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  #1  
Old 4 Jul 2008
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R75/5 Modifications

Oh Knowledgeable Ones,

I would like your thoughts on converting a 1973 R75/5 into a more enduro based adventure bike.

Thoughts on getting the bike a little higher off the ground, pipes up higher and getting the air intake higher, heavier suspension, bigger bore, etc.

OK - you may be thinking why? Is this guy an idiot (maybe)? There are newer better simpler bikes that already designed for this (1200 Adventure / F650 Dakar).

Here's the reason and purpose I seek the Holy Grail.

In 2006 I broke the TransAmericas World Record (Prudhoe Bay, USA to Usuaia, Argentina (see 34 for 40 - Benefitting the Pat Tillman Foundation)) on a fantastic BMW F650 Dakar. As much as l love that Dakar, I had a few Gremlins in the electronics.

My next journey is not going to be a race. It is going to be an adventure between me, my bike, and the (on and off) road. I want a bike that I personally rebuild over the next 12 months, with few electronics, very dependable, lighter weight than a GS 1200, road serviceable and worldwide parts friendly.

This has led me to the 1973 R75/5. I am told it is loyal, sturdy, and easier to work on than the Dakar. It also is close to the same HP and just a little heavier. However, a huge safety and environmental advantage of the Dakar was its tall seat and height.

I know there are old BMW experts on this forum. And I am also sure that BMW's of this age had to be modified by some of you over the years.

I seek those gurus who are willing to share their stories on such things as how they swapped the front end out of one of these bikes and made it an off road racer or equipped it with gear to make it a deep water crosser, or converted it to accept matching tire sizes on front and back.

Seeking your advice / opening myself up to ridicule,


Dave
nineosixdave
(my name)@34for40.com
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  #2  
Old 5 Jul 2008
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Thumbs up R75/5 mod's

Dave,

I think I speak for many of us in saying that I admire your gumption. Sometimes the bike IS the adventure, or becomes it through planning (as in your case) or lack of planning (as in it broke down and now I've got to fix it with a rock!).

This is a huge question with unlimited answers and possibilities based on how far you want or need to go. The R75/5 is certainly capable of getting you there, even in totally stock condition. I owned one of these as my introduction to BMW's. I loved many things about it (smooth, tractable, fixable, timeless) and hated others (the brakes, the front forks, the brakes, the antiquated electrics, the brakes). I know a lot more about Airheads now, but am still a neophyte by most standards.

Unless I missed something, you didn't really state why you thought of the 75/5 other then you have heard that it is reliable and light. In my opinion the /6 or (better) /7 version make far more sense in basically the same package. The early short wheel base /5's didn't handle that well, the drum brake was pathetic, and the 4 speed tranny limiting. The 1995 R100PD was essentially the same bike with years of development and refining. Sure they dropped the ball on a few issues over the years (build quality went down, I've heard) but most changes were improvements. I'm pointing this out because the later bikes are a far better starting point, so IMHO you need a reason to go back in time. I'm not saying there isn't one, but it is important to state the goal. Certainly the easiest method is to send $10,000 (plus base bike) to HPN and specify what you want done. 18 months later, you will undoubtedly have the most fit Airhead on the Trans American hwy.

The early Enduro bikes that eventually became the first R80G/S were modified /5,6,7's. These bikes are pretty modular and parts from different years and models can be swapped around. Many people think that the 750cc engine was the best of the lot. I like the fact that you can easily change the rear drive to get a different final gear ratio.

I spent quite a bit of time rebuilding and modifying my r80 G/S. In some ways, if I was to do it again I'd consider starting with an ST or an older /7 or early 80's road bike. By the time you've upgraded the front forks, rim, rear shock, and brakes you've got a GS anyway!

If you haven't already, I recommend joining Mica peaks Airhead and GS mailing lists - many of the Airhead 'gurus' hang out there, and it is invaluable for accurate and quick information. Lots of good info here as well - search the Hubb for R100 GS or R80 G/S and lots of excellent discussions will come up.

Good luck and hope we can all be of service to you.
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  #3  
Old 11 Jul 2008
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$10000.00 won't even get you started at HPN. If you strip the bike down yourself and send them the frame to be modified you can make it with only a modified frame, WP front forks ready to fit standard front wheel, Rear Shock and the 100mm lengthened cardan shaft.
But to be honest if the rest of the bike is in good shape you don't need more than this.
It will be nice to add the 1043cc Mahle converted motor but you can add another $10000.00 for this if you go the full hog with dual spark heads, sport crank, rally clutch, oil cooler & thermostat, larger sump, different cam to increase torque plus the labour to put this whole lot together.
Improving the brakes and adding tougher wheels and large 43l tank and rally fairing are all nice to haves and surely will make life easier in some instances.
The question is though do you want a bike a do you want to travel.
Few of us can afford both.
TransAfrica - Route
I agree though that a GS will make the best platform to start your modifications for off road use.
If you want to get more info look at my website Welcome to GS Workshop
There is also a link to HPN's site.
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  #4  
Old 14 Sep 2008
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How did it go ?
What did you build ?
What you are kind of referring to is the BMW ST model, but using "nippon" forks; i.e. the first versions of the /7 GS models.
Cool idea - feasable at a fair cost. HPN conversions is a whole different ball-game, and not a /5 and /6 project but a /7 development.

The simplicity of the beemer is -!
Realized that I have a bearing failiure in min 32/10 final drive today... 2h later I have my old 37/11 bolted on, adjusted and ready to go -
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Last edited by dc lindberg; 14 Sep 2008 at 20:24.
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  #5  
Old 14 Sep 2008
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Hello Dave, there is an article in issue 2 of BMW Motorcycle Magazine of a guy in Texas who has pretty much done what you intend to do using a /7 frame. Welded frame, RM forks WP shocks etc. Try BMW Motorcycle Magazine .
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  #6  
Old 14 Sep 2008
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Hi Brian,

BMW Motorcycle Owners of America published an article on that BMW-dirtbike contraption some years back. Very clever design! Not for Dave to fully use, but some of the ideas presented in the dirtbike conversion should he really nick! -
Simple, durable, service friendly - and affordable!
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  #7  
Old 15 Sep 2008
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Get a 21" rim laced onto the front hub, put some Metzler tyres on. It will give you a slightly chopper look but by the time you load the bike it will ok. You'll need a new front guard as well, even if you stick with a standard rim you need to increase the tyre to mudguard clearance as it will clog up with mud and bring you to a halt sooner or later.

Get a large capacity tank - 35 litres.

Have some pot racks made to carry some weight down low above each cylinder.

Maybe a bash plate for the sump.

Air cleaner is hard to relocate, the 75/5 just had holes in the metal shroud to let the air in. Maybe the R65 air cleaner can be made to fit with the intakes up above the engine, they are easy to close off as well if you have to wade the bike at sometime.

Get a Krauser pannier set and then make up a well braced carrier rack, then use a strap to go over the seat to take a bit of weight off the carrier.

DONT fart about trying to get increased ground clearance, it just wont work without almost starting again from scratch - by putting longer forks on the front you need to raise the back end. You cant raise the back end without building a new swingarm which has to be longer. You also have to tilt the engine/gearbox so that the universal at the back of the gearbox is not constantly being driven at its limit. You can only tilt the engine back by a few degrees because of the top tube - so you notch the top tube and need to brace it with additional welding. Then you need to muck around with the sump because the oil pickup will be suspect........it just goes on and on.

The standard road going BMWs used to go everywhere anyway, and it was only the introduction of the GS which saw the "must be an enduro" bike to go anywhere without bitumen that BMW have so successfully marketed.
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  #8  
Old 16 Sep 2008
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Quote:
Originally Posted by RogerM View Post
Maybe a bash plate for the sump.
Make it definatly a sump gard - 3mm thick al is good.

Quote:
Originally Posted by RogerM View Post
DONT fart about trying to get increased ground clearance, it just wont work without almost starting again from scratch
The G/S sump aids ground clearence .. you losse 2 litres of oil... and you'll need another dip stick ..

There are many /5s that have toured on many dirt roads .. all the modifications do is alow you to go faster .. fine if you are racing .. but if you are touring? .. add the sump plate .. tyres, fuel, luggage and go.
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  #9  
Old 16 Sep 2008
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Quote:
Originally Posted by Frank Warner View Post
The G/S sump aids ground clearence .. you losse 2 litres of oil... and you'll need another dip stick ..
You might need a new pick-up for the sump.
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  #10  
Old 16 Sep 2008
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Quote:
Originally Posted by AliBaba View Post
You might need a new pick-up for the sump.
Oh, err, that too.
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motorcycles BMW R80 G/S 1981, BMW K11LT 1993, BMW K75 G/S
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