Following the initial presentation of the K series in late 1983, BMW introduced the K100RT already in 1984, in parallel to the K100RS. The RT was produced until 1988, then officially superseded by the K100LT that had been introduced as a "luxury version" in 1986 and that was available until 1991.
According to its VIN, my particular K100LT was produced in 1989-10 and first put into circulation on 1990-05, exactly the same month like my trusty R80GS. I bought it from a Harley-Davidson rider who had owned the LT for 18 years in a row, and he said he used it "only when the Harley had a breakdown". Looking at the 8-year old tires on the LT, it would appear that HD are now quite reliable ;-)
The engine characteristics of the K series is completely different to that of the 2V boxer. The boxer is a spontaneous, jumpy engine; the K is somewhat sluggier (I'd compare it to a TDi car engine ;-), but at the same time also incredibly smooth - without the gear indicator, I would frequently look for a 6th gear on the highway, or try to take hairpins in 3th. In addition, gas consumption is not excessive: About 6 l/100 km on the highway, and 5.5 l/100 km when I'm on secondary roads, giving a range of 275 to 320 km before the reserve light light up. After that, there are still about 5 l left, but with a 285-kg bike I'm not too hot to try this ;-)
The well-designed fairing, the storage space, the smooth engine and the moderate gas consumption make the bike very convenient for long-distance riding, and indeed the K100LT replaces my R100TIC.
The "old" airhead RT carries about 40 kg less than the LT and has a very low center of gravity, providing superior handling on secondary roads and in city traffic. Together with the bigger wheels (19" front is standard in today's "Enduro" bikes!) and the long suspension travel, the airhead RT allows to go pretty fast on roads with a bumpy, deteriorated surface. On the downside, the frame and fork are rather weak, and the brakes may require "planning ahead". It's a perfect bike for long rides on country roads, no matter what surface.
On the other hand, the LT offers a superior frame, brakes and suspension, with the associated stability at any speed. The LT feels rather top-heavy and at low speeds the bike has a tendency to "drop" into a curve, but once you get used to this it is rather easy to manoever. Add to this the low seating position; even small riders can easily place both feet on the ground. The brakes (ABS!) are appropriate and easily deal with the half-ton weight of the fully loaded bike. On the downside, the reduced suspension travel of the LT is noticable on bumpy roads, where the rear shock sometimes bottoms out. Personally, I qualify the LT as a comfortable "mile muncher", preferably to be used on roads with a relatively good surface - and perfectly at ease on today's highways, which were by far not the strength of the airhead RT.
A short comparison of a few technical data to its predecessor, both historically and in my garage - the R100RT (using my R100TIC). Note that in particular the engine data refer to the Swiss version, which has less power and torque than the stock version.
| K100LT 1990 | R100TIC 1981 | |
| Engine | 4 cylinder in line, DOHC, liquid cooled | 2 cylinder boxer, OHV, air cooled |
| Displacement | 67×70 mm, 987 ccm | 94×70.6 mm, 980 ccm |
| Power | 58 kW (79 HP) at 6800/min | 49 kW (67 HP) at 7000/min |
| Torque | 83 Nm at 4000/min | 72 Nm at 5500/min |
| Compression ratio | 10.2:1 | 8.2:1 |
| Gas Tank and Consumption | 22 l, ca. 5.7 l/100 km | ca. 22 l, 6.0 l/100 km |
| Gross weight | 283 kg | 245 kg |
| Max weight | 480 kg | 440 kg |
| Wheelbase | 1516 mm | 1465 mm |
| Front wheel | 100/90-V18 on 2.50×18 MT H2 | 3.25H19 on 2.15B19 |
| Rear wheel | 130/90-V17 on 2.75×17 MT H2 | 4.00H18 on 2.50B18 |
| Front Brake | Double disk, 285 mm, ABS | Double disk, 260 mm |
| Rear Brake | Single disk, 285 mm, ABS | Simplex drum, 200 mm |
| Front Suspension | Telescopic fork, d=41.4 mm, 185 mm suspension travel | Telescopic fork, d=36 mm, 200 mm suspension travel |
| Rear Suspension | Monolever, 110 mm suspension travel | Twin shock, 125 mm suspension travel |
| Ground clearance | 175 mm | 165 mm |
| Alternator | 460 W | 280 W |
When I acquired the LT, the bike was equipped with Metzeler ME330 100/90-18 56V on the front and ME550 140/80-VB17 69V on the rear wheel. Their DOT number indicated that both tires are already a few years old (2002 - I acquired this bike in 2010!), so I plan on "finishing" the rear tire carefully, ensuite changing either to Avon AM-26 or to Bridgestone BT45 (110/80-18 58V @ 2.6 bar, 140/80B17 69V @ 3.0 bar).
The instruments of the K series are rather complete, providing e.g. a gear indicator (useful since I keep looking for a higher gear ;-), and a speedometer with astonishing precision (less than 2% error, compared to a GPSr). In addition, the LT features a rather complete set of instruments: Besides the usual set, you get a gauge for fuel (pretty "nervous") and for the coolant temperature (highly useful, since it can warn you early enough about fan failure).
Since the alternator voltage at idle is not enough to keep the battery charged, I installed one of my LED voltage indicators near the fuel gauge.
A distinct feature of the early RT fairings, be it the airhead or the K, is the weather and wind protection. The upper part of the K fairing is even better than that of the R, since there are almost no turbulences behind the fairing. On the downside, the protection for the rider's feet is less perfect on the K.
As far as I know, three different versions of the windshield were available from BMW:
Similar to the airhead RT, the 565-mm screen also causes quite some wind noise. The big difference is that this is "just" noise - no turbulences, no buffeting. But ... the noise is deafening!
My workaround is the same as on the airhead RT: Use the Fanta Spoiler, which works like a charm. Now I ride with an open visor most of the time, and conversation with a passenger at non-highway speeds is easily possible without shouting. The only time when I notice some buffeting is when I'm following a larger vehicle at highway speeds.
I got my K100LT in 2010-06, barely 20 years and 1 month young, with 37000 km on the counter.
The BMW workshop manual for the K100 series is available as a regular, official part from BMW. The booklet carries part number 01509798790 (german version); for the english version, the part number is 01519798791.
