BMW have always trod there own path when it comes to motorcycle design, no matter what the rest are doing, no amount of influence has ever found its way onto their drawing board. The R90S is one such example of how to do the same job in a very different way, MSL takes a look to see why this Beemer was, and still is, such a winner.
In 1974, the R90S was head and shoulders above the rest of the opposition and was the machine that kept the Japanese honest, so much so that many original ideas found in the Boxer design were widely imitated. Yamaha for instance used an identical shaft drive set up in their new range of sports tourers, while the handle bar fairing became both the norm for the next generation of sports machines and a much sought after accessory for those that came without one. The R90S does just about everything in a different manner, its chassis feels to be soft and compliant, in fact too much so for a sports bike, while the engine never gets into a screaming rage, and yet the package is superb in its own right, a deceptively fast weapon for the accurate rider to excel on.
It is difficult to conjure up an exact opponent for the BMW. On paper so many machines show potential, from the Honda Goldwing and Suzuki GT750 to the Moto Guzzi V7 and Kawasaki Z1. In reality none of those machines could match the total package that the BMW offered, very high speed, effortless mile munching and mechanical reliability. Add the 45mpg that the Beemer returns even up in treble figures and most machines pale into the distance with every petrol stop. The R90S was expensive however; nearly half the price again of the best of the Jap machines, making it the choice of a select few who could afford the ride.
With a design of such longevity, one would not expect much in the way of common faults and to a large degree this is correct. The R90 boxer engine is a tough old lump. The law of sod should dictate that the bike with the most comprehensively equipped tool kit is the least likely to need it and for once this is perfectly true. The R90S kit is over subscribed with sockets and spanners aplenty, all wrapped up in a snug tool roll complete with its own BMW badged towel to lay them out onto in the unlikely event of a road side malady. Even if something does go awry, the technology isn’t exactly hi-tech and most jobs can be tackled with the on board tools and a little no how, try that with a Jap 4’s electronic trickery by the side of the road on a rainy night.
Unlike many bikes from the period the R90S is still capable of day-to-day use or even long distance mile munching, the Boxer engine remaining reliable and willing too while the laid back attitude displayed by the chassis is ideal for relaxed riding two up and fully laden.
Quite surprising is the compactness and narrowness when sat upon the Beemer, what looks like a very big bike as you approach it becomes nothing more than a powerful middleweight once sat upon it. The bars are hardly wider than your shoulders and the footrests, off set to match the cylinder spacing, incredibly close together, around five inches further in than anything the Japanese have to offer. Even the large capacity tank isn’t in any way bulky, achieving its five and a quarter gallon capacity with length and height rather than width. This instantly endears itself to the rider as you sit fully on the centre line of the machine making the bike feel balanced and not at all the capacity it is. At low speed the bike is so easy to keep upright there is hardly a need for that comforting foot down, U turns can be performed with consummate ease, feet up all the way, while sitting at junctions and traffic lights can often be done the same way, The engine configuration helping no end in this aspect with very little weight above mid shin height.
Once on the move, little changes, the big German twin is agile, lithe and above all, fast. Ok, you can’t go rushing around in a ham-fisted manner or the bike will get upset with you, and the result will be one sour kraut. There is simply no point trying to run to the apex of a bend with the brakes on for instance. The front suspension is too soft and compliant to handle such abuse, while the engine is a bit on the lazy side to cope with the late downshifts required for this technique. Far better to work in unison and make small suggestions to your new Bavarian buddy, (actually the R90S was built at the new Munich factory) that way no one will get their nose put out of joint and you will be rewarded with a superlative ride. Get the braking, downshifting and turning done well in advance then, try to go through the apex on the gas, that way the whole plot tightens up nicely and you will be exiting the corner at a far higher speed than an equivalent Jap machine. Braking is positive and effective although some feel is lost as the front lever pulls a cable that in turn actuates the master cylinder deep under the petrol tank, this doesn’t afford the kind of feedback usually associated with hydraulic systems, but it works none the less.
Gear shifts need to be positive, no half hearted touches on the lever, a real prod is required to get the Boxer engine to shift ratios. When you get the engine revs correct the gear will shift both effortlessly and without the clutch, get it wrong and it is as if you are trying to shift a breezeblock with your left big toe. More difficult than going up the box is coming back down again, the engine speed must be accurately matched to the rear wheel speed to make this a smooth affair, mismatch either and the bike will let you know in a way that only trying to shift large engine components before they are ready creates.
BMW R90S Model history
The R90S concept, in one form or another, could have been produced as early as 1969 and had been considered for production during that period, Honda’s release of the CB750, along with doubts as to the publics’ readiness for a 900cc motorcycle, no doubt delaying BMW’s intentions
The engine was based heavily upon the previous R75/5 engine, sharing the same stroke, but with a larger bore, to give a capacity of 898cc. First seen in October 1973, the new machine quickly established itself as a front-runner with the press and public alike, being the first bike from BMW that could genuinely run with the leading sports machines from Italy and Japan.
Released alongside a less highly specified R90/6 model, the new sportster featured several lessons learnt on the racetracks of the world. Roller bearings replaced bronze bushes for the rocker arms and the big engine breathed through 38mm Dell’Orto carburettors whereas all other Beemer’s used Bing units. The electrical system was up rated over the /5 model Boxers, using a Bosch three phase system, although the R90s actually produced slightly less power into the regulator. This was largely attributable to a smaller diameter rotor, required to give a greater clearance between it and the stator windings, crankshaft flex being a major problem at high engine revs with the sportier version. For 1975 there were a few updates and modifications, a new crank shaft and front main bearing was implemented enabling a larger alternator to be fitted and the kick-starter was removed, although fitting one still remained an option. Wet weather braking was improved by drilling the two discs, and fitting a 17mm wheel spindle, in place of the 14mm one of the earlier model, reduced front-end flex. For the final year of production, the engine was radically altered in readiness for the 980cc R100 series, first seen in 1977.
BMW R90S Timeline
1974
The first of the R90S models was released to a surprised world come to expect sports bikes from any other manufacturer but BMW. The Hans Muth design was a success though and those that rode the machine quickly became a fan.
1975
A few minor updates were introduced for the second year of production. The crankshaft became stronger and had a new main bearing design to prevent flex along its length while new handle bar switches and controls were implemented to improve ergonomics.
1976
There was a new, larger capacity machine waiting in the wings and the final year of production reflected this. The R90S from 1976 already had the improved casings to facilitate the bigger pistons along with improved cylinders heads and many more modifications in readiness for the R100RS seen the following year.
BMW R90S Specifications
- Engine – air-cooled four-stroke opposed twin OHV
- Capacity – 898cc
- Bore/stroke – 90 x 70.6mm
- Power – 67bhp @ 7000rpm
- Torque – 56ft-lb @ 5500rpm
- Carburetion – 2 x 38mm Dell’Orto
- Transmission – 5-speed dry clutch shaft final drive
- Suspension – 36mm telescopic forks. Twin shock adjustable spring pre load
- Brakes – 260 mm discs single-piston floating-calipers. 200mm disc single-leading-shoe drum
- Wheels – 3.25 x 19 4.00 x 18
- Weight – 215kgs
- Top speed – 124mph
- Wheelbase – 1465mm
- Fuel capacity – 24ltrs
BMW R90S Gallery
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