Living with a 1974 BMW R75/6

This article was originally published in the paper version of Real Classic Magazine in November 2017(There is a link to their web page on the links sidebar).  The magazine version of this article is much better laid out than I can manage here, but the info is the same.  If you are considering long term ownership of an airhead, then hopefully a) you won't have a 1974 bike, and b) there might be something in here that you find useful.  Since I recently wrote about a starter motor problem I had had with this bike, I thought you all might enjoy the bikes back story.  It's a bit of a love/ hate thing.  Enjoy.

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There has been a lot written about BMW airheads in the last few years.  So, in the spirit of if you can’t beat them, join them, here are a few thoughts from my own perspective.

Firstly, a bit about my own bike; a picture or two of which might even decorate/ desecrate these pages.  I bought it 23 years ago after giving up the fight to find decent parts for a high mileage Honda Black Bomber which I had used and abused for 13 years.  While I had second thoughts about buying such an obvious ‘auld lads’ bike, the catalogues of easily available parts from the likes of James Sherlock, Motorworks and Moto-Bins, the bikes reputation for longevity, and of course, the relatively cheap purchase price made it too good to resist.  It is a 1974, R75/6.  A number of updates were introduced that year, the most notable of which are disc brakes and 5 speed gearboxes, so it didn’t appear too outdated compared to the brand new Mystics that were still available new back then.  New R100 Mystics were listed as having 60bhp, while an R75/6 had 50.  That didn’t seem like a sufficient difference to justify 20 years of development, the extra cc, or the thousands of extra pounds that the new bike would have cost.

I had ridden a few other Airheads, mostly newer ex-police ones, so my initial impressions on a test drive of the torque reaction trying to roll the bike to the right and the slow gear change weren’t a complete surprise.  It doesn’t take long for the bikes quirky nature to grow on you, like the strange switchgear that is a hangover from the earlier /5 models.  They actually start to seem normal after a while.  In fact, the engine and frame numbers on my bike show that it is not much more than 100 numbers into series 6 production, so there are a number of parts on it that must come from BMW using up the stock from the older bikes. 

There is one thing that should really sell old Airheads to any rider, and that will only happen when you start to ride long distance.  My own epiphany came shortly after getting the bike, when I rode down to Littlehampton for a friend’s wedding.  I live in Northern Ireland, so got a late boat to Stranraer that deposited me on Scottish tarmac ahead of a queue of lorries and cars at about 12 midnight.  I arrived at Littlehampton around 11 am, after numerous coffee stops a few motorway blasts, and as many excursions onto suitable A roads as I could find, still able to stand and function normally and completely hooked on the old Beemer’s capabilities.  By contrast, a few years later I used a Honda NTV 600 for a day trip to visit a friend in Donegal.  Riding there and back to my home, plus a little messing around in Donegal put up around 350 miles that day.  By the time I was coming home, leg cramps were crippling me, and when I pulled into our driveway, it took a real effort even to get my foot on the ground before the bike fell over!  Despite this, I put over 60,000 miles on the Honda’s odometer in seven or eight years, but have managed only about 46,000 on the R75 in 23 years.

From this you can see that BM’s are not perfect.  The ATE disk brakes for example were probably less than adequate even in the mid 70’s but they can be improved and long production runs facilitate upgrades like this.  The ATE disc brakes fitted to early Airheads used a cable from the handlebar lever to an under tank master cylinder, then hydraulics from there on, in my case to a single disc.  Many later, larger capacity bikes had twin discs, which can be retrofitted.  Converting to a handlebar master cylinder is also a good idea, as are braided hoses.   All this was made more difficult for me, because very early bikes have a 14mm wheel spindle, but all twin disc ones use a 17mm one.  Early bikes also had brake calipers with 38mm pistons, while later ones had 40mm ones.  Because of this, I had to change both fork legs and both calipers.  The handlebar master cylinders are available with a range of bores, from 12mm to 16mm, and are made for completely different switchgear, so I also had to sculpt a suitably curvaceous piece of alloy to fill the void and fit my own.  With that done, the bike stops in a manner that would shame many later bikes.

Very early 5 speed gearboxes (like mine), were also prone to failure as I discovered after a year or so while going to work one morning.  I don’t remember there being any warning rattle, just a loud and truly awful noise when changing to a suddenly non-existent gear. On dismantling the old box it soon became obvious that there was very major damage, so I had to fork out for a reconditioned one.  Another gearbox problem occurred years later when the bike suddenly stuck in second, with no other gears available.  Fortunately, this was simpler to fix.  A small return spring was the cause of the problem, but requires disassembly of the box to fit it, and my God, what a job it was to get the output flange off its taper!  I actually broke a mechanic friend’s rather substantial puller and had to fabricate a new version out of a piece of plate thick enough to shift the earth’s axis (and the flange) relative to the gearbox rather than just releasing that taper.  More modern versions of the gearbox were revised so that even if the return spring broke, gears could still be selected.  The BMW Club’s ever helpful Technical Officer (Hi Mike), told me that he has heard of people on tour having to turn their bike upside down which allows gravity to drop the recalcitrant selector into place.  They then select their choice of gear with which to limp home for a more permanent repair.

Of course, as with any older bike, problems can be caused by owners too.  Many years ago I was travelling through England on my way to a rally.  At one coffee stop I noticed a little oil on the left side of the engine, checked the oil level but since it was OK, I thought little more about it and rode on.  By the time I reached my destination things were getting worse.  A check showed that the oil level was now low, and it was leaking sufficiently that regular top ups were needed when going home.  Damage had been done however, and one of the tappet followers broke leaving scores on its bearing surface in the crank case.  Tappet follower breakages are not unknown in early bikes, and BMW revised this part too to strengthen it.  On investigation, the leakage was caused by the oil pressure gauge fitted by some previous owner.  They had used the metal part of an oil pressure switch that threads into the engine, and had welded a pipe fitting onto this to allow the gauge to be attached.  Their rather crude looking weld had failed. 

Twisty bits on the Antrim Coast.  Great bike roads!!

Most recently, the bike destroyed a relatively new Motobatt battery.  I checked the charging system by briefly connecting a large car battery to it with jump leads, and found a charge well in excess of 15 volts even at low revs.  The high output regulator that was fitted had only been there for about 15 or 16 years too.  Damn these shoddy modern parts!  Rather than a direct replacement for this, which is about £50, I decided to go the whole hog and replace the diode board as well with an RR45 ‘high side switched’ regulator/ rectifier.  These are made in the UK by Electrex World, are not vastly more expensive than the regulator alone, and should easily cope with anything this old bike can produce. This air cooled unit is now wholly out in the open air where before the diode board lived within the front timing casing.  It is a tight fit to mount in the space available under the petrol tank though.  They have an additional advantage in that by replacing the two wires that switch the starter motor, the whole original engine loom can be removed.  The bike got new electronic ignition at the same time.

Some issues with old bikes are easy to fix.  Like the pin in each carburettor that the floats hinge on.  At one point the right hand carb began flooding constantly.  The hinge pin was nearly rusted through, and was stopping the petrol supply from being sealed when the carb was full, (modern petrol or just age? Take your best guess).  A short length of 2mm stainless bar is cheap, easily cut to length and simple to fit.  Problem solved.

This is no show bike.  Between problems it is great, but it does seem to have spent more time off road than BMW’s reputation for reliability would suggest, thus I have had a whole series of other bikes during its ownership.  The fact that they have gone, while the BM, despite its many problems, remains must say something?  As a test of memory, I thought I would try and list all the evolutions that the bike now carries apart from loads of lovely stainless steel, along with a few comments on these.  If you have an Airhead, or plan to buy one, there may be one or two of these that you might like to try.  They are listed from front to back rather than in any chronological order.

Front brake upgrade to twin discs	Mentioned in the text.  The cast wheel arrived at this time too, and while I did plan to refit the spoked one, it works as is so it never happened.
High capacity alternator	Probably not needed, but provides reliable heated grips and loads of spare capacity should it ever be required.
Electronic ignition	This is the third version.  One failed, and the ECU was replaced.  I suspect the replacement was going the same way, so changed to a totally different system.
Starter motor	The original Bosch is heavy (cast iron), low powered with electromagnets and has a high load in use.  The replacement Valeo item is reputedly 6 pounds lighter, is easier on the battery and turns the engine over much more freely.  Starting is now instant.  Beware though, early bikes like mine need an 8 tooth bendix compared to the 9 tooth one fitted to these starters when they became standard fitment on the lighter flywheel Airheads.
Valve gear	BMW valve gear rattles; period.  Valve gear went through three versions during the Airhead lifetime.  Late models have shimmed rockers to reduce end float, but this cannot be fitted to early bikes like mine without machining.  The noise absolutely did my head in, so mine has been modified to accept some rather trick one piece CNC machined rocker mounts from a German company, Motoren Israel.  They still rattle, particularly when cold, but nowhere near as much.
Twin Plugging/ hardened valve seats	Modern petrol and a long flame path are not a good mix.  It's probably fine if you plan to tootle gently around the countryside in true classic bike mode, but if you like to use your bike as intended, then these mods might be worth thinking about.  One of the original twin outlet coils that were supplied with the mapped for twin plug ignition failed, so the bike is now fitted with Dyna Coils (Two 1.5 ohm coils wired in series).
Gearbox	Later, stronger version fitted as per the text.
Swinging arm.	At some point, while still in twin shock guise BMW decided that a shock absorber in the drive shaft would give the gearbox an easier time.  They are a direct bolt on replacement for the swinging arms on earlier bikes, and since gearboxes are expensive, why would you not fit one?  The shaft side of the swinging arm is of larger diameter tube to allow for the shock absorber, which prevented me from using the original brake light switch.
Seat	The original seat base rotted away, so was replaced by a pattern one.  The cover on this quickly ripped, so I tried this one (originally a Corbin one that soaked up water profusely). Because of the water logging problems, I had it recovered without the king and queen profile which I disliked anyway.  It’s not as pretty as the original seat, and it adds an inch or so the ride height, but it is comfortable.  I may change it again at some point, but it is low on the priority list.
Givi pannier frames	The bike came without luggage, which is unusual for a BM.  A second hand Givi rack was a whole £10 at the time (originally for an old Goldwing), and was modified to fit by friend and ace welder Red (who starred in a previous story on Real Classic).  I had one box already and picked up a few more over time.  The seat has been modified to lift off rather than hinge to clear these frames.
Rear Differential	Changing the gearing on a shaft drive bike will always be problematic.  My R75 constantly felt to me like it was under geared, like there should have been a sixth gear in there to make it a more leisurely road burner. I fitted the differential from a 900cc bike when I got the chance, and now it purrs at 70 rather than buzzes.  As for speedo accuracy, comparing it now against a sat nav, the mod seems simply to have reduced the rather optimistic speedo to a more realistic reading.

 

And now to the eternal question. Whatillitdomister?  Well, I’ve already said that it starts and stops in a reasonable manner.  Earlier Airheads had bigger flywheels that give them a more ‘vintage’ feel.  OK, I admit it; this tick over to maximum revs, get it into top and never change gear riding style suits me.  It is a great way to flow through the glorious A and B roads that predominate in this part of the world, and it is so much more relaxing than narrow power bands and racing crouches.  I’m too old for all that flat to the tank nonsense now anyway.   Despite this, I’m still childish enough to enjoy gunning the throttle a bit under low bridges just to enjoy the relaxed deep bass rumble from the exhaust.  It’s difficult to believe in these more regulated days, that in their time Airhead BMW’s were considered refined and silent!

I’d love to be able to use that old “all the controls fell easily to hand” cliché here, but I can’t.  The brake and clutch are fine, that switchgear though could never be described as such.  If, like many, you have more than one bike, then the right hand indicator switch will require a distinct mental adjustment.  As for their operation, while it is easy to put the indicators on with the knuckle of your thumb, turning them off again just requires a more exact hand movement.  The mirror stalks also tend to get in the way when trying for a quick visor wipe in wet weather.

The suspension soaks up modern potholes fairly well.  It doesn’t have modern trail bike levels of comfort, but it is good enough to be able to ride all day and not leave the rider feeling like they have been using a jack hammer.  There are Hagon units at the rear currently.  They are firmer than the originals without being harsh.  I have also used Fournales air shocks which were very good until one started leaking.  I must get them fixed some time.  The tyres are currently a mis-match; a Conti Go on the front, and a more classically profiled Heidenau rear.  The bike is twitchy over road lines.  Not an ideal trait when filtering through traffic like I do a lot of the time, so I will be changing the front at some point.

Twin plugs.

The obvious influence of Norton’s Featherbed on BMW frame design has been noted many times, and they handle acceptably well.  Slightly later bikes than mine (series 7 onwards), got extra bracing for the front down tubes to reduce flex.  A steering damper is standard on early bikes.  I keep in on the first damper setting, where on open roads it feels like the bike is on rails.  I have always been surprised that BMW didn’t use their substantial engine castings to brace the frame.  The engine is mounted to the bottom frame rails and nowhere else, so the whole parallelogram shape that surrounds the engine must be liable to a degree of flex?  At road speeds it really doesn’t matter, although regular track day use may require some extra bracing.  My only complaint about the bikes handling comes when it is fully loaded and two up.  Then the weight bias is too far back and the steering becomes a bit light.  The remedy for this is simply to keep the tank topped up for a little more weight at the front.

The old R75 would still happily cruise for as long as any rider could want at well over the speed limit.  70mph equates to a leisurely 4000rpm, at which speed there is plenty more to come should you need it.  Combining the earlier heavy flywheel, and the change of gearing as I have produces acceleration without the arm stretching adrenaline rush of a sports bike.  Planning is required when overtaking.   The relaxed nature of the power delivery can be deceptive though, and I have often glanced at the speedometer only to find that I am travelling rather more rapidly than planned.   The end result of all the modifications is good enough to surprise some modern bike riders.  People these days seem to think that to go further than the local corner shop requires huge capacity bikes with gargantuan lumps of plastic at the front and price tags that make the national debt of Venezuela pale into insignificance.  Some of the photos hereabouts should come from a photo competition held by my local BMW club a few years ago.  One picturesque stop is selected in each of the nine counties that make up the historic province of Ulster.  I chose to go around them all in a day (just over 455 miles), which seemed to surprise a few people in my local section.   On this bike; no problem.  In fact, by the time you read this, I may have done something similar for this year’s competition.

Wish you were here?

Don’t get me wrong, I like my bike and have spent a lot of time getting it set up as I want it, but if buying an airhead with the knowledge that I have of them now, I think I would go for the 1981 to 85 bikes, the last of the twin shock ones.  These are reasonably well developed, but have not yet been stifled by emissions regulations.  They are both faster and more economical than the later bikes.  I think too that I would plump for one with an RS fairing.  It’s an age thing; my old bones could do with a little extra protection these days.  Meanwhile following the failure of the Funduro that I was using for commuting duties, my R75 has been pressed back into service for that role.  You will see in some of the pictures that the rear sub frame and pannier rails could do with a coat of paint, and if I don’t find a suitable winter bike soon, this job will have to be done sooner rather than later.  As with any bike that is used, the eternal fight against deterioration continues.