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K75 GS conversion Fri May 26, 2017 9:00 am
jbt
Life time member
Here's a post about the bike figuring in my avatar.
Riding bikes for 30 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 hade chosen this model because it has a drum rear brake, so 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 biks. 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 panniers, 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.
Next ride, the Stella Alpina Rally in Italy, 2nd week end of July.
Riding bikes for 30 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 hade chosen this model because it has a drum rear brake, so 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 biks. 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 panniers, 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.
Next ride, the Stella Alpina Rally in Italy, 2nd week end of July.
Last edited by jbt on Fri May 26, 2017 10:45 am; edited 2 times in total
