the wheel.

Dunno Mike, maybe there was some sort of cross over period right at the end of the production run of K75C's. The bike under restoration is by all sources built in 3/90 but has a disk brake in the rear and as I mentioned, Max BMW (who may not be the final answer but who surely got their parts info from BMW) shows it by VIN to be a K75C.

It's no longer there but the official BMW Motorad site used to have spec sheets up for the old models.  The one for the K75 standard had production starting in 86 and ending in 96.

If you look at the parts at the RealOEM site, it only shows parts being available for the Gen 1 K75 available for Europe, not the US, so the Gen 1 (0561) standards may not have been exported.

And here's a listing of K75 production VINs by month:  http://www.bmbikes.co.uk/chassispages/enginechassisK75.htm

jjefferies wrote:

MikeP wrote:Most sources show the K75C ending production in 1990. From that point onwards the naked K75 is just that, listed as K75.

The easiest way to separate the two is by the rear brake (drum for K75C and disc for K75) and rear wheel. The 'C' has the drum cast into the wheel.

Dunno Mike, maybe there was some sort of cross over period right at the end of the production run of K75C's. The bike under restoration is by all sources built in 3/90 but has a disk brake in the rear and as I mentioned, Max BMW (who may not be the final answer but who surely got their parts info from BMW) shows it by VIN to be a K75C.

It could be that it has been swapped to a disc brake set-up at some stage. I personally know of one at least where that's been done using a front-end damaged K75S as a donor.

duck wrote:And here's a listing of K75 production VINs by month:  http://www.bmbikes.co.uk/chassispages/enginechassisK75.htm

That list highlights my point regarding accuracy; it lists K75T (US market only). That's because BMW do not recognise K75T which was a BMW(NA) parts-bin "special".

Look at the fiche that BMW dealers use (Max BMW is one). It lists the K75C and K75 giving the span of the production and they do not run concurrently.

To help identify your forks perhaps this thread will help
https://www.k100-forum.com/t4858-front-forks?highlight=forks

duck wrote:And here's a listing of K75 production VINs by month:  http://www.bmbikes.co.uk/chassispages/enginechassisK75.htm

Ok, the list does show the bike as a K75C on the last line of K75C/K75T. Last seven of the VIN are 0133119. And it came to me with a rear disk brake and the swing arm etc setup for disk. For whatever that matters.

jjefferies wrote:

duck wrote:And here's a listing of K75 production VINs by month:  http://www.bmbikes.co.uk/chassispages/enginechassisK75.htm

Ok, the list does show the bike as a K75C on the last line of K75C/K75T. Last seven of the VIN are 0133119. And it came to me with a rear disk brake and the swing arm etc setup for disk. For whatever that matters.

That's one of the last Cs made (March 90) so it may have come from the factory like that.  Or a prior owner converted it from drum to disc.  It's not very hard to do.

Just to help with information on K75 'S' forks, I have a complete set fitted to my 1987 K100RS (triple clamps, tubes, sliders and all). The slip right in with no rake/trail number differences. The immediate difference is that they have a much shorter, firmer spring rate, use just 280cc of oil per leg, and have a small 'S' scribed into the top end of the tube underneath the black plastic cap. Mine are the two-piece mudguard (fender) version. I prefer to use 5W fork oil as they're very stiff in comparo to the various other longer travel, softer sprung K versions. The 5W helps with small ripples in the bitumen (tarmac). Combined with an nicely adjustable but stiff, Italian-made Bitubo rear shockie I 'feel' the road beneath me.

I am certain I have the "s" forks on my K75s and find them far to hard. I had stripped and checked them before going on a long distance trip. Strangely the left leg dampened but the right had no dampening that I could notice. Also the internals are different with the right having big 10 or 12mm holes, which would certainly not dampen.

Nevertheless I went on the trip, and traveling on the dirt road the left leg started leaking at the seal. There was a clatter. I then checked, while on the trip, how much oil was lost and it was only about 10-20ml. It was better after having replaced the oil, but soon the clatter was back. (I think the addition of air did the trick)

I have now stripped the left leg to replace the seal but am not convinced that that will sort the clatter, dampening and hardness out?

Are the any threads that can guide and advise on this subject?

The parts in the left leg and the right leg differ. If all the parts are there in the correct order and the bushings aren't scuffed and springs aren't fatigued, experimenting with oil weight could help steady the ride. #6 is the right leg; #12 is the left leg. MaxBMW's diagrams often help with clarification. This is the front suspension section's diagram of a Sport (S) fork assembly.

If you've wondering if you've got S forks, and it sounds as though you do, the easiest way to ID them is to note the handy li'l "S" stamped into the top cap on each leg. I always used 5W oil (280cc per leg) measured carefully in them. They're ruff as guts over, well, rough roads, otherwise. When my '87 K100RS, on which I was using them, went away, I kept them and we fit the longer travel plushy originals back onto it. They'll go on another early K one day in the future. As Laitch suggested, if everything is tight and in good order inside, they shouldn't make unusual noises as they function. The right had side has no damping properties to it, it's all on the left hand side to do that part of the job.

Note:
Dampening or to dampen is to throw a wet blanket over a fire. Damping controls oscillations.

Thanks, I assembled and tested the forks after replacing the seal and filling 280ml 5w oil. (I had used 280ml 5W before). The parts are all there as per the diagram and the "s" mark too. Laitch, thanks for the Max BMW link.

The ride is certainly better but going over a rough section of road it does feel like it should be smoother. If the right side does no work, maybe one can use 15W or 20W instead of 5W on the right?

Note: noted 

Two Wheels Better wrote:Note:
Dampening or to dampen is to throw a wet blanket over a fire. Damping controls oscillations.

I nominate the author of this comment for consideration as an inductee into the grammarian branch of the Anglo-American Sisyphean Society.  

Like the scorpion (or spider, as the variations go) who requests safe passage from the turtle to cross the river but who stings his carrier midstream, leading to both their deaths, and crying out when asked, "But why did you do it?" "I can't help it. It's in my nature."

Had a look at this Ultima today, something more for the dirt road, than my s. Like one member said, the K75 is a trooper.

I see though that these forks differ from mine. The dust covers and the bottom clamping are different. The fork travel seems more and is a lot smoother than my "s". Would these have the OD and triple clamps?

The screen is yellow. Can that be brought back to clear?

Would these have the same stanchion OD and triple clamps?

Two Wheels Better wrote:"I can't help it. It's in my nature."

It is rumored that Sisyphus has an admirable set of pecs and quads, so the effort has been worth something after all. In fact, he may be posing in a selfie on TikTok right now.

Maybe there is something like an intellectual six-pack, too.

Laitch wrote:Maybe there is something like an intellectual six-pack, too.

I drank that ale a while ago and all I've got to show for it is the plastic can carrier which can't even be recycled, and has to be cut so's the little creatures don't get their heads stuck in the rings.

Rolf E wrote:

Would these have the same stanchion OD and triple clamps?

Whack an accurate calliper onto the stanchions. The offset, etc., would be the same, but I recall minute diametre differences between the longer travel forks. Enough to mean that fork seals are not the same and attempting to tighten things up might just ruin a perfectly sunny arvo.

Using the entire assembly is a direct bolt in, however, and it's likely time for some nice, new steering head bearings anyway.

There is a dimensional chart for Brick forks in post #2 of this thread. An Ultima would have a Showa fork. The axle end of the fork casting differs in each model. There is a recess between the pinch bolts of the Sport fork. The stanchions of the Showa are 0.3mm smaller in diameter than those of the S, so the triple clamps would be needed to swap one fork style for the other, as Two Wheels Better indicated.

Thanks Laitch, I eventually figured it out, but this helps, and is exactly as you described it.

Rolf E wrote:The screen is yellow. Can that be brought back to clear?

I got a kit from my local mechanic recently and did the headlights on my Hilux. Seemed to work fairly well and might be worth a try. Cleaning pad, drying cloth and a pad to spread some stuff over the plastic and leave a film.

Rockman wrote:

Rolf E wrote:The screen is yellow. Can that be brought back to clear?

I got a kit from my local mechanic recently and did the headlights on my Hilux. Seemed to work fairly well and might be worth a try. Cleaning pad, drying cloth and a pad to spread some stuff over the plastic and leave a film.

One issue with many BMW windscreens is that they are made with a film on them.

Cleaning that screen by hand would be a PITA. I wonder if you could use some sort of buffer or a very fine media blast. And then final polish by hand.

I have not yet acquired the bike. The seller wants too much for what needs to be done. So I'll still wait and see if something else becomes available.

A friend suggested this "PLASTX" product. So when the time comes, I'll test it.

BTW, on my K75s  I found the hydraulic brake pipe positioned in the steering column loose. Having sorted that out there is less noise but it still feels like the one or both of the stanchions are "bottoming" out either at the top or the bottom. When I had the stanchion apart I measured the spring lengths (one short and one long) but the Clymers Manual only gives the length of the normal spring, longer than the two combined.

Does anyone perhaps have the specs for the "s" spring lengths?

Rolf E wrote:Does anyone perhaps have the specs for the "s" spring lengths?

Until an authoritative answer to the spring length comes along, just assume they’re ok. However, if all the parts are there and can be placed in the correct order, you can determine if adjusting the left and right dampers into their specified lengths helps your cause. The instructions start as the bottom entry on p. 31-17 of BMW's K75 K100 2V service manual downloadable from a link on this page.

Also, testing different weights of fork oil should be in the program after cleaning, damper adjusting and assembly is finished. Start with 10 weight and let us know how that goes. Eventually you could feel the need to spend on cartridge emulators.

Rolf E wrote:
BTW, on my K75s  I found the hydraulic brake pipe positioned in the steering column loose. Having sorted that out there is less noise but it still feels like the one or both of the stanchions are "bottoming" out either at the top or the bottom. When I had the stanchion apart I measured the spring lengths (one short and one long) but the Clymers Manual only gives the length of the normal spring, longer than the two combined.

Does anyone perhaps have the specs for the "s" spring lengths?

From what I can see there are no specification for the 75S spring lengths.  Both my Haynes manuals, the 1993 & 1999 versions state that the S Specs are Not Available.  Even BMW's own repair manual only list the fork spring length of 395mm for K75 & K100.  So I guess it is one of those secrets lost to time or sitting on a shelf gathering dust in Munich.  Maybe one of the other K Bike forums may have an answer.  

The only possible way I can see to alleviate the bottoming out with your moto may be to fit emulators, a spacer in there or a thicker grade of oil.  Unless of course you can change springs or find there are internal components missing.

Oh, and what year is your 75S?

Rolf E wrote:I have not yet acquired the bike. The seller wants too much for what needs to be done. So I'll still wait and see if something else becomes available.

A friend suggested this "PLASTX" product. So when the time comes, I'll test it.

I use PlastX on my windscreens and face shields. It's for routine cleaning and polishing, not rehab projects.

TacKler wrote:
Oh, and what year is your 75S?

It's red like yours, 1989. With a low seat



I am going to try the heavier oil and hope for the best. But the emulators you guys suggest look like an answer. Our roads here are not of the best. Patched and patched and patched after 30 years of no maintenance. 

First off, regarding the windscreen.  I have not been successful rehabbing mine.  What I would suggest is doing a black paint job on it.  It's not like you are going to be looking through it.

Second, I was annoyed at the turbulence mine threw at my head and the noise in my helmet even when I was wearing ear plugs.  I made the situation a lot better by shortening the screen about 4-5 inches.  That put the turbulence on my chest and put my helmet in clean air.  Much quieter!

To paint the screen is an option to consider.

The previous owner did the opposite. He attached a deflector to the top of the screen. It worked well but does not look good. I took it off and am happy as is. 

Did not know the the K75 came with ABS option. Your bike is looking good and I think I will also fit additional lights.

The 10W oil made quite a difference.  It does not bottom out as much (hardly) but going over a rough road section it still does not feel as smooth as I think it should.

Been thinking about the assembly order. There are 2 springs and 2 spacers on top of each other respectively. (the springs at the bottom and spacers on top) Should one not have one spacer between the springs? i.e. not metal on metal bouncing against each other.

Is there any preference or performance difference between the Showa and the Brembo forks?

Rolf E wrote:Should one not have one spacer between the springs? i.e. not metal on metal bouncing against each other.

Is there any preference or performance difference between the Showa and the Brembo forks?

They aren't bouncing, Rolf. Everything is compressed together during assembly and becomes more compressed with the weight of the Brick itself and the rider. I tend to go along with the technical data and diagrams I'm given by BMW hoping that the engineers weren't drinking their lunches during design, that the editor and technical proofreaders were on the ball during the shop manual's composition, and that I'm using the correct manual.  I just want to finish up and ride.

The Showa fork is my only experience. I ride rough gravel roads approximately a third of my travel time. It would be more comfortable using suspension with greater travel than the 135mm/114mm front and rear stock set up, but I'm dancing with the one I brung and she's been pretty good for the last 60K miles over these roads. Maybe I'll get a DR650 if my Brick kicks the bucket. The DR wouldn't hold a candle to the Brick's comfort and speed on pavement, that's certain, but it would deal with misery more gracefully.

You can always try 15 weight oil. I'm using 10 in my Showa. Verify that the tubes are being filled with the correct amount of oil. I weigh 170lbs and side cases usually have 5–10lb loads each.

The ride home—Class 3 gravel, municipally maintained, 4%–6% grade with a couple of sphincter tightening slopes in the mix, usually smoother road surface but can be tender for days after heavy rain or road personnel shortages.

Laitch wrote:
The ride home—Class 3 gravel, municipally maintained, 4%–6% grade with a couple of sphincter tightening slopes in the mix, usually smoother road surface but can be tender for days after heavy rain or road personnel shortages.

Feck. I've seen/ridden better roads in the Himalayan Mountains!!!

I remember Vermont roads like those back in the '70s when I was first brought to 'Merca. The town of Mallets Bay didn't have many but when we moved over to Williston the school buses couldn't drive over some of the most rural roads, especially in late winter and springtime during "the melt."

Oh how I love to get into a K75 "S" fork thread 

My factory naked 1990 K75 (low seat, rear disk brake) came with the Sport Fork. Here is some info that our late guru Inge put me onto way back in regard to the correct layout of the springs and spacers and a image clip out of the K100/K75 service manual that I believe was compiled by the Flying Brick forum with assistance from BMW which is probably in our forum archive.



A photo of my parts (they were not in this order when I first stripped them, in fact the components were laid out differently in each leg)

The longer progressive spring is 263mm long
The short heavier spring is 87mm long
Longer spacer is 110mm long
Shorter spacer is 70mm long
Total length is 530mm

As always I'm sure there are lots of varying opinions out there. Have fun!

Thanks Guys 

I still have some work to do . . . on this one. The set-up I have is as per the manual and as per Gaz's pic. I also agree with you Laitch regarding the engineers doing a good job. One thing I am still going to check is the steering column bearing clearance, if there is any play and if the preload is sufficient.

Also the "s" is not really a dirt road bike and that is why I am keeping my eyes open for a K75C or Ultima which has more travel than the s. Here some of the roads we recently covered. Unfortunately one guy in the group wrote his R1250GS off. 

Rolf E wrote:Also the "s" is not really a dirt road bike and that is why I am keeping my eyes open for a K75C or Ultima which has more travel than the s. 

The K75C and Ultima are no more "dirt road bikes" than the K75S and according to BMW's shop manual, the suspension travel is essentially the same on all of them. The Showa fork on the Ultima could possibly have a few more millimeters but not make a significant difference. What can help control is having tires that aren't fatter than stock, throttle control and brakes that are responsive to light input, patience, and if ABS is in the picture, the ability to disconnect it for steep loose road surfaces.

If you want a more dirt road capable version, you already have a good template. There is no need to harpoon Moby Dick. Try to modify your current Brick like member jbt did his. That will keep you occupied for sure.

One thing you could do to make a K75 more dirt worthy is run some dual sport tires like the Conti Trail Attack 3. Even better, it is a radial so it makes street riding better too!