Tuesday, November 15, 2011

K75 GS Adventure?

While browsing through the Adventure Rider Motorcycle Forum on K Bike Cafe Racers, I vectored off on this thread...

Source: Frank Warner fwarner_au@yahoo.com.au  Dated 22 April 2002 http://www.geocities.ws/fwarner_au/mc_things/abmwk75gs.html

WHY a K75 G/S?
I have a BMW R80 G/S that has been very good, but it does not like high temperatures, creek crossings, constant high speeds and requires a fair amount of maintenance. The K model motors have been very reliable, economical and have reduced maintenance compared to airhead motors.
The K75 is lighter and smaller than the K100, heaver than the R80G/S but not as heavy as a R1100GS.
The electrical complexity of the K could be a problem if a fault occurs in the field. However the K electrics are more reliable than the airhead electrics. In part this is due to the water cooling of the K reducing the temperature fluctuations and maxima experienced by the electronics on the R series. And the fact that the K series are a little newer.
A second hand K75 GS has come up for sale. It has had the suspension modifications done, together with the frame, stands, bash plate and rear wheel modifications. Cheaper to buy it with these things done, and the previous owner has done some testing of the mods!

Engine and gear box

Torque Comparision of K and R.

The above torque curves are calculated from

  • the R80 G/S riders handbook (includes a torque diagram)
  • the K max torque and the 2,500 rpm torque from the BMW Profiles 4 book. BMW Order No. 01 09 0 035 281
The rest of the torque curve is estimated by me using the R80 curve, thinking they have the same shape. If you have a torque curve for any K75 well, I'd like to put it up here for the comparison.
The engine torque is multiplied by the gear box and final drive ratios to obtain the rear wheel torque.
As you can see the K has more torque over the entire speed range, and a wider speed range compared to the R.

Engine and gear box - Maximize engine flexibility so as to obtain a minimum "walking" speed to trickle through rough spots? The K gear box compare to the airhead is a closer ratio gearbox. The K has a top to bottom gear box ratio of 2.69 , 8.7 for the engine top to bottom speed making an overall maximum speed to minimum speed ratio of 23.40. The airhead has a top to bottom gear box ratio of 2.93, 7.4 for the engine top to bottom speed making an overall top to bottom speed ratio of 21.61. So the K is overall more flexible, but would be even better with a wider ratio gear box.

I am assuming that the idle speed is the minimum speed for both motors in first gear. This is where the K has a disadvantage compared to the R, the R has less torque at idle thus actual decreasing its speed more readily than the K does. So when loaded the R goes slower than the K.


Ideally set for a cruise speed of 140 km/h. Top speed say 180 km/h? That would be a 10% reduction in overall gearing, from a final ratio of 3.2 to 3.52 .. However the lowest available ratio is 3.36 i.e. a 5% reduction so that would lower the top speed from 200 km/h to 190 km/h. I would like a 5% lower bottom gear for the K gear box, to achieve a total 10% reduction. Is there such a thing? Maybe it will have to be a special, if so I think leaving the standard rear gearing, and lower bottom gear say 15%? This would keep the cruise speed and economy, and provide a good low speed gear at around 7.3 km/h with the clutch fully engaged. If so that would make the gear box as wide in ratio as the airhead.

Some thoughts on low speed running

The idle speed is set for smooth running with no load and no throttle. The motor will run at a lower speed - other wise the starter motor would have to spin the motor up to at least the idle speed to start the motor!
The K motor will run lower if the idle speed is adjusted down. The reason why you should not do this are the motor may run rough, possibly stall without added throttle, and reduced cam chain (and chain slider) life. So setting the idle speed down can be done, but I would only do it for the time I needed the speed/torque reduction. But I need to experiment with this. It is certainly a cheap option compared to a gearing change.
The following are some of my thoughts on making a K75 G/S before I bought the bike. If you want to see further mods then you will have to look at my other page on improvements to a K75 G/S.

Front Suspension

A K75GS by Paul Rooney. Replace front end with a dirt bike front end - Includes front wheel (21 inch), triple clamps, leavers,
Along with this goes the front brake, say 300 mm diameter disk with a 4 pot caliper. 6 pot calipers I think are overkill provided you are not carrying a lot of weight.
Ensure the forks are strong enough for the weight carried. The triple clamp may need to be made up to suit the bmw head stem, the new forks and the geometry required.
Suspension travel between 250 mm and 280 mm.
The actual bike came with 300 mm travel and a 6 pot caliper.

Rear Suspension

A K75GS by Paul Rooney. The R80 G/s has 170 mm travel on a 415 mm swing arm length (pivot to axle centre), which is around 12 degrees movement in both directions with respect to the gear box output flange. My K11LT has about 16 degrees on the rear paralever joint.
Replace rear shock, extend swing arm. New mounting points for the shock may be required.
The actual bike ended up with around 213 mm travel, around plus and minus 15 degrees.

Rear Wheel

All the K bikes come with alloy wheels. Enduro and Dirt motorcycles usually have spoked wheels... because they 'give' a little with impacts, this flexing makes them undesirable on hard smooth surfaces. But on dirt the flexing absorbs some part of the impact, reducing the possibility of damage. Some people think spoked wheels are stronger than alloy wheels. What ever, I would like a spoked rear wheel. Tradition? Maybe so, but if there was something better you would see them in competition.
The K75C came with a drum brake, this makes the wheel changes a little more complex involving the drum. It also can have a brake chatter as the suspension rises and falls and the brake is applied. It would be better if either the brake was cable operated or the pivot point for the brake was at the swing arm pivot point.
Better with the disk rear brake of the S or RT models as this over comes the wheel changing complication and the brake problem.
For spoked rims two possibility exist, the rear wheels off the R100R or R100GS. The K75S comes with a 130/90 - 17 tyre on a 2.75 rim (weight 229kg wet).
The R100 GS come with 130/80 17; 2.50 rim (weight 210kg wet), R1100 GS with 150/70; 4.00 rim (weight 243kg wet), R100R 140/80 17; 2.50 rim (weight 218kg wet).
Either the R100GS or the R100R rear wheels could be adapted to fit. Both of these are tubeless rims. Suitable tubeless dual sport tyres are available from Prielli (MT90) and Metzeller (Tourance and Enduro 4) in both 130 and 140 sizes. Adaptors to the hub are required, for the drum brake the shoes must be extended outwards, for the disk brake I think a spacer is required.

Strengthen and modify top shock mount to increase rear wheel travel.
Move rear mudguard back to accommodate longer swing arm if fitted.
One of the frame failure points is between the pillion and rider just after where the bottom brace joins the top part of the frame. Another failure point is around the steering head.
Lower foot pegs, replacing the cast aluminium mounts with a soft (bends rather than fracturing) fitting.
Seat height - not too high! Say 860mm? If the footpegs are lower then lower the seat height by a similar amount.

Bash Plate

Protection for the sump, water and oil pump. The bash plate can mount to the sump tube used for the crash bars. It should protect the sump area from rocks and sticks, while letting cooling air circulate. It must not significantly increase the time taken for oil changes, or oil filter changes. Possibly the front mount should pivot, so that once the rear mounts are undone the plate can be moved out of the way giving access to the sump. It should be light, quickly removable and easy to repair/replace.

Centre and side stands

A K75GS by Chris, at TTT rally. Extend lengths for the increased ground clearance, and broaden contact surface for sand and earth support. The centre stand bolts to the bottom of the gear box, this mounting could be moved downwards using spacers until it starts to reduce ground clearance. Possibly the new footpeg mounts could use these spacers for the bottom mounting points.
Exhaust mods may be needed for clearance?

Weight and balance

A K75GS by Paul Rooney. This bike is not intended for tight 'trials' type work, nor was the R80G/S... it is intended for touring dirt roads where a normal road bike has problems. Must carrying enough supplies for days away from stores, most dirt bikes don't have the load capacity. Probably why dirt dikes are so light, they don't need the strength to carry the weight, nor are they expected to last as long. The weight of the K compared to the airhead is a little more. The centre of gravity is not much higher as the airhead carries the alternator and starter motor high compared to the K where as the K has the radiator carried high. Water is lighter than metal..
Currently (April 2002) the bike weights about 205 (113 rear) kg wet (full tank of fuel, oils and coolant, but no tool kit) [I used the bathroom scales so inaccurate]. My old R80 G/S is suposed to be 192 kg (13 kg less) [with tools, I guess that the tools weight 2 kg?]

Electrics (simplify)

It has a husky speedo fitted that contains the idiot lights. This replaces the original instrument cluster. The idiot lights are not very visible. It does have a trip meter. Other than this it has the standard electrics.

A K75 G/S

What I got - I bought an existing K75 G/S - one of about 4 or 5 in OZ.


 This looks like a husky front, and it is very similar. The headlight, speedo are off a husky. The front suspension is a white power unit that was bought new as the best suspension available at that time. The wheel is 21 inch, with hub from a yamaha, disk from brembo, caliper from harrison (6 pot).


 The wheel is an R100 GS or R with spacers inside the K hub to fit the wheel to the centre and brake.
The suspension is bascily an R80G/S shock unit and a heavy duty spring for an XR650. The mounts were changed to obtain the required travel (welding two plates on top of the swing arm).
The rear part of the original frame was cut off and a new simpler and lighter frame made up.

Non BM Bits

Light and simple is the key. The seat base is a honda four unit, any light plastic base with a simple mounting system could be used. The rear mudguard top is a universal with brake light type. The front mudguard is now a honda XR unit - no holes directly behind the tyre so maybe less mud into the radiator. The side panels shown are YZ units. I've removed these now as they don't cover the battery area and were interfering with my pannier mounts. I've made a prototype one on the left that covers the battery area and should match the panniers when both are painted.


These have been extended for the increased ground clearance.

Improving a K75 G/S

The biggest areas of wanted improvement are currently the weight (and what dirt bike could not be improved by reducing weight?) and increasing the steering lock.

Present Mods

April 2003

Weight reduction

I have removed the standard battery holder and made up a new one from al sheet. The new battery holder also holds the ignition brain, fuel injection brain, fan circuit and fuel pump relay. The battery itself has the starter rely mounted to it as well but I might move it to the starter motor to save some space, it is also lower on the bike.

I will also fit the smallest Oddessy battery (about 9 Ah and 1Goddess less kg)to the bike. These batteries have more than enough cold cranking current to start the bike, but have limited capacity to run things like lamps and air pumps at a nights camp. Running the motor will provide the energy for these if needed.

The alternator is the 33Amp 460 watt type - the 60 Amp 700 watt variety is 0.7 kg more.
I have removed the light failure indicator, fuse box, electric blinker controller, horn relay. The plastic box containing these items have been remove too, thus freeing and area of about 4.5 litres for use as more fuel.

The left handle bar switches have been changed over to an old R airhead switch system. This has the advantage of using a simple rocker switch to control the indicators, thus using a simple indicator relay that is now mounted under the headlight.

I've fitted kawa KLR/X 250 front blinkers - these are light, large and bolt to the handle bars but tuck behind the bark busters so are well protected. The rear blinkers are more of a problem, at the moment these are Honda XL types that are small and tuck inside the frame so are well protected.
K75GSes near Tipboburra. Currently (April 2002) the bike weights about 205 (113 rear) kg wet (full tank of fuel, oils and coolant, but no tool kit) [I used the bathroom scales so inaccurate]. My old R80 G/S is supposed to be 192 kg (13 kg less) [with tools, I guess that the tools weight 2 kg?], a 2002 husky TE570 is supposed to be 117 kg dry add say 30 kg for the oils/fuel then 147kgs wet (58 kg less).

Steering Lock

The upside down forks reduce the steering lock considerably due to the increased diameter at the top. To increase the steering lock the top left and right side frame tubes have been modified by Lauarie Alderton. This frame modification is apparently done to racing sidecar rigs to increase their steering lock. The problem is now making good fork lock stops and modifying the tank to accommodate the increased lock.

Future Mods


Bottom of a K fuel tank. I'm going to use the original top section of the fuel tank, with a new bottom section. This means I don't have to have the metal forming and fuel filler manufacturing done for most of the bits that you see. The top section will sit some 8 highfighte cm higher to obtain more capacity.
The new bottom section can be made as simply as possibly yet maximise the fuel contained (I'm going for 35 litres) because of the simple shape that now exist under the tank. I'd like to move the fuel filter and the pressure regulator to a space on top of the ignition coils.
This would make servicing easier. The fuel pump remains in the tank using the surrounding fuel to dissipate heat. A plate that bolts to the bottom section of the fuel tank will have all the tubes that enter the tank, mount the fuel pump mounted to it.
I'll probably use a ford XF sedan fuel pump - it is cheaper and should be easier to replace around the country. The fuel pump is a Bosch pump part no 0 580 464 998.
To be done by August 2003. I'm trying for a fuel/riding range of 1000 kms and say 4 days. That is a lot of weight to carry.

Rear Stop Light

The single rear stop light is a bit dim in a following sun, it also fails. Two push bike LED lights will supplement the single stop light, but I've yet to make these have enough contrast in bright sunlight from off to on. A coat or two of red nail polish might do the trick? They certainly work too well at night!

Warning Lamps

The R100 Mystic has a nice set of instrument warning lamps that I might use for the oil and alternator warning lamps. The current lamps are too dim.

Side Stand

As the K75 (and K100) motor is known for burning oil if left on the side stand and the ring gaps happen to be on the low part of the cylinder. If the side stand is moved to the right then this will not occur.

The side stand can also be made adjustable in length by including a bolt and nut as part of the stand. The reason to make the length adjustable is due to the adjustably of the long travel suspension at both front and back can make the bike stand too tall or too sloped.

Bash Plate

I'm thinking of modifying this to incorporate sides that can contain parts and tools. Similar to those of the single cylinder trail bikes ... but I'm still thinking of this. Depends on weight bias front to back. The K 75 is heavy on the front wheel, so I don't know at this stage. The current plate is 3 mm, I may just leave that and add 2mm sides so damage to them might be easier to deal with.

Fuel Injection and Ignition System

The present system has two brains, one for the ignition system the other for the fuel. Combining the systems into one brain increases the reliability!
Think of it this way, a rickshaw pulled by two rickshaw pullers will fail when either of them fails, if the average life of rickshaw puller is 65 then probably one of them will fail before 65 the other past 65. A rickshaw pulled by one rickshaw puller will probably fail at 65(the average). Thats about as simple as I can put it?
The present ignition system only has the engine speed as an input, so throttle position and engine temperature cannot be used to gain better performance and economy. By using the one brain this information can be used for the ignition system.
The present fuel system fires the injectors all together, this is fine at high engine speeds where the air flow in the inlet is fairly constant. A better system has the injectors fire only when the inlet valve is open for that cylinder. This is used in vehicles with large injectors that cannot reduce the fuel supply low enough at idle. It aids the smooth running of these motors. I think it would help the K motor too.
The more recent K11 system has no air flow sensing, calculating this from the throttle position and engine speed. Thus removing the air mass flow sensor (weight about 1.5 kgs, and it is on top of the air cleaner) which is a component known to wear.
Duplication for redundancy in event of failure of both air and water temperature sensors is easy. Unfortunately the engine position sensors (there are 2, one for cylinder one, the other for cylinder two) cannot be done. However a third sensor can be added for cylinder three (thus any one of the three can fail and still have a running motor)! A replacement sensor is easy to carry and replace provided the units are bolted on and not riveted. The TPS is harder. However using a cheap light sensor eases the pain of carrying and replacing a faulty unit.
Also a knock sensor enabling lower octane fuels to be used, and retarding the ignition timing on low throttle settings to enable leaner mixtures and better fuel economy... There is room for improvement in fuel consumption!
A CO 2 (Lamba) sensor can also be fitted to provide a closed loop tuning system. This is like having a tune up every revolution of the motor. I think this is ok for occasional use, but not a requirement all the time. The problems are that the unit needs to be hot to work. Any deep creek crossing or really cold weather can temporarily disable the unit. Also the CO 2 (Lamba) sensor can be poisoned by poor fuel or oil burn off.
The air mass flow sensor on top of the air cleaner weight about 0.8 kg. On the later models this is not used; the air flow is simply calculated from the throttle position - thus saving some weight up high.
A pre filter would improve the life of the standard air filter and be easier to service if it was located as part of the extension to the air inlet. This would place it near the front right hand side of the fuel tank where less would need to be removed to gain access. This would encourage servicing.
The oil breathers will weep if the bike has fallen over. By running a tube from the current breather location over the top frame tube and back down this loss of oil could be prevented. This is particular important for the engine oil breather that will empty oil in to the air inlet track - and potentially contaminate the air fillter. If you need to maintain the pollution control then running the engine breather over the top frame tube then down to the bottom of the bike and then back up to the old fitting on the air box should do the trick.

More ideas:


  1. You can get K75 torque graph here:

  2. note: the K75 torque is WAY diffetent to R80. It's totally different engine design. water cooled inline-4 2v/DOHC compared to air-cooled opposed-twin 2v/2xSOHC.

  3. correction: INLINE-3 (not inline-4) for K75!!! SORRY!! My bad, that's what happens when I'm typing under stress at work!

  4. The BMW K75 is really good for long distance rides and I recommend the riders to take the BMW K75 Repairing Manual with them so that they can be able to repair their bike during long journey when you are not aware of the mechanic shop.

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