The R90S was a sport bike produced by BMW. It was first introduced in 1973. The R90S is considered by many to be one of the first superbikes, and the first such machine from BMW. However, in retrospect it may have been one of the earliest incarnations of what would later be known as a "sport touring" bike: a machine designed to cover long distances quickly and in relative comfort, particularly if the route between Point A and B was as twisty as possible.
Since its introduction, this motorcycle has become a kind of cult special and is of particular interest to vintage BMW motorcycle enthusiasts. There is even a dedicated National R90S Sport Owners Club, and the American Motorcyclist Association included the R90S in its The Dawn of the Superbike exhibit.
The first U.S.A. AMA Superbike Championship was won in 1976 by Reg Pridmore riding a R90S. This type of engine is a 247 BMW and known as Airhead (motorcycle). It is especially well regarded amongst sportbikes from 1973-1976. Overall, it is the definitive ’airhead’ BMW as regarded by owners and fans alike.
The R90s BMW was an OHV, two valve per cylinder, air cooled flat twin, or boxer engined motorcycle weighing 215 kg and running a five speed gearbox through a shaft final drive. The model was an extension of the previous 750cc series /5 machines. The R90s was one of two 900cc models introduced in 1973 (1974 in Australia); the other being the 900cc version of the existing R75/6 machine, the R90/6.
There were three series of R90S: - model year 1974 : 9-1973 to 8/1974 (6.058 units) - model year 1975 : 6/1974 to 9/1975 (6.413 units) - model year 1976 : 8/1975 to 6/1976 (4.984 units)
The first series of R90s 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 R90S had this kind of pinstripes. The following complaints of the customer did convince BMW to turn back to hand painted pinstripes). Later variants (6/74 on) were sold both in "Smoke Silver TT" and "Daytona orange" (gold/silver smoke with red pinstriping). The R90s 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 R90s sported a small but effective factory fitted Bikini Fairing, which held four dial instruments (Speedo, Tacho, Clock and Voltmeter). The first series R90S 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 250W.
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 R90s came equipped with standard 38mm Dell Orto ’pumper’ Carburetors, differing from the previous 26mm Bing slide carbs or 32mm Bing CV carbs of the standard /6 and /7 800/1000cc series machines (later 1977-80 1000cc machines sported 40 mm Bing CV carbs). There are many, small but important differences on the three series of R90S. On the first series, the brake discs are not drilled, and the handlebar switches are the same of the /5 series. Restoring a R90S might be difficult because it’s very important to respect the correct features. Just for exemple, the wheel axle diameter of the first series is smaller than the one of second/third series...
The bike’s stunning 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 R69S. This designer later went on to design the R65LS, and also the Suzuki Katana. The R90s 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 /5/6 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 winter gloves.
On the road, the R90s 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 380Km on a tank of fuel.
Its performance today is modest when one compares it to 180+ HP K series BMWs; but the 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 - not bad for a 67 BHP pushrod twin. Impressively, it is capable of maintaining long distances at high speeds in relative comfort. With car-like maintenance intervals, it made good sense as a long distance sports tourer for those that could afford one.
Tools and owner servicing were well thought out. The R90s 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 upright and 200 km/h on the tank. The bike will run at 190 km/h all day if required.
The bike will run close to the redline all day/night, 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 1,000CC, 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 ’74 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.
Another point of interest is that the bike was originally equipped with a manual engine crankcase breather, which was superseded by a reed valve design on the /7 series. An original R90s (as with many /5/6 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 Harleys continue with some form of point of difference to the rest of the world’s manufacturers in this regard, for reasons known only to themselves...
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 R90s 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 cardan shaft effect of either the back of the bike squatting or lifting.
The tyres too are minimal in comparison to modern bikes. The R90S’s 19 inch front/18 inch rear combination are undersized, 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/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 R90s 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 R90s is still regarded as ’the one to keep’ and owners tend to hang onto them for longer periods. While the overall numbers of R90s produced were smaller than the more mainstream /6/7 BMW R series models, the model is still regarded favourably, and for good reason.
From 1973 to 1976 17,455 R90s were sold. The R90s became the R100S in 1977, which maintained the R90s bikini fairing, but ran the full 1000CC engine, 40MM Bing CV carbys, and altered paintwork. The mantle of the lead BMW factory twin was passed to the R100RS, 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 R100CS were also produced in later years. Many later R series ’mono’ bikes still visually resembled the R90s in order to retain a visual link to this significant model.
In closing, BMW R90s 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 D Series Black Shadows 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 CB750 Honda, ZI900 Kawasaki and SS 750 Ducatis 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.
There are many books about BMW boxers (including R90S) but till now there is only one book dedicated to R90S: R90S by Ian Falloon, which describes all the differences of the three 90S series.
1975 BMW R90S
Configuration: Horizontally opposed air-cooled twin, pushrod operated valves, 2 per cylinder
Bore & Stroke: 90 x 70.6 mm
Capacity: 898 cc
Compression Ratio: 9.5 : 1
Maximum Power: 67 DIN hp at 7000 rpm
Maximum Torque: 55 ft/lb (7.6 mkg) at 5500 rpm
Fuel: Dell’Orto PHM 38mm slide type carburettors; Micronic air filter
Lubrication: Gear driven pump
Charging system: 12v 280w alternator
Starter: Electric starter 0.5 hp
Transmission: Gear driven primary drive, 3.00:1 bevel final drive (optional 2.91:1)
- 1st 4.40: 1
- 2nd 2.86:1
- 3rd 2.07:1
- 4th 1.67:1
- 5th 1.50:1
Clutch: Cable operated single plate dry clutch
Frame: Steel main and bolt-on subframe
- Weight: 215 kg wet
- Wheelbase: 1465 mm
- Length: 2180 mm
- Seat Height: 820 mm
Front Suspension: Hydraulic telescopic
Rear suspension: Boge dual dampers, springs adjustable for pre-load
- 1.85 x 19 front
- 2.15 x 18 rear
- Spoked alloy rims
- 3.25 H 19 front
- 4.00 H 18 rear
- Front: 260 mm stainless undrilled discs with floating single piston calipers
- Rear: 200 x 30 mm drum
Fuel Capacity: 24 Litres (5 1/4 Imperial gallons) including 2.5l reserve
- Upright: 195 kph (121 mph)
- Prone: 200 kph+ (125 mph+)
- Standing 400m: 13.3 sec
- Standing 1000m: 25.3 sec
- 0-62.5 mph: 4.8 sec
Fuel Economy: 55.7 mpg (Imp.) at 68 mph