Author Topic: K75GS build & travels  (Read 1916 times)

Offline jbt

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K75GS build & travels
« on: November 01, 2022, 06:35:41 AM »
Here's a post about a bike I've build during winter 2015.

Riding bikes for35 years, most of them were BMW GS or K (some of them were blue with sirens...)
I love the abilities of the GS to ride either on or off road, and love the confort and reliability of the K.
So I've been wondering why BMW never  made a K75 or K100GS, that would be a perfect long range trail bike.
It appears that apart some prototypes made for the Paris Dakar or bitzas made by some creative owners, this project never was made by BMW. So I decided to create it the way BMW should have done it. That means only with BMW parts.
And guess what? It's an easy job. Long but easy.

The donor bike is a 1985 K75C. Basic, with aprox. 100 000 kms, but in good genuine condition. I had chosen this model because it has a drum rear brake, so making it easier to fit a R100GS spoke wheel instead of the disc brake later K75.
Why a K75 and not a K100? Because I want a 21 inch front wheel and I may have not room enough at the front engine with a 4 cylinder.
I ran a few months with the K75 in its genuine condition to spot defects, then a friend of mine borrowed it...and crashed. So the paint job was financed and I could start the modification.
The rear first: gear box paralever and final drive come from a K1100LT. I had chosen this instead of the monolever to avoid issues due to the higher suspension. With 2 ball joint articulations, the paralever is more tolerant to a wider angle.
At first, I wanted to fit a R100GS final drive on the paralever with K1100 driveshaft. Bit if it fits easily the swinging arm, the bevel shaft is different and has no circlip on the GS. I would have to open it, shorten the shaft and create room for the circlip. So I keep the K11 bevel, to see...

This option led me to adapt a rear disc brake and a R1100GS rim. Yet the caliper is different on R11 and K11, and I had to mill a bit its support to fit to the bevel case. Another issue is that the rear shocks is moved apart from the case, because there's not enough room for it besides the R11 caliper. A longer stud and washers solves it. I used the K11 genuine shock, longer travel and very soft but facing the big compressions easily.
I also had to mount a K11 footrest support, to fit the with of the paralever pivots.

The front, now.

The fork, the wheel, the bar come from a R100GS. Easy to fit, nothing special, the bearings are the same. I supressed the genuine useless direction damper.
I modified a right fork stanchion to fit it to the left and allow to have a second caliper. (Drill larger for the wheel axle, grind to adjust the caliper fixings and to give room for the disc rivets).
I kept the twin discs from the K75, narrowed them to fit the GS hub, bolt on, and made two new brake lines.

The front light and cockpit come from a R80G/S.
I cut the high part to put the original K75 instruments, on gum absorbers to prevent vibrations from destroying it. It works but the angle is not very efficient, I'll have to re adapt this.

This was looking weird. Looks like the Franco's spanish guards...

So a white GS windshield hides this fascist looking of the cockpit...

Then, with long travel suspensions, I had to lower the stands to fit the extra height of the bike. Longer studs and spacers, and it remains easy to put on stand and well balanced.

I also kept the exhaust system. I don't intend to fall, nor to cross rivers, and it allows to use the genuine tragkorbs, so...Maybe I'll adapt a R100GS higher silencer later.

Then a new foam and seat cover, two knee pads painted in blues to evoque the genuine R80G/S color scheme, home made logos with the genuine police K75 GS and...that's it.

After a few rides, and 2 months searching for a bloody problem of pressure (the 2 inches long line that leads to the fuel filter into the tank was cracked! No way to see it, can't be detected when measuring pressure beacause the pressure drop is progressive, what a mess! I eventually found it because I inspected the inside of the tank, engine running at high RPM, and saw swirls at the surface...) it's a great trail bike.
First, on road, the extra wheel base due to the long fork and the paralever gives a grat stabilité to the bike. In spite of the Metzeler Karoo tyres, it handles very easily and there's grip on tar. The confort is excellent, better than on my K1100! Yet, the gear ratio is a bit long, but that's not a problem on road, and an advantage on highway.
Then, off road, the smooth suspensions are perfect for motricity. Anyway, they're designed to support bikes with a bigger weight and they'll cope with bumps or holes easily.
There's enought travel at the rear for casual tracks, and if it's too rocky, ride gentle and it wil pass.

I had some rides in the Alps, and never noticed defects for this use, but maybe this long ratio that sometimes makes delicate the ride in verry narrow pins. Well it's a trail bike, not a trial, and still easier than a R1200GS adventure...

The gummy footrests are not adapted to off road use, they're unprecise and should be changed for alloy spike footrests.
I'm considering converting it in a solo seat, with a long luggage rack.

Is the filiation evident enough?

Ready for the Stella Alpina rally, behind the Mont Blanc...

  • France
  • K100, K1100LT, K1, K100RS, K75GS, K100+Sidecar, K1+sidecar

Offline jbt

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Re: K75GS build & travels
« Reply #1 on: November 01, 2022, 06:56:58 AM »
Since the firsts rides, some improvements had to be done.
First, the sump cover plate, adapted from a R80 G/S.

The mountings were just provisory, and were remplaced by more solid ones. Maybe not a good thing, I prefer the mouting to bend instead to crack the engine case.

Then, I found a pair of Fehling crashbars. Much more protection than the OEM ones, they fit perfectly the GS look:

Later, the crash bar and the skid plate were joined together, with a front junction supporting a protecting plate for the water pump and the ignition trigger.

Then the footpegs: I tried to adapt R80G/S footpegs, but, to fit them on the K support,  I had to grind them too thin to support my weight. Even playing with MIG welder was not enough to strenghten then. So I failed to my rule to use nothing but BMW parts and adapted a pair of aluminium footpegs from a modern off road bike, with more meat to bite in.

And it's perfect.

A little improve of my K75GS: the OEM K1100LT shock was perfect for it, but I noticed that it was sometimes not stiff enough on big potholes when the bike was fully equiped. So I bought a used Ohlins shock for a K1, intending to fit it on the K75GS.

Two problems appeared:
- the soft line to the compression setting cracked and leaked. So the shock was rebuilt at a specialist nearby, and I got the spring resprayed.

- there's not enough room between the bottom of the shock and the brake caliper.
I thought it was easily solved by rotating the drive cover. But the holes are not symetrical, no position fits. The solution is to weld another fitting for moving the caliper back. I cut an old K100 cover to get the new fitting welded: 

The rear original fitting becomes the front fitting.

I asked many professional welders, but most of them refused to take the job as soon as they hear the magic word "brake": No, I won't take any responsability about brakes. Sad.
Eventually, I found a good welder who accepted. And made a very good weld. No finition required, nothing is too thick, it fits perfect.

I had to cut the breather that, now, is in the front of the caliper.

And my K75GS has now a beautiful and very efficient Ohlins shock. :cheers:

The compression setting device is hidden behind the battery, protected but reachable.
The preload setting is just at the front of the shock.
Click here to view travels done with this moto.
  • France
  • K100, K1100LT, K1, K100RS, K75GS, K100+Sidecar, K1+sidecar

Offline Laitch

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Re: K75GS build & travels
« Reply #2 on: November 01, 2022, 07:06:29 AM »
Bravo! For those who are under the impression that adding strap-ons, a two-bit speedometer, a clown shoe seat, knobby tires and a $9000 price tag to a clapped out K is "bike building," this is what it actually looks like.
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  • 1995 K75 89,000 miles

Offline Past-my-Prime

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Re: K75GS build & travels
« Reply #3 on: November 01, 2022, 10:03:27 PM »
I am very impressed. You chose to go back to disks for the rear. And dispensed with ABS. You did this without adding a lot of weight. How well do the OEM luggage hold, going across the bumps?
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Offline MEZ

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Re: K75GS build & travels
« Reply #4 on: November 01, 2022, 11:46:09 PM »

* Screenshot (335).png (76.85 kB . 768x512 - viewed 513 times)
I chose to build mine for a different purpose but the same reason, that 75 engine...!! I love what you have built and your approach Sir, well done.
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Offline frankenduck

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Re: K75GS build & travels
« Reply #5 on: November 02, 2022, 12:00:41 AM »
VERY impressive.

If you want more torque you can re-gear the paralever drive to 32:10 to be like the 32:10 of K75 monolever.
Once I had a Collie pup. Dug a hole and covered him up. Now I sit there by the hour. Waiting for a Collie-flower.
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Offline jbt

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Re: K75GS build & travels
« Reply #6 on: November 02, 2022, 01:27:24 AM »
Thanks for the comments.
The OEM luggage is OK, never had any issue. I'm not afraid to lose it but to be unable to take it off! But in case of crash, no surprise, plastic is not solid. Yet I prefer it to expensive aluminium cases, as they are very cheap (I have dozen of cases in the attic, nobody wants them) and repair is very easy by melting or soldering with acetone. I just take a pair of spare hinges.
ABS? Unnecessary weight for a fake security and inefficient braking? No, thanks, really. If it was equipped with it I would have removed.
About the gear ratio, on one hand, it would compensate the lack of torque at low rpm. But the K75 engine is soft enough to cope with it and I never had to use the clutch to get out of a tricky situation. On the other hand, I appreciate long ratio as an overdrive on long journeys and the  fuel economy it provides. And re-gearing the paralever will create an issue about the speen signal trigger: I'm not sure that the OEM K75 bevel drive fits the K1100 case, but if it does, I'm sure that shimming it is boring enough to appreciate the long gear ratio.

  • France
  • K100, K1100LT, K1, K100RS, K75GS, K100+Sidecar, K1+sidecar

Offline Rcgreaves

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Re: K75GS build & travels
« Reply #7 on: December 09, 2022, 11:11:30 PM »
terrific build, inspiring actually.  I'll do some serious gravel on my K75 next year, GS style tires and go. Thanks for sharing...
  • Livingston in Southwest WI. USA-"With the good earth all around."
  • 94' K75S, 85' GL1200 Aspencade, 96' VFR750F, 01' GL1800. Restoring: 95' K1100RS, 83' R80RT NEW: Motorvation Formula II
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