BMW R 90 S
From BMW Tech
The BMW R 90 S is a Sport bike that was produced by BMW between 1973 and 1976. The R 90 S is considered by many to be one of the first Superbike, and the first such motorcycle from BMW. The first United States AMA Superbike was won in 1976 by United Kingdom rider Reg Pridmore on a R 90 S.
This R 90 S is fitted with a type 247 engine, otherwise known as an airhead. The engine is an OHV, two Valve per Cylinder (engine), air cooled Flat-twin, or Flat engine. The R 90 S weighs 215 kg (474 lb) and has a five speed gearbox connected to a Driveshaft. The model is an extension of the previous 750 cc series "/5" machines. The R 90 S was one of two 900 cc models introduced in 1973 (1974 in Australia); the other being the 900 cc version of the existing R75/6 machine, the R90/6.
There were three series of R 90 S:
- model year 1974: September 1973 to August 1974 (6,058 units)
- model year 1975: June 1974 to September 1975 (6,413 units)
- model year 1976: August 1975 to June 1976 (4,984 units)
The first series of R 90 S sported a distinctive two tone paintwork (Black/Smoke) called "TT silberrauch" or "TT smoke silver" with adhesive gold pinstripes (no more hand painted by the factory; only the first series of R 90 S had this kind of pinstripes. The following complaints of the customer did convince BMW to turn back to hand painted pinstripes). Later variants (August 1974 on) were sold both in "Smoke Silver TT" and "Daytona orange" (orange/silver with red pinstriping). The R 90 S was an individual factory motorcycle - no two left the factory the same due to the individual differences across each motorcycle's paint job. Look at two identical machines from the same production run and you can see this difference today - which makes restoration a subjective task.
The R 90 S sported a small but effective Factory fitted Bikini Fairing, which held four dial instruments (Speedo, Tacho, Clock and Voltmeter). The first series R 90 S was only equipped with a 238 watt alternator (All other "/6" machines had at least a 280 watt alternator), which meant after market lighting or heated handgrip accessories would be less effective on this bike. That's why on the third series the alternator was upgraded to 250 watt.
The bike also possesses an adjustable hydraulic Steering damper activated via a knob located on the steering head. Suspension is by telescopic forks at the front and twin shocks at the rear. The rear dampers were adjustable for preload, which is the only suspension adjustment available. Aftermarket fork gaitors are often fitted to the bike but are not required due to the relative hardness of the fork stanchions.
Other critical engine differences are that the R 90 S came equipped with standard 38 mm Dell Orto 'pumper' Carburetors, differing from the previous 26 mm Bing slide carbs or 32 mm Bing CV carbs of the standard "/6" and "/7" 800 cc and 1000 cc series machines. Later 1977–1980 1000 cc machines sported 40 mm Bing CV carbs). There are many, small but important differences on the three series of R 90 S. On the first series, the brake discs are not drilled, and the handlebar switches are the same of the "/5" series. Restoring a R 90 S might be difficult because it's very important to respect the correct features. Just for example, the wheel axle diameter of the first series is smaller than the one of second/third series.
The bike's visual design was overseen by Hans Muth, who was brought into the BMW fold to create a machine with a unique presence far removed from the staid image offered by previous BMW offerings such as the fast, well built, but conservative R 69 S. This designer later went on to design the R 65 LS, and also the Suzuki Katana. The R 90 S possessed a redesigned seat, with a small, styled ducktail, which was regarded by some as a retrograde step for those that rode two up (given the relative comfort of the previous Denfield double seat on the standard "/5" and "/6" machines). However, this ducktail added a second underseat storage space to add to the original underseat tool tray - just perfect for lightweight waterproofs, extra Maps, or Motorcycle gloves.
On the road, the R 90 S was a sweet running and capable solo/two up sports machine. The addition of either Krauser or BMW branded hard panniers and either a rear rack or over the cylinder 'toaster racks' made the bike a capable tourer. Overall, the bike can exceed over 380 km (236 mi)) on a tank of fuel.
Its performance is modest compared with modern K Series BMW bikes producing more than 160 bhp (119 kW), but the 67 bhp (50 kW) pushrod-twin engined bike ran the quarter mile in around 13.5 seconds and went from 0-60 mph (0-100 km/h) in just over 5.4 seconds. It is capable of maintaining long distances at high speeds in relative comfort. With long maintenance intervals, it was suited as a long distance sports tourer for those that could afford one.
Tools and owner servicing were well thought out. The R 90 S came standard with a full toolkit, a hand pump, a first-aid kit and even a small hand towel (with an embroidered BMW logo on it). Owner maintenance is standard practice with these machines. Valve gear was adjusted by simple locknut, and timing was taken care of by points (later replaced by electronic ignition in many models). Most other maintenance tasks were easily achieved due to easy access to most mechanicals.
Maximum torque is delivered at about 5,500 RPM and redline is at 7,200 RPM. Top speed is 190 km/h (118 mph) upright and 200 km/h (124 mph) on the tank. The bike will run at 190 km/h all day if required.
The bike will run continuously at speeds close to the redline, but the relative fragility of the overhead valve gear is something that needs to be considered when running the bike over the redline, as it has no rev limiter as found on modern bikes. As the OHV engine pushrod valvegear components were designed ‘off centre’ by four degrees to accommodate future plans to expand the R series bikes to 1,000 cc, this slight offset produces additional tensions on the OHV gear at maximum revs that need to be considered by owners who ride over the limit.
As for brakes, two 230 mm drilled discs were gripped by single ATE callipers, with a 200 mm drum on the back wheel. The 1974 onwards model bikes make a distinctive noise under braking thanks to the drilled discs. Overall braking performance bettered the Japanese offerings of the time (especially in the wet), but were substandard to contemporary Brembo systems (which were subsequently fitted to BMWs from the "/7" series onwards).
The front brake's cable activation system for all BMW "/6" series bikes, including the R90s, is also different to other brands, as the master cylinder is located on the top tube of the frame and is activated by a cable from the lever to the cylinder. The argument was that this system offered greater protection for the master cylinder in the event of a crash. Later "/7" machines reverted to handlebar mounted Brembo master cylinders, with disc brakes replacing rear drums until the advent of the R100R Mystic models in the 1990s.
Other interesting points
Another point of interest is that the bike was originally equipped with a phenolic disc and spring engine crankcase breather, which was superseded by a reed valve design on the "/7" series. An original R 90 S (as with many "/5" and "/6" machines so fitted) makes a 'plopping' noise at idle as the crankcase breather manually opens and closes: later reed type breathers retrofitted to earlier bikes see this interesting auditory quirk removed.
The Electrical system was both adequate and reliable, with a H4 Headlight providing adequate illumination for legal speed touring at night. The switchgear was upgraded in 1975 from the previous "/5" system. The indicator switch now operated on the vertical plane, rather than the horizontal one used by most other manufacturers. Even in 2006, both BMW and Harley-Davidson continue with some form of point of difference to the rest of the world's manufacturers in this regard.
The ignition key is placed on the left headlight mounting point. While accessible, it is ill advised to ride round with a huge bunch of keys hanging off the switch. Not only does paint wear occur, the effect of gravity eventually wears out the switch, necessitating an expensive and avoidable rebuild. The steering lock is mounted in the steering headstock somewhere.
Handling wise, the R 90 S was OK so long as you are smooth in throttle application and sure of cornering line. Machines of this vintage are not 'point and squirt' types of bikes. Lines need to be thought through well in advance and stuck to. Steady power on through corners helps immensely. Backing off in corners, or sudden Throttle application, resulted in the Shaft effect of either the back of the bike squatting or lifting.
The Tyres too are minimal in comparison to modern bikes. The R 90 S's 19 inch front and 18 inch rear combination are narrow, but adequate for most riding activities. One advantage of such a narrow tyre combination is good stability on dirt roads: A downside is the effect that worn tyres places on the frame - a head shake or tank slapper can develop on worn tyres that mimics the feeling of loose/poor condition head bearings. Good tyres are essential to good handling manners.
The worst part of owning an R 90 S is the Gearbox. The five speed unit does work but requires such precise changes and gentle movements but firm application that it is impossible to make quick shifts either up or down the gearbox.
The R 90 S is still regarded as 'the one to keep' and owners tend to hang onto them for longer periods. While the overall numbers of R 90 S produced were smaller than the more mainstream /6 and /7 BMW R series models, the model is still regarded favourably.
From 1973 to 1976 17,455 R90S were sold. The R 90 S became the R 100 S in 1977, which maintained the R 90 S bikini fairing, but ran the full 1000 cc engine, 40 mm Bing CV carbs, and altered paintwork. The mantle of the lead BMW factory twin was passed to the R 100 RS, which by now sported a full fairing (note: 'specials' such as the 4V Krauser and Fallert BMW's are not compared in this article). Other factory variants such as the R 100 CS were also produced in later years, sporting spoked wheels and black-painted square valve covers; as a sort of 'licht' derivative that was available only in black bodywork color. Many later R series 'mono' bikes still visually resembled the R 90 S in order to retain a visual link to this significant model.
A later example of the R259 oilhead BMW, the R 850/1100 R, sported a similar two tone smoke grey paint scheme in homage to the original BMW 'superbike' colour scheme.
BMW R 90 S machines were the original 1970s superbike, and should be considered in the same league as other similar 'high performance' bikes from earlier years such as the Brough Superior SS 100 and the Vincent Black Shadow in terms of quality, exclusivity, technological advances, overall performance levels, cost and impact on the riding community at the time of release. While Japanese and other European offerings such as the Honda CB750, Kawasaki motorcycles ZI900 and Ducati Motor Holding SS 750 superseded the performance of the R90S in their own way, these bikes could not compete with the overall package of performance, longevity and sustainability of the BMW flagship. The title of the first superbike of the post 1970 era must rest with the BMWR90S.
- Falloon, Ian (2006). BMW R90S. Whitehorse Press. ISBN 1884313566.
- ↑ "Media Guide 2007" (PDF). AMA Superbike Championship. pp. 302. http://www.amaproracing.com/prorace/pdf/Superbike_Guide_07h.pdf. Retrieved on 2007-09-20.